INES-event
INES 2

Exceeding the required operating limits, aggravated by safety culture problems

On March 4, 1997 the reactor was in operation at full power, the operator realized that the neutronic flux distribution limits, set in the operating technical specifications, had been exceeded for several hours.

The neutronic flux difference between the upper and lower parts of the core must be limited to reduce the potential core damage in case of an accident, such as rod ejection or rod jamming. For this purpose, operating technical specifications include requirements regarding neutronic flux limits as a function of core power which define the control-diagram. When these limits are exceeded, the reactor power has to be reduced below 15% nominal power within one hour.

This diagram is processed by a software, in which parameters are updated monthly and manually, in order to take into account the actual core state. This up-dating falls within the competence of the "control/tests" service, whereas the reactor control in accordance with the diagram is within the competence of the operators in the control room.

On November 20, 1996 upon new parameter setting, one staff-member of the control/tests service made wrong calculations. Another staff member, in charge of checking, failed to detect the error and consequently the operators started to operate the reactor on the basis of a wrong control-diagram. Two days later, the staff-member of the control/tests service detected his error thanks to computer data processing he performed as requested. He immediately informed his boss. However, no corrective action was taken; the potential consequences of this event were not analyzed and the relevant services were not informed.

The rector, shutdown from November 25, 1996 to January 3, 1997 due to problems on the generator, started up again on January 4. On January 11, a parameter up-dating was again necessary . A similar scenario took place with another team of the control-tests service.

Correct values were finally set on February 13, 1997, during a new monthly test. The previous anomalies were however concealed from the plant management.
The analysis of power operation during the periods of time concerned showed that the neutronic flux distribution limits could have been exceeded during not more than 13 hours. Moreover, according to available safety studies, the safety margins related to rod ejection or rod jamming accidents remained sufficient.

Detailed circumstances related to these incidents were confirmed by a Safety Authority inspection performed on March 13; other inspectors' findings were poor plant skills regarding neutronics, deficiencies regarding staff training and qualification requirements, and coordination problems between the plant and the neutronic specialized EDF corporate department.

Exceeded required operating limits, repeated calculation errors, failure in controlling as well as in taking corrective actions, evidence of safety culture problems, aggravated by deliberate information concealment.

This incident is rated INES level 2.
The safety authority requests the operator to:
- propose actions aiming at improving staff skills and organization systems to prevent neutronic related deficiencies.
- reinforce actions already taken to prevent human factor related deficiencies, regarding both organization and individual behaviour.

Location: PALUEL-1
Event date: Fri, 07-03-1997
Nuclear event report
Legenda & explanation