GB2157880A - An improved nuclear reactor plant construction - Google Patents

Abstract

An improved nuclear reactor plant construction for a liquid metal reactor (LMR) which substantially reduces the size of and the amount of materials required in constructing a plant by eliminating the massive steel lined, reinforced concrete dome associated with conventional LMR plants. The plant design includes a pool of low pressure coolant, such as liquid sodium disposed within a vessel (62) which provides a first barrier between the coolant and the exterior atmosphere. The reactor vessel (62) is surrounded by a containment vessel (66) which provides a second barrier between the reactor vessel and the exterior atmosphere. A double barrier deck (73) covers the reactor vessel (62) and the containment vessel (66) and seals off the upper open ends of both vessels (62, 66). <IMAGE>

Description

SPECIFICATION An improved nuclear reactor plant construction The present invention relates generally to nuclear reactors and, more particularly, is directed to the plant construction of a liquid metals fast breeder reactor, hereinafter re ferred to as a liquid metal reactor (LMR).

A LMR, like other reactors, produces heat by fissioning of nuclear materials which are fabricated into fuel elements and assembled within a nuclear core situated in a reactor vessel. In commercial nuclear reactors, the heat produced thereby is used to generate electricity. Such nuclear reactors typically comprise one or more primary flow and heat transfer loops, and a corresponding number of secondary flow and heat transfer loops to which conventional steam turbines and electrical generators are coupled. A typical energy conversion process for commercial nuclear reactors, therefore, involves transfer of heat from a nuclear core to the primary coolant flow system, to a secondary coolant flow system and finally into steam from which electricity is generated.In a liquid cooled nuclear reactor, such as a liquid metal cooled breeder reactor, a reactor coolant, such as liquid sodium, is circulated through the primary coolant flow system. A typical primary coolant flow system comprises a nuclear core, a heat exchanger, and a circulation pump. In a "pool" type system, the nuclear reactor core, the heat exchanger and the circulation pump are located within a large pool of coolant housed within a single vessel, whereas, in a "loop" type system, the heat exchanger and circulation pump are removed from the vessel housing and the nuclear core, and relocated normally in separate vessels. Generally, there are several heat exchangers and circulation pumps associated with the nuclear core. The heat generated by the nuclear core is removed by the reactor coolant which flows into the reactor vessel and through the reactor core.

The heated reactor coolant then flows through the heat exchangers which transfers the heat to secondary flow systems associated therewith. The cooled coolant exits from the heat exchangers and flows to a circulation pump which again circulates the coolant to the reactor vessel, repeating the described flow cycle.

The alkali metals, in particular, have excellent heat-transfer properties and- extremely low vapor pressure temperatures of interest for power generation. Sodium is the most attractive because of its relatively low melting point and high heat-transfer coefficient. It is also abundant, commercially available in acceptable purity, and relatively inexpensive. It is not particularly corrosive, provided low oxygen concentration is maintained. Its nuclear properties are excellent for fast reactors. In the liquid metal fast breeder reactor, sodium in the primary loop collects the heat generated in the core and transfers it to a secondary loop in the heat exchanger, from which it is carried to the steam generator.

However, sodium does present an activitation problem because Na22 is formed by the absorption of a neutron and is an energetic gamma emitter with a 1 5-hr. half-life. Thus, the containing system requires extensive biological shielding. Further, sodium reacts violently with water, imposing severe problems in the design of sodium-to-water steam boilers.

Therefore, reactor safety is a foremost design requirement. Due to the aforementioned characteristics of the liquid metal coolant, sodium, the design must guard against the unlikely happening of loss of coolant around the reactor core. Coolant loss could result from the rupture of the reactor vessel or one of the main coolant circulating lines. Thus, a guard vessel surrounding the reactor and guards around the flow piping are provided to maintain the required level of coolant around the core should a rupture occur. Further, steel liners are provided adjacent the concrete wall structures of the containment building to prevent contact of the sodium with the concrete structures should leakage occur.Still further, the containment building includes a massive dome-shape containment structure comprised of a steel shell and thick concrete walls which extend over the reactor vessel to accomodate pressure build-ups and provide radioactive shieldings. Thus, due to the amount of construction commodities required, the labor and time involved in on-site construction, and the compound and complexity of designs required for adequate maintenance and safety standards, the capital costs of the LMR are formidable and investment in these plants has been unattractive up to the present time.

Therefore, it is the principal object of the present invention to provide a reactor which would afford significant capital cost reductions while maintaining or improving safety in order for LMR plants to compete economically in the near future.

With this object in view, the present invention resides in a nuclear reactor plant construction comprising a nuclear reactor including a core, a reactor vessel for holding a large pool of low pressure coolant, such as liquid sodium, and housing said core within said pool, said reactor vessel having an upper end and providing a first barrier between said coolant and the exterior atmosphere, and a containment vessel surrounding said reactor vessel and providing a second barrier between said coolant and the exterior atmosphere, said containment vessel having a top open end, characterized in that a deck is disposed above said vessels so as to cover and seal both of said vessels.

This design eliminates the need for the large concrete steel lined containment building common to conventional LMR's by adequately insuring against leakage and the loss of the reactor coolant and the resulting high pressure and contamination resulting therefrom. Consequently, the improved design has reduced seismic category I building volumes by nearly half, and non-seismic volumes by more than one-fourth that of conventional LMR designs. The use of fewer and simplier systems, smaller buildings and maximum shop fabrication translate to lower plant costs for equipment, construction materials and construction labor. These costs reductions are compounded by the use of simple straight wall foundations and the elimination of steel cell liners. Low cost safety is another significant advantage of this new design.Plant safety is achieved through the use of inherent natural processes to achieve high reliability, and passive inherently safe features that provide additional margins of plant protection.

Among the major passive safety features of the improved plant construction is a large voiume of low pressure coolant, which obviates loss of coolant conditions and heat rejection concerns, and provides intrinsic safe shutdown and shutdown heat removal capabilities.

Still further, the improved plant design features a dedicated reactor auxiliary cooling system, which removes heat directly from the reactor vessel to the atmosphere by natural circulation of sodium and air.

In the preferred embodiment, the improved nuclear reactor plant construction further includes a system for removing heat directly from the reactor vessel to the atmosphere by natural circulation of the coolant within the reactor vessel and air about the exterior of the containment vessel. Further, the improved plant construction preferably includes a concrete enclosure surrounding the containment vessel and supporting the deck. The enclosure defines a chamber wherein atmospheric air is circulated for cooling the containment vessel.

In an alternative embodiment, the nuclear reactor further includes circulation pumps and heat exchangers located exterior to the reactor vessel and the containment vessel, and means for communicating the pumps and heat exchangers with the large pool of coolant through the deck and the upper end of the reactor vessel. Preferably, the pumps and heat exchangers are housed within auxiliary vessels which are sealed at their respective ends to the deck.

The invention will become more readily apparent from the following description of a preferred embodiment thereof shown by way of example only in the accompanying drawings wherein: Figure 1 is a schematic illustration of the plant construction of a conventional liquid metal fast breeder reactor showing the containment building having an outer cylindrical steel-lined, reinforced concrete dome-shaped structure disposed about, above and overlying the reactor vessel, and an inner cylindrical steel-lined, reinforced concrete enclosure surrounding the guard and reactor vessels.

Figure 2 is a schematic illustration of the preferred embodiment of the improved plant construction of a LMR formed in accordance with the principles of the present invention.

Figure 3 is a schematic illustration of an alternative embodiment of the improved plant construction of a LMR formed in accordance with the principles of the present invention.

For a clearer and better understanding of the present invention, it is thought that a brief description of the prior art plant construction and its shortcomings would be helpful. Now, turning to Figure 1 of the drawings, there is schematically illustrated the conventional plant construction of a typical liquid metal cooled fast breeder reactor, being designated generally by the numeral 10. Plant 10 is of the type fully described in the EPRI report number NP-1 01 6-SY, Project 620-26,27, dated March 1979 and entitled "Large Pool LMFBR Design, Executive Summary". Since the plant is an exceedingly complex structure, as can be appreciated by those skilled in the art of reactors, only a simplified version of the main components of the prior art plant, which are relevant to the improved plant construction, are shown in Figure 1.

The prior art plant 10 is of the "pool" type which basically includes a hemispherical reactor vessel 1 2 which holds a large pool of coolant, such as liquid sodium, and houses a reactor core 14, a heat exchanger 1 6 and a circulation pump 18. The reactor vessel 12 has an open top end and is supported from a transverse deck 20 which, in turn, is supported on its outer ring girder 22-by-a rein--.

forced concrete cylindrical side wall 24 that extends upwardly from a concrete base 26.

Also supported on the base 26 are outer cylindrical vertical walls 28, 30 and intermediate walls 32 that are intertied by various horizontal walls 34 to the side wall 24 in a honeycomb fashion to define a plurality of individual rooms or cells 36 for housing various equipment associated with the reactor.

The reactor plant 10 also includes a guard tank 38 which surrounds the reactor vessel 1 2. While the sodium-filled reactor vessel 1 2 is suspended within the guard tank 38, the vessel 1 2 and tank 38 are spaced apart and supported independently of one another. On the one hand, the vessel 1 2 is attached at its open top end in any suitable manner, such as by a full penetration bimetallic weld, directly to the bottom of the deck 20. The deck 20 thus provides a seal or closure for the reactor vessel 1 2 for containment of reactor coolant, cover gas, fuel and other radioactive materials.On the other hand, the guard tank 38 is an open tank and has an upper flange 39 by which it is suspended in a reactor cavity 40 defined by the cylindrical concrete side wall 24, from a lower annular recessed ledge 42 formed in an upper portion of the cylindrical side wall 24. The tank flange 39 is bolted to the support ledge 42 so as to withstand vertical seismic loads. The guard tank 38 serves as a catch basin for reactor primary sodium that might escape from the reactor vessel 1 2 under faulted conditions. It also serves to insulate the reactor core 14 from the reactor cavity side and base walls 24, 26. The space between the reactor vessel 1 2 and the guard tank 38 is filled with nitrogen gas.

Thus, while the reactor vessel 1 2 is attached directly to the deck 20, the guard tank 38 is not attached to the deck 20 at all. As seen in Figure 1, the upper flange 39 is spaced outwardly from the perimeter of deck 20 and below its outer ring girder 22 where the deck 20 is supported on an upper annular recessed ledge 44 also formed in the upper portion of the cylindrical side wall 24. Therefore, although the reactor vessel 1 2 and deck 20 provide a primary boundary or barrier between the contents of the reactor vessel 1 2 and the external atmosphere, the guard tank 38 in reality does not provide a true secondary boundary or barrier between the reactor vessel 1 2 and exterior atmosphere.Any sodium leaking into the tank 38 from the reactor vessel 1 2 could eventually contact and escape through the joint between the concrete side wall 24 and outer girder ring 22 of the deck 20 or the ledge 39 of the tank 38.

Since regulatory requirements for nuclear reactors make the provision of a double boundary or barrier about the reactor mandatory, the concrete containment building 46 of the conventional plant 10 which houses all of the above-mentioned parts of the plant 10 includes an outer steel liner 48 which encompasses all of the plant parts. The liner 48 is exaggerated in cross-sectional thickness in Fig. 1 for purposes of illustration. Also, it should be understood that, while not shown in Fig. 1, in the upper dome 50 of the containment building 46, the liner 48 is spaced from the interior wall of the concrete structure of the building 46. Additionally, an inner steel liner 52 is provided adjacent the concrete side and base walls 24, 26 of the reactor cavity.While the liner 52 is also illustrated directly contacting the interior surfaces of walls 24, 26, it should be understood that a small gap is present between the liner and walls. The respective gap between liner 48 and dome 50 and between liner 52 and walls 24, 26 serve to impede the transfer of heat from within the dome 50 to the concrete structure of building 46 and from within the reactor cavity 40 to the concrete base and walls 24, 26.

Turning now to Fig. 2,/there is shown the preferred embodiment of the improved nuclear reactor plant construction of the present invention, being generally designated by the numeral 54. In the preferred embodiment of the improved plant 54, a nuclear reactor per se includes, generally the same basic components as those found in the prior art plant 10 of Fig. 1: a nuclear core 56, one or more circulation pumps 58 and one or more heat exchangers 60. Also similar to the prior art plant, the improved plant 54 includes a reactor vessel 62 for holding the large pool 64 of low pressure liquid coolant, for instance liquid sodium, and for housing the reactor core 56 in the coolant pool 64. In the preferred embodiment, the circulation pump 58 and heat exchanger 60 also extend into the coolant pool 64.

The improved plant 54 includes a containment vessel 66 and a support arrangement for it which differs markedly from that provided previously for the guard tank. The recognition that the mounting arrangement of the guard tank was the primary source of the problem of complex and costly containment structures led to an alternative approach: the provision of the containment perimeter as near as feasible to the nuclear reactor and use of passive, natural processes as much as feasible to achieve high reliability and enhanced safety. In the present invention, in addition to the pimary barrier provided by the reactor vessel 62 between the coolant 64 and the exterior atmosphere, the containment vessel 66 provides a secondary barrier between the reactor vessel 62 and the exterior atmosphere.The outer containment vessel 66 surrounds and is coextensive with, but spaced from, the inner reactor vessel 62. For neutralizing any leakage of liquid coolant from the reactor vessel 62 into the containment vessel 66, an inert gas, such as nitrogen, is contained within the chamber or space between the two vessels.

The reactor vessel 62 and containment vessel 66 are both supported and sealed at their open upper ends 68, and 70, respectively, from a lower plate 72 of a deck 73 of the improved plant 54. The deck plate 72 has an annular groove 74 into which the upper ends 68, 70 of the vessels 62, 66 are fitted and attached to the plate 72 by any suitable method such as by welding. In such manner, the deck plate 72 completes the primary and secondary containment barriers provided by the vessels 62, 66 by forming a closure and seal for the upper ends thereof. The sealed containment vessel 66 thereby ensures that the sodium level in the reactor vessel 62, even if the latter should leak, cannot be less than the minimum safe level in the reactor vessel 62.

In the improved plant 54, the deck 73 also has two separate compartments 76, 78 which are separated and sealed from one another by a wall 80. The lower compartment 76 contains an inert gas, such as nitrogen, which bolsters the effectiveness of the seal provided by the deck plate 72 for the vessels 62, 66.

Lower compartment 76 also houses upper portions of the pump 58 and heat exchanger 60 which extend therethrough into the upper compartment 78.

In view of the sealed, double barrier now provided by the inner reactor and outer containment vessels 62, 66, in combination with the lower plate 72 and compartment 76 of the deck 73 which supports the vessels, a reactor cavity or chamber 82 defined by a concrete enclosure 83 formed of side wall 84 and base 86 of the improved plant 54 may be utilized in cooling the containment vessel 66.

As shown in Fig. 2, the enclosure 83 has lower and upper openings 88, 90 in its side wall 84. A blower 92 is connected to the lower opening 88 for circulating cooler ambient air into the chamber 82 from the exterior atmosphere, while a conduit 94 is connected to the upper opening 90 for routing hotter air from the chamber 82 to some appropriate discharge location. Alternatively, air can be allowed to freely circulate by natural thermal circulation, eliminating the need for the blower. In such manner, atmospheric air may be circulated into, through, and out the chamber 82 for cooling the containment vessel 66.

The side wall 84 of the concrete enclosure 83 at its upper end also serves to support the deck 72.

The preferred embodiment of the improved plant 54 in Fig. 2 is referred to as a "pool" type system since the nuclear reactor core 56, the circulation pump 58 and heat exchanger 60 are all located within the large pool of coolant 64 in the reactor vessel 62. An alternative embodiment of the improved plant is shown in Fig. 3, being generally designated 96. It is referred to as a 'loop' type system in which it is seen that now the circulation pump 98 and heat exchanger 100 are located exterior to the reactor vessel 102 and containment vessel 1 04. Means, such as respective conduits 106 and 108, are provided for communicating the pump 98 and heat exchanger 100 with the large pool 110 of coolant in the reactor vessel 102 via the deck 112 and upper end of the vessel 102.Similar to the deck 73, the deck 11 2 has lower and upper compartments 114, 11 6 separated and sealed from one another by wall 118. The communicating conduits 106, 108 pass through the lower compartment 114 of the deck 11 2. As before, an inert gas is contained within the lower deck compartment 114. Furthermore, the pump 98 and heat exchanger 100 are housed in their own respective auxiliary vessels 120, 1 22 which are supported from and sealed by a lower plate 1 24 of the deck 11 2.

The concrete cylindrical enclosure 1 26 formed by cylindrical side wall 128, which supports the deck 112 at its upper end, and base 1 30 define a chamber 132. Like chamber 82 of the preferred embodiment, the chamber 1 32 receives cool atmospheric air being fed through a lower wall opening 1 34 by a blower 1 36. The air circulates within the chamber, receives heat from the containment vessel 104 and auxiliary vessels 120, 122, and discharges from the chamber through an upper wall opening 138. A conduit 140 connected to upper opening 1 38 routes the heated air to an appropriate discharge point.

As seen in Figs. 2 and 3, the dramatic result of the improved nuclear reactor plant construction of the present invention is the elimination of a significant amount of the concrete and steel liner superstructure and with it much of the complexity and cost of the LMR plants as known heretofore. Instead of the expensive concrete containment building, a lower cost steel structure can be used to contain the improved plant.

Claims (9)

1. A nuclear reactor plant construction comprising a nuclear reactor including a core (56), a reactor vessel (62) for holding a large pool (64) of low pressure coolant, such as liquid sodium, and housing said core (56) within said pool (64), said reactor vessel (62) having an upper end (68) and providing a first barrier between said coolant and the exterior atmosphere, and a containment vessel (66) surrounding said reactor vessel (62) and providing a second barrier between said coolant and the exterior atmosphere, said containment vessel (66) having a top open end (70), characterized in that a deck (78) is disposed above said vessels (62, 66) so as to cover and seal both of said vessels (62, 66).

2. A nuclear reactor plant construction as recited in claim 1, characterized in that said containment vessel (66) is spaced from said reactor vessel (62) thereby defining a chamber therebetween, and that an inert gas is contained within said chamber.

3. A nuclear reactor plant construction as recited in claim 1 or 2, characterized in that said deck (73) includes upper and lower compartments (78, 76) sealed from one another, and further comprising an inert gas contained within said lower compartment (76).

4. A nuclear reactor plant construction as recited in claim 3, characterized in that said lower compartment (76) houses various associated equipment for operating said reactor.

5. A nuclear reactor plant construction as recited in any of claims 1 to 4, characterized by a system for circulating air about the exterior of said containment vessel (66).

6. A nuclear reactor plant construction as recited in any of claims 1 to 5, characterized in that said nuclear reactor further includes circulation pumps (58) and heat exchangers (60), said pumps (58) and heat exchangers (60) being disposed in said large pool of coolant (64) within said reactor vessel (62).

7. A nuclear reactor plant construction as recited in any of claims 1 to 5, characterized in that said nuclear reactor includes at least one circulation pump (98) and heat exchanger (100) located exterior to said reactor vessel (102) and containment vessel (104), and means (106, 108) for placing said pump (98) and heat exchanger (100) in communication with said large pool of coolant (110) through said deck (112) and said upper end of said reactor vessel (102).

8. A nuclear reactor plant construction as recited in claim 7, characterized in that said decks (112) upper and lower compartments (11 6, 114) are sealed from one another and said means (106, 108) for communicating said pump (98) and heat exchanger (100) with said large pool of coolant (110) passes through said lower compartment (114) of said deck (112).

9. A nuclear reactor plant construction as recited in claim 7 or 8, characterized in that auxiliary vessels (120, 122) are provided for housing said pump (98) and heat exchanger (100) and said auxiliary vessels (120, 122) being sealed to said deck (112).

1 0. A nuclear reactor plant construction as recited in any of claims 1 to 9, characterized in that a concrete enclosure (83, 126) surrounds said containment vessel (66, 104) and supports said deck (73, 112), said enclosure defining a chamber (82, 132) wherein atmospheric air is circulated for cooling said containment vessel (66, 104).