For the first point, a simple indicator of the capture to fission balance is the reaction ratio a = Capture/Fission. This ratio is heavily dependent on the neutron spectrum, as can be seen in Table 17.2. For most TRUs, the most favorable (i. e. low) a ratio values are obtained in a fast neutron spectrum. The fact that MA isotopes act as ‘neutronic poisons’ (i. e. they have high a values) in a thermal neutron spectrum, indicates in principle that additional fissile enrichment is likely to be needed, if these isotopes are loaded into a thermal neutron core. In addition the table illustrates the well-known saying that the even-numbered isotopes of plutonium are not much fissionable and shows that this only applies to thermal reactors.

For the second point, a fast neutron spectrum reactor leads to fewer high mass isotopes since, compared to a thermal reactor, the TRU isotopes are more likely to fission. A typical example is the build-up of Cf-252 (a very strong neutron emitter by spontaneous fission), see Fig. 17.2 . A consequence of higher quantities of higher mass TRU isotopes in thermal reactor fuel (especially the shorter-lived Am and Cm isotopes, see Table 17.1) is an increase of the decay heat.

17.2 Cf-252 inventory in the core. (a) For grouped TRU multi­recycling in a LWR. (b) For grouped TRU multi-recycling in a FR.

A radionuclide-specific issue, whatever the neutron spectrum, is related to fuels containing large amounts of Am-241. Here, Cm-242 produced by neutron capture decays by a emission to Pu-238. a production leads to helium deposition in the fuel and this is sufficiently great to require special design measures to accommodate it.

For the third point, for a full understanding of the transmutation potential of different neutron fields one can use the very simple notion of total neutron consumption per fission, Dj, of an isotope family j, similar to that introduced in Ref. 4. In the case of a negative D (i. e., when the j-family produces more neutrons than it consumes), a core fuelled by j-nuclides can produce enough neutrons to destroy the source material at equilibrium if the neutron excess compensates for parasitic captures (e. g. by structures, fission products, etc.) and for neutron leakage. In the case of a positive Df neutron consumption by the j-nuclides dominates over neutron production and the core cannot support transmutation unless a supplementary neutron source is provided.

The global neutron balance of a core must consider the total neutron production (or consumption) of the relevant fuel families, the parasitic captures of other core components (C ) and of the accumulated fission products (CFP) and neutron leakage (Lcore). A very simple and general equation for the neutron surplus (NScore) can be written as:

— D) — C — CFP — L = NS [17.1]

fuel par FP core core

with all quantities of Eq. 17.1 normalized to one fission. For a critical system, the neutron surplus must equal zero and the neutron production of the fuel feed must be sufficient to overcome the other loss terms. Thus, D) , is a useful parameter for quantifying the transmutation potential of a given isotopic fuel mix.

Due to the fact that most MAs act as strong neutron ‘consumers’ in a thermal neutron spectrum (i. e. having unfavorable fission-to-capture rates, see Table 17.2), the D values corresponding to a LWR fuel loaded with MAs can become positive, indicating that the neutron balance is very tight (i. e. no ‘neutron excess’ is available) and that a further fissile enrichment would be needed (a potentially significant economic penalty). An example of NScore values is given in Table 17.3.