Driving forces of 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 cooling system, developed by General Electric, or the Emergency Condenser, developed by Siemens.

Passive systems without phase changes have much smaller driving forces with pressure differences in the range from 1 Pa to 1000 Pa. An example is the building condenser, developed by Siemens. The natural circulation with single-phase flow is valid both on the primary and on the secondary side, as long as the temperature differences are small. But small driving pressures are not equivalent with ineffective”. For example: heating up water from 5°C to 10°C gives a density difference of 0.3% kg/m3 (or 0.03%).

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.

In other passive systems natural circulation with pressure differences in the range <<1 Pa can determine whether a component can reach the right operating conditions or not. Assuming that radiolytic gas is accumulated from the original concentration 10"5 by a factor 100 in a “bull eye” of a conventional RPV level measurement, then the density difference is about 0.03 kg/m3. In a “bull eye” with a gas space of 5 cm height the driving pressure results to 0.015 Pa. Nevertheless in a “bull eye” that was properly formed this small pressure difference was sufficient to initiate a counter-current flow (a special form of natural circulation) of the enriched gas mixture against the fresh steam inside the connecting pipe. So in this “bull eye” no accumulation of radiolytic gas above the ignition concentration was possible. In another “bull eye”, which was not properly designed, the opposite was true.

So the effectiveness of systems with natural circulation is not primarily a question of high driving forces but of the proper design. Normally a simple calculation is sufficient for the designer to calculate the driving forces in a balance with flow resistances. It is not necessary to know each detail of the flow fields like degree of turbulence, exact velocity profiles in different regions of the circuit or the exact form and flow velocity of a raising plume in a pool. Rough calculations normally are sufficient to decide whether a system using natural circulation is properly designed or not.

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.