Evaluation of internal exposure to radionuclides of I, Cs and Co during maintenance operations on primary steam leak in a nuclear power station.

A case of internal exposure due to the inhalation of primary steam leaking from a boiling water reactor power station has been followed up by whole body counting and urinalysis. The thyroid burden of radioiodine and body burden of radiocesium and radiocobalt have been measured from the 3rd day to the 50th day after exposure. Urinary excretions of 1311 and 134+137C~ have also been followed up from the third day of exposure. The dose due to this internal contamination has been estimated on the basis of ICRP procedures. The effective half lives, percentage urinary excretions and body retention and elimination relationships for these radionuclides have been evaluated for this case. For all three nuclides, the body elimination has been observed to be exponential with fast and slow components. In the present case the half-times have been observed to be smaller than those given by ICRP, indicating a faster turnover. INTRODUCTION A PRIMARY steam leak was noticed in one of the boiling water reactor units at Tarapur Atomic Power Station and maintenance operations were undertaken to shut off the leakage. One of the members of the team sent his bioassay sample to the laboratory as per the normal periodical check up schedule. He was found contaminated with fresh fission products although to a minor extent. When this information was conveyed to the power station, the senior member of the team remembered that he had removed the respirator to readjust it and relieve discomfort and came to the laboratory 2 days later to get himself monitored. The body spectra showed contamination from radioiodine, radiocesium and radiocobalt with the radioiodine level slightly above the permissible level for chronic exposure. He was withdrawn from radioactive operations and taken up for detailed investigation for the purpose of dose evaluation and study of his excretion pattern. Insufficient data are available on the body elimination of these radionuclides for a normal Indian adult. The paper presents the results of a follow up of body retention and elimination of 1311, 13*+13'Gs and 6oCo for the subject from the third day following exposure. INSTRUMENTATION AND METHODS The whole body monitor used is a shadow shield moving bed, kept in stationary positions for trunk and thyroid monitoring. The detector is a shielded NaI(T1) crystal of 10 x 10 cm. The efficiencies for the three isotopes were determined using standard sources of 1311, 13'Cs and 'OCo in a masonite phantom corresponding to the positions described. The efficiencies observed were 0.028 % for 13'Cs and 0.015% for 6oCo. The total radiocesium was estimated from the combined 0.60 and 0.66 MeV peaks after correcting for contributions from 1311 and "Go and 40K. For thyroid counting with the bed moved to a position bringing the thyroid directly under the crystal, the efficiency was found to be 0.1 % for Sixteen-hour urine samples were examined by radiochemical separation of radioiodine and radiocesium.(l) Radiocobalt elimination was followed only by whole body counting since in an earlier inhalation exposure it was noticed that most of the cobalt gets eliminated through faeces.@) 1311. OBSERVATIONS AND RESULTS The 1311 thyroid burden evaluated from the 3rd day to the 50th day after exposure, urinary