The presented work is a result of actions taken in connection to analyses of root cause of damages (cracks and one brake) in the control rod shafts in Swedish BWR Forsmark 3. The damages were detected during the refueling outage 2008. It has been found that damages were caused by thermal fatigue. Extensive analyses of flow and temperature fields around the shaft were performed using transient CFD calculations [1, 2]. The character of the fluctuating thermal loadings on the shaft was confirmed by a limited experiment [3]. However the CFD-calculations of heat transfer between the water and the shaft have not been validated experimentally. In order to validate CFD-calculations of heat transfer between the water and the solid body the measurements of the non-stationary heat transfer are planned. The paper presents the method of determination of heat flux and temperature on the surface of the body based on temperature measurements at some discrete points beneath the surface and solving the inverse heat conduction problem (IHCP). Software was developed for performing measurements and calculations. Main parts of measurement system particularly design and manufacturing of measuring items, thermocouple installation, construction of test stand for initial testing and calibration are described. Verification of thermocouple locations was performed using computer tomography and the actual locations were introduced in to the calculation algorithm in order to improve accuracy of heat flux determination. Heat flux measuring error has been determined based on assumed random error in temperature measurement and accuracy of location verification (computer tomography). Results of initial verifying tests are presented and discussed. The measuring system is now ready for performing measurements of transient heat transfer in configurations that can occur in a reactor environment.

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