Core shroud cracking has been observed in several boiling water reactors (BWRs) since 1993. A current U.S. Nuclear Regulatory Commission concern is the response of a cracked core shroud to loads resulting from the main steam-line-break loss-of-coolant accident (MSLOCA). Core shroud loads and responses have been calculated by GPU Nuclear Corporation (GPUNC) for the Oyster Creek BWR/2 using the RELAP5 computer code. The objectives of the RETRAN-02 analysis performed by S. Levy Incorporated were to assess the capability of RETRAN-02 to simulate an MSLOCA and to obtain an independent validation of the GPUNC results.

A main steam-line break will result in rapid depressurization of the steam dome and an upward pressure load over the shroud head. This upward force has the potential to cause separation and displacement of the shroud head if the shroud head contains a 360-deg through-wall flaw.

The key parameters and phenomena affecting the core shroud head pressure differential following the initiation of the MSLOCA are critical flow through the vessel side of the steam-line break; pressure wave dynamics in the steam lines; depressurization rate of the vessel steam dome; flow inertia and pressure drop of the steam dryers, steam separators, and standpipes; and flashing of saturated liquid in the upper plenum and reactor core.

The key parameters and phenomena affecting the core shroud head lift are the core shroud head mass above the cracked weld, the core shroud head projected area, and the characteristics of the shroud weld crack leakage flow path from the core bypass to the vessel downcomer annulus.

Comparison of RELAP5 and RETRAN-02 calculation results shows good agreement for the transient core shroud head pressure drop and lift predictions by the two methods. An important element in simulating this rapid transient, for both RELAP5 and RETRAN-02, is the ability to calculate the shroud head loading and lift through the use of control block elements and to directly couple the effect of flow through the shroud weld crack leakage flow path to the upper plenum thermal hydraulics.