Severe accident research has been the main focus for reactor safety research over the past two decades. This largely started with the TMI-2 accident with phenomenological research, and the need to reduce severe accident risk further was re-enforced by the Chernobyl accident in 1986. For existing plants (Krugmann, 2001), measures have been introduced to reduce severe accident vulnerabilities, such as primary and secondary feed and bleed, filtered containment venting, hydrogen control by recombiners, igniters or by inerting, and filtration of control room air intake. For new designs, the IAEA has set more restrictive technical safety objectives (IAEA, 1999) such as severe core damage frequency less than 10 5 per plant operating year, elimination of sequences that could give rise to large early releases, and prevention of containment failure, thus limiting the need for off­site protection measures. These objectives have led to greater emphasis in reducing severe accident risk in the newer evolutionary designs.

There have been numerous research programmes over the past few decades to develop understanding of severe accident-related phenomena and also to develop guidelines for the prevention and mitigation of severe accidents. Much knowledge has been gained and at the present time, there is a reduction of effort on severe accidents R&D worldwide. Some research workers (Krugmann, 2001) believe that sufficient knowledge of severe accident phenomenology now exists. Confirmatory research, however, is still in progress in some areas. Also, clearly further work may be required to support a particular design in the event of new building. Recent research programmes are summarised in Table 15.5.

There have been relatively slowly changing trends in energy infrastructures over the past few decades but the mix of future energy providers is likely to change in the future. A number of countries are reviewing their energy policies for some time in the future. For example, the UK government published an Energy White Paper (Energy White Paper, 2003) in 2003, which proposed an energy policy looking forward to the year 2050. The paper covered all forms of energy requirement, from electricity generation, heating and lighting to transport, industry and communications. It was based on in-depth analysis following a report published by a UK-appointed strategy unit in 2002 (The Energy Review, 2002). Other countries are performing similar reviews, see e. g. the forward vision to 2030 published by VTT, Finland (Energy Visions 2030 for Finland, 2003). The strategy for the UK is outlined below, by way of example.

The major global challenges that need to be faced are:

— environmental and climatic change from carbon dioxide levels increase;

— decline of the world’s indigenous energy supplies, from oil, gas, and coal and how these may be replaced (e. g. by nuclear, renewables);

— the need to update national energy infrastructures over the next few decades to meet new energy mixes.

The goals of most of the industrialised countries are to:

— reduce carbon dioxide emissions with specific targets. In the UK the goal is to cut carbon dioxide emissions by some 60% by about 2050, with significant progress by 2020. Many, but not all, countries support the Kyoto Protocol;

— maintain reliability of energy supplies;

— promote competitive markets, raising the rate of sustainable economic growth and improving productivity. There is an increasing trend toward deregulation;

— meet other energy (non-electrical) requirements for industrial and domestic supply (e. g. to ensure every home is adequately and affordably heated).

To meet these goals, it is likely that an energy system will be required that is quite different from that of today. Much more diverse systems are envisaged. These will include a balance between imported energy and fuel, a mix of large power stations, that could include offshore marine plants, including wave, tidal and wind farms and also onshore wind farms. There would be an increase in local generation, including biomass, local wind and tidal generators and micro-generation from combined heat and power (CHP) plant, fuel cells or photovoltaics. Energy efficiency improvements would be expected from improved home design. Gas might be expected to form a large part of the energy mix whereas coal fired generation would either play a reduced part or be linked to carbon dioxide capture and storage.

There have been debates in many sectors (industry, learned societies), etc. on how goals for security of energy supply can be achieved and there are many different opinions. In the UK for example, the future of energy was the focus of the 2002 Parliamentary Links Day, organised by the Royal Society of Chemists (http://www. rsc. org/lap/parliament/linksday. htm). This included an audience of distinguished scientists and politicians and covered energy-related activities taking place in government and industry. The scope was broad across the energy spectrum, covering nuclear and non-nuclear, electrical and non­electrical applications.

Many of the presently operating nuclear plants will be shut down over the next two decades. In the UK, by 2020, the existing AGR nuclear power stations will almost all have reached the end of their lives and all the Magnox stations will have shut down. However, new build continues in Asia and some new plants are likely in Europe in the next few years. Nuclear power remains an option for the future for the UK. However, the Government White Paper did not propose it and stated that before any decision to proceed with the building of a new power station, there would need to be the fullest consultation and publication of a White Paper setting down the Government’s proposals. The arguments for a delay were both on economic grounds and concerned with the issue of waste disposal (sustainability).

It is increasingly recognised internationally (within the EC, US and Japan, as described in the section on hydrogen generation) that the ‘hydrogen economy’ has significant benefits as a clean and flexible energy system. In a report to the Parliamentary Links Day, the UK Government’s Chief Scientific Adviser also anticipates a significant move towards a hydrogen economy by 2020 (http://www. rsc. org/lap/parliament/linksday. htm). This view is also supported by the UK nuclear industry (Clegg, 2002) and others. The issue is how to produce hydrogen without releasing carbon dioxide.

In the Section 17.4, world events of recent years are examined. After that, a discussion is given on how these and other developments may shape developments in the near future. The remaining sections continue to look further into the future, covering likely developments over the next half-century.

It is likely that nuclear power within particular sectors will decline over the next 20 years. However, increasing competition will encourage utilities to seek plant life extensions, tending to slow this decline and contribute to reducing carbon dioxide emissions. It is probable that with appropriate investment and refurbishment, the lives of some plants may extend up to 60 years and beyond.

Many present-day reactors are now approaching the end of their design life. There are considerable efforts to extend the operational life of such plants by various means such as backfitting of systems, changes in operational practices, etc. For many countries, the economics of extending the life of existing plants, compared with the capital costs of building new plant, is very favourable. However, the Chernobyl accident in particular has shown that reactor safety is an international concern and economic benefits have to be considered against global acceptability. Decisions on the extension of life depend on a range of technical issues, principally materials performance, chemistry and availability of sophisticated inspection techniques. These and other more general issues (Table 2.9) are reviewed in this section.

Table 2.9. Extension of lifetime issues

Technical feasibility — effect of the processes of ageing?

Plant safety for intended period of operation — ageing of critical safety components? Regulatory framework — establishment of procedures for licence extension?

Social acceptability in national climate — changes during plant lifetime and public perception? Economic considerations — are the economics favourable?

The plant operating envelope is agreed between the licensee and the regulator as part of the plant safety case. It is usually defined (Pershagen, 1989) by a set of rules and guidelines to ensure safe operation of the plant but also containing some degree of flexibility to enable the plant to operate in an optimal way. The degree of optimisation or the operating margins that can be achieved must be compliant with these rules and guidelines.

They include technical specifications, Table 4.2, which define bounding values for key safety-related parameters. If exceeded, the plant would need to shut down and the regulator would require a full investigation before operation could restart. There are requirements on the functioning of safety systems and components in order that the conditions of plant operation are met. If not all these requirements are met a reduced mode

Bounding values for the safety parameters and reporting arrangements to safety authorities if limits are exceeded

Allowable conditions for plant operation, including systems availability — how operations must be restricted if such systems functions are not in place

Specification and schedule for testing and inspection of components and systems — restrictions, if testing is not carried out or functionality is impaired

Rules for both normal and abnormal operation — reporting procedures for operational events and design modifications

Pershagen (1989).

of plant operation may be imposed. Conversely, a more optimised mode of operation may require more stringent performance of the systems and components, possibly a need for plant modifications. Similarly, the degree of optimisation that can be achieved may depend on the outcomes of inspection and testing programmes. Finally, any change in operating conditions must meet the rules for both normal and fault conditions.

The operating rules cover all plant states from start-up to shut-down and in all modes of plant operation. These are documented in detail and may be updated in the light of new experience on changes in plant, e. g. modifications.

The strategy in most decommissioning activities is to reduce the radiological hazard in a systematic way, until the delicensing condition for the site is reached. As noted earlier, after operations have ceased in a reactor, the removal of fuel reduces the hazard by a significant degree. After that the POCO will result in a further reduction in level.

In general, radioactive decay will result in a reduction of radioactivity and deferral of operations may be of benefit. However, there may be an issue if radioactive daughter chains exist producing isotopes that present greater problems than with the parent isotope. For example (Twidale, 1999), the 241Pu isotope primarily emits Beta radiation but it has 241Am as a gamma emitter. Furthermore, the parent isotope has a half-life of 12 years but the daughter has a half-life of 432 years.

The activation of the construction materials of the reactor and the presence of gamma-emitting isotopes are a significant problem in decommissioning. The areas concerned are the core internals, the biological shielding and the pressure vessel. The most problematic isotopes are 60Co, 108Ag and 94Nb, which have half-lives of 5.27, 418 and 20,000 years, respectively. It is therefore possible to achieve reductions in activity from, e. g. 60Co after a timescale of several decades, but the other problematical radioisotopes will remain.

The quantity of activated components varies considerably with the type of reactor and the size of the vessel. A large PWR has a reactor vessel of diameter 4.5 m and total weight 600 Te compared with a Magnox reactor that has a vessel of about 20 m of weight 5000 Te. The PWR vessel can be moved as a single item but this would not be possible for a Magnox reactor (Twidale, 1999).

Many of the principles for design basis assessment have been established for present-day reactors over many years. These include the ‘defence-in-depth’ principle, needs for diversity and redundancy, Safety Analysis Report (SAR) assessments and so on. In this section, the principal design basis approaches that are likely to apply to reactor licensing in the future are reviewed.

Most of the licensing submittals for present-day plant have been submitted using the conservative evaluation model (EM) methodology (Table 8.3). Conservative modelling was required to overcome lack of detailed knowledge of the phenomena. This methodology is commonly referred to as ‘Appendix K’ referring to the relevant appendix in the US CFRs (10 CFR 50).

BE methods are likely to be a common goal for licensing in the future, including those for future reactors. BE methods have been accepted by the USNRC (and other regulators) c. f. Appendix K Revision in 1988.

The acceptance criteria for fault studies are established by good understanding of the physics of present designs. However, as designs evolve, these criteria may need to be re-evaluated. The additional changes may also be required to accommodate extensions in, e. g. mode of operation.

There are likely to be increased requirements for Probabilistic Safety Assessment (PSA) studies (Table 8.4) to underpin deterministic studies and to help estimate doses to the population, source term of release, etc. Probabilistic targets are likely to become more

Table 8.4. Safety approach for severe accidents

PSA approach Selection of most probable sequences leading to a core melt

Provision of preventative or mitigative measures Wide coverage of possible sequences

Good quantification of the benefits from proposed measures Depends on the status of PSA accident analysis

Deterministic safety Definition of containment challenges from core melt behaviour

analysis approach Assurance of containment integrity by design measures

stringent, e. g. on core damage frequency or on containment limits. The current trend is to use best estimate methods for the frequencies and probabilities in PSAs.

The move towards BE methods is being supported by regulators and utilities because more realistic margin estimates enable a better quantification of actual risk to be obtained and enable a wider operating window.

However, there are developments required before the methodology is likely to be regarded as a mature engineering tool. It is necessary to be able to quantify unbiased uncertainty limits on key parameters (e. g. peak clad temperatures) and as yet the methodologies are not yet very practicable for licensing studies.

Another factor is that there is generally reluctance to change from an established methodology that is accepted by all parties.

Looking further to the future, risk informed methods (Wahlstrom, 2003) are being put forward by the USNRC but these methods require further development. Traditionally, the safety of NPPs has been justified by a deterministic approach based on the defence — in-depth principle and single failure criterion for design basis accidents, etc. Probabilistic approaches have further developed and now the probabilistic safety analysis methodology is becoming well established. These provide a means of taking a systematic approach to determining the probability and therefore risk of various failure sequences.

The idea of risk-based or risk-informed approaches is to focus on the most important issues in terms of risk. If PSAs are used to determine the risk then clearly it is important that there is confidence in the PSA methodology used. Risk-informed approaches can be applied to new reactor design or indeed to assist modifications of old plants, to target maintenance actions and inspections. PSA methodology can also be used to identify the safety categorisation of components.

The USNRC has made a commitment to move towards a risk-informed regulatory regime. Other regulators are considering the development of the approach.

The notions of design basis and defence-in-depth have been well established in the licensing of present generation reactors. For some future systems, these notions may need to be revised in the light of newer technologies with very different designs, materials and fuel cycles.

11.1. INTRODUCTION/OBJECTIVES

The purpose of this chapter is to focus on how the design basis for evolutionary water reactors is being extended. The approach continues to be based on the defence-in-depth but a major difference is to attempt to include more severe (core melt) accidents within the design basis. This is achieved in evolutionary designs by the adoption of new technical features, not only to protect against present design basis events affecting the core and primary circuit, e. g. loss of cooling accidents (LOCA), steam line break (SLB) and steam generator tube rupture (SGTR) but also ultimately to protect against early and late containment failure.

Many evolutionary plant designs incorporate passive safety systems in place of active systems but in other respects do not vary substantially from current generation designs. In this chapter, the focus is again on water reactor technology for power generation since these reactors are such an important class of interest. Reviews of advanced light water reactor designs are given in IAEA-TECDOC-968 (1996), covering evolutionary medium — and large-size reactor designs for power generation. Further review of evolutionary designs including strategic issues and economic viability is given in IAEA-TEC- DOC-1117 (1999). A common feature is that decay heat is removed from the primary circuit to large tanks or pools via natural circulation. There are some new phenomena associated with decay heat removal in advanced designs with such components that are not found in present generation reactors. These are discussed in Relevant thermal-hydraulic aspects of advanced reactor design (1996). An issue here for the plant designer is to ensure that such systems have sufficient heat capacity and also initiate as intended. In addition, reactor coolant inventory is maintained using passive injection rather than active pump injection.

Different containment designs have been proposed, utilising steel, concrete or composites. Heat removal may need to be via natural circulation cooling of the containment wall in the case of steel or enhanced in concrete based containments using passive heat exchangers. These and other passive systems are covered in this chapter.

The design basis for the containment has traditionally been that it must survive the peak pressure arising from a double-ended guillotine break of the largest primary or secondary pipes. The design basis for more advanced plants will have to cover a broader selection of accident sequences, perhaps including significant core melting. This selection will be based on a combination of probabilistic and deterministic analyses. The lowest probability high consequence sequences will still need to be covered by engineering judgement or other means. There will be a tendency for deterministic analyses to be carried out by best estimate rather than conservative methodologies.

Advanced evolutionary water containments include other measures to ensure they survive under severe accident loads. Measures (IAEA-TECDOC-752,1994) are introduced to prevent fuel coolant interactions (FCIs) to prevent direct containment heating (DCH) and to control hydrogen. They are also designed to reduce the source term by improving leak tightness. This is achieved via inherent safety features in the design, utilising passive heat removal systems in many cases. In addition to internal events, external events such as aircraft crashes and seismic events are also receiving special attention.

A number of more revolutionary designs of water reactor have been put forward as ‘inherently safe’ designs. These eliminate almost entirely active systems, e. g. relying on reactivity control via careful management of boron concentration. Some of these approaches are also summarised briefly although these are unlikely to be developed further at the present time.

There are clearly significant areas of research required to realise the ADS technology. Some broad scope areas are given in Table 13.4. These relate to general requirements needed for most of the different fuel cycles and applications. There are also particular engineering-related materials issues associated with radiation damage, and the need to extend the methodologies developed for critical reactors to the more complicated ADS-coupled transport situation.

Severe radiation damage can occur as a consequence of high current, medium-energy protons being injected into the target (Takahashi and Gudowski, 1997). Neutrons and charged particles are generated at energies reaching those of the protons causing radiation damage to the target and surrounding structural materials. This stems from the

Table 13.4. Research requirements

Transmutation of commercial power plant waste, particularly reactor grade plutonium Deployment of weapons grade plutonium in power production Assurance of proliferation resistant fuel cycle Benefits and utilisation of the thorium fuel cycle

Impact of different ADS options on radiotoxicity of the fuel cycle reduction Materials-related research, e. g. radiation damage of the target regions ADS safety issues and their resolution

Methodologies development for ADS, e. g. necessary developments of critical reactor models

displacement of lattice atoms within the target and from the energy the atom receives following emission of a nuclear particle, e. g. g ray (Wechsler et al., 1995).

The primary concerns on the effects of damage relate to hardening and embrittlement and the changes in mechanical properties and stability. The embrittlement is characterised by radiation defect clusters, helium aggregation to form bubbles, ductile brittle transition effects, and impurities arising from transmutation products.

The areas of particular damage will be surrounding walls and the window, which therefore needs to be replaced frequently in high-energy accelerators. Thus, damage is likely to be worst for a high-power accelerator with a large sub-critical reactor. This may be mitigated by adopting a concept with a smaller current and smaller sub-criticality. Similarly the structural damage in an accelerator driven system might be expected to be higher than in a corresponding critical reactor (Takahashi et al., 1994).

The adoption of suitable materials for the beam window section and the target side walls is a subject for research.

BREST 300 is a lead-cooled, pool-type fast reactor design operating at close to atmospheric pressure (IEA/OECD (NEA)/IAEA, 2002). The reference rating is 300 MWe. It has been put forward by RDIPE, Russia. It incorporates a loop concept for primary circuit heat removal. It is based on a relatively simple and robust design with passive decay heat removal to the environment. It has similar characteristics to those of the other lead — cooled reactors described above.

It has an increased core outlet temperature compared with the PWR making it a better candidate for somewhat higher temperature process heat applications.

14.6.2 Energy Amplifier

Lead-cooled subcritical reactors driven by a proton accelerator, such as the energy amplifier, are also being considered for process heat applications (IEA/OECD (NEA)/IAEA, 2002).

15.8. FUTURE REACTOR RESEARCH

Research programmes for the innovative designs described in Chapter 12 are described in IEA/OECD (NEA)/IAEA (2002) and Background Report for the Three-Agency Study (2001). Compared with the level of R&D investment in the performance and safety optimisation of current generation reactors over the years, and in evolutionary designs, the level of investment in future generation reactors is small at the present time.

To facilitate further research, it will be advantageous to set up collaborative international R&D programmes if possible. However there are many diverse designs under consideration and collaboration will only be possible if there are common interests in a particular field or topic. There are also the issues of commercial interests and the sharing of proprietary information to be addressed.

It is suggested in IEA/OECD (NEA)/IAEA (2002) that the setting up of a comprehensive experience database may be a useful initial activity in a collaborative relationship. Reactor designers could access this database to collect information on existing experience on the advantages and disadvantages of different reactor types.

Below are sections on the areas of research that are likely to be required for future innovative reactor systems. There are programmes already in place on research of some evolutionary systems issues; these are seen as a step towards developing the later systems. The discussion in these sections focuses particularly on the designs put forward by the GIF for Generation IV systems.

In summary, there are many R&D activities that will need to be accomplished before most of the innovative systems are available. The main technical developments for the Generation IV systems are summarised in Table.15.7. Some of these R&D activities have already started, e. g. for the nearer term SCWR and HTR concepts. SCWR activities have been ongoing since 2000 in the US, Canada, Japan, South Korea and in the EU, on materials and corrosion research. For the HTR concepts there are plans for the building

Table 15.7. Generation IV technology research

R&D activities

The US Generation IV Implementation Strategy (2003), Newton (2002) and Institute of Nuclear Engineers (2004).

of a Next Generation Nuclear Plant (NGNP) at Idaho in the US for R&D as a step towards the VHTR. There are also plans for an experimental technology demonstration reactor (ETDR) looking forward to the advent of GCR technology.