بررسی حمایت کارشناسان هسته ای از فرایند تصمیم گیری برای محافظت از جمعیت در طی حالات اضطراری هسته ای Analyzing nuclear expertise support to population protection decision making process during nuclear emergencies

Chapter 1: Context Twenty-five years apart, Chernobyl and Fukushima nuclear accidents demonstrated the need to strengthen capacities to cope with such events in parallel of the continuous improvement of safety in nuclear facilities. In this order, nuclear emergency planning, preparedness and management are essential aspects of any country’s nuclear power program. Nuclear emergency management strategies are mainly based on a good coordination between the nuclear power plant owner’s actions to bring back the situation under control and the public authorities’ actions regarding population and environment protection duties. This paper focuses on this last aspect. In the case of a severe nuclear or radiological accident, efforts are oriented to avoid uncontrolled release of radiological materials in the environment. This aim is mainly achieved by a technical defense-indepth approach, which implies the design of several physical defense barriers between radioactive elements and the environment. However, a radiological release can occur when the situation is such that the last physical barrier (such as the containment) is threatened (deliberate controlled venting can be switched on to avoid containment explosion) or already damaged by events such as explosions or fires. In this case, radiological elements are emitted in the form of gas or aerosols firstly transported by atmospheric or water vectors, thus threatening public health. When the nuclear emergency situation is such that a release cannot be excluded in the following hours, general public countermeasures are set up with the aim to avoid short-term deterministic effects (acute harmful tissues reactions) and keep long-term stochastic effects as low as possible (cancers or hereditary effects) (ICRP, 2007). In a radiological or nuclear emergency, general public protection strategy relies on three main urgent countermeasures: evacuation, sheltering, and ingestion of stable iodine tablets. The two first protective actions aim at getting the population off the exposition to radiations and radioactive particles that can be emitted in the environment in case of a severe nuclear accident; the third especially aims at reducing the risk of thyroid cancer. The decision to implement these countermeasures is based on two strategies illustrated in Table 1. Population sheltering action can be ordered as reflex action in an emergency context. When the situation assessment states that a radiological release can occur quite soon (less than 6 h in the French response), sheltering reflex action can be triggered by the radiological facility owner acting on behalf of and under the control of the local government according to the emergency regulation. In this case, sheltering reflex action perimeter is defined during risk analysis prior any emergencies. However, as evacuation and iodine tablet prophylaxis, sheltering decision can also be implemented based on forecasted doses referencevalues. Indeed, in nuclear emergency situations, population protection countermeasures actions aim at avoiding acute effects relating to high dose exposure but also at reducing the probability of emergence of cancers or hereditary effects induced by radioactivity in the long term. For this purpose, protection countermeasures in a nuclear emergency are mainly implemented in relation to absorbed doses reference values expressed in Sievert (mSv, µSv) that take into account: (i) energy deposited in organs and tissues in the human body by radiations; (ii) the biological impact of different radiation types; (iii) organs and tissues sensitivity to ionizing radiation. Reference values contribute to the radiological situation assessment by providing a landmark to which real-time information regarding the situation and protective actions can be compared (ICRP, 2007). Nowadays, recommended dose guidance values play a crucial role in the population protection strategy in a nuclear or radiological emergency. However, the choice to order one or another of these emergency countermeasures need also to take into account several other factors such as additional risks, situation on the field, local data (population density, economical stakes, etc.). These data, in regards to dose exposure, are not related to guidance values that trigger decision about population’s protection countermeasures. By consequence, they play a critical role in the emergency decision process.