SAFETY
APPROACHESThe break preclusion
approach

OFABSTRACT
French Pressurized Water Reactors are designed according to the defense-in-depth principle. The safety approachconsiders equipment failure, notwithstanding the design and manufacturing requirements that are applied toprevent such breaks.
Due to the difficulty of limiting the consequences of the breaks of some components for these reactors, such
events are not considered in safety studies. The lack of realistic and demonstrable provisions to limit
consequences according to current knowledge and available techniques, is justified by the implementation ofparticularly stringent requirements for design, manufacturing and both initial and in-service reinforced inspection
aiming to guarantee that these components will not break: the “break preclusion” approach.
Regarding pressurized water reactors, these cases of “break preclusion” relate to large components of the main
primary system and main secondary systems.
Concerning pipework, “break preclusion” approach can only be considered for main primary pipings and main
secondary steam line pipings: “break preclusion” is then a choice. According to ASN guide n° 22, produced jointlywith IRSN, this choice may only be regarded as acceptable on the basis of justified major safety benefits.
Furthermore, the possibility to apply the “break preclusion approach” shall be assessed at an early stage of thedesign as it is a decisive choice for the design.
IRSN has established a non-exhaustive list of necessary conditions which shall be satisfied in order to consider
the application of the “break preclusion” approach for components or pipings, given that such an approach
shall remain an exception and shall be justified either by the absence of a physical means of mitigation or bymajor safety benefits.

RESUMELes réacteurs à eau sous pression français sont conçus selon le principe de défense en profondeur. La démarche desûreté consiste à considérer la défaillance d’équipements indépendamment des exigences de conception et defabrication des circuits qui visent à prévenir leur rupture.
Les difficultés de limitation des conséquences de la rupture de certains équipements mécaniques de ces réacteurs
ont conduit à ne pas postuler de tels évènements dans les études de sûreté. L’absence de disposition de limitation
des conséquences, raisonnable et dont l’efficacité peut être démontrée au vu de l’état des connaissances et destechniques disponibles, est alors justifiée par l’application d’exigences particulièrement fortes en matière deconception, de fabrication et par une surveillance initiale et en service renforcée visant à garantir la prévention
de la rupture de ces équipements : c’est la démarche d’exclusion de rupture.
Ces cas d'exclusion de rupture concernent, pour les réacteurs à eau sous pression, les gros composants du circuit
primaire principal et des circuits secondaires principaux.
Pour les cas des tuyauteries, la démarche d’exclusion de rupture ne peut être envisagée que pour les tuyauteries
primaires principales et pour les tuyauteries secondaires principales véhiculant de la vapeur : l’exclusion derupture résulte alors, sauf exception, d’un choix. Ce choix ne pourrait être accepté conformément au guide del’ASN n°22 élaboré conjointement avec l’IRSN que par la mise en évidence de gains significatifs pour la sûreté. Par
ailleurs, l’opportunité du recours à la démarche d’exclusion de rupture doit faire l’objet d’un examen à un stadeprécoce de la conception, celui-ci étant un choix structurant pour la conception.

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L’IRSN a établi une liste non exhaustive de conditions nécessaires pour que l’exclusion de rupture puisse êtreenvisagée pour un équipement mécanique étant entendu que le recours à une démarche d'exclusion de rupture
pour des équipements mécaniques doit rester exceptionnel et être impérativement justifié soit par l’absence demoyen physique permettant de faire face à la rupture soit par des gains significatifs en matière de sûreté.

KEY WORDS
Preclusion, break, requirements, defense in depth, pipework, large component, main primary system, main
secondary systems

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TABLE OF CONTENTS
1 INTRODUCTION ............................................................................... 4
2 THE DEFENSE-IN-DEPTH PRINCIPLE AND ITS ADAPTATION TO MECHANICALEQUIPMENT ...................................................................................... 5
3 THE BREAK PRECLUSION APPROACH: CURRENT PRACTICES ......................... 6
3.1 MAIN COMPONENTS (VESSELS) ....................................................................................6
3.2 PIPINGS ...............................................................................................................7

4 CONDITIONS TO BE SATISFIED ............................................................. 9
5 CONCLUSIONS................................................................................13

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1 INTRODUCTION
The break preclusion approach1 is based on the principle of not considering a break in a vessel or piping as a single
initiating event to be studied in a deterministic manner in the safety demonstration.
This document describes the position of IRSN (Institut de radioprotection et de sûreté nucléaire) on this approach.
After giving a reminder of the break preclusion approach in current practices, the document identifies the vessels
and pipings of pressurized water reactors (PWR) for which an operator may apply this approach in the safety
demonstration and defines the associated conditions.
This document is an English translation of the original report in French which is to be referred to for a guaranteed
content.

1

According to the definition in the French Official Journal of 23 September 2015, break preclusion is a "decision to not consider acomplete break of an equipment containing fluid as an initiating event in the safety analysis of this equipment".

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2 THE DEFENSE-IN-DEPTH PRINCIPLE AND ITS ADAPTATION TO
MECHANICAL EQUIPMENT
Pressurized water reactors are designed according to the defense-in-depth principle: a series of provisions aredefined, on the one hand to avoid any incidents or accidents occurring, and, on the other hand, to limit theconsequences of any which occur despite the preventive provisions applied. On this basis, various levels ofdefense-in-depth are differentiated, with each level aiming to limit the consequences of the failure of theprevious level(s) and prevent the next level being affected, in compliance with general safety objectives.
ASN guide n° 22, produced jointly with IRSN, specifies some elements which help to implement the defense-indepth principle, particularly the consideration of events with a potential effect on the nuclear safety of the
facilities. "in the nuclear safety demonstration [such events are] "precluded" or "postulated" ". An event "can" be"excluded" if it is demonstrated that this event is physically impossible or extremely unlikely with a high degreeof confidence, with regard to the safety objectives". However, for the special case of break preclusion for vessels
and pipings, some specific considerations are also recommended in ASN guide n° 22.
This document is related to the integration of the risk of breaks for vessels and pipework - hereafter calledmechanical equipment - based on these considerations, given that, in terms of the defense-in-depth principle, theobjectives for the different levels may be satisfied by:•at level one, by preventing the different types of damage by an appropriate design, manufacturing andinspection;•

at level two, by checking that the structural integrity2 of the equipment is kept by defining appropriate
equipment monitoring conditions during operation and in-service inspections in order to ensure nodamage;•beyond, by defining the failures considered, despite the precautions based on the applicablerequirements for the first two levels, and limit the consequences of such failures.

2

The structural integrity of an equipment is kept if it is not affected by any degradation mechanism which would question theprevention of degradation mechanisms made at the design stageErreur ! Source du renvoi introuvable..

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3 THE BREAK PRECLUSION APPROACH: CURRENT PRACTICESThe order of 30 December 2015 on pressurized nuclear equipment (ESPN) does not cover the safety demonstrationof reactors, and only considers the safety of each pressurized nuclear equipment separately. According to thisorder, all the components and pipings of the main coolant system (MCS or main primary system) and the main
secondary systems shall be classed as level N1, as defined in article 3, and designed on this basis. Thus, theoperator adopted the level 1 RCC-M rules3 when designing these components and pipings. The order does not referto a possible break preclusion for some of these components.

3.1 MAIN COMPONENTS (VESSELS)
Provisions can be applied in order to limit the consequences of most breaks, according to current knowledge andavailable techniques. Nonetheless, realistic and demonstrable provisions to limit consequences cannot be applied
to breaks in some main components: it cannot be ensured that the consequences are compatible with the safety
objectives adopted for the corresponding facilities. Efforts are therefore made to improve the prevention of thecorresponding events in order to eventually “preclude” them.
This applies for the castratrophic failure in main components such as the reactor pressure vessel, the pressure
boundary of steam generators (SG), the reactor coolant pump casing or the pressurizer. The mechanical effects ofa break in one of these components, should it occur, would have unavoidable consequences on the fuelassemblies, internal vessel structures, steam generator tubes or the containment, which would prevent PWR safety
objectives from being achieved.
For such main components, it appears that a break shall be regarded as extremely unlikely with a high degreeof confidence thanks to compliance with the reinforced design, manufacturing and monitoring requirements.
From the design phase for the initial PWR, the decision was reached to not consider accidents initiated by suchbreaks in deterministic safety studies, and therefore to not define additional equipment and procedures to limit
the consequences of these breaks as required by the 3rd level of the defense-in-depth. Consequently, particularly
stringent requirements were adopted for equipment design and manufacturing, and both initial and in-service
inspections to prevent breaks for these components. Therefore:•

the design choices aiming to prevent the occurrence of any degradation mechanism, combined with theapplication of manufacturing procedures and inspection so as to ensure a high level of quality;•

In-service inspection is used to check the absence of degradation mechanism or to detect any occurrence
sufficiently early.

On this basis, the break preclusion approach is mentioned in ASN guide n° 22, produced jointly with IRSN, which
covers the design of pressurized water reactors. In particular, this guide indicates "[that] “break preclusion” shallbe implemented for the large components of the main primary system and main secondary systems. This isbecause no reasonable measure to limit the consequences of their rupture - as an initiating event - could be
defined. These components are said to be "non-ruptible"”. The guide then mentions the following as part of theselected means for the design and manufacturing of these components: "the conservative determination of the3

Design and construction rules for mechanical equipment for PWR nuclear islands

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loads applied, the analysis of the behaviour of the structures under these loads, the existence of margins withrespect to the mechanical criteria in particular, the qualification of the manufacturing and procurement
processes, the choice, extent and precision of the inspection techniques with respect to the manufacturing
processes, the determining of acceptance criteria for manufacturing defects, the in-service accessibility of theareas to monitor and the extent of the associated checks, the integration of experience in the behaviour ofsimilar materials or installations, are means necessary for the implementation of this procedure. "Finally, it is important to remember that the selected manufacturing procedures and inspections shall take into
account "technical improvements and changing practices".

3.2 PIPINGS
Accident studies of breaks on piping are part of the safety demonstration of a facility. They lead to postulate
location and dimensions of breaks. In practice, the operator selects specific locations for these breaks. For
example, according to design rules, for level 1 auxiliary pipings, as defined in the RCC-M, one break shall beconsidered at the end of each section and at least at two intermediate points where the level of stresses is high.
Locations with special characteristics (particularly welds) are also considered. Other points can also be taken into
consideration, particularly in hazard studies. Finally, the positions of the breaks considered is partially based on
their estimated frequency of occurrence, however positions are also determined by convention.
Positioning the breaks at specific points on a piping, to study the consequences, implies that breaks in otherlocations will not be studied. This approach is not part of a break preclusion approach. In this case, the operator
shall justify that the events studied are representative and lead to an evaluation covering the consequences of all
break situations considered as plausible. This is the case for the main coolant lines (MCL) used in French reactors –excluding the EPR reactor at Flamanville, where breaks are not assumed to be precluded, and for which theconsequences of seven breaks, positioned at welds by convention, are studied.

By applying reinforced requirements to specific piping sections, it may be considered that the breaks to be studied
are necessarily positioned outside of these sections. Attention shall be paid to the requirement reference
document applied to the sections in question for this type of approach, which can be rightfully considered as abreak preclusion. Elements mentioned hereafter in the document also apply to this case.

On this basis, the break preclusion approach was adapted to the main steam lines (MSL) sections located between
the containment penetration in the reactor building and the main steam isolation valve4. This approach leads tothe notion of "SUPERPIPE", designed and produced to meet specific requirements, which correspond to a reinforced
design requirement compared to those applicable to safety level 2 pipings5.
Finally, the break preclusion approach was agreed, for the EPR reactor at Flamanville, for the MCL and the MSL, in
accordance with technical guidelines for the design and construction of the next generation of nuclear power
plants with pressurized water reactors and with conclusions of the Expert meeting (SPN) on 21 June 2005.
4

If MSL breaks between the penetration in the reactor building and the anchor point downstream from the isolation valve, this
could drain two steam generators if we assume the failure of the isolation valve on the broken pipe (due to the forces induced by
the break) and if we apply the single failure criterion to another isolation valve (taken from RCC-P).
5

MSL are classed as safety level 2 for French reactors, excluding the EPR reactor at Flamanville.

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However, technical guidelines still provide for the continuation of some studies considering "a double-ended
guillotine break in the MCL" with realistic assumptions, particularly without considering an aggravating failure.
On this basis, the break preclusion approach for pipings is also mentioned in ASN guide n° 22, produced jointlywith IRSNErreur ! Source du renvoi introuvable., which does however specify that it can only be considered for
the MCL and MSL. The aforementioned guide specifies the provisions required to implement this approach,particularly including, over and beyond similar provisions used to preclude breaks in main components, the need to"demonstrating that this choice is reasonable considering the advantages and drawbacks it brings to the overalllevel of safety of the installation and to radiation protection ". In fact, concerning pipings, unlike the main
components of the primary system, provisions can be defined in order to limit the consequences of a double ended
guillotine break in principle6.

6

However, it appears that the consideration of all effects induced by a double ended guillotine break cannot be demonstrated,particularly the dynamic effects on the reactor pressure vessel (RPV) internals.

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4 CONDITIONS TO BE SATISFIED
According to IRSN, the circumstances leading an operator to preclude breaks in some components or pipings as an
initiating event in the safety demonstration, can be considered as one of the following two cases:•

Case 1: breaks cannot be considered as, based on current knowledge and available techniques, thedefinition and implementation of realistic provisions to limit the inherent consequences are not feasible
or not demonstrable;•

Case 2: breaks can be considered and provisions to limit the inherent consequences can be both definedand implemented, however the operator decides to ignore such breaks and considers that breakpreclusion is possible to be applied based on specific design, manufacturing and inspection requirements.

Case 1
The consideration of a break in mechanical equipment is not feasible as the consequences of such a break cannot
be limited using dedicated provisions or their efficiency cannot be demonstrated using the usual study practices.
Break preclusion is mandatory in this case as no alternative option is available. Reinforced design, manufacturing
and inspection requirements are necessary in this case, to ensure that the break can be considered as extremely
unlikely with a high degree of confidence.
A complete break for main MCS components in French NPPs is typically covered by case 1.
In this case, the break preclusion approach and the practical elimination approach can be associated.
Case 2The consideration of a complete break for mechanical equipment, as an initiating event, is not postulated by theoperator, which deems that it can justify that its design, manufacturing and inspection requirements are such that
this break can be considered as extremely unlikely with a high degree of confidence.
In general, in this situation, the break preclusion is the outcome of a deliberate decision by the operator, based on
the high quality and appropriate application of design, manufacturing and inspection requirements, in view of the"deliberate" absence of provisions to limit consequences.
Thanks to this decision, the operator can optimize its resources to a certain degree as some studies, as well as the
deployment of provisions to limit consequences, are unnecessary.

To begin with, IRSN highlights that no regulations or prescriptive technical reference document defines therequirements to be satisfied by an operator to justify the break preclusion 7.
In addition, even defining these requirements is delicate as the equipment in question already benefits frommanufacturing provisions intended to ensure high quality levels. This is true for MCL piping and main componentsof the MCS, which are designed and manufactured according to RCC-M class 1 rules. On this basis, the difference
between equipment covered by a break preclusion approach and similar equipment not covered by this approach
could appear minimal. This could lead to the conclusion that applying RCC-M class 1 is equivalent to a breakpreclusion approach. IRSN rejects this conclusion.
The definition of mechanical criteria which could reinforce RCC-M level 1 criteria associated with the preventionof conventional damages has been mentioned previously, however no such works have been completed. In all
events, according to IRSN, such a proposal would not appear satisfactory as "safety coefficients" applied at the7

It is, however, noteworthy that the concept of "SUPERPIPE" for MSL has a clearly-identified definition.

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design stage are considered to be sufficient to prevent the equipment from failing. Increasing these coefficientswould not lead to any significant safety benefits. Thus, this proposal is not used to prevent conventional damage
and a different type of input shall be identified.
This different type of input could, in particular, include selecting provisions to simplify access to the equipment inquestion and the use of several inspection procedures, tried and tested manufacturing methods, enhanced
inspection during manufacturing and in-service, and in-service requalifications. Two conditions also appear as
indispensable prerequisites:•

There shall not have been failure caused by damage mechanisms detected on the type of mechanicalequipment in question;•

previsions shall be applied to guarantee that no loads are likely to lead to damage, other than thethermal and mechanical loadings considered during the design phase.

On this basis, IRSN has established a non-exhaustive list of conditions to be satisfied based on technicalimprovements and enhancing practices, in order to allow a break preclusion approach to be acceptable formechanical equipment. This list includes:a.

positive operating experience from the manufacturing and use of similar components, including thematerials used and the manufacturing procedures implemented. No loss of structural integrity shouldhave been reported. In addition, a detailed analysis of any difficulties faced by the selection ofmaterials and manufacturing process is required;b.

reinforced design requirements:oeasier access for the purposes of mandatory inspection, with 100% accessibility andinspectability;othe number of welds, the length of the piping and the distance between two fixed points
shall be reduced to the strict necessary;ono stresses exist other than the transient stresses described in the safety report. The
pessimistic values of the stresses assumed shall be justified. On this basis, significant
hydrodynamic effects, for example, shall be avoided. Stresses induced by the effects ofinternal aggressions shall be avoided as far as reasonably possible;oclose attention shall be paid to the risk of corrosion when selecting materials and defining
chemical specifications for fluids flowing through systems;oc.

mechanical design shall be at least equivalent to RCC-M level 1 rules8;

reinforced requirements applicable to manufacturing and associated inspection, taken from RCC-M
level 1 or better, completed by the requirements defined in the equipment specification, including:oprocurement qualification;ovolumetric inspection of products from procurement;omanufacturing processes to guarantee the homogeneity and compactness of products;oinspection of edges to be welded;

8

Equivalence shall be considered in terms of the types of damage covered and of the safety coefficients applied. RCC-M level 1
particularly requires detailed studies of fatigue and the risk of fast failure.

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o

welding using a qualified procedure specifically developed for the purposes of the breakpreclusion approach;orepresentative test specimens shall be tested for all load-bearing welds;odouble volumetric inspections for all load-bearing welds using different procedures. It is
important to aim to ensure the redundancy and diversification of these procedures, in linewith the type of defects to be detected;d.

reinforced in-service inspection requirements:oa in-service inspection program shall be defined – the intervals of these inspections is a key
parameter – and an initial zero point shall be determined. In particular, this program shall
include volumetric testing for all load-bearing welds;oe.

in-service inspection procedures shall be qualified.

appropriate suppliers and sub-contractors shall be selected for all manufacturing and inspection
activities. The selected parties shall be justified based on their experience and qualifications, inparticular.

In addition, feedback from the production of some items of mechanical equipment of the EPR reactor at
Flamanville recently revealed organisational shortfallings, both in terms of specifications and manufacturing, andthe associated inspections, despite the applicable requirements. This raises the question of the efficiency of thequality management system.
On this basis, IRSN considers that a reinforced management quality is required for the design andmanufacturing phases for mechanical equipment selected for break preclusion. This reinforced quality
management shall ensure that any non-compliance is detected at the earliest possible stage, in particular.

Furthermore, the justification of the extremely unlikeliness with a high degree of confidence cannot be simply
based on compliance with a probabilistic cutoff threshold, just like the safety approach in general. Some paststudies were completed to estimate the probability of a break in a piping of the nuclear section, but results varied
widely due to significant uncertainty. IRSN considers that these probabilistic tools are not suitable for integrating atype of damage mechanism whose properties are unknown during the study phase. Feedback for a few piping
breaks which have occurred at French nuclear facilities in recent years illustrates this point:•

the incident on the cooling system during an outage (RHR) at the Civaux 1 power plant was caused by alongitudinal break attributed to thermal fatigue during normal operation, which was not identified during
the design;•

between 2005 and 2008, several incidents led to significant leaks in the steam generator tube bundles at
French electronuclear facilities; clogging, which was not identified during the design phase, was detectedin the secondary side of the steam generators. Such clogging increases the risk of fluid-elastic instabilityof some tubes.

Probabilistic studies would not have forecast these incidents. Although the sections mentioned in these examples
are clearly not designed based on break preclusion, IRSN considers that phenomena which were not identified
during the design phase can also occur for sections designed based on break preclusion.
Finally, the possible contribution of probabilistic studies remains extremely limited in these conditions. IRSN
considers that probabilistic input cannot be used to justify the use of a break preclusion approach.
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According to feedback on manufacturing non-conformities for large forged components in the primary system inFrance and abroad, the level of quality required or specified for these components was not systematically reachedand this non-compliance was not detected during the manufacturing process. In the same way, welds in the main
steam pipework sections of the EPR reactor at Flamanville failed to comply with the break preclusion reference
document defined by the operator. On this basis, IRSN considers that the break preclusion approach should
only be used in certain circumstances.

For case no. 1, which covers "non-ruptible" large components, IRSN considers that the approach has beenadequately tested, particularly according to operating experience, and no reconsideration is necessary whenconstructing new PWR providing that the aforementioned conditions are correctly implemented and controlled.

For case no. 2, which covers MCL and MSL pipings, IRSN considers that the preferred approach is to consider
plausible breaks, study accident scenarios and implement provisions to limit consequences, if applicable, forexample, by implementing whip restraints, leaktighting of neighboring areas or selecting a double-walled piping,combined with a safety-classed leak detection system in the inner space.
Applying the "selected" break preclusion approach may be acceptable if provisions to limit the consequences of a
guillotine break lead to safety drawbacks. For example, the EDF operator highlighted that the presence ofrestraints on the MCL hampered in-service inspection and could induce stresses in the line if incorrect values of
the gaps were used. IRSN considers, as per guideErreur ! Source du renvoi introuvable., that use of the breakpreclusion approach could be acceptable if:• applying this approach is demonstrated to bring significant safety benefits;• above conditions a) to e) are satisfied.
In terms of potential significant benefits, we could list:• installing piping with forged connections, improving both robustness and inspectability;• optimising the layout of the pipings connected to the piping designed based on break preclusion, thanks tothe absence of restraints ;• reducing the doses absorbed during inspection thanks to an easier access to the equipment and systems to
be inspected9.
Safety benefits shall be analyzed on a case-by-case basis. IRSN therefore considers that, if an operator wishes to
use the break preclusion approach, it shall precisely justify the expected safety benefits from the initialproject phases.
It is important to realize that using a break preclusion approach is a structural design decision as this approach hasconsequences on the main design assumptions applied for some systems. Applying a break preclusion approach to
the MCL piping can have consequences on the design of the safety injection, RPV internals, the containment andinternal structures as well as on the qualification profiles of components. Whether or not this approach may be
used shall therefore be assessed during an early design stage.

9

Despite this, it appears difficult to compare such an argument with the drawbacks of a safety demonstration excluding a
guillotine piping break, as additional provisions to limit exposure can be defined.

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5 CONCLUSIONS
French Pressurized Water Reactors are designed according to the defense-in-depth principle. The safety approachconsiders equipment failure notwithstanding the design and manufacturing requirements that are applied toprevent such breaks. In-service inspections can be used to check that design assumptions are verified during
operations and that the structural integrity of the systems remains throughout the life of the facility.
Breaks are considered as initiating events and studied in a deterministic manner as part of the safety
demonstration in order to mitigate their consequences.

Due to the difficulty of limiting the consequences of breaks for some types of mechanical equipment ofpressurized water reactors, such events are not considered in safety studies. On this basis, the lack of realistic anddemonstrable provisions to limit consequences according to current knowledge and available techniques, isjustified by the implementation of particularly stringent requirements for design, manufacturing and both initialand in-service reinforced inspection aiming to guarantee that these items of equipment will not break: the “breakpreclusion” approach.
For these break preclusion cases, which we could qualify as "necessary", and which involve main coolant systemand the main secondary systems components for pressurized water reactors, IRSN considers that the approach
implemented for all operating reactors is satisfactory and no reconsideration is necessary when constructing newreactors, providing that the aforementioned conditions are correctly implemented and controlled, particularly
during manufacturing.

Concerning piping, “break preclusion” approach can only be considered for main coolant line and main steamlines. The break preclusion approach is therefore an active decision and IRSN considers that provisions aiming tolimit the consequences of a break shall be granted priority where possible. According to ASN guide n° 22, produced
jointly with IRSN, a break preclusion approach can only be accepted for piping if this break preclusion approach isproven to have significant safety benefits. The possibility to apply this approach shall be considered at an early
design stage as this choice establishes the design structure.

To conclude, IRSN considers that a break preclusion approach can only be used for mechanical equipment on
an exceptional basis, and such an approach shall be justified either by the absence of a physical means oflimiting the consequences of breaks or by major safety benefits. In both cases, the existence of positive
feedback in relation to similar items of equipment and the application of reinforced design, manufacturingand monitoring requirements are necessary.
IRSN has established a non-exhaustive list of conditions which shall be satisfied in order to allow a breakpreclusion approach for mechanical PWR main coolant system and the main secondary systems equipment.
These conditions could provide input for any operator wishing to apply a break preclusion approach to otheritems of equipment, including another type of facility (non-PWR). A case-by-case approach shall be adopted inall events.

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