There are two main issues associated with the TRU burning fast reactor cycle
using uranium-free metallic fuel in terms of practicability:
(1) Decrease in the absolute value of the negative Doppler reactivity coefficient
resulting from absence of uranium-238, which has the ability to absorb neutrons
at elevated temperatures. example,
metallic fuel with uranium: À1 Â 10
À3 Tdk/dT
metallic fuel without uranium: À6 Â 10
À4 Tdk/dT
(2) Increase in burn-up reactivity swing as fissile decreases monotonically in
uranium-free core. example,
metallic fuel with uranium: ~1 %dk/kk
0 /150 days
metallic fuel without uranium: ~6 %dk/kk
0 /150 days
To solve these issues, there are several candidates, as follows:
(1) Enhance Doppler feedback
– Introduce diluent material in the metallic fuel
– Introduce spectrum moderator
(2) Reduce burn-up reactivity swing
– Reduce the core height
– Introduce neutron absorber outside the core
– Increase the number of refueling batches
Generally, if it is conventional fast reactors with U-Pu fuel, the burn-up reactivity swing depends mainly on decrease of fissile amount and increase of neutron
parasitic capture of fission products and actinides from burn-up. Therefore, the
typical ways to reduce burn-up reactivity swing are to increase conversion ratio via
fissile enrichment reduction and to reduce neutron parasitic capture. Here, the
conversion ratio is defined as the amount of fissile materials production divided
by the amount of neutron absorption, that is, fission and capture, and natural decay
of fissile materials. It is difficult, however, for a uranium-free core to increase the
conversion ratio because fissile enrichment cannot be controlled in the absence of
uranium. Although the reduction of neutron parasitic capture by neutron spectrum
hardening improves burn-up reactivity swing, it also harms the Doppler effect. For
these reasons, when it comes to uranium-free core, increase of the fissile amount at
the beginning of the cycle makes sense because it reduces the ratio of the fissile
consumption to the fissile amount at the beginning of the cycle.
These candidates were parametrically surveyed to evaluate the feasibility of the
uranium-free TRU metallic fuel fast reactor core in light of aforementioned issues.
The targets assumed were the core performances with the Doppler reactivity
coefficient equivalent to a conventional U-Pu metallic fuel core. Furthermore,
constrains associated with fuel fabrication such as melting temperature was taken
into consideration because, in this evaluation, diluent material was assumed to be
used as a fuel slug alloy, not cladding material. Hence, the slug was assumed be
158
K. Ishii et al.
using uranium-free metallic fuel in terms of practicability:
(1) Decrease in the absolute value of the negative Doppler reactivity coefficient
resulting from absence of uranium-238, which has the ability to absorb neutrons
at elevated temperatures. example,
metallic fuel with uranium: À1 Â 10
À3 Tdk/dT
metallic fuel without uranium: À6 Â 10
À4 Tdk/dT
(2) Increase in burn-up reactivity swing as fissile decreases monotonically in
uranium-free core. example,
metallic fuel with uranium: ~1 %dk/kk
0 /150 days
metallic fuel without uranium: ~6 %dk/kk
0 /150 days
To solve these issues, there are several candidates, as follows:
(1) Enhance Doppler feedback
– Introduce diluent material in the metallic fuel
– Introduce spectrum moderator
(2) Reduce burn-up reactivity swing
– Reduce the core height
– Introduce neutron absorber outside the core
– Increase the number of refueling batches
Generally, if it is conventional fast reactors with U-Pu fuel, the burn-up reactivity swing depends mainly on decrease of fissile amount and increase of neutron
parasitic capture of fission products and actinides from burn-up. Therefore, the
typical ways to reduce burn-up reactivity swing are to increase conversion ratio via
fissile enrichment reduction and to reduce neutron parasitic capture. Here, the
conversion ratio is defined as the amount of fissile materials production divided
by the amount of neutron absorption, that is, fission and capture, and natural decay
of fissile materials. It is difficult, however, for a uranium-free core to increase the
conversion ratio because fissile enrichment cannot be controlled in the absence of
uranium. Although the reduction of neutron parasitic capture by neutron spectrum
hardening improves burn-up reactivity swing, it also harms the Doppler effect. For
these reasons, when it comes to uranium-free core, increase of the fissile amount at
the beginning of the cycle makes sense because it reduces the ratio of the fissile
consumption to the fissile amount at the beginning of the cycle.
These candidates were parametrically surveyed to evaluate the feasibility of the
uranium-free TRU metallic fuel fast reactor core in light of aforementioned issues.
The targets assumed were the core performances with the Doppler reactivity
coefficient equivalent to a conventional U-Pu metallic fuel core. Furthermore,
constrains associated with fuel fabrication such as melting temperature was taken
into consideration because, in this evaluation, diluent material was assumed to be
used as a fuel slug alloy, not cladding material. Hence, the slug was assumed be
158
K. Ishii et al.