Background of the Invention

The basis of the present invention is ultimately the problem of a reliable reverse flushing or cleaning of those strainers m a nuclear power plant that have the task of puri¬ fying water in the reactor containment before it is fed into a cooling system. In boiling water reactors the strainers are permanently under water at the bottom of the containment and convey water to a regularly working cooling system and/or to an emergency cooling system which is only started in case of a shutdown. Under normal circumstances, compressed water reactors lack water at the bottom of the containment. Here, the strainers shall only start working m connection with possible shutdowns, during which water is collected at the bottom of the contain¬ ment. However, m both cases the strainers shall be capable of working m a reliable way m so far as reverse flushing or cleaning is initiated without delay, if and when the strainers are clogged by impurities to a certain extent Hence, if the strainers are clogged to a too far-reaching extent, the flow of pure water to the cooling system is reduced to a level that may become dangerous. In practice, the degree of clogging of the strainers is controllable by measuring the pressure drop over the individual strainers. As long as the individual strainer is clean, i.e., free from clogging impurities, the pressure drop is minimal, but if fibres and/or other impurities begin to be deposited withm and without the apertures of the strainer, the pressure drop increases gradually, to eventually reach a value at which the flow of water to the cooling system becomes maccep ably low.

In practice, the only realistic possibility of deter¬ mining the pressure drop over a strainer is based upon a comparative measurement of gas pressures. More specifically, tests have been made to provide pressure indicators or sensors for each strainer, at the upper part of a couple of tube con¬ duits, of which one at its lower part is laid to the interior of the strainer, while tne other one has its lower part leading

into the water mass surrounding the strainer. Thereby, the firs _t sensor shall measure the pressure prevailing inside the strainer, while the second shall measure the reference pressure that prevails in the surrounding water. If the strainer is clean, i.e., it lets water through unobstructedly, these two measured pressures become equal, but as soon as the apertures of the strainers start to be clogged, the pressure within the strainer sinks. This manifests itself as a difference between the pressure that is measured by the two sensors, the magnitude of the pressure difference being a measure of the degree of clogging. Therefore, when the pressure difference has reached a given threshold value, reverse flushing or another cleaning shall be initiated. However, tests performed with measurements of pressure differences have not been successful due to diffi- culties in holding the tube conduits serving as reference legs clean, i.e., filled with gas only. Thus, if differently large water heads occur in the two reference legs, errors of measure¬ ment arise that make a reliable operation alarm or a steering of the strainer cleaning function impossible.

Objects and Fear.iπ-fls of the Inventinn

In a narrow aspect, the present invention aims at eliminating the above mentioned difficulties and create the prerequisites of a reliable measurement of pressure differences, preferably in order to make possible an adequate cleaning of strainers in nuclear power plants. In a wider aspect, the invention aims at providing a device which already in connection with elementary measuring of an absolute gas pressure functions in a satisfactory way. Thus, a primary object of the invention is to create a gas pressure measuring device having a tube conduit that is always kept dry and that is immersed in or immersable into a liquid.

In its widest aspect, the invention attains this object by the features that are defined in claim 1. In its narrower aspect, the invention attains its object by the features that are defined in claim 2. Preferred embodiments of the invention are further defined in the other dependent claims.

Brief Description of the Appended Drawings

In the drawings Fig 1 is a side view showing two strainers comprised in a nuclear power plant,

Fig 2 is a view in an angle of 90° to the view according to

Fig 1, Fig 3 is a schematic illustration of a gas pressure measuring device according to the invention, Fig 4 is a partly cross-sectional view showing a vessel of a first type comprised in the device, Fig 5 is a corresponding view showing a second type of vessel, and Fig 6 a partly cross-sectional end view of a vessel according to Fig 4 or 5.

Detailed Description of a Preferred Embodiment of the Invention

In Fig 1 and 2 reference numerals 1, 1' designate two strainers, which are mounted at the bottom of a reactor contain- ment, whose wall is designated 2 and whose bottom is indicated at 3. The strainers are partly immersed in a pit or depression 4 and are separately connected to conduits 5 for the feeding of water into a cooling system (not shown) . In the example, the nuclear power plant is thought to work with a compressed water reactor. This implies that under normal circumstances, the bottom of the reactor containment lacks water. However, at a possible shutdown water may accumulate at the bottom and create a water mass that entirely surrounds the strainers 1, 1' . In case the strainers were mounted to serve a boiling water reactor, the reactor containment would contain a bottom water mass in which the strainers would be fully immersed also under normal running circumstances .

Onwards, the invention is described under the presump¬ tion that the strainers work in water. To each strainer 1 and 1' , respectively, is connected a measuring device whose main components consist of two pressure indicators or sensors 6, 6' , two tube conduits 7, 7' and two

vessels 8, 8' . As indicated in Fig 3, the two vessels 8, 8' are located at one and the same level, i.e., in a common horizontal plane. The first vessel 8 opens substantially directly towards the surrounding main water mass, while the second vessel 8' is connected with the strainer 1 via a secondary tube conduit 9 that extends horizontally between the vessel and the strainer. Reference is now made to Fig 4 and 6, which show the nature of the two vessels 8, 8' in more detail. In both cases, the vessel comprises a cylinder or a cylindrical wall 10 which is sealed at its opposed ends by means of gable walls 11, 11 ' . The cylinder is long and narrow and placed horizontally. In practice, the cylinder may have a diameter within the range of 80 to 100 mm and a length that is 3 to 5 times larger than the diameter. Therefore, the volume of the vessel may amount to about 2 to 4 dm ³ . In the proximity of one of the gable walls 11' is provided a coupling means 12 connected to the upper part of the vessel, which coupling means may be connected to the previously mentioned tube conduits 7 and 7' , respectively.

In the opposed gable wall 11 ' is made an aperture 13 serving as an inlet, which aperture is placed near the bottom of the vessel and has a cross-sectional area which is considerably smaller than the cross-sectional area of the cylinder 10. At the vessel 8' according to Fig 5, a coupling means 12' is connected to the aperture 13 , which coupling means may be connected to the horizontal secondary tube conduit 9. Contrary thereto, at the vessel 8 according to Fig 4, a short piece of tube 14 is connected to the aperture 13 , which piece of tube is sealed at its free end by means of a gable 15. In the tube wall are recessed a plurality of small apertures 16, through which water can pass into the interior of the piece of tube and via the aperture 13 into the interior of the cylinder or the vessel. By the fact that the piece of tube includes these small apertures, which are substantially evenly distributed over the tube envelope, it is guaranteed that at least some of the apertures enable a liquid communication between the surrounding main water mass and the interior of the vessel, although some other would unintentionally be clogged by impurities. The water designated

by 17 forms a surface or a mirror 18, above which there is an air or gas cushion 19.

Each one of the two tube conduits 7, 7' that are connected to the vessels 8, 8 ' should have a limited diameter in order to guarantee that the total volume of the tube conduits becomes many times smaller than the volume of the vessels 8, 8' . In this way, it is guaranteed that the vessels receive a sufficient gas volume to keep the conduits filled with gas at all occurring pressure variations and water levels in the surroundings. In practice, the diameters of the tube conduits may be within the range 8 to 20 mm, preferably 10 to 15 mm, or most suitably 12 to 13 mm. As a practical example, it may be mentioned that the longest conduit 7 ' may have a length of 5 meters and a diameter of 12,7 mm. Then the conduit obtains a volume of 0,63 dm . At the same time, the vessel 8' may have a volume of 3,24 dm , i.e., a volume that is about 5,2 times larger than the volume of the tube conduit. In practice, the volume of the vessel should be 4 to 7, prefereably 5 to 6 times larger than the volume of the tube conduit. It is essential that the two vessels 8, 8' be located in a common horizontal plane, so that differences in the stati¬ cal liquid columns between the vessels do not influence measure¬ ments of pressure differences. More specifically, the two inlet apertures 13 shall be located at one and the same level.

The Function of the Device According to the Invention

Assume that a shutdown occurs and that the strainers 1, 1' start working in connection with an accumulation of water at the bottom of the reactor containment. The two vessels 8, 8', which are located approximately on a level with the appurtenant strainer, will then be filled with water substantially simul¬ taneously. Depending upon how high the surrounding main water mass rises in relation to the level of the vessels, a more or less high gas pressure will be created in the air cushion 19 and the interior of the tube conduits 7, 7' . These gas pressures may be detected by the sensors 6, 6' (or by a gauge indicator for pressure differences common for both conduits) . As long as the

individual strainer, e.g. the strainer 1, is clean, equally large air or gas pressures prevail in the vessels 8, 8' . However, if impurities start clogging the apertures in the strainer, the water pressure in it will decrease m relation to the pressure the surrounding mam water mass . This will have the consequence that the water level 18 in the vessel 8' sinks somewhat and that the gas pressure in this vessel is reduced. In other words, a difference arises between the gas pressures in the vessels 8, 8' and the appurtenant tube conduits, which difference may be read by the sensors 6, 6' . Here, the magnitude of the gas pressure difference constitutes a measure of the degree of clogging of the strainer. When the pressure difference has reached a predetermined magnitude, reverse flushing or any other suitable cleaning of the strainer is initiated a suitable way. In practice, the cleaning of the strainer may be accomplished either an automatic way or by the sensors starting an alarm which in turn is utilized by the staff to manually start the cleaning operation.

An essential advantage of the device according to the mvention is that the vertical parts of the measuring legs, i.e., the tube conduits 7, 7' , always are kept dry, whereby the pressure measuring or indicating is not influenced by varymg liquid columns. By the fact that the volume of the vessels 8, 8' is many times larger than the volume of the tube conduits, this condition is guaranteed also if the level of the surrounding main water mass would rise to a considerable level above the vessels . From the individual strainer extends the tube conduit 9, which serves as a measuring leg, horizontally to the appur¬ tenant reference level vessel; something that involves that the measurement is not influenced by the fact whether this tube conduit is wholly filled with water or contains air. The pressure difference at the difference pressure gauge indicator or the sensors is the same as at the measurement sites, mde¬ pendently of the surrounding pressure or liquid level. Sometimes wave formations or other sudden water motions may arise m the containment, which extreme cases may lead to that water an uncontrolled way is pressed up mto the con-

duits 7, 7' . However, by the fact that these conduits have a pronounced, albeit limited diameter (e.g. 12 to 13 mm) , it is guaranteed that the water is not withheld by the surface ten¬ sion, but may flow back down into the vessel when the conditions return to normal.

Feasible Modifica ions of the Invention

The invention is not restricted solely to the embodi¬ ment as described and shown in the drawings. Thus, in a wider aspect the invention is also applicable to the measuring of absolute gas pressures, i.e., without difference pressures being measured or indicated. In such cases, one single vessel may be included in a tube conduit and form a reference point or a reference vessel that forms a starting point from which a measurement may take place, independently of the level of the surrounding water or liquid mass. In this context it is under¬ lined that the vessel, by being elongated and horizontal, guarantees that the level of the liquid enclosed in the vessel will vary within narrow limits even if the level of the surroun- ding liquid mass varies quite considerably.