James Mancillas

Historical Rail Accident Analyses Identifying Accident Parameters That Could Impact Transportation of Spent Nuclear Fuel

As the regulatory authority for transportation of spent nuclear fuel (SNF) in the United States, the Nuclear Regulatory Commission (NRC) requires that SNF transportation packages be designed to endure a fully engulfing fire with an average temperature of 800 °C (1,475 °F) for 30 minutes, as prescribed in Title 10 of the Code of Federal Regulations (CFR) Part 71. The work described in this paper was performed to support NRC in determining the types of accident parameters that could produce a severe fire with the potential to fully engulf a SNF transportation package. This paper describes the process that was used to characterize the important features of rail accidents that would potentially lead to a spent nuclear fuel transport package being involved in a severe fire. Historical rail accidents involving hazardous material and long duration fires in the United States have been analyzed using data from the Federal Railroad Administration (FRA) and the Pipeline and Hazardous Materials Safety Administration (PHMSA). Parameters that were evaluated from this data include, but were not limited to, class of track where the accident occurred, class of hazardous material that was being transported, and number of railcars involved in the fire. The data analysis revealed that in the past 34 years of rail transport, roughly 1,800 accidents have led to the release of hazardous materials resulting in a frequency of roughly 1 accident per 10 million freight train miles. In the last 12 years, there have only been 20 accidents involving multiple car hazardous material releases that led to a fire. This results in an accident rate of 0.003 accidents per million freight train miles that involved multiple car releases and a fire. In all the accidents analyzed, only one involved a railcar carrying Class 7 (i.e., radioactive) hazardous material (HAZMAT).Copyright

Spatial Variability and Parametric Uncertainty in Performance Assessment Models

The problem of defining an appropriate treatment of distribution functions (which could represent spatial variability or parametric uncertainty) is examined based on a generic performance assessment model for a high-level waste repository. The generic model incorporated source term models available in GoldSim® , the TDRW code for contaminant transport in sparse fracture networks with a complex fracture-matrix interaction process, and a biosphere dose model known as BDOSE™. Using the GoldSim framework, several Monte Carlo sampling approaches and transport conceptualizations were evaluated to explore the effect of various treatments of spatial variability and parametric uncertainty on dose estimates. Results from a model employing a representative source and ensemble-averaged pathway properties were compared to results from a model allowing for stochastic variation of transport properties along streamline segments (i.e., explicit representation of spatial variability within a Monte Carlo realization). We concluded that the sampling approach and the definition of an ensemble representative do influence consequence estimates. In the examples analyzed in this paper, approaches considering limited variability of a transport resistance parameter along a streamline increased the frequency of fast pathways resulting in relatively high dose estimates, while those allowing for broad variability along streamlines increased the frequency of “bottlenecks” reducing dose estimates. On this basis, simplified approaches with limited consideration of variability may suffice for intended uses of the performance assessment model, such as evaluation of site safety.Copyright

Estimates of mean consequences and confidence bounds on the mean associated with low-probability seismic events in total system performance assessments

An approach is described to estimate mean consequences and confidence bounds on the mean of seismic events with low probability of breaching components of the engineered barrier system. The approach is aimed at complementing total system performance assessment models used to understand consequences of scenarios leading to radionuclide releases in geologic nuclear waste repository systems. The objective is to develop an efficient approach to estimate mean consequences associated with seismic events of low probability, employing data from a performance assessment model with a modest number of Monte Carlo realizations. The derived equations and formulas were tested with results from a specific performance assessment model. The derived equations appear to be one method to estimate mean consequences without having to use a large number of realizations.