Effect of Thermal Neutron Irradiation on Thermocouples and Resistance Thermometers

IV. Summary The purpose of this paper is to demonstrate for certain conditions that transmutation of elements can result in a temperature measurement error that is reasonably predictable. Methods are developed for estimating the errors for the purpose of: (1) Engineering judgment in selecting temperature measuring elements for application; (2) Obtaining the order of magnitude for expected error in nuclear applications; (3) Use as a guide in the evaluation of results from nuclear irradiation studies. Theoretical and experimental results show that the thermal emf of a Pt vs Pt-10% Rh is decreased (i.e., a temperature reading will be too low) when exposed to high neutron fluxes. The experimental results, together with theoretical considerations, indicate that the changes are due primarily to the transmutation of rhodium. Furthermore, the experimental results given here are in very close agreement with other irradiation data obtained independently by Oak Ridge National Laboratory. The results are believed to be sufficiently conclusive to permit an estimation of errors to be expected due to transmutations caused by nuclear reactions in this thermocouple. In order to evaluate the various resistance thermometers for use in nuclear reactors, from the transmutation point of view, the values arrived at for the numerical examples are listed below for room temperature.