Computational-experimental method of evaluating the measurement error in the fuel-element cladding temperature during the simulation of emergencies

Abstract

The objective of this work is to develop a computational-experimental method of determining the systematic error in measurements of the temperature of cooled fuel-element cladding using a thermocouple secured along the generatrix of the cladding. A method of nondestructive diagnostics of the quality of the thermal contact of the working end of a thermocouple with fuel-element cladding and the conditions for its heat exchange with the coolant and their effect on the measurement error in the temperature of the cooled cladding is examined. The method is based on thermal probing of a thermocouple by passing a current through the thermal electrodes and recording and processing the responses in the presence and absence of the coolant. A relation is derived for the systematic error with stationary and linearly growing cladding temperature. The experimental and computational results for the systematic errors are presented.Work on the safety of nuclear power plants includes the study of fuel-element cladding in a steam-hydrogen medium at high temperature. Accurate calculations of the thermoelastic state of cladding in emergency situations associated with overheating and bulging of the cladding are possible if precise data on the temperature distribution are available. However, when such studies are performed a thermocouple records a temperature that is different from the true temperature in the weld zone. The scheme for securing the thermocouple to the cladding of a fuel element is shown in Fig. 1.In [1], the effect of the main factors on the correction introduced is evaluated and recommendations for minimizing it are formulated. At the same time, it is noted that the computed temperature of the cladding can differ from the measured temperature because of the presence of factors that are difficult to take into account. A computational-experimental method for evaluating the error if a stationary and uniform temperature field is attained in the model assembly is examined in [2]. We shall present its basic assumptions.Let q 1 , α 1 , and S 1 be the heat flux density, the effective thermal conductivity of the contact, and the effective area of the contact on the cladding-soldered seam boundary, respectively; q 2 , α 2 , and S 2 are the same parameters at the soldered seam-coolant boundary. On the basis of Newton's lawwhere T 1 , T, and T 2 are the desired cladding temperature, the soldered seam temperature (measured), and the coolant temperature in the temperature-measurement zone, respectively.