Activity

The activity of the spent fuel is an intrinsic material property that does not depend on the geometry of the spent fuel elements. It affects the measures necessary for safe handling of the material as well as the chemicals that are used for reprocessing of the spent fuel. High radiation degrades the chemicals and they might react with the solvents in the separation process to form red oil which is easily exploding at temperatures above 135◦C (IRSN 2008). The activity of the spent fuel originates from two groups of elements in the fuel: the instable fission and activation products and the fissile isotopes, in this case the transuranium isotopes. The latter is also referred to as the intrinsic activity of the fuel. The contribution of the fission products rises with higher burn-up of the fuel elements since more fission products have already been produced then, while at the same time the fraction of the transuranium isotopes declines.

The activity of the spent fuel for different irradiation times is derived from the burn-up calculations for the EFIT-like fuel elements. Figure 7.7 depicts the total activity in Becquerel per volume for different times after discharge. The graphs start after a short cooling period to eliminate the contribution of the very short-lived fission products, some of them with half-lives of only several seconds. Naturally, activity coming from the fuel with a low burn-up is the smallest. With increasing burn-up, the values for the different irradiation times are converging in the long term. The longer time in the core increases the fraction of isotopes with intermediated half-lives that decay during irradiation. From the graph it is evident, that after four years other isotopes with longer half-lives become more important.

● ● ● ● ● ● ● ● _● ● ● ● ● ● ● ■ ■ ■ ■ ■ ■ ■ ■ _■ ■ ■ ■ ■ ■ ■ ◆ ◆ ◆ ◆ ◆ ◆ ◆ ◆ _◆ ◆ ◆ ◆ ◆ ◆ ◆ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ _▲ ▲ ▲ ▲ ▲ ▲ ▲ 0.1 0.5 1 5 10 50 100 5 × 1011 1 × 1012 5 × 1012 1 × 1013

Time After Discharge in Years

A c ti v it y in B q /c m 3

Figure 7.7.: Activity of spent P&T-fuel in dependence of the time after discharge for different irradiation times in the core. It takes almost five years for the activity to drop to one tenth of its initial value.

The combinedα- and β-activity of the spent fuel is 16.8 TBq per cm3 for the spent fuel with the highest burn-up. It takes almost five years for the activity to drop to one tenth of its initial value. After a cooling period of one century, the average value is roughly 400 GBq per cm3, about 2.5 % of the initial value. Calculations were done for fuel containing 43 % and 50 % fissile material, but only the values for the higher fraction are plotted. They correspond to fuel positioned in the outer

zone of the EFIT core. Looking at the activity per volume of the spent fuel corresponding to the inner elements, the values are less than 5 % lower at all times.

It is impossible to know activity limits at the current stage of R&D activities for the P&T fuel cycle. Hence, the best option to assess the resulting values is comparison to today’s conventional spent fuel. After 40 years of cooling, the spent fuel with higher burn-up from the outer zone of the transmutation facility has a combinedα- and β-activity of 7 · 1011Bq/cm3. Average BWR spent fuel with a burn-up of 48 MWd/kgHM and after the same cooling period has aβ-activity of 9· 1010Bq/cm3. This value is calculated assuming an average fuel density of 10 g/cm3(McGinnes 2002). For spent thermal reactor fuel, theα-activity is usually less than one tenth of the total activity after the cooling period of only one decade (ibid.).

Besides possible implications on reprocessing chemistry, the activity of the spent fuel is the deter- mining factor when assessing the effects of possible nuclear accidents and the impact on workers in fuel cycle facilities. In case of release of radioactive material, the possible exposure of the public has to be calculated. Even though in this regard usually only a few isotopes are relevant, a eight-fold increase in total activity is not negligible. For the implementation of a P&T fuel cycle, there will be additional nuclear facilities at several sites, increasing the risk of exposure to larger fraction of the population.

The calculated values for the total activity and theα-activity of the spent P&T-fuel after selected cooling periods are given in Table 7.12. Theα-activity is approximated by summing the activity of all transuranium elements except plutonium-241 and americium-242. This approximation is reasonable, since the transuranium elements in general account for more than 99% of the total α-activity of spent fuel in the short term (Fanghänel et al. 2010, p. 2982). Also, except for plutonium-241 and americium-242,α-decay is their most likely decay mode. The branching ratio for spontaneous fission is often up to ten orders of magnitude lower. The ratio ofα-activity and spontaneous fission rates changes with longer cooling periods due to the change in the most contributing isotopes. It can be seen from Table 7.12 that the contribution of the transuranium elements to the total activity increases with increasing cooling period. Directly after discharge, several short-lived fission and activation products are present in the spent fuel. With their vanishing, the contribution of the transuranium elements becomes more important. After a cooling period of 100 years, more than 80 % of the total activity is due toα-decay.

Table 7.12.: The total activity, theα-activity and the spontaneous fission rate per volume for one EFIT-like fuel element irradiated for 1080 full power days in the MYRRHA reactor. The spontaneous fission rate is calculated only considering plutonium-238, plutonium-240, plutonium-242, curium-242, and curium-244.

Cooling Period Activity α-Activity Spont. Fission Rate Bq/cm3 Bq/cm3 1/(s·cm3) 0. years 5.11· 1013 _{9.05· 10}12 _{1.16· 10}6 0.1 years 1.68· 1013 _{7.93· 10}12 _{1.09· 10}6 0.6 years 7.54· 1012 _{4.19· 10}12 _{8.48· 10}5 1. years 5.17· 1012 2.71· 1012 7.48· 105 5. years 1.70· 1012 9.18· 1011 5.52· 105 10. years 1.38· 1012 _{8.32· 10}11 _{4.56· 10}5 40. years 7.58· 1011 _{5.37· 10}11 _{1.46· 10}5 100. years 4.10· 1011 3.40· 1011 1.63· 104

For the spent fuel from the small, fast reactor analyzed in chapter 6, the activity 40 years after discharge is6.34· 1010Bq/cm3 and the α-activity 7.66 · 108Bq/cm3. The absolute values are significantly lower than in the P&T-fuel. Theα-decay plays a less important role in this spent fuel,

it comprises less than 1 % of the total activity. Alpha particles can be easily shielded. Theα-activity is in principal relevant for the case of accidental ingestion and the interaction of the spent fuel with the reprocessing chemicals.

The neutron background originating from the spent fuel determines the required shielding for fuel handling. The higher it is, the heavier the shielding must be. Neutrons are produced in the spent fuel by spontaneous fission, nearly exclusively by plutonium-238, plutonium-240, plutonium-242, curium-242, and curium-244. Preliminary calculations were performed to show that it is sufficient to consider these five isotopes for the neutron activity of the spent fuel from the EFIT-like fuel elements. The last column of Table 7.12 lists the calculated spontaneous fission rates.

The spontaneous fission rate is 1.16·1061/(s·cm3) directly after discharge. It declines visibly, because the contributing actinides have comparably short half-lives (cf. Table 1.2). After a cooling period of 100 years, the rate is about 1.5 % of its original value. The neutron activity for the fuel during the first year after discharge from a fast breeder is reported in Orlov et al. (1974). After a cooling period of twelve months, the number of produced neutrons per second per kilogram is reported to be0.7· 106. Assuming an average MOX density of 10 g/cm3 and approximating the average number of neutrons per fission with three, this results in a spontaneous fission rate of about 2,333 1/(s·cm3_{). The neutron rate generated by the fast breeder reactor fuel is more than}

two orders of magnitude lower than the neutron rate from the transmutation fuel after a cooling period of one year. Again, this result shows that the build-up of curium during burn-up poses severe challenges to reprocessing and fuel fabrication in a P&T fuel cycle due to its high spontaneous fission rate (cf. Table 1.2).