PARAMETRIC STUDY OF RECRITICALITY IN A BOILING WATER-REACTOR SEVERE ACCIDENT

Recriticality is possible in a severe accident if unborated or low bor on concentration water is added to a damaged core after control rod me lting but before fuel melting. Recriticality in a severe accident in a boiling water reactor was parametrically investigated using the TWODA NT code. Eigenvalue calculations for a unit central fuel cell with ref lective boundary conditions were performed by solving the two-dimensio nal multigroup steady-state Boltzman transport equation using TWODANT. Two sets of calculations were performed in this work. The first set o f calculations was carried out under three types of normal operating c onditions to provide reference values for the accident calculations: ( a) cold rodded condition, (b) cold unrodded condition, and (c) hot ful l-power condition. The eigenvalues at these conditions were found to b e 1.055, 1.208, and 1.098, respectively. The second set of calculation s was carried out after the melting of the control element and during the reflood phase, under the following reflood conditions: (a) reflood with unborated water and (b) reflood with borated water. For the refl ood case with unborated water, five values of void fractions were cons idered (100, 60, 40, 20, and 0%). Decreasing void fractions represent greater refill levels during the reflood process. The system pressure was taken to be 7 MPa, while the moderator temperature was set to 560 K. Plotting the eigenvalue compared with the fraction of control mater ials lost indicates recriticality is only possible if nearly 100% of t he control material is lost from the core. Eigenvalue calculations wer e repeated for short- and long-term recovery conditions of the reflood phase corresponding to maximum moderator density at 4 MPa pressure an d 525 K moderator temperature and for 1 MPa pressure and 325 K moderat or temperature, respectively. Recriticality was again observed to be a concern only after losing 95% or more of control materials from the u nit cell. Although a three-dimensional treatment might not require suc h a large percentage loss, these results suggest recriticality is not a major issue as there is likely to be significant fuel slumping befor e 95% of the control material leaves the core.