Until recently, applications of MSRs have been limited to thermal-neutron — spectrum graphite-moderated technology. From 2005 onwards, studies have concentrated on creating fast-spectrum MSRs, i. e. MSFRs, which combine the efficiency of fast neutron reactors with the specific benefits of molten salt fluorides discussed in the previous section. MSFR systems have been recognized as a long­term alternative to solid-fuelled fast neutron systems. They offer several advantages, including negative feedback coefficients, smaller fissile inventories, easy in-service inspection and a simplified fuel cycle. The main characteristics of MSFRs are summarized in Table 13.8. The first system developed was the ORNL molten salt breeder reactor (MSBR) project (Nuttin et al., 2005; Mathieu et al, 2006). Since then a variety of core arrangements, reprocessing methods and salt compositions have been proposed, most noticeably a graphite-free core (i. e. with no graphite moderator) (Fig. 13.13).

Two types of fuel cycle have been suggested (Renault et al., 2009): [23] [24]

Table 13.8 MSFR reference design characteristics (Renault et al., 2009)

Reference value(s)

Thermal power (MWt) 3000

Fuel molten salt composition (mol%) LiF-ThF4_233UF4 or LiF-ThF4_(Pu-MA)F3 with

LiF = 77.5 mol%

Fertile blanket molten salt composition (mol%) Melting point (°C) Operating temperature (°C)

Initial inventory (kg)

Density (g/cm[25])

Dilatation coefficient (/°C) Core dimensions (m)

Fuel salt volume (m3)

Blanket salt volume (m3)

Thorium consumption (ton/year) 233U production (kg/year)

Breeding ratio (233U-started MSFR)

13.13 Molten salt fast reactor (MSFR).

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produces an improved doubling time of 35 years. The existence of other fissile elements within the spent fuel would reduce 233U consumption, making MSFR systems more feasible (Renault et al., 2009).