PWR lower head rupture at high system pressure: Analysis of thrust load on pressure vessel during blowdown

In a hypothetical severe accident at a nuclear power plant it is conceivable that the reactor core melts down, while the primary system pressure remains on a high level (17 MPa). If accident management measures fail, the molten corium relocates into the lower plenum, several hours after the accident initiating event, and comes in direct contact with large portions of the lower head of the pressure vessel. The melt subjects the inner surface of the vessel wall to high temperatures (1700 K). As the temperature increases through the thickness of the wall, the strength of the steel is dramatically reduced, until the cooler outer portions of the vessel wall can no longer withstand the high internal pressure. That ultimately leads to a rupture of the lower head causing a violent blowdown. The discharge of a mixture of water, steam, and corium debris exerts a thrust load on the pressure vessel in upward direction extremely demanding the supporting structure of the pressure vessel. If this load cannot be carried by the supporting structure, the vessel could move upwards crashing through the reactor cavity covering and endangering the containment hall. The blowdown is accompanied by a heavy pressure build-up in the reactor cavity putting a strong strain on the cavity walls. Further, the cavity pressure increases the load on the vessel support ring, which presents a considerable area to the pressure. The high pressure failure of the vessel is being studied within a new R and D activity at KfK investigating ultimate containment loads as a result of severe accidents not getting under control. The activity is related to an initiative of Eibi, Hennies and Kessler, who proposed a new containment concept for large (1300 MW (e)) next generation pressurized water reactors. The concept is aimed at passive mechanisms that can safely confine core-melt accidents, whatever remote their likelihood might be. In contrast to other approaches, the concept demands a deterministic exclusion of large radioactivity releases and disastrous impacts from severe nuclear accidents to man and environment. The paper deals with the estimation of hydraulic loads during blowdown of the failed reactor pressure vessel. The information obtained provides a force-function input for structural dynamics analyses for a new core-melt resistant containment design. The analysis based on calculations with the light water reactor transient analysis thermal-hydraulics computer code RELAPS/MODS is more detailed than the analytic approach in the German risk study (phase B)