Fuel cycle and waste management. 3. Analysis of PWR Equilibrium Fuel Cycles Using Nuclide Importance

Energy generation by nuclear reactors entails production of plutonium and radioactive waste. To utilize the plutonium and to minimize the long-term radio-toxic waste, an option is a closed fuel cycle strategy employing reprocessing and recycling of actinides. Since commercial operation of fast reactors is not considered to be realized in the near future, plutonium and minor actinide recycling in light water reactors (LWRs) is considered, although LWR neutron economy is not good. In this study, uranium enrichment, natural uranium requirements, and toxicity of discharged heavy metals (HMs) are evaluated for a pressurized water reactor (PWR), whose design parameters are given in Table I. The following fuel cycles are investigated, where all fission products (FPs) and final products of HMs (Tl-Fr) are discharged from the reactor at a standard rate (25%/yr): Case 1: All HMs are discharged with the standard rate. Case 2: All HMs except Pu are discharged with the standard rate; Pu is discharged at the rate of one-half of the standard rate. Case 3: All HMs except Pu are discharged with the standard rate; Pu is confined. Case 4: All HMs except U are confined; U is discharged with the standard rate. Case 5: All HMs are confined. The infinite multiplication factor k can be expressed by using the nuclide importance (fission neutron importance fj and absorbed neutron importance aj ) as k = (Σj fj sj)/(αΣj aj sj), where sj = atomic percent of uranium isotopes (234U, 235U, and 238U ) in the supplied fuel α = correction factor for estimating neutron absorption by non-fuel-originating nuclides, such as coolant and construction materials. A detailed description of nuclide importance and calculation method is given in Ref. 1. The value k is set to be 1.02, and sj are evaluated from this equation and the following ones: s24 + s25 + s28 = 100 and 100s24 - 0.9937s25=-0.1925. The second equation is given by enrichment conditions. The group cross-section set is generated with the SRAC code system using the JENDLE-3.2 library. Table II shows some of calculation results from this study. The importance changes in different ways between 235U and 238U, and changes for 235U are larger. However, since the sj of 238U is much larger than 235U, effects of 238U are dominant. The enrichment as well as the required amount of natural uranium decreases considerably with increasing number of confined heavy nuclides when uranium is discharged from the reactor. The burnup changes inversely proportional to the amount of charged fuel. Figure 1 (see next page) shows the change of toxicity ratio of discharged HMs to that of fed fuel a long time after discharge of fuel from the reactor. The value for case 5 is zero and is not shown. For case 4, the toxicity of discharged fuel becomes slightly smaller than one of charged uranium. (authors)