NUREG/CR-6672: ACCIDENT SEVERITY AND RELEASE FRACTIONS

Simple models for severe accident source term severity and release fractions are described. Severity fraction equations are developed for collision accidents that do not initiate fires, for collision accidents that do initiate fires, and for fires not initiated by collisions. Rod-to-cask and cask-to-environment release fraction equations are developed for noble gases, for vapors that contain Cs or Ru, for CRUD, and for fuel fines. The rod-to-cask release fractions for fuel fines treat fuel pellet fracturing, particle bed formation, and particle filtering by those particle beds. The effects of fuel oxidation on Cs and Ru release fractions are discussed. Also discussed are the use of GIS methods of analysis to develop improved occurrence frequencies for shipment route wayside surfaces and the use of tank car accident data to develop probabilities for cask failure by puncture or shearing. Finally, the set of severity and release fractions developed for the transport of PWR spent fuel in a steel-lead-steel rail cask are presented to illustrate the results obtained using these severity and release fraction expressions.