Effect of molybdenum release on UO/MOX fuel oxidation under severe light water reactor accident conditions

Liu, J.; Miwa, Shuhei; Karasawa, Hidetoshi; Osaka, Masahiko

Nuclear Materials and Energy (Internet), 37, p.101532_1 - 101532_5, 2023/12

https://doi.org/10.1016/j.nme.2023.101532

Chemical interaction between Sr vapor species and nuclear reactor core structure

Mohamad, A. B.; Nakajima, Kunihisa; Miwa, Shuhei; Osaka, Masahiko

Journal of Nuclear Science and Technology, 60(3), p.215 - 222, 2023/03

https://doi.org/10.1080/00223131.2022.2092227

Tritium inventory and its temporal variation in Fukushima Front Sea Area; Comparison between coastal and offshore tritium inventories and 1f treated water and operational target values for discharge per year

Machida, Masahiko; Iwata, Ayako; Yamada, Susumu; Otosaka, Shigeyoshi*; Kobayashi, Takuya; Funasaka, Hideyuki*; Morita, Takami*

Nihon Genshiryoku Gakkai Wabun Rombunshi (Internet), 22(1), p.12 - 24, 2023/01

https://doi.org/10.3327/taesj.J22.001

We estimate inventory of tritium in two sea areas corresponding to coastal and offshore ones around Fukushima Daiichi Nuclear Power Plant (1F) based on the measurement results of sea-water tritium concentration monitored constantly from 2013 to Jan. 2021 by using Voronoi tessellation scheme. The obtained results show that the offshore area inventory and its temporal variation amount correspond to approximately 1/5 and 1/40 of that of the treated-water accumulated inside 1F, respectively. These results just suggest that the presence of tritium already included in sea-water as the background is non negligible in evaluating the environmental impact by releasing the accumulated treated-water into the sea region. We also estimate the offshore area inventory before 1F accident and find that it had exceeded over 1F stored inventory over about 30 years from 1960s to 1980s with approximately 4 times larger in the peak decade, 1960s. This fact means that we had already experienced more contaminated situation over 30 years in the past compared to the conservative case appeared by just releasing whole the present 1F inventory. Here, it should be also emphasized that the past contaminated situation was shared by the entire world. We further extend the estimation region into a wider region including an offshore area from Miyagi to Chiba prefectures and find that the area average inventory is now comparable to a half of the present 1F one. Finally, we estimate internal dose per year via ingesting fishes caught inside the area when 1F inventory is just added inside the area and kept for a year. The result indicates that it approximately corresponds to 1.0 of the dose from natural radiation sources. From these estimation results, it is found that all the tritium inventory stored inside 1F never contribute to significant dose increment even when it is instantly released into the area.

High-temperature gaseous reaction of cesium with siliceous thermal insulation; The Potential implication to the provenance of enigmatic Fukushima cesium-bearing material

Rizaal, M.; Nakajima, Kunihisa; Saito, Takumi*; Osaka, Masahiko; Okamoto, Koji*

ACS Omega (Internet), 7(33), p.29326 - 29336, 2022/08

https://doi.org/10.1021/acsomega.2c03525

Phase stability of Cs-Si-O and Cs-Si-Fe-O compounds on stainless steel

Suzuki, Chikashi; Nakajima, Kunihisa; Osaka, Masahiko

Journal of Nuclear Science and Technology, 59(3), p.345 - 356, 2022/03

https://doi.org/10.1080/00223131.2021.1971576

During a severe accident (SA) such as the Fukushima Daiichi Nuclear Power Plant accident, fission products (FP) can be retained on the surface of structural materials in reactors. Cesium (Cs) is an important FP, and various Cs compounds such as Cs silicates are formed on the surface of stainless steel (SS) in a reactor during a SA. We calculated total energies of Cs-Si-O compounds for evaluation on phase stability within an adiabatic approximation. The calculations indicate that CsSiO is the most stable of the Cs-Si-O compounds. We calculated, furthermore, total energies of Cs-Si-Fe-O compounds. These calculations indicate that Cs-Si-Fe-O compounds are more stable than C-Si-O compounds and that CsSiFeO is the most stable of these C-Si-O and Cs-Si-Fe-O compounds within an adiabatic approximation. The results of our present calculations and our previous experiments lead to the conclusion that Cs-Si-Fe-O compounds can be stably formed on SS surface by Cs chemisorption.

Estimation of temporal variation of discharged tritium from port of Fukushima Dai-ichi Nuclear Power Plant; Analysis of the temporal variation and comparison with released tritium from Japan and major nuclear facilities worldwide

Machida, Masahiko; Iwata, Ayako; Yamada, Susumu; Otosaka, Shigeyoshi*; Kobayashi, Takuya; Funasaka, Hideyuki*; Morita, Takami*

Nihon Genshiryoku Gakkai Wabun Rombunshi, 21(1), p.33 - 49, 2022/03

https://doi.org/10.3327/taesj.J20.036

We estimate monthly discharge inventory of tritium from the port of Fukushima Daiichi Nuclear Power Plant (1F) from Jun 2013 to Mar 2020 by using the Voronoi tessellation scheme, following that the tritium monitoring inside the port has started since Jun 2013. As for the missing period from the initial month, Apr 2011 to May 2013, we calculate it by utilizing the concentration ratio of tritium to that of Cs in stagnant contaminant water during the initial direct discharged period to Jun 2011 and the discharge inventory correlation between tritium and Cs for the next-unknown continuously-discharged period up to May 2013. From the all- estimated results over 9 years, we find that the monthly discharge inventory sharply dropped just after closing the sea-side impermeable sea-wall in Oct. 2015 and subsequently coincided well with the sum of those of drainage and subdrain. By comparing the estimated results with those in the normal operation period before the accident, we point out that the discharge inventory from 1F port is not so large compared to those during the normal operation. Even the estimated one in year 2011 is found to be comparable to the maximum of operating pressurized water reactors discharging relatively large inventory in the order. In the nation level, the whole Japan domestic discharge inventory significantly decreased after the accident due to operation shutdown of most plants. Furthermore, 1F and even Japanese total discharge inventory are found to be entirely minor when comparing those of nuclear reprocessing plants and heavy-water reactors in world-wide level. From the above, we suggest that various scenarios can be openly discussed on the management in tritium stored inside 1F with help of the present estimated data and its comparison with the past discharge inventory.

Study on solubility of cesium iodide and cesium molybdate in water at around room temperature

Imoto, Jumpei; Nakajima, Kunihisa; Osaka, Masahiko

Nihon Genshiryoku Gakkai Wabun Rombunshi, 20(4), p.179 - 187, 2021/12

https://doi.org/10.3327/taesj.J20.037

Some of the Cs inside the Fukushima Daiichi Nuclear Power Station would be deposited in chemical forms such as CsI and CsMoO. Since Cs compounds are generally water-soluble, it is predicted that the migration of Cs through the aqueous phase occurs in the long term. Knowledge of the solubility in water is required as basic data for such migration behavior evaluation. Therefore, this study was conducted to investigate the dissolution properties of CsI and CsMoO in water at 20C and 25C. The solubilities of CsI at 25C calculated using thermodynamic data and the Pitzer ion interaction model were in good agreement with the literature value. It was found that the literature value of CsI at around room temperature is highly reliable. The experimental value of CsI at 20C obtained by the OECD test guideline 105 flask method (test guideline) was also in good agreement with the literature value. The measured solubility of CsMoO was 256.8 6.2 (g/100 g HO) at 20C using the test guideline. This measured solubility of CsMoO was found to be comparable to those of other alkaline molybdates and considered to be more reliable than the literature value.

An Approach toward evaluation of long-term fission product distributions in the Fukushima Daiichi Nuclear Power Plant after the severe accident

Uchida, Shunsuke; Karasawa, Hidetoshi; Kino, Chiaki*; Pellegrini, M.*; Naito, Masanori*; Osaka, Masahiko

Nuclear Engineering and Design, 380, p.111256_1 - 111256_19, 2021/08

https://doi.org/10.1016/j.nucengdes.2021.111256

It is essential to grasp the long-term distributions of FP as well as fuel debris all over the Fukushima Daiichi Nuclear Power Plant (1F) for safe completion of its decommissioning projects. The fuel debris is going to be removed from the plant under the severe conditions of FP being scattered during major decommissioning work, and then, the decommissioning projects are going to be terminated by storing safely the removed debris as recovered fertile materials or as materials for final radioactive disposal. In order to determine the FP distribution in the plant for the long period from the accident occurrence to the termination of the plant decommissioning, procedures for analyzing multi-term FP behaviors were proposed. The proposed procedures should be improved by applying the FP data measured in the plant and validated based on the feedback data. Then, the accuracy-improved procedures should be applied to estimate FP distribution during each period of the decommissioning projects.

Experimental study on transport behavior of cesium iodide in the reactor coolant system under LWR severe accident conditions

Miyahara, Naoya; Miwa, Shuhei; Goullo, M.*; Imoto, Jumpei; Horiguchi, Naoki; Sato, Isamu*; Osaka, Masahiko

Journal of Nuclear Science and Technology, 57(12), p.1287 - 1296, 2020/12

https://doi.org/10.1080/00223131.2020.1782281

In order to clarify the cesium iodide (CsI) transport behavior with a focus on the mechanisms of gaseous iodine formation in the reactor coolant system of LWR under a severe accident condition, a reproductive experiment of CsI transport behavior was conducted using a facility equipped with a thermal gradient tube. Various analyses on deposits and airborne materials during transportation could elucidate two mechanisms for the gaseous iodine formation. One was the gaseous phase chemical reaction in Cs-I-O-H system at relatively high-temperature region, which led to gaseous iodine transport to the lower temperature region without any further changes in gas species due to the kinetics limitation effects. The other one was the chemical reactions related to condensed phase of CsI, namely those of CsI deposits on walls with surface of stainless steel to form CsCrO compound and CsI aerosol particles with steam, which were newly found in this study.

Seven-year temporal variation of caesium-137 discharge inventory from the port of Fukushima Daiichi Nuclear Power Plant; Continuous monthly estimation of caesium-137 discharge in the period from April 2011 to June 2018

Machida, Masahiko; Yamada, Susumu; Iwata, Ayako; Otosaka, Shigeyoshi; Kobayashi, Takuya; Watanabe, Masahisa; Funasaka, Hideyuki; Morita, Takami*

Journal of Nuclear Science and Technology, 57(8), p.939 - 950, 2020/08

https://doi.org/10.1080/00223131.2020.1740809

After direct discharges of highly contaminated water of the Fukushima Daiichi Nuclear Power Plant (1F) from April to May 2011, Kanda suggested that relatively small amounts of run-off of radionuclides from the 1F port into the Fukushima coastal region subsequently continued by his estimation method. However, the estimation period was limited to up to September 2012. Therefore, this paper estimates the discharge inventory up to June 2018. In the missing period, the Japanese government and Tokyo Electric Power Company Holdings have continued efforts to stop the discharge, and consequently, the radionuclide concentration in seawater inside the 1F port has gradually diminished. We show the monthly discharge inventory of Cs up to June 2018 by two methods, i.e., Kanda method partially improved by the authors and a more sophisticated method using Voronoi tessellation reflecting the increase in the number of monitoring points inside the 1 F port. The results show that the former always yields overestimated results compared with the latter, but the ratio of the former to the latter is less than one order of magnitude. Using these results, we evaluate the impact of the discharge inventory from the 1F port into the coastal area and radiation dose upon fish ingestion.