ENVIRONMENTALLY SEQUESTERED SPENT FUEL POOL

Abstract

An environmentally sequestered nuclear spent fuel pool in one embodiment includes sidewalls and a base slab that confine a water impoundment. The pool includes fuel racks containing spent fuel assemblies which heat the water via radioactive decay. A dual liner system enclosing the pool forms an impervious barrier providing redundant provisions for preventing leakage of contaminated pool water into the environment. An interstitial space is formed between the liners which may be maintained at sub-atmospheric pressures by a vacuum pump system that evacuates the space. By maintaining the pressure in the space at a negative pressure with corresponding boiling point less than the temperature of the pool water, any leakage through the inner-most liner into the interstitial space will vaporize and be extracted via the pump for signaling a potential leak in the liner system.

Claims

1. A spent fuel pool system comprising: a fuel pool comprising: a base slab; a plurality of vertical sidewalls extending from the base slab; and a cavity defined by the vertical sidewalls and the base slab, the cavity being at least partially filled with pool water; a pool liner system located within the cavity, the pool liner system comprising: an outer liner positioned adjacent to the vertical sidewalls; an inner liner positioned adjacent to the outer liner such that the inner liner is in contact with the pool water; and wherein the inner and outer liners are separated by an air gap.

2. The spent fuel pool system according to claim 1 wherein both of the inner and outer liners are positioned within the cavity of the fuel pool.

3. The spent fuel pool system according to claim 1 further comprising an embedment plate embedded into the vertical sidewalls and circumscribing the cavity, the embedment plate comprising an inner surface that faces the cavity.

4. The spent fuel pool system according to claim 3 wherein the inner liner is sealably coupled to the inner surface of the embedment plate at a first position and the outer liner is sealably coupled to the inner surface of the embedment plate at a second position, the first position being closer to an open top end of the cavity than the second position.

5. The spent fuel pool system according to claim 4 wherein a hermetically sealed flow plenum is formed between a top terminal edge of the inner liner and a top terminal edge of the outer liner, wherein the flow plenum is in fluid communication with the air gap.

6. The spent fuel pool system according to claim 5 further comprising a flow passageway formed through the embedment plate, a first end of the flow passageway being fluidly coupled to the flow plenum.

7. The spent fuel pool system according to claim 6 further comprising a vacuum system located outside of the cavity of the fuel pool, the vacuum system comprising a vacuum pump that is fluidly coupled to the air gap by way of the flow passageway of the embedment plate.

8. The spent fuel pool system according to claim 7 wherein the vacuum pump is configured to maintain the air gap at a vacuum pressure.

9. The spent fuel pool system according to claim 8 further comprising a pressure sensor configured to detect any rise in pressure within the air gap that would be indicative of a potential leak in the pool liner system.

10. The spent fuel pool system according to claim 5 wherein the flow plenum extends around an entire perimeter of the spent fuel pool.

11. The spent fuel pool system according to claim 1 wherein the air gap extends interrupted from an inner surface of the outer liner that faces away from the vertical sidewalls and towards the cavity to an outer surface of the inner liner that faces away from the cavity and towards the vertical sidewalls.

12. The spent fuel pool system according to claim 1 further comprising a plurality of storage racks disposed on the base slab and submerged in the pool water, each of the storage racks having a plurality of cells each configured for holding a spent nuclear fuel assembly containing nuclear fuel rods.

13. The spent fuel pool system according to claim 1 wherein the inner and outer liners are located entirely within the cavity of the fuel pool.

14. A spent fuel pool system comprising: a fuel pool comprising: a base slab; a plurality of vertical sidewalls extending from the base slab; and a cavity defined by the base slab and an inner surface of the vertical sidewalls, the cavity being filled with pool water; a pool liner system comprising: an outer liner positioned adjacent to the inner surfaces of the vertical sidewalls; and an inner liner positioned adjacent to the outer liner, the inner liner being spaced apart from the outer liner by an air gap, and wherein the inner liner is in contact with the pool water; and a vacuum system fluidly coupled to the air gap, the vacuum system comprising a vacuum pump that is configured to maintain the air gap at a vacuum pressure.

15. The spent fuel pool system according to claim 14 further comprising a pressure sensor configured to detect any rise in pressure within the air gap that would be indicative of a potential leak in the pool liner system.

16. The spent fuel pool system according to claim 14 further comprising a flow plenum formed between a top terminal edge of the outer liner and a top terminal edge of the inner liner, the flow plenum being fluidly coupled to the air gap between the inner and outer liners.

17. The spent fuel pool system according to claim 16 wherein the vacuum pump is fluidly coupled to the air plenum through a flow passageway formed into an embedment plate that is embedded within the vertical sidewalls.

18. A spent fuel pool system comprising: a base slab; a plurality of vertical sidewalls extending from the base slab; a cavity defined by the vertical sidewalls and the base slab, the cavity being filled with pool water; at least one fuel storage rack disposed in the cavity, the fuel storage rack holding a nuclear spent fuel assembly containing nuclear fuel rods that heat the pool water; a pool liner system located in the cavity and comprising an outer liner adjacent the sidewalls and base slab, an inner liner adjacent the outer liner and wetted by the pool water, and an interstitial space formed between the outer and inner liners; and a vacuum pump fluidly coupled to the interstitial space, the vacuum pump being configured to evacuate air from the interstitial space to maintain a pressure of the interstitial space below atmospheric pressure.

19. The spent fuel pool system according to claim 18 further comprising a pressure sensor configured to detect any rise in pressure within the air gap that would be indicative of a potential leak in the pool liner system.

20. The spent fuel pool system according to claim 19, further comprising a computer processor based plant monitoring system operably coupled to the pressure sensor to monitor the pressure of the interstitial space.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] The features of the exemplary embodiments will be described with reference to the following drawings where like elements are labeled similarly, and in which:

[0012] FIG. 1 is a cross sectional diagram of a known approach used to monitor the integrity of weld seams for leakage in a single spent fuel pool liner system;

[0013] FIG. 2 is a side cross-sectional view of an environmentally sequestered nuclear spent fuel pool having a dual liner and leakage collection and monitoring system according to the present disclosure;

[0014] FIG. 3 is a top plan view of the fuel pool with liner and leakage collection/monitoring system of FIG. 2;

[0015] FIG. 4 is a detail taken from FIG. 2 showing a bottom joint of the liner system at the intersection of liners from the sidewalls and base slab of the fuel pool;

[0016] FIG. 5 is a detail taken from FIG. 2 showing a top joint of the liner system at the terminal top ends of the sidewall liners;

[0017] FIG. 6 is a perspective view of an example nuclear fuel assembly containing spent nuclear fuel rods; and

[0018] FIG. 7 is a schematic diagram of a vacuum leakage collection and monitoring system according to the present disclosure.

[0019] All drawings are schematic and not necessarily to scale. Parts shown and/or given a reference numerical designation in one figure may be considered to be the same parts where they appear in other figures without a numerical designation for brevity unless specifically labeled with a different part number and described herein. References herein to a figure number (e.g. FIG. 1) shall be construed to be a reference to all subpart figures in the group (e.g. FIGS. 1A, 1B, etc.) unless otherwise indicated.

DETAILED DESCRIPTION

[0020] The features and benefits of the invention are illustrated and described herein by reference to exemplary embodiments. This description of exemplary embodiments is intended to be read in connection with the accompanying drawings, which are to be considered part of the entire written description. Accordingly, the disclosure expressly should not be limited to such exemplary embodiments illustrating some possible non-limiting combination of features that may exist alone or in other combinations of features.

[0021] In the description of embodiments disclosed herein, any reference to direction or orientation is merely intended for convenience of description and is not intended in any way to limit the scope of the present invention. Relative terms such as lower, upper, horizontal, vertical,, above, below, up, down, top and bottom as well as derivative thereof (e.g., horizontally, downwardly, upwardly, etc.) should be construed to refer to the orientation as then described or as shown in the drawing under discussion. These relative terms are for convenience of description only and do not require that the apparatus be constructed or operated in a particular orientation. Terms such as attached, affixed, connected, coupled, interconnected, and similar refer to a relationship wherein structures are secured or attached to one another either directly or indirectly through intervening structures, as well as both movable or rigid attachments or relationships, unless expressly described otherwise.

[0022] Referring to FIGS. 2-6, an environmentally sequestered spent fuel pool system includes a spent fuel pool 40 comprising a plurality of vertical sidewalls 41 rising upwards from an adjoining substantially horizontal base wall or slab 42 (recognizing that some slope may intentionally be provided in the upper surface of the bottom wall for drainage toward a low point if the pool is to be emptied and rinsed/decontaminated at some time and due to installation tolerances). The base slab 42 and sidewalls 41 may be formed of reinforced concrete in one non-limiting embodiment. The fuel pool base slab 42 may be formed in and rest on the soil sub-grade 26 the top surface of which defines grade G. In this embodiment illustrated in the present application, the sidewalls are elevated above grade. In other possible embodiments contemplated, the base slab 42 and sidewalls 41 may alternatively be buried in sub-grade 26 which surrounds the outer surfaces of the sidewalls. Either arrangement may be used and does not limit of the invention.

[0023] In one embodiment, the spent fuel pool 40 may have a rectilinear shape in top plan view. Four sidewalls 41 may be provided in which the pool has an elongated rectangular shape (in top plan view) with two longer opposing sidewalls and two shorter opposing sidewalls (e.g. end walls). Other configurations of the fuel pool 40 are possible such as square shapes, other polygonal shapes, and non-polygonal shapes.

[0024] The sidewalls 41 and base slab 42 of the spent fuel pool 40 define a cavity 43 configured to hold cooling pool water W and a plurality of submerged nuclear spent fuel assembly storage racks 27 holding fuel bundles or assemblies 28 each containing multiple individual nuclear spent fuel rods. The storage racks 27 are disposed on the base slab 42 in typical fashion. With continuing reference to FIGS. 1-6, the spent fuel pool 40 extends from an operating deck 22 surrounding the spent fuel pool 40 downwards to a sufficient depth D1 to submerge the fuel assemblies 28 (see, e.g. FIG. 6) beneath the surface level S of the pool water W for proper radiation shielding purposes. In one implementation, the fuel pool may have a depth such that at least 10 feet of water is present above the top of the fuel assembly.

[0025] A nuclear fuel assembly storage rack 27 is shown in FIGS. 2 and 3, and further described in commonly assigned U.S. patent application Ser. No. 14/367,705 filed Jun. 20, 1014, which is incorporated herein by reference in its entirety. The storage rack 27 contains a plurality of vertically elongated individual cells as shown each configured for holding a fuel assembly 28 comprising a plurality of individual nuclear fuel rods. An elongated fuel assembly 28 is shown in FIG. 6 holding multiple fuel rods 28a and further described in commonly assigned U.S. patent application Ser. No. 14/413,807 filed Jul. 9, 2013, which is incorporated herein by reference in its entirety. Typical fuel assemblies 28 for a pressurized water reactor (PWR) may each hold over 150 fuel rods in 1010 to 1717 fuel rod grid arrays per assembly. The assemblies may typically be on the order of approximately 14 feet high weighing about 1400-1500 pounds each.

[0026] The substantially horizontal operating deck 22 that circumscribes the sidewalls 41 and pool 40 on all sides in one embodiment may be formed of steel and/or reinforced concrete. The surface level of pool water W (i.e. liquid coolant) in the pool 40 may be spaced below the operating deck 22 by a sufficient amount to prevent spillage onto the deck during fuel assembly loading or unloading operations and to account to seismic event. In one non-limiting embodiment, for example, the surface of the operating deck 22 may be at least 5 feet above the maximum 100 year flood level for the site in one embodiment. The spent fuel pool 40 extending below the operating deck level may be approximately 40 feet or more deep (e.g. 42 feet in one embodiment). The fuel pool is long enough to accommodate as many spent fuel assemblies as required. In one embodiment, the fuel pool 40 may be about 60 feet wide. There is sufficient operating deck space around the pool to provide space for the work crew and for staging necessary tools and equipment for the facility's maintenance. There may be no penetrations in the spent fuel pool 40 within the bottom 30 feet of depth to prevent accidental draining of water and uncovering of the spent fuel.

[0027] According to one aspect of the invention, a spent fuel pool liner system comprising a double liner is provided to minimize the risk of pool water leakage to the environment. The liner system is further designed to accommodate cooling water leakage collection and detection/monitoring to indicate a leakage condition caused by a breach in the integrity of the liner system.

[0028] The liner system comprises a first outer liner 60 separated from a second inner liner 61 by an interstitial space 62 formed between the liners. An outside surface of liner 60 is disposed against or at least proximate to the inner surface 63 of the fuel pool sidewalls 41 and opposing inside surface is disposed proximate to the interstitial space 62 and outside surface of liner 61. The inside surface of liner 61 is contacted and wetted by the fuel pool water W. It bears noting that placement of liner 60 against liner 61 without spacers therebetween provides a natural interstitial space of sufficient width to allow the space and any pool leakage there-into to be evacuated by a vacuum system, as further described herein. The natural surface roughness of the materials used to construct the liners and slight variations in flatness provides the needed space or gap between the liners. In other embodiments contemplated, however, metallic or non-metallic spacers may be provided which are distributed in the interstitial space 62 between the liners if desired.

[0029] The liners 60, 61 may be made of any suitable metal which is preferably resistant to corrosion, including without limitation stainless steel, aluminum, or other. In some embodiments, each liner may be comprised of multiple substantially flat metal plates which are seal welded together along their peripheral edges to form a continuous liner system encapsulating the sidewalls 41 and base slab 42 of the spent fuel pool 40.

[0030] The inner and outer liners 61, 60 may have the same or different thicknesses (measured horizontally or vertically between major opposing surfaces of the liners depending on the position of the liners). In one embodiment, the thicknesses may be the same. In some instances, however, it may be preferable that the inner liner 61 be thicker than the outer liner 60 for potential impact resistant when initially loading empty fuel storage racks 27 into the spent fuel pool 40.

[0031] The outer and inner liners 60, 61 (with interstitial space therebetween) extend along the vertical sidewalls 41 of the spent fuel pool 40 and completely across the horizontal base slab 42 in one embodiment to completely cover the wetted surface area of the pool. This forms horizontal sections 60b, 61b and vertical sections 60a, 61a of the liners 60, 61 to provide an impervious barrier to out-leakage of pool water W from spent fuel pool 40. The horizontal sections of liners 60b, 61b on the base slab 42 may be joined to the vertical sections 60a, 61a along the sidewalls 41 of the pool 40 by welding. The detail in FIG. 4 shows one or many possible constructions of the bottom liner joint 64 comprising the use of seal welds 65 (e.g. illustrated fillet welds or other) to seal sections 60a to 60b along their respective terminal edges and sections 61a to 61b along their respective terminal edges as shown. Preferably, the joint 64 is configured and arranged to fluidly connect the horizontal interstitial space 64 between horizontal liner sections 60b, 61b to the vertical interstitial space 64 between vertical liner sections 60a, 61a for reasons explained elsewhere herein.

[0032] The top liner joint 65 in one non-limiting embodiment between the top terminal edges 60c, 61c of the vertical liner sections 60a, 61a is shown in the detail of FIG. 5. The top of the spent fuel pool 40 is equipped with a substantially thick metal embedment plate 70 which circumscribes the entire perimeter of the fuel pool. The embedment plate 70 may be continuous in one embodiment and extends horizontally along the entire inner surface 63 of the sidewalls 41 at the top portion of the sidewalls. The embedment plate 70 has an exposed portion of the inner vertical side facing the pool which extends above the top terminal ends 60c, 61c of the inner and outer liners 60, 61. The opposing outer vertical side of the plate 70 is embedded entirely into the sidewalls 41. A top surface 71 of the embedment plate 70 that faces upwards may be substantially flush with the top surface 44 of the sidewalls 41 to form a smooth transition therebetween. In other possible implementations, the top surface 71 may extend above the top surface 44 of the sidewalls. The embedment plate 70 extends horizontal outward from the fuel pool 40 for a distance into and less than the lateral width of the sidewalls 41 as shown.

[0033] The embedment plate 70 has a horizontal thickness greater than the horizontal thickness of the inner liner 61, outer liner 60, and in some embodiments both the inner and outer liners combined.

[0034] The top embedment plate 70 is embedded into the top surface 44 of the concrete sidewalls 41 has a sufficient vertical depth or height to allow the top terminal edges 60c, 61c of liners 60, 61 (i.e. sections 60a and 61a respectively) to be permanently joined to the plate. The top terminal edges of liners 60, 61 terminate at distances D2 and D1 respectively below a top surface 71 of the embedment plate 70 (which in one embodiment may be flush with the top surface of the pool sidewalls 41 as shown). Distance D1 is less than D2 such that the outer liner 60 is vertical shorter in height than the inner liner 61. In one embodiment, the embedment plate 70 has a bottom end which terminates below the top terminal edges 60c, 61c of the liners 60, 61 to facilitate for welding the liners to the plate.

[0035] In various embodiments, the embedment plate 70 may be formed of a suitable corrosion resistant metal such as stainless steel, aluminum, or another metal which preferably is compatible for welding to the metal used to construct the outer and inner pool liners 60, 61 without requiring dissimilar metal welding.

[0036] As best shown in FIG. 5, the top terminal edges 60c, 61c of inner and outer liners 60, 61 may have a vertically staggered arranged and be separately seal welded to the top embedment plate 70 independently of each other. A seal weld 66 couples the top terminal edge 61c of liner 61 to the exposed portion of the inner vertical side of the embedment plate 70. A second seal weld 67 couples the top terminal edge 60c of liner 60 also to the exposed portion of the inner vertical side of the embedment plate 70 at a location below and spaced vertical apart from seal weld 66. This defines a completely and hermetically sealed enclosed flow plenum 68 that horizontal circumscribes the entire perimeter of the spent fuel pool 40 in one embodiment. The flow plenum 68 is in fluid communication with the interstitial space 62 as shown. One vertical side of the flow plenum is bounded by a portion of inner liner 61 and the opposing vertical side of the plenum is bounded by the inner vertical side of the top embedment plate 70.

[0037] The top flow plenum 68 may be continuous or discontinuous in some embodiments. Where discontinuous, it is preferable that a flow passageway 105 in the top embedment plate 70 be provided for each section of the separate passageways.

[0038] Seal welds 66 and 67 may be any type of suitable weld needed to seal the liners 60, 61 to the top embedment plate 70. Backer plates, bars, or other similar welding accessories may be used to make the welds as needed depending on the configuration and dimensions of the welds used. The invention is not limited by the type of weld.

[0039] In one embodiment, the outer and inner liners 60, 61 are sealably attached to the spent fuel pool 40 only at top embedment plate 70. The remaining portions of the liners below the embedment plate may be in abutting contact with the sidewalls 41 and base slab 42 without means for fixing the liners to these portions.

[0040] It bears noting that at least the inner liner 61 has a height which preferably is higher than the anticipated highest water level (surface S) of the pool water W in one embodiment. If the water level happens to exceed that for some reason, the top embedment plate 70 will be wetted directly by the pool water and contain the fluid to prevent overflowing the pool onto the operating deck 22.

[0041] According to another aspect of the invention, a vapor extraction or vacuum system 100 is provided that is used to draw down the air pressure in the interstitial space between the outer and inner liners 60, 61 to a relatively high state of vacuum for leakage control and/or detection. FIG. 7 is a schematic diagram of one embodiment of a vacuum system 100.

[0042] Referring to FIGS. 5 and 7, vacuum system 100 generally includes a vacuum pump 101 and a charcoal filter 102. Vacuum pump 101 may be any suitable commercially-available electric-driven vacuum pump capable of creating a vacuum or negative pressure within the interstitial space 62 between the pool liners 60 and 61. The vacuum pump 101 is fluidly connected to the interstitial space 68 via a suitable flow conduit 103 which is fluidly coupled to a telltale or flow passageway 105 extending from the top surface 71 of the top embedment plate 70 to the top flow plenum 68 formed between the pool liners 60 and 61. Flow conduit 103 may be formed of any suitable metallic or non-metallic tubing or piping capable of withstanding a vacuum. A suitably-configured fluid coupling 104 may be provided and sealed to the outlet end of the flow passageway 105 for connecting the flow conduit 103. The inlet end of the flow passageway penetrates the inner vertical side of top embedment plate 70 within the flow plenum 68. The flow passageway 105 and external flow conduit 103 provides a contiguous flow conduit that fluidly couples the flow plenum 68 to the vacuum pump 101. A one-way check valve is disposed between the flow plenum 105 and the suction inlet of the vacuum pump 101 to permit air and/or vapor to flow in a single direction from the liner system to the pump.

[0043] The absolute pressure maintained by the vacuum system 100 in the interstitial space 62 between the liners 60, 61 (i.e. set pressure) preferably should be such that the bulk water temperature of the spent fuel pool 40 which is heated by waste decay heat generated from the fuel rods/assemblies is above the boiling temperature of water at the set pressure. The table below provides the boiling temperature of water at the level of vacuum in inches of mercury (Hg) which represent some examples of set pressures that may be used.

TABLE-US-00001 Pressure in inch, HgA Boiling Temp, deg F. 1 79 2 101 3 115 4 125 5 133

[0044] Any significant rise in pressure would indicate potential leakage of water in the interstitial space 62 between the liners 60, 61. Because of sub-atmospheric conditions maintained by the vacuum pump 101 in the interstitial space, any water that may leak from the pool into this space through the inner liner 61 would evaporate, causing the pressure to rise which may be monitored and detected by a pressure sensor 104. The vacuum pump 101 preferably should be set to run and drive down the pressure in the interstitial space 62 to the set pressure.

[0045] In operation as one non-limiting example, if the vacuum pump 101 is operated to create a negative pressure (vacuum) in the interstitial space 62 of 2 inches of Hg, the corresponding boiling point of water at that negative pressure is 101 degrees Fahrenheit (degrees F.) from the above Table. If the bulk water temperature of pool water W in the spent fuel pool 40 were at any temperature above 101 degrees F. and leakage occurred through the inner pool liner 61 into the interstitial space 62, the liquid leakage would immediately evaporate therein creating steam or vapor. The vacuum pump 101 withdraws the vapor through the flow plenum 68, flow passageway 105 in the top embedment plate 70, and flow conduit 103 (see, e.g. directional flow arrows of the water vapor in FIGS. 5 and 7). Pressure sensor 104 disposed on the suction side of the pump 101 would detect a corresponding rise in pressure indicative of a potential leak in the liner system. In some embodiments, the pressure sensor 104 may be operably linked to a control panel of a properly configured computer processor based plant monitoring system 107 which monitors and detects the pressure measured in the interstitial space 62 between the liners on a continuous or intermittent basis to alert operators of a potential pool leakage condition. Such plant monitoring systems are well known in the art without further elaboration.

[0046] The extracted vapor in the exhaust or discharge from the vacuum pump 101 is routed through a suitable filtration device 102 such as a charcoal filter or other type of filter media before discharge to the atmosphere, thereby preventing release of any particulate contaminants to the environment.

[0047] Advantageously, it bears noting that if leakage is detected from the spent fuel pool 40 via the vacuum system 100, the second outer liner 60 encapsulating the fuel pool provides a secondary barrier and line of defense to prevent direct leaking of pool water W into the environment.

[0048] It bears noting that there is no limit to the number of vapor extraction systems including a telltale passageway, vacuum pump, and filter combination with leakage monitoring/detection capabilities that may be provided. In some instances, four independent systems may provide adequate redundancy. In addition, it is also recognized that a third or even fourth layer of liner may be added to increase the number of barriers against leakage of pool water to the environment. A third layer in some instances may be used as a palliative measure if the leak tightness of the first inter-liner space could not, for whatever reason, be demonstrated by a high fidelity examination in the field such as helium spectroscopy.

[0049] While the foregoing description and drawings represent exemplary embodiments of the present disclosure, it will be understood that various additions, modifications and substitutions may be made therein without departing from the spirit and scope and range of equivalents of the accompanying claims. In particular, it will be clear to those skilled in the art that the present invention may be embodied in other forms, structures, arrangements, proportions, sizes, and with other elements, materials, and components, without departing from the spirit or essential characteristics thereof. In addition, numerous variations in the methods/processes described herein may be made within the scope of the present disclosure. One skilled in the art will further appreciate that the embodiments may be used with many modifications of structure, arrangement, proportions, sizes, materials, and components and otherwise, used in the practice of the disclosure, which are particularly adapted to specific environments and operative requirements without departing from the principles described herein. The presently disclosed embodiments are therefore to be considered in all respects as illustrative and not restrictive. The appended claims should be construed broadly, to include other variants and embodiments of the disclosure, which may be made by those skilled in the art without departing from the scope and range of equivalents.