 INES 2 |
FAILURE TO COMPLY WITH TECHNICAL SPECIFICATIONS FOR OPERATION AT REACTOR 4 |
On Friday 3 June, while the operator was carrying out the pre-planned reduction of reactor power, some of the control rods remained inside the core at a level below that set in the technical specifications for operation.
An a posteriori inspection showed that this limit had in fact been overstepped several times in the period 19 May to 3 June.
The nuclear operator has two main ways of controlling the nuclear reaction in the reactor core:
- modifying the concentration of boron in the reactor coolant system water (boron can absorb the neutrons released in the nuclear reaction),
- dropping the control rods into the reactor core or removing them altogether: these control rods contain neutron-absorbing materials.
In order to rapidly shut down the reactor should this prove necessary, some of the control rods must be left at an adequate level as stated in the technical specifications, in part so that they can effectively smother the nuclear reaction as they drop, and in part to ensure an even spread of the neutron flux.
In the present case, following inadequate planning, the power shedding measures meant that on several occasions the rods were left below the limit set in the technical specifications. The shift controllers failed to take rapid measures under these conditions, even when a control room alarm signalled the malfunction.
The safety authority subsequently carried out an inspection at the site on 13 June. At this time the nuclear operator set out the intitial results of the analysis actually carried out by the main individuals involved in the incident. Significant malfunctions were reported: in order to follow the national grid load, the shift controllers had not responded to the alarm signalling that the control rods had been inserted to the limit level on 88 occasions since the previous start-up and 173 times during the previous operating cycle. Some of these infringements lasted several hours. It also appears that the power variation demands on the reactor were too strict and also difficult to implement within the licensed limits for operation.
The site shift controllers did not identify these infringements as failures to comply with the technical specifications for operation, and so did not report them to the safety authority. No steps were therefore taken to prevent them recurring.
Finally, during the inspection, the nuclear operator indicated that further ten identical infringements of the limit level had arisen at Tricastin 3. Two similar infringements were also reported in April 1993 and 1994 at Blayais 4 and Gravelines 5.
This incident was initally ranked at Level 1 on the scale, pending additional information from the 13 June inspection.
In the light of the failure on several occasions to comply with the technical specifications for operation and the shortcomings in the safety culture, this potentially generic incident was upgraded, in response to the safety analyses carried out, to level 2 on the International Nuclear Event Scale (INES). This incident could be downgraded to Level 1 in the light of more detailed analyses.
Following this incident, the Nuclear Installations Safety Directorate (DSIN), the French regulator, took the following steps:
1) The DSIN requested Nuclear Power Plant Operations corporate management to transmit its analysis of the incident and to psecify the immediate measures taken with regard to operations at the Tricastin site;
2) It informed EDF of tis decsion to suspend the national grid load following license for Tricastin Power Plant;
3) It asked EDF to identify the plants at which such incidents had already arisen and to present its opinions on the ability of these units to follow grid load under normal conditions for operation. If necessary, this operating mode would have to be restriced.
Justification of rating:
Table 1/C, basic level 1 + lack of safety culture + level 2
Event date: Fri, 03-06-1994
