Patent Number: 046506357
Section: summary

FIELD OF THE INVENTION The invention relates to a process for the monitoring of leaks in the primary circuit of a pressurized water nuclear reactor. The primary circuit of nuclear reactors which are cooled with water under pressure corresponds to the part of this reactor which contains water under pressure for cooling the reactor core. This primary circuit therefore comprises the reactor vessel enclosing the core, the primary part of the steam generators, the inner volume of the pressurizer and of the pipes for circulating pressurized water, connecting each of the steam generators to the vessel independently, each of the parts of the circuit comprising a steam generator and a system of pipes which are connected to the vessel and form a loop of the primary circuit. The primary circuit is also connected to auxiliary circuits including the circuit for volumetric and chemical monitoring of the pressurized water. This auxiliary circuit which is arranged branching on the primary circuit makes it possible both to maintain the quantity of water in the primary circuit by replenishing, when required, with measured quantities of water and to monitor the chemical properties of the cooling water, particularly its content of boric acid which is involved in the operation of the reactor. During the periods when the chemical properties of the reactor water are adjusted, it may be necessary to carry out tappings or injections into the primary circuit, the quantities tapped or injected being known and controlled in a highly accurate manner. Outside these periods of injections or tappings, the valves connecting auxiliary circuits other than the circuit for volumetric and chemical control to the primary circuit are closed. The primary circuit is then theoretically isolated and completely sealed, with the result that the quantity of water in this primary circuit is theoretically constant. In practice, however, it is observed that this quantity of cooling water diminishes during the operation of the reactor, as a consequence of unavoidable leaks which can be monitored and perfectly evaluated or, on the contrary, unmonitored. The unmonitored leaks can themselves be localized or unlocalized and, in the latter case, the evaluation of the magnitude of these leaks is particularly difficult. It is nevertheless very important to have good knowledge of the leakage rate of the primary circuit, in order to undertake preventive actions before accidental leaks become more serious and call into question the safety of operation of the nuclear reactor. PRIOR ART Various processes have been proposed for detecting leaks in a closed circuit. It has been proposed, for example, to employ sound detectors to show the presence of a leak greater than a limit value at a location in the circuit. This process, however, does not make it possible to estimate the total of the leaks from the circuit and indications given are not really quantitative. There has also been proposed, in French Patent No. 2,214,992, a process making use of a level control in an expansion vessel arranged in the circuit and a measurement of the temperature of the fluid in the closed circuit. If the changes in level in the expansion vessel are incompatible with the change in the mean temperature of the fluid, it is concluded from this that the changes in level are due to leaks. A replenishment of liquid is then carried out in the expansion vessel until the level returns to a predetermined level which is a function of the mean temperature of the fluid. This process, which permits the presence of leaks to be detected, does not however allow them to be estimated quantitatively in a precise manner. U.S. Pat. No. 3,712,750 describes a process for detecting leaks in the cooling circuit of the primary circuit of a nuclear reactor. The leaks are collected in a drainage sump under the reactor vessel and the leak liquid collected evaporates in the reactor safety housing. During the treatment before recirculation of air in the reactor housing the traces of tritium which may be present in the water vapor present in this air are measured. The radioactivity in the reactor housing air can also be measured directly. This process, which permits the leaks to be determined in a more or less quantitative manner, is, however, highly complex in use. SUMMARY OF THE INVENTION The aim of the invention is therefore to offer a process for monitoring leaks in the primary circuit of a pressurized water nuclear reactor comprising a vessel enclosing the reactor core, at least two steam generators connected independently to the vessel by pipework for circulating water under pressure, a pressurizer, at least one auxiliary circuit for monitoring and replenishment of the water under pressure and stocktanks inserted in the primary circuit and in the auxiliary circuit, this monitoring process permitting a quantitative determination of the total leaks from the primary circuit with very high accuracy and employing only conventional, easily operated means of measurement. To this end, a sectioning of the internal volume of the primary circuit and of the circuit for volumetric and chemical monitoring, excluding the stocktanks, is determined, as a function of the characteristics of the primary circuit, into a set of volume elements in which the temperature and the pressure of the water are equal at any point in the volume element with a predetermined margin of error, during the operation of the reactor, at fixed time intervals, called "steps", during the operation of the reactor: the level of the pressurized water is measured inside each of the stocktanks, the pressure and the temperature of the pressurized water are measured in each of the volume elements, the mass of water in each of the stocktanks and in each of the volume elements is calculated as a function of the measured temperatures, pressures and levels, the total mass of water in the primary circuit is calculated by adding the masses of water in the volume elements and in the stocktanks, and for periods of time which correspond to a multiple of the step and which are offset by at least one step, the mean of the masses of water in the primary circuit and the difference of the mean values corresponding to two successive periods of time are calculated, the difference representing the leakage flow of the primary circuit.