No abstract

No abstract

Economic and safe operation of modern nuclear power plants requires that the coolant temperature in the reactor be monitored [1]. In a VVI~R reactor the coolant temperature at the exit from the fuel assemblies is measured with chromel-alumel cable thermocouples, placed in special protected temperature-monitoring channels [2, 3]. The thermocouples are replaced during the operating life of the reactor. When the replacements are made, it is necessary to check that the end of the thermocouple cable with the "hot" junction is inserted into the end of the temperature-monitoring channel, because this influences the error in the measurement of the coolant temperature. If the end of the thermocouple cable is not inserted into the receptacle or it is inserted only partially, then the width of the air gap between the cable end and the channel adaptor increases, and therefore the thermal resistance of the air gap also increases, which in turn increases the dynamic error of the measurements [4] as well as the error due to the radiative heating of the thermocouple [5].The radiation conditions at the time the thermocouples are replaced make it difficult to determine directly whether or not the thermocouples have been inserted properly into the channels. We describe below a method for determining indirectly whether or not the thermocouples have been properly inserted. The method is based on analysis of the amplitude of the signal from the thermocouples after they are heated by an electric-current pulse passed along the hot electrodes. To determine whether or not the thermocouples have been properly inserted into the adapter receptacle in the temperature control channel it is only necessary to estimate the thermal resistance of the air gap between the thermocouple and the adapter.The method for passing current through thermocouples is well known. It is used for different purposes: checking the electrical resistance between hot electrodes [6] and between electrodes and a cable sheath [6], for making it easier to extract wedged-in thermocouples [7], and for determining the thermal-inertia indicator [8, 9].We now consider a model of a thermocouple in the temperature-monitoring channel that makes it possible to describe the response of a thermocouple, which has been only partially inserted into the receptacle of the channel adapter, to the heating of the thermocouple by the current passing through the hot electrodes.To determine the response of such a thermocouple, it is sufficient to study a system of two heat-balance equations [10] for the thermocouple cable in the region of the hot junction and the adapter of the temperature-control channel:(1) T~(t = O) = r2(t = O) =-T~(O),where the indices 1 and 2 refer to the thermocouple cable and the adapter of the temperature-monitoring channel, respectively; Ti(t) is the temperature averaged over the volume V i (t = 1, 2); qi(t) is the specific Joule heating; Pi, ci are the density and specific heat capacity; V1, 2 are the volume per unit length of the thermocouple cable and the channel a...