MEASUREMENTS OF SPENT FUEL USING THE RRFC-II

Measurement of the fissile content of foreign-returned Material Test Reactor (MTR) spent fuel is one of the most important safeguards problems for the Department of Energy (DOE). The DOE/Savannah River Site funded LANL in 1996 to develop a prototype of an assay instrument, the Research Reactor Fuel Counter (RRFC). The RRFC was built and installed in the Receipts Basin for Offsite Fuel (RBOF) at the Westinghouse Savannah River Site (WSRS). It has been used successfully to assay a limited range of MTR-type spent fuel. In 1998, the DOE Office of Safeguards and Security funded the development of a second-generation instrument, the Research Reactor Fuel Counter, RRFC-II. This instrument has a greater range of applicability and ease of use by the operators. It was fabricated and tested at LANL, installed at SRS “L” basin in May 2002, and measured its first spent fuel assembly in November 2002. The purpose of the RRFC-II is to measure the remaining 235U content in foreign spent MTR fuel using a combination of passive and active neutron coincidence counting, and active-multiplicity analysis. The fuel to be measured started as fresh fuel nominally with 93% enriched uranium. The RRFC-II is designed to measure fuel with an initial 235U enrichment of 80–95%, and a remaining 235U mass of 50–400g with a one standard deviation accuracy of 10%. Fuel burnups of up to 65% can be measured without the use of additional calibration curves. The RRFC-II may also be used to assay other nonstandard items, although with diminished accuracy. Initial measurements of six spent MTR fuel assemblies took place in November 2002. Five assemblies had initial 235U inventories of ~179 g, and the sixth assembly had 74.5 g. All six assemblies had burnups of ~24.4%. Assemblies were measured for 10 minutes in the RRFC-II in the “active-minus-passive” mode. The measured masses agree very well (?? 3.0% absolute value) with the declared masses for the 179 g assemblies, and reasonably well (10.7%) for the 74.5 g assembly. The difference in the total measured and declared mass over the entire measurement campaign, which included a measurement of the fresh fuel standard, was 1.2%.