The experimental investigations using MM models were preceded by numerical ones mainly aimed at the selection of geometric and operating regime parameters for the MM and reactor unit as well as the prediction of their behaviour under accident conditions. In the process of calculations for normal operating conditions, the limiting MM power value was obtained and the dynamics of natural circulation development at varying power, etc. were determined.

Calculations were also performed for typical accidents such as the disrupture of the pipeline between the MM and the pressurize, the MM vessel failure and others. In view of the complexity of hydrodynamical and heat transfer processes under such condition some problems should be experimentally verified. These questions will be touched upon later when discussing experimental data in Section 4 and now we shall consider some calculational results for beyond-design accidents in more detail.

In a beyond-design accident with postulated instantaneous MM uncovery, under the reactor emergency shutdown conditions, the maximum fuel temperature at a fuel assembly power of 1070 kW was 1070 C. The fuel melting takes place when the pipeline connecting the MM with the pressurize breaks just near the MM vessle with the simultaneous failure of the reactor emergency protection or when the MM vessel breaks simultaneously with the emmergency protection system failure and the disrupture of the pipeline between the MM and pressurizer. In both cases, the fuel melts only in one MM subjected to the failure.

In beyond-design accidents accompanied by the failure of secondary circuit heat removal systems, the absorber cooling system ensures that the maximum fuel element cladding (1200 C) was not exceeded. If this system fails also, the element temperature of 1200 C is riched for ~ 12 hours because of the large heat capacity of the reactor. During this time it is necessary to take adequate measures to monitor the accident process. Table 2 gives an idea on the radiation exposure as a consequence of different normal and accident conditions.