Application of Uncertainty Analyses with the MAAP4 Code

Abstract

Uncertainty analyses are an important element associated with using integral computer codes to evaluate the response of a reactor/containment system to off-normal situations. The more severe the off-normal transient, the more important the uncertainty analyses. How should such uncertainty analyses be formulated? How should the results of the uncertainty approach be applied? To address these questions for the MAAP4 code, an approach has been developed to uncertainty evaluations defining the importance of individual physical process (Table 1) and establishing a structure on how phenomena should be evaluated and quantified with respect to the integral assessment.

Documentation of the technical basis for uncertainty bounds is essential to meaningful uncertainty analyses. In particular, the technical basis for determining oxidation rates, cooling rates, combustion rates, etc. must come from a composite of separate effects and integral experiments, as well as industrial experience. How this technical basis is developed and how it should be used must be documented so that the user has a clear understanding what is, or is not, included in the technical basis for the phenomena of interest.

This paper will discuss the approach to developing the technical basis for uncertainty evaluations related to the phenomenon of RCS failure which includes the influence of natural circulation within the reactor coolant system. This discussion is an example of how relevant experiments and analyses must be documented to create the uncertainty bounds for each of the physical processes of interest. How these uncertainty bounds should be used in plant analyses will be discussed.

As addressed by the plant specific PSAs/IPEs, there is a low frequency, for which severe accidents could occur and the core debris would not be cooled within the vessel, i.e. the reactor vessel would fail and core debris would be released to the containment. Under these conditions, the objectives of accident management actions remain the same, i.e. cool the core debris by submerging it in water. However, the debris cooling rate has significant uncertainties under these conditions.