GB1567264A - Liquid metal cooled fast breeder nuclear reactors - Google Patents

Description

(54) LIQUID METAL COOLED FAST BREEDER NUCLEAR REACTORS (71) We, NUCLEAR POWER COMPANY LIMITED, of of 1, Stanhope Gate, London, W1A 1EH, a British Company, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to liquid metal cooled fast breeder nuclear reactors.

In one known construction of liquid metal cooled fast breeder nuclear reactor the reactor core is submerged in a pool of liquid metal coolant in a primary vessel which is housed in a concrete vault. The core is carried by a strong back and is surrounded by an impermeable barrier bounding an inner or hot region of the pool and an outer or cool region of the pool. There is a plurality of coolant pumps in the outer region which circulate coolant upwardly through the core by way of the strongback thence to a plurality of heat exchangers in the hot region which discharge to the cool region.

In order that the diameter of the primary vessel and therefore the magnitude of the concrete vault shall be as small as possible.

the pumps and heat exchangers are arranged alternately and generally side-by-side in the primary vessel and the barrier has un dulating contours to enable it to pass between them. As the temperature of the inner pool is approximately 5400C and that of the outer pool is approximately 370"C thermal insulation is provided on the hot side of the barrier to reduce temperature degradation of the inner pool and to reduce thermal stresses induced in the barrier. However, because there is a pressure differential across the barrier during operation of the reactor the barrier is constructed to pressure vessel standards but design and manufacture to meet such exacting standards is made very difficult because of the irregular shape and the complexity of the many induced stresses.

It is an object of the invention to provide a construction of liquid metal cooled fast breeder nuclear reactor in which the complexity of design and manufacture of a barrier for bounding inner and outer pools of coolant is reduced.

According to the invention in a liquid metal cooled fast breeder nuclear reactor of the kind comprising a nuclear reactor core submerged in a pool of coolant within a primary vessel and having a barrier surrounding the core and bounding inner and outer regions of the pool, a coolant pump in the outer region for circulating coolant through the core and through a heat exchanger disposed in the inner region, the barrier is permeable and comprises a thermally insulating medium and the upper region of the barrier is surrounded by a continuous impermeable membrane spaced from the barrier, the interspace forming a cooling jacket and having coolant flow ducts extending through it.

In a preferred construction the coolant flow ducts extending through the interspace comprise a continuous rank of upstanding pipes connected at their lower ends to a header which is fed with coolant by the coolant pump, the upper ends of the pipes opening into capping thimbles which extend over the depth of the membrane. The coolant pipes are in heat exchange with pool coolant flooding the interspace thereby creating the cooling jacket.

From another aspect the invention can be said to reside in a liquid metal cooled fast breeder nuclear reactor wherein the conventional inner tank constituting a fluid tight barrier for defining the inner and outer regions of the pool and for supporting the thermal insulation is dispensed with, the inner tank being replaced by a permeable thermal insulation supporting skeletal framework which is not subject to complex stresses caused by large temperature differentials.

In a preferred construction the thermal insulation medium is of the kind disclosed in a co-pending application of the same date by Hodgsoo Howard, Dale and Hall. The insulating medium comprises a plurality of spaced layers of sheet material, each layer lying substantially parallel to a supporting skeletal framework and comprising rectilinear panels secured to the framework in spaced array in vertical and horizontal rows, and closure members for the spaces therebetween, the closure members being of cruciform shape and the arms thereof being arranged to overlap opposed faces of adjacent panels.

Thermally insulating medium of this kind although permeable forms a substantial barrier to flowing coolant and has adequate clearances distributed over the area of the layers to allow superficial thermal expansion to take place without causing distortion of the layers or setting up complex stresses. In the event of failure of the attachment of a panel to the frame the released panel is retained in position by the adjoining panels and strips thus providing for reliability in service.

A construction of liquid metal cooled fast breeder nuclear reactor according to the invention is described, by way of example only, with reference to the drawings accompanying the Provisional Specification wherein, Figure 1 is a sectional view of the construction, Figure 2 is a fragmentary plan view, partly in cross-section of the construction of Figure 1, Figure 3 is a fragmentary sectional view on line 111-111 of Figure 2, Figure 4 is a fragmentary front view of thermal insulation used in the construction of Figure a framentary plan view 1, Figure 5 is a fragmentary plan view in section on line V-V of Figure 4; and Figure 6 is a fragmentary plan view in section on line VI-VI of Figure 4.

In the construction shown in Figure 1 the reactor core 1 is submerged in a pool of liquid sodium coolant 2 in a primary vessel 3 which is housed in a concrete vault 4. The core is carried by a strongback 5 and is surrounded by a barrier 6 which bounds inner and outer regions 7, 8 of the pool.

There are eight coolant pumps 9 (only one being shown in Figure 1) in the outer region 8 for circulating coolant though the core by way of the strongback 5 and thence to eight heat exchangers 10 (again only one being shown in Figure 1) disposed in the inner region 7. The heat exchangers finally discharge the coolant into the outer region.

The primary vessel 3, a leak jacket 11 for the primary vessel, the strongback 5, heat exchangers 10 and coolant pumps 9 are all suspended from the roof of the vault and the roof includes a double rotating shield 12 from which control rods 13 extend to the top of the core. A neutron shield 15 surrounds the core within the barrier 6 which is described in greater detail hereinafter. A secondary liquid sodium coolant flowing through the heat exchangers conveys the heat energy derived from the core to steam generating plant not shown in the drawings.

In operation of the reactor, the coolant in the inner region of the pool is at temperature approximately 540"C and that in the outer region is at temperature approximately 3700C. The pressure differential across the inlet and outlet ports of the pumps 9 causes a differential in the levels of the coolant in the regions the levels being designated L1 and L2.

Referring now to Figure 2 which shows in plan view, in a 900 sector, some details of the layout of the heat exchangers 10, coolant pumps 9 and barrier 6, there is a skeletal framework 14 upstanding from the strongback 5. The framework 14 embraces the coolant pumps 9 and supports thermal insulation forming the barrier 6. There is also a continuous flow baffle 17 which partly embraces each heat exchanger 10.

The barrier forming thermal insulation comprises a plurality of spaced layers of stainless steel sheet 18 defining a radial series of compartments 19 for containing substantially static sodium. The upper region of the barrier 6 is surrounded by a continuous impermeable membrane 20 extending downwardly and as shown in Figure 3 spaced from the insulation to form a cooling jacket 21 for the continuous membrane. A continuous rank of coolant conducting pipes 22 connected at their lower ends to a header (not shown) at strongback level is fed with coolant from the coolant pumps, the pipes extending upwardly between the thermal insulation and the continuous membrane 20.

Each coolant pipe 22 is reduced in diameter over the upper part of its length co-extensive with the membrane 20 and the open upper end has a capping thimble 23 extending downwardly over the depth of the continuous membrane 20.

In use of the reactor, sodium flooding the compartments 19 formed by the membranes 18 of the insulation is substantially static thereby providing a resistance to heat transfer between the inner and outer regions of the pool of coolant. However, owing to the difference in levels L1, L2 of the inner and outer pool level by the continuous mem brane 20, there is no heat transfer across the membrane over the differential levels between the inner pool sodium and the sodium in the outer pool so that supplementary cooling means is required for the continuous membrane. The supplementary cooling means is provided by cool liquid sodium flowing from the outer pool upwardly through the pipes 22 then downwardly through the capping thimbles 23 in heat exchange with sodium flooding the interspace between continuous membrane and thermal insulation. Instead of the coolant being supplied to the header by the pumps 9 it may, alternatively, be supplied by electromagnetic pumps.

Referring now to Figures 4, 5 and 6 the thermal insulation comprises rectilinear panels 24 secured to the framework 14 by central retaining studs 25. The panels 24 are spaced apart and arranged in vertical and horizontal rows. The spaces between the panels are closed by closure members 18 also secured to the framework 14 by studs 25. The closure members 18 are of cruciform shape each comprising a cruciform spacer 27 intermediate a pair of cruciform sealing strips 26. The inner (relative to the clad side of the framework) cruciform strip 26 of each member is welded to the spacer 27 whilst the outer is free for assembly after placing the complementing panel 24. The sealing strips 26 of each member 18 are disposed to overlap opposed faces of a panel 24 and each arm of the cruciform member co-operates with an arm of a neighbouring cruciform member to extend along and overlap adjacent sides of adjacent panels. The combination of cruciform strips 26, spacer 27 and panel 24 form a labyrinth barrier serving to restrict flow of coolant through each layer of panels. The studs 25 are arranged in two lattices of square pitch, one lattice being displaced relative to the other by one half pitch in both horizontal and vertical directions and each stud carries alternately a panel 24 and a closure member 18 so that the vertical rows of panels in one layer are displaced relative to the vertical rows of panels in an adjacent layer, the displacement being one half of the pitch of the rows in both horizontal and vertical directions. By displacing the panels in one layer relative to the panels in an adjacent layer coolant flow across the insulation due to convection currents is reduced.

The panels are 0.9 metre square and 0.55 mm thick and are disposed on a 1 metre square lattice pitch. The cruciform strips of the sealing members are 0.55 mm thick and the cruciform spacing members are 0.70 mm thick. Stud spacers 29 10 mm thick serve to space the closure members.

A clearance 32 is provided between the cnds of the arms of the cruciform strips to provide for thermal expansion but the joints are closed to fluid flow by lapping strips 30 attached to selected arms of the cruciform strips. Two of the arms of each spacer 27 are longer than the other two, long and short arms of adjoining strips being assembled together and providing an expansion clearance 33 which is displaced from the expansion clearance 32 of the strips. pacers 31 are also attached to selected arms of the cruciform strips to hold adjacent strips of adjacent membranes in place.

Thermal insulation of the described form provides a substantial barrier to flow of coolant and accommodates both lateral and longitudinal expansion. The expansion is absorbed gradually over a small area of a membrane and thereby substantially avoids distortion and complex stresses. The insulation is easily erected because the components are small and can be handled by an operator and components can be readily repaired or replaced on site.

WHAT WE CLAIM IS:- 1. A liquid metal cooled fast breeder nuclear reactor of the kind comprising a nuclear reactor core submerged in a pool of coolant within a primary vessel and having a barrier surrounding the core and bounding inner and outer regions of the pool there being a coolant pump in the outer region for circulating coolant through the core and through a heat exchanger disposed in the inner region wherein the barrier is permeable and comprises a thermally insulating medium and the upper region of the barrier is surrounded by a continuous impermeable membrane spaced from the barrier, the interspace forming a cooling jacket and having coolant flow ducts extending through it.

2. A liquid metal cooled fast breeder nuclear reactor according to claim 1 wherein the coolant flow ducts extending through the interspace comprise a continuous rank of upstanding pipes connected at their lower ends to a header which is fed with coolant by the coolant pump, the upper ends of the pipes opening into capping thimbles which extend over the depth of the membrane.

3. A liquid metal cooled fast breeder nuclear reactor according to either of claims 1 and 2 wherein the insulating medium comprises a plurality of spaced layers of sheet material each layer lying substantially parallel to a supporting skeletal framework and comprising rectilinear panels secured to the framework in spaced array in vertical and horizontal rows, and closure members for the spaces therebetween, the closure members being of cruciform shape and the arms thereof being arranged to overlap opposed faces of adjacent panels.

4. A liquid metal cooled fast breeder nuclear reactor substantially as hereinbefore described with reference to the drawings accompanying the Provisional Specification.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (4)

1. **WARNING** start of CLMS field may overlap end of DESC **.

   being supplied to the header by the pumps 9 it may, alternatively, be supplied by electromagnetic pumps.

   Referring now to Figures 4, 5 and 6 the thermal insulation comprises rectilinear panels 24 secured to the framework 14 by central retaining studs 25. The panels 24 are spaced apart and arranged in vertical and horizontal rows. The spaces between the panels are closed by closure members 18 also secured to the framework 14 by studs 25. The closure members 18 are of cruciform shape each comprising a cruciform spacer 27 intermediate a pair of cruciform sealing strips 26. The inner (relative to the clad side of the framework) cruciform strip 26 of each member is welded to the spacer 27 whilst the outer is free for assembly after placing the complementing panel 24. The sealing strips 26 of each member 18 are disposed to overlap opposed faces of a panel 24 and each arm of the cruciform member co-operates with an arm of a neighbouring cruciform member to extend along and overlap adjacent sides of adjacent panels. The combination of cruciform strips 26, spacer

   27 and panel 24 form a labyrinth barrier serving to restrict flow of coolant through each layer of panels. The studs 25 are arranged in two lattices of square pitch, one lattice being displaced relative to the other by one half pitch in both horizontal and vertical directions and each stud carries alternately a panel 24 and a closure member

   18 so that the vertical rows of panels in one layer are displaced relative to the vertical rows of panels in an adjacent layer, the displacement being one half of the pitch of the rows in both horizontal and vertical directions. By displacing the panels in one layer relative to the panels in an adjacent layer coolant flow across the insulation due to convection currents is reduced.

   The panels are 0.9 metre square and 0.55 mm thick and are disposed on a 1 metre square lattice pitch. The cruciform strips of the sealing members are 0.55 mm thick and the cruciform spacing members are 0.70 mm thick. Stud spacers 29 10 mm thick serve to space the closure members.

   A clearance 32 is provided between the cnds of the arms of the cruciform strips to provide for thermal expansion but the joints are closed to fluid flow by lapping strips 30 attached to selected arms of the cruciform strips. Two of the arms of each spacer 27 are longer than the other two, long and short arms of adjoining strips being assembled together and providing an expansion clearance 33 which is displaced from the expansion clearance 32 of the strips. pacers 31 are also attached to selected arms of the cruciform strips to hold adjacent strips of adjacent membranes in place.

   Thermal insulation of the described form provides a substantial barrier to flow of coolant and accommodates both lateral and longitudinal expansion. The expansion is absorbed gradually over a small area of a membrane and thereby substantially avoids distortion and complex stresses. The insulation is easily erected because the components are small and can be handled by an operator and components can be readily repaired or replaced on site.

   WHAT WE CLAIM IS:- 1. A liquid metal cooled fast breeder nuclear reactor of the kind comprising a nuclear reactor core submerged in a pool of coolant within a primary vessel and having a barrier surrounding the core and bounding inner and outer regions of the pool there being a coolant pump in the outer region for circulating coolant through the core and through a heat exchanger disposed in the inner region wherein the barrier is permeable and comprises a thermally insulating medium and the upper region of the barrier is surrounded by a continuous impermeable membrane spaced from the barrier, the interspace forming a cooling jacket and having coolant flow ducts extending through it.
2. 2. A liquid metal cooled fast breeder nuclear reactor according to claim 1 wherein the coolant flow ducts extending through the interspace comprise a continuous rank of upstanding pipes connected at their lower ends to a header which is fed with coolant by the coolant pump, the upper ends of the pipes opening into capping thimbles which extend over the depth of the membrane.
3. 3. A liquid metal cooled fast breeder nuclear reactor according to either of claims 1 and 2 wherein the insulating medium comprises a plurality of spaced layers of sheet material each layer lying substantially parallel to a supporting skeletal framework and comprising rectilinear panels secured to the framework in spaced array in vertical and horizontal rows, and closure members for the spaces therebetween, the closure members being of cruciform shape and the arms thereof being arranged to overlap opposed faces of adjacent panels.
4. 4. A liquid metal cooled fast breeder nuclear reactor substantially as hereinbefore described with reference to the drawings accompanying the Provisional Specification.