Dragon Operations Second Quarterly Progress Report September to November, 1965. Dragon Project Report

When the reactor power reached 5 MW with low gas outlet temperature a start was made on plotting primary circuit characteristics, i.e., variation of inlet temperature and outlet temperature with circulator speed at various by-pass valve positions. As the reactor outlet temperature was increased control rod sticking incidents were encountered and when conditions were met in which ten rods showed sticking in various degrees, the planned programme was discontinued in favour of diagnostic tests of control rod behaviour. These tests have demonstrated, in conjunction with analysis, that sticking is caused by bowing of the control absorbers due to the temperature gradient produced by heat flow from the core into the reflector blocks. Photographs of a discoloured control rod extracted from the Reactor also support this theory (Fig. 1). (author)