Radiological Safety Assessment of Workers for 1 ton/day Spent Resin Treatment Facility

Abstract

The conservative and realistic radiological safety assessment of worker was performed according to various operating scenarios for the 1 ton/day spent resin treatment facility. First scenario is maximum amount of spent resin (600 kg) mixture in the facility. In the case of close work for 250 hours per year, the annual dose of the worker at 1 m from the facility was 1.98E+01 mSv, which was less than the average annual dose limit of the worker. Therefore, it was expected that the worker would have to work at least 1 m away from the facility when performing close work. The annual dose for 2,000 hours exceeded the dose limit regardless of the distance. Internal dose of worker was derived as 3.77E-02 mSv. For the realistic condition, the annual doses (2.03E+00 mSv ~ 8.25E+00 mSv) for 250 hours were less than the dose limit regardless of the distance. In the case of annual dose of 2,000 hours, it was confirmed that it was less than the dose limit from more than 180 cm (1.84E+01 mSv). The remote worker's doses for conservative and realistic assessment were 1.44E+01 mSv and 3.00E+00 mSv, which were less than the worker's dose limit. Therefore, it was judged that the close worker should work at least 180 cm away from the facility or perform remote work to verify the radiological safety of worker. The realistic internal dose was derived as 7.85E-03 mSv. The second scenario is consideration of lead shielding. The maximum doses (250 hours) of close worker for conservative and realistic assessment were 6.75E+00 mSv and 1.43E+00 mSv for lead thickness of 0.5 cm, 3.25E+00 mSv and 6.75E-01 mSv for lead thickness of 1.0 cm, 1.65E+00 mSv and 3.75E-01 mSv for lead thickness of 1.5 cm, and 9.75E-01 mSv and 2.00E-01 mSv for lead thickness of 2.0 cm. By applying lead shielding, the dose limit was satisfied regardless of the distance of close workers. The conservative annual dose for 2,000 hours of close worker was exceeded in the case of distance of 200 cm for 0.5 cm, 80 cm for 1.0 cm. Otherwise, the realistic annual doses were less than the dose limit in the range of close work. Therefore, it was confirmed that the application of lead shielding in the facility would be suitable in terms of radiological safety because it was less than the dose limit even if the worker works at a close distance. The dose range of remote worker according to lead shielding for 250 and 2,000 hours of work were 1.10E-01 mSv ~ 5.25E-01 mSv and 8.80E-01 mSv ~ 4.20 mSv (Realistic dose range of 250 and 2,000 hours were 2.35E-02 mSv ~ 1.08E-01 mSv and 1.88E-01 mSv ~ 8.60E-01 mSv). The radionuclides that most affect workers from spent resin mixtures were 137Cs (86.2 %), 60Co (7.44 %), and 152Eu (4.97 %). It is expected that lead shielding must be applied to MWR in order to prevent the release of 137Cs, a volatile nuclide, due to accidents when treating spent resin with microwave. The third scenario is consideration of elapsed operating time. The conservative and realistic annual doses of close worker according to the operating time of the treatment facility were derived as 7.38E+01 mSv ~ 2.98E+02 mSv and 1.62E+01 mSv ~ 6.60E+01 mSv. Realistic dose up to 160 cm and conservative dose in all close work range exceeded the dose limit. The conservative and realistic annual dose of the remote worker were 1.33E+01 mSv and 3.00E+00 mSv, which were less than the average dose limit. The fourth scenario is leakage of spent resin mixture. Close worker removing the leaked spent resin mixture had the most exposure dose (2.70E-01 mSv/h ~ 3.50E-01 mSv/h for conservative assessment and 5.63E-02 mSv/h ~ 7.29E-02 mSv/h for realistic assessment) in the case of leakage from the MWR. For remote workers, the conservative and realistic range of exposure doses were derived as 7.20E-03 mSv ~ 8.20E-03 mSv and 1.50E-03 mSv ~ 1.71E-03 mSv. The annual dose which was conservatively evaluated (1.80 mSv ~ 2.05 mSv for 250 h/year of work and 1.44E+01 mSv ~ 1.64E+01 mSv for 2,000 h/year of work) was also less than the worker's annual dose limit. The ranges of committed effective doses for worker who does not wear air-purifying respirators and worker who wears air-purifying respirator were derived as 2.08E-02 mSv ~ 1.88E+01 mSv and 4.16E-04 mSv ~ 3.77E-01 mSv, respectively. Without the air-purifying respirator, the outflow from the MWR had the most effect (1.88E+00 mSv ~ 1.88E+01 mSv) on worker. These conservatively evaluated values were also less than the dose limit. In the case of considering the air-purifying respirator, workers' dose limit was conservatively satisfied regardless of the outflow rate and leakage part. In realistic assessment, the range of the committed effective dose for the case where APF is considered and not considered were 8.66E- 05 mSv ~ 7.85E-02 mSv and 4.33E-03 mSv ~ 3.92E+00 mSv, respectively. When considering the sum of the internal and the external dose, it was confirmed that the annual dose of the worker without APF exceeded the dose limit when 80 % or more (2.03E+01 mSv ~ 2.22E+01 mSv) was leaked from the MWR. Otherwise, the annual dose of worker with APF was less than the dose limit regardless of outflow rate and leakage part (maximum dose: 1.83E+01 mSv). Therefore, respirator must be considered when performing work to remove leaked spent resin mixture. In addition, since the exposure from leakage of MWR had the greatest effect on worker, shift work should be required to conservatively satisfy the dose limit.