Consideration of Nuclear Criticality When Directly Disposing Highly Enriched Spent Nuclear Fuel in Unsaturated Tuff—II: Geochemical Constraints

Rechard, Robert P.; Sanchez, Lawrence C.; Trellue, Holly R.

doi:10.13182/nt03-5

Cited by 4 publications

(20 citation statements)

References 30 publications

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“…Combining the mass transport analysis with the results of the neutronics analysis, which has been performed partially in this study, some important points for selecting a site for criticality safety can be suggested. These include (1) iron existing in the host rock reduces the likelihood of criticality significantly; (2) low host rock porosity is preferred for criticality safety; (3) the conservatism could change when comparing heterogeneous geometries for different fracture apertures, in other words, the planar fracture geometry applied in the previous CSA for geological disposal [6][7][8][9][10][11]15] would not necessarily yield conservative results against the homogeneous uranium deposition because the k eff for heterogeneous geometry can be smaller than that for homogeneous one in case of larger width of fracture aperture; and (4) the importance of the mass of the deposition increases when it is smaller. To make these more reliable and specific, further studies in the neutronics and mass transport are crucially important.…”

Section: Resultsmentioning

confidence: 99%

“…Corresponding to these two questions, CSA consists of two parts: the neutronics analysis for evaluating critical mass of TFM [4] and the analysis for transport and deposition of TFM in the near and far fields [5]. For the first part, in the CSA for YMR, various neutronics models were developed [6][7][8][9][10][11][12][13] by considering compositions of TFM, groundwater, rocks, and geometries of TFM depositions for various conditions in Nevada tuff rock.…”

Section: Introductionmentioning

confidence: 99%

“…In the previous studies for repository criticality safety such as [6][7][8][9][10][11]15], the planar fracture model was used for representing heterogeneity of C 2014 Atomic Energy Society of Japan. All rights reserved.…”

Section: Introductionmentioning

confidence: 99%

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Conditions for criticality by uranium deposition in water-saturated geological formations

Liu

Ahn

Hirano

2014

Journal of Nuclear Science and Technology

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This study focuses on neutronic analysis to examine the criticality conditions for uranium depositions in geological formations resulting from geological disposal of damaged fuels from Fukushima Daiichi reactors. MCNP models are used to evaluate the neutron multiplication factor (k eff ) and critical mass for various combinations of host rock and geometries. It has been observed that the k eff for the deposition become greater with (1) smaller concentrations of neutron-absorbing materials in the host rock, (2) larger porosity of the host rock, (3) heterogeneous geometry of the deposition, and (4) greater mass of uranium in the deposition. This study has revealed that the planar fracture geometry applied in the previous criticality safety assessment for geological disposal would not necessarily yield conservative results against the homogeneous uranium deposition.

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Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Conditions for criticality by uranium deposition in water-saturated geological formations

Liu

Ahn

Hirano

2014

Journal of Nuclear Science and Technology

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“…The present article and two companion articles ( 6,7 ) present additional arguments on the improbability of a critical event. In these articles, only the characteristics of the tuff geologic subsystem that prevent criticality are considered, not the planned features of the engineered subsystem that include neutron absorbers.…”

Section: Introductionmentioning

confidence: 97%

“…After closure, several geochemical constraints exist that prevent reassembly of fissile material, and these geochemical constraints ( 4,5 ) have been reported in two companion articles. ( 6,7 ) This article discusses the potential for a critical event after the proposed repository is closed but volcanism disrupts the repository. In 1997, Rechard et al .…”

Section: Introductionmentioning

confidence: 99%

Improbability of Igneous Intrusion Promoting a Critical Event in Spent Nuclear Fuel Disposed in Unsaturated Tuff

Rechard¹,

Tierney²

2005

Risk Analysis

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In their regulations, the U.S. Environmental Protection Agency and the U.S. Nuclear Regulatory Commission permit the omission of features, events, or processes with probabilities of <10(-4) in 10(4) yr (e.g., a constant frequency of <10(-8) per yr) in assessments of the performance of radioactive waste disposal systems. Igneous intrusion (or "volcanism") of a geologic repository at Yucca Mountain for radioactive waste is one disruptive event that has a probability with a range of uncertainty that straddles this regulatory criterion and is considered directly in performance assessment calculations. A self-sustained nuclear chain reaction (or "criticality") is another potentially disruptive event to consider, although it was never found to be important when evaluating the efficacy of radioactive waste disposal since the early 1970s. The thesis of this article is that the consideration of the joint event--volcanism and criticality--occurring in any 10,000-year period following closure can be eliminated from performance calculations at Yucca Mountain. The probability of the joint event must be less than the fairly well-accepted but low probability of volcanism. Furthermore, volcanism does not "remove" or "fail" existing hydrologic or geochemical constraints at Yucca Mountain that tend to prevent concentration of fissile material. Prior to general corrosion failure of waste packages, the mean release of fissile mass caused by a low-probability, igneous intrusive event is so small that the probability of a critical event is remote, even for highly enriched spent nuclear fuel owned by the U.S. Department of Energy. After widespread failure of packages occurs, the probability of the joint event is less than the probability of criticality because of the very small influence of volcanism on the mean fissile mass release. Hence, volcanism plays an insignificant role in inducing criticality over any 10(4)-yr period. We also argue that the Oklo reactors serve as a natural analogue and provide a rough bound on probability of criticality given favorable hydrologic or geochemical conditions on the scale of the repository that is less than 0.10. Because the product of this bound with the probability of volcanism represents the probability of the joint event and the product is less than 10(-4) in 10(4) yr, consideration of the joint event can be eliminated from performance calculations.

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Hazards and scenarios examined for the Yucca Mountain disposal system for spent nuclear fuel and high-level radioactive waste

Rechard

Freeze

Perry

2014

Reliability Engineering & System Safety

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Consideration of Nuclear Criticality When Directly Disposing Highly Enriched Spent Nuclear Fuel in Unsaturated Tuff—II: Geochemical Constraints

Cited by 4 publications

References 30 publications

Conditions for criticality by uranium deposition in water-saturated geological formations

Conditions for criticality by uranium deposition in water-saturated geological formations

Improbability of Igneous Intrusion Promoting a Critical Event in Spent Nuclear Fuel Disposed in Unsaturated Tuff

Hazards and scenarios examined for the Yucca Mountain disposal system for spent nuclear fuel and high-level radioactive waste

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