Patent GB-984193-A (GB)
Title: Improvements in or relating to nuclear reactors
IPC: G21C7/27
Filed: 1961-04-14
Abstract: 984,193. Nuclear reactors. BABCOCK & WILCOX CO. April 14, 1961 [April 14, 1960], No. 13503/61. Heading G6C. A heterogeneous nuclear reactor has a core comprising fissile material disposed for operation of the reactor with moderator material of relatively high neutron slowing-down power and in quantity greater than that required for operation of the reactor with said moderator material and comprising a filling of moderator material of relatively low slowing-down power, means for increasing the slowing-down power of the moderator material in the core and a blanket of fertile material disposed around the core. A thermal breeder reactor, described by way of example, has a moderator comprising initially 80% light water and 20% heavy water with means for increasing the percentage of light water during the operation of the reactor to compensate for fuel burn-up, the conversion of fertile into fissile material, and fission product build-up. The core is made up of uniformly spaced elongated fuel assemblies 42 each comprising a number of spaced long slender fuel pins (not shown) containing fertile material at each end with a mixture of fertile and fissile material between each end. The fissile material includes both U-233 and U-235 in the form of uranium oxide while the fertile material is in the form of thorium oxide. About the lateral periphery of the core containing the fuel assemblies 42 there is a plurality of elongated blanket assemblies 44. These have the same shape as, and are arranged in parallel relationship with, the fuel assemblies 42 but contain only fertile material in the form of thorium oxide and provide a fertile blanket about the sides of the core, the fertile material at each end of the pins of the fuel assemblies 42 providing fertile blankets across the ends of the core. The fuel assemblies 42 marked C form a central inner zone in which the ratio of fissile to fertile material is lower than in an outer zone formed by the fuel assemblies marked B, the purpose of this being to provide a more uniform power distribution within the core. For the same reason, the ratio of fissile to fertile material may vary longitudinally within the pins of the fuel assemblies. In the core, a group of elongated cruciform shaped, movable control rods 48 of neutron-absorbing material are arranged in a regular symmetrical pattern in the spaces 46 between the fuel assemblies 42 while about the core there is disposed a pair of thermal shields 50, 52 and a pressure vessel 54. To start-up the reactor, the control rods 48 are fully inserted and the moderator-coolant system charged with a mixture of light and heavy water in which the volume fraction of heavy water is 20%, the percentage amount required to control the fission chain reaction when the core first goes critical, and the control rods 48 are then removed. As operation of the reactor proceeds, the ratio of light to heavy water must be increased as explained above and at the end of the core lifetime, there will be 90-95% light water and 10-5% heavy water. The percentage of heavy water is not reduced to zero in order to provide the continual opportunity for control of the reactor by variation of the water mixture composition. When it is required to shut-down the reactor, the control rods 48 are reinserted into the core. The ratio of light to heavy water may be varied for making both fine and coarse adjustments in the reactivity of the reactor. This method of control makes neutrons not required for fission available for breeding by regulating their speed of slow-down towards thermal energy and not by non-productive absorption as in the case of control rods, while the blanket of fertile material provides for the productive absorption of neutrons in the resonance absorption range as they escape from the core.