The Thermal Electromotive Force Stability Of Type K Mineral-insulated Metal-sheathed (MIMS) Thermocouples For Pressurized Water Nuclear Reactors Applications

Mineral-insulated metal-sheathed(MIMS)thermocouple is one of the most important sensors in nuclear Electrical equipment.It provides important informations for the safety operation,abnormal condition treatment and the accident monitoring of nuclear power plants.In order to meet the requirements of the third generation pressurized water reactor(PWR),the thermocouple sensors applied in Severe Accident and the in-core temperature measurement sensor were employed.In this paper,the influence of ?irradiation,temperature and cold working on the thermal electromotive force(EMF)stabilities of type K MIMS thermocouples was investigated for the third generation Pressurized Water Nuclear Reactors applications.The type K MIMS thermocouples are used to investigate the effect of high dose ? irradiation(accumulated 4500 kGy)on the calibration characteristics.The results indicate that after ? irradiation,the EMF of the irradiated thermocouples increased significantly at the temperature of 100 ?~400 ?,especially increased up to +173 ?V at 400 ?.Moreover,the whole EMF drifts included reversible and irreversible changes.Owing to the short order arrangement occured in the KP thermoelement during ? irridiation,the reversible EMF drifts of the thermocouple was positive and descreased the hysteresis loop and changes the Cp of the KP alloy.A small amount of inhomogeneous oxidation in the KP thermoelement and the aging were attributed to the irreversible EMF changes of the irradiated thermocouples.Therefore,the irreversible EMF drift of the MIMS thermocouple changed towards negative directions at 200 ? and positive directions at 300 ? and 400 ?.Two pairs of typical commercial thermocouple alloys were selected to investigate their EMFs stability behaviors under the temperature conditions such as normal condition,unusual condition and Severe Accident for nuclear plant applications.At the normal condition of in-core temperatute of 350 ?,the Seebeck coefficient is sensitive to the constituent elements.Hence even a little inhomogeneous would generate the change of EMFs.The stability of EMFs would be worse,if the thermocouple alloy contents the Fe element,which would alter the Curie temperature of the thermoelements.The EMFs drift towarded to positive direction after aging at the abnormal temperature 500 ? for 168 h.The irreversible EMF changes was due to the surface oxidation of the thermocouple wires with only several ?m.When using slight oxidized thermocouple wires to manufacture MIMS thermocouples,the oxidation film decreased the diffusion of oxygen atom or metal cation and inhibited the oxidation during aging.Aging at 500 ? for 168 h,the reversible EMFs changed towards to positive direction.That was because of the amount of short ordered NiCr crystal lattice.An appropriate stabilization processing would form a center amount of short ordered NiCr lattice and decrease the discrepancy of the EMFs hysteresis during heating and cooling calibration.The EMFs stability of the thermocouples at the temperature of Severe Accident condition was depend on the compatibility of the thermocouple alloy and sheath alloy.Using 316 L as a sheath alloy,the EMFs drift was attributed to inhomogeneous oxidation especially the KN thermoelement.The solution elements were consumed and diffused toward the surface of the thermocouple wires,which generated an inhomogeneous of the composition even in the un-oxidized area.Hence,the EMF had a rapid negative component suddenly aging at the temperature of 1000 ?.Using Inconel 600 as a sheath alloy,if the discrepancy of the atomic diameters of the solution elements and Ni element was larger,there would be a natural tendency to reject the supersaturated.The precipitation of solution elements at the grain boundary or the inner grain in the Ni matrix generated the inhomogeneous compositions that result in a larger EMF drift of the thermocouple.According to the requirement of assembly of drawing in the manufacture of in-core measurement sensor,the effect of cold working on the calibration characteristic and long-term stabilities of the thermocouple was investigated.The results showed that the EMFs of the thermocouple and EMFs VS Pt of each thermoelement were decreased with the increase of calibration temperature after cold working.The main drift reason was attributed to KP thermoelement.Because of the acceleration of short order arrangement,the hysteresis loops were larger with the increase of cold working.The deformation reduced the diffusion active engine of element and speeded up the elements diffusion,so the EMFs of thermocouple with larger deformation had a greater fluctuation in a consequence of instability aging at 350? for 720 h.Aging at 500??600? and 700?,the EMFs increased with the prolong of aging time.After aging at 500? for 500 h,an optimal amount of short order arrangement lattices was formed,therefore,the maximal EMFs hysteresis loops was-6?V.While after aging at 600? and 700?for 500 h,the EMFs of the thermocouples with a cold working of 28% and 38% were still lower than that of un-worked thermocouple.Thus,the effect of cold working on thermocouple could not be recovered.What's more,the hysteresis loops of the cold worked thermocouples were lower than that of un-worked thermocouples.