Patent GB-964841-A (GB)
Title: Nuclear reactors cooled by liquid metal
Inventor: WEBB JOHN
IPC: G21C1/03
Filed: 1962-02-14
Abstract: 964,841. Reactors. UNITED KINGDOM ATOMIC ENERGY AUTHORITY. Feb. 7,1963 [Feb. 14, 1962], No. 5737/62. Heading G6C. A reactor comprising a core contained in a tank and liquid metal within the tank which is pumped through the core as a primary coolant during normal operation but is free to circulate through the core by natural convection, is characterized by an auxiliary cooling coil submerged in the primary coolant such that the coil abstracts heat from the primary coolant and promotes its circulation by natural convection, an auxiliary heat exchanger outside the reactor tank and connected to the auxiliary cooling coil, and an auxiliary coolant within the auxiliary coil which is free to circulate by natural convection to convey heat from the primary coolant to the auxiliary heat exchanger. As shown, Fig. 1, a double-wall tank 10 having inner and outer walls 14,11 defining a leak jacket 15 is filled with liquid sodium covered by an argon blanket. A reactor core is surrounded within the tank by four pumps 18 and four heat exchangers 19 whereby the sodium primary coolant is pumped from the tank through the core and heat exchangers and back to the tank. Each pump 18, Figs. 1, 5, has its intake 101 open to the primary coolant and offers little impedance to the flow of coolant when not in operation, the pump output being connected by a conduit 47 to a plenum 50, Fig. 3, beneath the core 16. Thus, should the pumps 18 fail the coolant circulates through the core by natural convection due to a temperature differential between the coolant above the core and the coolant at the pump intakes. This natural convection is promoted by an auxiliary cooling coil 104 mounted around each pump intake 101, each coil being connected by conduits 105,106 to an auxiliary heat exchanger 107 within a stack 36 outside the reactor containment 25. The auxiliary cooling system is filled with a low melting point sodiumpotassium alloy, the auxiliary coolant circulating by natural convection and losing its heat in the exchangers 107 to air flowing up the stack 36. During normal operation of the reactor when the pumps 18 are operating the air flow through the stack is reduced by dampers 109 to reduce heat loss from the reactor; also,the flow of the auxiliary coolant is reduced by a valve 111. Reactor construction. The core 16 is surrounded by a neutron shield 21 of graphite plugged at its upper end by a rotating shield 23a having eccentrically mounted therein a smaller rotating shield 23b. A concrete shield 22 surrounds the tank 10 and nitrogen is circulated in the space therebetween; moreover, nitrogen fills the space between the reactor and the containment 25 below the level of charge floor 26. Undue heating of the shield 21 is prevented by a stainless steel thermal barrier 17 and by a by-pass flow of primary coolant in the space between the barrier and shield. Fuelling is performed by a charge machine 27 and a discharge machine 28 in conjunction with shoots 29 extending to a fuel handling room 31 and with a normally plugged bore 72 in the rotary shield 23b. The bore 72 may be positioned over any of the fuel assemblies 37 in the core,the plug removed and replaced by a guide tube 73 dipping into the primary coolant. Within the tube 73 is a slidable rod 76 carrying a grab 77 for grasping an assembly 37. The core 16 comprises the fuel assemblies 37 arranged in inner and outer zones (Fig. 4, not shown) the inner zone being a fissile zone containing PuO 2  fuel and the outer zone being a breeder zone containing fertile material UO 2 , the breeder zone being surrounded by a reflector 41 of steel rods. Storage positions are provided in the core outside the reflector 41 for the decay of spent fuel assemblies before they are removed. The core is supported on two spaced grids 43,44 providing the inlet plenum 50 for the primary coolant, the heated coolant leaving through a conduit 53 lagged with a thermal barrier 54. A thermocouple tube 81 enables the temperature of the primary coolant to be measured as it leaves the core. The reactor is controlled by verticallymovable rods 83 coupled to sub-assemblies 85 (shown in their lower shut-down position). The sub-assemblies 85, comprising clusters of fuel pins, increase the reactivity of the core as they are raised. Electromagnetically operated emergency shut-down rods 87 are also provided, these rods being coupled to sub-assemblies 88 comprising clusters of fuel elements crowned by a block 89 of neutron absorbing material. Specification 857, 959 is referred to.