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Journal Articles

Accumulation of gadolinium isotopes in used nuclear fuel

Suyama, Kenya; Kashima, Takao

Proceedings of International Conference on Nuclear Criticality Safety (ICNC 2015) (DVD-ROM), p.273 - 282, 2015/09

In the technical development of the criticality safety control of the fuel debris of Fukushima accident in Japan, there have been a discussion on a possibility of adopting BUC with FP. The Expert Group on Burnup Credit Criticality Safety (EGBUC) under the Working Party on Nuclear Criticality Safety (WPNCS) in OECD/NEA Nuclear Science Committee had carried out an international burnup calculation benchmark "Phase-IIIB" and "Phase-IIIC" for BWR fuel assemblies. In these benchmarks the difference of the calculation results of $$^{155}$$Gd among the participants obtained keen interests because it showed rather larger difference among the participants. Authors has been carried out additional analyses on the accumulation of the gadolinium isotopes in the used nuclear fuel during the burnup. Without cooling time, the assembly-averaged amount of $$^{155}$$Gd against the burnup value depends on the burnout property of gadolinium in the burnable poison rods. However, after few year cooling time, $$^{155}$$Gd increase drastically by the decay of $$^{155}$$Eu. In this case, the amount of gadolinium isotopes in the burnable poison rods has less importance. It means that the adopted parameters and data concerning the $$^{155}$$Eu generation have much more importance than the burnup treatment of the burnable poison rods for better prediction of $$^{155}$$Gd.

Journal Articles

A Study on the criticality safety for the direct disposal of used nuclear fuel in Japan; Application of burnup credit to the criticality safety evaluation for the disposal canister

Yamamoto, Kento; Akie, Hiroshi; Suyama, Kenya

Proceedings of International Conference on Nuclear Criticality Safety (ICNC 2015) (DVD-ROM), p.228 - 237, 2015/09

Japan has recently started to study the technical issues for the direct disposal of the used nuclear fuel (UNF) to prepare various disposal options. The criticality safety is important for the direct disposal because of the presence of certain amount of the fissile nuclides in UNF. This paper gives the outline of the research to be addressed in this field and the relevant studies in Japan. Especially, it presents the first result of the criticality safety evaluation for a disposal canister model adopting burnup credit. The uncertainties of effective neutron multiplication factor ($$k_{eff}$$) caused by the depletion calculation errors as well as the effect of the axial burnup profile and the horizontal burnup gradient on $$k_{eff}$$ were also evaluated. It was found that the $$k_{eff}$$ including these uncertainties and conservatism was below 0.95 for the representative used PWR fuel when the fuel assemblies and the disposal canister were assumed to keep intact.

Journal Articles

Inter-code comparison of TRIPOLI${textregistered}$ and MVP on the MCNP criticality validation suite

Brun, E.*; Zoia, A.*; Trama, J. C.*; Lahaye, S.*; Nagaya, Yasunobu

Proceedings of International Conference on Nuclear Criticality Safety (ICNC 2015) (DVD-ROM), p.351 - 360, 2015/09

This paper presents a joint work conducted at CEA Saclay and JAEA Tokai aimed at comparing the Monte Carlo codes TRIPOLI${textregistered}$ and MVP on a selection of ICSBEP benchmarks. Our goal is to establish a common set of Monte Carlo input decks, as a basis for rigorous inter-code comparison in criticality-safety. As a reference, we will use the MCNP Criticality Validation Suite: other Monte Carlo developers might easily join that effort in the future. For the purpose of inter-code comparison, the TRIPOLI${textregistered}$ and MVP input decks have been translated from those of MCNP, without any further assumptions. Both TRIPOLI${textregistered}$ and MVP have been run with the same ENDF/B-VII.0 evaluated nuclear data, and as far as possible the same simulation options as in the original LANL work. In this abstract, we present preliminary results on the BIGTEN benchmark. In the full paper these will be extended to the 31 benchmarks of the MCNP Criticality Validation Suite. In the future, this database will also help in the analysis of sensitivity to nuclear data, CPU times and figures of merit.

Journal Articles

Nuclear criticality safety standard for a fuel reprocessing plant assuming burnup credit published by the Atomic Energy Society of Japan

Nakajima, Ken*; Itahara, Kuniyuki*; Okuno, Hiroshi

Proceedings of International Conference on Nuclear Criticality Safety (ICNC 2015) (DVD-ROM), p.496 - 502, 2015/09

An outline of the standard "Procedures for Applying Burnup Credit to Criticality Safety Control of a Reprocessing Facility: 2014" (AESJ-SC-F025: 2014) published in April 2015 by the Atomic Energy Society of Japan (AESJ) is presented. The AESJ published more than 60 Standards. However, many of them were in the field of nuclear power reactors or radioactive wastes. Ten years ago the AESJ published "Basic Items of Criticality Safety Control: 2004" (AESJ-SC-F004:2004), which prescribed basic ideas, requirements and methods on nuclear criticality safety controls of facilities handling with nuclear fuel materials in general for preventing a nuclear criticality accident. However, it did not include any specific procedures for adopting burnup credit. Therefore, a new standard was envisaged as the first Standard for fuel reprocessing plants, which clarified the specific procedures to apply burnup credit to designers, operators, maintenance persons and administrators.

Journal Articles

Study on criticality control of fuel debris by Japan Atomic Energy Agency to support Nuclear Regulation Authority of Japan

Tonoike, Kotaro; Yamane, Yuichi; Umeda, Miki; Izawa, Kazuhiko; Sono, Hiroki

Proceedings of International Conference on Nuclear Criticality Safety (ICNC 2015) (DVD-ROM), p.20 - 27, 2015/09

From the viewpoint of safety regulation, criticality control of the fuel debris in the Fukushima Daiichi Nuclear Power Station would be a risk-informed control to mitigate consequences of criticality events, instead of a deterministic control to prevent such events. The Nuclear Regulation Authority of Japan has set up a research and development program to tackle this challenge. The Nuclear Safety Research Center of Japan Atomic Energy Agency, commissioned by the authority, has launched activities such as computations of criticality characteristics of the fuel debris, development of criticality risk assessment method, and preparation of criticality experiments to support them.

Journal Articles

Criticality characteristics of MCCI products possibly produced in reactors of Fukushima Daiichi Nuclear Power Station

Tonoike, Kotaro; Okubo, Kiyoshi; Takada, Tomoyuki*

Proceedings of International Conference on Nuclear Criticality Safety (ICNC 2015) (DVD-ROM), p.292 - 300, 2015/09

The damaged Unit 1-3 reactors of the Fukushima Daiichi Nuclear Power Station may contain fuel debris of a significant amount that is in a form of molten-core-concrete-interaction (MCCI) product with porous structure. Such low density MCCI product including fissile material is a great concern for its criticality control, especially under submerged condition, due to its fairly good neutron moderation. This report shows computation results of basic criticality characteristics of the MCCI product, which will facilitate criticality risk assessments during decommissioning of the reactors. The results imply that water bound in concrete may raise the risk from the viewpoints of possibility of criticality events and of effectiveness of mitigation measures such as neutron poison injection into coolant water.

Oral presentation

Criticality safety control of fuel debris produced by the accident of Fukushima Daiichi Nuclear Power Plants; Technical development in Japan from 2012 and the future

Suyama, Kenya

no journal, , 

After the accident of Fukushima Daiichi Nuclear Power Plants, Government of Japan setup the council for the decommissioning the plants and it released the Mid-and-Long-Term Roadmap towards the Decommissioning. For this purpose, under the umbrella of International Research Institute for Nuclear Decommissioning (IRID), the technical development required for the criticality safety control of the Fukushima fuel debris has been carried out. Furthermore, JAEA has a plan to use the criticality assembly STACY for obtaining the neutronics characteristics of the fuel debris. This presentation will summarize the on-going research program in Japan for the criticality safety control in order to remove and storage the fuel debris from the Fukushima site in Japan.

Oral presentation

Development of criticality risk evaluation method for fuel debris in Fukushima-Daiichi NPS

Yamane, Yuichi; Tonoike, Kotaro

no journal, , 

After the accident of Fukushima Dai-ichi Nuclear Power Station, there is still damaged nuclear fuel in the reactor buildings, which is called fuel debris and contains an amount of fissile materials more than the minimum criticality amount. A new method, which has been developed by Japan Atomic Energy Agency to technically support the Nuclear Regulatory Authority in Japan, NRA, is designed to estimate criticality risk based on a probability distribution of k-eff due to uncertainty of composition and geometry of fuel debris, which method will provide risk information for the criticality safety of fuel debris. A trial analysis by using the procedure defined in the method shows that some events which can initiate criticality can be supposed to happen during maintenance period, and that analysis also indicates countermeasures should be considered as soon as possible in order to reduce criticality risk for public and workers.

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