Monitoring of Interloop Density of a Propulsion Nuclear Power System Cooled by Pressurized Water

Abstract

Tritium accumulation in the first-loop coolant of a propulsion nuclear power system is examined. The results confirm that tritium can be used as a reference radionuclide for monitoring interloop density. The sensitivity of the radiometric method of tritium monitoring for determining coolant leakage in the first loop during operation of the power system at power and in an idling regime is estimated.Heat-exchange equipment is potentially dangerous, from the standpoint of interloop leakage, in propulsion nuclear power systems [1]. As a result of the strap which always present in the piping system of steam generators, when even the smallest leak appears coolant from the first loop enters the second loop. This ultimately results in contamination of occupied rooms, containing the second-loop equipment and, possibly, other adjoining rooms, with radioactive gases and aerosols [2].The sealing of the steam generators of propulsion nuclear power systems during operation at power is monitored by two methods, which differ by the manner in which the technological operations are performed. The instrumental method of monitoring does not require removal of steam from the loop, while the sampling method is based on laboratory radiochemical analysis of reference radionuclides in samples of the scheme condensate. The latter method is used for the earliest possible detection of leaky steam generators and for confirmation of the results of instrumentational method of monitoring with a small leak in a steam generator and supplements the first method.Monitoring in an idling regime, using the instrumentational method and laboratory chemical analysis, is impossible because the reference radionuclides 16 N and iodine isotopes have short half lives [3]. Monitoring by disassembly and subsequent visual examination of the interior cavities of the steam generators is also impossible because of the circulation of the first-loop coolant in the interpipe space and the structural features of the piping system of the second loop [2]. Ultimately, there is no information making it possible to perform an objective analysis of the reliability and safety of physical barriers.Tritium as a reference radionuclide has not been considered for instrumentational and radiochemical monitoring of interloop seal. At the same time, it has substantial advantages for expanding the possibilities of monitoring interloop seal and obtaining information about the sealing of heat-exchange equipment for all three loops.The main source of tritium formation in the reactor of a propulsion nuclear power system is fissioning of nuclear fuel. One fission of 235,238 U and 239 Pu in the fuel of white-water reactors produces (in units 10 -4 ) 0.85, 2, and 2 tritium atoms, respectively [4]. As a result of diffusion through the cladding of a fuel element and cracks and microcracks in cladding, tritium can migrate from the fuel into the coolant. Leakage from fuel elements with corrosion-resistant steel cladding is 1%, and leakage from zirconium alloy cladding is 0.1% of the total amount...