Methodological errors in in-reactor temperature measurements

Abstract

The requirement of increased reliability and economy of atomic power stations necessitates high accuracy of in-reactor measurements of the fuel-element temperatures. Thermoelectric converters are mainly used as the primary temperature converters for this purpose [i]. In most cases, they are positioned along the fuel-element axis, where the temperature gradients are least. Accordingly, there are longitudinal holes in the fuel element, into which the thermoelectric converter is introduced directly or as part of a thermometric assembly, where the converter is placed in a technological (protective) sleeve [2]. The changes in thermoemf observed over time suggest [3] that one cause of such changes, in addition to integral effects [i], may be the presence of methodological errors ATqv due to radiational energy liberation in the constructional materials of thermoelectric converters and the technological sleeve. Variation in the contact thermal resistance between the working junction of the converter and the fuel leads to oscillation of the methodological errors. Theoretical estimates of these oscillations do not ensure that the basic modifying factors are taken adequately into account: the initial deviation of the geometric dimensions of the constructional elements from the specification, the actual heattransfer conditions in the gas gaps, their variation on account of change in the geometric parameters of the elements under the action of the medium, the temperature, and radiation in the course of operation, and the thermophysical characteristics of the constructional materials in the given conditions. Thus, it is necessary to develop a theoretical-experimental method of finding the methodological error directly in the operating conditions of the thermoelectric converters, ensuring that the complex of factors determining the contact thermal resistance between the working junction and the object being studied is taken into account.