Some Examples of Recurring Events in Belgian NPP's: Root cause and Prevention

Some examples of recurring events that occurred in Belgian NPP's are discussed and analysed: - Between 1997 and 2001, 5 spurious draining of auxiliary nuclear circuits and an inadvertent level drop of primary coolant during (second) mid-loop operation, requiring safety injection occurred during outages in the same Belgian NPP. All these events could be linked to tagging process or tagging control deficiencies (wrong configuration of circuits). During the 2000 outage, the occurrence of three events (including the primary level drop) having the same root cause pointed out that the actions defined in agreement with the Regulatory Organisation (improvement of the tagging process) were not implemented in a timely manner by NPP. - In 1997, a periodic control of Radiation Monitoring systems (equipment verification) was not performed at the frequency requested by the Technical Specifications, during a weekend (due to the absence of a Radiation Protection agent for two successive shifts). A deficiency in the organisation of the Radiation Protection department was identified. Utility was asked to implement corrective actions at the organisation level. A very similar event occurred in 2001 in one other unit of the same nuclear site, which shows that the actions were not sufficiently implemented (not at the adequate level of the NPP organisation). - Two examples of recurring unavailability's of safety equipments are described and analysed. A common root cause for the recurrence is identified: a lack of internal reporting and analysis of events with potential impact on safety (i.e. waste production, contamination risks, safety system capability weakening, TS violation...). As a result, no effective actions were taken. Serious and real incidents (like the primary level drop at mid-loop in 2000) are needed to trigger the utility analyse and reporting process. Regulatory Organisation concluded that NPP experience feedback process had to be improved. Since October, 2000, thus after most of events, a new NPP organisation has been implemented. The new experience feedback organisation is too recently in place to already draw conclusions but it seems that it will have a favourable impact to avoid recurring events. The role of the Regulatory Organisation (RO) as a stakeholder is described. All cases (and they potential consequences on safety) where discussed during inspections with the Belgian RO in order to define corrective actions. How to prevent recurring events more effectively: - Utility has to improve analyses of events in order to better define and implement corrective actions to prevent recurring events. The recurring character of the events described above was identified by the RO, which is not its role. Nevertheless, this is an example of the possible contribution of stakeholders. - Utility has to document the corrective actions for preventing recurring events with potential safety impact. The potential consequences and the root cause of events that had no direct or significant impact on safety (or operation) but that could be precursors of more severe events are to be analysed more deeply. - Utility has to develop a process able to detect recurring unavailability's of safety equipments. The purpose of this paper is to try to identify general causes to the recurrence of some events encountered in Belgian, Nuclear Power Plants in order to propose corrective actions. (author)