Influence of increasing hydrogen concentration on the heat transfer phenomena during boildown

A previous paper has studied boildown transients on a single pin using a simple computer model. The computer model has now been improved to allow for the production of hydrogen by the exothermic zirconium/water reaction and this paper describes the model and studies how the previous results are changed. The same test cases were run with the new model as before; single pin boildowns at 80 bars. For the particular configuration (inlet temperature, fuel pin length, axial power distribution etc.) that was studied some conclusions were reached. Provided the cladding does not rise above melting temperature, the volume fraction of hydrogen does not rise above a value of 6%. The chief effect of the hydrogen is to reduce the thermal capacity of the coolant. Thus the clad rises to higher temperatures than would otherwise be the case. When the cladding rises above melting temperature the modelling is more approximate but there are indications that the concentration of hydrogen can rise to higher values. In these cases the oxidation reaction rate may be limited by diffusion of steam through the coolant