Discussion of Developing HTGR Emergency Action Levels Applying Probabilistic Risk Assessment

Emergency action level (EAL) is a pre-determined, site specific, observable threshold for a plant initiating condition that places the plant in a given emergency classification level. The original EAL scheme was developed in the post-Three Mile Island accident era and documented in NUREG- 0654/FEMA-REP-1, "Criteria for Preparation and Evaluation of Radiological Emergency Response Plans and Preparedness in Support of Nuclear Power Plants". After that a series of technical documents named as "Methodology for Development of Emergency Action Levels," (NEI 99-01) give a detail description on developing EAL scheme for Power Water Reactor (PWR) and Boiling Water Reactor (BWR). The most recent outcome NEI 07-01 focus on the advanced passive light water reactors. However, neither of these documents are focus on the high temperature gas-cooled reactor (HTGR). High Temperature Gas-cooled Reactor (HTGR) has specific safety characteristics which are different from water-cooled reactor to some extent. Because of inherent design features of the HTGR, a significant reduction is achieved in the potential for an offsite radiological release. The tri-structural isotropic (TRISO)-coated fuel is particularly critical to the prevention of radiological releases besides other fission-product barriers. The accident transients occur over hours and days, not seconds. No fast- acting active safety systems are required to maintain the fuel within design limits and so on. These characteristics are significant to emergency planning and will affect the HTGR EAL development. It is not suitable for applying water-cooled reactors' EAL to HTGR. The paper discusses the EAL differences between HTGR and water-cooled reactor. Probabilistic risk assessment technology is suggested to develop appropriate HTGR EALs.