For the safe application of nuclear power, radioactive waste processing
is one of the most important problems. In particular, the handling
of Minor Actinide (MA) wastes such as neptunium and americium
nuclides with very long half-lives of several thousand years to
several million years is most difficult. As one of the major themes
in the Neutron Science Project, JAERI performs a research and
development on an accelerator-driven transmutation plant which
effectively process these radioactive wastes. In the plant, MA
wastes are used as fuel materials in a subcritical transmutation
core and the chain reaction in the core is maintained by spallation
neutrons generated in the target bombarded by a high energy proton
beam from an accelerator. As a result, this plant can transmute
the MA's and can supply the power necessary to operate its own
accelerator by converting the MA's fission energy to electric
power.
As a first step of the development, JAERI has carried out a conceptual
design study of an experimental reactor to simulate the commercial-type
transmutation plant loaded with MA fuels, in which several fuel
handling difficulties are anticipated. At present it has been
concluded that the newly proposed uranium nitride fueled core
can achieve about 10% of the MA transmutation reaction ratios.
In the design study, its core parameters are adjusted to give
an effective neutron multiplication factor of about 0.93 (neutron
multiplicity of 14). The concept of the experimental reactor is
shown in Fig. 10-3 and the characteristics is given in Table 10-1.
It is also found that the neutron spectrum in the experimental
reactor can reproduce very well the one obtained in designing
a commercial-type core as shown in Fig. 10-4. The design result
has also given good performance in transmuting the long-lived
fission products such as technetium and iodine, large amounts
of which are also included in the spent fuel. The present study
has given us a clear guideline to establish a method for designing
a transmutation demonstration experiment system. |
