Nuclear safeguards considerations for pebble bed reactors (PBRs)

Abstract

Recent reports by the Department of Energy National Laboratories have discussed safeguards considerations for low enriched uranium (LEU)-fueled pebble bed reactors (PBRs) and the need for bulk accountancy of the plutonium in “used fuel.” These reports fail to account for the degree of plutonium dilution in the graphitized-carbon pebbles that is sufficient to meet the International Atomic Energy Agency (IAEA) “provisional” guidelines for termination of safeguards on “measured discards.” The thrust of this finding is not to terminate safeguards but to limit the need for specific accountancy of plutonium in stored used fuel. While the residual uranium in the used fuel is not sufficiently diluted to meet the IAEA provisional guidelines for termination of safeguards, the estimated quantities of the uranium minor isotopes 232U and 236U in the used fuel at the target burnup of ∼90 Gigawatt-days per metric ton (GWD/MT) exceed standard specification limits for reprocessed uranium and will require extensive blending with either natural uranium or uranium enrichment tails to dilute the 236U content to fall within specification. Hence, the PBR used fuel is less desirable for commercial reprocessing and reuse than that from light water reactors. Also the PBR specific activity of a reprocessed uranium isotopic mixture and its A2 values for effective dose limits if released in a dispersible form during a transportation accident are more limiting than the equivalent values for light-water-reactor used fuel at 55GWD/MT without accounting for the presence of the principal carry-over fission product (technetium, 99Tc) and plutonium contamination. Thus, the potentially recoverable uranium from PBR used fuel carries reactivity penalties and radiological penalties likely greater than those for reprocessed uranium from light water reactors. These factors impact the economics of reprocessing, but a more significant consideration is that reprocessing technologies for coated particle fuels encased in graphitized carbon have not progressed beyond laboratory-scale demonstrations. However, key equipment that has been tested in the past (such as graphite burners and electrolytic disintegration/dissolution devices) is not listed on either the “Trigger List” or the “Dual Use List” for mandatory export controls. If gross burnup determined from fission-product gamma-ray inspection of a discharged pebble cannot be correlated acceptably with predicted plutonium content of the pebble, development and testing may be required on detector concepts for more directly measuring the plutonium content in a discharged pebble to ensure that its placement in the spent fuel storage tanks is for an acceptable measured discard of diluted plutonium.