Between power operation and decommissioning: Why GRS deals with safety aspects of the post-operational phase of nuclear power plants
When a nuclear power plant is finally shut down and no electricity is fed into the supply grid any longer, the so-called post-operational phase begins. This can take several years and ends when the licence holder receives from the competent supervisory authority a licence for the decommissioning and dismantling of the nuclear power plant. During the post-operational phase, the licence holder, in coordination with the competent supervisory authority, can already make preparations for the dismantling of the plant. Currently, there are three plants in post-operation in Germany.
Modifications in the post-operational phase
Even when the reactor is no longer in power operation, the main safety functions of subcriticality, heat removal and retention of activity must still be guaranteed. For this reason, the systems for accident control for example must be kept operational. These include i.a. the emergency diesel generators of the plant. They ensure that the electrical energy required for the operation of safety-relevant components is available in the event of a failure of the external power grid.
In the course of post-operation, however, numerous structures, systems and components in nuclear power plants are no longer needed or need to be adapted. For example, there is no longer any need to cool the fuel assemblies once they have been removed from the reactor or from the spent fuel pool. Accordingly, the requirements for monitoring, inspection and maintenance of the plant change during the course of post-operation. In addition, there are organisational and personnel changes that now need to be implemented. While turbine operators, for example, are no longer needed in the plant, more radiation protection specialists are required to accompany dismantling. In addition, preparatory measures for decommissioning can already be taken in the post-operational phase, e.g. the decontamination of plant components.
In many cases, changes in operational management or at the level of the personnel are due to the fact that the licence holder endeavours to simplify procedures in the post-operational phase and to reduce running costs. This can be achieved, for example, by shutting down safety systems of the reactor after the fuel has been unloaded in order to be able to dispense with the otherwise mandatory so-called in-service inspections and maintenance of these systems. It is also possible to strive for lowering of the requirements for the qualification of the shift personnel in order to reduce costs.
When a nuclear power plant is finally shut down and no electricity is fed into the supply grid any longer, the so-called Modifications of the safety systems and other "essential modifications" in terms of the Atomic Energy Act may, however, only be carried out with the approval of the supervisory authority. The granting of such a licence presupposes that, as part of a safety assessment, it has been verified and confirmed that the relevant safety requirements will continue to be met even after the planned modification. It poses a challenge for the supervisory authorities and their authorised experts that the nuclear regulations are largely not explicitly related to the post-operational phase and that therefore certain requirements for the operation or the shut-down states of nuclear power plants should be interpreted in light of the specific circumstances of this phase. By carrying out its studies, GRS helps to improve the foundations for such inspections.
Studies of GRS in connection with the post-operational phase
Since 2016, for example, experts of GRS have been working on a project – funded by the Federal Ministry for the Environment – dealing with the generic safety assessment of nuclear power plants in the post-operational phase. Here, the generic requirements for the post-operational phase are described by way of example for a pressurised water reactor and a boiling water reactor (deterministic approach). One requirement is e.g. that the emergency core cooling and residual-heat removal chain, including the supply of electrical energy, must be maintained at the required level of redundancy until it is no longer needed. This also comprises the in-service inspections and the maintenance concepts for these systems. A particular challenge is to pinpoint where the boundaries are to the operational systems that might no longer be needed. In a further step, a probabilistic assessment of the deterministically identified minimum requirements is made in order to calculate the probability of certain event sequences. Finally, the results of the deterministic and probabilistic analyses are to be summarised so that if necessary, they can be considered in the safety assessments of licence holders’ modification applications and in the updating of the regulations.
In another project, which was completed in 2017, GRS dealt with possible failure mechanisms in the post-operational phase of nuclear installations. To this end, the experts analysed the engineered systems and evaluated numerous events that occurred in nuclear power plants in Germany and abroad. Post-operational events as well as events that already occurred during power operation and may have increased relevance for post-operation were considered. Relevant mechanisms such as corrosion due to changes in flow velocities in pipes, the crashing of containers, or errors in the planning of plant modifications were identified and described. The published GRS report "Safety-related failure mechanisms in the post-operational phase" is available here (in German).