An alternative approach is to take more advantage of inherent forces such as gravity in the design of safety systems.

An additional advantage is that this approach results in simplified systems since it can eliminate the need for some redundancy, e. g. in emergency power supply systems. Some of the important reactor types in this category are listed in Table 11.2. Details of the various passive features of these plants were given in the previous chapter.

In evolutionary passive designs there are three typical components to protect against faults and accident conditions:

— cooling of the core via natural circulation, e. g. in intact circuit faults;

— gravity-driven cooling systems to mitigate LOCAs, and;

— passive containment cooling systems (PCCS).

In passive systems, the active components, e. g. pumps, diesels, fans, etc. are dispensed with and so there is no need for the redundant active safety grade systems associated

Table 11.2. Evolutionary water reactors incorporating passive systems

Reactor Description

AP600, 1000 Innovative decay heat removal via passive systems

EPP

SWR 1000 Utilisation of natural forces and phenomena,

e. g. gravity, natural circulation, passive injection

ESBWR

VVER-640 Passive containment cooling

CANDU 6(E)

Table 11.3. Passive plant reduction in components

50% fewer valves 35% fewer pumps

80% less pipe (no safety grade pumps)

80% fewer heating, ventilating and cooling units (safety grade) 35% less seismic building volume 70% less cable

AP1000: Set to Compete (2002).

with active systems. This leads to simplification and hence potential scope for capital cost reduction. Similarly, the main ultimate heat sink is often the ambient air and hence there is no need for active service water systems.

Passive safety systems do not require the framework of safety support systems that are needed for current generation plant, including AC power, HVAC, cooling water systems and associated seismic buildings containing these components. Table 11.3 illustrates the consequential reductions in a number of key components compared with current generation plant (AP1000: Set to Compete, 2002).

Examples of evolutionary passive plants also include both light and heavy water reactors: PWRs — AP1000, AP600, EPP; BWRs — SWR 1000, ESBWR; and HWR — CANDU 6(E).

Considerable experience has been gained with lead-bismuth eutectic cooled reactors in the Russian Federation. This has been largely in connection with the development and operation of submarine propulsion reactors (IAEA-TECDOC-1289, 2002). Studies have been carried out by the Russian Federation Institute of Physics and Power Engineering (IPPE) and EDO Gidropress. Lead-bismuth offers some potential advantages, compared with lead, as a coolant and also some disadvantages; these are discussed below.

Lead-bismuth systems are being considered within the GIF Generation IV initiative. Several design concepts have been studied by the Russians. SVBR-75 is designed to produce 75 MWe and to operate for 10 years without refuelling. A smaller transportable combined heat and power version, ANGSTREM, has also been studied, generating 6 MWe. There is also a 25 MWe version being investigated by the Russians.

12.6.3.1 LFR (Gen IV). The main characteristics of the GIF reference design for lead cooled systems in general were discussed above.

12.6.3.2 BRUS-150. An integral type lead-bismuth reactor, generating 150 MWe, is being considered in the BRUS-150 project. All the lead-bismuth is contained in the reactor vessel, which contains the core, pumps and steam generators. This reactor is designed for the burning of weapons grade plutonium and the transmutation of minor actinides. In the present design of this (and other lead cooled) reactors, there is no intermediate circuit between the primary coolant and the water/steam secondary side. This is a concern in the event of steam generator leakage, which might result in Pb or Pb — Bi/water/steam interactions.

Pb-Bi and Pb share a number of similarities in terms of their thermal-hydraulic properties and also some advantages compared with sodium. For example, they have high boiling temperatures and relative chemical inertness compared with sodium. Pb-Bi has some advantages over Pb as a coolant in that it has a lower melting point (123.5°C) compared with Pb (327°C). A disadvantage in the use of Pb-Bi coolant is the formation of the volatile alpha emitter, polonium (210Po) produced from bismuth (and to some extent from Pb). Therefore, leakage poses a hazard to the operators and to the environment in the event of a cover gas release. Careful chemistry control of the primary circuit is also required to avoid the formation of lead oxide and other impurities.

It is concluded in IAEA-TECDOC-1289 (2002) that there are problems in applying much of the experience and data gained from the Pb — Bi cooled submarine studies to commercial-sized lead cooled power plants. This is because of the much greater annual load factor required, the higher temperature of the lead primary circuit, and additional corrosion phenomena at the commercial plant scale. Thus, there is considerable R&D required to extrapolate from the lead -bismuth submarine experience to the civil commercial nuclear plant situation.

12.2. MOLTEN SALT REACTORS

Molten salt reactor (MSR) technology has been available since the 1960s. It was developed at Oak Ridge National Laboratory (ORNL) and the MSR Experiment (MSRE) which operated for nearly 3 years during the late 1960s (IEA/OECD (NEA)/IAEA, 2002). Examples are listed in Figure 12.6.

The UNITHERM Russian reactor concept (Adamovich et al., 1997) is based on a transportable small-sized PWR concept that can be delivered to site in components. It would be appropriate for remote areas. It is designed for a life of 20 years with a single fuel loading for flexibility of location and with no requirement for a local cooling water supply. It comprises a dual loop system, the primary loop driving a turbine, the secondary system supplies steam or hot water for the intended process. The UNITHERM power plant was found (Grechko etal., 1998) to have a higher freshwater cost than fossil fuel plants but these plants could be found to be more economic than alternatives in remote access regions.

There has been considerable research over the years on whether steam explosions pose a risk to structural (containment) failure. Experimental programmes include FARO, KROTOS, ECO and BERDA (Jorge and Chaumont, 2001). There is evidence of pre­mixing of melt and water during the core relocation phase providing a mitigating effect. Experiments such as ECO (FZK) have shown that energy conversion factors now seem to be much lower than were originally envisaged. However, in, e. g., the French safety analysis process, in-vessel and ex-vessel steam explosion risks are still considered (Jorge and Chaumont, 2001). The R&D needs for in-vessel steam explosions are mainly concerned with gaining a better understanding of material effects and better characterisation of experiments (water, fuel and vapour fractions) (Seiler et al.). For ex­vessel melt, the main challenge is to establish the lack of explosive potential from realistic corium melt flow into water.

The nuclear industry may be unique among the industrialised industries in regard to the safety standards expected from it. Increasingly higher standards will be placed in the future. Having operated successfully (in the main) for over half a century and having met these standards, any lapse would be quickly seized upon. Thus the most important aim in the near future is to ensure that safe and reliable operation continues.

As observed earlier, there will be increasing emphasis on ensuring that the environment is protected from the operations of industries. The nuclear industry will have to meet increasingly stringent limits on radiological releases; it will need to pay greater attention to emergency preparedness planning, etc. As already stated, the measures that are being considered to reduce greenhouse gas release should benefit the case for nuclear power, which in this respect is a clean source of power.

For the de-regulated utilities, there is a need to create more investor confidence. In the US, for example there is evidence that investors now perceive the industry more positively. It is seen to be a stable industry in the more competitive market of today and can offer advantages over its competitors. In the recent years, low and stable operating costs have been realised. A number of large multi-unit sites are generating at a little under 2-2.2 cents per kW h (Sinco, 2003).

In many countries a commitment to new build will be a business decision against other generator alternatives. The target in the US is around $1100 per kW if it is to be competitive with combined-cycle gas.

Another factor in engendering investor confidence is to ensure a stable predictable licensing process in order to reduce uncertainties for the owner/utility. There have been moves towards design certification in both Europe and the US, which is an important step. However, there are still areas of uncertainty, e. g. associated with the time taken to gain the plant operating licence, following construction.

1.1. INTRODUCTION/OBJECTIVES

In the early days of nuclear power development, many different reactor types were considered and indeed prototypes were built. These included light water, heavy water, gas reactor and liquid metal-cooled fast reactor systems. The majority of the reactors in operation in the world today are light water reactors (LWRs) but there is also a sizeable fraction of heavy water reactors. Most of these reactors were built in the 1970s and 80s; only a few new reactors have been built during the last decade. Regarding other types, gas reactors continue to operate in the UK. There are only one or two prototype fast reactors still in operation, although interestingly both gas and fast systems are now starting to be reconsidered for next generation plants. These will be described in the succeeding chapters. Details will be given on the latest designs that are being proposed.

This is an introductory chapter to summarise briefly and review the designs of currently operating reactor systems. It presents the achievements of the technologies to date. It covers the principal reactors in operation today including light, heavy water, gas and other reactor types that have operated successfully, e. g. liquid metal-cooled reactors. The chapter defines the starting point for discussion of future designs in subsequent chapters. Thus, only the main features of the various reactor designs are highlighted below. Detailed descriptions of these reactors are included in a number of sources, see Leclercq (1986), Ramsey and Modarres (1998), Hewitt and Collier (2000) and Mounfield (1991).

The scale of current nuclear power plant operation worldwide is given in Table 1.1, which shows International Atomic Energy Agency (IAEA) data for 2002. This indicates that in 2002, there were a total of 441 units in operation in 30 countries, generating 358,661 MWe (Net) of electricity.

Nuclear safety is a major topic of interest within the activities of a number of international organisations, see for example Hall (1998). The IAEA has taken a lead role in promoting the role of atomic energy in all aspects. This includes operational safety and the agency is helping to set up regulatory frameworks throughout the world. The IAEA is helping to define common standards and understanding, one example is the setting of agreed event scales, which aim to provide a safety significance marking for a particular event. Another example is ‘safety culture’, recently reported on by the IAEA’s International Nuclear Safety Advisory Group. The intention is to instil an awareness of safety significance in all those responsible for the safety of a nuclear plant.

The IAEA carries out operational safety reviews through operational safety review teams (OSARTs). The OSARTs investigate particular operational safety issues, identify lessons learned and monitor corrective actions.

Another important international body is the World Association of Nuclear Operators (WANO), formed in 1989. The members of this organisation are solely utilities. WANO’s aim is to maximise the safety and reliability of nuclear power plant operation through exchange of information amongst its members. WANO conducts many different progra­mmes. These include exchange of operators between different stations. Operator exchanges have taken place between many Western and Eastern European countries. WANO has set indicators for plant performance (Table 3.1). Examples of such plant performance indicators relevant to safety include unplanned scrams, levels of radiation exposure, accident frequency and so on. WANO has also instigated various peer review programmes.

Prior to WANO, the Institute of Nuclear Power Operators (INPO) was set up by US utilities to promote safety culture in the US, working with other US organisations, the Electric Power Research Institute (EPRI) and the American Nuclear Society (ANS).

There have been a number of international initiatives to transfer Western safety culture to Central and Eastern Europe and Russia. Aid has been provided by the G7 countries via the European Bank for Reconstruction and Development (EPRD) and also by the European Commission through the Poland and Hungary Aid for the Restructuring of the Economy (PHARE) and the Technical Assistance to Commonwealth and Independent States (TACIS) programmes. Although much of this support has been spent on technical consultancy, a significant proportion has been spent on plant improvements and training of operator personnel.

Other activities either directly or indirectly supporting improved operator safety have resulted from the activities of the NEA of the OECD (NEA/CSNI/R, 2001).

Table 3.1. WANO performance and safety indicators

Unit capability factor — % of maximum energy generation that a plant is capability of supplying to the electrical grid

Unplanned capability loss factor — % of maximum energy that a plant is not capability of supplying to the grid because of unplanned energy losses

Unplanned automatic scrams per 7000 h critical — mean scram (automatic reactor shutdown) rate per year (approximately)

Collective radiation exposure — monitor of effectiveness of total personnel radiation exposure controls

Industrial safety accident rate — number of accidents resulting in lost work, restricted work or fatalities per 200,000 work-hours

Safety system performance — availability of three important standby safety systems at each plant

Fuel reliability — progress in preventing defects in the metal cladding that surrounds fuel

Chemistry performance — progress in controlling chemical parameters to retard deterioration of key plant materials and components during the operational lifetime

Collectively these international programmes contribute to improved operational safety of the world’s power plants. There are many areas of complementary and collaborative activities. The incident reporting system (IRS) for example is managed by both the IAEA and the NEA/OECD, and both liase with WANO. WANO collaborates with IAEA in many of its work programmes including the scheduling of peer reviews, operational safety review missions and so on.

The fuel design and fabrication process is clearly very dependent on the type of nuclear power plant in question. Only some gas cooled plants (e. g. Magnox) and some small reactors still use uranium metal fuel; all other types use uranium (or possibly mixed) oxide fuel.

Fuel design and fabrication costs are relatively small at about 3% of the nuclear electricity cost (Bertel and Wilmer, 2003). However, fuel design and fabrication have an important influence on the successful operation of the plant. There is strong competition among fuel vendors to achieve better utilisation and higher plant availability. Fuel designs to increase the discharge burn-up are an objective of current fuel vendors. Competition between vendors is also heightened since at the present time there is LWR manufacturing capacity around 50% in excess of annual requirements.

Future nuclear power plant operation will have to compete with coal and gas-fired power plants, certainly for large base-load operation (Hudson et al., 1999). Comparative cost estimates from the last OECD study were given in Chapter 2. Within the countries that provided data, nuclear power (at the time of the survey) was found, in about half the countries, to be the cheapest option at a 5% discount rate. However, not surprisingly, at higher discount rates the nuclear option becomes less attractive.

The main factors enhancing the competitiveness of evolutionary water-cooled reactors are summarised in Table 7.3. Simplification of plant design to minimise the number of systems, valves, pumps, etc. consistent with maintaining the plant’s safety envelope is a key objective. These, together with improved man-machine interfaces help to minimise operator demand and reduce risk. In the US, there has been considerable progress towards better co-operation between plant vendors and regulatory bodies in respect of the licensing process. The aim has been to develop the ‘one-step licensing process’. An important objective in achieving low costs is to use a standardised approach. Thus design and engineering costs can be amortised over many units, licensing costs can be reduced, construction methods can be optimised and operator training can be made more efficient.

Construction duration can be kept to a minimum by adherence to the above principles. A significant fraction of the design should be completed before construction starts. The EPRI URD has introduced a quantitative criterion that 90% of design drawings must be 100% complete. Modularisation whereby plant components can be assembled in a factory helps to ensure fabrication takes place in a controlled environment, also with more automation and higher productivity.

Another way to improve competitiveness is to aim for multiple unit sites. This can be more efficient by taking advantage of better construction scheduling and the use of common administrative buildings and facilities.

Thus much can be done to improve competitiveness by reducing capital cost, which contributes to over one half of the total generation cost of a nuclear plant.

Two countries whose programmes are characterised by standardisation and technology self-reliance are France and Korea. In the case of France, large series orders have characterised the French programme. A 2% productivity gain is claimed for each unit after the second one on a given site. Similarly in Korea, for the Korean Standard Nuclear Power Plant (KSNP), the total cost of the fifth and sixth units is 15% less than that for the first and second units. For the Korean Next Generation Reactor (KNGR), a 1300-MWe PWR, there is expected to be a greater than 17% capital cost reduction compared with the KSNP.

Changes in the economic landscape associated with de-regulation of the electricity market pose particular challenges to capital intensive technologies such as nuclear energy. Flexibility in generating strategies is likely to be a requirement, e. g. building smaller size plants with relatively low investment costs and shorter pay back times. This would be coupled with a requirement for simplified technologies and infrastructure.

Concerning external costs or benefits related to electricity production costs (but not directly carried by producers or consumers), there are issues associated with job creation, resource management, sustainability and health and environmental impacts of emissions. Of these, environmental impacts are potentially the most significant. A European Commission study showed that external costs for nuclear power are lower than those for coal and gas due to the greater environmental emissions of fossil fuel plants.

For the French plants, the costs associated with health impacts were on average, 0.022 million per kWh for the current 1300 MWe plant design compared with 0.026 mill per kWh for the 900 MWe plant. For normal operation, the differences between the two types of PWR were not significant.

The Netherlands’ sole remaining nuclear plant, Borssele, continues to operate (Foratom e-Bulletin, 2003c) but in 2002, it only contributed to 4% of the electricity generation (International Atomic Energy Agency, 2002). In 1994, the Netherlands declared a moratorium on the building of new nuclear power reactors (European Commission, 2000).

The Dodewaard BWR was closed for economic reasons in 1997, and the last remaining spent fuel assemblies have been shipped to Sellafield in the UK. The plant site will be decommissioned with the intention of returning to a green field site after 40 years.

The Netherlands is however looking to ensure continuation of medical isotopes production after the Petten research reactor reaches the end of its operational life in 2015. The intention is to site a new research reactor at Petten (Foratom e-Bulletin, 2003b).