WO2008051203A2

WO2008051203A2 - Advanced radial core reflector

Info

Publication number: WO2008051203A2
Application number: PCT/US2006/039021
Authority: WO
Inventors: Leonard J. Balog
Original assignee: Westinghouse Electric Company Llc
Priority date: 2005-10-13
Filing date: 2006-10-05
Publication date: 2008-05-02
Also published as: WO2008051203A3

Abstract

An advanced radial core neutron reflector RCR is provided for a nuclear reactor core. The RCR comprises a minimal number of tiers and employs a number of improved restraint mechanisms to provide vertical and horizontal support of the RCR. Specifically, the RCR has four tiers, a bottom tier which functions as a foundation that is precisely aligned and secured with an improved dowel and fastener arrangement, two intermediate tiers, and a top tier. A spigotting staircase-type geometry is employed to radially secure abutting, adjacent tiers. At least the intermediate tiers are segmented into quadrants to facilitate making of the RCR, and a clamping assembly secures the seams between quadrants. One or both of a keying or an alignment pin assembly secures the interfaces between tiers, and a secondary vertical support may be employed at the top tier.

Description

ADVANCED RADIAL CORE REFLECTOR

BACKGROUND OF THE INVENTION Field of the Invention The present invention relates generally to nuclear reactors and, more particularly, to a radial core reflector structured to surround the fuel assemblies in the active core of a nuclear reactor. Background Information

The reactor core within a typical commercial nuclear power reactor, such as for example, a pressurized water reactor (PWR), is formed by numerous elongated fuel assemblies arranged in a cylindrical vessel. Certain reactor designs utilize a radial core neutron reflector (RCR) that surrounds the fuel assemblies in the active nuclear core.

In general, the RCR provides a containment envelope for both core and main coolant flow. It also acts as a fast neutron barrier which shields the pressure vessel wall. Furthermore, the radial core reflector is a load bearing structure which is designed to absorb impact loads while remaining stable and maintaining a predetermined core geometry, which is often critical to ensuring coolability of the . core. Still further advantages provided by the RCR include such things as improved fuel utilization, elimination of mechanical fasteners and/or welded core baffle structures within zones of relatively high radiation, increased pressure vessel life by reducing neutron leakage and damaging radiation fluence on the reactor vessel associated therewith, improving core neutron utilization, and total removability of the RCR for repair, maintenance or modification. However, known radial core reflector designs also suffer from a number of disadvantages. For instance, radial core reflector designs such as the one disclosed in U.S. Patent No. 5,680,424, typically comprise eight or more tiers of reflector segments. The use of eight tiers, for example, results in the presence of seven seams between adjacent tiers, and an eighth seam between the bottom tier and the lower core support plate. Each seam requires precise alignment, with even the slightest mismatch at any one of the seams posing a potential hang-up for peripheral fuel assemblies being inserted or withdrawn from the core. Additionally, long term irradiation and aging-induced swelling may cause seams to open, disadvantageously creating a gap in the horizontal plane between tiers. Such gaps present a number of disadvantages, including the possibility of coolant water jetting which can result in fuel rod damage, and neutron streaming to the pressure vessel. Additionally, six of the seams or gaps of the eight tier radial core reflector are disposed in the high fluence region of the core. It is desirable, therefore, to minimize the number of tiers and thus the number of seams in the radial core reflector.

Horizontal and vertical load containment, and shifting of one tier relative to an adjacent tier have also been known to pose substantial problems in conventional radial core reflector designs. In an attempt to accommodate, for example, horizontal seismic and loss of coolant accident (LOCA) loads in shear, known radial core reflector designs have been known to employ a plurality of large dowels at each interface between adjacent tiers and at the lower core support plate interface. For example, the aforementioned conventional eight tier reflector design requires 36 such dowels. In the vertical direction, a plurality of tension rods pass vertically through all eight of the tiers and thread into the lower core support plate. Thus, in the same eight tier reflector example, there are typically eight such tension rods, each of which is stud tensioned and held with a threaded nut at the top tier level, in order to clamp the entire RCR assembly together and thereby resist vertical uplift forces. Vertical forces, such as for example, safe shutdown earthquake forces (SSE), individually or in combination with loss of coolant accident forces (LOCA), can be substantial (e.g., one-million pounds (453,592 kilograms) or more. Accordingly, existing RCR designs further require eight large radial support pins, four at the top reflector flange and four at the bottom reflector flange. Such support pins are welded into the lower core barrel. It is, therefore, also desirable to reduce the number of components and the associated complexity of known RCR designs, and to overcome the aforementioned disadvantages associated therewith, as well as other disadvantages, such as weld distortion of the lower core barrel induced by welding the radial support pins in place. There is, therefore, room for improvement in several aspects of radial core reflectors for nuclear reactor cores. SUMMARY OF THE INVENTION

These needs and others are met by the present invention, which is directed to an advanced radial core reflector for a nuclear reactor core. Optimized geometric features combined with a reduced number of components provide efficient constraint and load distribution while reducing the number of seams in the core framing structure, thereby minimizing the potential disadvantages known to be associated therewith.

As one aspect of the invention, a radial core reflector is provided for a nuclear reactor core including a pressure vessel, a lower core barrel disposed within a lower portion of the pressure vessel, an upper core plate with radial support pins coupled to an upper portion of the lower core barrel, a lower core support coupled to the bottom of the lower core barrel, and a plurality of fuel assemblies extending longitudinally between the upper core plate and lower core support. The radial core reflector comprises: a plurality of tiers stacked one on top of another, the tiers being structured to surround the fuel assemblies and to be disposed within the pressure vessel in the space between the fuel assemblies and the lower core barrel. A first one of the tiers is structured to be coupled to the lower core support. Yet, the tiers are structured to be secured together independently with respect to the lower core support. The plurality of tiers may include four tiers, a first or bottom tier structured to be coupled to the lower core support by a base restraint, a second or lower center tier structured to be stacked on top of the bottom tier thereby forming a first interface therebetween, a third or upper center tier structured to be stacked on top of the second tier thereby forming a second interface therebetween, and a fourth or top tier structured to be stacked on top of the third tier, thereby forming a third interface therebetween. The bottom tier may include a flange having a plurality of through holes wherein the base restraint includes a plurality of first fasteners structured to be inserted through the holes in order to secure the bottom tier to the lower core support via threaded holes in that member. The second, third and fourth tiers may be secured together and to the bottom tier using a plurality of second fasteners independent with respect to the lower core support. The first fasteners may be socket head cap screws and the second fasteners may be tension rods structured to be inserted through all four tiers of the radial core reflector. The base restraint (i.e., bottom tier attachment), may further include a plurality of dowels structured to be received within the flange of the bottom tier in order to facilitate alignment of the bottom tier. Each of the tiers may comprise a plurality of radial segments wherein the segments are coupled together using a clamping assembly. The plurality of segments may comprise four separate quadrants wherein adjacent quadrants form a vertical seam which is secured together using the clamping assembly. The clamping assembly may comprise a corner angle clamp including a structural angle, a plurality of fasteners, and a plurality of dowels, the dowels being structured to align the structural angle with respect to the vertical seam, resist shear loads and prevent shifting of the quadrants, the fasteners being structured to tighten the clamping assembly, thereby securing the adjacent quadrants together. Each interface between tiers may include an alignment and securing mechanism selected from the group consisting of a keying assembly and an alignment pin assembly. The keying assembly may comprise a seat protruding from the abutting surface of one of the tiers and cooperable with a corresponding slot in the abutting surface of an adjacent tier. The alignment pin assembly may comprise a locating pin structured to be inserted within a first bore and a second bore when the first and second bores are properly aligned, the first and second bores being disposed in the abutting surfaces of the adjacent abutting tiers. As another aspect of the invention, a nuclear reactor core comprises: a pressure vessel; a lower core barrel disposed within a lower portion of the pressure vessel; an upper core plate; a lower core support plate disposed adjacent the bottom of the lower core barrel; a plurality of fuel assemblies extending longitudinally between the upper core plate and the lower core support; and a radial core reflector surrounding the fuel assemblies, the radial core reflector comprising: a plurality of tiers stacked one on top of another in order to form a plurality of interfaces therebetween, the tiers surrounding the fuel assemblies within the pressure vessel in the space between the fuel assemblies and the lower core barrel. A first one of the tiers is coupled to the lower core support and the tiers are structured to be secured together independently with respect to the lower core support.

Individual fuel rods comprising a fuel assembly may be secured by a number of transverse grids along the assembly length to provide spacing between fuel rods and to prevent them from vibrating. Tier lengths are selected so that seams between tiers align with fuel assembly grids. By so doing, any coolant jetting through a seam impinges the grid and not an unsupported area of a fuel rod, thus negating any vibration issue. This predisposition of seam and grid is sometimes referred to as "gap

The tiers may comprise a plurality of radial segments wherein adjacent abutting segments form a vertical seam which is secured together using a clamping assembly. The segments may be quadrants of the tier of the radial core reflector and the vertical seams between adjacent abutting quadrants may be disposed at about the 40-220 degree azimuth axis and at about the 130-310 degree azimuth axis.

The plurality of tiers may include a top tier including a flange and the radial core reflector may include a secondary vertical support structured to limit vertical uplift of the RCR assembly during a severe seismic event, thus limiting tie rod extension and bottom tier clamping screws extension below failure levels. The core barrel, in the vicinity of the upper plate core may include a plurality of upper core plate alignment pins and the secondary vertical support may comprise a base plate coupled to the top tier, and a support post disposed on top of the base plate and structure to be positioned beneath the alignment pins of the core barrel with a calculated cold gap. One or more of the interfaces between the tiers may include a staircase geometry in which adjacent abutting surfaces of the adjacent tiers are spigotted with respect to one another in order to cooperate in precise alignment and to absorb horizontal shear loads which thereby eliminates the need of large dowel pins between tiers.

The lower core barrel may include a number of weld bands disposed proximate the interfaces between tiers of the radial core reflector wherein each of the weld bands is structured to provide a predetermined gap between the radial core reflector and the lower core barrel, while ensuring radial securement of the radial core reflector at core operating temperatures. The lower core barrel may have an inner diameter and the core may include a number of irradiation specimen holders wherein at least one of the irradiation specimen holders is disposed in the space between the inner diameter of the lower core barrel and the exterior of the radial core reflector. At least one thermocouple may be disposed within one of the tiers at an elevation of the radial core reflector at which the temperature during operation of the core is maximized, or at such locations where measured values can corroborate calculated values.

BRIEF DESCRIPTION OF THE DRAWINGS A full understanding of the invention can be gained from the following description of the preferred embodiments when read in conjunction with the accompanying drawings in which:

Figure 1 is a cross-sectional view of the bottom two-thirds of a nuclear reactor vessel and core including a radial core reflector, with certain internal structures removed for simplicity of illustration;

Figures 2A, 2B and 2C are cross-sectional views of peripheral portions of the nuclear reactor core of Figure 1 , showing a conventional radial core reflector having eight tiers and some of the hardware necessary to secure the same and a device to extract the radial core reflector as a total assembly; Figure 3 is a vertical elevational view of a portion of a nuclear reactor core and an advanced radial core reflector in accordance with the present invention;

Figure 4 is a cross-sectional view of a peripheral section of the advanced radial core reflector of Figure 3, showing a fastening and spigotting mechanism therefor; Figure 5 is a plan view of the reactor core of Figure 1 with internal structures showing radial core reflector cooling hole patterns, and tension rods and irradiation specimen basket intended locations.

Figure 6 is a cross-sectional view of a peripheral section of the radial core reflector of Figure 3, showing a dowel arrangement for aligning the bottom tier thereof;

Figures 7A and 7B are plan and vertical electrical views, respectively, of an assembly for securing tier segments of the radial core reflector in accordance with an aspect of the invention;

Figures 8A, 8B and 8C are cross-sectional, exploded and plan views, respectively, of a keying assembly for the interfaces between tiers of the radial core reflector in accordance with an embodiment of the invention; Figures 9A, 9B and 9C are cross-sectional, exploded, and plan views, respectively, of an alignment pin assembly for the interfaces between tiers of the radial core reflector in accordance with another embodiment of the invention;

Figure 10 is a cross-sectional view of a peripheral section of the radial core reflector of Figure 3, showing a coolant flow path through the lower core support;

Figure 1 1 is a cross-sectional, vertical elevational view showing a spigotting staircase-type geometry and gap at the interface between abutting tiers of the radial core reflector; Figures 12 and 12B are cross-sectional, vertical elevational and plan views, respectively, of the top tier radial restraint for the radial core reflector;

Figures 13 A and 13B are plan and vertical elevational, partially sectioned views, respectively, of a secondary vertical seismic support in accordance with an aspect of the invention; Figures 14A and 14B are plan and vertical elevational views, respectively, of an irradiation specimen holder configuration in accordance with an aspect of the invention; and

Figures 15A and 15B are plan and vertical elevational, partially sectioned views, respectively, of a thermocouple configuration for the radial core reflector in accordance with another aspect of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS For purposes of illustration, the invention will be described as applied to an exemplary nuclear reactor core having 157 fuel assemblies, although it will become apparent that it could also be applied to smaller nuclear reactor plants (e.g., without limitation, plants having a reactor core design with about 121 fuel assemblies or less) as well as to reactor plants employing a larger core design (e.g., without limitation, about 248 fuel assemblies or more). Two representative, nonlimiting examples of known commercial nuclear reactors employing RCR concepts are also discussed for purposes of comparison. These examples include the Westinghouse Electrical Company LLC AP-600 reactor which has 145 fuel assemblies, and the Mitsubishi Heavy Industries Ltd. Tsuruga 3 reactor which has 248 fuel assemblies. Directional phrases used herein, such as for example, left, right, top, bottom, upper, lower, and derivatives thereof, relate to the orientation of the elements shown in the drawings and are not limiting upon the claims unless expressly recited therein. As employed herein, the term "fastener" refers to any suitable connecting or tightening mechanism expressly including, but not limited to, dowels, pins, a number of interlocking components, screws including socket head cap screws (SHCS), tie rods, bolts and the combinations of bolts and nuts (e.g., without limitation, lock nuts) and bolts, washers and nuts. As employed herein, the statement that two or more parts are

"coupled" together shall mean that the parts are joined together either directly or joined through one or more intermediate parts.

As employed herein, the term "number" shall include a quantity of one or a quantity more than one (i.e., a plurality). Reactor Core

Figure 1 shows a nuclear reactor core 30 for a commercial nuclear power reactor. The components and functional details of the reactor core 30 are generally old and well known in the art. For example, a detailed description of the core 30 can be found in The Scientific Encyclopedia, Van Nostrand, pp. 2208-2209, 8th ed., 1995. In general, the core 30 is housed within a reactor vessel 32 designed to contain a plurality of fuel assemblies 34, shown in simplified form in Figure 1, rod cluster control assemblies 36 for controlling the vertical movement of control rods within the fuel assemblies 34 within the core 30, and a variety of additional internal structures required to support the core 30. A more detailed description of the fuel assembly 34 is provided, for example, in U.S. Patent No. 4,061,536.

Typically, the reactor vessel 32 is an Inconel clad, thick-walled, carbon steel pressure vessel comprised of a cylinder with two hemispherical halves, an upper half (not shown) and a lower half 38. The upper half is joined by a forged ring, or vessel flange (not shown). The lower half 38 is welded. The vessel 32 includes inlet and outlet nozzles 40, 42 located radially below the vessel flange. The fuel assemblies 34 are disposed within a lower core barrel 44, which is defined by a core shroud 46 or a core baffle/former configuration. Both have an outer cylindrical configuration concentric with respect to the lower core barrel 44 and both concepts present a staircase-type geometry which forms a perimeter surrounding the array of fuel assemblies 34. The upper core plate 48 is an integral part of the upper internals assembly (partially shown), upper core plate alignment pins 49 are affixed to the lower core barrel 44, and the lower core support plate 50 is affixed to the lower core barrel 44 by a complete circumferential weld. A radial core reflector (RCR) 52 (shown in simplified form in Figure 1) surrounds the fuel assemblies 34 and is disposed in the generally irregular space between the fuel assemblies 34 and the core barrel 44. Radial Core Reflector (RCR)

As previously discussed, and as shown in Figures 2A, 2B and 2C, known radial core reflectors 52 typically employ a plurality of tiers or sections 54. The example shown is representative of the radial core reflector 52 for the aforementioned Westinghouse AP-600 reactor, and includes eight tiers 54 which undesirably result in the presence of seven seams 56 between adjacent, abutting tiers 54, and an additional eighth seam 57 between the bottom tier 54^' and the lower core plate 50. A plurality of large dowels 58 (Figure 2B) measuring, for example, about 6 in. (15.2 cm) in diameter, are required at the interface or seam 57 between the bottom tier 54' and the lower core support plate 50, as shown in Figure 2B. The AP-600 example requires 36 such dowels 58 disposed around the periphery of the RCR 52. In the vertical direction, a plurality of tension rods 60 (Figure 2A), which pass vertically through all eight of the tiers 54, are required. For ease of illustration, only one tension rod 60 is shown in Figure 2A. However, there are typically eight such tension rods 60 peripherally spaced in order to hold the RCR 52 together. Each rod 60 is about 1.5 in. (3.8 cm) in diameter and, as shown in Figure 2A, is threaded into the lower core support plate 50. The rods 60 are then stud tensioned and held with a threaded nut 62 at the top tier level 64. In this manner, the entire RCR assembly stack 52 is clamped together and to the lower core support plate 50 in order to resist vertical uplift forces. As shown in Figure 2C, the design also requires eight large (e.g., about 7 in. (17.8 cm) in diameter) radial support pins 66, four at the top RCR flange 68, and four at the bottom RCR flange (not shown in Figure 2C). The support pins 66 (for ease of illustration, only an upper pin 66 is shown in Figure 2C) are welded to the lower core barrel 44.

Referring now to Figure 3, and comparing the same with the AP-600 example of Figures 2 A, 2B and 2C, the advanced radial core reflector 152 of the present invention provides an improved design with a reduced number of components while improving structural integrity and safety. In particular, the number of tiers 151,

153, 155, 157 and thus the number of seams 150, 154, 156 therebetween, are dramatically reduced. Specifically, the RCR 152 example of Figure 3 includes four tiers, a first or bottom tier 151 , a second or lower center tier 153, a third or upper center tier 155, and a fourth or upper tier 157. Accordingly, the number of seams 150,

154, 156 has been reduced from eight (see seams 56 and 57 of Figures 2A, 2B and 2C) to three, a first seam 150 between the first and second tiers 151, 153, a second seam 154 between the second and third tiers 153, 155, and a third seem 156 between the third and fourth tiers 155, 157. Therefore, the aforementioned potential problems of, for example, long term irradiation and aging induced swelling concerns, which are associated with seam gaps, are advantageously proportionally reduced, as is the undesirable potential for misalignment between tiers 151 , 153, 155, 157. For new plants featuring longer core lengths (e.g., 168 in. (426.7cm)), nine tiers would be required thereby creating 8 seams, conventionally. This advanced concept would still only produce three seams. The superior neutron shielding afforded by the RCR 152 of the invention is significant for new nuclear plants and for existing plants looking for extended contract life, i.e., 40 years to 60 years (calendar). With conventional shielding, solutions to extend life include increasing the pressure vessel inside diameter, employing thicker stainless steel neutron panels, or using more exotic neutron panel materials like tungsten or beryleum. However, each of these choices has its own attendant problems. For example, increasing the pressure vessel inside diameter means increasing wall thickness and vapor containment volume, which is cost intensive, thicker neutron panels adversely affect coolant downcomer velocity and pressure drop parameters, and new materials are of a concern because of little or no plant operating experience. Bottom Tier Foundation

Among other advantages of the advanced reactor core reflector design 152 of the invention, is the fact that the bottom RCR tier 151 has been redesigned to advantageously serve as a foundation block which is positioned and then fixed to achieve a precise configuration with respect to the core barrel 44 and the lower core support plate 50. This precise configuration is then maintained with each successively higher tier 153, 155, 157 being precisely positioned and aligned upon the foundation 151. Accordingly, any potential for misalignment and propagation of the same from tier to tier is substantially reduced for the exemplary four tier RCR design 152 compared to known eight tier RCR designs (e.g., RCR 52 of Figures 2A, 2B and 2C).

More specifically, as shown in Figure 4, the bottom tier or foundation 151 of the advanced RCR 152 is affixed to the lower core support plate 50 with fasteners, such as the socket head cap screws (SHCS) 164, shown. The exemplary RCR 152 uses between about 20 to about 24 SHCS 164, which are between about .875 to about 1.00 in. (2.22 to 2.54 cm) in diameter and are radially spaced about the periphery of the core 30, as shown in the plan view of Figure 5 (20 SHCS shown, for example). During assembly, when the bottom tier 151 is located at its optimum position, it is transfer drilled to receive positioning dowels 158 (Figures 5 and 6), and SHCS 164 (Figures 4 and 5) through existing holes 166 in the bottom tier flange 167. Transfer drilling is a well known machining process which involves the creation of an aperture or hole which aligns with an opposing existing aperture or hole, by using the existing hole as a guide or reference when performing the machining operation. Accordingly, contrary to conventional designs such as the AP-600 model RCR 52 (Figures 2A, 2B and 2C), which require tapped holes 59 (Figure 2A) in the lower core support plate 50 for receiving the tension rods 60 (Figure 2A), the advanced RCR design 152 of the invention contemplates providing threaded holes 166 in the flange 167 (best shown in Figure 4) of the bottom tier 151. This avoids potentially damaging the lower core support plate 50 which, unlike the bottom tier 151 , is irreplaceable. Accordingly, the four tiers 151, 153, 155, 157 of the exemplary RCR 152 are clamped together via the eight tension rods 160 (one tension rod 160 is shown in Figure 3), while the bottom tier 151 is independently fixed and aligned to the lower core support 50 by the SHCS 164 and dowels 158 shown in Figures 4 and 6, respectively. In other words, unlike conventional RCR designs, the tension rods 160 of the advanced RCR 152 of the invention are not directly threaded into the lower core support 50. Thus, loads transmitted through the tiers 151, 153, 155, 157 of the RCR 152 and, in particular, the tension rods 160, are effectively decoupled from the lower core support 50. As such, the likelihood of damage to the lower core support 50 is minimized.

Once the exemplary bottom tier or foundation 151 is precisely aligned and secured, the second, third and fourth tiers 153, 155, 157 are sequentially installed while maintaining such precise alignment. Additional mechanisms for aligning and securing the tiers 151 , 153, 155, 157 and maintaining alignment therebetween in accordance with the invention, will be discussed hereinbelow. Gaps in the Horizontal Plane Between Tiers of the RCR

As previously noted, gaps at the seams (e.g., seams 56 and 57 of Figures 2A, 2B and 2C) between tiers (e.g., tiers 54 of Figures 2 A, 2B and 2C) undesirably present the possibility of coolant water jetting inflicting fuel rod damage, and neutron streaming to the pressure vessel 32 (Figure 1 ). As previously discussed, the RCR 152 of the invention addresses these concerns by reducing the number of tiers 151, 153, 155, 157 from eight to four, and the number of seams 150, 154, 156 and thus gaps potentially associated therewith, from eight to three. This results in only two seams 154 and 156 (Figure 3) in the high fluence region of the core 30, as opposed to conventional RCRs (e.g., RCR 52 of Figures 2 A, 2B and 2C) wherein there are six seams in the high fluence region. Specifically, tier heights have been chosen so that the three circumferential gaps at seams 150, 154, 156 of the exemplary RCR 152 coincide in elevation with grid locations 35 on the fuel element 34, as shown in Figure 3. Therefore, the likelihood of fuel rod damage in the event of coolant jetting through any such gaps, is significantly reduced. All of the aforementioned gaps are in the horizontal plane between tiers 151, 153, 155, 157 of the advanced RCR 152.

As will be discussed, the advanced RCR 152 of the invention does result in the creation of four longitudinal gaps 174 at the interfaces of the four radial segments which comprise each of tiers two and three (Figure 7A). However, as shown, for example, in Figure 7A, the longitudinal gaps 174 are disrupted by a corner angle clamp 172 which is structured to shut off water and neutron streaming paths. More specifically, the horizontal gaps of conventional RCR designs have been eliminated by virtue of replacing seven toroidal tiers (e.g., tiers 54 of Figures 2A, 2B and 2C), commonly referred to in the art as pancake type tiers, with the aforementioned lower center and upper center tiers 153, 155 each about 62 in. (157.5 cm) long in the case of an extended core plant, and each comprised of four radial segments 173, 175 (shown in Figure 7A) each measuring about 90 degrees in arc length and connected at abutting joints 174, with a novel clamping, dowel assembly 176 which will be discussed. As shown in Figure 5, this geometry produces two tiers 153, 155 each having four vertical seams 174 at about the 40-220 degree azimuth axis 180, and about the 130-310 degree azimuth axis 178, rather than the aforementioned conventional horizontal gaps. The vertical seams 174 do not present any new gap problems because the exemplary clamping assembly 176 (discussed below) (Figures 7 A and 7B), closes off both water jetting and neutron streaming paths. Furthermore, the neutron fluence level at the azimuths of the vertical seams 174 is only about 60% of the peak fluence, which exists at about the 0-180 degree axis 182 and about the 90-270 degree axis 184.

The reason that eight vertical tiers (total) have been conventionally chosen instead of four are: (1) forging capacity weight, and (2) the fact that the machine carriage that is used to machine (i.e., plane) the core faces (i.e , the staircase geometry shown, for example, in Figure 5) on the inside perimeter of each tier generally cannot fit through the closed geometry of the toroid in order to complete the machining thereof. As previously noted with respect to Figure 5, the invention addresses these issues by segmenting the tiers 153, 155 (Figure 3) in four parts or quadrants 173, 175 (Figure 7A), which results in the planing machine, for example, having no such access restriction during production. With respect to forging weight, because each tier 153, 155 of the advanced RCR 152 of the invention deals with quadrants 173, 175, the length of each quadrant 173, 175 can be four times the length of the conventional pancake tiers and yet still satisfy forging weight limitations. Casting the RCR tiers remains an option and these same criteria would apply. It is, however, important to note that one potential problem is posed with regard to the extended length of the exemplary tiers 151, 153, 155, 157 namely, how to facilitate the fabrication of cooling flow holes 186 therein. More specifically, as will be appreciated with reference to the plan view of Figure 5, there are about 750 cooling holes about 0.75 in. (1.9 cm) in diameter each, which extend through the 61.6 in. (156.5 cm) length of the exemplary tiers 153, 155. Conventional gun drilling machines, for example, cannot accommodate the weight represented by the tiers 153, 155 (segments). One solution is to set up the workpiece (i.e., a tier 153 segment) and bring the machining device, (not shown), to the set up rather than the more common, vice versa method of moving the workpiece. This proposed method is well known to be done with success, for example, when steam generator tube sheets are gun drilled. With regard to the extended length of the tiers 153, 155, one method of fabrication could entail drilling halfway through from each end of the tier 153, 155. This works well because a precise, true position of the cooling holes 186 is important only at both end faces to ensure that they match up with the continuance of the cooling holes 186 on adjacent, abutting tiers 151, 157 and the interface of 153, 155. Additionally, any runout or mismatch would occur at the center of the tier 153, 155, is of nominal concern, because these holes 186 are merely flow paths to cool the material. Exact alignment is not required. Mismatch is expected to be about 0.060 in. (.2 cm), maximum.

Clamping of the RCR Segments

As discussed previously, securing the 90 degree segments or quadrants 173, 175 of the tiers 153, 155 is achieved by way of a clamping assembly 176. Figures 7A and 7B illustrate an exemplary embodiment of such assembly 176. Corner angle clamps (structural angles) 172, four for each long tier

153, 155 (one angle 172 and one tier 153 are shown in Figures 7A and 7B) bring the segments 173, 175 together and secure them via 20 SHCSs 190 about 0.75 in. (1.9 cm) in diameter, after dowels 192 that are about 0.75 in. (1.9 cm) in diameter, align each vertical seam 174. In the example of Figures 7 A and 7B, 80 screws 190 and 24 dowels 192 are used for each of the two 61.6 in. (156.5 cm) long tiers 153, 155 (one tier 153 is shown). However, it will be appreciated that any known suitable alternative fasteners in any suitable number and configuration, could be employed. Even welding is a possibility.

The SHCS 190 and dowels 192 see relatively little radiation because of their core position azimuth (see Figure 5) and location on the outside of the RCR 152 instead of the core side (see Figure 5). Two additional dowel pins 194 (Figure 7A) which, in the example shown and described herein are hollow and measure about 1.12 in. (2.8 cm) in diameter and 3.0 in. (7.62 cm) long, are added in the vertical plane centered on each seam 174, one at the top and one at the bottom of each tier 153, 155 (only top dowel 194 is shown for ease of illustration) in order to fix the position of the joint between adjacent tiers (e.g., 153, 155 (Figure 3)) and to resist any shifting of one tier segment 173, 175 relative to the other. These dowels 194 also provide an additional barrier to neutron streaming, for a relatively short length, through the 45 degree joints 174 (best shown in Figure 7A). In the example of Figure 7A, the dowels 194 are hollow because they are positioned directly over a cooling hole 186. Thus, it will be appreciated that in operation, coolant flows through the dowels 194.

Horizontal and vertical load containment, and shifting of one tier relative to an adjacent tier have posed substantial design problems in conventional RCR designs. As previously discussed, safe shutdown earthquake forces, (SSE), combined with LOCA forces, can be substantial with vertical forces of more than about one-million pounds (453, 592 kilograms) or more being possible depending on plant size and site location. As noted hereinbefore, previously these issues have been addressed through use of relatively large dowels 58 (Figure 2B) (i.e., about 6.0 in. (15.2 cm) in diameter), four at each interface, between tiers and at the lower core support plate interface 57 (Figures 2A, 2B and 2C). For instance, in the AP-600 example discussed hereinbefore, 36 such dowels are employed to counter horizontal seismic and LOCA loads in shear and in the vertical direction, the tension rods 60 (Figure 2A), which are about 1.5 in. (3.8 cm) in diameter, pass vertically through all of the tiers 54, 54^' (Figures 2A, 2B and 2C), and thread into the threaded hole 159 (Figure 2A) in the lower core support plate 50. Eight such rods 60 (Figure 2A) were employed, with each rod 60 being stud tensioned and held with a threaded nut 62 (Figure 2A) at the top tier level 64 (Figure 2A), in order to clamp the entire RCR assembly stack 52 (Figure 2A) together, and thereby resist vertical uplift forces. As previously discussed with respect to Figure 3, vertical restraint of the advanced RCR 152 of the invention also contemplates use of tension rods 160 but, in an independent support assembly which does not require the rods 160 to be threaded directly into the lower core support plate 50. Additionally, as shown in Figures 8A, 8B and 8C and 9A, 9B, and 9C, the advanced RCR 152 of the invention also contemplates replacing the aforementioned 36 relatively large dowel pins 58, with a keying assembly 200 (Figures 8A, 8B and 8C) or an alternative alignment pin assembly 210 (Figures 9 A, 9B and 9C), for tier-to-tier alignment purposes.

Figures 8A and 8B, by way of example, show seam or interface 154 between tiers 153 and 155 (tiers 253 and 255 of Figures 9A and 9B). It will be appreciated, however, that assemblies 200 and 210 (Figures 8 A, 8B, 8C, 9A, 9B and 9C) are substantially similar at the other interfaces (not shown) of the RCR 152 assembly. In the example of Figures 8A, 8B and 8C, the keying assembly 200 (two per tier interface) of the invention generally comprises a slot 202 machined in the adjacent abutting surface of one tier (e.g., 155) which is cooperable with a corresponding key 204 on the adjacent, abutting surface of the mating tier (e.g., 153). Two positioning dowels 206 which are of a reduced diameter, about 1.75 in. (4.4 cm), are used at each tier interface 154, for a total of twelve dowels 206, and two additional longer dowels 158 (Figure 6), about 2.25 in. (5.7 cm) in diameter, are employed at the bottom tier and lower core support plate 50 interface (Figure 6). Figure 8C shows a plan view of the key assembly 200 and the positioning of dowels 206 and a fastener 208 for securing the same. In the example of Figure 8C, the fastener is a centrally located SHCS 208 disposed between dowels 206 of the key assembly 200.

The alternative, tier interface alignment pin assembly 210 generally accomplishes the same alignment and support goals as key assembly 200, but through an alternative geometry, which is shown in Figures 9A, 9B and 9C. Rather than the aforementioned slot and seat configuration, alignment pin assembly 210 generally includes a locating pin 212 which is received in corresponding bores 214, 216 in the adjacent tiers 253, 255, respectively, when they are properly aligned (best shown in Figure 9A). The pin 212 is hollow in the example of Figures 9A, 9B and 9C and thereby provides a continuation of a cooling hole 286. Figure 9B shows the assembly 210 prior to joining the tiers 253, 255 at interface 254 (Figure 9). Figure 9C provides a plan view illustrating the general orientation of the assembly 210 and alignment pin 212 therefor, as disposed proximate the periphery of the advanced RCR 252. Accordingly, the tier interface assemblies 200 (Figures 8A, 8B and 8C) and 210 (Figures 9A, 9B and 9C) reduce the number of dowels from 36 to 14 and 36 to 8, respectively, and also reduces the size of all of the dowels significantly. Additionally, the advanced RCR 152 (Figures 8A, 8B, and 8C) and 252 (Figures 9A, 9B and 9C) of the invention eliminates the eight relatively large (i.e., about 7.0 in. (17.8 cm) in diameter) radial support pins, four at the top and four at the bottom of the RCR flanges, which are welded into the lower core barrel 44 in conventional RCR designs, as previously discussed. These pins and the high cost and core barrel distortion associated with them are, therefore, eliminated by the advanced RCR 152 of the invention. The following further explains how this is possible. Horizontal Shear Loads

The alignment pin assembly 210 or keying assembly 200, depending on which design the RCR 152 employs between the abutting tiers 153, 155, will provide precise centering of one tier to the next. Horizontal shear loads are substantially borne by spigotting each tier 151, 153, 155, 157 to its mating companion and the bottom tier 151 to the lower core support plate 50, as shown in Figure 4. As employed herein, the term "spigotting" and derivatives thereof refers to use of a series of precisely dimensioned, machined (i.e., turned and bored) interconnecting geometries (i.e., diameters). The exemplary interfacing geometry is a staircase-type geometry between adjacent abutting tiers.

Specifically, referring back to Figure 4 and also to Figures 10 and 1 1, the spigot fit engagement at the lower core support 50 interface 149 with bottom tier 151 , is shown. With the exception of the geometry used to introduce main coolant water into the RCR sectors positioned on the cardinal axes of the core (illustrated in Figure 10), Figure 4 shows the flow path at all other typical cross sections. Referring to Figure 4, coolant water passing up through the lower core support plate enters the core, but a small percentage (on the order of 5 percent), passes through the feet of the peripheral fuel assemblies 34 and is gated into the cooling hole plenum 171 through apertures 188 cut through the core-side faces of the RCR. The number of apertures and the size of their opening will be selected to produce the desired pressure drop and flow volume based on the chosen number of RCR cooling holes and their diameter. To produce the spigot fit, the bore in the lower core support plate 50 is measured, and then the mating diameter on the bottom tier flange 167 (Figure 4) is made (i.e., turned) to provide a radial gap 170 of about .018 in., ±.005 (.046 cm, + .013 cm). This close fit can be achieved by first measuring the bore in the lower core support 50 and then machining the turn on the bottom tier flange 167 to reduce tolerance stack- up, and second, heating the lower core support 50 and cooling the bottom tier 151 before attempting engagement of the two. This gap 170 will close by about .015 in. (.038 cm) at operating conditions due to thermal expansion. In accordance with this design, any faulted load (e.g., horizontal load) will first close the nominal hot radial gap of about .003 in. (.008 cm) straining dowel pins 158 (Figure 6) to that extent. It will then encounter a shear path through the ledge 177 (i.e., rim) of the lower core support plate 50 which compromises a section of material about 1.25 in. (3.2 cm) thick, over an assumed arc of about 32 degrees or approximately 44 in.² (11 1.8 cm²) of material. In other words, the exemplary structure is well suited to accommodate significant shear loads. Specifically, it can sustain a horizontal load of about 450,000 Ib. (204,1 16.6 kg) before yielding in shear. Additionally, a tight fit is provided between core barrel 44 and the RCR diameter, at about 21 inches (53.3 cm) above the interface 150 between the bottom tier 151 and next tier 153. This limits the shear force at the bottom of the assembly considerably, as compared to the conventional design. This design also eliminates the need for the aforementioned four large alignment pins 66 (Figure 2C) which had to be welded to the lower core barrel 44, and engage slots in the bottom tier flange 67 (Figure 2B) in the conventional design shown in Figures 2A, 2B and 2C. As shown in Figure 1 1 , at the next three tier interfaces 150, 154, 156, as at the interface between the foundation or bottom tier 151 and lower core support 50, substantially the same spigotting profile is used to accommodate the shear loads. For ease of illustration, only interface 154 between tiers 153, 155, is shown. Specifically, a full diameter (not shown) exists only on the four cardinal horizontal axes (see Figure 5). Elsewhere, arcuate sections exist (see e.g., arcuate sections of Figures 8C and 9C), each having arc length of, for example, about 28 degrees. At these locations, a small radial cold clearance 220 of about 0.039 in. (.099 cm), substantially closes due to thermal expansion, as previously discussed.

It will be appreciated that the spigotting dimensions and configurations shown and described herein represent merely one possible configuration. Any known or suitable alternative configuration could be employed. For example, without limitation, an alternate possibility would be to provide a larger radial gap (see, for example, gap 170 of Figure 1 1) of, for example, about 0.62 in. (1.6 cm), in order to facilitate assembly. The gap .062 in. (.157cm) (not shown) could be measured upon assembly and a ribbon of shim stock not shown, for example, could be customized and threaded into the gap from both ends of the 28 degree arc segment (see, e.g.,

Figure 5 and the arcuate segment of Figure 9C). Arc length is about 33 in. (83.8 cm) long. Therefore, each ribbon or shim would be about 16 in. (40.6 cm) long, about 0.5 in. (1.3 cm) wide, and about .003 in. (.008 cm) to about .005 in. (.013 cm) less than the gap width (i.e., 0.058 in. (.147 cm)) thick. Such shims could be installed, for example, at eight places at two tier interfaces 150, 154 for a total of sixteen shims. It will also be appreciated that the shims could be secured in any suitable manner, such as by tack welding. Accordingly, there would be essentially no room for the tiers 153, 155 to shift relative to each other even if the dowels 206 or locating pin 212 were to yield in shear. Continuing to refer to Figure 1 1 , precise gaps 220, 222 are also provided between the outside diameter (OD) of the RCR 152 and the inside diameter (ID) of the lower core barrel 44, proximate each tier interface (interface or seam 154 is shown in Figure 1 1). Such gaps 220, 222 provide for additional support and better load distribution over the length of the RCR 152, which is about 182.25 in. (462.9 cm). In the example of Figure 11, the gaps 220, 222 are achieved by using weld build-up 224 on the ID of the core barrel 44 at these locations, and then machining them to provide a true circular opening. Core barrel rolling tolerances of plus or minus about 0.25 in. (0.6 cm) on the diameter makes this machining step a necessity. However, the weld bands 224 only have to coincide with the 28 degree arc length of the OD of the RCR 152 on each cardinal axes (see, e.g., the plan view of Figure 5), or only about 31% of the total circumference. To facilitate assembly and removal of the RCR assembly, the machined diameter of these four weld bands (one weld band 224 is shown) become discreetly smaller as they descend in elevation with respect to the core barrel 44. The gaps 220, 222 are structured such that they close to about .010 in. (.025 cm) clearance at operating conditions (i.e., due to thermal expansion), as previously discussed and perhaps to zero at end of plant heat up. The core barrel 44 is at a lower temperature during operation than the RCR tiers 153, 155. The cold radial gaps 220, 222 in the aforementioned example will, therefore, be on the order of about 0.039 in. (.099 cm) and the engagement length of the welds 224 will be about 2 in. (5.1 cm). Accordingly, in view of the spigotted, stepped diameter features at each interface 154 between tiers 153, 155 during assembly of the RCR 152, the weld bands 224 will all engage the tiers 153, 155 simultaneously as the RCR assembly 152 nears the bottom of its descent into the lower core barrel 44, thereby providing for easier insertion or removal and a secure fit.

As shown in Figure 12A at top tier 157, as on the bottom tier 151, previously discussed, there is a full circumferential flange 159 which is located about 7.5 in. (19.1 cm) below the upper core plate (not shown in Figure 12)(flow holes through the flange are not shown). As shown, the staircase profile at this location extends between a pad 226 at the periphery of the top tier 157 and the top surface 163 of the top flange 159. At this elevation, and at eight strategic locations around the circumference, the pad which is a customized radial bearing pad 226, is designed to take horizontal forces and transfer them into the core barrel 44 via the aforementioned machined weld bands 224. Such pads 226 are fitted and then bolted and doweled into slots 228 in the upper tier flange 159, as shown in Figures 12A and 12B.

During assembly of the RCR 152 and, in particular, at the final shop fit-up assembly of the RCR 152 and the core internals, measurements are taken from the back of the slots 228 to the opposing ID of the core barrel 44 (machined weld build up). Based upon these measurements, an outside radius is machined on the pads 226 in order to provide a desired cold radial clearance or gap 230 of about 0.39 in. (.99 cm) nominal, at these eight circumferential locations (one location is shown in Figure 12). Two of the bearing pads 226 will "straddle" (i.e., one on either side) (not shown) the axis of the pressure vessel inlet nozzle 40 (Figure 1), in regard to their positions on the flange 159. Since horizontal LOCA loads will be highest along such axis, fuel impact loads will be effectively transferred into the lower core barrel 44 via the four pads 226 (one pad 226 is shown in Figure 12). The remaining four pads (not shown) will generally be equally distributed around the circumference on the top tier flange 159 to resist oscillating movement of the RCR structure.

Significant, with respect to the aforementioned concept of very small radial gaps 220, 222 between the RCR assembly 152 and the ID of the core barrel 44 at four elevations above the lower core support 50, is the result that any overturning moment caused, for example, by large horizontal loads, is essentially negated. There is, therefore, minimal concern over a restraining couple which could, for example, introduce upsetting forces in the clamping bolts, (e.g., at the RCR bottom flange 167 to lower core support plate 50 (Figure 4)), tension rods 160 (Figure 3), or any core derangement in the particular portion of the RCR 152 which is affected. Furthermore, the foregoing radial restraints of the invention (e.g., between the OD of the RCR 152 and the ID of the core barrel 44 at four elevations above the lower core support plate 50) better distribute and dissipate large horizontal, faulted condition loads, thereby reducing local stresses, increasing safety margins, and assuring core stability. Vertical Loads-Secondary Vertical Seismic Support

Vertical loads, arising during postulated seismic and LOCA events, for example, are absorbed by the fasteners 164 (Figure 4) which attach the bottom tier 151 of the RCR 152 to the lower core support plate 50 and the tension rods 160 (Figure 2A) which clamp the three upper tiers 153, 155, 157 to the bottom tier 151 , as previously discussed. However, concern over the possibility of relaxation of the tension rods 160 over time and larger than anticipated vertical loads suggests a need for additional vertical load management. Accordingly, a secondary vertical seismic support 300 is provided in accordance with another aspect of the invention. As shown in Figures 13A and 13B, the secondary vertical seismic support 300 generally comprises support posts 302 (one is shown) positioned at four locations on top of the top tier flange 159, directly beneath the upper core plate 48 alignment pins 49. The post 302 will come to bear on the underside of the alignment pins 49 after the preload in the bottom tier bolts 164 (Figure 4) and the tension rods 160 (Figure 2A) is overcome and clamping items, such as the bolts 164 and rods 160, begin to yield. In the example shown, the support post 302 is coupled to the top tier upper flange 159 using four fasteners 304. A number of shims 306 are employed between the base 308 of the post 302 and a notch 161 in the top surface 163 of the flange 159, as best shown in Figure 13B.

Sizing of the components of the secondary seismic support 300 and, in particular, the shims 306 therefore, can take place at the same time that the radial gap 220, 222, 230 dimensions are taken when customizing the radial shim pads for the top tier 157, previously discussed. Specifically, vertical dimensions will be taken for the measurement between the seating areas on the top tier flange 159 and the underside of the upper core plate alignment pins 49. From these dimensions, the seismic support base plates 308 will be custom machined, or a shim stock 306 will be selected so as to provide a vertical cold gap 310 between each support post 302 and alignment pin 49 on the order of about 0.1 10 in. (.28 cm). This will result in a gap of 0.015 in. (.04 cm) at operating temperatures. Closure of this gap, for example in response to a fault scenario, will limit the strain and extension in the fasteners 160, 164, 304 thus resisting fracture, and containing the upset condition without any adverse affects on the core 30 (Figure 1 ).

In order to remove the RCR assembly 152, should the need arise, the four secondary seismic supports 300 will need to be disengaged and removed first. This does not pose a problem because mechanical locking devices (not shown) which secure the support clamping bolts, can be readily undone. Additionally, geometric features provided on the posts 302, such as tapped holes or gripping areas for remote handling tools will facilitate handling of the assembly. Accordingly, the secondary seismic supports 300 in accordance with the invention can be easily dissembled and then re-assembled, as needed. Supplemental Torsional Restraint In a severe seismic episode, the torsional rigidity of the RCR assembly

152 must be considered. The advanced RCR 152 of the invention also contemplates providing additional torsional restraint to address this concern. Specifically, the bottom tier 151 , with the clamping affect of the threaded fastener and the doweling to the lower core support 50 will resist any rotational forces. At the interfaces 150, 154, 156 of the upper tiers 153, 155, 157, the friction force associated with the clamping provided by the tension rods 160 will resist rotation to some extent. If this is determined to be insufficient, an anti-rotation feature (not shown) will be adopted which, through a number of dowel pins, the top tier 157 will be connected to the upper core plate 48. As the upper internals are inserted, through holes in the upper core plate 48 will engage several dowel pins fixed to the top tier 157 of the RCR 152. Any rotation forces on the RCR assembly 152 must then be conveyed to the upper core plate 48 via the aforementioned dowels. Rotation of the upper core plate 48 is prohibited by the virtue of its engagement with the four upper core plate alignment pins 49. Thus, torsional stability will be achieved. The dowels (not shown) on the top tier 157 will be precisely located by placing the upper core plate 48 in its final position above the RCR assembly 152 centered on the alignment pins 49, and then transferring centers of the through holes in the core plate 48 onto the top tier 157 upper surface to locate dowel pin centers. In this manner, only small clearances will exist between dowels and core plate through holes. Irradiation Specimen Holders

Irradiation specimen holders house samples of pressure vessel material which are removed and analyzed periodically to predict the condition of the pressure vessel shell 32 (Figure 1). Embrittlement is the primary concern. Generally, four such holders 39 are sufficient, (one is shown in Figure 1), positioned on the OD of the core barrel 44 at an elevation roughly equivalent to mid-core height, and three more at spaced intervals around the circumference of the core 30. Generally, these locations must meet the criteria of being at a radial distance from the core's axial centerline and at an azimuthal position such that the material specimens within the holders 39 do not lead the ID of the pressure vessel 32 by more than a factor of about three and no less than a factor of about one in radiation fluence attrition (i.e., accumulation of high energy fast neutrons). The specimens must also be at a temperature that is no greater than about 25°F (-3.89⁰C) different than the temperature at the ID of the pressure vessel 32.

As shown in Figures 14A and 14B, in accordance with another aspect of the advanced RCR design 152 of the invention, the specimen holders 439 are optionally relocated inside the lower core barrel 44 in the clearance between the ID of the core barrel 44 and the outside of the RCR geometry 152 (best shown in Figure 14A). The specimen holders 439 are fixed to the RCR 152 by fasteners, such as the socket head cap screws 441 shown, and by dowel pins 443 in a manner which is generally similar to that employed with corner clamping and dowel assembly 176 discussed hereinbefore in connection with Figures 7 A and 7B. The purpose of this relocation is fourfold. First, it is desirable to remove the specimen holders 439 from the high- flow, turbulent, downcomer region between the pressure vessel 32 (Figure 1) and lower core barrel 44. Second, such relocation would eliminate the need to provide cut-outs in the pressure vessel ledge (not shown), which is about 1 1.0 in. (27.9 cm) below the pressure vessel mating surface. Such cut-outs are required to provide free passage when inserting or removing lower reactor internals. The relocation also eliminates the requirement of machining holes in the lower core barrel flange directly over the specimen baskets. Thus, the plugs which are required to fill such holes when the reactor is operating, and the requirement to remove and re-insert such plugs when retrieving material samples, are also avoided. Third, in the new location, a good visual picture (i.e., from a plan perspective) is provided when a material sample is removed from the specimen holder. Conventionally, the sight picture is restricted through a small hole in the lower internals upper flange (not shown) which rests on the pressure vessel ledge. The design in accordance with the invention would include a larger flow hole (not shown) in the top tier flange located over the specimen positions. Finally, the relocation removes the specimen holders 439 to a location where they are less likely to be harmed or to cause harm when handling or transporting the lower reactor internals. Conventionally, as shown in

Figure 1, the specimen holders 39 protrude from the OD of the core barrels 44 and are more susceptible to damage during handling.

It will be appreciated that the foregoing aspect of the invention, which relates to the specimen holders 439, like all aspects of the invention discussed herein, may be employed in combination with one or more of the other features or aspects of the advanced RCR 152 of the invention, or alternatively, can be employed individually. For example, the specimen holders 439 may be positioned in the conventional location on the OD of the core barrel 440 in combination with the aforementioned four tier RCR design 152. This may be required if, for example, it is discovered that the 25°F (-3.89⁰C) temperature limitation cannot be met when the specimen holder 439 is positioned internally. Specifically, all coolant flow to the annulus between core barrel 44 and RCR assembly 152 is conventionally introduced at an elevation near the lower core support plate 50 through radial holes (not shown) which extend through the core barrel wall 44. To improve cooling for the revised internal specimen holder location, some of this bypass coolant could, for example, be let through the core barrel 44 at the mid-plane height of the specimen holders 439. This would more efficiently cool and reduce the operating temperature of the holders 439. Thermocouples

Figures 15A and 15B show a thermocouple 500 in accordance with another aspect of the advanced RCR 152 of the invention. The thermocouple 500 is provided in the RCR top tier 157. More specifically, a number of thermocouples 500 (one is shown) are buried at the elevation at which the RCR maximum temperature has been predicted to occur (i.e., through study of thermal models of the conventional RCR design, such as the RCR 52 of Figures 1, 2A, 2B, 2C. This location is a hot junction and through use of the thermocouples 500, valuable temperature data can be obtained that will permit, for example, refinement of analytical stress and temperature calculations, and lead to possible future design improvements. Therefore, although the thermocouples will have to be retracted about 50.0 in. (162 cm) before removing the upper internals (not shown), for example, for a refueling outage, any inconvenience with respect thereto is outweighed by the value of the data that the thermocouples 500 afford. It is also worth noting that the thermocouples could be permanently removed after several fuel cycles if no changes in temperature readings occur.

Generally, the hot junction is disposed near the bottom of the top tier 157 on the periphery of the RCR 152 at some angular location dictated by the thermal analysis. The thermocouple 500 passes through a thermocouple conduit 502 which exits an upper support column and is connected at a target location on the upper core plate 48 by a weld or threaded mechanism. The thermocouple pathway resumes below the upper core plate and is shown in Figure 15B as the thermocouple conduit extension (506). The extension directs the thermocouple 500 into a drilled hole in the upper tier 157 which extends to a depth where the thermocouple hot junction will stop. This is the point where the metal temperature of the RCR will be monitored. Note the recess in the underside of the upper core plate. This geometry is chosen so that the exposed portion of the thermocouple, with a diameter of about 0.125 (.405 cm), is not exposed to coolant cross-flow which might induce vibration and hence fretting between the thermocouple and conduit. Conduit bore diameter and bend radii are selected to insure free passage of the thermocouple as it is either inserted or removed. However, it will be appreciated that the foregoing dimensions and configuration are merely provided as an example embodiment of the thermocouple concept in accordance with the present invention, and are not limiting upon the scope of the invention. It will also be appreciated that any known or suitable sensors (e.g., without limitation, pressure transducers) other than the exemplary thermocouples 500 could be employed to gather valuable data at various locations on or about the exemplary RCR 152.

In summary, the invention provides an advanced radial core reflector RCR with a number of features that improve upon known RCR designs in many ways. Among them are the fact that the advanced RCR 152 of the invention provides a snug radial fit at four elevations of the core 30 between the RCR 152 and the ID of the core barrel 44, and at a fifth elevation between the bottom tier 151 and the lower core support 50, for purposes of providing superior horizontal load absorption. In the application shown in Figures 6, 9A, 9B and 9C, six dowel pins of about 1.75 in. (4.5 cm) in diameter and two of about 2.25 in. (5.7 cm) in diameter replace the known 36 dowels of about 6 in. (15.2 cm) in diameter and the eight conventional radial alignment pins of about 7.0 in. (17.8 cm) in diameter, which were welded into the lower core barrel wall at the top and bottom RCR flange elevations (only the alignment pins 66, at the top of the RCR 152 are shown in Figure 2C).

Eight or nine pancake-type toroidal tiers 54, depending on core height, (Figures 2A, 2B and 2C) are replaced with two toroidal tiers 151 , 157 and two tiers 153, 155 each made up of four quadrant segments (e.g., 173, 175 of Figure 7A) aligned by dowels and clamped to form a rigid assembly which provides an equivalent opening for bounding the core 30 (Figure 1 ). This also replaces seven or eight circumferential gaps with three (Figure 3), thus minimizing the possibility of pressure driven main coolant water jetting radially through gaps between tiers and damaging fuel rods in the peripheral fuel elements. Also, without the gaps, there can be no fast neutron streaming to the pressure vessel wall 32 (Figure 1). Pressure vessel life is thereby extended. Additionally, any such coolant water jetting, should it occur, is only possible at grid height evaluations, where the fuel assembly is substantially stiffened, thereby further protecting the fuel rods.

A secondary vertical seismic support 300 limits the strain and resists fracture in the primary fasteners (e.g., 164) which clamp the RCR assembly 152 and the lower core support 50. Therefore, although unlikely, in the event that combined LOCA and safe shut down earthquake (SSE) vertical load components overcome the preload in the clamping screws 164 and tension rods 160, further strain or stretching, of those fasteners 160, 164 will be limited to about 0.015 in. (.04 cm) at steady state operating conditions before the increasing vertical load will be safely assumed by the upper core plate alignment pins 49.

The bottom tier 151 of the RCR 152 is designed as a separate entity (i.e., a foundation block), which is precisely positioned and then fixed to the lower core support 50 by screws 164 and dowels 158. Accordingly, although it remains an option, no threading of the eight tension rods 160 into the lower core support 50 or securement of the entire tier stack directly to the lower core support 50, is likely to be required.

Tension rod 60 location and installation is improved, generally. Specifically, conventional designs position the rods in passage holders which must be machined through the entire length of the tier stack 54, which is a significant amount of costly machining (i.e., about 1.75 in. (4.45 cm) in diameter through about 181 in. (459.7 cm) of material in eight places (see, for example, tension rod 60 of Figure 2A). The top tier flange 159, Figure 12, of the invention is configured so that when drilling through its thickness (i.e., about 4.5 in. (1 1.4 cm)) for the tension rod 160 passage, directly beneath the flange 159 is free space until reaching the bottom tier flange 167 about 174 in. (442 cm) below. The tension rods 160 of the invention then engage threaded holes (see Figure 3) in the bottom tier flange 167 (Figure 4), rather than screwing directly into and potentially damaging the lower core support 50. Besides the savings in machining, the tension rods 160, which are now in the annulus between the core barrel 44 and the RCR 152, operate at a slightly cooler temperature and receive less radiation than the RCR tier stack that they clamp together. This allows for lower torque at cold conditions, because the rods 160 will be stretched at operating temperatures and thus achieve their maximum clamping force. Also, the lower operating temperature and reduced neutron attrition address concerns over relaxation in the rods.

The irradiation specimen holders 439 are relocated in the annulus between the ID of the lower core barrel 44 and the outside perimeter of the RCR assembly 152 which, among other benefits, provides a safer location with regard to flow forces, and reduces susceptibility to damage during handling.

Core barrel distortion concerns are also substantially alleviated by eliminating the eight conventional alignment pins where four upper pins were located directly beneath the four upper core plate alignment pins and four immediately above the lower core support plate, and required precise positioning. In accordance with the invention, as discussed hereinbefore, such tight tolerance requirements are achieved more easily without requiring a substantial weldment which could distort the surrounding area. Additionally, compared to the conventional eight tier RCR 52 discussed hereinbefore, there is relatively little difference in manufacturing costs. Specifically, although the advanced RCR 152 of the invention may require more precise machining in several areas, including additional fit-ups and measurements during fabrication, and some additional hardware, the costs associated with these differences are substantially offset by (1) the elimination of the 36 large diameter dowels and the associated machining at the tier interfaces, (2) the elimination of eight large precision alignment pins, with associated machining and welding, at the top and bottom tier elevations, (3) reduced tier machining for tension rods, (4) elimination of the need to handwork 4 or 5 of the circumferential seams for fit and finish, and (5) elimination of slotting or cutting of the pressure vessel ledge and lower internals flange for irradiation specimen holder passage. Accordingly, the advanced RCR and the various features thereof, whether employed independently or in combination, provide numerous design improvements and benefits.

While specific embodiments of the invention have been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be_. developed in light of the overall teachings of the disclosure. For example, although aspects of the invention have been described herein specifically with regard to 157 fuel assembly plants, the novel features and designs of the invention are scalable to smaller plants, for example, of the 121 fuel assembly variety, and larger plants of 193, 248 or more fuel assemblies. Accordingly, the particular arrangements disclosed are meant to be illustrative only and not limiting as to the scope of the invention which is to be given the full breadth of the claims appended and any and all equivalents thereof.

Claims

What is claimed is:

1. An advanced radial core reflector for a nuclear reactor core including a pressure vessel, a lower core barrel disposed within a lower portion of said pressure vessel, an upper core plate with alignment pins coupled to an upper portion of said lower core barrel, a lower core support coupled to the bottom of said lower core barrel, and a plurality of fuel assemblies extending longitudinally between said upper core plate and lower core support, said radial core reflector comprising: a plurality of tiers stacked one on top of another, said tiers being structured to surround said fuel assemblies and to be disposed within said pressure vessel in the space between said fuel assemblies and said lower core barrel, wherein a first one of said tiers is structured to be coupled to said lower core support, and wherein said tiers are structured to be secured together independently with respect to said lower core support.

2. The radial core reflector of claim 1 wherein said plurality of tiers includes four tiers, a first or bottom tier structured to be coupled to said lower core support by a base restraint, a second or lower center tier structured to be stacked on top of said bottom tier thereby forming a first interface therebetween, a third or upper center tier structured to be stacked on top of said second tier thereby forming a second interface therebetween, and a fourth or top tier structured to be stacked on top of said third tier, thereby forming a third interface therebetween.

3. The radial core reflector of claim 2 wherein said bottom tier includes a flange having a plurality of through holes; wherein said base restraint includes a plurality of first threaded fasteners structured to be inserted through said holes in order to secure said bottom tier to said lower core support; and wherein said second, third and fourth tiers are secured together and to said bottom tier using tension rods independent with respect to said lower core support.

4. The radial core reflector of claim 3 wherein said first fasteners are socket head cap screws; and wherein said second fasteners are tension rods structured to be inserted through all four tiers of said radial core reflector, or through the top three tiers while threading into said bottom tier.

5. The radial core reflector of claim 3 wherein said base restraint further includes a plurality of dowels structured to be received within said flange of said bottom tier in order to facilitate alignment of said bottom tier.

6. The radial core reflector of claim 2 wherein each interface between tiers include an alignment and securing mechanism selected from the group consisting of a keying assembly, an alignment pin assembly, and a combination of keying assembly and an alignment pin assembly.

7. The radial core reflector of claim 6 wherein said keying assembly comprises a key protruding from the abutting surface of one of said tiers and cooperable with a corresponding slot in the abutting surface of an adjacent tier.

8. The radial core reflector of claim 6 wherein said alignment pin assembly comprises a locating pin structured to be inserted within a first bore and a second bore when said first and second bores are properly aligned, said first and second bores being disposed in the abutting surfaces of said adjacent abutting tiers.

9. The radial core reflector of claim 1 wherein each of said tiers comprises a plurality of radial segments; and wherein said segments are coupled together using a clamping assembly.

10. The radial core reflector of claim 9 wherein said plurality of segments comprises four separate quadrants; wherein adjacent quadrants form a vertical seam which is secured together using said clamping assembly; and wherein said clamping assembly comprises a corner angle clamp which is a structural angle, a plurality of fasteners, and a plurality of dowels, said dowels being structured to align said structural angle with respect to said vertical seam, said fasteners being structured to tighten said clamping assembly, thereby securing said adjacent quadrants together.

1 1. The radial core reflector of claim 1 wherein said nuclear reactor core includes a number of transverse support grids which secure said fuel assemblies within said core; and wherein said interfaces between tiers of said radial core reflector are structured so as to be disposed adjacent the locations of said .support grids in said core.

12. A nuclear reactor core comprising: a pressure vessel; a lower core barrel disposed within a lower portion of said pressure vessel; an upper core plate having a plurality of upper core plate alignment pins; a lower core support plate disposed adjacent the bottom of said lower core barrel; a plurality of fuel assemblies extending longitudinally between said upper core plate and said lower core support; and a radial core reflector surrounding said fuel assemblies, said radial core reflector comprising: a plurality of tiers stacked one on top of another in order to form a plurality of interfaces therebetween, said tiers surrounding said fuel assemblies within said pressure vessel in the space between said fuel assemblies and said lower core barrel, wherein a first one of said tiers is coupled to said lower core support, and wherein said tiers are structured to be secured together independently with respect to said lower core support.

13. The nuclear reactor core of claim 12 wherein said plurality of tiers includes four tiers, a first or bottom tier coupled to said lower core support by a base restraint, a second or lower center tier stacked on top of said bottom tier in order to form a first interface therebetween, a third or upper center tier stacked on top of said second tier in order to form a second interface therebetween, and a fourth or top tier stacked on top of said third tier in order to form a third interface therebetween.

14. The nuclear reactor core of claim 13 wherein said bottom tier includes a flange having a plurality of through holes; wherein said base restraint includes a plurality of first threaded fasteners structured to be inserted through said holes and tightened in order to secure said bottom tier to said lower core support; and wherein said second, third and fourth tiers are secured together and to said bottom tier using a plurality of tension rods independent with respect to said lower core support.

15. The nuclear reactor core of claim 13 wherein said fuel assemblies are secured within said core using a number of transverse support grids; and wherein said interfaces between tiers of said radial core reflector are structured so as to be disposed adjacent the locations of said support grids in said core.

16. The nuclear reactor core of claim 12 wherein each of said tiers comprises a plurality of radial segments; wherein adjacent abutting segments form a vertical seam which is secured together using a clamping assembly; and wherein said clamping assembly comprises a corner angle clamp which is a structural angle, a plurality of fasteners, and a plurality of dowels, said dowels being structured to align said structural angle with respect to said seam between said adjacent segments, said fasteners being structured to tighten said clamping assembly, thereby securing said adjacent abutting segments together.

17. The nuclear reactor core of claim 16 wherein said segments are quadrants of said tier of said radial core reflector; and wherein said vertical seams between adjacent abutting quadrants are disposed at about the 40-220 degree azimuth axis and at about the 130-310 degree azimuth axis.

18. The nuclear reactor core of claim 12 wherein each interface between tiers includes an alignment and securing mechanism selected from the group consisting of a keying assembly and an alignment pin assembly; wherein said keying assembly comprises a key protruding from the abutting surface of one of said tiers and cooperable with a corresponding slot in the abutting surface of an adjacent tier; and wherein said alignment pin assembly comprises a locating pin structured to be inserted within a first bore and a second bore when said first and second bores are properly aligned, said first and second bores being disposed in the abutting surfaces of said adjacent abutting tiers.

19. The nuclear reactor core of claim 12 wherein said plurality of tiers includes a top tier including a flange; and wherein said radial core reflector includes a secondary vertical support secured to said top tier thereby further securing said radial core reflector from excessive vertical seismic loads.

20. The nuclear reactor core of claim 19 wherein said upper core plate includes an alignment pin; and wherein said secondary vertical support comprises: a base plate coupled to said top tier; and an integral support post disposed on top of said base plate and structured to be positioned under said alignment pin of said upper core plate.

21. The nuclear reactor core of claim 12 wherein one or more of said interfaces between said tiers includes a staircase geometry in which adjacent abutting surfaces of said adjacent tiers are spigotted with respect to one another in order to cooperate in precise alignment and to prevent shifting between tiers.

22. The nuclear reactor core of claim 12 wherein said lower core barrel includes a number of weld bands disposed proximate said interfaces between tiers of said radial core reflector; and wherein each of said weld bands is structured to provide a predetermined gap between said radial core reflector and said lower core barrel, while ensuring securement of said radial core reflector at core operating temperatures.

23. The nuclear reactor core of claim 12 wherein said lower core barrel has in inner diameter; wherein said core includes a number of irradiation specimen holders; and wherein at least one of said irradiation specimen holders is disposed in the space between the inner diameter of said lower core barrel and the exterior of said radial core reflector.

24. The nuclear reactor core of claim 12 including at least one thermocouple disposed within one of said tiers at an elevation of said radial core reflector at which the temperature during operation of said core is maximized or of other particular interest.