JPS61161488A - Weak absorbing control rod for nuclear reactor - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION This invention relates to nuclear reactors, and more particularly to an improved weakly absorbing control rod for insertion into a nuclear fuel assembly.

In conventional pressurized wood reactors, the core is designed to have excess reactivity at startup, so even if the reactivity decreases as the core approaches the end of its life, it remains There is still sufficient reactivity left to maintain the core functioning over time. However, since excessive reactivity is expected in the core at the beginning of the core life, it is necessary to appropriately control the reactivity at that point.

In a typical nuclear reactor, the core is comprised of a plurality of elongated fuel assemblies, each consisting of a plurality of elongated fuel elements or rods. - Pumping liquid coolant upward through the core to extract heat from the core and convert it into useful work. The thermal power of the reactor core is typically regulated with various movable rods, such as control rods, weak absorption control rods, and/or water rejection rods. In a pressurized wood reactor, a typical fuel assembly has a plurality of guide tubes or "guide simples" within which the aforementioned rods can reciprocate.

These three different types of rods all have approximately the same dimensions, but have somewhat different purposes due to the different materials from which they are constructed. Displacement rods, which are hollow or contain low neutron absorbing material, are used to exclude reactor coolant and thereby control reactor moderation. Control rods containing materials with high neutron absorption function primarily to control the reactivity of the reactor core by absorbing neutrons. Weak-absorbing control rods are similar to exclusion rods, but use materials with moderate neutron absorption, such as stainless steel, that allow for high reactivity. These weakly absorbing control rods absorb neutrons with a smaller neutron capture cross section than the control rods, and 2. they also exclude coolant to control core reactivity.

Weak absorption control rods are also called fine adjustment auxiliary control rods. Such an auxiliary control rod phosphor, disclosed in European Patent Application No. 8430B376.9, consists of stainless steel upper and lower tubes closed at the inner end with a middle plug and at the outer end with an end plug. The lower tube contains solid zirconium oxide pellets stacked between its middle and end plugs;
These solid pellets provide radial support to the lower tube. To equalize the pressure inside and outside the walls of the upper tube,
The upper tube is perforated to admit the coolant inside.

The performance of this type of auxiliary control rod is quite satisfactory and serves its purpose under the operating conditions for which it was designed. However, it has been found that solid zirconium oxide pellets can radially expand upon reaching high internal temperatures of 300' to 600 DEG C., leading to undesirable distortions in the coating.

A principal object of the present invention is to provide an improved weakly absorbing control rod that overcomes this problem.

Accordingly, the present invention provides a weakly absorbing control rod inserted into a nuclear fuel assembly including an array of fuel rods, the elongated tubular member being sealed at both ends and made of a material with a small neutron capture cross section. and pellets made of a non-fuel material and stacked end-to-end within a tubular member, the tubular member being positioned within the array of fuel rods when inserted into the fuel bed assembly. and a second longitudinal portion located essentially outside the array of fuel rods, the pellets being disposed within the pellet receiving space defined within the first longitudinal portion. stacked on top of each other, each pellet having an annular and penetrating cavity of sufficient diameter to provide radial support to the adjacent wall of the first longitudinal section, The second longitudinal section has a hollow space that does not contain pellets, and the space is provided with means for increasing its radial collapse resistance to prevent the second longitudinal section from being dented by external pressure. The weak absorption control rod is characterized by being provided with a weak absorption control rod.

The use of annular rather than solid pellets minimizes the risk of undesirable distortion of the coating. This is because the internal temperature of annular pellets tends to be very low, so radial expansion is usually minimal, as there is an internal cavity that absorbs most of the thermal expansion, no matter how small. This is because it is present in each annular pellet. moreover,
The lighter weight of annular pellets than solid pellets reduces the mass of the weakly absorbing control rod, which is advantageous for the operation of the weakly absorbing control rod during use.

A preferred embodiment of the invention uses a spiral spring to hold the annular pellets in a stack within the pellet receiving space of the first longitudinal portion of the tubular member. The spiral spring is kept under compression within the hollow space of the second longitudinal section and extends longitudinally over substantially the entire length of the hollow space. Furthermore, the diameter of the spiral spring is large enough to provide radial support so that the walls of the second longitudinal section do not cave in due to external pressure. The spiral spring thus serves the dual purpose of retaining the stacked pellets within the pellet receiving space as well as increasing the radial collapse resistance of the second longitudinal section. Elastic spiral springs are used to hold the stack of pellets in place and maintain their integrity so that the axial expansion of the pellets can be accommodated without distortion. The annular pellets are preferably made of Zircaloy material, the well-known non-pulverizing Zircaloy alloy. In a preferred embodiment, the tubular member having the first and second longitudinal portions is a unitarily molded member to enhance the alignment and straightness of the bar.

Hereinafter, preferred embodiments and modifications of the present invention will be described in detail with reference to the accompanying drawings.

In the following description, the same reference symbols designate the same or equivalent parts throughout the several views of the drawings, and also indicate
Terms such as "rear", "left", "right", "upper", "lower", etc. are for convenience and do not have a limiting meaning.

FIG. 1 shows a fuel assembly, generally indicated by the reference numeral 10, an upper core support plate 12 disposed above and extending across the top of the fuel assembly 10, and an upper core support plate 12 disposed above the upper core support plate 12. The spider assembly 14 shown in FIG.

The fuel assembly 10, shown in vertically foreshortened form, is essentially a lower end supporting the fuel assembly on a lower core plate (not shown) in the core of a nuclear reactor (not shown). a lower nozzle 16 and a plurality of guide tubes or guide simples 1 extending longitudinally upward from the lower nozzle 16;
8, a plurality of grids 20 spaced axially along the guide simple 18, and elongated fuel rods 2 supported in a transversely spaced manner by the grids 20.
2, an instrumentation tube 24 centrally located in the fuel assembly, and an upper end structure or nozzle 26 attached to the upper end of the guide simple 18. The fuel assembly 10 constitutes an integral unit that can be handled in a conventional manner without damaging its components.

As mentioned above, the fuel rods 22 are maintained in spaced relationship with each other by grids 20 spaced along the fuel assembly.

Each fuel rod 22 contains nuclear fuel pellets (not shown) and is closed at both ends by end plugs (not shown). Nuclear fuel pellets made of fissile material are the source of the nuclear reactions that occur in nuclear reactors. Water or boron-laden water is pumped upwardly through the fuel assemblies of the core to extract heat from the core and convert it into useful work.

The upper core support plate 12 extends across the top of all fuel assemblies in the core and has coolant passages, such as passages 28, extending therethrough. At least some of these passages 28 are guided by the guiding simple 18.
The weak absorption control rods, such as the weak absorption control rod 30, and other control rods (not shown) are aligned axially with the core plate 1.
It is possible to descend into the respective guide simple 18 via two passages 28 .

As best seen in FIGS. 2 and 3, the spider assembly 14 includes a central hub 32, a plurality of vanes 34 extending radially from the hub 32, and a weak absorption control system mounted on the vanes 34. It has a plurality of fingers 36 connected to the upper end of the rod 30. Preferably, central hub 32 is formed as an elongated tube with threads 40 at its upper end for connection to a drive (not shown) used to raise and lower the spider assembly, as is well known in the art. . The tubular hub 32 has a coil spring 42 kept in compression and a weak absorbing control rod 3 within the core plate passage 28 and guide simple 18.
core plate 12 to help maintain correct alignment.
It houses a load absorbing device having a nipple 44 that seats in a shallow cavity (not shown) formed in the top surface of the .

As is well known, the primary purpose of the load absorbing device is to fully insert the weak absorbing control rod 30 into the guide simple 18.
The purpose is to prevent the spider assembly 14 from hitting the core plate 12 and applying an impulsive load to the core plate 12 and the fuel assembly 10.

4 and 5, a weak absorption control rod 30 embodying the present invention includes a tubular member 46, an annular support pellet 48 stacked within the tubular member, and the pellets. 48 within the pellet receiving space 52 in the lower longitudinal portion of the tubular member 46, the spiral spring 50 is defined in the upper longitudinal portion of the tubular member 46. The spiral spring is disposed in a partially compressed state in a pellet-free hollow space 54, and the diameter of the spiral spring is large enough to provide radial support to the wall of the upper longitudinal section. It may be noted that the spiral spring 50 extends axially over substantially the entire length of the hollow space 54.

More specifically, the tubular member 46 is a cylindrical tube 56 closed at each end with an end plug 58,60. The upper end plug 58 is integrally formed with an upwardly extending stem 62 which has an externally threaded end 64 for connection to an internally threaded lower end 66 of one of the spider fingers 36. has. The lower end plug 60 is conical. The cylindrical tube 56, like the end plugs 58, 60, is a thin-walled sheath made of stainless steel. Since the sheath is made of stainless steel, it has a small neutron capture or absorption cross section, and its thin walls minimize the weight and therefore moment of inertia of the weak absorption control rod 30.

Each of the stacked pellets 48 is hollow and annular in shape to fit concentrically within the tube 56 and has a central cavity 68 therethrough. The use of hollow pellets 48 not only reduces the mass of the weak absorption control rod 30 but also makes the tube 56 sufficiently dent-resistant. Weak absorption control rod 3
Since 0 has a low mass, the internal temperature of the weakly absorbing control rod remains at an acceptable level, and therefore the distortion of the coating is small. Additionally, the void space 68 within each pellet 48 helps to maintain the internal temperature of the pellet itself at a low level, so that radial expansion and therefore coating distortion is minimized.

Each pellet is preferably made of Zircaloy material.

This Zircaloy material does not appreciably absorb neutrons and does not become powder during gradual insertion/extraction of the weakly absorbing control rods 30 from the fuel assembly.

As mentioned above, the spiral or compression spring 50 serves a dual purpose. That is, it is first of sufficient length to remain compressed between the stacked pellets 48 and the upper end plug 58 to keep the pellets pressed against the lower end plug 60; Any slight longitudinal expansion of the individual pellets due to heat is absorbed. Thus, the spiral spring 50 holds the pellets 48 in a stacked state within the pellet receiving space 52 of the lower tube section. This lower tube portion is the portion that is located within the fuel rod array when the weak absorption control rod is inserted into the fuel assembly 10, and the upper tube portion is the portion that is essentially located at the position where the weak absorption control rod is inserted. is outside the fuel rod array but within the associated passageway 28 in the upper core plate 12. The second role of the spiral spring 50 is to internally support the thin-walled tube 56 throughout the hollow space without adding weight beyond that of a lightweight spring.

FIG. 6 shows a variation 3 of a weak absorption control rod similar to the weak absorption control rod 30 of the preferred embodiment in that it has a tubular member 46° containing a stack of hollow annular pellets 48°.
0' is shown. However, unlike the weak absorbing control rod 30, this weak absorbing control rod 30' is such that the wide elongate tube 56' is comprised of separate lower and upper tube sections 70, 72, and these tube sections 0. Adjacent ends of 72 are joined together by 4° to a solid plug 74 therebetween. The lower tube section 70 defines a pellet receiving space 52'' containing 8 DEG of stacked pellets, and the upper tube section 72 defines a hollow space 54 DEG containing no internal structure. In order to equalize the pressure within the hollow space 54° and the pressure outside the weak absorption control rod, holes or openings 76 are provided in the wall of the upper tube section 72 for admitting reactor coolant into the hollow space 54°. is formed. The two tube sections o, 72 consist of thin-walled jackets, these tube sections together with the solid plug 74 are made of stainless steel. Pellets 48° are made of a non-fuel material with a negligible coefficient of thermal expansion, one example of which is zirconium oxide. In particular, because the pellets are annular and void 68 is formed, they do not extend appreciably in length and are therefore held tightly stacked between solid plug 74 and end plug 60.

[Brief explanation of the drawing]

FIG. 1 is a fragmentary elevation of the core of a nuclear reactor showing the fuel assembly, the upper core support plate extending across the top of the fuel assembly, and the spider assembly supporting the weak absorption control rods; FIG. 3 shows the core support plate in cross-section and partially cut away to expose the weakly absorbing control rods of the present invention, and the fuel assembly is shown vertically shortened. FIG. 2 is an enlarged plan view of the spider assembly. FIG. 3 is a cross-sectional view of the spider assembly taken along line 3--3 in FIG. FIG. 4 is an enlarged partial sectional view showing a weak absorption control rod according to a preferred embodiment of the present invention, shortened in the vertical direction. FIG. 5 is an enlarged sectional view taken along line 5--5 in FIG. 4. FIG. 6 is an enlarged view showing a weak absorption control rod which is a modified example of the present invention and is shortened in the vertical direction. (Explanation of main reference symbols) 10...Fuel assembly 22...Fuel rod 30.30゛...Weak absorption control rod 46.46°...Tubular member 48.48° ... Pellet 50 ... Spiral spring 52.52' ... Pellet storage space 54.54'.
・;・Hollow space 68・・Vacancy 76・・hole or opening FIG, 5

Claims (1)

[Claims] 1. A weak absorption control rod inserted into a nuclear fuel assembly including an array of fuel rods, which is an elongated weak absorption control rod sealed at both ends and made of a material with a small neutron capture cross section. a tubular member;
pellets made of a non-fuel material and stacked end-to-end within the tubular member, the tubular member having a first pellet located within the array of fuel rods when inserted into the fuel assembly. and a second longitudinal portion located essentially outside the array of fuel rods, the pellets being stacked within a pellet receiving space defined within the first longitudinal portion. , each pellet has an annular interior through-hole having a diameter sufficient to provide radial support to an adjacent wall of the first longitudinal section and a second longitudinal section of the tubular member. The longitudinal section has a hollow space that does not contain pellets, and the space is provided with means for increasing the radial collapse resistance to prevent the second longitudinal section from being dented by external pressure. A weak absorption control rod characterized by: 2. The weak absorption control rod according to claim 1, wherein the elongated tubular member is covered with a thin wall. 3. The weak absorption control rod according to claim 2, wherein the coating is made of stainless steel. 4. The means for increasing the radial collapse resistance of the second longitudinal portion is arranged in the hollow space so as to elastically retain the pellets stacked in the pellet receiving space within the space. a spiral spring held in partial compression, the spiral spring extending longitudinally over substantially the entire length of the hollow space, the diameter of which radially supports the walls of the second longitudinal section; Claim 1, 2 or 3, characterized in that the size is sufficient for
Weak absorption control rod as described in section. 5. The weak absorption control rod according to claim 1, 2, 3 or 4, wherein the pellets are made of Zircaloy material. 6. The first and second longitudinal portions are separate tubes, and the tubes are connected to each other by a solid plug provided between them, and the annular pellet is inserted into the pellet receiving space by the solid plug. Claims 1, 2, or 3 are held in a stacked state.
Weak absorption control rod as described in section. 7. The weak absorption control rod according to claim 6, wherein the annular pellet is made of zirconium oxide material. 8. Said means for increasing the radial collapse resistance of the second longitudinal section are arranged such that said means for increasing the radial collapse resistance of the second longitudinal section includes a wall of the second longitudinal section for equalizing the pressure between said hollow space and the exterior of the weakly absorbing control rod; 8. A weak absorption control rod as claimed in claim 6 or claim 7, characterized in that it comprises at least one hole formed through the portion.