 INES 2 |
UNAVAILABILITY OF THE IODINE FILTERING OF REACTOR CONTROL ROOM VENTILATION SYSTEM |
On 22 January the operator discovered that the iodine filtering circuit of the control room ventilation system was unavailable. This anomaly was due to the presence (since 23 December 1991) of plugs in both the lines of the circuit. This common mode failure of a system important for safety due in particular to inadequate procedure is classified at level 2.
Additional information by public release (MAGNUC):
On Wednesday 22nd January, while the reactor was running at rated power, the operator noticed that the iodine filtering circuit of the control room ventilation system was unavailable. This anomaly was due to the presence of plugs in both the lines of the circuit.
The control room of a nuclear reactor is equipped with an external air filtration and ventilation circuit to maintain suitable ambient conditions for the personnel. This circuit includes an iodine filtering device for us in the case where an accident situation has resulted in radioactive releases at the site, to protect the operating shifts in the control room.
The plugs were fitted to carry out tests on the filtering device on 23 December 1991. After the tests, the operator forgot to remove the two plugs blanking the two lines of the circuit.
On discovering the anomaly, the plugs were removed and the control room ventilation circuits of the other reactors at the site were checked. No anomalies were found in the other three reactors.
It is to be noted that in the last few years a certain number of anomalies have been detected in the control room ventilation circuits of other reactors. These affected Penly 1 in December 1990, Belleville in May 1991, Gravelines 3 in September 1991 and Cattenom 1 in November 1991. These incidents resulted in the safety authority addressing a number of requests to EDF regarding experience feedback. In particular, the DSIN requested the operator to verify the operability of the iodine filtering circuits of the other nuclear power generating reactors.
Furthermore, this incident must be considered in the light of that which occurred in the same reactor (Dampierre 1) in August 1989, in which plugs were found in the hydrogen trapping and mixing circuit inside the containment. At the time, arrangements were made to prevent this type of incident from recurring. In particular, EDF announced its intention to implement "requalification" procedures making it possible to detect any errors after maintenance operations. Also, measures to improve the management of temporary fittings used during fieldwork (such as plugs) were established at national level. The various measures taken, both at national and local levels, do not appear to have been as effective as expected.
An inspection was made on Wednesday 29th January by the safety authority to determine the reasons for this anomaly and the lessons which could be drawn from it, particularly as regards integration of the corrective measures determined in the context of feedback of experience.
The properties of this ventilation circuit and the circumstances of the event avoided the safety of the reactor being compromised by this specific anomaly. It is however grounds for concern due to its implications: this new "common mode" incident (i.e. in which the same error simultaneously made all the redundant streams of a device classified as "important for safety" unserviceable) shows that further efforts are necessary to ensure the effectiveness of certain corrective measures announced by EDF.
In accordance with the criterion laid down in the safety scale, this common mode failure of a system important for safety is classified as level 2.
BASIS FOR RATING:
DIFFICULTIES IN RATING:
Event date: Wed, 22-01-1992
