Figure 10.28 shows the principal steps in reprocessing LMFBR fuel. Feed quantities are for a plant fed with 5 MT/day of irradiated heavy metal (uranium plus plutonium). Feed is combined core and blanket assemblies from LMFBRs operated under conditions nearly the same as those on which Fig. 3.34 and Tables 8.8 and 10.20 were based. The head-end steps 1 through 6 follow one alternative of several sketched in Report ORNL-4422 [05].

Fuel assemblies for the core and axial blanket consist of long bundles of stainless steel tubes, each about 0.6 cm in diameter, in which the spent fuel and fission products are sealed.

Table 10.20 Principal differences between irradiated fuel from LMFBR and LWR

Reactor

The lower end of each tube contains irradiated depleted U02, the middle portion irradiated mixed depleted U02 and Pu02, an upper portion irradiated depleted U02, and the top a plenum to accommodate buildup of fission-product gases. The rod bundles are surrounded by a square or hexagonal stainless steel sheath to the top and bottom of which are attached end fittings to direct sodium flow in the reactor and to facilitate handling outside. Fuel assemblies for the radial blanket are of the same length but contain rods of larger diameter charged initially with depleted U02.

In Fig. 10.28 it is assumed that assemblies from the core and radial blanket are reprocessed in the proportion in which they are discharged from the reactor. The average composition of feed to the reprocessing plant then is 10 w/o plutonium, 3.56 w/o fission products, and 86.44 w/o uranium. The 5000 kg of fuel processed per day is associated with 6858 kg of stainless steel and an indeterminate amount of metallic sodium that coats exterior surfaces of the assembly and possibly has penetrated imperfections in some of the fuel rods. Sodium is used as coolant in the LMFBR and is a likely candidate for removing decay heat in shipping irradiated fuel from the reactor to reprocessing.

The first step in Fig. 10.28 is deactivation of sodium coating the outside the fuel rods, either by dissolving it off or converting it to a less reactive sodium compound. In the second step, as much of the stainless steel as possible is removed without permitting fission products to escape. End fittings are removed and fuel rod bundles are extracted from the enclosing sheath, if possible. In the third step, the plenum is sheared from fuel rod bundles, thus releasing some of the fission product gases to a retention system. The portion of the rod bundle containing

fuel and blanket material is sheared into short lengths to facilitate subsequent processing. In step 4, voloxidation, the sheared fuel is heated to 550 to 600°C first in argon, to which is then added an increasing amount of air, to react with possible entrained sodium, convert U02 to U308, and release tritium. In step 5, the fuel is dissolved in 8 M nitric acid to which sufficient gadolinium nitrate, boric acid, or other soluble poison is added to control criticality. Undissolved residues rich in Pu02 are treated with special reagents. In step 6, feed adjustment, nitric acid concentration of solvent extraction feed is brought to З M and plutonium is made tetravalent by addition of N204. In the Purex process, step 7, solvent extraction with 30 v/o TBP is used to separate dissolver solution into high-level waste, decontaminated uranyl nitrate, and decontaminated plutonium.