Numerical Estimation of the Spent Fuel Ratio at Elevated Fracture Temperatures

Sabotage of spent nuclear fuel casks remains a concern nearly forty years after the potential for such attacks against shipment casks were first analyzed and has a renewed relevance in the post- 9/11 environment. A limited number of full-scale tests and supporting efforts using surrogate materials, typically depleted uranium dioxide (DUO2), have been conducted in the interim to more definitively determine the source term from these postulated events. In all the previous studies, the postulated attack of greatest interest was by an explosively focused high velocity jet of copper that is much more efficient than bulk explosives. However, the validity of these large-scale results remain in question due to the lack of a defensible Spent Fuel Ratio (SFR), defined as the amount of respirable aerosol generated by an attack on a mass of spent fuel compared to that of an otherwise identical surrogate, as determined through testing. Previous attempts to define the SFR in the 1980’s have resulted in estimates ranging from 0.42 to 12 and include suboptimal experimental techniques and data comparisons. In part because of the large uncertainty surrounding the SFR, estimates of releases from security-related events may be unnecessarily conservative. Credible arguments exist that the SFR does not exceed a value of unity. A defensible determination of the SFR in this lower range would greatly reduce the calculated risk associated with the transport and storage of spent nuclear fuel in dry cask systems. In the present work, the shock physics codes CTH (developed at Sandia National Laboratories) and ALE3D (developed at Lawrence Livermore National Laboratory) were used to simulate SNF (Spent Nuclear Fuel) and DUO2 targets impacted by high-velocity jets. The shock-physics code results are combined with the available empirical data for respirable particle production of DUO2 and SNF samples under high-energy loadings to provide an estimate of the SFR. Previous efforts considered the average imparted energy density over large portions of the target ceramic such that the average temperatures were below the UO2 brittle-ductile transition point. The current work looks at more localized zones of material where maximum energy density is expected. Not only do portions of the material fracture above the brittle-ductile transition point but under the point of impact the UO2 will melt. The targets considered are fresh fuel, uniform density SNF and SNF with a bimodal radial density profile representing a high burnup rim. A methodology for accounting for elevated fracture temperatures and melting on the SFR estimate is presented.