Dedicated transmuter reactors

A ‘dedicated’ transmuter reactor13 should be able to burn any amount of TRUs or MAs, according to the chosen strategy. ‘Transmutation’ in this case means essentially ‘fission’. In order to compare the ‘transmutation’ effectiveness of different systems one has to compare the system performance at the same power (i. e. at the same number of fissions). Then, what really matters is the composition of the fuel loaded in the ‘dedicated’ transmuter reactor, since that determines which isotopes will be fissioned. Because for each actinide ~1 g is burnt (by fission) for 1 MW-day, the total mass MFi (in Kilograms) of isotope i burnt by fission in a year is given by:

M_ =—xW x365xl0"3

F’’ f,

where y is the load factor and f is the ratio of the total number of fissions in the system (all isotopes, all regions) to the fissions in the core due to isotope і and W (MW) is the power. If MTot. is the total mass of isotope і, consumed both by fission and by capture, then:

M0t. = MF. x (1 + a)

Tot, i F, i v v

where a. is the average capture-to-fission ratio of isotope і.

This shows that the ‘burning’ potential of a core is related to its power, i. e. to its fission rate. Because of this, any core with the same power will show a comparable transmutation potential. Power apart, the real difference between one core and another is determined by the ‘quality’ of the fuel that each core allows. One way of maximizing transmutation performance is to use fertile-free fuel; in essence, this means uranium-free. Unfortunately, such fuel cannot be used in a critical fast reactor because it would make it difficult to operate the reactor safely. This is a consequence of the absence of uranium, the most important result of which is a very low fraction of delayed neutrons — see e. g. Ref. 14. As an alternative, sub-critical systems (or accelerator driven systems, ADS, see e. g. Refs 15-17), have been proposed, since they could, in principle, provide a way around these potential difficulties. More recently, fission-fusion hybrid systems have also been considered.

The use of fertile-free fuel would enable an ADS to reach high transmutation levels so that a relatively small number of them would be needed to handle the TRUs arising from the electricity-producing reactor fleet. They may be thought of as a separate stratum of the fuel cycle, leaving the fuel cycle stratum devoted to electricity production ‘uncontaminated’ by the presence of MAs (see Section 17.4).

As of today, no uranium-free fuels have been definitively identified, even if some promising candidates have been pointed out.1 8-20 This suggests that, if TRU ‘burning’ is a priority objective, it may be worth exploring alternatives to ‘dedicated’ transmuter reactors loaded with uranium-free fuel. Examples are ADSs with some uranium in the fuel or fast reactors with a low conversion ratio (the conversion ratio, CR, is defined as the ratio of the fissile produced to the fissile destroyed). It has been shown,2 1 for example, that the TRU consumption rate reaches -80% of the maximum theoretical value for uranium-free fuel when the CR is of the order of -0.4-0.5 both for metal or oxide fuels and for MA/Pu ratios varying in the range -1 to 0.1, see below. Results reported in Ref. 22 indicate that, in terms of reactor control, cores with conversion ratios as low as -0.25-0.40 are in principle feasible. Since these cores allow TRU consumption, whatever the

Pu/MA ratio and fuel type, at close to 80% of the maximum theoretical consumption, it seems that U-free fuels could possibly be avoided, regardless of the scenario and specific P&T strategy.

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