Elemental analysis and radioactivity evaluation of aerosols generated during heating of simulated fuel debris; The Urasol project in the framework of Fukushima Daiichi fuel debris removal

Tsubota, Yoichi; Porcheron, E.*; Journeau, C.*; Delacroix, J.*; Suteau, C.*; Lallot, Y.*; Bouland, A.*; Roulet, D.*; Mitsugi, Takeshi

Proceedings of International Conference on Environmental Remediation and Radioactive Waste Management (ICEM2023) (Internet), 6 Pages, 2023/10

https://doi.org/10.1115/ICEM2023-110064

In order to safely remove fuel debris from the Fukushima Daiichi Nuclear Power Station (1F), it is necessary to quantitatively evaluate radioactive airborne particulate generated by the cutting of nuclear fuel debris. We fabricated Uranium-bearing simulated fuel debris (SFD) with In/Ex-Vessel compositions and evaluated the physical and chemical properties of aerosols generated by heating the SFDs. Based on these results, we estimated the isotopic composition and radioactivity of aerosols produced when 1F-Unit2 fuel debris is laser cut, which is a typical example of a heating method. Plutonium, mainly Pu,Am, and Cm were found to be the alpha nuclide, and Pu, Cs-Ba, and Sr-Y were found to be the beta nuclide of interest.

In-situ alpha aerosol monitoring in harsh environments

Tsubota, Yoichi

Isotope News, (787), p.28 - 31, 2023/06

In the fuel debris retrieval process during the decommissioning of the Fukushima Daiichi Nuclear Power Station (1F), radioactive particulates are generated in the primary containment vessel (PCV). Particulates containing nuclides (-aerosols), which have a particularly large internal exposure effect, are important to be monitored in addition to confinement measures in the PCV. The research group of the Japan Atomic Energy Agency (JAEA) summarized the four requirements for measuring -aerosols in the harsh environment of the 1F-PCV as follows: "reliable operation in a high humidity environment", "measurement of -aerosols without using filter paper", "measurement of high concentration of -aerosols", and "selective measurement of -aerosols". The IAAM was developed as an instrument that satisfies these four criteria. The IAAM was developed as an "in-situ" -aerosol measurement device that combines air heating, a flat-type flow path, direct measurement of -aerosol, usage of multi-channel photomultiplier tube, and an optimized scintillator thickness. Performance verification tests have demonstrated that the IAAM can be used for real-time measurement of -aerosol concentration in air, and that it can measure more than 30 times of -aerosol concentration expected in the 1F-PCV. In the future, we will continue to verify the measurement of -aerosol concentration at various sites and aim to apply the system to such sites.

Experiences from the cutting of metallic blocks from simulant Fukushima Daiichi fuel debris

Journeau, C.*; Molina, D.*; Brackx, E.*; Berlemont, R.*; Tsubota, Yoichi

Proceedings of International Topical Workshop on Fukushima Decommissioning Research (FDR2022) (Internet), 5 Pages, 2022/10

CEA has manufactured a series of Fukushima Daiichi fuel debris simulants, either with depleted uranium oxide or with hafnium oxide as a surrogate of UO. In ex-vessel compositions resulting from an interaction between corium and concrete, the oxidic phase density becomes lighter than that of the metallic phase, which segregates at the bottom. Three of these metallic phases have been mechanically cut at CEA Cadarache with handsaw and with core boring tool in FUJISAN facility. It appeared that two of these metallic blocks were extremely hard to cut (one from a fabrication with uranium oxide, the other from a simulant block) while the last one was more easily cut. The similarities and differences in metallographic analyses (SEM-EDS and XRD) of these three metal blocks will be presented and discussed. This experience provides useful learnings in view of the cutting and retrieval of fuel debris from Fukushima Daiichi.

Joint meeting of young researchers in the field of radiation protection and radiation biology

Kataoka, Noriaki*; Nakajima, Junya; Miwa, Kazuji; Hirota, Seiko*; Tsubota, Yoichi; Yamada, Ryohei; Fujimichi, Yuki*; Ishikawa, Junya*; Sunaoshi, Masaaki*

Hoken Butsuri (Internet), 54(2), p.140 - 145, 2019/06

https://doi.org/10.5453/jhps.54.140

It is the written report of the joint meeting of young researchers in the field of radiation protection and radiation biology.

Development of a Si-PM based alpha camera for plutonium detection in nuclear fuel facilities

Morishita, Yuki; Yamamoto, Seiichi*; Izaki, Kenji; Kaneko, Junichi*; Toi, Kohei*; Tsubota, Yoichi*

Nuclear Instruments and Methods in Physics Research A, 747, p.81 - 86, 2014/05

https://doi.org/10.1016/j.nima.2013.12.052

Alpha particles are monitored for detecting nuclear fuel material (i.e., plutonium and uranium) at nuclear fuel facilities. In this study, we developed a new alpha-particle imaging system by combining an Si-PM array, which is insensitive to noise, with a Ce-doped GdAlGaO (GAGG) scintillator, and evaluated our developed system's fundamental performance. The scintillator was 0.1-mm thick, and the light guide was 3.0 mm thick. An Am source was used for all the measurements. We evaluated the spatial resolution by taking an image of a resolution chart. A 1.6 lp/mm slit was clearly resolved, and the spatial resolution was estimated to be less than 0.6-mm FWHM. The energy resolution was 13% FWHM. A slight distortion was observed in the image, and the uniformity near its center was within 24%. We conclude that our developed alpha-particle imaging system is promising for plutonium detection at nuclear fuel facilities.

Application test of GPS scintillator plates for an alpha particle dust monitor

Toi, Kohei*; Shimaoka, Takehiro*; Kaneko, Junichi*; Izaki, Kenji; Tsubota, Yoichi*; Higuchi, Mikio*; Nishiyama, Shusuke*

no journal, , 

no abstracts in English

Development of a Si-PM based alpha camera for plutonium detection in nuclear fuel facilities

Morishita, Yuki; Yamamoto, Seiichi*; Izaki, Kenji; Kaneko, Junichi*; Toi, Kohei*; Tsubota, Yoichi*

no journal, , 

Light yield maximization of triclinic La-GPS scintillator by optimizing La and Ce concentration

Tsubota, Yoichi; Waki, Takaaki*; Kaneko, Junichi*; Higuchi, Mikio*; Ishibashi, Hiroyuki*

no journal, , 

GdSiO:Ce (GPS) is a rare-earth oxide scintillator with large light yield (LY) exceed NaI:Tl and can be used under high temp. environment. The triclinic La-GPS, which substituted Gd site for La, also have LY close to NaI:Tl. Although several trials on growth of La-GPS were reported, systematic research on LY were not sufficiently carried out. In this report, the optimization of La and Ce concentration of La-GPS was conducted to maximize LY. Sintered (LaxGd1-x-yCey)SiO (x= 0.1, y= 0 - 0.16) and (x= 0 - 0.15, y= 0.025) were fixed on glass plates and mechanically polished to be approx. 100 micro-meter thick. For the evaluation of scintillation LY, alpha-particle response measurement using Am was performed. For the Ce= 2.5% samples, La-GPS with 5 - 6.7% of La showed the highest LY. LY was then decreased as the La increased. For the La=10% samples, light yield increased as Ce increased in the range of Ce=0.5-2.5% that corresponded with the increase of luminescent center of Ce. Decrease of LY due to concentration quenching was observed in the range of Ce4%.

Development of direct-type alpha dust monitor for in-situ measurement of airborne concentration during fuel debris retrieval and decommissioning of nuclear fuel cycle facilities

Tsubota, Yoichi; Honda, Fumiya; Nakagawa, Takahiro; Kawasaki, Takashi; Ikeda, Atsushi; Tamakuma, Yuki*; Tokonami, Shinji*; Momose, Takumaro

no journal, , 

A large number of radioactive aerosols, especially alpha particles, are expected to be generated during fuel debris retrieval on Fukushima - Daiichi (1F) site and decommissioning of nuclear fuel cycle facilities. For conventional alpha dust monitor, 1-channel of silicon surface barrier detector (SSBD) is often used as a detector which has not so high limit of count rates. Moreover, since radioactive aerosol collected on a filter paper are measured by a detector, it is difficult to calculate an airborne concentration at the sampling site because a conventional dust monitor measures an integrated radioactivity on a filter paper. In this work, we developed a prototype of alpha dust monitor aiming at an in-situ measurement of high concentration airborne alpha particles. The efficiency for UO checking source was 80.8%. For high count rate test using 3 MBq of Am checking source, 2.1 10 cpm was achieved which was more than 10 times higher compared to an conventional alpha dust monitor using SSBD. For follow-up test to the airborne concentration of alpha particles using radon chamber, counting rate of prototype alpha dust monitor was smoothly followed with respect to the radon concentration in a radon chamber.

Development of elemental technology for alpha-ray dust monitor for in-situ measurement of radioactive aerosol in dismantling and decommissioning

Tsubota, Yoichi; Honda, Fumiya; Nakagawa, Takahiro; Kawasaki, Takashi; Tamakuma, Yuki*; Tokonami, Shinji*; Momose, Takumaro

no journal, , 

A large number of radioactive aerosols, especially alpha particles, are expected to be generated during fuel debris retrieval on Fukushima - Daiichi (1F) site and decommissioning of nuclear fuel cycle facilities. In the case of conventional alpha dust monitor, silicon surface barrier detector (SSBD) is often used as a detector which has not so high limit of count rates. Moreover, SSBD is not suitable for high humidity environment. In this work, we developed a prototype of alpha dust monitor aiming at an in-situ measurement of high concentration of airborne alpha particles. Approx. 20M cpm of counting rate was achieved which was more than 10 times higher compared to a conventional alpha dust monitor using SSBD. For follow-up test, counting rate of prototype alpha dust monitor was smoothly followed with respect to the airborne concentration of alpha particles.