EPR Reactor: Information Letter n°11 - Highlights of ASN’s inspection efforts at the Flamanville EPR construction site

During the first half of 2011, ASN continued its inspection activities at the Flamanville 3 EPR construction site. Highlights from recent months include:

Temporary suspension of work on prestressing sheaths

Since 2008, several inspections by ASN have examined the prestressing system[1] of the Reactor Building containment. During construction of the first concreting lifts of the inner containment wall, EDF notified ASN of noncompliance issues relating to the prestressing system (see Information Notice No. 8).

In May 2011, new noncompliance issues regarding positioning of portions of the prestressing sheaths were identified. In addition to occasional deviations, ASN found that recurring anomalies in the system indicated a lack of preparation, skills and training in safety culture on the part of workers and shortcomings in EDF’s monitoring of its subcontractors. As a result, ASN requested EDF to suspend concreting of the inner containment wall on 23 June 2011 and to present an action plan to avoid any further deviations in constructing the prestressing sheaths.

In particular, ASN’s requests concern:

• increased training in and awareness of safety culture on the part of workers;
• increased monitoring by EDF;
• impact analysis of anomalies observed to date taking into account any cumulative effect.

In the days which followed, EDF presented an action plan, which included measures to improve the skills of teams responsible for prestressing sheaths.

On 1 July 2011, considering that the actions adopted by EDF would result in the proper conduct of work on the prestressing sheaths, ASN authorised EDF to pursue concreting activities on the inner containment wall. An unscheduled inspection was performed on 12 July 2011 on this issue. The conclusion of the inspection is that implementation of EDF’s action plan is satisfactory overall (see follow-up letter INSSN-CAE-2011-0662).

Reminder regarding complex concreting requirements

On 7 July, EDF notified ASN that rock pockets[2] had been detected in certain concrete walls of the Reactor Building cavity. This information follows several requests for additional information and inspections by ASN regarding the method chosen for concreting the structures. In addition to handling such occasional deviations, for which repairs were already planned, ASN sought to verify if they demonstrated possible organisational lapses; strict requirements for the worksite require correct performance of operations to avoid the need for future corrections.

On 12 July 2011, ASN’s inspectors examined measures taken by EDF and its primary civil engineering subcontractor responsible for concreting the Reactor Building cavity. Inspectors also examined the defects on site before repair. The technical exchange revealed that defects result from the difficulty of pouring concrete in complex shapes with dense reinforcement using the adopted method. Among other things, inspectors noted that cleaning prior to pouring was incorrectly performed and inspected.

On 10 August 2011, ASN thus requested EDF to:

• provide a report on the quality of the concrete walls affected by the defects, after repairs are completed;
• identify if such complex concreting could have led to defects for which visual inspections are not possible;
• define appropriate preventive measures, particularly concerning inspection and skill of workers, so that future concreting operations are fully mastered;
• present operating experience feedback or useful lessons contributing to mastering the activity prior to the next complex concreting operations.

EDF has already given its response, which addresses worker training and increasing monitoring of activities by the firms concerned.

Ongoing examination of instrumentation and control (I&C) system

The Flamanville 3 EPR I&C system comprises two platforms:

• the Téléperm XS platform, specifically developed for the nuclear industry and designed to protect the reactor during incidents and accidents;
• the SPPA T2000 platform, a conventional industrial component used for normal reactor operation with certain functions designed to protect the reactor during incidents and accidents.

In response to ASN's request dated 9 July 2010, EDF presented design measures that differ from those initially planned. The measures include grouping certain safety functions that were not part of the Téléperm XS platform in a “hard kernel” system. The measures enable handling a situation which includes complete loss of the SPPA T2000 platform combined with certain accident situations.

At the same time, EDF, in coordination with the designers and manufacturers concerned, completed a significant effort to demonstrate that certain safety functions could be included on the SPPA T2000 platform.

At ASN’s request and on the basis of an analysis performed by IRSN, the Advisory Committee on Nuclear Reactors examined this information during its meeting of 16 June 2011. Overall, the Committee found the information provided by EDF to be satisfactory. ASN will give its position on this subject in the near future. As with all positions taken by ASN after consideration by the Committee, the Committee’s assessment and ASN’s letter will be found on ASN’s website.

Examination of accident study methods

As part of its examination of the request to begin reactor operation, ASN is examining the methods used to perform studies of several accident transients. Several methods have changed significantly from those previously used for the operating fleet.

A new method, called MTC 3D, has been developed by EDF to study transients related to steam line breaks (SLBs) The new method makes use of the main steps of the current method used for the operating fleet but substitutes three-dimensional models of the core.

In April 2010, after extensive research by IRSN, ASN determined that:

•  the MTC 3D method may be used to study SLB transients with operation of reactor coolant pumps on condition that requirements concerning validation of software required by the method and intercomparison of results from various software programmes are fulfilled;
• application of the method to SLB transients with shutdown of reactor coolant pumps could not be approved in the current state since the choice of modelling adopted by EDF in these situations was not sufficiently representative of physical phenomena involved.

EDF thereupon submitted additional materials for its application which were examined by ASN and its technical support.

On 7 July 2011, finding that the additional information was still insufficient to approve application of the method for SLB transients with reactor coolant pump shutdown, ASN requested EDF to pursue its case further or research other methods to perform research into SLB transients.

Occupational health and safety inspection activities at the Flamanville 3 worksite

During the first half of 2011, fatal accidents occurred on 24 January and 11 June at the worksite, and an automobile accident on 24 June took the life of employee during his commute between work and home. ASN’s occupational health and safety division submitted its conclusions about the accident of 11 June to the legal and administrative authorities. For the accident of 24 June, it submitted information for the use of the regular competent occupational health and safety authority. An inquiry into the accident of 24 January continues.

ASN also investigated the conditions for declaring labour accidents at the Flamanville 3 worksite as well as the administrative conditions for employment of European Union workers temporarily assigned to the worksite. The effort is being carried out in close cooperation with other government departments.

[1] Prestressing is a construction technique for improving the mechanical strength of a structure. For a nuclear reactor, it involves tensioning steel cables that run through sheaths that are embedded in the concrete. The EPR Reactor Building containment is formed of two walls with a space between them; the inner containment wall is composed of prestressed concrete covered with a steel liner on the inside.

[2] Rock pockets are areas in the concreting lifts which have a concentration of aggregate and a lack of cement which require repair.