Verification of the check program for calculating leakage in steam generators in nuclear power plants with a VVÉR reactor

Abstract

A method for calculating one of the main parameters of nuclear and radiation safety of a nuclear power plant with a VVI~R reactor --coolant leaks from the first loop into the boiler water of the steam generators --was studied in [1, 2].Computer codes for such a computation were studied in [3]. The computational algorithm is based on an analysis of a system of linear differential equations describing radionuclide transport through systems of the second loop of a nuclear power plant with a VVI~R reactor.The leakage G is estimated on the basis of a periodically measured specific activity of the isotopes 131q35I, 24Na, and 42K in the coolant in the first loop and the purge water of steam generators taking account of the degree of reliability of these data (measurement error). The method makes it possible to calculate this quantity both with a known flow rate Q of the purge water for purification and in the absence of this information. The latter case is especially important, since there often occur cases when the indications of standard equipment for monitoring the flow rate of the purge water for one reason or another are approximate, and in some cases, for example, in the presence of large corrosion deposits on the inner surfaces of the pipeline [4] and throttling rings, they can be unreliable or completely absent. On account of possible pressure differences in steam generators operating in parallel, it is not always correct to determine Q by dividing the total flow rate of the purge water by the number of steam generators in the power-generating unit, even in the case of a regime where the steam generators are purged continuously and simultaneously [5]. This can occur as a result of both their different steam productivity and the difference of the lengths of the corresponding steam pipes. This factor necessarily affects the flow rate of the water in parallel purge lines combined into a common mainline. Thus, in the case of the indicated scheme of operation of the steam generators under certain conditions hydrodynamic instability can appear. For this reason, a long-time decrease of the purge water flow out of a steam generator is possible. This possibility must be taken into account when estimating G.It should be noted that leakage of coolant from the first loop into boiler water is not a directly measurable physical quantity. Information about its values can be obtained only on the basis of indirect data. There arises a danger that a quantity which is functionally coupled to the leakage is actually taken as the leakage of the steam generator. This makes it impossible to make a direct experimental check of the authenticity of the theoretical assumptions of the method for calculating G. However, in cases of this kind the following idea can be used to verify the CHECK program.As we have noted, the fundamental feature of the method is the calculation not only of the point and interval estimates of the leakage but also the analogous characteristics of the purge water flow.We shall examine two independent methods for de...