ABSTRACT INTRODUCTION

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1 A UK regulatory view on the analysis and design of nuclear structures subjected to dynamic and impact loadings C.M. Pratchett & A.M. Coatsworth EM Nuclear Installations Inspectorate, Health and ABSTRACT During the last decade all new safety related structures at nuclear power and chemical plant sites in the United Kingdom have been designed to resist the effects of external and internal hazards. The regulation of nuclear installations in the UK is carried out by means of a licensing system which is administered on behalf of the Health and Safety Executive by HM Nuclear Installations Inspectorate. A major part of the licensing process lies in the assessment of licensees' safety submissions for new and existing plant. The purposes of this paper are to describe the Inspectorate's approach to: the assessment of safety submissions on dynamic and impact loading; validation of design; areas of uncertainty; and to indicate areas of current and future research. INTRODUCTION Since the late 1970s, new nuclear power and chemical plant in the UK has been analysed and designed to resist the effects of earthquake loadings. HM Nuclear Installations Inspectorate (Nil) has been concerned with the assessment of safety submissions for these plants. In the case of the Sizewell B pressurised water reactor (PWR), which is currently being commissioned, and the proposed Hinkley Point C PWR, this involved Nil submitting evidence to and being cross examined at Public Inquiries [1,2]. The objective of this paper is to outline the approach adopted by Nil in the assessment of safety submissions on dynamic and impact loading for nuclear plant licensed in the UK. This includes a description of the legislative process in the UK, Nil's Safety Assessment Principles and our approach to: validation of analyses; justification of design; use of expert judgement and the treatment of uncertainty. The paper also reviews current and possible future developments in dynamic and impact loading which may help reduce uncertainty and provide a greater knowledge of the performance of structures.

2 394 Structures under Shock and Impact LEGISLATIVE BASIS In the United Kingdom no site may be used for the purpose of installing or operating any commercial nuclear installation unless a nuclear site licence has been granted by the Health and Safety Executive (HSE). The Nil is that part of HSE which is responsible for administering this licensing function. HSE's powers in this regard derive from the Health and Safety at Work etc Act 1974 and those parts of the Nuclear Installations Act 1965 which are relevant statutory provisions. Most importantly, these include the power to attach conditions, which are legally binding, to site licences in the interests of safety. There are thirty five standard licence conditions currently in use dealing with such matters as restriction on dealing with the site, the reporting of incidents, operating instructions and decommissioning. Licence condition 14 is particularly important in the context of this paper; it requires the production of safety cases to justify safety throughout the design, construction, manufacture, commissioning, operation and decommissioning phases of the installation. Under UK law responsibility for nuclear safety rests with the licensee. The role of Nil is to ensure that the appropriate standards are developed, achieved and maintained by the licensee, to ensure the necessary safety precautions are taken and to regulate and monitor the actions of the licensee by means of HSE's powers under the site licence. The Nil independently reviews and assesses licensees' safety cases to confirm compliance with the licence conditions throughout the plant life. The licensees are free to develop their own safety standards and plant safety cases using appropriate methodologies, provided these are in general conformity with Nil's Safety Assessment Principles [3]. Nil SAFETY ASSESSMENT PRINCIPLES The process adopted by Nil in making decisions on granting of a site licence requires the safety case and supporting reports to be submitted by the licensee for assessment by NIL The Inspectorate needs to adopt a consistent and uniform approach to the assessment process; to this end it is necessary to provide a framework which can be used as a reference for the technical judgements assessors have to make. The Nil's Safety Assessment Principles (SAPs) form such a framework. The 1992 publication of the Safety Assessment Principles for Nuclear Plants updates and consolidates earlier publications of its separate nuclear power and chemical plant principles. The safety submission from a licensee for a new installation will show that the design meets their safety standard, but the Inspectorate assesses the submission to ensure compliance with the SAPs. The following parts of this section identify general principles which are applicable to the determination of

3 Structures under Shock and Impact 395 the seismic hazard, analysis and design and impact loading. Most of the examples chosen also show an adequate level of robustness should be included in the calculations to make an allowance for uncertainties. Design Data And Models There are a number of general principles which cover design data; these are covered in principles P86-89 and 175. The main emphasis of these principles is to ensure adequate verification and validation is carried out. P86 and 175 are typical examples :- P86 Theoretical models should be employed as appropriate in support or confirmation of a design basis or as a means of describing safety related conditions in a plant at any time. Such models should be based on a sound scientific understanding and any necessary assumptions or approximations should demonstrably bias results on the safe side. PI75 Where analyses have been carried out on civil structures to derive static and dynamic structural loadings for the design, the methods used should be adequately verified and validated, if necessary by model test. Design Principles The structural integrity section of the SAPs refers to the use of sound engineering concepts and proven design features, the analysis of potential failure modes, the use of proven materials and the application of high standards of manufacture. An important aspect of design against internal and external hazards is covered in the last sentence of PI47; this is to ensure a level of robustness in design. P147 For safety related structures, a schedule of all loading combinations within the design basis, together with their frequency, should be used as the design basis against operating, testing and accident conditions. For more severe loadings, predicted failure modes should be gradual and detectable. TOPICS IN DYNAMICS AND IMPACT The assessment of safety submissions on dynamic and impact loadings involves reviewing a wide variety of technical issues, establishing the main elements of the safety case and having an understanding of the SAPs. This will enable the assessor to form a judgement on the adequacy of the case presented. In the following paragraphs some examples of our approach to assessment are presented. These cover: model testing; validation of analyses; demonstration of ultimate structural failure; dropped loads and missile/aircraft impact.

4 396 Structures under Shock and Impact Seismic Analysis - Validation By Model Testing As indicated above, P86 and 175 ask for analyses to be validated. In the case of seismic loading, validation of analyses has generally been carried out either by using other models or known mathematical solutions. Model testing has only been carried out to a limited extent. One of the most important scale model tests carried out in recent years has been at Lotung in Taiwan, where a 1/4 scale model of a typical PWR plant has been subjected to actual earthquakes. This has given analysts and designers the opportunity to use their analytical methods and modelling techniques to compare their results against those measured. Nil expressed reservations on the extent of soil structure interaction analysis validation of new nuclear power plant in the UK and asked the licensee to consider validating its methodologies via the Lotung experiment. The analyses were subsequently carried out using the raw data such as soil properties and real earthquake time histories and using the same computer program and modelling assumptions used for design. Once the soil properties had been adjusted following a forced vibration test the results from the predictive analyses were found to be generally consistent with actual results. In our opinion the results obtained from the validation exercise demonstrate the positive effects of scale model testing, which we consider help to provide confidence in the analytical and modelling methodologies used for design. Therefore where uncertainty exists due to limited data being available the designer should consider advantages of model testing to provide additional confidence in the capability of the item under consideration. Soil Degradation In Soil-Structure Interaction A recent reappraisal of the soil conditions at nuclear power plant site by a licensee concluded: the low strain modulus values should be changed, the degradation of shear modulus with shear strain be reduced (ie more linear behaviour), and the damping values be reduced. The changes in the soil properties appeared reasonable; indeed in the USA the Nuclear Regulatory Commission (NRC) had made changes to its Standard Review Plan [4], which stated that reports on recent earthquakes seemed to show that there may not be a decrease in shear modulus or an increase in damping under high strains. Nil commissioned an independent review of the licensee's submission since the conclusions represented a significant change in the behaviour of soil properties under earthquake loading. The study concluded that field data from recent earthquakes did not generally support the view that soils remained elastic up to about 10"" % shear strain and several reference papers in reality merely showed that one dimensional propagation models worked, or that linear behaviour was observed at very low excitation levels. In our opinion this demonstrates the need to ensure that where changes to accepted practice are

5 Structures under Shock and Impact 397 proposed, these should be thoroughly researched and subject to expert peer review. Ultimate Structural Failure, Analysis And Model Tests To ensure an adequate safety margin over design pressure in Prestressed Concrete Pressure Vessels British Standard BS 4975 requires an ultimate load analysis to demonstrate that at ambient temperature the vessel can normally withstand a substantially greater pressure (normally 2.5 times design pressure). In the past this has been achieved by either analysis or a combination of analysis and model test. For the PWR reactor containment structure at Sizewell B a scale model was constructed and an analysis carried out to determine the likely failure pressure and mode of the structure. Predictive analyses were also carried out by Nil and other interested groups from the UK, France and USA. The purpose of the predictive analyses was to validate the analytical methodologies used and to establish the failure pressure of the containment. This would also satisfy SAPs 86 and 175. The pressure test on the model was carried out in July Results from the test confirmed the target ultimate load pressure was exceeded, and therefore the test had fulfilled its requirements. Following this an interpretation of the results was made to identify the location of failure and review displacements and strain recordings. The data were then made available to those organisations which had carried out predictive analyses for them to establish how the results of their analytical modelling compared. In general the failure modes identified from the scale model had been predicted, although most organisations recognised the necessity to have reviewed the results from certain areas in more detail. In the model analysed for Nil, failure of the base had not been predicted since the analysis was insufficiently detailed in this area. However, on closer inspection and greater refinement the regions of high stress were identified. Since the PWR containment was a new technology in the UK we consider there were clear advantages in carrying out this work. These were: greater confidence in the analytical methodologies; confirmation of structural capability and identification of likely failure mechanisms and assurance the actual structure should meet the design intent. Dropped Loads An assessment by a licensee of the consequences arising from a dropped load (a fuel flask) into a spent fuel cooling pond initiated a programme of model tests at 1/4 and 1/15 scale. The licensee's contractor concluded that, although the pressure measured on the base of the model flask was substantially less than the water hammer pressure, this was attributable to air being trapped between the base of the flask and the water surface.

6 398 Structures under Shock and Impact Nil considered this effect would not be replicated at full scale, even in the presence of a trapped air layer. Assessment of the hydrodynamic pressures measured on the pond wall of the model indicated they were lower than expected and that their duration was variable. To determine the implications of these results, scaling effects were considered in deriving the pulse duration at full scale. Theoretical studies carried out showed the measured wall pressures were low due to the entrained and entrapped air and that neither effect was likely at full scale. This therefore highlights the need to ensure experimental data are scrutinised. Missile/Aircraft Impact The hazard resulting from aircraft crash has been the subject of much discussion with the licensees and at Public Inquiries. To obtain more knowledge on the topic Nil has commissioned several research studies. A recent project was carried out into the probability of aircraft debris, either falling onto or being thrown forward during a crash into a nuclear installation. This study was initiated because of events such as Lockerbie (1989) where aircraft debris was scattered over a wide area. Investigations into debris thrown forward during an aircraft crash concentrated on engines rather than other potential missiles. This was because the engines are generally the largest and most energetic missile which could be thrown the furthest distance from the crash site. It was found that distances of less than 500m are most likely, but distances up to 2000m have been observed. Long engine throw forward distances can result from impacts at low descent angles on a variety of surfaces. The engine throw forward distance, regardless of descent angle appears to have little dependence on impact velocity. Initial conclusions indicate the frequency of impact is not likely to be significant. APPROACH TO UNCERTAINTY From our experiences of assessing seismic safety case submissions we have identified several areas which are of concern due to uncertainty of the data and the accuracy of the analyses carried out using the information. It must be remembered our overall objectives are compliance with the SAPs and to determine whether the risk has been reduced to as low a level as is reasonably practicable. Our approach to assessment is generally to ask the "what if questions. The reason for this is: to test the key elements of the case; establish whether it has addressed the broader issues; has assessed all reasonable uncertainties adequately and is robust. It is also important to recognise that the public are becoming more aware of the issues surrounding nuclear power and are far more capable of understanding technical issues This has been demonstrated in recent

7 Structures under Shock and Impact 399 Public Inquiries in the UK. Any change in standards would be closely scrutinised. In the following sections a number of areas have been identified which we consider to be important in assessing the level of uncertainty in a safety case submission. These cover expert judgement, analysis and testing. Expert Judgement We consider judgements are inferences or evaluations which go beyond statements of fact or the conventions of discipline. Any judgement requiring expertise is an "expert judgement". Expert judgement is used in many areas of analysis, design and testing. Human judgement holds a central position in the analysis of difficult technical problems. For example, in general the decision of how to model a structure is based upon human judgements on the essential elements of a load resisting system and the distribution of mass. Expert judgement can also be used to allow for uncertainty. When using expert judgement, we consider a methodology is required to demonstrate the basis of the decisions which have been made. For topics where the number of experts is limited we have identified several concerns in the way in which justifications have been presented. We are concerned that a full range of views are considered. The reasons for the decisions made should be clearly laid out in an auditable trail which enables assessors to understand how arguments have been developed and conclusions reached. Uncertainty In Each Stage Of Analysis/Testing The design of a modern nuclear power station is a multi-disciplinary activity involving many design teams. With limitations on project timescales and lack of firm design information or uncertainty in basic data there is an understandable tendency for each team to incorporate some conservatism within its package, in part so as to be insensitive to input parameter changes over which it has no control. This approach to uncertainty is also incorporated in PI 66 of the SAPs, which states that the data used in any analysis should be demonstrably conservative. This creates particular difficulties in soil-structure interaction, where in the past many engineers have accepted that the soil modulus profile with depth should be bounded by half and twice the best estimate. No single set of soil properties is demonstrably conservative for all the multitudinous secondary and tertiary systems in the dynamic analysis. Attempts to incorporate the uncertainty through enveloping the floor response spectra are inevitably quite conservative.

8 400 Structures under Shock and Impact CURRENT AND FUTURE TRENDS As described earlier in the paper the role of Nil is to ensure that the appropriate standards within the nuclear industry are developed achieved and maintained by the licensee. To achieve this objective there is a need to keep abreast of current development and promote research where uncertainties exist and further knowledge is required. The need for further research is subject to review by both the licensees and NIL Part of this work is carried out under a programme of work which is currently funded by a levy on the licensees and is managed by the Health and Safety Executive. Current areas under review are: the validation of soil structure interaction analyses; the behaviour of shear walls under dynamic loading; ductility of shear walls and dynamic testing of structures. As discussed in the paper we consider much information can be gained from model testing which can be used to help provide confidence in methodologies used for design. Therefore the role of model testing should be promoted where appropriate. In addition further developments should be considered on: full scale dynamic testing of structures which may help validate analytical methodologies and investigations into the reliability of structures under dynamic loading. ACKNOWLEDGEMENT The authors wish to thank the Chief Inspector of Nuclear Installations of the Health and Safety Executive for permission to publish this paper. The views expressed are those of the authors and do not necessarily represent those of the Inspectorate. REFERENCES 1. HM Nuclear Installations Inspectorate. Sizewell B Public Inquiry: Proof of NII/P/2, HM Nuclear Installations Inspectorate. Hmkley Point C Public Inquiry: cc, HSE (Nil), Health and Safety Executive. Safety Assessment Principles for Nuclear, London, United States Nuclear Regulatory Commission. Standard Review Plan for the Review of Safety Analysis Reports for Nuclear Power Plants. LWR Edition. NUREG Formerly issued as NUREG -75/087. Revision 2 to SRP Section "Seismic System Analysis", November 1989.