Safety assessment of PyroGreen waste disposal operation in underground intermediate level radioactive waste repository

Abstract

This thesis study has been conducted for safety assessment of PyroGreen waste disposal operation in a hypothetical underground intermediate level radioactive waste repository. PyroGreen is an innovative partitioning technology has been developed at Seoul National University based on existing pyrochemical technology being tested at Korea Atomic Energy Research Institute (KAERI), Korea. Improvement of Decontamination Factors (DFs) for producing only Intermediate Level Waste (ILW) to remove long-term uncertainty in safety has been the principal reason for the modification. PyroGreen wastes meet WIPP requirements both on alpha-emitter concentration and heat density of waste packages. Geometry and disposal environment for the hypothetical PyroGreen waste repository are patterned after data from the Gyeongju underground repository, which is the only radioactive waste repository that can accept ILW to some extent in Korea. The safety assessment of the radioactive waste repository can be divided into one for the post-closure storage period and the other for the disposal operational period. To date, extensive studies have been made with focus on the former issue including efforts for reducing uncertainty in long-term safety assessment. Because operational phase is under high surveillance and management, the latter issue has not received adequate studies. Since the truck fire accident and waste drum explosion accident on February 5 and February 14, 2014, respectively, at the US Waste Isolation Pilot Plant (WIPP) the safety during the waste disposal operational period receives increasing scrutiny. Especially in the case of an explosion event, consecutive failure of waste packaging inspection and filter system caused radionuclides release to biosphere. The US Department of Energy (DOE) has identified twelve areas of risk contributors including the failure of characterization program, training and qualifying operators and supervisions through the accident investigation report. Through this accident, it has been found that multiple management system installed to ensure defense-in-depth during operation can be failed. Therefore, it is necessary to analyze whether or not the safety can be assured even if an accident occurs, and the accident scenarios discussed in the existing studies are sufficient. Accident scenarios analyzed in this thesis have been derived from based on existing operational safety assessment scenarios and actual accidents at existing ILW repositories. Flooding is also included because the disposal structures are assumed to have been located below the sea level. Therefore, the final scenarios selected for this study is as follows

1) fire, 2) deflagration, 3) drop of a box containing drums, 4) seismic event, 5) flooding, 6) rock drop. The source term from PyroGreen wastes in each accident scenario is calculated by the five factor formula as follows

MAR: Material at Risk DR: Damage Ratio ARF: Airborne Release Fraction RF: Respirable Fraction LPF: Leak Path Factor Each of five factors is defined as below

[1] Materials at risk= Amount of radioactive material involved in the event. Damage ratio= Fraction of material impacted by the accident conditions Airborne release fraction= Fraction of material that can be suspended in the atmosphere and made available for airborne transport Respirable fraction= Fraction of airborne radionuclides inhaled into the human respiratory system (commonly assumed to include particles 10 microns aerodynamic equivalent diameter or less). Leak path factor = Factor of representing the division of plume pathway (1.0 is assumed) To simulate the release of radionuclides based on the Gaussian plume model the atmospheric dispersion factor is used. Atmospheric dispersion factor is a parameter for quantifying airborne concentration [Bq/m3] to unit release rate [Bq/s] which is affected by wind speed, atmospheric stability, and distance from accident point. Nuclear Safety and Security Commission Notice No. 2012-19, Survey on Evaluation Criteria of Meteorological Conditions of Reactor Site, presents a method for evaluating nuclides transport using atmospheric dispersion factor during hypothetical accident based on U.S. NRC Regulatory Guideline 1.145. Based on the calculated radioactivity source term and atmospheric dispersion factor, the consequence from radionuclides release is calculated by GoldSim®. To validate the performance of model, the model was applied to the Gyeongju near surface disposal facility. The public dose results were compared with the results of the radiation environmental impact assessment [2] and it was confirmed that both results agree well. Fire and explosion scenarios, for single drum damage case, are assessed for both underground and surface facilities. It is clearly shown that the impact is much greater if it occurs at a surface facility closer to workers and public. For all scenarios except for flooding, it is assumed that the filtering function of ventilation systems fails for pessimistic evaluation. In the case of accidents occurring in the underground silo, the all radionuclides pass through upper part of the silo and move to the ground area. In flooding scenario, it is assumed that leaching occurs from the surface of all the drums in silo to all the way into the sink ocean. Therefore inhalation dose from local sediment dust and aerosol in marine water, ingestion dose from fish, crustacean, and seaweed, and external dose is calculated by GoldSim® Radionuclide Transport (RT) module which provides solution for contaminant transport equation. All results confirm that the calculated doses meet the respective regulatory standards with adequate safety margin. As a result, the safety of PyroGreen waste disposal operation in an intermediate level waste repository has been demonstrated for six types of scenarios

1) fire, 2) deflagration, 3) drop of a box containing drums, 4) seismic event, 5) flooding, 6) rock drop.