ASN rates on INES level 2 a pipe nonconformity in two Cattenom NPP fuel storage pools

On 18th January 2012, EDF notified ASN that the absence of a "siphon breaker" orifice on the fuel storage pools of reactors 2 and 3 had been detected during an internal inspection.

In every reactor, a pool is provided for the storage of fuel assemblies pending their use in the reactor core or pending disposal. The fuel assemblies are kept under water and permanently cooled. A significant drop in the water level would lead to an exposure of the fuel assemblies and to their damage. Alarms indicate any drop in this level, with the necessary action being triggered.

The cooling water is injected at the bottom of the pool by a pipe. In the event of an incident, for example, inadvertent operation of certain valves, the injection pipe could extract the water from the pool through a siphon effect, instead of injecting it, which would lead to a drop in the water level. An orifice, called the "siphon-breaker", is drilled in this pipe near the surface of the pool, in order to stop any siphon effect if initiated.

During an inspection carried out as part of the measures taken following the post-Fukushima complementary safety assessments, the licensee found out that these siphon-breakers were indeed present on the Cattenom NPP reactors 1 and 4, but not on reactors 2 and 3.
On 24th January 2012, ASN conducted an inspection on this subject, in particular concerning the steps taken by the licensee. Following this inspection, it asked the licensee:

• to take immediate compensatory measures, to prevent any risk of accidental emptying of the pool,
• to remedy these nonconformities within 10 days.

The anomalies observed were corrected by repairs to the pipes between 1st and 3rd February 2012; an ASN inspector went on site to check that the required modifications were correctly implemented.
This nonconformity is a deviation from the design baseline safety requirements. It had no impact on the workers or the environment but did compromise defence in depth. Owing to its potential consequences, this event was rated level 2 on the INES scale. The INES scale, which comprises 8 levels from 0 to 7, applies to:

• events with radiological consequences,
• events with no radiological consequences which compromised defence in depth.

 Press contact: Evangelia Petit, Press Department, tel.: 01 40 19 86 61, evangelia.petit@asn.fr