Patent Number: 046997555
Section: summary

FIELD OF THE INVENTION The invention relates to an ultrafiltration circuit for the primary cooling fluid of a pressurized-water nuclear reactor. Such reactors incorporate a primary circuit, in which the pressurized water cooling the fuel assemblies of the reactor core circulates. The primary circuit communicates with the inner volume of the reactor vessel containing the core and incorporates primary-fluid circulation pumps, steam generators and a pressurizer which are connected by means of large-diameter pressure-resistant pipelines. The primary circuit as a whole is located inside a containment shell constituting one of the protective structures of the nuclear reactor. Auxiliary circuits arranged either completely or partially within the containment shell are taken off from the primary circuit. In particular, to keep the quantity and chemical quality of the primary fluid constant, a circuit called a volumetric and chemical monitoring circuit is used, and this makes it possible to sample some of the fluid circulating in the primary circuit, carry out various treatments on this primary fluid and reintroduce into the primary circuit specific quantities of fluid having precisely defined chemical characteristics. The volumetric and chemical monitoring circuit taken off from the primary circuit comprises a discharge branch and a charge branch which both pass through the wall of the containment shell to be connected, outside this containment, to various devices for purifying and treating the primary fluid. These treatments are carried out on a low-temperature and low-pressure fluid, cooling and depressurizing devices being arranged on the discharge branch of the circuit upstream of the treatment units. The primary fluid is depressurized and partially cooled before it leaves the containment shell, thus limiting the risks entailed by the presence of pressurized primary fluid outside the containment shell. Arranged on the charge branch of the volumetric and chemical monitoring circuit are various means of injecting additives or make-up water and a charge pump, making it possible to reintroduce into the primary circuit a fluid which is at a pressure a little higher than the pressure in this circuit. The charge branch of the volumetric and chemical monitoring circuit likewise passes through the secondary side of a heat exchanger serving to cool the primary fluid sampled by means of the discharge branch. The cooling fluid consisting of pressurized water circulates in the primary circuit at a temperature of approximately 300.degree. C., at a pressure in the neighborhood of 155.times.10.sup.5 Pa and with a flow rate of the order of 25,000 m.sup.3 /h in each of the loops of the circuit. During the various operating phases of the reactor which result in increases or decreases in power, the physical or chemical parameters of the reactor have to be modified, and this is achieved particularly by means of the volumetric and chemical monitoring circuit. During its circulation in the primary circuit or in the auxiliary circuits, the cooling fluid comes in contact with many components, most of which are made of or covered with a nickel alloy which makes it possible to limit the degree to which they are attacked by the primary fluid. However, some components, such as the seats of valves and of cocks, or even certain portions of piping experience a certain wear, with the result that the primary fluid becomes laden with particles of very small dimensions which are torn off from these components. These particles tend to circulate together with the primary fluid and therefore pass through the reactor core, where they are subjected to intense neutron bombardment, the effect of which is to activate them. In particular, wear-resistant alloys containing a certain proportion of cobalt cause highly activated particles to occur. These particles accumulate in certain parts of the components of the reactor, and this presents problems which are very difficult to solve during the reactor maintenance operations, since these operations require preliminary decontamination phases which are very difficult to carry out. On the other hand, the make-up water and additives introduced into the primary fluid by means of the volumetric and chemical monitoring circuit likewise contain solid particles of various origins which are activated when the primary fluid passes through the reactor core. It is therefore necessary to treat the primary fluid periodically or continuously to reduce the content of activated or activatable particles in this primary fluid. Arranged in the volumetric and chemical monitoring circuit are units for treating the primary fluid with ion exchanger resins of the mixed-bed type, but these resins only make it possible to retain certain types of chemicals dissolved in the primary fluid, excluding particles of small dimensions transported by this fluid. Mechanical filters in the form of filter cartridges are also associated with these treatment units, but these filters do not allow particles of a size less than 5 microns to be retained, whereas the Gaussian probability curve representing the proportion of particles transported as a function of their size is centered around 0.5 microns. On the other hand, the solubility of the particles in the primary fluid is inversely proportional to the temperature, with the result that the treatment conditions in the volumetric and chemical monitoring circuit outside the containment shell are not conducive to capturing the particles effectively. In French Patent Application No. 83-15130, in the name of the assignee of the present invention, it has been proposed to purify the primary fluid at a pressure and a temperature near its operating pressure and temperature by means of ultrafiltration using a device located inside the containment shell of the reactor, for example on the discharge branch of the volumetric and chemical monitoring circuit. The filtrate recovered at the filter outlet is subsequently cooled and depressurized, before being conveyed outside the containment shell into the treatment units of the volumetric and chemical monitoring circuit. To prevent the clogging of the ultrafiltration wall, the circulating concentrate is kept in contact with this wall during the operation of the ultrafilter. The concentrate is thus constantly laden with radioactive impurities, and the primary part of the filter has to be emptied as soon as the proportion of impurities in the concentrate becomes too high for the filter to operate under good conditions. The concentrate has to be discharged towards a liquid-waste treatment installation, thus complicating the maintenance operations in the nuclear power station. In addition to this disadvantage, there is also that associated with intermittent operation of the filter. An additional disadvantage arises because the fluid recycled in the primary circuit via the charge branch of the volumetric and chemical monitoring circuit contains particles which come from the make-up water or additives and which are activated when they pass through the core. The quantity of active particles transported by the primary fluid consequently remains relatively large. SUMMARY OF THE INVENTION The object of the invention is, therefore, to propose an ultrafiltration circuit for the primary cooling fluid of a pressurized-water nuclear reactor incorporating, inside a containment shell, a primary circuit which communicates with the inner volume of the reactor vessel containing a core consisting of fuel assemblies and in which the pressurized water constituting the primary fluid circulates, and at least one auxiliary circuit taken off from the primary circuit and comprising a discharge branch, on which are arranged means of cooling and depressurizing the primary fluid sampled by means of the auxiliary circuit and which passes through the containment shell of the reactor, and a charge branch intended for returning the fluid into the primary circuit and likewise passing through the containment, and, outside the containment, means of treating the cooled and depressurized fluid, this ultrafiltration circuit making it possible to purify the primary fluid effectively both by eliminating the activated particles in the core and by eliminating non-active particles introduced via the auxiliary circuit, without increasing the quantity of radioactive waste to be treated in the power station. To achieve this object, the ultrafiltration circuit according to the invention comprises inside the containment shell: a first loop taken off from the discharge branch of the auxiliary circuit and incorporating a pipe for extracting and conveying fluid at its operating pressure and temperature into a first ultrafiltration device located in the first loop, a pipe for discharging filtrate at the outlet of the ultrafiltration device and for returning this filtrate into the discharge branch downstream of the point where fluid is extracted by means of the extraction pipe, a valve being inserted in the discharge branch between the two pipes, and a concentrate discharge pipe which passes through the wall of the containment and on which are arranged, inside the containment, means of cooling and depressurizing the concentrate before it is introduced into the discharge branch of the auxiliary circuit downstream of the cooling and depressurizing means located on this discharge branch, and a second loop taken off from the charge branch of the auxiliary circuit and comprising a pipe for extracting and conveying fluid at its operating temperature and pressure into a second ultrafiltration device located in the second loop, a pipe for discharging filtrate at the outlet of the ultrafiltration device and for returning this filtrate into the charge branch downstream of the extraction point, a valve being inserted between these two pipes, and a concentrate discharge pipe connected to the discharge branch of the auxiliary circuit, inside the containment, upstream of the cooling and depressurizing means arranged on this branch.