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Event Report Forms (ERFs) are filed to the IAEA. Sometimes there are multiple event reports, updating the rating, the event description. Here you only see the latest event report

ERF send date: Mon, 01-07-1991
ERF description: On Friday, 24 May, while the reactor was shut down for a full inspection and fuel reloading, the pumps of the shutdown cooling system (RRA) showed signs of abnormal functioning. this anomaly became apparent during emptying of the primary circuit, which takes place before the vessel is opened. The facility's control team reacted immediately by interrupting the emptying operation and adding make-up water to the reactor vessel. It should be recalled that this stage of emptying of the primary circuit, which is carried out while the core is loaded with fuel, is rather tricky as the water level must be reduced while still keeping a sufficient reserve above the core. Indeed, excessive reduction of the level in this situation could cause damage to the pumps of the RRA system and hence interfere wiht reactor core cooling.
Probabilistic safety studies carried out in 1990 had shown, among other things, that pressurized water reactors are particularly vulnerable to this type of event. Under the circumstances, Electricite de France (EDF) decided, at the request of the DSIN, to introduce special precautionary measures, including the use of permanent redundant systems to monitor the water level in the primary circuit.
On 24 May 1991 at Belleville reactor no. 2, the two different devices designed to monitor the water level were incorrectly adjusted and produced and erroneous high reading (the actual level was 15cm below the value shown in the control room).
It was only after analysis of the information provided by the device which was directly monitoring the operation of the coolant pumps that the operator was able to detect the anomaly and adjust the level in the primary circuit.
The significance of this incident became apparant only gradually, from the results of the investigations carried out by the operator. On 31 May, a team of engineers from the IPSN (Institute for Radiation Protection and Nuclear Safety), which provides technical support for the DSIN, arrived on the site to consider the first results of the investigations.
An inspection was carried out on Friday, 7 June to consider the causes of the incident in greater detail and particularly the lessons to be drawn from it which would be relevant to other nuclear factilities.

















BASIS FOR RATING: In view of the two independent failures of safety-relevant systems during a single sequence, this incident is classified at level 2.
DIFFICULTIES IN RATING:
ERF rating date: 01-01-1991
ERF contact person: Mr. J. Breuil
ERF address: 99 rue de Grenelle, F-75700 Paris, France
ERF affiliation: Ministere de l'industrie, Service central de surete des, installations nucleaires

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