5.2 Differences from LWR Fuels

Table 10.20 lists the principal differences between irradiated fuel to be reprocessed from an LMFBR and an LWR. The data have been excerpted from Figs. 3.34 and 3.31 and Tables 8.8 and 8.7.

Because some of the sodium coolant used in the LMFBR fuel that may have adhered to the cladding or penetrated leaks in it would react vigorously with water or nitric acid, it is necessary to oxidize all sodium by exposing the fuel to an inert gas containing a controlled amount of water vapor before the dissolution step. LMFBR fuel may not be stored with water cooling till after all sodium has been removed.

Use of stainless steel cladding in the LMFBR instead of zircaloy has little effect on mechanical decladding or on dissolution. Stainless steel, like zircaloy, is not rapidly dissolved by nitric acid of the concentrations used in the Purex process.

Fuel in the core of the LMFBR is operated at a specific power over three times that of the LWR. During the cooling period, the specific power of LMFBR core fuel from radioactive decay remains about three times that of LWR fuel cooled for the same length of time. This

makes shipping, handling, and storage of LMFBR fuel prior to reprocessing much more difficult than LWR fuel.

To reduce the specific power somewhat in reprocessing, it is planned to combine irradiated fuel from the LMFBR core with irradiated fuel from the LMFBR blankets in proportion to the rates at which they are discharged from the reactor. Even so, the specific power of LMFBR fuel cooled 150 days is 1.4 times that of LWR fuel cooled the same length of time.

The bumup of fuel in the LMFBR core is two or more times that of LWR fuel, leading to higher concentrations of fission products, gaseous and solid, and greater radiation effects on cladding and fuel. The average bumup of combined LMFBR core and blanket material is about 15 percent higher than that of LWR fuel.

The concentration of plutonium in combined core and blanket fuel from the LMFBR is more than 10 times that of LWR fuel. This is the most significant difference between the two fuels with respect to reprocessing. Other important differences are the greater amounts of tritium and 1311, the 140 percent greater ruthenium activity, and the 60 percent greater overall specific activity of 150-day cooled LMFBR fuel.

Because of the high plutonium content of spent fuel from the LMFBR, there is strong

&

Figure 10.27 Aqueous phase concentration

at which second organic phase forms. ——-

n-dodecane [W6];——— Ultrasene [S23].

economic incentive to return this plutonium to the reactor with minimum delay for cooling, reprocessing, and refabrication. Consequently, the foregoing comparison of LMFBR and LWR reprocessing conditions for equal cooling periods of 150 days does not tell the whole story. For example, if LMFBR fuel were reprocessed 90 days after discharge from the reactor instead of 150 days, the activity of 8.05-day 1311 would be

2(iso-90)/8.os _ ]75 (10.16)

times greater, and the specific power of fuel from the core would be 1.5 times greater.

The following discussion of reprocessing LMFBR fuels outlines the principal process steps, lists the main problem areas, and discusses possible solutions. Since 1973, international dissemination of reprocessing information has been restricted. This discussion of reprocessing LMFBR fuel is thus less complete and less up to date than would be desired.