PLENTIFUL ENERGY

like breeding, does increase such damage. But three- or four-year residence times
for fuel are perfectly satisfactory, and in fact are typical for all power reactors.
14.5 Considerations for Burner vs. Breeder
There have been suggestions that higher actinides created in other reactors
should be destroyed by irradiation in reactors with a fast neutron spectrum.
Certainly at present, when plutonium stocks from LWRs are building up
worldwide, and some weapons plutonium stocks having been declared surplus, a
case can be made for reactor configurations that burn more of these actinide
elements than they create. The IFR core can be designed as a burner or breeder of
fissile isotopes, or indeed of all actinides. All are fissionable in the IFR neutron
spectrum. If the core shown in Figure 14-5 is surrounded by reflector assemblies
(steel components that tend to reflect neutrons back into the core, thus conserving
neutrons), the core itself will not breed enough to break even in fissile conversion,
and the core becomes a net burner of fissile isotopes and total actinides. If the
reflector assemblies are replaced by blanket assemblies of depleted uranium,
leakage neutrons are captured by the uranium and produce plutonium that can later
be harvested by pyroprocessing and used to manufacture new fuel pins. If adequate
blankets are provided (including axial as well as radial), the breeding ratio (the ratio
of fissile production to fissile destruction) can substantially exceed unity. Selfsufficiency
only, a breeding ratio of unity, indeed any burner or breeder
configuration desired, is easily achieved by such blanket adjustments during
planned refueling outages at any time during the reactor‘s lifetime.
As a rule of thumb, an LWR operating for thirty years generates enough
actinides to start up an IFR. The LWRs currently operating worldwide, a capacity
of about 375 GWe, operating for a sixty-year lifetime, will have generated
sufficient actinides to start up about 750 GWe IFRs. Even if only a fraction of the
LWR spent fuel is reprocessed, a large number of IFRs can be built without the
need yet to breed. The penalty of such breeding is only that blanket assemblies will
have to be processed, which can mean more heavy metal throughput than for the
core fuel itself. If reflectors are installed, this cost can be avoided until such time as
breeding is required.
There are two figures of merit that have been put forward for designers
concentrating their attention on actinide burning instead of breeding. [13] Clearly
there is incentive to burn actinides in IFRs. Where burning actinides instead of
replacing them by normal breeding has been emphasized, the net actinide
destruction rate in terms of kg/GWe-yr is used as a figure of merit. This emphasizes
non-fertile fueling (little or no uranium in the fuel), and even non-reactor options
such as accelerator-driven subcritical systems emerge as possible optimum
strategies.
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