Uncertainties in TRAC plenum pressures for the FI phase of a DEGB LOCA

The TRAC-PF1/MOD1 code (TRAC) is used to perform best-estimate analyses of certain postulated Design Basis Accidents (DBAs) in SRS production reactors. Currently, the most limiting DBA in terms of reactor power level is an instantaneous double-ended guillotine break (DEGB) loss of coolant accident (LOCA). For this accident, TRAC is used to analyze only the first 5 seconds following the DEGB, which encompasses the Flow Instability (FI) phase of the DBA. The TRAC analysis provides time-dependent plenum and tank bottom pressures for use as boundary conditions in the FLOWTRAN code. The quantification of uncertainty is an important element of determining safe operating power levels for SRS reactors. A detailed methodology for the determination of uncertainty for the FI phase of a DEGB LOCA has been developed. This report presents estimates of the uncertainty in the time-dependent plenum pressures for the DEGB LOCA calculated by TRAC. The plenum pressure uncertainty was estimated by means of comparing TRAC results with steady-state data measured in L Reactor, and confirmed by comparisons with transient LOCA results calculated by an independent group with the RELAP5 code. An overview of the limits methodology is given and discusses the L Reactor data. The methodology for estimating the plenum pressure uncertainty is presented along with the results.