ESTABLISHING BOUNDING INTERNAL DOSE ESTIMATES FOR THORIUM ACTIVITIES AT ROCKY FLATS
As part of an evaluation of a Special Exposure Cohort petition filed on behalf of workers at the Rocky Flats Plant, the National Institute for Occupational Safety and Health (NIOSH) was required to demonstrate that bounding values could be established for radiation doses due to the potential intake of all radionuclides present at the facility. The main radioactive elements of interest at Rocky Flats were plutonium and uranium, but much smaller quantities of several other elements, including thorium, were occasionally handled at the site. Bounding potential doses from thorium has proven challenging at other sites due to the early historical difficulty in detecting this element through urinalysis methods and the relatively high internal dose delivered per unit intake. This paper reports the results of NIOSH’s investigation of the uses of thorium at Rocky Flats and provides bounding dose reconstructions for these operations. During this investigation, NIOSH reviewed unclassified reports, unclassified extracts of classified materials, material balance and inventory ledgers, monthly progress reports from various groups, and health physics field logbooks, and conducted interviews with former Rocky Flats workers. Thorium operations included: (1) an experimental metal forming project with 240 kg of thorium in 1960; (2) the use of pre-formed parts in weapons mockups; (3) the removal of 228Th from 233U; (4) numerous analytical procedures involving trace quantities of thorium; and (5) the possible experimental use of thorium as a mold coating compound. The thorium handling operations at Rocky Flats were limited in scope, well-monitored and documented, and potential doses can be bounded.
Rolling and Forming of Thorium Metal
One vacuum induction melted and cast nuclear grade thorium ingot was satisfactorily rolled and formed into a hemispherical shape. Attempts to roll and form two vacuum induction melted and cast alloy grade thorium ingots were not successful. Rolling and forming procedures are described and metallurgical evaluation is reported.
Determination of aluminium in plutonium metal by differential linear sweep oscillographic polarography.
A procedure has been developed for the polarographic determination of aluminium in plutonium. The plutonium is separated by anion-exchange in a nitrate medium, and the aluminium is determined in the eluate by use of Superchrome Garnet Y. The aluminium concentration range in the polarographed solutions is 0.020-0.800 mug ml . The average recovery of aluminium was 99.3 %, relative standard deviation 7.2%. Of 20 common impurities found in plutonium métal, only molybdenum and titanium cause significant interference; both can be removed by ion-exchange.