Following of reactivity during the start-ups of Cofrentes NPP

During the start-up process, Cofrentes NPP follows the sequence of control blade withdrawal that corresponds to the time of the cycle. The amount of reactivity that is inserted into the core when a control blade rod is removed depends on the fuel assemblies loaded in that cell and the core conditions at the time of start-up.

On 11 September 2021 at 3:07 a.m., during the reactor start-up maneuver following an unscheduled shutdown during Cycle 23, the Reactor Protection System was automatically triggered by a very high scale signal from the intermediate power range neutron instrumentation, IRM.

Following the root cause analysis carried out after the event, a series of measures have been implemented with the aim of eliminating the possibility of a repetition of a similar event.

Among the improvements adopted, it is worth highlighting the creation of the Reactivity Insertion Map (RIM), specific for each start-up, which represents the reactivity insertion path that will occur in each control rod withdrawal movement.

The RIM allows visual detection of the specific points within each sequence where high reactivity insertion with high neutron activity will occur, resulting in significant changes in the range of IRMs.

In addition, it identifies graphically the region where the reactor may become critical, being the area where the operator must pay, if possible, more attention to the neutron instrumentation (IRM).

The RIM is obtained for different conditions of moderator temperature and Xe concentration, which allows knowing in advance the evolution of the expected criticality range with time.

The map is a very powerful tool, which helps to reduce human error by providing information on the evolution of the reactivity with each control rod movement.

On the other hand, as a result of the root cause analysis and lessons learned, other improvements have been made such as specific training programs, training and simulation of start-ups with specific conditions and similar to the previously occurred scenario (Just in Time Training).

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