Assessment of radiological safety for irradiated thorium fuel handling operations

Thorium oxide rods irradiated in a nuclear reactor contain actinides like 232U, 233U and fission products and hence pose special radiological safety problems while handling. 228Th, the daughter of 232U has an alpha-decay half life of 1.9116±0.0016 y and has a chain of daughter products of which 220Rn (thoron), 208Tl and 212Bi are of major radiological concern. Special ventilation measures are required for 220Rn, a noble gas with a half life of 55.6 ±0.1 sec, which contributes to inhalation exposure hazards along with its particulate progeny. Hard gamma emitters, 208Tl and 212Bi contribute to external exposure hazards in thorium fuel-cycle operations. Radiological source term estimation, the gamma dose rate evaluation methodology and the results of calculations are presented in this paper. Special emphasis is placed on the effect of neutron cross section data employed on estimated values of radionuclide source term in thorium bundles irradiated in PHWRs. Theoretically estimated values of radioactivity source terms and gamma dose rates in PHWR irradiated thorium rods were compared with experimentally measured values. Dose rate values generated using modified neutron cross-section data for ORIGEN code shows better agreement with the experimental values in contrast to the original cross-section data set that was part of ORIGEN2. Modified neutron cross section data for simulating thorium irradiation in Indian PHWR yields a value of 550 ppm for 232U in irradiated thorium oxide whereas the ORIGEN based cross section data yields a value of 90 ppm of 232U against the experimental value of 500 ppm. Radiological safety evaluation of irradiated thorium oxide fuel reprocessing is discussed. Radiological aspects of handling freshly fabricated 233U fuel containing ppm levels of 232U are also discussed in this paper. Contact gamma dose rate on fresh AHWR (Advanced Heavy Water Reactor) fuel cluster containing about 1.5 kg of 233U is estimated to be 750 mSv/h, 3 years after fabrication due to build up hard gamma emitting daughters from 232U. Validation of simulated gamma dose rate values generated by radiation transport methods was done with experimental measurement results of external gamma ray exposure rates on irradiated thorium bundles and natural thorium pellets, Experimental validation of radioactivity source terms and the dose rate calculations indicate that the theoretical simulation methods adopted for assessment of radiological safety provide adequate margin of safety for protection of both occupational workers and the public, during handling/processing of irradiated thorium oxide rods and freshly fabricated 233U fuel bundles. (author)