In the framework of the TMI-2 VIP (Vessel Investigation Programme) considerable research efforts were devoted to the Reactor Pressure Vessel (RPV) behaviour under severe accident conditions. The major topic was the behaviour of the lower pressure vessel head in case of core melt situations. Phenomena like the flow and distribution of the core debris on the vessel bottom, and the thickness of the crust insulation layer were considered in various numerical simulation programmes, as they determine the temperature distribution and the creep behaviour. These models enabled to extrapolate first results to different vessel geometries. The objective of the project is to establish a consensus on the present capabilities to describe the thermal load and the structural response of the lower pressure vessel head in case of meltdown scenarios. An extension of the Project is foreseen to investigate impact effects on the upper vessel head. A major portion of the project is the generation of a materials database for typical pressure vessel materials, with existing experimental results and with the results of specific new experiments for extreme values of temperature and loading. Benchmark calculations are performed to determine the capabilities of existing models to describe the thermal loading and the structural response of the pressure vessel lower head and to assess the sensitivity of the models to certain input parameters.
For the input into the databank creep and tensile data of RPV steels and cladding material produced in the framework of a German research programme and data from the TMI VIP and former reports published by US institutions were digitalized and physical data collected from the literature.

Structural models

In the development of structural models describing the behaviour of the RPV lower head with an inside pool of molten corium, the geometry of the lower head was in all cases simplified by a hemisphere, sometimes with an adjacent cylinder of different wall thickness and a taper in between.

Some of the most important results of the calculations are:

- When only elastic plastic effects are considered, the temperature gradient has a minor effect while the mean temperature of the wall is most important. However the temperature gradient has a large effect on creep deformations as soon as the temperatures are high enough.

- The influence of the dead weight of the corium is significant at low pressures (< about 1 MPa).

- At high pressure creep and plastic failure take place at the bottom of the hemisphere.

- A hot spot with a diameter of 1m is too small for a uniform plastic instability to develop, therefore the pressures resulting in failure are considerably higher than for a bottom with uniform high temperature.

- The lifetime of the vessel decreases strongly with rising external temperature, an important fact for outside cooling.

Thermo-hydraulic models

A simplified model of heat transfer from a volume heated pool is developed for use in accident codes.

Integrated models

Here the LOWHED code has to be mentioned, which is an integrated model for debris behaviour and thermal attack on the lower heat and internal structures, with a simplified treatment of lower head failure.

Screening calculations for dynamic loadings

Screening calculations of the response of the vessel lower head to steam explosions have been performed with PLEXUS. They should give the order of magnitude of the maximum energy input which can be sustained by the vessel for different ways of energy input.
Work programme The development of the materials database takes into account the most recent experimental results relevant to pressure vessels, analyzing the data with respect to median values and uncertainty bands. The data are provided in ready-for-use format for finite element codes. GRS/Cologne performs calculations of failure modes and time sequences of a PWR lower head without penetrations under various severe accident conditions. Slug impact tests on the upper vessel head are being performed in the BERDA facility of KfK: the calculations focus on strain-rate effects and energy dissipation by structural deformation. Dynamic loadings on a pressure vessel under severe accident conditions are computed by the participants in the framework of a benchmark exercise about plastic and creep failure. Thermal calculations of a turbulent heated pool of molten corium are performed by CEA-IPSN/Fontenay. ENEA-DRI/Rome and ENEL-VDN/Rome are examining jointly outside-vessel cooling techniques on the basis of corium/RPV interaction calculations, with and without water in the vessel. They are using the computer codes TRIO-EF and CORIUM-2D for the thermo-hydraulics and CASTEM-2000 for the thermo-mechanics. They are also developing, with the University of Pisa and ANPA/Rome, dynamic structural loading models under in-vessel steam explosion and H2 detonation conditions. This helps in particular in predicting stresses and deformations for the bolted flange of the upper vessel, needed for the precalculations of the BERDA experiments. The experiments performed by CEA-DRN/Saclay include new creep and rupture data at 700 and 1000 C. New constitutive equations under multi-axial conditions are derived from rupture experiments with non-uniform thickness tubes in connection with penetration tube failure analysis. The creep tests performed at KFA/Jülich focus on large specimen effects (Diameter=70 mm and height= 1000 mm) at 700 and 1000 C. Effects of strain-rate, pressure and temperature drop are investigated by SIEMENS-KWU/Erlangen, and radiation & heat flux distribution effects in the case of outside-vessel cooling.