GB1126367A - Improvements in or relating to nuclear reactors - Google Patents

Abstract

1,126,367. Reactors. UNITED KINGDOM ATOMIC ENERGY AUTHORITY. 2 Dec., 1965 [8 Feb., 1965], No. 51152/65. Heading G6C. A nuclear reactor is controlled by means comprising a number of fuel free zones disposed throughout the core which are each bounded by absorbing material capable of absorbing thermal neutrons but transparent to neutrons of higher energy, such that so long as the neutron spectrum adjacent the zones has a strong epithermal component, control can be effected by introducing or withdrawing a moderator into and out of the fuel free zones. The movement of moderator in this way can, by thermalizing those epithermal neutrons which enter the fuel free zones, so ensure their capture. In an integral pressurized water reactor (Fig. 1, not shown), the core (51) housed within a pressure vessel (50) is cooled and moderated by water which fills the vessel (50) up to a certain level (indicated at L), the water being pressurized by a gas-vapour mixture confined in the space (indicated at S) above the water level. To prevent any part of the core from becoming uncovered should there be a gross leakage of water from the primary circuit, a moderator container (56) is disposed in close proximity to the core. The pressurized water is circulated by means of centrifugal pumps (57) mounted in the bottom of the pressure vessel, passing up between a thermal shield (54) and the moderator container (56), over the rim of the latter and down over a shroud (51a) to the bottom of the core. Thence the water heated by the core passes through the pipe banks of a heat exchanger (52), these being arranged in inner and outer annular zones. A flow splitter (59a) directs the water through the zones in series and down over the outside of the thermal shield (54) to the pumps (57). The core is divided into a number of cells each having a fuelled zone (65) through which extends a fuel free zone or void (66), the latter being bounded by a layer (67) of a neutron absorbing material. The voids 66 are in two groups 66a, 66b, as shown diagrammatically in Fig. 2, those in group 66a which are nearer to the centre of the core serving for reactivity control to compensate for fuel bum-up and those in group 66b at intermediate and outer regions of the core serving for reactor shut-down. The upper ends of the voids are connected to inlet pipes 70a, 70b which pass from the pressure vessel via a central duct 63, while the lower ends of the voids are connected by pipes 68a, 68b to an open-bottomed chamber 69 within which a free surface of moderator is supported by the pressurizing gas. The moderator level is maintained by a pipe 71 which returns excess gas to the pressurizing space S. The upper ends of the control voids in group 66a are connected by pipes 70a via control valves VI with a pressure line 72 containing gas from the space S raised to a higher pressure by positive displacement pumps 73. The upper ends of the shutdown voids in group 66b are connected by pipes 70b with the pressure line 72 by way of the central duct 63, a common outlet pipe 75, a pipe 77 and parallel valves V3. The pipe 75 can be communicated with the space S by branch pipes 76 having valves V2 and connected by a common pipe 78 to the space S. Initially all the voids are flooded and, to start up the reactor, the shut-down voids in group 66b are blown down by opening the valves V3. Any selected number of control voids in group 66a may then be blown down by opening the appropriate valves V1 but first the valves V3 must be closed otherwise the reactor power may be caused to fluctuate. Having blown down the control voids, the valves V3 are re-opened. In an emergency, when it is desired to shutdown the reactor, any shut valves V1 and the valves V2 are opened and the voids fall to the pressure in space S and flood with water. The rate of flooding may be increased by fitting a closed vessel to the outlet side of the shutdown valves V2 and V3, the pressure within this vessel being held at a lower value than that in the space S. For marine application, the chamber 69 is in the form of a long trough and the reactor is positioned so that the longer sides of the trough are disposed fore and aft of the ship. By this means only the pitching motion of the vessel will affect the level of the water in the voids and not to any extent the rolling motion. The pitching motion, provided it is not more than a few degrees, will not cause the water level to change by any significant amount.