(An) investigation of the fission product release from the fuel pellet-cladding gap into the coolant through a cladding breach

Abstract

The fission product release from the fuel pellet-cladding gap into the coolant through a cladding breach has been investigated for two release processes, i.e., prompt and delayed release processes. For the prompt release process, a prompt fission gas release model that includes the resistance to gas flow in the gap has been developed and the effects of failure location on prompt fission gas release from the cladding breach have been assessed. A porous media concept (permeability) is used to define the resistance to the gas flow in the tortuous gap. The process of prompt fission gas release from the plenum and the gap into the coolant has been modeled in accordance with three major phenomena: (1) transient gas flow in the gap, (2) the growth of the fission gas bubble while it is still attached to the breach, and (3) the detachment of the fission gas bubble from the breach and mixing with the coolant. The fission gas bubble is assumed to be detached from the cladding of breach as soon as its diameter reaches the predictable detachment diameter. The plenum pressure history and the cumulative mass release fraction by the present model incorporating the friction in the gap have been calculated for the case of Younggwang 3 & 4 nuclear fuel rod as a typical example. The results show that the release behavior of the prompt fission gas with time is different from the frictionless model which has frequently been used in the simplified approach, and that the location of cladding failure is another key factor of the prompt fission gas release process due to the important role of the resistance in the gap on the prompt fission gas release process. For the delayed release process, a Windows computer code entitled as "CAAP (Coolant Activity Analysis Program)" has been developed to evaluate the number, the degree of failures, and the location of failed fuel rods using primary coolant radioactivity data obtained from operating PWRs. New models are developed and incorporated int...