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OCR for page 107
Appendix C Contamination Concerns Relating to Radon Gas Spread From Tables 2.4 and 3.l, an upper-bound estimate of the amount of uranium-233 (233U) present in Molten Salt Reactor Experiment (MSRE) drain tanks is 33 kg, 160 parts per million (or 5.28 g) of which is 232U. This 232U emits ~14 curies (Ci) of alpha particle decays that, when added to the decays of the six alpha-emitting equilibrium daughters, gives a total alpha emission of 684 Ci at full equilibrium (about 20 years). Radon (2 °Rn) gas is a member of this decay chain and has a half- life of about ~ minute but grows a daughter product, lead-212 (2~2Pb), with a half-life of 10.5 hours that feeds thallium-208 (208TI ), which emits a 2.6-MeV (million electron volt) gamma ray. As long as the decay chain is intact from thorium-228 ~ Th) onward, Rn gas and T} gamma rays are emitted with a 1.9-year half-life (about a 20-year total life). Radon is a monoatomic gas that permeates organic polymers such as neoprene gloves and Plexiglas windows and causes many safety problems because of its highly radioactive particulate daughters. A 4.5- kg sample of salt will contain about 0.5 Ci of alpha emitters of which 0.~l Ci (2.5 x 10~ alpha disintegrations per minute) will be gaseous radon. If analytical evaluations on kilogram-sized samples are performed in a standard "hot cell," a large fraction of this gas, at least 10~° alphas per minute, will be released to the cell atmosphere and spread throughout the facility by rapid diffusion through the atmosphere. Radon is known to spread upwind through ventilation ducts and other equipment, leaving alpha particulate contamination wherever it was present. Although the short-lived daughters may not be considered a real hazard, they are impossible to distinguish from an actual particulate shill, since the alpha contamination will appear everywhere and the 2 Th parent has a 1.9-year half-life. For this reason, great caution should be exercised with any MSRE salt samples taken for analysis or evaluation. C.1
OCR for page 108
C.2 AN EVALUATION OF DOE ALTERNATIVES FOR MSRE Contamination spread from the radon gas will contaminate all equipment with alpha-emitting daughters, some of which emit 2.6-MeV gamma rays as well. The implications of sampling MSRE `drain salts should be considered carefully before a commitment to sample the salts is made, and adequate containment enclosures should be available to handle and analyze samples ofthis type.
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