A Main-Steam-Line-Break accident in a CANDU nuclear steam generator produces a blowdown in which all the pressurised water is boiled off in a few seconds. The resulting high transient loading on the heat exchanger tubing could lead to their rupture, resulting in the release of radioactive materials out of containment. A better understanding of this phenomenon will permit the development of improved design tools to ensure steam generator safety in the event of such an accident. The paper presents a commissioned experimental rig and instrumentation system, for which a two-phase experimental program has been developed. Using R134a as the working fluid, measurements of temperature, pressure, and tube loading, as well as simultaneous high-speed flow visualisation, have been taken at conditions simulating a full-scale operating steam generator. The experimental results will be used to develop theoretical modelling tools for single and two-phase blowdown, such that an estimate of tube loading during Main-Steam-Line Break can be predicted from initial conditions.

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