Code-to-code comparison for analysing the steady-state heat transfer and natural circulation in an air-cooled RCCS using GAMMA+ and Flownex

• The GAMMA+ and Flownex codes are used in the analyses of the air-cooled RCCS system.
• Radiation heat transfer comprises the bulk of the total rate of heat transfer.
• It is possible to obtain reverse flow through the RCCS standpipes.
• It has been found that the results obtained with the two codes are in good agreement.
• RCCS remain functional for very high blockage ratios thus supporting the safety case.

The GAMMA+ and Flownex codes are both based on a one-dimensional flow network modelling approach and both can account for any complex network of different heat transfer phenomena occurring simultaneously. However, there are notable differences in some of the detail modelling aspects, such as the way in which the convection in the reactor cavity is represented. Despite this, it was found in the analyses of the air-cooled RCCS system that the results provided by the two codes compare very well if similar input values are used for the pressure drop coefficients, heat transfer coefficients and view factors.The results show that the radiation heat transfer comprises the bulk of the total rate of heat transfer from the RPV surface. It is also shown that it is possible to obtain a stable and sustainable steady-state operational condition where the flow is in the reverse direction through the RCCS standpipes, resulting in excessively high values for the concrete wall temperature. It is therefore crucial in the design to ensure that such a flow reversal will not occur under any circumstances.In general the good comparison between the two codes provides confidence in the ability of both to correctly solve the fundamental conservation and heat transfer relations in an integrated manner for the complete RCCS system. Provided that appropriate input values are available, these codes can therefore be used effectively to evaluate the integrated performance of the system under various operating conditions. It is shown here that the RCCS should remain functional and continue to provide sufficient cooling even for very high blockage ratios at the inlet to the riser ducts, which supports the safety case.