GB1133427A - Improvements in or relating to nuclear reactors - Google Patents

Abstract

1,133,427. Controlling reactors. UNITED KINGDOM ATOMIC ENERGY AUTHORITY. 14 Feb., 1968 [8 March, 1967], No. 11017/67. Addition to 1,093,029. Heading G6C. A nuclear reactor has control elements of large neutron absorbancy for normal control and shutdown functions and additional control elements of small neutron absorbancy intended for deep insertion in the reactor core during full power operation for controlling the excess reactivity provided for xenon over-ride and for operation by auto - control where provided, each additional control element being in co-positional relationship with a control element of large neutron absorbancy and each telescopically cooperating with a further control element of small neutron absorbancy which is captive on the respective additional control element. As shown in Fig. 2, a control rod 14 of conventional construction fits with clearance within a control channel 1, being suspended by a chain 10<SP>1</SP> by means of which it can be inserted into or withdrawn from the channel 1 from a winch unit (not shown). The length of the control rod 14 is <SP>2</SP>/ 3 to ¥ the core height, and the rod 14 carries at its lower end an additional control rod 15 of such length that the total rod length equals the full core height. The rod 15 is of smaller diameter than the rod 14 and has an external flange 16 at its lower end which fits with clearance within the bore of a tubular further control element 17 provided with an internal flange 18 at its upper end. The length of the element 17 is the same as that of the rod 15. A support stool 19 supports the closed lower end of the element 17 level with the bottom of the core 2. The rod 14 is of high absorbancy for thermal neutrons ("black"), and the rod 15 and the element 17 of small absorbency ("grey"). When the rod 15 is wholly within the element 17, their total absorbancy is of a value approaching "black" and withdrawal of the rod 14 progressively exposes the "grey" rod 15 and reduces the "blackness" of the element 17. When the flange 16 reaches the flange 18. between <SP>2</SP>/ 3 and ¥ of the assembly will be "grey" from the bottom of the core. Further withdrawal raises the element 17, leaving no absorber at the lower end of the channel 1 and progressively reducing the amount of "black" absorber at the upper end of the channel. It may prove desirable for the rod 15 to have its absorbanoy for thermal neutrons to be somewhat increased, but not to fully "black", by incorporation therein of a suitable material such as cobalt.