The standard vertical fuel tube assembly of the CP-5 research reactor was modified to permit diversion of heavy water coolant into the thimble for temperature-controlled irradiation experiments of capstdes containing fuel specimens. The modification entailed revisions to the thimble, outer top shield plug and center shield plug, and helium sweep system. The modified thimble is provided with inlet and outlet ports which circulate heavy water from the reactor flow system through the thimble cavity. The coolant in the thimble removes fission heat from the encapsulated samples introduced into the thimble cavity. Flow tests of the thimble were first performed on a flow stand, with final flow characteristics being determined in the reactor. A helium purge system was incorporated to remove dissociation gases trapped in the thimble cavity. Four configurations of capsules and capsule baskets were flow tested \uider conditions simulating operating flow conditions from 2 to 10 Mw reactor power. Irradiation capsules are now being satisfactorily operated in the modified thimbles at 2-Mw reactor power.
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