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Enhancing Nuclear Safety


Research

Theses in progress

Transient boiling: heat transfer model


Host laboratory: ​​Loss of Coolant Accident and Uncertainties Modeling Laboratory (LIMAR)

Beginning of the thesis: October 2015

Student name: Valentin Scheiff



Subject description

During the hypothetical case of a RIA (Reactivity Initiated Accident) in the core of a nuclear reactor power plant, a power excursion can trigger the formation of a vapour film around the fuel rod, leading to an important rise of the rod temperature. The evolution is so rapid that no heat transfer regime has time to settle. Heat transfer is therefore considered as transient.

 

A few experiments, performed under reactor conditions, provide transient boiling curves. But their modeling lacks of reliability. This has motivated the IRSN to launch a research program. So, a model experiment has been designed and constructed at the IMFT. It considers the flow of a refrigerant in a semi-annular section duct bounded by a metal foil rapidly heated by Joule effect, simulating the heating of a fuel rod.

 

Infrared thermography is used to measure the temperature of the metal foil, painted with a black paint. This paint allows increasing wall emissivity and therefore improving temperature measurement. But it also increases the thermal resistance of the wall that induces a bias in the measurement. A correction of the measurement with an optimization of the temperature estimation has been developed during this PhD.

 

The experiments should make it possible to study the heat transfer during film boiling. The initial control system allowed only to impose a predefined power signal as a function of time. This made such study impossible. Therefore a temperature control of the metal foil using a P.I.D. (Proportional, Integrator, Derivator) regulation system of the power has been implemented. The results in nucleate boiling regime are analyzed both in steady and transient conditions. The film boiling, during the heating or cooling of the wall can be stabilized and its study will make it possible to model all the regimes of the transient boiling.

 

Finally, considering all the results obtained and comparing them with the tests already carried out under conditions of nuclear reactors of the NSRR and PATRICIA type, it will be possible to propose models improvement for clad to coolant heat transfer in the SCANAIR software.



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