FISSION PRODUCT LEAK DETECTION IN A NUCLEAR REACTOR

Abstract: The invention relates to a method and apparatus for carrying out leak detection of a fuel assembly (2) for a pressurised-water nuclear reactor. A substantially vertical hollow mast means (1) open at the bottom end and with side walls which are pierced by one or more apertures (8) houses the fuel assembly (2) to be examined. A leak detection device (4, 4') has extraction means (5, 5', 6, 6', 9, 9') for extracting fluid through said apertures (8) from inside said mast means (1).

Description: Fission product leak detection in a nuclear reactor This invention relates to a method and an apparatus for carrying out leak detection of fuel assemblies for nuclear reactors, in particular fuel assemblies for a pressuπsed-water reactor (PWR). BACKGROUND OF THE INVENTION Leakages which occur in fuel assemblies in nuclear reactors lead to the reactor water and hence the different parts of the primary circuit of the reactor being contaminated with the radioactive fission products When a contamination of the reactor water has been determined or is suspected to have occurred, it is of the utmost importance that the leakage is located so that leaky fuel assemblies can be replaced A fuel assembly most often includes several fuel rods Identification of leaking assemblies is performed by elevating fuel assemblies individually and investigating if any radioactive fission products are leaking from them It is known from US patent apphcanon 08/341 1 53 to use a substantially vertical hollow telescopic mast open at the bottom to house the fuel assembly to be examined The mast is positioned over the fuel assembly to be examined in the reactor core. The fuel assembly is drawn up inside the mast. The mast has an internal diameter which is slightly larger than the squaremetπc cross-section of a fuel assembly In order to reduce the water resistance encountered when drawing the fuel assembly into the mast, the wall of the mast may have one or more vertically spaced apertures in it which permit water ώsplaced by the incoming fuel assembly to easily stream out of the mast A fuel assembly is lifted into the mast to a relatively low ambient pressure position in which the fuel assembly is provided near but below the water surface of water inside the nuclear reactor. This reduction in the pressure exerted on the fuel assembly means that if the casing of the fuel rod in the fuel assembly is not leak-proof then any fluid which is inside the rod will be forced out through the leak. The water to be examined for the presence of fission products is extracted from the mast from a position inside the mast above the fuel assembly. In the case that the mast is provided with apertures, this method, known as "sipping", which extracts the water to be sampled from the top of the inside of the mast leads to a low concentration of fission products. This is due to water containing fission products leaking out through the apertures during lifting of the fuel assembly and water, which dilutes the water containing fission products which leaks out of the fuel assembly, entering through the apertures during extraction of the water for examination. OBJECT OF THE INVENTION An object of the present invention is to provide a fuel leak detector method and apparatus which permits "sipping" to be used on the type of masts provided with apertures. According to the invention this is accomplished by providing a leak detection device which extracts water from the apertures in the wall of the mast. DESCRIPTION OF THE INVENTION According to the invention an apparatus is disclosed for carrying out leak detection of a fuel assembly especially for a pressurised- water nuclear reactor. The apparatus comprises: a substantially vertical hollow mast means, open at the bottom end and with side walls which are pierced by one or more apertures, wherein the mast is intended to house the fuel assembly to be examined; and, the apparatus further comprises leak detection device having extraction means for extracting fluid through said apertures from inside said mast means. In a further aspect of the invention a method of using the apparatus to detect leaks is disclosed. BRIEF DESCRIPTION OF THE DRAWING The invention will now be described in more detail with reference to the accompanying drawings, in which: figure 1 illustrates schematically a first embodiment of a leak detection device according to the invention arranged on a mast means surrounding a fuel assembly; figure 2 shows in perspective the leak detection device of figure 1 ; figure 3 shows in perspective part of a second embodiment of a leak detection device according to the invention; figure 4 shows in perspective part of a third embodiment of a leak detection device according to the invention; and, figure 5 shows in perspective part of a fourth embodiment of a leak detection device according to the invention. figure 6 illustrates schematically a fifth embodiment of a leak detection device according to the invention arranged on a mast means surrounding a fuel assembly DESCRIPTION OF THE PREFERRED EMBODIMENTS Figure 1 shows a hollow lifting mast means 1 open at the base for elevating fuel assemblies preferably in a pressurised- water reactor. The mast means 1 surrounds a fuel assembly 2 which has been lifted into the mast means. This lifting has been performed by means (not shown) known from the state-of-the-art such as a gripping device provided at the end of a wire provided inside the mast means 1 which is adapted to grip a lifting handle on a fuel assembly 2 in the reactor core and subsequently be drawn into the mast means 1 by a motor In order to increase the pressure differential between any fluid trapped inside a fuel rod inside the fuel assembly 2 and the ambient pressure the mast means 1 with the fuel assembly 2 can be optionally lifted up m the reactor so that the fuel assembly comes nearer to, but remains below the surface 3 of the water in the reactor The mast means 1 is surrounded by leak detection device 4, 4' which is connected to pipe means 5 leading to an analysis device (not shown) Leak detection device 4, 4' compπses two co-operating hollow frames 4, 4' which have nozzles 6 and optionally blanking plates 7 mounted on them at vertically spaced distances which correspond to the distances which separates apertures 8 in the side of the mast means Apertures 8 go all the way through the side of mast means 1 and allow fluid to pass between the interior and exteπor of mast means 1 More apertures 8 can be added if necessary Nozzles 6 and blanking plates 7 are positioned near to, respectively over apertures 8 Blanking plates 7 can cover apertures 8 so that in the event of lateral displacement of the mast fluid is prevented from enteπng or exiting from any aperture 8 which does not have a nozzle 6 positioned near it By applying suction to pipe means 5 the fluid inside mast means 1 is sucked via nozzle 6 from apertures 8 into hollow frames 4, 4' to pipe means 5 and subsequently to the analysis device In this embodiment the fluid extracted from mast means 1 is replaced by fluid flowing in through the bottom of mast means 1 and any uncovered apertures 8 although it is also conceivable to recirculate the extracted fluid back to the mast means (as mentioned below) Figure 2 shows in more detail the first embodiment of leak detection device 4, 4' which is intended to be fitted around a mast means 1 which is immersed in a reactor As the upper cross-section of the mast means 1 is greater than the cross-section of the part of the mast which receives fuel assembly 2 for testing it is complicated to produce a leak detection device which can be lowered past this part of the mast means and yet still extend inwardly and correctly position nozzles 6 and blanking plates 7 in relation to apertures 8 in lifting mast means 1 Leak detection device 4, 4' is therefore made of a plurality of parts which can be separately lowered at a distance from the mast means 1 to the required depth and subsequently joined together to perform the examination. In this embodiment leak detection device 4, 4' comprises two frames 4, 4' each comprising hollow pipes 9 joined to a flange 10, 10'. Flanges 10, 10' have the shape of an arc of a circle with an inner diameter greater than the outer diameter of the mast means 1 at the portion of mast means 1 intended to surround a fuel assembly 2 during examination. The flanges 10, 10' each extend through an angle which is more than 180 degrees but which does not extend so far that the open distance between the ends of a flange 10, 10' is less than the outer diameter of the part of the mast means 1 which the flange 10, 10' is intended to partially surround during use. Flange 10 has a guide hole 11 at each end positioned 180 degrees apart and flange 10' has a tapered guide pin 12 at each end positioned 180 degrees apart. Guide pins 12 on flange 10' co-operate with guide holes 1 1 on flange 10 to form an axially movable locating and joining means for holding the two frames 4, 4' together around mast means 1. This is achieved by frame 4' first being positioned around mast means 1 at a suitable position, frame 4 being positioned vertically above frame 4' and then lowered onto frame 4' so that guide pins 12 locate into guide holes 1 1 and thereby hold frames 4, 4' together with flange 10 above flange 10'. The vertical and circumferential position of leak detection device 4, 4' can then be adjusted as necessary so that nozzles 6 and blanking plates 7 are coπectly positioned by apertures 8. The invention is not limited to the guide means above and any other suitable guide means could be used, for example: horizontally orientated guide pins and co-operating holes; or, mutually attracting permanent magnets or electromagnets. Furthermore nozzles 6 can be of any appropriate size and it is conceivable that each nozzle 6 covers a plurality of apertures 8 but not necessarily all of them. In the same manner each blanking plate 7, if used, can cover a plurality, but not necessarily all, of apertures 8 which do not have an associated nozzle. The method of using the apparatus is as follows: if the apparatus has not been mounted on mast means 1 then it is mounted on mast means 1 in the following manner; frame 4' is positioned next to one side of mast means 1 so that nozzles 6' and any blanking plates T are positioned by apertures 8; frame 4 is positioned diametrically opposite frame 4' on the other side of mast means 1 with guide holes 1 1 vertically above guide pins 12; frame 4 is then lowered onto frame 4' so that guide pins 12 locate into guide holes 1 1 and thereby hold frames 4, 4' together with flange 10 above flange 10'; the vertical and circumferential position of leak detection device 4, 4' is adjusted as necessary so that nozzles 6, 6' and blanking plates 7, 7' are coπectly positioned by apertures 8; once the apparatus has been mounted on mast means 1 an examination cycle can be started as follows; if it is necessary to have a background reading of radioactivity in the reactor to determine a base-line before examining fuel assemblies then suction can be applied to nozzles 6, 6' and the fluid in the empty mast means 1 sampled; the fuel assembly 2 to be examined is grappled and lifted up into mast means 1 - if suction is not already being applied to nozzles 6,6' then it can be applied now so that fluid inside mast means 1 is extracted through nozzles 6, 6' and conducted via pipes 9 and pipe means 5 to an analysis device; mast means 1 is moved closer to the surface of the water and if necessary moved laterally to a fuel assembly storage position - blanking plates 7, T prevent water being forced into the mast means 1 through apertures 8 not associated with nozzles 6, 6' and hence prevents unwanted dilution of any fluid leaking out of fuel rods in fuel assembly 2; once the analysis of the extracted fluid is complete fuel assembly 2 is lowered out of masi: means 1 and a new examination cycle can be commenced. In a second embodiment of the invention shown in figure 3 frames 4, 4' are movably hinged together about a longitudinal axis buy hinge means 13 so that leak detection device 4, 4' can first be opened out so as to pass around mast means 1 and then closed to bπng nozzles 6 and blanking plates 7 into position Lock means 14 can releasably hold frames 4, 4' together In a third embodiment of the invention shown in figure 4 frames 4, 4' are hinged together by hinge means 15 at the base about an axis perpendicular to the longitudinal axis of the frames to form a leak detection device which is V-shaped when open, seen from the side This open leak detection device can be manoeuvred around the mast means 1 and then closed to bπng nozzles 6, 6' and blanking plates 7 into position Lock means 16 can releasably hold frames 4, 4' together In a fourth embodiment of the invention shown in figure 5 frames 4, 4' are essentially permanently joined together to form a substanttally cyhndπcal apparatus which can be positioned under mast means 1 and lifted into position from below In a fifth embodiment of the invention shown in figure 6 frame 4' is connected by pipe 5' to the output of the analysis device Nozzles 6' on frame 4' can then be used to return the fluid from the analysis device to the mast means for subsequent extraction by nozzles 6 on frame 4 and recirculation through the analysis device This recirculation of the test fluid through the analysis device maximises the chances that any fission products in the test water are detected The method of using the first embodiment of the invention descπbed above can naturally be adapted as necessary for use with all other embodiments of the invention by taking into account the differences between the embodiments Furthermore although the apparatus has been described with fuel assemblies being elevated substantially vertically in relation to the mast means and possibly moved in other directions while in the vertical orientation it is also possible to adapt the apparatus and method of the invention to sampling where the fuel assembly is moved substantially horizontally into mast means or other containing means and then possibly moved while in the horizontal orientation. It is furthermore possible to adapt the apparatus and method of the invention to the cases where the fuel assembly is subjected to a combination of vertical and horizontal movements and in which it takes up a variety of orientations. While the invention has been described with reference to a number of specific embodiments, it is to be understood that the invention is not limited to these embodiments but is to include all modifications and alterations that may be made which fall under the scope of the accompanying claims.

Claims: Claims 1. Apparatus for detecung leaks m a fuel assembly (2) for a nuclear reactor compπsing: a. substantially immersed m a fluid, substantially veracal hollow mast means (1) into which said fuel assembly (2) is housable. wherein said mast means (1) has a side wall having one or more through apertures (8); means for drawing said fuel assembly (2) into said mast means (1); characteπsed by extracπon means (5. 5' . 6. 6 ' , 9, 9') for extracπng fluid from said mast means (1 ) through at least one of said apertures (8) 2. Apparatus according to claim 1 characteπsed in that said extracπon means (5. 5\ 6, 6' . 9. 9' ) compπses at least one nozzle (61 wherein said nozzle (6) is adapted to connect to one aperture (8 ) 3 Apparatus according to claim 1 characteπsed in that said extracnon means (5. 5', 6. 6\ 9. 9 " ) compnses at least one nozzle (6 > adapted to connect to a plurality of apertures (8). 4 Apparatus according to any of claims 1 -3 characteπsed m that it compnses blanking means (7) for closing one or more apertures (8) 5. Apparatus according to any of claims 1 -4 charactensed in that said extracnon means (5. 5 " , 6. 6\ 9. 9' ) compπses at least one pipe (9 ) with a plurality of substannally vemcallv spaced sucπon nozzles (6) each adapted to connect with one or more apertures (8) 6 Apparatus according to any of claims 1-5 charactensed in that said extracπon means (5. 5\ 6. 6 " . 9. 9' ) compπses a frame (4. 4") having an internal diameter which is greater than the external diameter of the lower end of said mast means ( 1 ) 7 Apparatus according to any of claims 1 -6 charactensed in that the internal diameter of said frame (4, 4' ) is less than the external diameter of the widest part of said mast means (1) 8 Apparatus according to any of claims 1 -7 characteπsed in that said frame (4, 4') consists of a plurality of sub-frames (4. 4') jomable together to form a frame having an internal diameter which is equal to or greater than the external diameter of the lower end of said mast means ( 1 ) and less than the external diameter of the widest part of said mast means (1) 9 Apparatus according to anv of claims 1 -8 characteπsed in that in an open condinon said sub-frames (4 4' ) are hinged together bv hinge means ( 1 15 ) to form a closed frame (4 4') having an internal diameter which is equal to or greater than the external diameter of the lower end of said mast means ( 1 ) and less than the external diameter of the widest pan of said mast means ( 1 ), said closed frame (4. 4') being openable to form an open frame with an opemng with an internal dimension which is equal to or greater than the external diameter of the pan of said mast means ( 1 ) intended to accommodate said fuel assembly (2) 0 Apparatus according to any of claims 1 -8 characteπsed in that it compπses outlet nozzles (6') for feeding fluid into at least one oi said apertures (8) 11 Method for detecnng leaks in a fuel assembly (2) for a nuclear reactor compnsing a, substantially immersed in a fluid, substanπally vertical hollow mast means ( 1 ) into which said fuel assembly (2) is housable, wherein said mast means ( 1) has a side wall having one or more through apertures (8) and further has means for drawing said fuel assemblv (2) into said mast means (1 ). compnsing the steps of drawing a fuel assembly (2) into said mast means (1), sampling fluid from said mast means ( 1 ) characterised by sampling of the fluid being performed by an apparatus of the type according to any of claims 1-10. 12. Method according to claim 11 characterised in that after being sampled the fluid from said mast means ( 1 ) is recirculated back into said mast means ( 1 ).

Inventors: COLLIN PER, DELERYD ROLAND, NORBAECK JAN, COLLIN, PER, DELERYD, ROLAND, NORBAECK, JAN
Applicants: ASEA ATOM AB, COLLIN PER, DELERYD ROLAND, NORBAECK JAN, ABB ATOM AB, COLLIN, PER, DELERYD, ROLAND, NORBAECK, JAN
Application year: 1997
Main classification: G21C17
IPC codes: G21C17