| 摘要 |
1,029,712. Reactors. UNITED KINGDOM ATOMIC ENERGY AUTHORITY. Feb. 11, 1965 [Feb. 11, 1964], No. 5624/64. Heading G6C. A nuclear reactor core has fuel-containing zones including fissile material and moderator to form a critical assembly having an epithermal neutron spectrum, and fuel-free zones bounded by absorbing material of predominantly thermal captive characteristics; provision is made for introducing moderator into the fuel-free zones so as to thermalize neutrons in these zones so that they are then absorbed by the absorbing material. The reactor is preferably water cooled and moderated, although the moderator may be solid graphite rods, particles or a slurry and the coolant a gas. Both shut-down and power control are effected by varying the level of moderator in the fuel-free zones or various parts of the zones, and this control may be supplemented by the inclusion of burnable poisons. Liquid moderator may be introduced from above or below the core. The reactor is stated to be particularly suitable for marine propulsion. The reactor (Fig. 5) comprises a core 51, heat exchangers 52 and a control system 63, 66, 70. The core comprises (Fig. 1, plan view) fuel assemblies 5, comprising vertical fuel plates, arranged in groups of four to define square fuelfree zones 6 which are lined by square section tubes of neutron absorber, such as boron-steel, or a cadmium alloy. Water coolant is circulated by three pumps 57 as indicated by the arrows. Immediately surrounding the core is a moderator container 56 which is intended to keep the core totally immersed in the event of a major leakage in the coolant system. A thermal shield 54 is also provided. Operation and control.-In the shutdown condition, all the moderator control channels 66 are filled with water via conduit 68a, 68b, 69; thus fast neutrons, which would normally pass through the empty channels, are moderated to thermal neutrons and consequently absorbed by the preferential absorbing material of the tubes. At full power, all water in the control channels is displaced by gas supplied through conduits 70 in tunnel 63; thus no moderation takes place of fast neutrons passing through the control channels. The control channels are divided into annular zones so that different levels of moderator may be induced throughout the core for optimizing flux distribution and start-up techniques. It is preferred to use the central region for fine control and the intermediate and outer regions for shut-down. The fuel preferably comprises 80% Pu 239, 16% Pu 240 and 4% Pu 241.
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