Irradiation History and Criticality

The overall design of the EFIT reactor is based on a plutonium and minor actinide vector derived from an exemplary stock, coming from spent LWR fuel at a burn-up of 45 MWd/kgHM and cooled for a certain period. But this composition is only valid for one possible initial load of the reactor core. In this section it is referred to as the fresh isotopic vector. Already one irradiation cycle changes the isotopic composition and therefore the behavior of the core and its criticality. A simplified approach was taken, because it was not possible to perform burn-up calculations for the EFIT- reactor: the spent fuel from the EFIT-like fuel elements irradiated in the MYRRHA core were filled into the EFIT reactor geometry to perform criticality calculations. Therefore, the effect of irradiated plutonium and minor actinides on the criticality of the EFIT reactor could be approximated. Since the irradiation period in the EFIT core is not yet clear and to account for the higher neutron flux in the MYRRHA core, the criticality was calculated for fuel with different bu irradiation histories11. Table 7.8 lists the composition of the plutonium vector after various irradiation times. It shows a decreasing plutonium-239 fraction. The plutonium-238 fraction increases. It is much higher than in light water reactor spent fuel. Plutonium-238 is mainly produced by breeding from neptunium-237 or via α-decay from curium-242. Both isotopes are present in an unusually high fraction due to the admixture of minor actinides into the fresh fuel. Compared to the average reactor-grade plutonium (cf. Table 5.4), there are more plutonium isotopes with an odd mass number present in the irradiated transmutation fuel. This is also the reason why a light water reactor fleet is commonly regarded as not being suitable to use the plutonium discharged from an accelerator-driven system for energy generation. The resulting plutonium isotope vector as depicted in Table 7.8 is not sufficiently fissionable in a thermal neutron spectrum.

Table 7.8.: Evolution of the plutonium vector in the P&T-fuel for different irradiation histories of the EFIT-like fuel elements in the MYRRHA IPS. All values are in weight percent and derived from irradiation of EFIT-like fuel elements in the MYRRHA IPS.

Pu-238 Pu-239 Pu-240 Pu-241 Pu-242

0 Days 3.7 46.4 34.1 3.8 11.8

270 Days 6.4 41.7 34.7 4.0 13.1

540 Days 10.9 36.8 34.4 4.0 13.9

810 Days 14.8 32.6 34.0 4.0 14.6

1080 Days 17.9 29.1 33.8 4.0 15.2

The spent fuel is separated into different material streams after irradiation in the accelerator-driven system. It is then fabricated into fresh fuel elements for the next transmutation cycle. To obtain the best possible transmutation efficiency, it is now assumed that the fraction of plutonium to minor actinides is reset to the optimum fraction as derived based on the 42-0 concept (cf. section 7.2.2). Yet, the calculated plutonium and minor actinide ratio might not hold true for the new composition.

11 _{For the change in the material composition, the total neutron flux is an important factor. The exposure to a certain}

But since this ratio is a reference point of the design efforts, the fraction of 45.7 % plutonium is used in all the following calculations.

For a first set of calculations, the minor actinides are taken from the stockpile and combined with previously irradiated plutonium into fuel for the EFIT reactor. Figure 7.6 shows the evolution of the calculated k_{e f f}. The irradiation period assigns the days the plutonium had been irradiated prior to fuel fabrication. With the initial composition, criticality of the system is at 0.96, a value consistent with the published values for k_{e f f} (Mansani et al. 2012). With plutonium aging, criticality drops massively to a value of less than 0.92 after a previous irradiation of 1080 FPD. This reactivity swing is by far higher than values derived in other calculations for various oxide fuels rich in minor actinides (NEA 2005). For MYRRHA, a reactivity swing of 1500 pcm (∆k ≈ 0.015) was calculated12 (Sarotto et al. 2013). For EFIT, publications even list a value of only 200 pcm per cycle (Artioli et al. 2007). It seems reasonable that these are ranges controllable by the beam current than the approximately 4000 pcm visible in the calculated figures.

Table 7.9.: Evolution of the minor actinide vector in the P&T-fuel for different irradiation times. All values in weight percent. The fraction of Am-243 is constant at 16 %.

Np-237 Am-241 Am-242m Cm-242 Cm-243 Cm-244 Cm-245 0 Days 3.9 76. 0.25 0. 0.066 3. 1.1 270 Days 3.8 71. 1.2 2.8 0.094 4.2 1.2 540 Days 3.8 68. 2.1 3.2 0.14 5.4 1.2 810 Days 3.9 66. 2.8 3.2 0.18 6.6 1.4 1080 Days 3.9 64. 3.4 3.1 0.22 7.8 1.6

To meet the objective of significant minor actinide reduction, several irradiation periods in the reactor are necessary. Table 7.9 shows the evolution of the minor actinide vector derived from the EFIT-like fuel elements. The fraction of americium decreases while at the same time more curium is generated. Curium has large fission cross sections in fast spectrum, e.g. more than seven barn for curium-243 compared to less than two barn for plutonium-239. The combined compositions from Table 7.9 and 7.8 are used for criticality calculations of EFIT. Figure 7.6 shows the resulting values for k_{e f f}.

Naturally, the initial value for k_{e f f} is also 0.96, because there is no irradiation history of the fuel. But with minor actinides and plutonium previously irradiated, ke f f increases. As a reference value,

the upper limit of 0.983 for the effective multiplication factor during design based conditions is shown as well (Mansani et al. 2012). When the EFIT-like fuel elements were irradiated in the MYRRHA reactor in-pile test sections for 540 days or more, the neutron multiplication factor of EFIT is higher than allowed during operation. Therefore, recycling of the fuel is only possible with shorter irradiation times.

Several publications on EFIT mention k_{e f f} = 0.97 instead of 0.96 as used in the computer model of this thesis. Modifying the computer model to start at 0.97 would have led to an offset resulting in the fact that the upper limit for the reactor during design based conditions is reached after even shorter irradiation histories.

In theory, one option to stabilize the criticality level could be to mix the plutonium and minor actinides in fractions according to their effect on reactivity. Plutonium and minor actinides are separated into different material streams during reprocessing anyway. Consequently, the fraction of plutonium in the newly fabricated fuel might as well be adjusted. It would imply detailed

12 _{The unit pcm assigns percent milliρ with the reactivity ρ defined as ρ =} ke f f−1

ke f f . It represents the fractional

change of the neutron population per generation (United States Department of Energy 1993b). It is zero for steady-state operation and consequently the preferred figure for automatic control of the neutron population. 86

● ● ● ● ● ■ ■ ■ ■ ■ ◆ ◆ ◆ ◆ ◆ 0 200 400 600 800 1000 0.92 0.94 0.96 0.98 1.00

Irradiation Period in Days

ke

ff

Figure 7.6.: Evolution of criticality in the EFIT reactor in dependence of the irradiation history of plutonium (plutonium and minor actinides) prior to fuel fabrication. The values derived from calculations with the initial minor actinide composition and the plutonium vector from previous irradiation periods are shown in green. The blue graph depictsk_{e f f} for fuel made of both previously irradiated minor actinides and plutonium. As a reference, the upper limit for criticality during design basis conditions for EFIT is also plotted.

knowledge on the current isotopic composition, since not only the fission cross sections but also the neutron yield has to be taken into account for the new fuel. More importantly, this procedure might work against the premise for optimum minor actinide transmutation depending on the plutonium fraction in the fuel as well.