ASN Report 2017

336 ASN report on the state of nuclear safety and radiation protection in France in 2017 Chapter 12  - EDF Nuclear Power Plants ཛྷ ཛྷ the ventilation systems, which confine radioactive materials by depressurising the premises and filtering all discharges; ཛྷ ཛྷ the fire protection water systems; ཛྷ ཛྷ the instrumentation and control system, which processes the information received from all the sensors in the NPP. It uses transmission networks and sends orders to the actuators from the control room, through the programmable logic controllers or operator actions. Its main role with regard to reactor safety is to monitor reactivity, control the removal of residual heat to the heatsink and take part in the containment of radioactive substances; ཛྷ ཛྷ the electrical systems, which comprise sources and electricity distribution. The French nuclear power reactors have two external electrical sources: the step-down transformer and the auxiliary transformer. These two external sources are supplemented by two internal electrical sources: the backup diesel generators. Finally, in the event of total loss of these external and internal sources, each reactor has another electricity generating set comprising a turbine generator and each NPP has an ultimate backup source, the nature of which varies according to the plant in question. 2. Monitoring of nuclear safety 2.1 Fuel 2.1.1 Changes to fuel and fuel management in the reactor In order to enhance the availability and performance of the reactors in operation, EDF, together with the nuclear fuel manufacturers, is developing improvements to fuels and their use in the reactor. ASN ensures that each change in fuel management is the subject of a specific safety case. Any change in the fuel or its management must first be examined by ASN and may not be implemented without its consent. When these changes are significant, their implementation requires an ASN resolution. Fuel behaviour is an essential element in core safety in normal operation or accident conditions, and its reliability is therefore of prime importance. The leaktightness of the fuel rods, of which there are several tens of thousands in each core and which constitute the first containment barrier, are therefore the subject of particular attention. In normal operation, leaktightness is monitored by EDF through permanent measurement of the activity of the radionuclides contained in the primary system. Any increase in this activity beyond predetermined thresholds indicates a loss of leaktightness in the fuel assemblies. During shutdown, EDF must look for and identify the assemblies containing leaking rods, which may not then be reloaded. If this activity in the primary system becomes too high, the General Operating Rules require reactor shutdown before the end of its normal cycle. ASN ensures that EDF looks for and analyses the causes of the observed leaktightness losses, in particular by examining the leaking rods in order to determine the origin of the failures and prevent them from reoccurring. Preventive and remedial actions may therefore affect the design of rods or assemblies, their manufacture, or the reactor operating conditions. Furthermore, the conditions of assembly handling, of core loading and unloading, and the measures taken to exclude foreign material from the systems and pools are also the subject of operating requirements, some of which contribute to the safety case and for which EDF’s compliance is verified by ASN spot-checks. ASN also carries out inspections to check the nature of EDF’s monitoring of its fuel suppliers. Finally, ASN asks EDF to learn the lessons from fuel operating experience feedback. Fuel assembly.