Based on a peculiar property of the Fifth Unstable Nuclide Series（FUNS） in developing stage

namely the transformation rate of parent fuel-

238

U consumption to fission production being increased with the fuel’s reactor core life

it was shown that the used fuel taken out from the reactor core of PWR could be used again as PWR fuel

and the post processing for the used fuel needed only to de-contaminate the fission Products from the used fuel and then to rebuild it into its normal form.Scheme A in the article discussed the condition of using such rebuilt used fuel in PWR. Another peculiar property of FUNS is characterized by the fact that the equilibrium saturate concentrations of the derived nuclides in FUNS are different for different types of reactor. It is well known that the saturate concentration of

239

Pu in PWR is much larger than that in HWR. Scheme B in the article

i.e. a method of using the rebuilt used fuel from PWR to replace the natural uranium fuel for HWR

was discussed.