Speaker
Description
It has been 15 years since the Fukushima Daiichi nuclear power plant accident, and we are still working
toward our mission of achieving safe decommissioning, which is expected to require approximately 40 years. One of
the most difficult challenges is the presence of approximately 880 tons of nuclear fuel debris in the reactors that melted
during the accident. Key questions include: (1) how can the debris be safely removed, (2) how can nuclear fuel be
separated from alpha-contaminated waste within the debris, and (3) how can long-term storage of the debris be man-
aged? The answers depend strongly on the actual composition of the debris.
Japan Atomic Energy Agency (JAEA) has successfully obtained two debris samples, which were transported
to the JAEA campus in November 2024 and April 2025. From isotopic and elemental analyses using ICP-MS and
SEM, majority of the components, such as uranium and iron, have been identified. However, trace components (<1%),
including 243Am, 243Cm, 248Cm, 238Pu, 239Pu, and 10B, are difficult to identify due to interfering nuclides, even though
these nuclei are critical for controlling criticality and ensuring safe decommissioning.
To analyze such trace components using advanced nuclear physics and chemistry techniques, we employ
(1) Coulomb excitation activation analysis coupled with a Ge detector array and (2) alpha–gamma coincidence meas-
urement techniques. As a validation experiment for method (1), “mock” debris targets were prepared using electrodep-
osition techniques and irradiated with a 58Ni beam from the JAEA-Tokai tandem accelerator. The mock debris targets
were prepared by mixing natZr, natFe, natCr, natNi, and natPb to reproduce the composition of actual debris samples. In
addition, 248Cm was intentionally added as a trace component at approximately the 1% level. Rotational band peaks
from 248Cm were clearly observed, demonstrating the feasibility of the method.
Actual debris samples will be provided to our research group in 2026. We will then perform both alpha–
gamma coincidence measurements and Coulomb excitation activation analysis using real debris samples.