1.2.1. Single phase flow

In Section 2 several nuclear reactor designs as well as natural circulation applications have been described. The single phase natural circulation flow is driven by a gravity head induced by coolant density differences; the mass flow is established according to the balance between driving head and flow resistance losses. Because the (one component) density is a function of the temperature there is a functional interaction between heat exchange and natural circulation flow.

In general, the determination of the main parameters, e. g. average velocities, pressure drops, heat transfer and 1-D temperature fields for flow inside pipes or around structures, is based on established engineering practise. These parameters can be calculated with industrial codes as well as with thermal hydraulic codes used for nuclear reactor system behaviour.

Some uncertainties exist if two natural circulation flows with different densities caused by different temperatures are mixed. Then diffusion processes, turbulences and other mixing processes become important. Specific experiments would be capable to reduce the uncertainties; in some cases related experiments are necessary for a design.

If 2-D or 3-D flow fields can establish, the use of capable codes isnecessary; the codes listed in Section 3.2.2.2 can be used for calculations of this type.

Mixing processes and 3-D natural convection flow is important for deboration accidents for PWRs. For BWRs similar complicated flow fields exist for sequences with boron injection. For these processes experiments with detailed instrumentation are underway; the data can be used also for code validation.

The capability to calculate single phase natural convection flows with high confidence does allow the optimisation of systems and components, e. g. a decrease in flow resistances and an appropriate arrangement of heat sources and heat sinks.