Fuel storage pool siphon-breakers: ASN has verified the measures taken by EDF on all reactors

On 18 January 2012, EDF notified ASN of the absence of a “siphon breaker” orifice on the fuel storage pool cooling pipes for reactors 2 and 3 at the Cattenom NPP. This significant safety event (ESS), detected during the course of an internal inspection, was rated level 2 on the INES scale, which comprises 8 levels from 0 to 7 (see press release of 6 February 2012).

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 by a circulating water system referred to as PTR. The cooling water is injected through a pipe at the bottom of the pool.

In certain situations, for example incorrect operation of certain valves or a break in a pipe connected to the PTR cooling system, the injection pipe could extract water from the pool by acting as a siphon, instead of injecting water, which would lead to a drop in the water level in the pool. A significant drop in the water level would expose the fuel assemblies, meaning that they would no longer be cooled and would heat up until damage occurred. An orifice, called the “siphon-breaker”, is made in this pipe near the surface of the pool, to prevent any initiation of a siphon.

Following analysis of this event by ASN and IRSN, ASN asked EDF to run a systematic check on the presence of these siphon-breakers on all the storage pools of all its reactors, as well as on their dimensions.

For the discharge piping on the PTR system, these checks revealed that the diameters of the siphon-breakers on the Belleville 1 and Golfech 1 pools were 15 and 17 mm respectively, as compared with an expected diameter of 20 mm; conformity work was carried out on these siphon-breakers. Two reactors (Nogent 1 and Penly 2) also had siphon-breakers with a diameter of between 19 and 20 mm. EDF presented an analysis concluding that the diameters measured were enough to ensure that the siphon-breakers would function satisfactorily.

For the P4 plant series (comprising the reactors at Paluel, Flamanville and Saint-Alban), these systems use a different design, with a smaller diameter of about 10 mm. The diameters measured on the reactors concerned are in conformity with this design requirement. EDF presented a study confirming the effectiveness of the “siphon-breaker” of the reactors of that series, ASN asked EDF some additional information that will be analyzed in the coming weeks.

Moreover, in order to reinforce the robustness of the facilities to scenarios which had not been envisaged when the siphon-breakers were installed, for example scenarios involving a complete piping break, ASN asked EDF, as part of the on-going safety reviews, to make provision for a modification widening the siphon-breakers on all the reactors. Implementation of this modification began in 2011. Following the complementary safety assessments carried out following the Fukushima Daiichi accident, ASN enjoined EDF to speed up this modification, so that it is in place on all the reactors no later than March 2014.