CA2568963C

CA2568963C - Gravitational settling bed for removal of particulate impurities in a nuclear steam generator

Info

Publication number: CA2568963C
Application number: CA002568963A
Authority: CA
Inventors: Richard G. Klarner
Original assignee: Babcock and Wilcox Canada Ltd
Current assignee: BWXT Canada Ltd
Priority date: 2006-11-24
Filing date: 2006-11-24
Publication date: 2010-02-02
Anticipated expiration: 2026-11-24
Also published as: CA2568963A1; FR2909161A1; JP2008209106A; KR101587736B1; KR101477797B1; KR20140141539A; KR20080047303A

Abstract

A low velocity region above the primary deck in a recirculating nuclear steam generator is formed through which flows a small percentage of recirculated liquid. The liquid flow is established by strategically sized and positioned inlet and outlet ports. Suspended particles within the low flow zone will settle onto the horizontal primary deck plate and are thereby removed from the recirculated flow.

Description

GRAVITATIONAL SETTLING BED FOR REMOVAL OF
PARTICULATE IMPURITIES IN A NUCLEAR STEAM GENERATOR
[0001] FIELD AND BACKGROUND OF THE INVENTION

[0002] The present invention relates generally to the field of nuclear steam generators for electric power production and, in particular, to an improved apparatus for removing particulate impurities from such nuclear steam generators.

[0003] For a general discussion of the principles of operation of various types of nuclear power plants and the equipment provided in such plants, the reader is referred to STEAM/its generation and use, 41st Edition, Kitto and Stultz, Editors, Copyright 2005, The Babcock & Wilcox Company, and particularly Section VIII, Chapters 46 through 50.

[0004] Fig. 1 is a schematic illustration of a known recirculating steam generator (RSG) design used in the production of steam. Thermal energy is extracted from the core of a nuclear reactor (not shown) by means of the primary coolant 12. The primary coolant 12 is conveyed through the inside of the U-tubes 14 of the RSG. The U-tubes 14 are located within the lower shell 201 of the RSG, where they are supported at their U-bend region 40c by U-bend restraints or supports 20d and lower down along their lengths by lattice grid tube supports 20e. The incoming primary coolant 12 passes into the RSG via a reactor coolant inlet 20h and passes out via a reactor coolant outlet 20f, both of which are located on the surface of the RSG's primary head 20i. The coolant 12 entering via the inlet 20h is separated from the coolant 12 exiting via the outlet 20f by a fully welded primary divider plate 20g. A secondary coolant or secondary side water 16 passes into the RSG as feedwater 26, where it is fed by a feedwater header 20n down a downcomer annulus 20j, past a tubesheet 20p, around the bottom of a shroud or wrapper 20k, and over the outside of the U-tubes 14. Impurities from the secondary coolant 16 are removed by a blowdown outlet 20o. As the secondary coolant 16 passes over the U-tubes 14, it absorbs heat_ from the primary coolant 12 and is partially converted to steam, resulting in a steam/water mixture 18. The steam/water mixture 18 is then conveyed to the upper shell or steam drum 106 of the RSG, above the water level 20a to primary steam separation equipment 20b and then to secondary steam separation equipment 20c, which remove the residual water and recirculate 22 it back to the RSG tube bundle 14 for further evaporation. The substantially moisture-free steam 24 is then sent out the steam outlet 110 to the steam turbine generator equipment (also not shown) to produce the electric power. Feedwater 26 supplied to the RSG
replaces the portion of the water which is converted to steam and conveyed to the steam turbine generator.

[0005] As described in Chapter 48 of the aforementioned STEAM 41 st reference, high efficiency steam/water separation is extremely important. Low moisture carryover in the steam improves steam turbine efficiency and total power output, and minimizes the carryover of contaminants into the steam turbine. Similarly, low steam carryunder (steam entrainment) within the downcomer return water maximizes the downcomer annulus driving head, thereby maximizing the intemal circulation rate. Low separator pressure drop also increases the natural circulation rate through the U-tube bundle by lowering the overall resistance.

[0006] Fig. 2 illustrates the steam separation equipment 20 provided in certain types of RSGs manufactured by the assignee of the present invention. As shown, the steam separation equipment combines curved arm primary (CAP) separators 30 and cyclone separators 32, both of which are centrifugal type separators, to accommodate the necessary steam throughput. A plurality of such separators are provided in each steam generator and located and supported on top of a plate known as the primary separator deck 34. Although not shown in Fig. 2, the primary separator deck 34 is also stiffened in the out-of-plane direction, for normal and accident loading, by horizontal stiffeners between rows of separators. Apertures 36 are provided through the primary separator deck 34 for each separator and the steam/water mixture 18 produced in the U-tube bundle 14 below the primary separator deck 34 is conveyed upwardly through these apertures 36 into risers 38 of their respective separators.

[0007] The risers 38, whose length varies depending on the application, convey the steam/water mixture 18 upwardly into the CAP separators 30 which act to separate the water 30a from the steam 30b. During the separation process, a film of water develops on the inner wall of the return cylinder 40 and spirals down to the main inventory of water for recirculation. The return cylinder 40 extends above the top of the curved-arms where there are several small diameter perforations 30c and a retaining lip 30d, which are used to improve the water removal capabilities of the separator at high steam and water flows. The steam exits the top of the primary separators into an interstage region 30e, which is used to more evenly distribute the steam prior to entering the secondary compartment 30f where the secondary cyclones 32 are located.

[0008] The majority of the water is separated in the primary stage of separation, resulting in an inter-stage quality of about 95%. Cyclone secondary separators associated with each of the CAP separators 30 and located thereabove complete the removal of the water from the steam. The relatively small size of the separators allows for more efficient use of space in the drum. The "dry" steam 24 exits from the top of each cyclone secondary separator through a steam outlet, while the separated water is returned downwardly via a drain line 30h along the return cylinder 40 onto the top of the primary separator deck 34. Fig. 3 contains close-up views, the lower in section and the upper in plan, of the region of the RSG 10 of Fig. 1 in between the lower portion containing the U-tubes 14 and the upper steam drum portion 106. The primary separator return cylinders 40 return water 22 separated from the steam/water mixture riser flow 18 by the steam separation equipment 20 to the primary separator deck 34, where it flows over the edges thereof to form the downcomer return flow 140.
The downcomer return flow 140 mixes with the feedwater flow 26 and is conveyed to a lower portion of the RSG 10.

[0009] Steam and water are not chemically inert physical media. As discussed in Chapter 42, page 42-1 of the STEAM 41st reference, pure water dissociates to form low concentrations of hydrogen and hydroxide ions, H+ and OH-, and both water and steam dissolve some amount of each material that they contact. They also chemically react with materials to form oxides, hydroxides, hydrates and hydrogen. Water used in boilers or steam generators must be purified and treated to inhibit scale formation, corrosion and impurity contamination of steam. Two general approaches are used to optimize boiler water chemistry. First, impurities in the water are minimized by purification of makeup water, condensate polishing, deaeration and blowdown.
Second, chemicals are added to control pH, electrochemical potential, and oxygen concentration. Chemicals may also be added to otherwise inhibit scale formation and corrosion. The primary goals of boiler water chemistry treatment and control are acceptable steam purity and acceptably low corrosion and deposition rates. In the case of nuclear steam generators, the chemistry programs aim to minimize both corrosion product transport and the corrosion of steam generator tubes. The nuclear power industry has developed very specific secondary-side water chemistry guidelines for the proper control of operating chemistry conditions.

[00010] Deposition and corrosion still occur in nuclear steam generators, despite the precautions and efforts described above. In particular, field observations made during secondary side visual inspections of certain RSGs report that deposition occurs preferentially in low flow regions on the top of the primary separator deck.
The highest concentration of impurities occurs in the recirculated water at the top of the primary separator deck, before mixing with the cleaner feedwater in the downcomer.

[00011] It is thus desirable to provide for an improved apparatus for a nuclear steam generator which would reduce the concentration of impurities in the recirculated water and/or facilitate their removal from the system.

[00012] SUMMARY OF THE INVENTION

[00013] One aspect of the present invention is drawn to a mud drum sludge collection system which is installed on the top of the primary separator deck support.

The primary separator deck comprises a flat horizontal plate that supports the primary separators. The primary separator deck is stiffened in the out-of-plane direction, for normal and accident loading, by horizontal stiffeners between rows of separators. To provide for the mud drum sludge collection system, a peripheral ring plate is added to the primary separator deck, along with a top plate covering the stiffeners and peripheral ring plate. A low velocity region above the primary deck in a recirculating nuclear steam generator is thus established within the recirculating steam generator, defined by the primary separator deck plate, the top plate located above the primary separator deck plate, and the peripheral stiffener ring adjacent a periphery of the primary separator deck plate and the top plate. Means are provided for conveying a bleed stream of recirculated water created within the recirculating steam generator to the low velocity .
region. The bleed stream comprises a small percentage of the recirculated liquid within the recirculating steam generator. The liquid flow through the mud drum sludge collection system is established by strategically sized and positioned inlet and outlet ports. Suspended particles within the low velocity flow zone will settle onto the horizontal primary deck plate and are thereby removed from the recirculated flow.

[00014] Another aspect of the present invention is drawn to a recirculating nuclear steam generator. The recirculating steam generator comprises a pressure vessel containing a bundle of U-tubes for conveying primary coolant from an external heat source into and out of the pressure vessel for heating feedwater provided into the pressure vessel to generate a steam/water mixture. The bundle of U-tubes is provided at a lower portion of the pressure vessel. A steam drum is provided at an upper portion of the pressure vessel having steam/water separating means for separating water from the steam/water mixture, the steam exiting from the pressure vessel via a steam outlet, the separated water being recirculated back to the lower portion of the pressure vessel for reheating. Finally, a mud drum sludge collection system is provided for removing sludge from a bleed stream of the recirculated water and conveying the bleed stream to a low velocity region established within the pressure vessel.

[00015] The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and the specific benefits attained by its uses, reference is made to the accompanying drawings and descriptive matter in which preferred embodiments of the invention are illustrated.

[00016] BRIEF DESCRIPTION OF THE DRAWINGS

[00017] In the Figures:

[00018] Fig. 1 is a schematic illustration of a known recirculating steam generator (RSG) design to which the present invention may be applied;

[00019] Fig. 2 illustrates steam separation equipment provided in the RSG of Fig.
1;

[00020] Fig. 3 are close-up views, the lower in section and the upper in plan, of the region of the RSG of Fig. 1 in between the lower portion containing the U-tubes and the upper steam drum portion;

[00021] Fig. 4 is a sectional side view of an RSG employing the mud drum sludge collection system according to the present invention;

[00022] Fig. 5 are close up views, the upper view being of the region of an RSG
employing the mud drum sludge collection system according to the present invention, and the lower view including a Detail of the upper view;

[00023] Fig. 6 is perspective view, partly in section, of the portion of the RSG
illustrated in Fig. 5, according to the present invention;

[00024] Fig. 7 is a perspective view of the mud drum sludge collection system illustrating the components and flow paths established thereby according to the present invention; and

[00025] Fig. 8 is a graphical representation of the effect of the mud drum sludge collection system on deposition rates.

[00026] DESCRIPTION OF THE PREFERRED EMBODIMENT

[00027] Referring to the drawings generally, wherein like reference numerals designate the same or functionally similar elements throughout the several drawings, the basic mud drum sludge collection system 50 design according to the present invention is illustrated in Figs. 4 - 7. Fig. 4 is a sectional side view of an employing the mud drum sludge collection system 50 according to the present invention.

[00028] The RSG 100 comprises a pressure vessel 102 on a sliding base support 40d. The vessel 102 contains a bundle of U-tubes 14 for conveying primary coolant 12 from an extemal heat source (not shown) into and out of the pressure vessel 102 for heating feedwater 26 provided into the pressure vessel 102 to generate a steam/water mixture 18, the bundle of U-tubes 14 provided at a lower portion 104 of the pressure vessel 102. A steam drum 106 is provided at an upper portion 108 of the pressure vessel 102 having ninety-nine steam/water separating means 20 (only one shown) for separating water from the steam/water mixture 18, the steam 24 exiting from the pressure vessel 102 via a steam outlet 110 equipped with a nozzle with a flow restrictor, the separated water 22 being recirculated back to the lower portion 104 of the pressure vessel 102 for reheating. The water level in the steam drum at 100% power 40a is significantly higher than the water level at 0% power 40b. In accordance with the invention, a mud drum sludge collection system or sludge trap 50 is provided for removing sludge from a bleed stream of the recirculated water 22 and conveying the bleed stream to a low velocity region established within the pressure vessel 102.

[00029] Referring now to Figs. 5 and 6, it will be seen that the mud drum sludge collection system 50 according to the present invention is installed on the top of the primary separator deck 34 support. The primary separator deck 34 comprises a flat horizontal plate that supports the primary separators 30. The primary separator deck 34 is stiffened in the out-of-plane direction, for normal and accident loading, by one or more horizontal stiffeners 120 between rows of separators 20 (see Fig. 6). The stiffeners 120 may be located between every second row of separators 20 but that spacing is not -$

required; other placements or locations may be used. Field observations made during secondary side visual inspections of RSGs 10 report that deposition occurs preferentially in low flow regions on the top of the primary separator deck 34. The highest concentration of impurities occurs in the recirculated water 22 on the top of the primary separator deck 34.

[00030] From a review of Figs. 4 - 7, it will be recognized that the mud drum sludge collection system 50 according to the present invention employs the following structural features in addition to a conventional primary separator deck design:

[00031] addition of a peripheral stiffener ring plate 122 with a foreign object weir 50a to the primary deck 34; and

[00032] addition of a top plate 124 covering the stiffeners 120 and peripheral stiffener ring plate 122.

[00033] The mud drum sludge collection system 50 is designed to create a bleed stream flow 126, the bleed comprising approximately 1% of the recirculated flow from the primary separators 30. This represents approximately 4.0 to 5.0% of the steam flow. The flow 126 enters the mud drum sludge collector 50 through central annular openings 128 in the top plate 124 around a few (advantageously three, but a different number could be involved) primary separators 30 closest the centre-line of the recirculating steam generator 100. The flow 126 exits the sludge collector 50 through openings 130 in the peripheral ring 122, the latter being located adjacent a periphery of the primary separator deck plate 34 and the top plate 124. Upon entering the sludge collector 50, the flow area greatly expands, resulting in a low velocity flow region 132 established within the RSG 100 in between the primary separator deck plate 34 and the top plate 124 and including several chambers created by the primary deck stiffeners 120 which promotes the settling of particulate materials from the bleed flow stream 126.
Typically, the primary separator deck 34 is circular and the primary deck stiffeners 120 extend across the primary separator deck 34 in chordal fashion. The stiffeners 120 are welded both to the primary separator deck 34 and to the top plate 124, resulting in a compact, and.robust support structure for the primary separators 30.

[00034] As shown in Fig. 7, the bleed stream flow 126 must migrate past the primary deck stiffeners or ribs 120, which have their lower edges adjacent the primary separator deck plate 34, their upper edges adjacent the top plate 124, and which have at least one flow opening 134 in the upper half of the stiffeners 120. The stiffeners 120 thus create recirculating eddies, which promote settling and serve to prevent particulates from passing to an adjacent chamber. The stiffeners 120 have ends located adjacent the peripheral stiffener ring 122. The peripheral stiffener ring 122 also has a lower edge adjacent the primary separator deck plate 34 and an upper edge adjacent the top plate 124.

[00035] The flow openings 130 in the peripheral ring 122 are also located in the top half of the ring 122 to have a similar effect. In addition, the lower edge of the peripheral stiffener ring 122 is provided with at least one drain opening or drain hole 136, and at least one small stiffener drain opening or drain hole 138 is provided at the bottom edge of the stiffeners 120 adjacent one of the ends of the stiffener 120 to facilitate complete draining of the mud drum 50 for maintenance outages. These drain holes 136, 138 were modeled in a 3-D Computational Fluid Dynamics (CFD) qualification of the mud drum 50, described in the followirng, and have an insignificant effect on the normal operation of the mud drum 50.

[00036] Analytical Qualification of the Mud Drum Design

[00037] A 3-D CFD analysis of the mud drum design was performed to investigate the flow fields within the mud drum and to optimize the size, shape and location of the various flow openings and stiffeners. The solution domain of the model consists of the following flow regions:

[00038] The primary separator return flow, with detailed modeling of the flow exiting each of the primary separators;

[00039] The bleed flow path from the separator return into the mud drum;

[00040] The inside flow paths of the mud drum, including all stiffeners, flow obstructions, flow and drain openings; and

[00041] The downcomer return from the primary deck and the mud drum, down to the elevation of the feed-ring.

[00042] As shown in Figs. 6 and 7, the steam/water separator means 20, particularly the primary separator 30, is supported by the primary separator deck plate 34. The steam/water separator means 20 has riser inlet pipes 38 extending upwardly from the primary separator deck plate 34 through the top plate 124. The return cylinders 40 of the steam/water separators 20 extend downwardly towards the top plate 124 but a clearance is provided between their lower ends and the top plate 124. The bleed flow stream 126 taken from the recirculated flow 22 from the steam/water separator means 20 enters through the 3 annular openings 128 in the top plate around the 3 primary separator riser inlet pipes 38 which extend upwardly through the top plate 124 at the centre of the RSG 100. The center-most stiffener 120 effectively distributes the bleed flow 126 across the width of the mud drum 50 and reduces the flow velocities below 0.5 m/s in almost all of the regions of the mud drum 50. The bleed flow 126 slowly migrates across the stiffeners 120 to the peripheral exit holes 130, where the flow 126 re-joins the downcomer flow 140.

[00043] To investigate the effectiveness of the mud drum, an algorithm was used to study the flow path and settling of various size particles. For calculation of a dispersed phase by a third-party computer software modeling program, CFX-TASCflow, the Lagrangian Particle Tracking (LPT) method is employed. The basic concept of this approach is to calculate the particle motion in a continuous fluid medium under the action of forces that are due to difference- in velocity between the particle and the fluid and due to displacement of the fluid by the particle. Each particle that is tracked represents a sample of particles that follow an identical path. The behaviour of the tracked particles is then used to describe the average behaviour of the dispersed phase.

[00044] The basic assumptions in the LPT model are:

[00045] particles are spherical;

[00046] particle/particle interactions are not included; and

[00047] turbulence is not modified by the solid phase.

[00048] The influence of turbulent fluid fluctuations on particle motion is incorporated in the code by expressing turbulent velocity, eddy lifetime and length in terms of locally calculated turbulent kinetic energy and dissipation.

[00049] In order to have a statistically significant sample, a total of 200,000 particles were injected into domain and their trajectories calculated.

[00050] Simulations were performed to track the motion and settling of 1 and micron diameter particles, which are the typical size of steam generator sludge particles. These studies have shown that the efficiency of particle sedimentation within the mud drum is between about 68% to about 74%. The efficiency is higher for the larger size of particles.

[00051] Reduction in RSG Deposition Rate due to the Mud Drum

[00052] The analytical impact of the mud drum on general RSG deposition was assessed by Atomic Energy of Canada Limited (AECL) using a third-party computer software code called SLUDGE, code where the following conditions were modeled:

[00053] 4 ppb crud in feedwater;

[00054] 1 % blowdown;

[00055] 0.1 % carryover;

[00056] Magnetite deposition with morpholine; and

[00057] 0.5 year simulation with two hour simulation time steps (linear after days).

[00058] SLUDGE is a 3-D code for calculating transient behaviour of sludge in a recirculating steam generator and was developed by C. Turner et al. at the AECL Chalk River laboratories. SLUDGE uses thermal hydraulic inputs taken from either of two other third-party software programs, THIRST or ATHOS.

[00059] The SLUDGE code uses deposition and removal equations fitted to deposition data for aqueous systems accounting for chemistry and medium effects.

[00060] Under the. applied fouling model, the fouling rate is linear for times significantly greater than 10 days; e.g. to obtain the results for 5 years of operation, simply multiply the results of 0.5-y simulation by 10. The results are summarized in Table 1 below and Fig. 8.

[00061] Results

[00062] The results shown in Table 1 and Fig. 8 show that a crud trap situated on the primary separator deck plate that process only 1%o of the flow provides an effective means of removing particles from the separated water, and thus lowers the rate of deposit accumulation in other sinks, i.e., in the tube bundle and horizontal plates, including the tubesheet. A trap with an efficiency of 75% (meaning that 75% of the particles transported to the trap are removed from the separated water) is predicted to remove 45 % of the crud that is transported to the SG with the feedwater.
Total crud removed by blowdown and the trap is important because this represents crud that is not deposited on other surfaces, where it can lead potentially to steam generator degradation. For a blowdown rate of 1%, total crud removed by blowdown and the sludge trap is predicted to increase from 37% for 0% trap efficiency to 70%
for a trap efficiency of 100%.

[00063] Table 1: Effect of Crud trap efficiency on deposit distribution in a recirculating steam generator with no integral preheater

[00064]
Trap Horizontal Run # Efficiency Tubes Surfaces Blowdown Carryover Trap % % % % % %
ST275 0 52.2 6.8 37.1 3.9 0 ST276 25 40.9 5.3 29.1 3.0 21.7 ST277 50 33.6 4.4 23.9 2.5 35.6 ST278 75 28.6 3.7 20.3 2.1 45.3 ST279 100 24.9 3.2 17.7 1.8 52.4

[00065] With an efficiency of 75% and 1% blowdown, the mud drum sludge collection system removes 45% of the crud introduced in the feedwater. Total crud removal (blowdown and mud drum) increases from 37.1% with no mud drum to 65.6%
with a 75% effective mud drum. Tube deposition is reduced from 52% to 29%
(about 45%) with the addition of the mud drum sludge collection system (Fig. 8).

[00066] While specific embodiments of the invention have been shown and described in detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles. For example, the present invention may be applied to new RSG
installations, whether for CANDU nuclear systems, or for U.S. design PWR nuclear power plants, or to the replacement, repair or modification of existing RSG steam generators where enhanced particulate removal capabilities are desired. In some embodiments of the invention, certain features of the invention may sometimes be used to advantage without a corresponding use of the other features. Accordingly, all such changes and embodiments properly fall within the scope of the following claims.

Claims

I claim:

1. A mud drum sludge collection system for a recirculating steam generator, comprising:
a low velocity region established within the recirculating steam generator and defined by a primary separator deck plate, a top plate located above the primary separator deck plate, and a peripheral stiffener ring adjacent a periphery of the primary separator deck plate and the top plate; and means for conveying a bleed stream of recirculated water created within the recirculating steam generator to the low velocity region.

2. The mud drum sludge collection system according to claim 1, comprising at least one primary separator deck stiffener interposed between the primary separator deck plate and the top plate.

3. The mud drum sludge collection system according to claim 2, wherein the at least one primary separator deck stiffener has a lower edge adjacent the primary separator deck plate and an upper edge adjacent the top plate, the upper edge of the primary separator deck stiffener being provided with at least one flow opening.

4. The mud drum sludge collection system according to claim 2, wherein the at least one primary separator deck stiffener has ends located adjacent the peripheral stiffener ring.

5. The mud drum sludge collection system according to claim 4, comprising at least one stiffener drain opening located adjacent one of the ends of the primary separator deck stiffener.

6. The mud drum sludge collection system according to claim 1, wherein the peripheral stiffener ring has a lower edge adjacent the primary separator deck plate and an upper edge adjacent the top plate, the lower edge of the peripheral stiffener ring being provided with at least one drain opening.

7. The mud drum sludge collection system according to claim 6, comprising at least one peripheral edge opening provided on the upper edge of the peripheral stiffener ring.

8. The mud drum sludge collection system according to claim 4, wherein the primary separator deck is circular and the at least one primary deck stiffener extends across the primary separator deck in chordal fashion.

9. The mud drum sludge collection system according to claim 1, comprising steam/water separator means supported by the primary separator deck plate.

10. The mud drum sludge collection system according to claim 9, wherein the steam/water separator means has riser means extending upwardly from the primary separator deck plate through the top plate.

11. The mud drum sludge collection system according to claim 10, wherein the means for conveying a bleed stream of recirculated water created within the recirculating steam generator to the low velocity region comprises several annular openings in the top plate for several of the riser means extending upwardly therethrough for receiving a bleed stream of recirculated flow from the steam/water separator means.

12. The mud drum sludge collection system according to claim 11, wherein the several annular openings are provided around several of the riser means located closest to a centre-line of the nuclear steam generator.

13. The mud drum sludge collection system according to claim 1, comprising a plurality of primary separator deck stiffeners interposed between the primary separator deck plate and the top plate, each primary separator deck stiffener having a lower edge adjacent the primary separator deck plate and an upper edge adjacent the top plate, the upper edge of the primary separator deck stiffeners being provided with at least one flow opening, the primary separator deck stiffeners having ends located adjacent the peripheral stiffener ring.

14. A recirculating nuclear steam generator, comprising:
a pressure vessel containing a bundle of U-tubes for conveying primary coolant from an external heat source into and out of the pressure vessel for heating feedwater provided into the pressure vessel to generate a steam/water mixture, the bundle of U-tubes provided at a lower portion of the pressure vessel;
a steam drum provided at an upper portion of the pressure vessel having steam/water separating means for separating water from the steam/water mixture, the steam exiting from the pressure vessel via a steam outlet, the separated water being recirculated back to the lower portion of the pressure vessel for reheating;
a mud drum sludge collection system for removing sludge from a bleed stream of the recirculated water and conveying the bleed stream to a low velocity region established within the pressure vessel, the low velocity region defined by a primary separator deck plate for supporting the steam/water separating means, a top plate located above the primary separator deck plate;
and a peripheral stiffener ring adjacent a periphery of the primary separator deck plate and the top plate; and means for conveying a bleed stream of recirculated water created within the recirculating steam generator to the low velocity region.

15. The recirculating nuclear steam generator according to claim 14, comprising a plurality of primary separator deck stiffeners interposed between the primary separator deck plate and the top plate, each primary separator deck stiffener having a lower edge adjacent the primary separator deck plate and an upper edge adjacent the top plate, the upper edge of the primary separator deck stiffeners being provided with at least one flow opening, the primary separator deck stiffeners having ends located adjacent the peripheral stiffener ring.

16. The recirculating nuclear steam generator according to claim 15, wherein the mud drum sludge collection system receives a bleed stream of recirculated water comprising about 1% of the recirculated water from the steam/water separating means.

17. The recirculating nuclear steam generator according to claim 14, wherein the bleed stream of recirculated water is provided to the low velocity region via several apertures provided through the top plate and located closest to a centre-line of the nuclear steam generator.

18. The recirculating nuclear steam generator according to claim 17, wherein the steam/water separator means has riser means extending upwardly from the primary separator deck plate through the top plate.

19. The recirculating nuclear steam generator according to claim 18, wherein the several apertures through the top plate are annular openings established between the several of the riser means extending upwardly therethrough for receiving the bleed stream of recirculated water from the steam/water separator means and conveying same to the low velocity region.