Fast Reactors and Accelerator Driven Systems Knowledge Base

Conference Article: Use of plutonium in pebble bed HTR's in a two ball type concept

Abstract

Most of reactors can achieve only incomplete utilization of plutonium. Insertion of plutonium as a mixed oxide with uranium (PuO2+UO2) brings only the partial burnup of Pu, as it is in parts compensated by the Pu-build-up process of U-238 breeding. Comparing with other reactors the HTR has some unique features that can be used for the more complete burning of plutonium. The different fuel materials can be inserted into different coated particles. In pebble bed reactors the different coated particles can be loaded in different balls, and the balls can have different number of passes through the reactor until they reach the target burnup. The two ball's concept proposes use of plutonium feed coated particles with a diameter of 220mu, packed in feed balls with heavy metal loading 3g_HM/ball. Thorium with the admixture of high enriched uranium is inserted in coated particles with a diameter of 500mu and packed in breed balls with a content of heavy metal of 20g_GM/ball. It should be interesting to notice, that both types of balls can be exposed by 60% longer in the reactor without any additional requirements of plutonium or feed-uranium inserted into the fuel elements. The burnup of plutonium increases up to 90%, which corresponds the limitations for feed particles. The study shows that pebble bed reactors, operated in Pu/Th-cycle provide a simple possibility for achieving very high destruction of plutonium. This is possible because Pu and Th can be inserted into different balls, and each type can be circulated through the reactor as often, as the constraints of target burnup allow.

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Reference:

Advanced fuels with reduced actinide generation.Technical committee meeting, Vienna (Austria). 21-23 Nov 1995

International Atomic Energy Agency, Vienna (Austria)

IAEA-TECDOC--916, pp:65-69