Patent Number: 042499941
Section: summary

This invention relates to a method for unclogging an electromagnetic filter and to an installation for carrying out said method. It is known that an electromagnetic filter is essentially constituted by a casing of non-magnetic material filled with a magnetizable packing and placed within the interior of a winding. The passage of an electric current in the winding results in the appearance of a magnetic field which has the effect of magnetizing the packing. The packing can be fixed in the form of padding, of woven steel-wire fabric or of a stack of grids. In the majority of instances, however, the packing is formed by a bed of steel beads. The application of a magnetic field to the packing beads results in magnetization of these latter and correlatively in the appearance of high magnetic-field gradients in the spaces between beads. When a fluid charged with ferromagnetic impurities passes through the bed of beads which have thus been magnetized, the impurities are transferred from the zones of low magnetic field to the zones of high magnetic field, that is to say towards the magnetic poles of the beads. The action of the magnetic forces is such that the ferromagnetic impurities adhere to the beads and the packing thus performs the function of a filter. The use of electromagnetic filters of this type has already been contemplated for a large number of installations and especially nuclear reactors in which they can be installed either in the primary circuits or in the secondary circuits. This use in nuclear reactors is described in particular in French Pat. No. 72 25870 filed on July 18, 1972 and entitled "Water treatment installation for steam generators in nuclear power plants of the pressurized-water reactor type " and in French Pat. No. 72 45355 filed on Dec. 20, 1972 and entitled "Water purification device for a nuclear power plant of the pressurized-water reactor type." In order to unclog a filter of this type, the operation is performed as follows. The first step consists in isolating the filter from the installation in which it is inserted. This operation is performed by closing valves. The packing is then demagnetized by applying a low-frequency alternating-current voltage to the terminals of the winding, the amplitude of said voltage being such as to decrease progressively to zero. Finally, a stream of liquid derived from a wash duct which is independent of the water circulation systems of the nuclear reactor is passed into the filter in order to wash the latter. This upwardly flowing liquid stream dislocates the bed of beads which accordingly undergo disordered motion during which they come into collision with each other many times. This has the effect of detaching the clogging products which are carried away by the wash liquid. On completion of this operation, the beads fall back into position at the bottom of the filter and reconstitute the bed, simply under the action of gravity. The bed of beads is then remagnetized and the filter is ready to be used again. This unclogging operation lasts a few minutes approximately. In known methods of this type, the wash water is not withdrawn from the primary (or secondary) circuit of the nuclear reactor and, in general, is therefore neither at the temperature nor at the pressure of the water which circulates within said circuit. Moreover, the unclogging operation takes place by means of a stream of water which has essentially a continuous character and is circulated upwards within the filter. Methods of the type described are subject to a large number of disadvantages. The first disadvantage arises from the fact that they call for a very large quantity of wash water which, in the case of each regeneration, is of the order of 1% of the hourly quantity of water treated by the filter. By way of explanation in the case of a filter which is capable of treating 1000 metric tons of water per hour, 15 metric tons of water are required in order to effect unclogging of the filter by means of a method of the prior art. It is therefore impossible by means of this method to withdraw such a large quantity of water from the nuclear reactor circuit over a short period of time. For this reason, it is necessary to have recourse to an auxiliary source connected to the filter by means of a wash water supply duct. This need to make use of a large quantity of water further leads to two difficulties: in the event that the filter is placed in the primary circuit of a nuclear reactor, the effluents discharged from the filter are radioactive and represent a large quantity of water to be treated, thus constituting an appreciable capital investment in the exploitation of the reactor. In the event that the filter is placed in the secondary circuit of a nuclear reactor, the water employed in this filter is usually conditioned and especially de-aerated and the need to employ a large volume of water is again objectionable in this case. The second disadvantage arises from the temperature difference observed in methods of the prior art between the filtering stage and the unclogging stage. The wash water is usually at a lower temperature than that of the water circulated within the nuclear reactor circuit. Unclogging therefore calls for a reduction in temperature at the beginning of the cycle followed by an increase in temperature at the end of the cycle; the length of the unclogging operation is increased accordingly. As a secondary consideration, it can be observed that the steels constituting the beads which form the filter bed are usually liable as a result of chemical corrosion to form products of corrosion in a different form and especially in a less magnetizable form. In consequence, it is also an advantage from this point of view to carry out washing of the filter at a temperature which is as high as possible. The present invention overcomes the foregoing disadvantages in that it proposes a method of unclogging which calls for the use of a much smaller quantity of wash water than the quantity employed in methods of the prior art. This permits withdrawal of said water from the nuclear reactor circuit and therefore the introduction of the water into the filter substantially at the temperature and pressure of withdrawal since the unclogging operation takes place in a series of washing and draining-off cycles and not by means of a continuous flow of wash water as in the prior art. By way of explanation, when the method in accordance with the invention is employed in the case of a filter which is capable of treating 1000 metric tons of water per hour it is possible to unclog the filter with only 3 to 4 tons of water instead of the 15 tons which were required by the methods of the prior art. This small quantity of water can accordingly be withdrawn from the water circuit of the nuclear reactor (namely either the primary or secondary circuit); in consequence, the filter receives water to be filtered and wash water which are substantially at the same temperature and at the same pressure. In the wash cycles which take place in accordance with the method of the invention, the bed of beads undergoes successive displacements of small amplitude with a sufficiently high degree of efficacy to dispense with any need to cause complete dislocation of the bed of beads within a relatively large free internal space provided at the bottom of the filter. It is therefore always possible to employ a filter which is packed to practically the full height of this latter, as was not the case with methods of washing in the prior art. So far as the installation is concerned, the invention finally provides further advantages which are related in particular to elimination of the wash duct and of the auxiliary source of wash water. This modification of the technology of the installation results in a reduction of capital cost of this latter. In more exact terms, the present invention is therefore directed to a method for unclogging an electromagnetic filter having a magnetizable packing and placed in a water circuit of a nuclear reactor. The method essentially consists first in isolating the filter from the circuit, then in subjecting the filter to a series of washing and draining-off cycles. The washing operation consists in withdrawing from said circuit a fraction of the water which is circulated therein and in introducing said water into the filter substantially at the temperature and pressure of withdrawal and under such conditions as to impart turbulent flow motion to said water within the packing. The draining-off operation consists in discharging the wash water contained in the filter. It is preferably ensured that, when the filter is placed in the primary circuit of a nuclear reactor of the pressurized-water type, unclogging takes place at a temperature within the range of 200.degree. C. to 300.degree. C. It is also preferably ensured that the pressure of the water employed for the unclogging operation is within the range of 100 to 160 bar. When the filter is placed in the primary circuit of a pressurized-water reactor comprising a pressurizer, withdrawal of the wash water is preferably carried out within said pressurizer. The invention is also concerned with an installation for the practical application of the method hereinabove defined and for unclogging an electromagnetic filter of the magnetized-packing type, said filter being placed in the water circuit of a nuclear reactor. The installation essentially comprises means for isolating said filter from the circuit, means whereby part of the water which circulates in said circuit is withdrawn therefrom, means for introducing the water into the filter substantially at the temperature and pressure of withdrawal under conditions which impart turbulent flow motion to said water within the packing, and means for discharging the wash water contained in the filter. In a first alternative embodiment, the installation comprises a closed wash circuit constituted by an accelerating pump connected at the upstream end by means of a pipe fitted with a valve to the top portion of the filter and at the downstream end by means of a pipe fitted with a valve to the bottom portion of said filter, said circuit being connected at the top portion thereof to the water circuit of the nuclear reactor by means of an introduction valve end and at the bottom portion thereof to an effluent tank by means of a drain-off valve. In a second alternative embodiment, the installation comprises a water admission duct which connects the bottom portion of the filter to the water circuit of the nuclear reactor by means of a water introduction valve and a drain-off pipe which is connected to the bottom portion of the filter and is fitted with a drain-off valve. When the filter is placed in the primary circuit of a nuclear reactor of the pressurized-water type comprising a pressurizer, the installation comprises a pipe connected to said pressurizer and a valve placed in said pipe. When the filter is placed in the secondary circuit of a pressurized-water reactor, said circuit being equipped with at least one high-pressure heater followed by a steam generator, the filter is accordingly placed between said high-pressure heater and said steam generator. It is preferably ensured that the magnetizable packing is constituted by a bed of beads.