Study on evaluation method of EAL effectiveness in nuclear emergency

The Nuclear Emergency Preparedness and Response Guide in Japan requires that the emergency planning zones around a nuclear power plant (NPP), the emergency classification and emergency action levels (EALs), according to the NPP conditions, shall be implemented in advance. Based on the Guide, before a release of radioactive materials, precautionary protective actions are implemented, and after the release, urgent and early protective actions are also implemented according to the measured radioactivity. Recently, the Guide was revised to set more reasonably foreseeable conditions based on the concept of eliminating excessive conservative assumptions in EALs for NPPs.

In relation to the above issues, a study on the evaluation method of EAL effectiveness set for NPPs has been performed. Evaluation of times that met or exceeded EAL in various accident scenarios of typical PWRs and BWRs has been conducted. EALs in Japan are generally categorized into three types: system malfunction-based, plant condition-based and radiation level-based. Figure 1 shows an example of the relationship of times that met or exceeded EALs for the case of a small loss of coolant accident (SLOCA), loss of main feedwater (LOMFW) and station blackout (SBO) in a typical 4-loop PWR plant. The vertical axis shows the times that met or exceeded EAL defined by fission product (FP) barrier thresholds in each accident progression (plant condition-based EAL), and the horizontal axis shows the times that met or exceeded all applicable EALs including the FP barrier EALs. From these results, it was confirmed that the emergency classification would be appropriately implemented by the current EAL framework, as the system malfunction-based EALs were met or exceeded prior to the plant condition-based EALs.

For future evaluation of EAL effectiveness, we will conduct a study on comprehensive evaluation method using not only such information on results from deterministic analyses but also risk information on core damage and radioactive material release.

 

Contact
Mr. Kazumi Miyagi
Regulatory Standard and Research Department,
Secretariat of Nuclear Regulation Authority, Japan
kazumi_miyagi@nsr.go.jp