Patent GB-1084090-A (GB)
IPC: G21C7/22
Abstract: 1,084,090. Controlling reactors. UNITED STATES ATOMIC ENERGY COMMISSION. June 6, 1966 [Sept. 15, 1965], No. 25103/66. Heading G6C. A control system for a boiling water nuclear reactor comprises an arrangement in which at least part of either the fuel assembly or reflector assembly is buoyant in water so as to effect changes in its vertical position with changes in the water level in the reactor containment vessel, which changes in turn cause changes in the reactivity of the reactor, and means for varying the flow rate of water entering the containment vessel in proportion to changes in steam load demand on the reactor. The means for varying the flow rate comprises a surge tank communicating with the containment vessel to which it supplies water under high pressure, a pump sump for receiving water under low pressure from the turbine condenser, and a feedwater pump for transferring water from the low pressure pump sump to the high pressure surge tank. In one embodiment, excess fissile material is initially disposed within the fuel assembly enabling it to reach criticality while substantially unreflected, the reflector assembly which is buoyant in water being initially disposed below the fuel assembly and gradually moving upward around the fuel assembly to increase the neutron reflection as the excess fissile material decreases. The reflector may be a hollow cylinder of beryllium or beryllium oxide, or heavy water contained in a hollow cylindrical tank. Alternatively, the fuel assembly comprises upper and lower fuel regions spaced apart vertically, the upper fuel region being fixed and acting as a superheater and the lower fuel region being buoyant in water and acting as an evaporator. The lower fuel region gradually moves upward toward the upper fuel region to maintain criticality as the excess fissile material contained in the fuel assembly decreases during reactor operation. A hollow, stationary, coupling cylinder of a neutron reflecting material such as clad thorium hydride, which has a moderate neutron capture cross-section and is useful in creating U-233, surrounds the upper and lower fuel regions and increases the reactivity coupling between them so that criticality occurs while there is still a substantial separation between the two regions. Feedwater flows down from the upper to the lower fuel region between the coupling cylinder and the reactor vessel. Increases in the steam load at the turbine decrease reactor pressure and cause an increase in feedwater flow from the surge tank. In either of the above embodiments, an increase in feedwater flow inserts reactivity into the reactor system, causing an increase in reactor power sufficient to boil the increased feedwater flow. As a safety feature, feedwater flow is limited to a value which the reactor can safely boil away. In addition, a stop is provided to limit upward movement of the floating reflector or fuel assembly, the position of the stop being varied over the reactor lifetime to compensate for reactivity losses due to fuel burn-up and fission product build-up. To shut down these reactors, safety dump valve ports are opened to lower the water level in the reactor vessel. The reflector or fuel assembly comes to rest on stops on the base of the reactor vessel, the water itself acting as a damper to gently stop the downward motion of the assembly.