The molten salt reactor (MSR) (Fig. 13.12) is the most innovative of the potential systems identified by GIF, departing significantly from current technologies. The MSR was first developed in 1954 by the US military. Further research was undertaken in the US in the 1960s, with two demonstration reactors built at the Oak Ridge National Laboratory (Abram and Ion, 2008; Renault et al., 2009). Instead of solid fuel elements, the MSR system uses a circulating salt mixture that contains the fissile material, typically a liquid mixture of fluorides of sodium, zirconium and uranium, which acts as both fuel and coolant. It circulates continuously through a graphite core, and then through a heat exchanger, where it transfers heat to a secondary salt circuit. A proportion of the salt is then diverted through a processing plant where fission products are removed and new fissile material is introduced. This continual processing of the fuel allows operation without refuelling.

Control rods

Coolant

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Emergency dump tanks

73.72 Molten salt reactor (MSR).

The design of the MSR means that it is a closed cycle system, which makes efficient use of U, Th, Pu and MAs. This design also has a unique safety feature, since the fuel can be easily drained from the reactor in the event of an accident (Abram and Ion, 2008). Compared with solid-fuel reactors, MSR systems do not need fuel elements, have lower fissile inventories, lower radiation damage that can limit fuel burn-up, efficient actinide burning, a mechanism for continuous FP removal, the possibility of adding makeup fuel as needed, which precludes the need for providing excess reactivity and extends fuel resources, and a homogeneous isotopic composition of fuel in the reactor (Schenkel et al., 2009).

However, MSR systems present a number of challenges. The processing of the highly radioactive salt mixture requires robust processing equipment and materials. The combination of a corrosive and radioactive salt, with an isotopic composition, which changes over time, and a high neutron fluence also places extreme requirements on the primary circuit components. The graphite core also receives a high radiation dose, which means almost certain replacement during the life of the reactor. These challenges mean that the MSR system is the least developed system with less chance of achieving commercial viability by 2030 than other designs (Abram and Ion, 2008). However, given the potential of the system, the MSR provisional system steering committee (PSSC) of GIF decided in 2008 to modify the Generation IV Roadmap agreed in 2002 to include research on fuel and coolant salts.

There is a range of developments that may help in the development of MSR systems, including Brayton power cycles (rather than steam cycles), which eliminate many of the historical challenges in building MSRs (Schenkel et al., 2009). Two main design concepts have subsequently been developed (Renault et al., 2009):

1 The fast-spectrum MSR (MSFR) as a long-term alternative to solid-fuel fast neutron reactors, characterized by large negative temperature and void reactivity coefficients, a unique safety characteristic not found in solid-fuel fast reactors.

2 The fluoride-cooled high-temperature reactor (FHR), a high-temperature reactor, which is more compact than the VHTR and has the potential for passive safety from small to very high unit power (>2400 MWft).

Developments in MSFR and FHR are discussed in more detail in the following sections.

Research has also been undertaken into the use of liquid salt technology in other reactor system (both nuclear and non-nuclear), including other Generation IV systems (Schenkel et al., 2009). Liquid salts could be used, for instance, as primary coolants in an FHR, as an alternative to secondary sodium in sodium fast reactors (SFRs), and to intermediate helium in VHTRs. Investigations into high — temperature salts as coolants may lead to other nuclear and non-nuclear applications. Possible examples include heat transfer for nuclear hydrogen production, concentrated solar electricity generation, oil refineries and shale oil processing facilities. Liquid salts have two key advantages (Renault et al., 2009):

1 their higher volumetric heat capacity allows for smaller equipment size

2 the absence of chemically exothermic reactions between them and the coolants used in the intermediate loop and power cycle coolants

There are a number of research priorities in improving understanding of liquid salt chemistry, including (Schenkel et al., 2009):

• the physico-chemical behaviour of coolant and fuel salts, including fission products and tritium

• the compatibility of salts with structural materials for fuel and coolant circuits, as well as fuel processing material development

• the on-site fuel processing for MSFR; the maintenance, instrumentation and control of liquid salt chemistry (redox, purification, homogeneity)

• safety aspects, including interaction of liquid salts with sodium, water, air, etc.