Systematic errors in VVER-440 coolant temperature measurement

Stable operation of current nuclear power stations requires on-line temperature monitoring within the reactor. Experience with VVER power reactors suggests that a necessary condition for safe operation of a station containing VVER-440 is that the coolant temperature should be monitored at the outlet from the fuel-pin assemblies and in the main circulation loop. It is possible to reduce the error in the reactor temperature measurements to determine the heat production nonuniformity coefficients over the core more accurately, as well as the underheating of the coolant relative to the saturation temperature at the exit from a fuel-pin assembly, together with other thermophysical parameters important for safe and effective power station operation. Measurements within reactors may be accompanied by systematic deviations in the thermocouple readings that are comparable in magnitude with the limiting permissible errors. This paper discusses the most important components of the systematic deviations: errors due to calibration drift during use, errors due to radiation heating, and dynamic measurement error. The authors consider the basic features in the method of determining and balancing out the first of these components. 17 refs., 3 tabs