GB2090042A - Improved Configuration for Loop-type Liquid Metal Fast Breeder Reactor - Google Patents

Abstract

An improved configuration for a liquid metal fast breeder reactor of the loop type. The reactor has a pressurized inlet plenum 3, a flat cold top deck 2, internally routed loop piping 43 with penetrations through the flat deck and a gas annulus 26 between the primary vessel and the volume 7 containing hot liquid metal. The reactor vessel is suspended from the cold deck and is free of all penetrations. <IMAGE>

Description

SPECIFICATION Improved Configuration For Loop-type Liquid Metal Fast Breeder Reactor This invention may have been conceived during performance of a contract with the United States Government, designated DE-AC-14-79-ET 37107.

This invention relates generally to liquid metal fast breeder reactors (LMFBR), particularly LMFBR's of the "loop" type.

The types of nuclear reactors which are now used commercially to generate electriciXy are mostly light water reactors that use low-enriched uranium fuel. This type of reactor can extract only a small fraction of the energy potentially available from uranium. Intensive efforts are underway in several countries to develop LMFBR's which would make practicable the extraction and use of a large amount of the energy available in the world's uranium and perhaps thorium resources.

A number of demonstration LMFBR's have been constructed for purposes of research and development of technology intended to result in a commercial reactor design.

There are two basic alternative LMFBR design configurations: the "loop" and the "pool". The loop reactor uses a plurality of piping systems, or loops, external to the reactor, containing pumps, heat exchangers, instrumenation, and perhaps valves, to remove and convert heat from the reactor core. The pool-type reactor has all such heat removal equipment integral within the reactor vessel itself: the pumps and heat exchangers are immersed in the reactor coolant.

A problem important to all LMFBR's is the design and support of the reactor vessel, especially as relating to the ability of the vessel to withstand earthquake-induced stress.

LMFBR's characteristically use liquid sodium, at a very high temperature, as the reactor coolant.

This high temperature sodium heats the inside reactor vessel surface to a high temperature.

During power operation of the reactor, the coolant temperature can rapidly change by several hundred degrees. As a result, high thermal gradients can occur in the vessel wall. If the vessel wall is thick, the high thermal gradients across the vessel wall cause high stress levels, particularly during the aforementioned temperature transients.

Reactor vessels are generally cylinders, for which an "aspect ratio" is defined as the ratio of the cylinder height to the cylinder diameter. Loop reactors are usually tall, relatively small-indiameter cylinders, thus having a large aspect ratio. A cylinder of large aspect ratio is more prone to oscillations during earthquakes than cylinders of more squat geometry.

The logical progression of LMFBR technology would be the extrapolation of existing demonstration plants up to reactors of commercial power reactor size. It develops that this increase in vessel size may not be possible, especially for the high aspect ratio loop reactors.

The increase in vessel wall thickness needed to render the vessel sufficiently strong for earthquakes would also cause undesirable thermal stress in the wall. The problem is further complicated for loop reactors since the loop piping usually must penetrate or otherwise interfere with the vessel support means.

It is the principal object of the present invention to provide a liquid metal cooled breeder reactor with high structural integrity and relatively low complexity.

With this object in view, the present invention resides in a liquid metal fast breeder reactor of the loop type, being part of a reactor system having coolant pumps and heat exchangers located separately in the vicinity of said reactor and having loop piping providing fluid communication with said reactor, said reactor comprising a flat structural to deck with all loop piping to said reactor penetrating said deck thereby entering said reactor, a primary vessel attached to said top deck and surrounding and containing a lower portion of said loop piping which portion is within said vessel, and a plenum located in a bottom region of said primary vessel, which connects with, and receives coolant under pressure from said loop piping, characterized in that a shield cylinder is located in an upper region of said primary vessel and adapted to form between said shield cylinder and said primary vessel a gas-filled annulus to thermally insulate said upper region of said primary vessel.

The inlet and outlet pipes ingress and egress the cylindrical primary reactor vessel via the flat, cold top deck, and traverse the reactor axially downward within insulating standpipes. The inlet pipes extend into a pressurized inlet plenum wherein each inlet pipe terminates with a diffuser.

The annulus filled with a gas provides thermal protection to the adjacent vessel section wall.

The reactor core is surrounded and supported by a conical structure part of which forms a portion of the previously mentioned pressurized plenum. The top deck is a rigid structural weldment, and has a series of metal plates arranged beneath it to provide thermal protection for the deck.

The invention will become more readily apparent from the foliowing description of a preferred embodiment thereof shown, by way of example only, in the accompanying drawings, in which: Figure 1 is a sectioned view of the reactor; and Figure 2 is a plan view of the reactor taken as indicated in Figure 1; Figure 3 is a sectional view of an outlet pipe taken as indicated in Figure 2; Figure 4 is an expanded portion of Figure 1; Figure 5 is a plan view taken as indicated in Figure 4; Figure 6 is a plan view taken as indicated in Figure 4; and Figure 7 is an expanded portion of Figure 4.

The layout of the reactor is illustrated in Figures 1 and 4. These Figures include only one of a plurality of coolant loops.

The liquid sodium is pumped by sodium pumps (not shown) through an externally located heat exchanger (not shown) and into the reactor via inlet pipes 1. This inlet pipe 1 enters the reactor through the top deck 2 and extends downward into a pressurized inlet plenum 3. The inlet pipe 1 terminates with a diffuser 4 which has a plurality of holes 5 arranged and designed to promote mixing of the flow from the plurality of coolant loops within the plenum, and to avoid the direct impingement of any flow stream on a reactor component. The sodium flows upward through the core 6 where it is heated by a nuclear reaction. After entering the hot outlet plenum 7, the sodium flows through an outlet pipe 8 to the sodium pump.

The reactor core 6 is supported by a cone 9 which is welded to the primary vessel 10. This cone 9 is approximately seven inches thick. The cone 9 forms part of the boundary of the inlet plenum 3. The remainder of the boundary of the inlet plenum 3 is formed by a pressure shell 11 which is welded to the cone 9. The pressure shell 1 1 is sized to contain the pressure loading. The inlet plenum 3 is pressurized by the sodium pumps (not shown) to approximately 8.4 kg/cm2 to impel the loop sodium flow through the reactor. The lower plenum 12 has a pressure established by the static head of sodium above, which is approximately 1 5 psi.The inlet plenum 3, a new feature in loop LMFBR design, results in the primary vessel 10 being free of the operating pressure load of the pumps and any thermal transients originating in one loop since the flow from all loops is mixed together within the inlet plenum 3, and remains isolated from the reactor vessel.

The reactor is provided with a "cold" structural deck 2 at the top. Within the context of LMFBR's, there is a division of technology relating to "cold" and "hot" top decks. A "hot" deck is one which is heated to temperatures above 490C, such that human contact must be avoided. A "cold" deck is one which by some mechanism remains at temperatures below 490C. The cold deck of this design consists of three distinct sections. The uppermost section is a five-foot thick concrete shield 13. The middle section is an 2.5 m thick chamber, the floor of which, a steel plate 14, is the primary system boundary. The deck 2 is supported from the top of a reactor cavity wall 1 5 which surrounds the entire reactor. The primary vessel 10 is suspended from the plate 14.The plate 14 also supports the lower section of the overall top deck 2; a structure of approximately thirty stainless steel reflective plates 1 6. These reflective plates 1 6 are arranged in a gas space 1 7 of the reactor which is filled with either argon or helium. The reflective plates 1 6 function to reflect heat from the sodium-gas interface 18 away from the top deck 2 such that the top deck 2 is maintained at a lower "cold" temperature.

The use of the reflective plates 1 6 and the gas space 1 7 is well known to the prior art. There is a false floor 1 9 about eight inches above the plate 1 4 which establishes a volume 20 through which cooling gas can be passed for cooling the plate 14. A large volume 21 exists above the false floor 1 9 which allows for personnel entry therein via access plugs 47.

Refer to Figures 2, 3, 4, 5 and 6. The top deck 2 consists of three concentric cylinders which are interconnected by radial rib plates 35 and four annular plates. The innermost cylinder 31 supports a rotating plug system 44, which completes the closure, and which is used to replace fuel assemblies. The plug system 44 also supports the upper internal structure 32. The construction of the plug system 44 is the same as that of the deck 2. The middle cylinder 33 is an xtension of the primary vessel wall 1 0. The outer cylinder 34 is used to transmit the weight of the reactor to the reactor cavity wall 1 5.

The bottom-most annular plate 14 spans between the innermost cylinder 31 and the primary vessel wall 10. The annular base plate 36 extends from the middle cylinder 33 to one foot beyond the outermost cylinder 34. The annular base plate 36 rests on the vessel support embedment plate 38 with 24 radial-keys 39 (see Figure 2) which carry lateral seismic loads. The middle annular plate 40 extends from the innermost cylinder 31 to the outer cylinder 34, and serves to support the concrete shield 13. The top plate 41 serves as a floor for the reactor head compartment 42.

The flat deck 2 is intended to accommodate all required reactor penetrations so that there are no penetrations through the reactor vessel.

The inlet pipes 1 and outlet pipes 8 penetrate the plate 14, turn 900, and exit radially through the cavity wall 1 5 above the reactor support plate 38. At the penetration of the plate 14, the pipes have conical nozzle-type supports 24. These must be joined to the plate 14 by dissimilar metal welds since the pipes and conical nozzle supports 24 are stainless steel. The thermal gradient of the outlet pipe (about 3900C) is distributed along the length of the conical support 24, and may require special cooling and insulation. Of particular advantage from the above-described design is the fact that the pipes exit from the reactor in the gas space 1 7 rather than below the sodium-gas interface 1 8 such that thermal transients occurring in the liquid sodium are not imposed on the penetrations. In addition, the pipes do not interfere with the primary vessel support means, and neutron streaming into the equipment cells (not shown) through the pipe chaseways is avoided.

Refer to Figures 4 and 7. The inlet pipes 1 and outlet pipes 8 are encased within a double walled standpipe 43 which provides for a gas space 45 and a sodium filled space 46. The gas space 45 is open at the top to the gas space 1 7. The arrangement provides for thermal insulation of the pipe.

The sodium exiting from the core 6 enters a hot outlet plenum 7. This hot outlet plenum 7 is partially defined by a shield cylinder 25 of about one-half inch thickness and which extends upward from the core support cone 9. The shield cylinder 25 forms a gas annulus 26 between the hot outlet plenum 7 and the primary vessel 10 and this annulus contains reflective insulation plates 27. The cover gas can commute between the gas annulus 26 and the gas space 17 due to a joining of these volumes at passage 28 (see Figure 4). This new annulus 26 design eliminates the need for a vessel thermal liner and cooling bypass flow, and reduces the primary vessel 10 design temperature from 51 00C to 4000C. The reduction in temperature may eliminate a need for inelastic analysis as a vessel design requirement.

The reactor has an intermediate plenum 29 partially formed by a lateral baffle plate 30. This plenum contains relatively stationary sodium which will form stratified layers and provide for a uniform transition in temperature from the hot outlet plenum 7 to the inlet plenum 3 and lower plenum 12. The lateral baffle plate 30 need not be flat as shown in Figure 1 but may be otherwise shaped, perhaps conical, if desired to provide a stronger and thinner structure.

The reactor has an intermediate plenum 29 described in detail herein or appearing in the drawings because these are not considered to be part of this invention.

Table I has been included to disclose design information presently considered part of the preferred embodiment.

Table I vessel bottom head outside radius 2.5 m vessel bottom head thickness 2.5 cm primary vessel (10) ID 15.2 m primary vessel (1 0) thickness 2.0 cm overall height 1 7.8 m primary vessel weight 245 tons top deck (2) thickness 5.0 m number of reflective plates (16) 30 sodium in vessel 1650 m3 The changes in the configuration for a looptype LMFBR described above are considered to be of importance. The primary vessels of other looptype LMFBR's must be of sufficient bulk and strength to accommodate the pressure and seismic loads, but such bulk is not consistent with the imposed thermal environment. In this invention, an inlet plenum 3 assumes the pressure containing function. In the area of the vessel where the highest temperatures occur, provisions are made to protect the primary vessel (the gas annulus 26 and reflective plates 27).These features are locally provided precisely where needed, such that the entire primary vessel 10 may be fabricated with thin sections which are compatible with the existing large temperature gradients inherent with LMFBR's.

The reactor vessel diameter is increased by the entry of the loop piping through the top deck and the axially downward routing of the pipes inside the reactor. This increase in reactor diameter is achieved without a correponding increase in the size of the containment building around the reactor because the reactor size increase occurs through the volume otherwise occupied by the loop piping outside the reactor. The effect is a lessening of the aspect ratio of the reactor. It has been found that this change in the aspect ratio improves the response of the reactor vessel to earthquake induced vibrations by increasing the bending stiffness of the vessel, thereby altering the natural harmonic frequencies of the structure such that these do not correspond to expected earthquake frequencies. This allows the primary vessel to be of reduced bulk with higher structural design margin.

It is the combination of the above features which make a substantial reduction in primary vessel thickness possible. The primary vessel may now be less than two inches thick, rather than four inches as required for previous designs. It is estimated that the metal required for the vessel may be 245 tons as compared to the approximately 840 tons required with other designs. Since the vessel material is stainless steel, an expensive commodity, such a reduction in the quantity required is indeed of importance.

While in the foregoing there has been described a general invention which comprises improvements in the design of a loop-type LMFBR, it should be understood that various changes and modifications may be made without departing from the true spirit and scope of the invention.

For example, the gas annulus 26, the pressure shell 11, the cone 9, and other components described which achieve the above-described improvements, can all be given various geometries which accomplish the same results.

Therefore, the foregoing disclosure should be interpreted as illustrative rather than limiting.

Claims (5)

Claims

1. A liquid metal fast breeder reactor of the loop type, being part of a reactor system having coolant pumps and heat exchangers located separately in the vicinity of said reactor and having loop piping providing fluid communication with said reactor, said reactor comprising: a flat structural top deck with all loop piping to said reactor penetrating said deck thereby entering said reactor; a primary vessel attached to said top deck and surrounding and containing a lower portion of said loop piping which portion is within said vessel; and a plenum located in a bottom region of said primary vessel, which connects with, and receives coolant under pressure from, said loop piping; characterized in that a shield cylinder (31) is located in an upper region of said primary vessel (10) and adapted to form between said shield cylinder (30) and said primary vessel a gas-filied annulus (17) to thermally insulate said upper region of said primary vessel (10).

2. A breeder reactor according to claim 1, characterized in that a space (21) is formed in said deck adapted to permit personnel entry therein.

3. A breeder reactor according to claim 1 or 2, characterized in that a double walled standpipe surrounds loop piping (18) within said reactor vessel (10) with a gds-space between-said double walls for thermal protection of said piping.

4. A breeder reactor according to claim 1, characterized in that all reactor penetrations are accomplished through said flat deck (2) such that said primary vessel (10) is unperforated.

5. A breeder reactor according to claim 1, characterized in that said flat deck (2) supports said primary vessel (10).