Implementation and assessment of improved models and options in TRAC-BF1

A summary of modifications and options introduced in TRAC-BF1 is presented and is shown that the predicting capabilities of the modified version of the code are greatly improved. These changes include the introduction of a different heat transfer package during reflooding, the implementation of a simple single-phase limit procedure for forcing the two phases to acquire the same velocity if one phase disappears, a close assessment of the annular flow interfacial shear correlation, implementation of a simple radiation model which seems to alleviate some numerical-oscillation problems induced by the existing highly complex model. Furthermore, different options were introduced and tested like upwinding some terms of the momentum equations (which seems to solve a number of problems reported in the past), the second upwind scheme for the convective terms of the phasic momentum equations and the implementation and assessment of a completely different annular flow interfacial shear correlation. The modified TRAC-BF1 is assessed against some bottom-flooding separate-effect experiments, a 'benchmark' top flooding simulation as well as against the TLTA test No. 6423. In the process of this task, the different options are assessed and discussed and is shown that the predictions of the modified code are physically sound and close to the measurements, while almost all the predicted variables are free of unphysical spurious oscillations. The modifications introduced solve a number of problems associated with the frozen version of the code and result in a version which can be confidently used for LB-LOCA analyses. (C) 1997 Elsevier Science S.A.