Concentration of Long-lived Fission Products in the Spent P&T-Fuel

Recent publications claim that the total inventory of minor actinides in the spent fuel can be significantly reduced (Abderrahim 2013; Sarotto et al. 2013). But the argument often ends at this point. There is hardly an assessment of related effects on the spent fuel. There is only a small note saying that the disproportional generation of plutonium-238 and its effect on the decay heat from the spent fuel elements should be further investigated in NEA (2006, p. 17). Therefore, the question to which extend long-lived fission products are generated is investigated in this section. These nuclides are known to be relevant for the long-term safety analysis of a deep geological repository, as already discussed.

Usually, concentrations of long-lived fission products within the spent fuel are given in Becquerel per ton heavy metal inventory. This is an obstacle for the comparison of the fraction of long-lived fission products in the spent fuel. Only conventional nuclear fuel comprises a heavy metal matrix, P&T-fuel consists of a magnesium-oxide matrix.

For further analysis the weight percentage of single radionuclides as a part of all fission products is considered. Figure 7.13 shows the derived weight fractions for the relevant long-lived fission products in the spent P&T-fuel. It was irradiated for 1080 FPD and cooled for 40 years. Shorter irradiation periods and a cooling period of only 90 days after discharge did not alter the result visibly. Cesium-135 has the highest fraction with more than 4 %. Hence, a first result is that additional long-lived fission products are produced in a P&T fuel cycle which have to be dealt with in a deep geological repository. The fraction of selenium-79 is not shown because its concentration is 0.006 % and would appear as zero. The fact the concentration of cesium-137 is also disproportionally increased is also not depicted13.

For comparison, the estimated values for the German spent fuel inventory in 2022 are also plotted (Schwenk-Ferrero 2013). The list is not comprehensive regarding relevant fission products. The values for selenium-79, zirconium-93, and tin-126 are missing in the publication and therefore also in Figure 7.13. They could however be estimated using the production rates listed in Table 7.15. The total amount of fission products is not mentioned in the paper directly, but can be calculated by summing up the figures for explicitly given isotopes and "other fission products". For 2022, the calculated German inventory for fission products is 415.65 tons. Details are discussed later on in the following section.

There are comprehensive inventories for Swiss spent fuel elements available. They originate from pressurized water reactor uranium-oxide, boiling water reactor uranium-oxide and pressurized water mixed-oxide fuel (McGinnes 2002). In Figure 7.13 and the following, these are referred to as Swiss PWR, Swiss BWR, and Swiss MOX. The inventories for different burn-ups are given in Becquerel per ton heavy metal for all unstable isotopes and in mol per ton heavy metal for all stable isotopes; directly after removal from the core and after a 40 year cooling period. For this study,

13 _{Cesium-137 is the dominating contributor to the gamma dose rate exposure of workers in conventional nuclear}

Zr-93 Tc-99 Sn-126 I-129 Cs-135 0.00 0.01 0.02 0.03 0.04

Important Long-lived Fission Products

F ra c ti o n o f A ll F is s io n P ro d u c ts

Figure 7.13.: Weight fraction of selected fission products as a part of all fission products for various types of spent fuel (see text). For the total German reference inventory expected in 2022, values for zirconium-93 and tin-126 are not published. The values for P&T-fuel are derived from the EFIT-like fuel elements after a 1080 FPD irradiation period in the MYRRHA core 40 years after discharge.

the values of a burn-up of 48 GWd/kg and a decay time of 40 years are used as reference values. In 2022, a certain (small) fraction of the accumulated spent fuel will still be originating from the core almost without cooling period. Analysis of the P&T-fuel has, however, shown that the fractions of the long-lived fission products of all fission products does hardly change in dependence of the cooling period after discharge. The focus lies on long-lived fission products. The overall amount of fission products is approximately constant because spontaneous fission and decay of the heavy metal have only low reaction rates.

Conversion of the activities and amounts of substances is done using Mathematica and the isotope half-lives implemented in the program. The calculated specific activities and masses for all considered isotopes are listed in Appendix C. Concentrations are published per 1,000 kg heavy metal. Summing up the calculated masses results in 996 kg (PWR), 995 kg (BWR) and 936 kg (MOX). The slight differences in comparison to the nominal 1,000 kg can be explained by the cut-off of unstable isotopes with an activity of less than 1 GBq/tHM and possible discrepancies in the used isotope data. Oxygen is not included in the list of isotopes for the spent fuel. All listed isotopes up to hafnium-178 are considered to be fission products. Figure 7.13 depicts the fractions for the long-lived fission products in the swiss spent fuel for the different reactor types as well.

In general, the concentrations of long-lived fission products for fuel originating from different sources are comparable. The zirconium-93 and technetium-99 fraction is higher for the spent fuel from light water reactors. The cesium-135 concentration in the P&T-fuel is present in nearly a four- fold higher concentration. Already the MOX fuel shows an increase in cesium-135 concentration. Hence this effect can most likely be attributed to the increased content of transuranium elements in the fresh fuel.

From the swiss figures, concentrations in the German spent fuel inventory can be derived for the missing three long-lived fission products selenium-79, zirconium-93, and tin-126. The German nuclear waste is modeled by combining the three vectors from BWR, PWR, and PWR-MOX spent

fuel. Therefore the fraction F_i of a certain isotope i in the German spent fuel can be estimated using equation

F_{Ger man y}i = κF_Swissi _{,PW R}+ λF_Swissi _{,BW R}+ µF_Swissi _{,M OX}. (7.13) Beginning with the known fraction for technetium-99, iodine-129, and cesium-135 in the German spent fuel, it can be derived that the variables are

κ = 0.662, λ = 0.058, and µ = 0.105.

(7.14)

Table 7.16.: Concentration of long-lived fission products of all fission products in the spent fuel in weight percent. Values are given for the accumulated German spent fuel in 2022 and for P&T-fuel based on own calculations.

Se-79 Zr-93 Tc-99 Sn-126 I-129 Cs-135 German SNF 0.004 % 2.00 % 2.23 % 0.09 % 0.55 % 1.31 % P&T-fuel 0.006 % 1.45 % 2.25 % 0.11 % 0.65 % 4.36 %

For the German spent fuel inventory, the concentration of all six important long-lived fission products is calculated using equation 7.13 and 7.14. Results are given in Table 7.16, together with the results for the P&T-fuel derived from the simulation of EFIT-like fuel elements in the MYRRHA reactor. For all considered isotopes except zirconium-93 and in particular for cesium-135, the fraction is higher for the P&T-fuel. Since the amount of fission products increases over burn-up, the ratio of the fission products under consideration to all fission products is almost independent of burn-up. Some fission products undergo further reactions, such as neutron absorption or decay. The choice to neglect these reactions seems reasonable. It was validated by calculating the ratios also for P&T-fuel that was irradiated for shorter times in the core.