Patent Number: 054266793
Section: summary

TECHNICAL FIELD OF THE INVENTION This invention relates to a strainer device for filtering water to an emergency cooling system in a nuclear power plant of the type comprising a reactor arranged in a containment whose bottom part forms a pool for water, the strainer device being placed in the pool and serving to filter water which, if required, is taken from the pool and supplied to nozzles in the emergency cooling system in order to cool the reactor core in the event of an inadmissible temperature rise therein, the strainer comprising at least one housing with one or several apertured strainer walls through which the water can be sucked from the outside and into the housing and thereafter be fed to the emergency cooling system via a tube conduit connected to the housing. BACKGROUND OF THE INVENTION Strainer devices of the above related sort can be divided into two main types, to wit a first type being equipped with means for back-flushing of the strainer housing, and a second type which completely lacks such means. The first mentioned strainer devices are advantageous from a safety point of view, as long as they permit a cleaning of the apertured strainer walls of the housing at locations where these walls would unintentionally be clogged or blocked by fibres or other impurities circulating in the containment. This is effected by feeding clean wash-water into the strainer housing through a special feeding conduit with a pumping unit being connected thereto, the latter being actuable when necessary. However, an important inconvenience of such strainer arrangements is that the pumping unit as well as the special wash-water conduit are costly to produce and install. Moreover, they require considerable space in the area outside the strainer housing. It is true that the other type of strainer arrangements, i.e., those which completely lack back-flushing means, are comparatively inexpensive and space-saving, but they are limited in regard to the safety aspect, since they stop functioning if the holes in the strainer walls become clogged. In case fibres cumulate on the outside of the apertured envelope surface on a strainer housing, the fibres will form a continuous, circumferential mat. With previously known strainer arrangements, considerable difficulties have been encountered when detaching this fibre mat in connection with a backflushing. The washwater which is brought to flow from the inside in a direction radially outwards through the perforations in the strainer wall, does not bring about any complete and immediate release of the fibre mat; initially the water flow merely stretches the mat during simultaneous breaking up of the fibre structure. The removal of the mat is thus accomplished in such a way that individual fibres are successively released and removed from the mat. It is only after considerable hydromechanical action that the mat starts getting weaker and is gradually divided into chunks that leave the strainer wall. In order to cope with these difficulties, it has recently been suggested to provide radially protruding wings on the outside of the strainer wall, which wings split a cumulated fibre mat into several, peripherally separate sections, each one of which easily detaches from the strainer wall in connection with a backflushing. This is disclosed in PCT/SE93/01042. According to preferred embodiments of the present invention, radially protruding wings of the above mentioned type are foreseen, which are thus known per se. SUMMARY OF THE INVENTION The present invention aims at obviating the above-mentioned inconveniences of prior-art strainer devices and providing a simple and inexpensive strainer device with strainer walls which are always kept clean without any necessity of back-flushing. Thus, a main object of the invention is to provide a strainer device with at least two separate strainer walls or strainer wall surfaces of which one is always automatically kept clean in order to reliably make possible a feeding of water from the water pool to an emergency cooling system as soon as the necessity arises. A further object of the present invention is to provide a strainer device having means for keeping the holes of the strainer walls clean or open, which means are arranged so as to be capable of being built in the strainer housing itself, while avoiding any form of space-demanding connection components in the area outside the strainer housing. Yet another object of the present invention is to provide a strainer device for filtering water to an emergency cooling system in a nuclear power plant of the type having a reactor arranged in a containment zone and wherein the containment zone has the portion adapted to form a water pool, and wherein the system includes a strainer device adapted to be placed in the pool of water and functioning to filter water, and wherein the strainer includes at least one housing with at least one apertured strainered wall through which water can be drawn from the outside through the apertures into the housing, and wherein water may be fed to an emergency cooling system via at least one conduit connected to the housing, the improvement comprising flexible shield means mounted between separate strainer walls or strainer wall surfaces, said shield means being positioned in a first position or condition when water is drawn through one of said separate strainer walls or surfaces in which first position said shield means is operable to interrupt a fluid connection with the other strainer wall and said conduit, said shield means being capable of assuming a second position in which there is provided an open connection between said second strainer wall and said conduit when said first strainer wall is blocked by impurities sufficient to create a low-pressure zone between the first strainer wall and said shield means, said low-pressure zone being effective to effect said shield means between said first and second positions. In case fibres cumulate on the outside of the apertured envelope surface on a strainer housing, the fibres will form a continuous, circumferential mat. With previously known strainer arrangements, considerable difficulties have been encountered when detaching this fibre mat in connection with a backflushing. The washwater which is brought to flow from the inside in a direction radially outwards through the perforations in the strainer wall, does not bring about any complete and immediate release of the fibre mat; initially the water flow merely stretches the mat during simultaneous breaking up of the fibre structure. The removal of the mat is thus accomplished in such a way that individual fibres are successively released and removed from the mat. It is only after considerable hydromechanical action that the mat starts getting weaker and is gradually divided into chunks that leave the strainer wall. In order to cope with these difficulties, it has recently been suggested to provide radially protruding wings on the outside of the strainer wall, which wings split a cumulated fibre mat into several, peripherally separate sections, each one of which easily detaches from the strainer wall in connection with a backflushing. This is disclosed in PCT/SE93/01042. According to preferred embodiments of the present invention, radially protruding wings of the above mentioned type are foreseen, which are thus known per se.