Gamma Thermometer Developments for Light Water Reactors

Gamma thermometers depend upon the heating of a metal block by gamma rays (~90%) and high energy neutrons (~10%). The heat so generated is proportional to the specific power of the nearby fuel rods. Heat generated in the block of metal is permitted to escape to a sink only through a controlled heat path of closely held dimensions. The temperature drop along that heat path is directly proportional to heat rate (watts/gm) in the heater and therefore proportional to power, not neutron flux, in adjoining fuel rods. A thermocouple, or thermocouples, arranged to measure the temperature drop along the controlled heat path produces a signal proportional to this power. Usual material of construction is stainless steel. The heat rate in the stainless steel at full power ranges from 0.5 to 7.5 watts/gm. Temperature drops have been selected in the range from 15°C to 250°C at full power. Above 500°C the instruments remain reproducible but tend to become nonlinear with power as the radiation component of heat loss becomes significant. Gamma thermometers have been employed for power distribution monitoring in large heavy water moderated reactor cores for over 15 years. This experience has established the following advantages: signal proportional to local power, non-changing calibration, ruggedness, long service life.