15.1 Introduction
For sustainable nuclear power deployment, not only ensuring its enhanced safety
but also reduction of the environmental burden associated with radioactive waste
management is a challenging issue for the international community. History has
shown that obtaining public support is difficult for waste management plans that
involve mass disposal of radioactive waste with a half-life of tens of thousands of
years. Therefore, as one of the solutions, Toshiba has been developing a system that
takes into account that, for the time being, light water reactors (LWRs) have a
leading role in commercial nuclear power plants, which enables toxicity and
radioactivity of high-level waste to be reduced to those of natural uranium within
a few hundred years. This system is mainly characterized by a fast reactor core that
does not contain uranium in its fuel, that is, uranium-free TRU fuel. The use of
uranium-free TRU fuel makes it possible to maximize the TRU transmutation rate
in comparison with fuel containing uranium because it prevents the fuel itself from
producing new plutonium and minor actinides.
Although there was much research focused on TRU transmutation with uraniumfree fuels, each of these seems to have drawbacks from some aspect. First, for
instance, candidates such as Tc-based and W-based oxide fuel, inert matrix fuel
such as the rock-like oxide fuel containing mineral-like compounds, and
MgO-based oxide fuel provide solutions against issues associated with uraniumfree operation, that is, decrease in Doppler reactivity feedback and increase in
sodium void reactivity [1–3], but such types of inert matrix fuel may require new
technologies for reprocessing. Additionally, many processing phases necessary for
fabrication are costly. Second, an accelerator-driven transmutation system coupled
with a fast reactor using uranium-free metallic fuel is another candidate that also
can relax the issue of the reduced Doppler effect owing to its subcritical system [4–
7], but installation of the accelerator facility at a fast reactor site is less cost
competitive, especially when the system is not only a TRU burner but also a
commercial power plant. Thus, it is worthwhile to develop the TRU transmutation
system with uranium-free TRU fuel from the aspect of technological maturity and
simplicity, which results in lower cost. Subsequently, the concepts for the TRU
burner system with uranium-free TRU are derived from this background: fewer
R&D needs and a simple system.
First, by contrast with inert matrix fuels, metallic fuel can be fabricated by the
well-known injection casting method [8]. Moreover, metallic fuel is compatible
with pyro-process reprocessing that has been developed since the 1960s [9]. Application of an accelerator-driven system for transmutation needs further R&D than
that of a fast reactor system. Thus, the metallic fuel fast reactor is preferred for the
system.
Second, we aim to develop the TRU-burning system in commercial power
reactors while avoiding cost impact. For this reason, a system that can employ
the pyro-process for fuel reprocessing would be preferable because it does not need
156
K. Ishii et al.
For sustainable nuclear power deployment, not only ensuring its enhanced safety
but also reduction of the environmental burden associated with radioactive waste
management is a challenging issue for the international community. History has
shown that obtaining public support is difficult for waste management plans that
involve mass disposal of radioactive waste with a half-life of tens of thousands of
years. Therefore, as one of the solutions, Toshiba has been developing a system that
takes into account that, for the time being, light water reactors (LWRs) have a
leading role in commercial nuclear power plants, which enables toxicity and
radioactivity of high-level waste to be reduced to those of natural uranium within
a few hundred years. This system is mainly characterized by a fast reactor core that
does not contain uranium in its fuel, that is, uranium-free TRU fuel. The use of
uranium-free TRU fuel makes it possible to maximize the TRU transmutation rate
in comparison with fuel containing uranium because it prevents the fuel itself from
producing new plutonium and minor actinides.
Although there was much research focused on TRU transmutation with uraniumfree fuels, each of these seems to have drawbacks from some aspect. First, for
instance, candidates such as Tc-based and W-based oxide fuel, inert matrix fuel
such as the rock-like oxide fuel containing mineral-like compounds, and
MgO-based oxide fuel provide solutions against issues associated with uraniumfree operation, that is, decrease in Doppler reactivity feedback and increase in
sodium void reactivity [1–3], but such types of inert matrix fuel may require new
technologies for reprocessing. Additionally, many processing phases necessary for
fabrication are costly. Second, an accelerator-driven transmutation system coupled
with a fast reactor using uranium-free metallic fuel is another candidate that also
can relax the issue of the reduced Doppler effect owing to its subcritical system [4–
7], but installation of the accelerator facility at a fast reactor site is less cost
competitive, especially when the system is not only a TRU burner but also a
commercial power plant. Thus, it is worthwhile to develop the TRU transmutation
system with uranium-free TRU fuel from the aspect of technological maturity and
simplicity, which results in lower cost. Subsequently, the concepts for the TRU
burner system with uranium-free TRU are derived from this background: fewer
R&D needs and a simple system.
First, by contrast with inert matrix fuels, metallic fuel can be fabricated by the
well-known injection casting method [8]. Moreover, metallic fuel is compatible
with pyro-process reprocessing that has been developed since the 1960s [9]. Application of an accelerator-driven system for transmutation needs further R&D than
that of a fast reactor system. Thus, the metallic fuel fast reactor is preferred for the
system.
Second, we aim to develop the TRU-burning system in commercial power
reactors while avoiding cost impact. For this reason, a system that can employ
the pyro-process for fuel reprocessing would be preferable because it does not need
156
K. Ishii et al.