GCFR systems have been considered in the past but were not developed. The GCFR is now being considered as a longer-term option in the second phase Generation IV programme (GEN IV-B) (Sinco, 2003).

The GCFR is one type of fast neutron system that is being put forward for use in a closed fuel cycle, thereby reducing the problem of long-term proliferation concerns (Overview of Generation IV Roadmap). The GCFR shares many of the attributes of the high-temperature thermal reactor with high outlet temperatures, enabling efficient electricity generation, hydrogen production or process heat applications. It has the additional benefit of enabling the full recycle of actinides minimising long-lived radioactive waste. Being a fast spectrum, it would utilise fissile and fertile fuel more efficiently than the high-temperature thermal systems with a once-through fuel cycle. Fast spectrum systems such as those based on GCFR technology are not expected to be available as early as the thermal systems. GCFRs are projected to be available commercially towards 2030-2050.

Sodium-cooled fast reactor (SFR) technology has been established over several decades and medium-scale prototype plants have been built and operated in several countries, including, e. g., France, UK, and elsewhere.

The SFR is also a Gen IV-B technology which is being put forward at both a medium — and large-size scale (Sinco, 2003). It is seen at present as mainly for the management of plutonium and other actinides and high-level waste (Overview of Generation IV Roadmap). As with all fast spectra systems, it offers an efficient utilisation of fissile and fertile materials in a closed fuel cycle. It is possible that it could be used as an electricity generator but at present capital costs are too high. A 2030-2050 timescale is again the projected timescale for the commercial SFR.

The lead-cooled fast reactor (LCFR) is another Gen IV-B system (Sinco, 2003). It utilises lead or lead — bismuth eutectic cooling in a fast spectrum system with the attributes of full actinide recycle fuel cycle and efficient conversion of fertile uranium (Overview of Generation IV Roadmap). It offers the prospect of a very long core life up to around 30 years with the obvious proliferation benefits.

It could be put forward at a range of different ratings from a small ‘battery’ scale, a medium-scale modular version, or a large scale of the greater than 1000 MWe range. It, therefore, offers a flexible option for distributed generation of electricity on small grids and for other energy products, including hydrogen products or desalination, through to large-scale electricity generation. The LCFR requires significant materials advancements for application in corrosive high-temperature environments. It is not expected to be available commercially until around the 2030-2050 timescale.

The molten salt reactor (MSR) is another Generation IV technology. It offers a full actinide recycle within an epithermal spectrum reactor system (Overview of Generation IV Roadmap). It is envisaged as a large-scale plant of the order of 1000 MWe operating with a high outlet temperature with therefore good thermal efficiency. It is a flexible system offering efficient utilisation of plutonium and MA management. As currently envisaged, there is a relatively complicated heat exchanger system with a large number of sub­systems. Therefore, the economics are less favourable than for some of the other future plants that are being proposed. Its main application would be for electricity generation and plutonium and actinide destruction. The timescale for a commercial plant would also be around 2030-2050.

17.7.2 Accelerator Driven Systems

Accelerator driven systems (ADS) are hybrid systems combining a subcritical reactor together with a high-energy particle accelerator in order to produce a self-sustained reaction. ADS can be designed for both fast and thermal neutrons systems. They can utilise different fuel forms (solid, liquid), different fuel cycles, and different coolants and moderators. These have similarities with corresponding critical reactor systems, both in terms of the materials used and the applications that are possible. The objective of some ADS is the nuclear transmutation of Pu and MA in waste, with or without energy production; the objective in others is to utilise the thorium fuel cycle for energy production (IAEA-TECDOC-985, 1997).

Fast neutron systems are available with U/Pu solid fuel cycles, Na or Pb cooled, also with U/Pu liquid fuel with molten chlorides or Pb/Bi; both being suitable for MA incineration. The Th/U solid fuel cycle is Pb cooled and suitable for energy production or waste transmutation. Thermal ADS include solid Pu fuel systems with heavy water, for Pu weapons burning. There are quasi-liquid U/Pu graphite particle beads systems, He/heavy water-cooled, for MA management. There are liquid fuel systems encompassing U/Pu with molten salt for Pu, MA and FP management; Th/U with molten salt for energy production and U/Pu with heavy water for MA and FP transmutation, and energy production. Most concepts are based on linear accelerators, but some on a proton cyclotron concept.

ADS have some advantages and some disadvantages compared with critical reactor systems (NEA/OECD Expert Group Study, 2002). In terms of advantages, they allow the possibility of operating with a neutron multiplication factor of less than unity. They can be designed as pure transuranics (TRU) or MA burners and therefore would minimise the fraction of dedicated transmutors on a site. Reactor power is proportional to accelerator current, which simplifies control. From a safety perspective, the reactivity margin to prompt criticality can be increased, without dependence on delayed neutrons. Excess reactivity can be eliminated, allowing more flexibility in core safety design.

With regard to disadvantages, there is a reduction in net plant efficiency and the overall plant is more complex. The accelerator must have high reliability against thermal shocks. There are extreme stress, corrosion and irradiation loads on the beam window and target. There is also increased power peaking because the neutron source is external. There are compromises that have to be made between the neutron multiplication factor and the power produced. From a safety point of view, there are new types of reactivity and source transients that need to be taken into account, because the external neutron source can vary rapidly and the feedbacks from TRU and MA cores are weak.

Finally, in this last chapter, a few comments are made on the status of fusion research.

The fusion reactor is still on the horizon for long-term energy generation. It is difficult to forecast the timescales for the development of the technology as a commercial power source. The UK Energy White Paper anticipates that nuclear fusion will be at an advanced stage of research and development by 2020 (Energy White Paper, 2003). Other commentators believe the reactor will still be in the development phase by 2030 (Energy Visions 2030 for Finland, 2003). Commercial realisation is unlikely to be before 2050 + . The fusion reactor is more attractive as a sustainable energy resource than the fission reactor since there are limitless fuel resources, there are no long-lived nuclides in the waste produced and the worst accident situations are of relatively low consequence.

Fusion research has and is being conducted in a number of collaborative international programmes. During the 1990s, the Joint European Torus (JET) project has made progress in generating significant amounts of energy. For the future generation of Tokamaks, interested nations will participate in the International Tokamak Experimental Reactor (ITER) project.

Even without the building of new nuclear plants, IAEA projections indicate that global nuclear generation will continue at least at the present level or higher, until around 2020. Large decreases in Western Europe and to a lesser extent in the US will be compensated by significant increases in the Far East and to a lesser extent in Eastern Europe.

Decisions on nuclear power continuation will be country dependent and will depend on the perceived benefits against the risks and alternatives for other forms of energy generation. There will be strong economic competition from the fossil fuel generators, e. g. combined cycle gas plants.

In deregulated industries, for nuclear new build, there will need to be frameworks in place to enable power companies to accept their large capital investment risk, in particular for them to have confidence that building cost forecasts and construction schedules can be met. There is also the issue of long-term operational risk (stability of electricity prices) and eventual decommissioning costs. Finally, risks arising from delays in the regulatory licensing process must be acceptable. There is progress in some countries towards resolving these issues, e. g. in the US.

For new build, there will most likely be a need for a nuclear obligation from governments to enable suppliers or operators to sign up for long-term contracts. It will also be necessary to put in place some kind of Price-Anderson act to limit insurance risks.

Another important factor regarding the continuation of nuclear power will be whether an acceptable solution to the legacy and future waste problem becomes available. Further, utilities will probably require some type of fixed price contract from governments for managing their waste, i. e. governments will have to accept liabilities for waste.

The long-term future of nuclear energy may be influenced by increased global environmental legislation to limit carbon emissions, if the rate of ‘greenhouse’ gases continues to rise. On the assumption that the latter does occur, nuclear power will need to compete for acceptance against alternative carbon-free (renewables) energy generators. The economic case will depend heavily on whether there exist carbon premiums on generation, e. g. carbon taxes or permits.

Miscellaneous small reactors are needed for many different applications including materials testing and irradiation, isotope production, and reactor and nuclear physics training. Further applications include neutron detector calibration, neutron activation trace element analysis and delayed neutron counting for evaluating fissile content and basic research applications.

A matter of growing concern is the reducing numbers of such reactors that remain in service. However, many of these reactors are ageing and are approaching 50 years of life. They are, therefore, reaching the end of their operational lives. In particular, the EC is currently evaluating the future needs of material test reactors in Europe (Parrat et al., 2003) which provide valuable services within Europe and worldwide. Materials testing facilities are likely to be needed for the development of some of the advanced Generation IV concepts that will include corrosive materials resulting in chemically and physically demanding environments.

Most of the therapeutic isotopes required by industry are currently produced using neutron irradiation in research and small reactors. However, with a potential 10-fold intensity increase in compact cyclotrons, some charged particle reactions are becoming accessible for producing some of the newer isotopes. Reliance on research reactors may diminish as accelerator-based techniques are developed and able to provide adequate technical capability at prices industry can support (Lewis).

There are many novel applications of nuclear energy in medicine at various stages of development. Examples include boron neutron capture therapy (BNCT), a technique being pioneered at Birmingham University for the treatment of cancer. This involves injecting boron into the patient, which concentrates in the affected organ and which is then irradiated. Another example involved a technique that has recently been applied in Italy, where a patient with liver cancer, had the organ removed, irradiated and replaced with successful remission of the tumour.

There are fewer nuclear engineering degree courses now available at the Universities and fewer small reactors available for teaching purposes. In the UK, collaborative research programmes between academia and industry are being undertaken by the University of Birmingham. Current projects in the Nuclear Physics Group relate to modelling of nuclear materials assay equipment and the study on nuclear waste transmutation (http://www. np. ph. bham. ac. uk/research/npt. htm). Academic research is conducted at the Imperial College research reactor, situated in Silwood Park (http://www. imperial. ac. uk/publication/pbb/ env_sci/intro. htm).

Other interesting applications of nuclear energy concern topics such as food irradiation. This is a growing international business (www. sercoassurance. com/answers). The process involves the use of high-energy gamma radiation, produced by a source, to kill bacteria in food and preserve it. Other possible applications include sterilisation of materials and implements for the medical industry.

Small reactors operate with different fuel cycles compared with large power reactors. There are research reactors of diverse design in a number of countries, including Australia (heavy water), India (pool type) and Japan (fast reactor) (http://www. world-nuclear. org/ info/inf61.htm).

17.7.1 Water Reactors

Light water reactors are the most widely used type of reactor in service at the present time and much work is taking place in optimising the performance and safety of advanced evolutionary designs. A similar approach is being adopted in the development of evolutionary heavy water reactors The emphasis has been to improve the operating economics and also to simplify design to reduce construction costs.

Recent focus has been on large power generation (1300-1500 MWe) but smaller and medium-sized plants are in consideration. There is an increased tendency to introduce more passive systems but some passive safety systems are less appropriate for large power generation.

More innovative types of water reactors are being considered within the first phase of the Generation IV programme (Gen IV-A) (Sinco, 2003). The supercritical water-cooled reactor (SCWR) is a high-temperature super-critical pressure reactor that could be developed from present water reactor technology (Overview of Generation IV Roadmap). It would be primarily for electricity generation. However, there are two core design options, offering an open fuel cycle with a thermal spectrum or a closed fuel cycle with a fast spectrum to enable actinide management. The projected time for commercial deployment of the thermal spectrum option is around 2020-2030. Table 17.3 shows approximate timescales for the different advanced nuclear reactor technologies.

Gas reactors have been operating successfully in the UK over many years. High — temperature gas reactors has been operated in the UK, US and Germany and new smaller plants are in operation in China and Japan. There is a revived interest in HTRs and in particular the South African Pebble Bed Modular Reactor (PBMR) (Clegg, 2002; http:// www. bnfl. com/website. nsf/researchmenu. htm; Hittner, 2002). R&D activities can, there­fore, build on considerable previous experience.

The very-high-temperature reactor (VHTR) system is another one of the thermal reactor types under consideration in the Gen IV-A programme (Sinco, 2003). It would operate at very high core outlet temperatures, > 1000°C and have very high efficiency compared with current generation plant. It could be used for electricity generation or hydrogen production using water cracking technology (Overview of Generation IV Roadmap). It could also be used in the process heat and chemical industries. A timescale of 2020-2030 is envisaged for commercial deployment of these systems.

12.6.2.1 BREST-300. Lead cooled reactor systems are under study at the Institute of Physics and Power Engineering (IPPE) and the Kurchatov Institute (IAEA-TEC- DOC-1289, 2002).

In the BREST-300 designs, developed by RDIPE and Kurchatov there is a two circuit design, there are four parallel loops including pumped lead flow removing heat from the reactor core. Lead inlet and outlet temperatures are 420 and 540°C, respectively. The design is integral with a supercritical pressure (24.5 MPa) steam water cycle. The uranium and plutonium nitride fuel implies low moderation and absorption of neutrons hence it is possible to achieve a core breeding ratio equal to one.

The BREST-300 reactor has various safety features such as negative void temperature coefficient; it operates with a breeding ratio of near unity with consequently minimal excess reactivity and there are no soluble poisons in the reactor coolant (IEA/OECD (NEA)/IAEA, 2002). Regarding the coolant, there is decay heat removal by passive systems, increased reactor coolant inertia, and the system pressure is low.

It has good thermodynamic efficiency due to high core outlet temperature, reduced number of components in the nuclear steam plant and reduced containment design requirements. The relatively small size implies reduced capital cost and this together with increased core outlet temperature means that the plant is also applicable to process heat applications.

There are, however, some penalties in using lead. There is a much greater pressure drop (about 7 times greater than sodium) across the core for otherwise similar conditions of reactor power, coolant flow cross section area in the core and fuel element length. This is caused by the lower thermal capacity of lead compared with sodium. The higher density of lead compared with sodium does not compensate. Lead cooled reactors therefore need to have a reduction in fuel fraction and increase in core diameter to reduce the hydraulic resistance. This implies that the core dimensions of the BREST reactors are large.

12.6.2.2 BREST-600. The plant has been scaled up to 600 MWe by RDIPE in co-operation with RRC Kurchatov (IAEA-TECDOC-1289, 2002). The characteristics of BREST-300 and BREST-600 are similar.

There is also an active programme on lead cooled reactors in Japan.

12.6.2.3 LCFR. Design studies of lead cooled fast reactors (LFRs) with nitride have been performed by the Japanese (IAEA-TECDOC-1289, 2002) as part of their programme to improve uranium resource utilisation and for the transmutation of high-level waste nuclides. The Japanese studied the impact of plant size on seismic issues and ways of developing more compacted and integrated plant designs.

The LCFR has negative void reactivity but a high breeding ratio of 1.26. The design is integral with the core, support structure and primary heat exchange systems situated within the reactor vessel. On the secondary side, the once through steam generator and its helical tubes are situated around the core and core diagrid. Regarding safety characteristics, the design is to reduce the propensity for lead-steam interaction.

12.6.2.4 General. Lead cooled systems have both advantages and disadvantages compared with sodium systems. Lead is much less reactive with air and water compared with sodium. In terms of other advantages in regard to minimum reactivity excess, transmutation of old actinides and fission products, proliferation safeguards consider­ations, safety in accident situations and economic competitiveness, lead and sodium systems have comparable properties.

There are however some negative aspects of lead associated with its corrosiveness; it may freeze in the steam generator in the case of feed-water heater failure. Repair and maintenance and remote re-fuelling operations are carried out at high lead temperatures of over 400°C. There is a potential for fuel subassembly blockage caused by lead/water/steam interactions.