Considerations on Reference Level and Assessments of Radiological Consequences of Emergency during Transport of Radioactive Materials

When an accident occurs during the transport of radioactive materials, pre-determined emergency responses is conducted immediately. For example, cordoned off areas may be established to prevent public exposures. In Japan 5 mSv has been used as the reference level to determine the extent of the area and it is reported that 10 mSv is used in France. However, technical basis of them doesn’t seem clear. IAEA GSR Part7 recommends the range of 20 - 100 mSv as the reference level to establish emergency measures. Recently Japanese Nuclear Regulatory Authority made intensive discussions on it and emphasized the importance to consider not only radiological risks but also various non-radiological influences as a lesson learned from the Fukushima-Daiichi Accident. Finally 100 mSv was adopted for nuclear installations in October 2018. It is considered that the radiological consequences of a transport accident are not significant basically because proper packages containing limited radioactivity are used for transport. However, transport accidents may occur in the public domain and it is crucial to prepare proper emergency response measures in advance to prevent public from exposures. In addition if there are important infrastructures near the accident site, social impacts should be considered as well as radiological risks to protect over all safety of public. Therefore, it is necessary to establish the balanced reference level based on both radiological and social risks from transport emergency. Furthermore, emergency response measures should be determined appropriately based on the precise assessments of radiological consequences from transport emergency. In general safety analysis reports for packages is too conservative because they include several safety margins and it may be inappropriate to use them directly for the assessments. For emergency, most likely conditions should be considered, but these conditions include uncertainties and both the best estimations and extents of variations should be considered. In the paper, the discussions on the reference level for transport emergency are reviewed and important aspects to determine it are proposed. And this paper shows basic concepts to assess precise radiological consequences including uncertainties in transport emergency.