COMPLETE DOCUMENT (1862 kb) - OECD Nuclear Energy Agency

Figure II.15 Concept of double stratum fuel cycle
Both critical reactors (burner reactors) and sub-critical reactors (hybrid systems or
accelerator-driven systems) are potential candidates as dedicated transmutation systems used in the
second stratum. MA transmutation in critical reactors can be either homogeneous or heterogeneous. The
extreme case of heterogeneous recycling is a core loaded only with MAs (or with MAs and Pu). This
could maximise the transmutation rate in dedicated systems. Systems loaded with MA based fuel,
however, pose crucial problems related to reactivity coefficients (increase in the void reactivity
coefficient and decrease in the Doppler effect), and to the small delayed neutron fraction.
To mitigate these problems for critical systems, a considerable amount of U should be added
to the MA based fuel [94], preferably highly-enriched U to avoid undesirable accumulation of MAs, but
this results in reduced transmutation rate. For hybrid systems, its sub-criticality mitigates the problems
without adding U and thus allows the maximum transmutation rate.
2.3.4.1 Dedicated minor actinide burner reactor [95,96]
Two types of actinide burner reactors (ABRs) are designed at JAERI. The fuel material of
these ABRs is a MA-U nitride mixture. One type is lead-cooled with pin fuel (L-ABR) and the other is
He-cooled with pellets-type fuel (P-ABR). Nitride forms were selected as fuel material for these ABRs
because of good thermal properties, applicability to very high burn-up, expected stability and mutual
solubility of the various heavy-element component. The reactor core design parameters of these ABRs
are given in Table II.11. Their neutron energy spectrum is very hard with the core-averaged neutron
energy around 720 keV. These hard neutron spectra are very effective for direct fission of those MAs
which have fission thresholds above 600 keV. The MA fission in the ABRs is 190 to 200 kg/GWt-year.
164