Patent GB-1023317-A (GB)
Title: Nuclear reactors
IPC: G21C7/22
Filed: 1963-06-04
Abstract: 1,023,317. Controlling nuclear reactors. SAMENWERKENDE ELECTRICITEITS PRODUKTIE BEDRIJVEN N.V. June 4, 1963 [June 4, 1962], No. 22263/63. Heading G6C. A nuclear reactor is controlled by the displacement of a neutron-absorbing liquid within control passages, closed at their upper ends and extending vertically in groups within the reactor core, wherein each group has a common supply passage connected to a pressureproducing device and an inert gas is trapped between the control liquid and the closed end of each passage. A required non-uniform distribution of the control liquid throughout the core is at least in part achieved by the passages extending to different upper levels in the core and/or by different quantities of gas being trapped in different passages. In addition, the cross-sectional area of each control passage may vary along the length of the passage, being of larger cross-section where greater neutron absorption is required. A boiling water reactor, as shown in Fig. 1, has a reactor vessel 1 containing a core 2 within which are situated control tubes 5 branching from main supply tubes 8 which connect with risers 8a outside the vessel 1. These risers 8a include locally enlarged portions 9 holding a sufficient quantity of the control liquid to supply the amount required to fill the control tubes 5 to the highest level under " scram " conditions. Above the liquid level in the risers 8a, gas may be admitted from a control vessel 12 by control means 10, e.g. valves, cocks or gas dosage means, provided in supply lines 11. The inert gas trapped in the ends of the control tubes 5 serves to cushion the impact of the control liquid against the tube ends in the event of an emergency shut-down and also obviates sticking of the liquid against the tube ends when the reactor is again made critical. A " scram " vessel 14 contains gas at a higher pressure than the control vessel 12 and by means of a number of valves or cocks 15 it is possible to submit all the risers 8a suddenly to the maximum " scram " pressure from vessel 14. For normal control purposes only the control means 10. are operated and these permit the gas pressure in each riser 8a to be set at the required value for holding the control liquid at predetermined levels in the associated control tube branches in the reactor core. The branches associated with each riser 8a are of different diameters so that the control liquid is differently distributed in the different parts of the core traversed by these branches. The pressure in the risers 8a may be relieved by operating valves or dosage means 16 permitting release of the gas to a space wherein a comparatively low pressure prevails. Reservoirs 17, 18 and 19 contain liquid metal and additives from which the required control liquid composition is obtained by mixing. In a modification (Fig. 2, not shown), the " scram " vessel 14 is connected through individually operable valves or cocks to a number of cavities containing diaphragms supported on the side of the vessel 14 by grids. When the valves or cocks are opened, the diaphragms burst under the high pressure of the gas flowing from the vessel 14 and this pressure consequently acts on the control liquid to force it to the level of the upper boundary of the core in all of the control tubes 5. In further modifications (Figs. 3, 4 and 5, not shown), the control tubes 5, some of which are of different height from the remainder, all branch from a single supply tube connected to either a control vessel (as in Fig. 1), a bowl fitted with an elastic membrane exposed to the working pressure inside the reactor vessel, or a chamber housing a membrane and having a gas line which communicates with the steam outlet of the reactor. The top and bottom of the reactor core are left comparatively free from obstruction and this facilitates the incorporation of a pump arrangement for the circulation of the coolant, which arrangement (Fig. 7, not shown) may comprise a pump 56 on a shaft rotated by a turbine 57 disposed so as to be driven by the steam issuing from the reactor. Hg is very suitable as the control liquid. In order to increase the absorption of thermal neutrons, Cd may be added to the Hg, a mixture with 10 atom per cent Cd being liquid at room temperature. A mixture with 20 atom per cent Cd and 8 atom per cent Tl (as resonance absorber) stays liquid at room temperature in spite of the high Cd content and has a neutron absorption capacity three times that of pure Hg. Another suitable control liquid is Hg with the addition of 5 atom per cent Cd and 15 atom per cent In, the In having an appreciable effect on the neutron absorption in the epithermal region.