Fission gas retention in spherical beryllium oxide based fuel elements for the conceptual aborigine reactor.

Fission gas release from spherical BeO-based fuel elements designed for the conceptual ABORIGINE reactor and from 200 micron diameter fuel particles of U02-Th02 solid solution has been measured by in-pile sweep capsule testing In the temperature range 600ºC to 1000ºC, Release parameters (R/B ratios) of 10-7 to 10 s were measured on several fuelled spheres, and 10-4 to 10-2 on bare fuel particles. Two spheres with bonded cladding maintained good gas retention at 1000ºC for fast neutron doses (6 x 1020 nvt) which were three times that required of the ABORIGINE fuel element; however, the fission burnup (1.1 percent U + Th) was a factor of nine below the design burnup. It has thus been shown that BeO coatings on fuelled BeO spheres can in principle retain fission gases to the high standard required for ABORIGINE. Further work would be needed, (a) to assess the reliability of the fabrication process, (b) to test to high burnups fuelled spheres designed so that the coating is subjected only to fast neutron damage and which therefore should not crack from burnup effects, (c) to eliminate other possible cracking mechanisms such as thermal stress, and (d) to measure the release from cracked spheres under a range of conditions and thus to determine the allowable cracking rate more accurately