For advanced monitoring of nuclear reactor components
The European research project THERFAT was initiated to advance the accuracy and reliability of instrumentation and procedures currently applied for assessing the integrity of pipe tee-connections. High and frequent temperature fluctuations as a result of mixing hot and cold fluids may cause the tee-connections' material to degrade and thermal fatigue cracks to appear. Such cyclic phenomena may not be properly detected by thermocouple instrumentation. Thus, evaluations of tee-connections' integrity rely on approximations determined by experiments and mathematical models. Scientists at the VTT Technical Research Centre of Finland concentrated on verifying selected aspects of the assessment of tee-connections against the results from critical experiments and numerical analyses. Initial stresses induced by thermal loads were estimated by using practical engineering methods, like approximate calculations of the thermal shock parameters or advanced computational fluid dynamics analysis. Common code procedures were proven to provide accurate predictions of the threshold values for crack initiation, which were verified with data from damage tests. The typical crack caused by a thermal shock occurs as a network of surface cracks. This random crack pattern was not considered to deteriorate the tee-connections' integrity as long as dominant cracks did not develop. For assessing the resistance capability of tee-connections to thermal shocks, models that provide an estimate of the crack growth rate as a function of temperature should be used. To transfer the results on crack initiation and propagation into improved practical methods for predicting thermal fatigue, a combined experimental and analytical approach was proposed. While focused on identifying the conditions under which thermal fatigue cracks advance or are in arrest, it can support suggestions on how to mitigate significant thermal loads. This holistic approach for tee-connections' integrity evaluation has been defined as the first step towards a new methodology that can be used to optimise nuclear plant operation conditions.
