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Journal Articles

Recent activities in the field of reactor physics

Fukushima, Masahiro; Tojo, Masayuki*

Journal of Nuclear Science and Technology, 56(12), p.1061 - 1062, 2019/12

 Times Cited Count:0 Percentile:0.34(Nuclear Science & Technology)

Reactor Physics that treat the essentials of how fission nuclear reactors work fundamentally has important roles on safe operations and design studies of various types of nuclear reactors. From the latest activities in the field of reactor physics, this report summarizes some outstanding researches and developments published in scientific journals including the Journal of Nuclear Science and Technology.

Journal Articles

Study on loss-of-cooling and loss-of-coolant accidents in spent fuel pool, 2; Fuel cladding oxidation

Nemoto, Yoshiyuki; Kaji, Yoshiyuki; Kanazawa, Toru*; Nakashima, Kazuo*; Tojo, Masayuki*

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

Oxidation behaviour of Zr cladding in SFP accident condition was evaluated by using a thermobalance in this work, and the obtained data were applied to construct oxidation model for SFP accident condition. For the validation of the constructed oxidation model, oxidation tests using a long cladding tube 500mm in length were conducted in conditions simulating SFP accidents, such as flow rate of the atmosphere in spent fuel rack, temperature gradient along the axis of cladding, and heating-up history. Thickness of oxide layer formed on the surface of cladding samples was evaluated by cross sectional observation, and compared with calculation results obtained by using the oxidation model. The detail of experimental results and validation of the oxidation model will be discussed.

Journal Articles

Study on loss-of-cooling and loss-of-coolant accidents in spent fuel pool, 4; Investigation of fuel loading effects in BWR spent fuel rack

Tojo, Masayuki*; Kanazawa, Toru*; Nakashima, Kazuo*; Iwamoto, Tatsuya*; Kobayashi, Kensuke*; Goto, Daisuke*; Nemoto, Yoshiyuki; Kaji, Yoshiyuki

Proceedings of 27th International Conference on Nuclear Engineering (ICONE-27) (Internet), 13 Pages, 2019/05

In this study, fuel loading effects in BWR spent fuel rack accidents are widely investigated using three-dimensional analysis methods from both nuclear and thermal hydraulics viewpoints, including: (a) Decay heat of spent fuel after discharge, (b) The maximum temperature of spent fuel cladding in the spent fuel rack depending on heat transfer phenomena, and (c) Criticality of the spent fuel rack after collapsing of the fuel due to a severe accidents in the BWR spent fuel pool (SFP).

Journal Articles

Study on loss-of-cooling and loss-of-coolant accidents in spent fuel pool, 1; Overview

Kaji, Yoshiyuki; Nemoto, Yoshiyuki; Nagatake, Taku; Yoshida, Hiroyuki; Tojo, Masayuki*; Goto, Daisuke*; Nishimura, Satoshi*; Suzuki, Hiroaki*; Yamato, Masaaki*; Watanabe, Satoshi*

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

In this research program, cladding oxidation model in SFP accident condition, and numerical simulation method to evaluate capability of spray cooling system which was deployed for spent fuel cooling during SFP accident, have been developed. These were introduced into the severe accident codes such as MAAP and SAMPSON, and SFP accident analyses were conducted. Analyses using Computational Fluid Dynamics (CFD) code were conducted as well for the comparison with SA code analyses and investigation of detail in the SFP accident. In addition, three-dimensional criticality analysis method was developed as well, and safer loading pattern of spent fuels in pool was investigated.

Journal Articles

Study on oxidation model for Zircalloy-2 cladding in SFP accident condition

Nemoto, Yoshiyuki; Kaji, Yoshiyuki; Onizawa, Takashi*; Kanazawa, Toru*; Nakashima, Kazuo*; Tojo, Masayuki*

Proceedings of Annual Congress of the European Federation of corrosion (EUROCORR 2018) (USB Flash Drive), 8 Pages, 2018/09

The authors proposed oxidation models based on oxidation data which previously obtained in high temperature oxidation tests on small sample of Zircalloy-2 (Zry2) cladding in dry air and in air/steam mixture environment. The oxidation models were implemented in computational fluid dynamics (CFD) code to analyse oxidation behavior of long cladding sample in hypothetical spent fuel pool (SFP) accident conditions. The oxidation tests were conducted using Zry2 cladding sample 500 mm in length. The oxide layer growth in dry air was well reproduced in the calculation using the oxidation model, meanwhile which in air/steam mixture was overestimated atmosphere composition change anticipated in the spent fuel rack during the accident, and its influence on the oxidation behaviour of the cladding were discussed in consideration of the oxidation model improvement.

Journal Articles

Influence of the air/steam mixing ratio in atmosphere on zirconium cladding oxidation in spent fuel pool accident condition

Nemoto, Yoshiyuki; Kaji, Yoshiyuki; Ogawa, Chihiro*; Nakashima, Kazuo*; Tojo, Masayuki*

Proceedings of 2017 Water Reactor Fuel Performance Meeting (WRFPM 2017) (USB Flash Drive), 10 Pages, 2017/09

To cope with the hypothetical severe accident in spent fuel pools (SFPs), it is important to understand the high temperature oxidation behavior of the Zirconium claddings exposed in the air or in the atmosphere of air/steam mixture. In this study, oxidation tests on Zircaloy-2 (Zry2) and Zircaloy-4 (Zry4) short samples were conducted in the atmosphere of air - steam mixture, and mixing ratio was varied to evaluate its influence on the oxidation kinetics in the temperature range from 600 to 1100$$^{circ}$$C. From 900 to 1000$$^{circ}$$C for Zry2, and from 800 to 1000$$^{circ}$$C for Zry4, oxidation rates appeared higher in air - steam mixture than in dry air or in steam without air. This tendency was appeared more evident in post-breakaway transition phase after fracture of dense oxide layer on the surface of specimens. These results suggest importance of the oxidation model development in consideration of the air - steam mixture environment for the SFP accident analysis.

Journal Articles

Investigation of Zircaloy-2 oxidation model for SFP accident analysis

Nemoto, Yoshiyuki; Kaji, Yoshiyuki; Ogawa, Chihiro; Kondo, Keietsu; Nakashima, Kazuo*; Kanazawa, Toru*; Tojo, Masayuki*

Journal of Nuclear Materials, 488, p.22 - 32, 2017/05

AA2016-0383.pdf:0.86MB

 Times Cited Count:1 Percentile:15.39(Materials Science, Multidisciplinary)

The authors previously conducted thermogravimetric analyses on zircaloy-2 in air. By using the thermogravimetric data, an oxidation model was constructed in this study so that it can be applied for the modeling of cladding degradation in spent fuel pool (SFP) severe accident condition. For its validation, oxidation tests of long cladding tube were conducted, and computational fluid dynamics analyses using the constructed oxidation model were proceeded to simulate the experiments. In the oxidation tests, high temperature thermal gradient along the cladding axis was applied and air flow rates in testing chamber were controlled to simulate hypothetical SFP accidents. The analytical outputs successfully reproduced the growth of oxide film and porous oxide layer on the claddings in oxidation tests, and validity of the oxidation model was proved. Influence of air flow rate for the oxidation behavior was thought negligible in the conditions investigated in this study.

Journal Articles

Oxidation behavior of fuel cladding tube in spent fuel pool accident condition

Nemoto, Yoshiyuki; Kaji, Yoshiyuki; Ogawa, Chihiro; Nakashima, Kazuo*; Tojo, Masayuki*

Zairyo To Kankyo, 66(5), p.180 - 187, 2017/05

In spent fuel pool (SFP) under loss-of-cooling or loss-of-coolant severe accident condition, the spent fuels will be exposed to air and heated by their own residual decay heat. Integrity of fuel cladding is crucial for SFP safety therefore study on cladding oxidation in air at high temperature is important. Zircaloy-2 (Zry2) and zircaloy-4 (Zry4) were applied for thermogravimetric analyses (TGA) in different temperatures and different air flow rates in this work. Oxidation rate increased with temperature. In range of air flow rate predicted in spent fuel lack during SFP accident, influence of flow rate was not clearly observed below 950$$^{circ}$$C for Zry2 and below 1050$$^{circ}$$C for Zry4. Over these temperature, oxidation rates appeared obviously higher in higher air flow rate, and this trend became clearer when temperature increased. Oxide layers were carefully examined after the oxidation tests and compared with the mass gain data in TGA to investigate detail of air oxidation process. The results revealed that mass gain data in the pre breakaway transition stage reflects growth of the dense oxide film on specimen surface, and in the post breakaway transition stage, it reflects growth of porous oxide layer beneath the breakaway cracking of the oxide film.

Journal Articles

Study on oxidation behavior of cladding for accident conditions in spent fuel pool

Nemoto, Yoshiyuki; Kaji, Yoshiyuki; Ogawa, Chihiro; Nakashima, Kazuo*; Tojo, Masayuki*; Goto, Daisuke*

Fushoku Boshoku Kyokai Dai-62-Kai Zairyo To Kankyo Toronkai Koenshu (CD-ROM), p.23 - 24, 2015/11

In order to clarify the air oxidation behavior of the cladding at high temperatures for study on improvement of safety for accident conditions in spent fuel pool, the oxidation tests for both small specimens under constant temperature conditions and long specimens under loss of coolant simulated temperature conditions were carried out, and the knowledge for influence of both temperature gradient and preoxide film on oxidation behavior of the cladding were obtained in this study.

Oral presentation

Study on improvement of safety for accident conditions in spent fuel pool, 2; Air oxidation behavior of the cladding

Nemoto, Yoshiyuki; Kaji, Yoshiyuki; Nakashima, Kazuo*; Tojo, Masayuki*; Goto, Daisuke*

no journal, , 

For the upgrading of the severe accident code aiming to analyze the coolant loss accident in spent fuel pool (SFP), it is required to model the air oxidation behavior of the cladding at high temperature. Authors proceeded air oxidation tests using small samples and long samples. For the long samples, temperature distribution simulating the SFP accident condition was given. The results were evaluated for the construction and validation of the air oxidation model. The results of the experiments and the outline of the program will be presented as well.

Oral presentation

Study on improvement of safety for accident conditions in spent fuel pool, 1; Whole plan

Kaji, Yoshiyuki; Nemoto, Yoshiyuki; Takase, Kazuyuki; Tojo, Masayuki*; Goto, Daisuke*; Iwata, Yutaka*; Otake, Yukihiko*; Nishimura, Satoshi*; Suzuki, Hiroaki*

no journal, , 

We will make quantitatively clear the phenomenon of fuel air oxidation mechanism, fuel failure mechanism after loss of coolant and criticality risks during and after sever accidents of Spent Fuel Pool(SFP) and clarify the quantitative effects of enhancements of SFP safety such as Spray and fuel loading patterns. We will report the whole plan in this presentation.

Oral presentation

Study on improvement of safety for accident conditions in spent fuel pool, 3; Evaluation of decay heat and cladding temperature during accident

Kobayashi, Kensuke*; Goto, Daisuke*; Tojo, Masayuki*; Ikehara, Tadashi*; Kaji, Yoshiyuki; Nemoto, Yoshiyuki

no journal, , 

We evaluate fuel temperature based on the steady state heat transfer analysis with considering accurate fuel history and storage conditions.

Oral presentation

JAEA studies on spent fuel pool severe accident, 1; Results of cladding air oxidation experiments

Nemoto, Yoshiyuki; Ogawa, Chihiro; Kaji, Yoshiyuki; Nakashima, Kazuo*; Tojo, Masayuki*; Goto, Daisuke*

no journal, , 

To improve the severe accident codes for the analysis of spent fuel pool (SFP) accident with water level decrease, it is necessary to model the air oxidation behavior of the cladding based on experimental data. For the air oxidation modeling, it is necessary to understand the influence of temperature distribution along axial direction of the cladding in SFP accident condition. In addition, it is important to evaluate the influence of oxide film formed on surface of cladding during operation in nuclear power plant. In this study, bare specimens and pre-oxidized specimens of Japanese zircalloy-4 were investigated. Longer specimens of cladding were tested in air with temperature distribution along the axial direction of the cladding simulating the SFP accident condition. Shorter specimens were adopted for tests in thermogravimetry to obtain basic data. Experimental data were compared and the authors discussed on influence of the temperature distribution and oxide film for the air oxidation behavior.

Oral presentation

Study on cladding oxidation behavior on spent fuel pool severe accident, 2

Nemoto, Yoshiyuki; Kaji, Yoshiyuki; Ogawa, Chihiro; Nakashima, Kazuo*; Tojo, Masayuki*; Kanazawa, Toru*; Goto, Daisuke*

no journal, , 

In order to investigate the air oxidation behavior of cladding materials for study on improvement of safety in spent fuel pool loss of coolant accident condition, both isothermal oxidation tests by short length specimens and oxidation tests with temperature gradient by long length specimens were conducted and the knowledge on influence of temperature and air flow rate on cladding oxidation behavior was obtained in this study.

Oral presentation

Study on improvement of safety for accident conditions in spent fuel pool, 7; Investigation for a modeling of the cladding air oxidation

Nemoto, Yoshiyuki; Kaji, Yoshiyuki; Ogawa, Chihiro; Nakashima, Kazuo*; Tojo, Masayuki*

no journal, , 

It is necessary to develop oxidation models of the cladding materials in air environment to improve the severe accident code for spent fuel pool (SFP) accident investigation. High temperature oxidation tests in the dry air on the zircaloy-2 cladding were conducted in different temperature and air flow rate to develop oxidation models. Oxidation tests on cladding tubes in a length of 500mm and computational fluid dynamics (CFD) analysis using the developed oxidation model were processed and the results were compared for the evaluation of the oxidation models. Outline of the whole project will be presented in the session as well.

Oral presentation

Study on improvement of safety for accident conditions in spent fuel pool, 9; Temperature evaluations of BWR fuels by three dimensional steady-state heat transfer analyses

Goto, Daisuke*; Tojo, Masayuki*; Kobayashi, Kensuke*; Nemoto, Yoshiyuki; Kaji, Yoshiyuki

no journal, , 

Steady-state heat transfer analysis method for the BWR spent fuel rack system was adopted to evaluate the cladding temperature in spent fuel pool accident condition. The cladding temperature seems to depend on spent fuel distribution, and the evaluation results were compared with the results obtained by using the MAAP. Ideas for the deployment of the fuels to prevent temperature elevation during the spent fuel pool water level decrease will be discussed.

Oral presentation

Study on improvement of safety for accident conditions in spent fuel pool, 10; Effects on criticality due to the differences of water level between in and out of channel box of BWR fuel

Kobayashi, Kensuke*; Tojo, Masayuki*; Goto, Daisuke*; Nemoto, Yoshiyuki; Kaji, Yoshiyuki

no journal, , 

It is suggested that effective multiplication factor would increase if there is a water level difference between in and out of channel box of BWR fuel. Monte Carlo criticality analysis was conducted in consideration of the water density and the water level in and out of the channel box which were indicated in the evaluation using void fraction correlation obtained in previous works simulating several irradiation conditions of BWR fuels.

Oral presentation

Study on improvement of safety for accident conditions in spent fuel pool, 13; Investigation of cladding oxidation model

Nemoto, Yoshiyuki; Kaji, Yoshiyuki; Ogawa, Chihiro*; Nakashima, Kazuo*; Kanazawa, Toru*; Tojo, Masayuki*

no journal, , 

Concerning the severe accident analysis in spent fuel pools (SFPs), it is important to construct the high temperature oxidation model of the Zirconium claddings exposed in loss-of-coolant accident condition. In this study, an oxidation model of Zry2 in air environment was constructed by using these basic data. The constructed oxidation model was applied for simulation of oxidation tests using long cladding tubes of Zry2 in hypothetical SFP accident conditions. Reproducibility of the calculation was evaluated in comparison with the results of experiment to validate the oxidation model.

Oral presentation

Influence of environment and temperature on oxidation behavior of fuel cladding

Nemoto, Yoshiyuki; Kaji, Yoshiyuki; Ogawa, Chihiro*; Nakashima, Kazuo*; Tojo, Masayuki*

no journal, , 

To cope with the hypothetical loss-of-cooling or lost-of-coolant severe accident in spent fuel pools (SFPs), it is important to understand the high temperature oxidation behavior of the Zirconium claddings exposed in the air or in the atmosphere of air/steam mixture which are presumed for the environment in spent fuel rack internal in the SFP accident. The authors have investigated the detail of high temperature oxidation behavior of the Japanese fuel cladding materials in dry air with flow rate in the hypothetical SFP accident condition, and proposed oxidation models as well. Since there is not enough knowledge about influence of air / steam mixing ratio in atmosphere on cladding oxidation in SFP accident condition, the authors conducted high temperature oxidation tests of claddings in atmosphere of air / steam mixture with flow rate of hypothetical SFP accident condition, and investigated the influence of air / steam mixing ratio on cladding oxidation behavior.

Oral presentation

Study on improvement of safety for accident conditions in spent fuel pool, 21; Investigation of cladding oxidation model

Nemoto, Yoshiyuki; Onizawa, Takashi*; Kaji, Yoshiyuki; Nakashima, Kazuo*; Kanazawa, Toru*; Tojo, Masayuki*

no journal, , 

Cladding oxidation model construction and its validation is necessary for the upgrading of severe accidentcode which will be utilized for analysis of the spent fuel pool (SFP) accident. Cladding oxidation tests and analyses using the proposed oxidation model were coducted in the simulated SFP accident condition, and their results were compared. Modification of the oxidation model will be discussed in the lecture.

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