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Enhancing Nuclear Safety


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11/04/2007

The Flamanville 3 EPR reactor

On April 10th 2007,  the french government authorized EDF to build a nuclear installation comprising an EPR-type pressurized water reactor on the Flamanville site (Manche). This nuclear reactor designed by AREVA NP and EDF is the first unit of a generation which sooner or later is likely to replace the French nuclear reactors currently in operation, at least partly.


As part of its mission to provide technical support to the Nuclear Safety Authority (ASN), the IRSN has successively widely contributed to :

  • defining the general safety objectives assigned to this new generation of nuclear pressurized water reactors;
  • analysing the safety options proposed by EDF for the EPR project;
  • going deeper - prior to the authorization decree procedure - into the assessment of the safety approach and the design provisions adopted by EDF so as to comply with the safety objectives notified to it.

  

Definition of general safety objectives

The first studies on the safety of possible future nuclear reactors in France were initiated by the IRSN (at the time IPSN) in 1987 and 1988. The IRSN then already stressed the need for significant improvements in safety. Particular attention was paid to reactor core melt accidents in terms of both prevention and mitigation of short, medium and long-term consequences for man and the environment.

The studies started by the IRSN in the late 1980s were jointly continued with its German counterpart, GRS, as a result of the will then expressed by the French and German governments to increase cooperation between the two countries in the field of nuclear power. Such cooperation led in particular to the setting up of NPI, a FRAMATOME and SIEMENS subsidiary, and to the signature of a collaboration agreement between the IRSN and the GRS in 1989 which was subsequently extended in 1998. Further to discussions between the two organizations and at the request of the safety authorities in both countries, the IRSN and GRS formulated joint proposals concerning the approach and the general safety objectives to be adopted for future nuclear pressurized water reactors. These proposals were jointly examined by expert groups in both countries (Advisory Committee for Nuclear Reactors in France and Reaktorsicherheits Kommission in Germany). They were approved by the two safety authorities in 1993 and made public.

The general safety objectives defined in this document are deliberately ambitious. Moreover, the document clearly states that the appropriate strategy for the construction of a series of nuclear reactors in the early 21st century consists in designing evolutionary installations compared with existing reactors. For this reason, the general safety objectives set out in 1993 only apply to improved pressurized water reactors similar to existing reactors in France and in Germany. The improvements in question are intended to take into account the experience gained with reactors in operation and the results of safety studies carried out on these reactors, in particular probabilistic safety studies, as well as a number of innovative measures, notably as regards reactor core melt accidents.

The general safety objectives thus defined are as follows :

  • a reduction in the individual and collective doses received by workers together with a limitation of radioactive effluent releases and a reduction in the quantity and activity of radioactive waste;
  • a reduction in the number of significant incidents, which implies improvements in equipment and systems, with the aim of reducing the frequency of transients and incidents and limiting the possibility of accidents;
  • a significant reduction in the probability of reactor core melt. The implementation of improvements in terms of defence in depth in installations should result in an overall probability lower than 10-5 per reactor and per year, in the light of all uncertainties and all types of failures and internal and external hazards;
  • a significant reduction in radioactive releases which may result from any conceivable accident, including reactor core melt accidents. 

 
With regard to accidents not involving a reactor core melt, no protective measures are required for the population living in the vicinity of the damaged installation (no evacuation, no sheltering).

Accidents involving a reactor core melt which might result in substantial short-term releases must be " practically eliminated ". In other words, if they cannot be considered physically impossible, design measures must be implemented to eliminate them. This applies in particular to core melt accidents under high pressure.

Other accidents involving a reactor core melt must be handled in such a way as to ensure that the maximum conceivable associated releases would require only very limited public protective measures both geographically and in time: no permanent rehousing, no need for emergency evacuation beyond the immediate vicinity of the installation, limited sheltering, no long-term restrictions on food consumption.

 

Examination of safety options and drawing up of technical guidelines

While the safety organizations were defining the safety objectives to be adopted for possible future nuclear reactors, the EPR project was being set up by industrial companies consisting of EDF, a consortium of German utilities and the NPI company. The industrial partners then put forward proposals for general design options intended to satisfy the general safety options adopted. As a result of discussions about these proposals, the application of the general safety objectives to a specific project was gradually defined. Thus, from 1994 to 2000, numerous meetings were held by the Advisory Committee for Nuclear Reactors (GPR) together with the Reaktorsicherheits Kommission up to 1998. After 1998, German experts continued to participate individually in GPR meetings relating to the EPR reactor project. All these GPR meetings were based on reports prepared jointly by the IRSN and GRS presenting the conclusions of the safety examination and design choices proposed by industrial companies.

The provisions adopted by the EPR project are intended to provide significant improvements in accident prevention, including reactor core melt accidents, in the light of operating experience feedback and results of probabilistic safety assessments relating to existing reactors. Here are a few examples of such improvements :

  • the main systems ensuring safety functions, as well as the supporting systems (cooling circuits, power supplies, instrumentation and control), consist of four independent trains separated geographically (called "safety trains" below);
  • the prevention of the total loss of power supplies is ensured by four identical main diesel generator sets (one per safety train) and two diesel generator sets of a different design (with a view to reducing the possibility of common mode failures);
  • the water tank ensuring direct reactor core cooling in the event of a break in the reactor coolant system is located inside the containment and consequently is protected from external hazards;
  • the reactor building, the spent fuel storage building and two out of the four buildings containing safeguard systems and their supporting systems are protected against external hazards by a thick concrete structure;
  • the standard seismic resistance of buildings in the nuclear island and safety-related equipment is designed by adopting a response spectrum more stringent than for reactors currently in operation, which should result in improved margins for the whole possible sites in France. In addition, the buildings of the nuclear island are built on a single basemat, which guarantees a better overall behaviour in the event of an earthquake.

Besides, a set of design provisions is to ensure the confinement of radioactive substances, even in the event of a reactor core melt, in accordance with the above-mentioned safety objectives defined in 1993. These provisions notably include a molten core catcher located at the bottom of the containment. This catcher is cooled by specific means and operates passively for at least twelve hours.

In October 2000, the Advisory Committee for Nuclear Reactors and associated German experts approved a document entitled " Technical Guidelines for the Design and the Construction of the Next Generation of Nuclear Power Plants with Pressurized Water Reactors ", based on a proposal made by the IRSN and GRS. This document which recalls the general safety objectives defined in 1993 takes account of the discussions relating to the EPR project from 1993 to 2000. It specifies the general safety objectives and puts forward acceptable measures together with conditions.

In September 2004, the ministers in charge of nuclear safety sent EDF a letter indicating that, at that stage of the examination of the EPR reactor project, the adopted safety options taken as a whole satisfied the objective set for overall safety improvement. In addition, the letter mentioned that this general appraisal was to be confirmed by the analysis of a certain number of detailed design studies which would require particular attention in the framework of the examination of a possible application for authorization for an EPR reactor in France. This applied to the following :

  • prevention of human errors, improvement of workers' radiological protection, reduction in radioactive releases and reduction in waste quantity and activity;
  • design, construction and operation provisions for the main lines of the primary coolant system and if necessary of secondary systems insofar as a double-ended guillotine break would be ruled out in the accident studies presented;
  • material architecture of instrumentation and control;
  • design of the molten core catcher for core melt accident management;
  • compatibility of the features for the standard EPR reactor project with the site which would be proposed;
  • protection of the installation against malevolence.

The French translation of the "Technical Guidelines" was appended to this letter.

 

Safety assessment carried out prior to the authorization decree

Between 2001 and 2005, before EDF officially decided to submit an authorization application to build an EPR-type reactor, the safety assessment of the project carried out by the IRSN as part of its mission of technical support to the ASN consisted in studying topics which seemed to require a more thorough examination at that stage, on several accounts :

  • either because they were important to ensure compliance with general safety options;
  • because they introduced innovations compared with the French reactors in operation in terms of design provision or approach;
  • or because they were intended to take account as far as possible of events occurred on operating reactors in the EPR reactor design.

First, the IRSN made sure that the design provisions adopted for the EPR reactor were appropriate to ensure compliance with the general safety objectives. On this account, the IRSN examined the following topics: human factor study programme, containment design, elimination of situations involving core melt resulting from heterogeneous boron dilution or involving by-pass containment, design of the molten core catcher, approach to hazards consideration and requirements reference standards for some of these hazards, workers' radiological protection and radioactive waste and effluents management.

Besides, the IRSN carried out an in-depth analysis of innovations brought by EDF compared with the French reactors currently in operation in order to check that they had no negative impact on the safety level of the installation. The topics examined by the IRSN in this context are the following: provisions intended to rule out the possibility of a double-ended guillotine break of the main primary piping and steam lines of steam generators, qualification of equipment to accident conditions, digital instrumentation and control and preventive maintenance of safeguard systems in power operation.

Lastly, the IRSN paid particular attention to design improvements which should prevent recent events in nuclear reactors in operation so that they are well identified by EDF and integrated into the EPR project. The topics examined in this context are the following: risk of sump plugging in the safety injection system, risk of external flooding, consideration of heat wave situations.

The IRSN also conducted a thorough review of probabilistic safety assessments presented by EDF at that stage in the design, for a further assessment of the design provisions adopted by EDF. This review resulted in strengthening some provisions, notably as regards the use of more diversified equipment to perform certain safety functions so as to reduce the risk of common mode failures.

During this period from 2001 to 2005, IRSN examinations were carried out on the basis of technical reports submitted by EDF, drafts of the preliminary safety report, EDF answers about IRSN questionnaires as well as joint technical meetings between the IRSN, the ASN and EDF.

For these examinations, the IRSN largely used its knowledge in the reactor design of previous French standardised plant series and its good knowledge of German Konvoi reactors resulting from its long collaboration with GRS which led to certain outstanding evolutions in the EPR reactor compared with previous French reactors. These examinations were also based on operating experience resulting from the analysis of incidents occurred in French reactors, probabilistic safety assessments carried out by both EDF and the IRSN, as well as periodic safety reviews and modifications brought to these reactors.

Likewise, the IRSN analysis relating to core melt accidents was based on its major research and development programmes carried out for many years, including small-scale and large-scale experiments and the working out of calculation programmes modelling the various physical phenomena likely to occur in the event of such accidents. This analysis also benefited from IRSN's knowledge acquired in the framework of international collaborations.

The conclusions of the technical examinations carried out by the IRSN over the period 2001-2005 were presented to the Advisory Committee for Nuclear Reactors (GPR) during six meetings held between July 2002 and January 2006. These conclusions focused on requests for justifications, additional studies and modifications to some approaches or design provisions. Following each of these meetings, GPR sent the ASN its opinion and its recommendations. Besides, during these meetings, EDF stated its position and committed itself to take actions which were formalized by letters sent to the ASN. The ASN then forwarded its positions and requests on the corresponding topics.

Besides, the IRSN took part in the review carried out by the ASN on the design choice for nuclear pressure equipment, in particular steam generators and control rod drive mechanisms. The conclusions were presented to the Standing Nuclear Section (SPN) of the Central Commission for Pressure Vessels (CCAP) which sent the ASN its opinion and its recommendations. The ASN then forwarded its positions and requests on the corresponding topics.

 

The authorization decree procedure

In May 2006, EDF submitted an authorization application to the ministers in charge of nuclear safety for the construction of an EPR-type reactor at the Flamanville site (Manche), called "Flamanville 3". At the same time, EDF transmitted the preliminary safety report for this reactor project to the ASN.

During the GPR meetings held on 6th and 11th July 2006, the IRSN submitted its conclusions on technical subjects for which an examination had been deemed necessary before a possible construction authorization and which had not been yet presented to GPR. Besides, the IRSN drew up a detailed account of the actions taken by EDF concerning all its committments and of the ASN requests resulting from GPR meetings held after the adoption of technical guidelines (July 2002 to January 2006).

Following the meetings of 11th July 2006, the Advisory Committee for Nuclear Reactors sent the ASN a technical notice in which it concluded that after reviewing the subjects submitted it had not identified anything which at that stage could hinder the continuation of the authorization decree procedure.

On this basis, on 16th February 2007, the ASN Board of Commissioners gave a favorable opinion on the draft decree authorising the creation of the installation which was signed by the Prime minister on April 10th  2007. 

The next regulatory stage will be the authorization application for "Flamanville" fuel loading. In support of this application, EDF will submit a detailed safety report together with operating safety rules whose technical examination will be carried out by the IRSN.

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