Transient boiling during RIA: Out-of-pile testing and in-pile design for treat

Cladding-to-coolant heat transfer during transient irradiation conditions remains a critical area of uncertainty in understanding nuclear reactor safety. This uncertainty impacts analytical predictions of both accident progression and fuel performance behavior. This uncertainty manifests itself in the use of extremely conservative models that widely bound the onset and duration of transition phenomena. Improved understanding and predictive models to describe the transitions to film boiling and its duration could meaningfully increase safety and design margins in both the current commercial fleet and advanced light water reactors (LWRs) (including small modular reactors). An accurate and reliable description of this behavior may also provide the primary building block to allow for a change in the current fuel safety criteria and enable safe operation at higher powers. This would result in a substantial increase in the performance of current and future reactors. The complexity of the phenomena and the variety of thermal-hydraulic scenarios of interest requires establishing a mechanistic description of the behavior and a body of dedicated experimental data to validate the model. An experimental program has been developed at Idaho National Laboratory to investigate transient boiling behavior in the TREAT reactor. The experimental strategy includes an out-of-pile transient boiling facility to perform parametric studies and qualify advanced instrumentation for the in-pile experiments.