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Reactor safety experiments for studying the reactions of a molten core (corium) with water and/or concrete involve materials at extremely high temperature. Such high temperature severely restricts the types of sensors that can be employed to measure characteristics of the corium itself. Yet there is great interest in improving instrumentation so that the state of the melt can be established with more precision. In particular, it would be beneficial to increase both the upper range limit and accuracy of temperature measurements. The poor durability of thermocouples at high temperature is also an important issue. For experiments involving a water-quenched melt, direct measurements of the growth rate of the crust separating the melt and water would be of great interest. This is a key element in determining the nature of heat transfer between the melt and coolant. Despite its importance, no one has been able to directly measure the crust thickness during such tests. This paper considers three specialized sensors that could be introduced to enhance melt characterization: (1) A commercially fabricated, single point infrared temperature measurement with the footprint of a thermowell. A lens assembly and fiber optic cable linked to a receiver and amplifier measures the temperature at the base of a tungsten thermowell. The upper range limit is 3000 C and accuracy is {+-}0.25% of the reading. (2) In-house development of an ultrasonic temperature sensor that would provide multipoint measurements at temperatures up to {approx}3000 C. The sensors are constructed from tungsten rods and have a high temperature durability that is superior to that of thermocouples. (3) In-house development of an ultrasonic probe to measure the growth rate of the corium crust. This ultrasonic sensor would include a tungsten waveguide that transmits ultrasonic pulses up through the corium melt towards the crust and detects reflections from the melt/crust interface. A measurement of the echo time delay would provide the location of the interface. These three sensors would provide a considerable upgrade of the instrumentation used in our reactor materials tests. The infracouple is a commercial product that could provide an immediate improvement in temperature measurements. The sensor could also serve to corroborate thermocouple data by providing a measurement based upon a different physical principle. The ultrasonic temperature sensor would involve a greater investment and longer time frame than the infracouple, but offers all the advantages of the infracouple along with miniaturization and the ability to measure at multiple locations. In addition, the UTS is the platform from which we would begin development of the crust detector. Of the three sensors, the crust detector requires the most effort and entails the greatest uncertainty. However, a real-time crust thickness measurement has never before been made and such data would be unique and of great benefit to reactor materials experiments

OECD MCCI project enhancing instrumentation for reactor materials experiments, Rev. 0 September 3, 2002.