An alternative to the SFR is the lead-cooled fast reactor (LFR) (see Fig. 13.22). The LFR features a fast-neutron spectrum and a closed fuel cycle for efficient conversion of fertile uranium. It can also be used as a burner of MAs from spent fuel and as a burner/breeder. A summary of the main characteristics of different LFR designs is provided in Table 13.14. The plan is to develop the system towards commercial operation within the next five years.

The LFR is similar to the SFR, except that the coolant is either lead or a lead — bismuth eutectic (LBE). This improves safety given that lead is a relatively inert coolant (Cinotti et al., 2009). The lead coolant is contained inside a reactor vessel

surrounded by a guard vessel. Lead is preferred to LBE to avoid the formation of alpha-emitting 210Po (formed from 209Bi by neutron capture) and to avoid relying on Bi which is a scarcer material. Pb is a coolant with very low neutron absorption and moderation, so it is possible to maintain a fast neutron flux even with a large amount of coolant in the core. This allows an efficient utilization of excess neutrons and reduction of specific U consumption. Reactor designs can readily achieve a breeding ratio of about 1, and long core life and a high fuel burn-up can be achieved (Cinotti et al., 2006). The use of Pb means the LFR has a simpler design and thus has lower capital costs than the SFR, making it more competitive for electricity generation (Cinotti et al., 2006). Pb is also important in the design of sub-critical accelerator-driven transmutation systems (ADS), because the coolant can also serve as a spallation target, and because the nuclear cross sections of Pb allow high-energy neutrons to be utilized particularly efficiently in a process known as adiabatic resonance crossing (Abram and Ion, 2008).

There are, however, some disadvantages to using Pb. Experience using Pb coolant is limited compared to Na. There is a need for more research in such areas as system design, components and innovative fuel and fuel cycle development

(Vezzoni, 2011). Pb is more than 11 times denser than Na and thus requires significantly higher pumping power. This greater density also makes it harder to achieve a seismically safe design. The greatest challenge, however, is the corrosive and erosive nature of Pb, which requires careful oxygen control and the use of highly corrosion/erosion-resistant materials (Abram and Ion, 2008).

There are two reactor designs being developed within the GIF framework (see Table 13.15) (Cinotti et al., 2009):

1 a 600MWe design (Fig. 13.23) based on the previous European lead-cooled system (ELSY), now called ELFR (European lead fast reactor) (Mansani, 2011)

Table 13.15 Summary of the main characteristics of primary and secondary systems of different LFR designs (Colombo et al., 2010)

13.23 600 MWe ELFR (Mansani, 2011).

2 a small modular design of 20 MWe (Fig. 13.24) based on the small secure transportable autonomous reactor (SSTAR).

The 600 MWe design has a simple and compact primary circuit with removable components. The reactor has a secondary water loop with steam generators feeding a turbine. This simple design should mean lower capital costs and construction time. The compactness of the design also means a smaller reactor building. The ELSY core consists of an array of open fuel assemblies of square pitch, surrounded by reflector assemblies to reduce the risk of coolant flow

blockage while a closed hexagonal arrangement of assemblies has been proposed for ELFR (Mansani, 2011). The core is self-sufficient in Pu and can burn its own generated MAs with a content at equilibrium of about 1% heavy metal (Cinotti et al., 2009).

The 20MWe design uses natural circulation in the primary lead loop, with a secondary supercritical CO2 loop for power conversion in a direct Brayton cycle. The combination of compact size, Pb coolant, nitride fuel containing TRU elements and a fast spectrum core all promote a high conversion ratio. This improves proliferation resistance, fissile self-sufficiency, autonomous load following, simplicity and reliability of operation, transportability and a high degree of passive safety. However, the SSTAR design relies on further developments, including a high-performance code-qualified TRU nitride fuel (Cinotti et al., 2009).