Feasibility study of using permanent magnets as indirect temperature monitor in irradiation capsule of nuclear research reactor

Abstract

In sodium cooled nuclear fast reactors, materials used for fuel cladding and structural components can undergo significant physical and dimensional changes due to exposure to high energy neutrons. At high temperature and radiation conditions prevailing in the nuclear reactor, material properties change considerably. Information about the temperature of exposure during specimen irradiation in a nuclear reactor is important since the temperature is having a strong influence on the mechanical and physical properties of materials. A proper evaluation of the effects due to radiation on materials needs a precise knowledge of the irradiation temperature. Many times, standard techniques for measuring temperature are not practicable in reactors due to the availability of limited instrumented positions. Therefore, the use of small-sized indirect temperature monitors such as melt wires, permanent magnets, etc. which can be incorporated along with the samples and examined after the irradiation are preferred. In the present study, permanent magnets of neodymium grade N38 and N38SHX of different sizes have been studied for their possible use as an indirect temperature monitor. The study was carried out first for their compatibility with sodium since the monitor will be exposed to sodium during irradiation experiments in fast reactors, and then by exposing to different temperatures in an electrical furnace for the determination of Curie point to study the feasibility of using these materials as indirect temperature monitors. Since the material loses its magnetic properties when the temperature of exposure exceeds the Curie point, by using a series of magnets having progressively increasing curie points, the temperature of exposure can be determined by testing these magnets for the retention of their magnetic properties. Specifically, since there is variation in the Curie temperature as reported in the literature, the experimental method was developed to determine the Curie temperature accurately. This paper discusses the details of the experiments carried out in our laboratory and the results obtained from these studies.