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

Improvement of model for cesium chemisorption onto stainless steel in severe accident analysis code SAMPSON (Joint research)

Miwa, Shuhei; Karasawa, Hidetoshi; Nakajima, Kunihisa; Kino, Chiaki*; Suzuki, Eriko; Imoto, Jumpei

JAEA-Data/Code 2021-022, 32 Pages, 2023/01

JAEA-Data-Code-2021-022.pdf:1.41MB
JAEA-Data-Code-2021-022(errata).pdf:0.17MB

The improved model for cesium (Cs) chemisorption onto stainless steel (SS) in the fission product (FP) chemistry database named ECUME was incorporated into the severe accident (SA) analysis code SAMPSON for the more accurate estimation of Cs distribution within nuclear reactor vessels in the TEPCO's Fukushima Daiichi Nuclear Power Station (1F). The SAMPSON with the improved model was verified based on the analysis results reproducing the experimental results which were subjected to the modeling of Cs chemisorption behavior. Then, the experiment in the facility with the temperature gradient tube to simulate SA conditions such as temperature decrease and aerosol formation was analyzed to confirm availability of the improved model to the analysis of Cs chemisorption onto SS. The SAMPSON with the improved model successfully reproduced the experimental results, which indicates that the improved model and the analytical method such as setting a method of node-junction, models of aerosol formation and the calculation method of saturated CsOH vapor pressure can be applicable to the analysis of Cs chemisorption behavior. As the information on water-solubility of Cs deposits was also prerequisite to estimate the Cs distribution in the 1F because Cs can be transported through aqueous phase after the SA, the water-solubility of chemisorbed Cs compounds was investigated. The chemisorbed compounds on SS304 have been identified to CsFeO at 873 K to 973 K with higher water-solubility, CsFeSiO at 973 K to 1273 K and CsSiO at 1073 K to 1273 K with lower water-solubility. From these results, the water-solubility of chemisorbed Cs compounds can be estimated according to the SA analysis conditions such as temperature in the reactor and the CsOH concentration affecting the amount of chemisorbed Cs.

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.

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.

Investigation of high-temperature chemical interaction of calcium silicate insulation and cesium hydroxide

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

Journal of Nuclear Science and Technology, 57(9), p.1062 - 1073, 2020/09

https://doi.org/10.1080/00223131.2020.1755733

The interaction of cesium hydroxide and a calcium silicate insulation material was experimentally investigated at high temperature conditions. A thermogravimetry equipped with differential thermal analysis was used to analyze thermal events in the samples of mixed calcium silicate and cesium hydroxide under Ar-5%H and Ar-4%H-20%H0 with maximum temperature of 1100C. Prior being mixed with cesium hydroxide, a part of calcium silicate was pretreated at high temperature to evaluate the effect of possible structural changes of this material due to a preceding thermal history and also the sake of thermodynamic evaluation to those available ones. Based upon the initial condition (preliminary heat treatment) of calcium silicate, it was found that if the original material consisted of xonotlite (CaSi0(0H)), the endothermic reaction with cesium hydroxide occurred over the temperature range 575-730C meanwhile if the crystal phase of original material was changed to wollastonite (CaSi0), the interaction occurred over temperature range 700-1100C. Furthermore, the X-ray diffraction analyses have indicated on both type of pretreated calsils that regardless of Ar-5%H and Ar-4%H-20%H0 atmosphere, cesium aluminum silicate, CsAlSi0 was formed with aluminum in the samples as an impurity or adduct.

Room-temperature adsorption behavior of cesium onto calcium silicate insulation

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

Mechanical Engineering Journal (Internet), 7(3), p.19-00563_1 - 19-00563_10, 2020/06

https://doi.org/10.1299/mej.19-00563

The adsorption of cesium (Cs) on calcium silicate insulation of primary piping system is postulated to contribute in high dose rate of surrounding pedestal area in Fukushima Daiichi NPP unit 2. In this study, room-temperature experiment of Cs adsorption on calcium silicate has been studied as an initial approach of Cs adsorption behavior toward higher temperature condition. As the result of analyzing of Cs adsorption kinetics, it was expected that the underlying adsorption mechanism is chemisorption. Furthermore, analysis of adsorption isotherm suggested unrestricted monolayer formation followed by multilayer formation.

Study on chemisorption model of cesium hydroxide onto stainless steel type 304

Nakajima, Kunihisa; Nishioka, Shunichiro*; Suzuki, Eriko; Osaka, Masahiko

Mechanical Engineering Journal (Internet), 7(3), p.19-00564_1 - 19-00564_14, 2020/06

https://doi.org/10.1299/mej.19-00564

A large amount of cesium (Cs) chemisorbed onto stainless steel is predicted to be present especially in the upper region of reactor pressure vessel (RPV) during light water reactor severe accident (LWR SA) and a chemisorption model was developed for estimation of such amounts of Cs for stainless steel type 304 (SS304). However, this existing chemisorption model cannot accurately reproduce experimental results. Therefore, in this study, a modified Cs chemisorption model which accounts for silicon content in SS304 and concentration of cesium hydroxide (CsOH) in gaseous phases was constructed by combining penetration theory for gas-liquid mass transfer with chemical reaction and mass action law for CsOH decomposition at interface between gaseous and solid phases. As a result, it was found that the modified model was able to reproduce the experimental data more accurately than the existing model.

An Experimental investigation of influencing chemical factors on Cs-chemisorption behavior onto stainless steel

Nishioka, Shunichiro; Nakajima, Kunihisa; Suzuki, Eriko; Osaka, Masahiko

Journal of Nuclear Science and Technology, 56(11), p.988 - 995, 2019/11

https://doi.org/10.1080/00223131.2019.1633968

In order to contribute to improvement of Cs chemisorption model used in severe accident analysis codes, the influence of chemical factors (temperature, atmosphere, concentration of affecting chemical elements etc.) on the Cs chemisorption behaviour onto stainless steel was investigated experimentally. It was found that the surface reaction rate constant used in the current Cs-chemisorption model was influenced by not only temperature, as already known, but also atmosphere, cesium hydroxide (CsOH) concentration in the gas phase and silicon content in SS304. Such chemical factors should be considered for the construction of the improved Cs-chemisorption model. Another important finding is that the chemisorption behavior at lower temperatures, around 873 K, could differ from those above 1073 K. Namely, Cs-Fe-O compounds would form as the main Cs-chemisorbed compounds at 873 K while Cs-Si-Fe-O compounds at more than 1073 K.

Study on chemisorption model of cesium hydroxide onto stainless steel type 304

Nakajima, Kunihisa; Nishioka, Shunichiro; Suzuki, Eriko; Osaka, Masahiko

Proceedings of 27th International Conference on Nuclear Engineering (ICONE-27) (Internet), 8 Pages, 2019/05

Cesium chemisorption models were developed for estimation of amount of cesium chemisorbed onto stainless steel type 304 (SS304) during light water reactor severe accident. However, existing chemisorption models cannot accurately reproduce experimental results. In this study, a modified cesium chemisorption model was constructed based on a penetration theory for gas-liquid mass transfer with chemical reaction and was able to adequately describe effects on concentration of cesium hydroxide in gaseous phase and silicon content in SS304. It was found that the modified model can more accurately reproduce the experimental data than the existing model.