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After the Fukushima Daichi accident in 2011 design extension conditions (DEC) came into focus. Most regulatory regimes now require that design extension conditions are considered in the design of new reactors, while existing reactors should try back fitting safety enhancements to be able to deal with design extension conditions as far as reasonably achievable. DEC are divided in categories A and B, where DEC-A scenarios are characterized by multiple failures of safety systems but with the reactor core remaining intact. A phenomenon that plays a significant role in DEC-A scenarios involving a containment bypass is Iodine Spiking (IS). In this paper we analyse a PRImary to SEcondary leaking accident (PRISE) with the assumption of a stuck open atmospheric relief valve at the first opening at two different reactor designs (VVER-1000, Generic PWR). For the thermal hydraulic best estimate simulation we use RELAP5-3D, for the transport of iodine we use the Relap5-3D radionuclide transport model. Additionally, to analyse the Iodine Spiking (IS) phenomenon for the examined accident scenario, we created an empirical IS model which is based on the existing NRC IS model to estimate how much iodine is released to the primary system from the fuel. The results of this study show that the Relap5-3D radionuclide transport model is capable of providing a bounding analysis on the iodine released to the environment. The analysis also shows that good results can be achieved for the analysed scenarios using an empirical model to estimate the iodine released to the primary system. The analysis also shows that during said scenario significant quantities of radioactive fission products are released into the environment. [ABSTRACT FROM AUTHOR]

We present the Laplace-transformed analytical solution (LTAS) to the one-dimensional radionuclide transport equation for an arbitrary length decay-chain through an arbitrary combination of multiply fractured and porous transport segments subject to an arbitrary time-dependent release mode at the entrance point to the series of transport segments. The LTAS may be numerically inverted to obtain the time-dependent concentration of the radionuclides of interest at an arbitrary down gradient location. For a special case, where the source function is defined as the band release with a single radionuclide without precursors, the Laplace inverse transformation could be performed analytically, yielding a closed-form analytical solution. A computer code, TTBX, has been developed by implementing the LTAS, and benchmarked against the closed-form analytical solution. Numerical examples are presented to demonstrate the utility of these solutions and the importance of increased fidelity in the transport pathway for reliable performance assessment for the geological disposal of spent nuclear fuels. © 2013 Elsevier Ltd. All rights reserved.

Abstract This study aims to assess the likelihood for criticality in the far field of a repository for direct disposal of commercial light water reactor used nuclear fuel. Two models are used in combination: (1) a neutronics model to estimate the minimum critical masses of spherical, water-saturated depositions of fissile material; (2) a transport model to simulate the dissolution of waste packages arranged in an array and the subsequent transport of fissile solutes through fractured bedrock to a single accumulation location. The neutronics model shows that heavy metals from different types of used fuel present the same minimum critical mass behavior in the parameter space of initial enrichment and burnup, dictated largely by the fissile content. However, the magnitude of the minimum critical mass varies significantly within that parameter space, and secondary effects like the presence of absorbing nuclides play a minor role. The transport model employs various subsurface transport scenarios, and for each scenario the mass of each isotope and the overall fissile content of the accumulation is reported from the time of canister failure up to one hundred million years at various distances from the repository edge. Taking the results of the two models in concert, it is shown that even if the accumulation of a critical mass is possible under conservative conditions, these conditions are unlikely to be present in the vicinity of a carefully engineered repository. Based on the results of each model, recommendations for risk mitigation in terms of waste characteristics and repository design are given. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents a new radionuclide transport model for performance assessment and design of a geologic repository for high-level radioactive waste. The model uses compartmentalization of a model space and a Markov-chain process to describe the transport. The model space is divided into an array of compartments, among which a transition probability matrix describes radionuclide transport. While similar to the finite-difference method, it has several advantages such as flexibility to include various types of transport processes and reactions due to probabilistic interpretation, and higher-order accuracy resulting from direct formulation in a discrete-time frame. We demonstrated application of this model with a hypothetical repository in porous rock formation. First we calculated a three-dimensional steady-state heterogeneous groundwater flow field numerically by the finite-element method. The transition probability matrix was constructed based on the flow field and hydraulic dispersion coefficient. The present approach has been found to be effective in modeling radionuclide transport at a repository scale while taking into account the effects of change in hydraulic properties on the repository performance. Numerical exploration results indicate that engineered barrier configuration and material degradation have substantial effects on radionuclide release from the repository. [ABSTRACT FROM AUTHOR]