NC in the containment

In many passive safety systems natural circulation is an essential part of the system. The main aim is to use density differences in a pool or in a closed loop to transfer thermal energy from a source to a heat sink without using other energies than gravitational energy. This process can take place with or without phase changes. 

Passive systems with phase changes can develop considerable driving forces. The pressure differences often are in the range from 1 kPa-100 kPa. Examples are the passive containment 

If the pool height is 5 m, then the maximum driving pressure is about 15 Pa. This corresponds to a vertical flow velocity of 0.17 m/s. Suggesting that the rising plume has a diameter of 2 m, the mass flow is about 500 kg/s and the thermal flux is about 10.5 MW. 

Fundamental research seems to be necessary in all cases where components normally are tested with pure steam instead with nuclear steam, which always has a small content of radiolytic gas. To avoid handling pure hydrogen and pure oxygen in a laboratory it could be possible to generate radiolytic gas by an electorolytical apparatus continuously and in small quantities. Such experiments can be very instructive about the accumulation of the radiolytic gas, about the natural circulation effects in components with stagnating steam atmosphere and about the possibilities to get a passive transport of the enriched steam back to the RPV or to another line with intensive steam flow. 

The opposite of natural circulation is stratification. In most cases where stratification takes place, it is neglected both by the designer and by the computer code. Experiments in the PANDA facility showed that stratification could become the essential effect in containment s atmosphere. So a deeper knowledge about the counterparts natural circulation and stratification seem to be unavoidable to do the right calculations with mixed or with separated media in a containment or in a pool. 

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