15.57. Prior to about 1970, metallic fuel was thought to be unacceptable because of its poor irradiation behavior. This picture was reversed com­pletely by new alloy designs and very favorable irradiation experience at EBR-II during the 1970s. Metal fuel that can achieve a peak burnup of 12.9 TJ/kg (150,000 MW • d/t) now appears feasible.

15.58. The thermal expansion and high thermal conductivity of metallic fuel provides important safety advantages. For example, in a loss-of-flow — without-scram (LOFWS) event, the exit coolant temperature will increase during flow coast-down. The resulting thermal expansion of the fuel as­semblies provides a negative reactivity feedback which reduces the power. Although during power reduction, the stored Doppler reactivity effect appears as a positive contribution, it is not significant because of the high thermal conductivity of the fuel. This behavior was demonstrated dra­matically at EBR-II in 1986 when a deliberate LOFWS experiment resulted in automatic reactor shutdown within about 7 minutes, with a maximum coolant temperature rise of about 200°C, which occurred within the first minute [8].

15.59. The reference fuel for the ALMR is an alloy consisting of ura­nium with 27 percent Pu and 10 percent Zr. Blanket assemblies contain an alloy of depleted uranium with 10 percent zirconium. A heterogeneous core configuration includes 199 assemblies of various types as shown in Fig. 15.6 sodium-cooled fast reactors generally use hexagonal geometry for the assembly ducts instead of the square design employed in LWRs. Since the coolant is not a moderator, the tighter triangular rod pitch can be used to maximize the fuel volume fraction and improve heat transfer characteristics.

15.60. For the equilibrium fuel loading cycle, one-third of the core is replaced after each 18 months of operation. Since the metallic fuel yields a breeding ratio of about 1.2, more fissile material is produced than con­sumed. In a planned on-site facility, electrorefining using a liquid cadmium anode and a fused chloride salt will be used to extract the discharged fuel uranium-plutonium mixture from the dissolved mixture of fuel and fission products. New fuel rods would then be prepared by a casting process.