Patent Number: 042253896
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the construction shown in FIG. 1 a nuclear reactor fuel assembly 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 fuel assembly is carried by a strongback 5 and is surrounded by a barrier 6 which defines inner and outer regions 7, 8 of the pool. There are eight coolant pumps 9 (only one being shown in FIG. 1) in the outer region 8 for circulating coolant through the fuel assembly by way of a diagrid 5a and thence to eight heat exchangers 10 (again only one being shown in FIG. 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 fuel assembly within the barrier 6 and the internal wall surface of the barrier is clad with thermal insulation 16. A secondary liquid sodium coolant flowing through the heat exchangers conveys the heat energy derived from the fuel assembly 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.degree. C. and that in the outer region is at temperature approximately 370.degree. C. 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. The thermal insulation 16 comprises a plurality of spaced layers of stainless steel sheet, each layer lying substantially parallel to the wall surface and comprising rectilinear panels secured to the wall surface in spaced array in vertical and horizontal rows. The spaces between adjacent panels are closed by members of cruciform shape the arms of the members being arranged to overlap opposed faces of adjacent panels. As shown in detail in FIGS. 2, 3, and 4 the rectilinear panels designated 24 are secured to the internal wall surface of the barrier 6 by central retaining studs 25. The spaces between the panels are closed by closure members 18 secured to the wall by further 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 wall surface) cruciform strip 26 of each member is welded to the spacer 27 whilst the outer is free for assembly after placing the complementing panels 24. The sealing strips 26 of each member 18 are disposed to overlap opposed faces of adjacent panels 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 meters square and 0.55 mm thick, and are disposed on a 1 meter 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 to the closure members part. A clearance 32 is provided between the ends 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. Spacers 31 are also attached to selected arms of the cruciform strips to hold adjacent strips of adjacent membranes in place. In an alternative construction of liquid metal cooled fast breeder nuclear reactor generally similar to that described in respect of the first embodiment of the invention the panels 24 each comprise two opposed membranes of stainless steel sealed together along their edges to define a sachet which is charged with inert gas. As shown in FIG. 5, the panels 24 and closure members 18 are each adapted to engage a stud 25 by central bosses or hubs, designated 24a, 18a respectively, and the peripheral regions 24b of the panels 24 are extended by a single thickness of membrane to complement the closure members 18. One of the cruciform strips designated 18b of each closure member is also of sachet form so that, in effect, substantially the full area of each layer of insulation comprises a gas filled layer. The inert gas contained in the sachets is argon at sub-atmospheric pressure under external conditions of normal temperature and pressure and each sachet contains dimpled stainless steel foil 34 which serves as stiffening against collapse of the thin walls of the sachet. For this reason each sachet is also seam welded in quilt like manner to form a plurality of compartments. Thermal insulation of the described forms provides a substantial barrier to flow of coolant and accommodates superficial thermal expansion. The expansion is accommodated by the clearances between the panels and closure members 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.