ABSTRACT:

Recriticality is possible in a severe accident if unborated or
low boron concentration water is added to a damaged core after control rod
melting but before fuel melting. Recriticality in a severe accident in a BWR
was parametrically investigated using the TWODANT code. Eigenvalue calculation
for a unit central fuel cell with reflective boundary conditions were perform
by solving the two-dimensional multigroup steady state Boltzman transport
equation using TWODANT. Two sets of
calculations were performed in this work. The first set of calculations was
carried out under three types of normal operating conditions to provide
reference values for the accident calculations: 1) cold rodded condition
2) cold unrodded condition and 3) hot full-power condition. The eigenvalues
at the above conditions were found to be 1.055, 1.208, and 1.098 respectively.
The second set of calculations was carried out after the melting of the contro
element and during the reflood phase, under the following reflood conditions:
1) reflood with unborated water and 2) reflood with borated water. For the
reflood case with unborated water, five values of void fractions were consider
( 100%, 60%, 40%, 20%, 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 eigen-
value against fraction of control materials lost indicates recriticality is
only possible if nearly 100% of the control material is lost from the core.
Eigenvalue calculations were repeated for short and long term
recovery conditions of the reflood phase corresponding to maximum moderator
density at 4 MPa pressure and 525 K moderator temperature and for
1 MPa pressure and 325 K moderator temperature respectively. Recriticality
was again observed to be a concern only after losing 95% or more of control
materials from the unit cell. Although a three dimensional treatment might
not require such a large percentage loss, these results suggest recriticality
is not a major issue as there is likely to be significant fuel slumping
before 95% of the control material leaves the core.