On 30 November 2016, IRSN submitted to ASN its assessment of the risks of fracture in steam generators (SG’s) with abnormally high levels of carbon in the steel that makes up their channel head. The local carbon levels measured in these channel heads are in the region of 0.4 % compared to the maximum expected value of 0.22 %. This anomaly meant that a re-examination of the risks of fracture of those steam generators was required since the mechanical properties of the steel have been altered.
The assessment relates to the SG’s in 900 MWe reactors whose channel heads were manufactured by the company Japan Casting and Forging Corporation (JCFC).
The evaluation of the risks of failure in the SG’s consists of examining the risks of fracture initiation resulting from a known or postulated crack in the steel. Thus, IRSN evaluated in particular the extent of the cracks postulated by EDF, the loads that give rise to stresses likely to open the postulated cracks, i.e. the thermal shocks that can affect the SG’s under all reactor operating conditions, as well as the mechanical properties of the steel, taking account of the preliminary data provided by EDF for the steel showing 0.4 % carbon levels.
The evaluation conducted by IRSN has led it to conclude that there is no risk of fracture for the SG’s manufactured by JCFC and fitted in the 900 MWe reactors covered by the assessment (Bugey 4, Dampierre 3, Fessenheim 1, Gravelines 2 and 4, Saint-Laurent-des-Eaux B1, Tricastin 1, 2, 3 and 4),
with the exception of three reactors (Bugey 4, Fessenheim 1 and Tricastin 4) subject to the recommendations of IRSN relating to limiting the extent of the potential thermal shocks and the results of the inspections requested by ASN. For the three aforementioned reactors, the study still needs to be finalised and submitted by EDF.
In order to reaffirm its assessment of the mechanical properties of the steels showing high carbon levels, IRSN has called on the assistance of its Belgian expert assessor counterpart, BEL-V, which is a member of the European ETSON network. ASN and IRSN also went to Japan to visit the manufacturer JCFC in order to better understand the cause of the excessive carbon levels observed.
The IRSN assessment has been published on its website and presented to the press during a joint conference with ASN on 5 December 2016. It has been be presented to the High Committee for Transparency and Information on Nuclear Safety (HCTISN) at its meeting on 6 December 2016.
Moreover, in conjunction with the National Association of Local Information Commissions and Committees (ANCCLI) and ASN, a meeting has been organised for February 2017 to hold an exchange of views with civil society stakeholders as an extension to the technical dialogue launched with ANCCLI, the Flamanville Local Information Commission and ASN on the topic of similar anomalies affecting the reactor vessel at the EPR in Flamanville.
Download IRSN information notice "IRSN assessment of the safety of reactors equipped with steam generators whose channel heads contain an abnormally high level of carbon" (PDF)