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.