More generally, and as in previous years, ASN in 2023 observed that discharges remain well managed on most of the sites. However, certain events are indicative of weaknesses reflecting operating defects in certain equipment such as the oil removers or cooling tower anti-scaling treatment installations using sulphuric acid. With regard to waste management, the checks carried out by ASN reveal that its operational management needs to be further improved, even if the positive trend observed in 2022 continued in 2023. During its inspections, ASN still finds cases of non-compliance with the operating baseline requirements, in particular with regard to storage durations and inventory-keeping, as well as non-conforming storage facilities. 2.7 RADIATION PROTECTION OF WORKERS 2.7.1 Exposure of workers to ionising radiation Exposure to ionising radiation in a nuclear power reactor comes primarily from the activation of corrosion products in the primary system and fission products in the fuel. All types of radiation are present (neutrons, α, β and γ), with a risk of internal and external exposure. In practice, most of the doses received come from external exposure to β and γ radiation and are primarily linked to maintenance operations during reactor outages. During the course of 2023, maintenance work related to the SC problem on the primary system auxiliary lines continued on several reactors and contributed to the increase in the average collective dosimetry (see Graph 5 next page) and the average dose received by the workers for one hour of work in the controlled area (see Graph 6 next page) in relation to 2022. Graph 7 (see page 312) shows the breakdown of the workers according to whole body external dosimetry. In 2023, 76% of workers were exposed to a dose of less than one millisievert (mSv), which is comparable to the previous years. The annual regulation limit for whole body external dosimetry (20 mSv) was exceeded on no occasion in 2023. Graph 8 (see page 312) shows the trend in whole body average individual dosimetry according to the categories of disciplines of the workers in the NPPs. As in previous years, those workers most exposed are the personnel in charge of heat insulation. The other categories of disciplines most exposed also remain unchanged: welders, personnel in charge of non-destructive tests, mechanical and ancillary activities. For these latter categories, the average individual dose increased in 2023, in particular for those workers in charge of non-destructive tests (33% increase in the average individual dose by comparison with 2022) and welders (13% increase) who were particularly in demand owing to the work related to the SC problem. Significant contamination events In 2023, EDF notified two significant worker contamination events in the NPPs, as against six in 2022. The first involved exposure greater than one quarter the annual regulation limit for the skin and was rated level 1 on the INES scale. The second concerned exposure greater than the annual regulation limit for the skin and was rated level 2 on the INES scale (see box page 313). In both situations, the workers concerned by these events were given care and the radioactive particles responsible for their contamination were removed. A period of heatwave and drought has three main consequences for the operation of nuclear reactors. Operation of equipment participating in reactor safety during a heatwave Heatwaves lead to high air temperatures, causing an increase in the temperature in the NPP premises. Within these premises, the correct working of the equipment contributing to the safety of the nuclear reactors is guaranteed up to a certain ambient temperature. Ventilation and air‑conditioning equipment prevents this temperature from being exceeded. The temperatures that nuclear reactors are required to deal with are regularly reassessed, notably during the periodic safety reviews. These reassessments take account of climate change. Removal of the heat produced by the reactors in a situation with high watercourse temperatures To help cool its reactors, an NPP takes water from a watercourse or from the sea. This water is then returned to the watercourse or the sea at a higher temperature, either directly (“once through” reactor), or after cooling in cooling towers (“closed loop” reactor), enabling some of the heat to be discharged to the atmosphere. This water discharged by the NPP leads to an increase in the temperature of the watercourse between the upstream and downstream of the discharge point. Depending on the reactor, this increase ranges from a few tenths of a degree (closed-loop) to several degrees (once-through). In order to manage the consequences for the environment, the thermal conditions of these discharges are regulated by ASN resolutions specific to each NPP. The prescriptions apply limit values for the temperature of the cooling water discharged into the natural environment and the heating downstream of the NPP, along with the environmental monitoring procedures. Thus, when the temperature of the watercourse upstream of the NPP is too high, EDF must reduce the power output by the reactors, or even shut them down, in order to meet the limit values associated with the downstream temperature. Since 2006, ASN has incorporated measures into the regulations covering NPP discharges, to ensure advance definition of the operations of NPPs in exceptional climatic conditions leading to significant warming of the watercourse. These special provisions are applicable if the security of the electricity grid is at stake. Temporary relaxation of the limit values for the thermal discharges may also be authorised by ASN, at EDF’s request, if needed by the electricity grid, as was the case during the heat waves of 2003, 2006 and 2022. In this case, environmental monitoring is reinforced. Management of radioactive effluents during periods of drought The discharge flow of the watercourse can also prevent EDF from discharging the liquid effluents from the nuclear reactors. In order to limit the impact of these discharges on the receiving medium, ASN determined a minimum watercourse discharge flow value, for each riverside NPP, below which no radioactive effluent discharge is permitted. Below these values, EDF must store this effluent until the return of favourable watercourse discharge flow conditions. The NPPs have emergency tanks with additional effluent discharge capacity in order to deal with exceptional situations. ASN must issue prior approval before they can be used. OPERATION OF NUCLEAR REACTORS DURING HEAT WAVES 310 ASN Report on the state of nuclear safety and radiation protection in France in 2023 • 10 • The EDF Nuclear Power Plants
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