Comments on the Analysis of the Fuel Debris Sample Retrieved from the Fukushima Daiichi Unit 2 Reactor
By Sakurai Megumu, former nuclear engineer
- The Events So Far
A fuel debris sample from the Unit 2 reactor of Tokyo Electric Power Co.’s (TEPCO’s) Fukushima Daiichi Nuclear Power Station has been extracted and analyzed. It was taken from the surface of the floor at the bottom of the pedestal using a telescopic device inserted through the hole designated as “X-6 penetration” in the Unit 2 containment vessel. The sampling work was completed on 7 November 2024, and the sample was taken to the Japan Atomic Energy Agency (JAEA) Oarai Nuclear Engineering Institute for analysis on 12 November. Non-destructive analysis was conducted there, and the results were announced at the end of January 2025. In addition, portions of the sample were transported to four institutes (JAEA Nuclear Science Research Institute, JAEA Harima, MHI Nuclear Development Co., and Nippon Nuclear Fuel Development Co., Ltd.) for subsequent detailed analyses.
According to the published results of the non-destructive analysis1,2, the sample’s appearance was heterogeneous and reddish-brown overall, with black spots and glossy portions visible on parts of the surface. The sample’s size was approx. 9 mm by approx. 7 mm, weighing 0.693 g. The surface dose rate was about 8 mSv/h. X-ray computed tomography showed pores widely dispersed throughout the sample. Since gamma-ray analysis detected americium-241, which can only be derived from the fuel components, and surface elemental analysis using X-ray spectroscopy detected uranium, the sample was thought to contain fuel components. The uranium was heterogenously but widely distributed on the sample’s surface. In addition, zirconium from the fuel cladding and other components, as well as iron, chromium and nickel from structural materials, were detected in the surface elemental analysis. Silicon, calcium and magnesium were also found, possibly derived from seawater. Thus, so far, the results have not differed from anticipations, and the goal of the non-destructive analysis can be considered to have been achieved. The detailed analyses to be carried out at the five institutes will take from half a year to about a year. TEPCO is planning to retrieve additional debris samples using the telescopic device currently installed at the Unit 2 reactor and is considering starting this work this spring3.
- Stated Goals of the Debris Analysis
Table 1 below lists the items and details to be evaluated in the detailed analyses1. JAEA has defined the purposes of these analyses as “1) Grasping the condition of the sampled area,” and “2) Estimation of formation process of fuel debris.” The latter, in particular, is estimated from the composition and crystal structure, so information such as the materials involved in the formation of the debris, temperatures reached, and degree of oxidation is also pertinent. By utilizing existing knowledge such as materials science, it is possible to infer what kind of debris will be generated under what conditions in the reactor. Conversely, by examining the debris generated, it is also possible to infer the conditions present at the time of its generation and the history of its development. In other words, they are seeking information on the accident scenario— “what was going on at that time.” The purposes JAEA has stated for its debris analyses are to gain a more precise understanding of the conditions in the reactor by comparing the results of the analyses with existing accident scenarios and results of internal investigations, and to serve as a basis for study toward full-scale debris removal. Previously, JAEA has analyzed uranium-containing particles from samples such as sediments and deposits (including from reactors other than Unit 2) obtained through its investigations of places like the interior of the containment vessel. This way they could get a grasp of the environment in the reactor at the time by making inferences about the processes generating those particles2, so by finally analyzing the “real” debris, they hope to improve the certainty of their previous findings.
- Is This a Good Way to Proceed?
Although the debris sample that was obtained this time was tiny, at 0.7 g, it is “huge” from the standpoint of analytical chemistry and will probably provide more information than most people can imagine. Nevertheless, it constitutes no more than a tiny portion of the enormous amount of debris, the properties of which may vary greatly depending on where it was found within the unit and its location within the reactor.
In the author’s opinion, this analysis is similar to that of the asteroid sample fetched by the Hayabusa spacecraft (in fact, many methods used to analyze these two samples have a lot in common). The analysis of the Hayabusa sample was “interesting,” and that is fine, but I think this Fukushima debris analysis also has aspects of satisfying researchers’ intellectual curiosity. Researchers performing simulations of scenarios, in particular, want to confirm their conjectures with the “real thing” (but I would like to point out that extracting and analyzing the “real thing” involved exposing workers to it).
These debris analyses are being conducted in the name of providing a basis for considering debris removal, as a means of justifying the Nuclear Damage Compensation and Decommissioning Facilitation Corporation’s goal in “Technical Strategic Plan 2024 for Decommissioning” to “retrieve fuel debris safely after thorough and careful preparations, and bring it to a state of stable storage that is fully managed.”4 However, it does nothing beyond “providing a basis,” and the connection with the decommissioning of the reactor, which is the main concern, is unclear. I wonder if it would be going too far to say that they are simply doing what they can for the time being, giving the impression of making progress, and showing enthusiasm about the results.
TEPCO says it will collect debris samples again from the same place in the same facility as this time. Although I acknowledge the significance of accumulating knowledge from multiple samples, that information is still limited. What do they plan to do after that? Moreover, the detailed analyses of this one sample alone will take a half a year to a year to complete. And what is their schedule for “considering debris removal” as they say? While how this relates to what happens beyond that remains unclear, will it not just result in large sums of money and human resources continuing to be poured aimlessly into it?
After all, shouldn’t we be having a wide-ranging national debate about the way the Fukushima Daiichi plant should be decommissioned, including the pros and cons of continuing to pursue debris recovery? Even if they are just taking one debris sample and analyzing it this time, one cannot help but think that.
1JAEA & TEPCO, “Non-destructive analysis results (follow-up report) and fractionation results of fuel debris sample,” Mid-to Long-Term Roadmap Materials, 30 January 2025.
2Ikeuchi Hirotomo, “Closing in on the properties of fuel debris deposited in the reactor—A sample analysis approach—,” JAEA Fukushima Research and Engineering Institute, Annual meeting to report results of 2024, 31 January 2025 (including questions and answers at the debrief meeting).
fukushima.jaea.go.jp/info/Templates/PDF/20250131-shiryo_ikeuchi.pdf (in Japanese)
3IRID & TEPCO, “Fukushima Daiichi Nuclear Power Station Unit 2 PCV Internal Investigation/Status of Fuel Debris Trial Retrieval,” Mid-to Long-Term Roadmap Materials, 30 January 2025.
4Nuclear Damage Compensation and Decommissioning Facilitation Corporation, “Technical Strategic Plan 2024 for Decommissioning of the Fukushima Daiichi Nuclear Power Station of Tokyo Electric Power Company Holdings, Inc.,” p.45, 27 September 2024.
dd-ndf.s2.kuroco-edge.jp/files/user/pdf/en/strategic-plan/book/20241216_SP2024eFT.pdf
