Anyone who has ever cooked a roast knows the value of a meat thermometer. Although you could use a formula that factors weight, temperature and cooking time, it’s better to have a device that tells you the meat’s internal temperature.

For advanced types of nuclear fuel, such precision is not just a convenience — it’s a necessity. Yet until recently, it has been impossible to directly measure temperatures inside nuclear reactor cores. Idaho National Laboratory’s High-Temperature Irradiation-Resistant Thermocouple (HTIR-TC) solves that problem. It faithfully provides nearly continuous, nearly instantaneous temperature readings from inside the reactor.

Surprisingly, until now equipment limitations in nuclear energy have forced engineers to calculate reactor core temperatures based on readings taken at some distance from the fuel. With the HTIR-TC, engineers can place a sensor into a reactor’s heart, even inside a fuel rod itself, getting direct temperature readings in real time. Researchers have actually begun exploring the possibilities of fabricating fuel rods using 3D printing, plasma-spraying ultra-thin thermocouples onto cladding or embedding them in the fuel itself.

Today, nuclear power production is entering its next phase, embracing new technological developments that involve the generation of high heat for advanced manufacturing, hydrogen production and desalination. As these concepts take shape, it is imperative that critical temperatures, such as those within the protective fuel cladding, are measured directly, accurately and reliably, said Dr. Richard Skifton, a researcher in INL’s Measurement Science Department.

A thermocouple consists of two wire legs made from different metals, welded together at one end. Due to a phenomenon known as the Seebeck effect, one can use the difference in voltage across the two materials to calculate a temperature reading. Thermocouples have been used for decades to measure temperatures in ovens, kilns, furnaces and jet engines, as well as in pasteurization and food production.

In nuclear research, however, the high temperatures and radiation levels inside reactor cores create conditions in which conventional thermocouples cannot survive for any great length of time. Even specialized thermocouples made of platinum, which have been shown to withstand temperatures over 2,000°C, are susceptible to neutron bombardment that causes the wire strands to deteriorate, granulate or transmute into different element compositions.

Because of its patented molybdenum-niobium construction, the HTIR-TC is the only high-temperature (greater than 1,250° C) probe that can withstand the environments inside reactors and nuclear fuel rod assemblies for sustained periods of time...