Abstract:
A CRUD collection system for removing deposits from a nuclear reactor fuel rod submerged in pool water. The system includes a sealable collection head for containing a portion of the fuel rod. The collection head includes at least one seal positioned to receive the fuel rod. Also provided is a liquid inlet communicating with an interior portion of the collection head, the inlet is configured to inject a liquid into the interior to displace the pool water therefrom. At least one blade is provided that is positionable in scraping engagement with the fuel rod. Also included is a liquid outlet for eliminating a CRUD sample and the liquid to a filter.

Description:
CROSS-REFERENCED AND RELATED APPLICATIONS 
     This application is related to and claims priority on the basis of Provisional Application Serial No. 60/208,188, filed on May 31, 2000. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a device and method for removing relatively uncontaminated CRUD samples from nuclear fuel assemblies, and more particularly, from nuclear fuel rods, for analysis. 
     BACKGROUND OF THE RELATED ART 
     Pressurized water reactors (“PWRs”) which are used to generate nuclear power include reactor cores that have fuel assemblies. Fuel assemblies are usually are made up of elongated hollow metallic tubular cladding, known as “fuel rods,” that contain nuclear fuel. Cladding functions to prevent fission products from being released from the fuel into the coolant/moderator and to prevent contact and chemical reactions between the fuel and the coolant/moderator. Common cladding maters include zirconium, zirconium alloys, and stainless steel. Zirconium based alloys—in which the major component is zirconium—have been used in the cladding of nuclear fuel rods or elements for several decades. 
     Irradiated fuel rods that have been expended in a reactor core are often stored in water-filled pools for deactivation. As time passes, CRUD collects over an oxide layer which forms on the nuclear fuel rods. CRUD can also collect on rods while they are in use. CRUD is an acronym for “chalk river unidentified deposit,” referring to highly radioactive substances originally found on the inside of piping and components at the Chalk River Nuclear Reactor in Ontario, Canada. CRUD has now become standard nomenclature referring to minute, solid, corrosion products that travel into the reactor core, become highly radioactive, and then flow out of the reactor into other systems in the plant. CRUD can settle into crevices or form coatings on piping, fuel rods, and other surfaces in a reactor. The major components of CRUD are iron, cobalt, chrome, and manganese. CRUD is known to be a concentrated source of radiation and represents a significant radiological risk because of its insolubility. Therefore, not surprisingly, nuclear plant workers are known to receive radiation doses directly from CRUD exposure during refueling and maintenance outages. As such, it is important to analyze CRUD samples to determine the nature and amount of the deposits. Specifically, the samples can be analyzed to determine their elemental and isotopic composition and to provide an estimate of the activity of the CRUD material resident on the fuel rods. 
     It is known in the art to use an essentially manual device to retrieve a CRUD sample from nuclear fuel rods. Such a device usually consists of a scraping head at the end of a manually operated pole. The scraping head is often a stone substance that crumbles during use and can score the surface of the cladding. Additionally, biological protection is not typically provided in such a system itself. This can lead to handling difficulties both technically and in terms of governmental regulations. 
     Also known are mechanized devices for removing deposits from irradiated fuel elements. For example, one such device is disclosed in U.S. Pat. No. 4,483,205 to Bellaiche et al. This patent describes an apparatus that scrapes fuel rod surfaces by means of a remote-controlled scraping device. The scraped sample is sucked into an intermediate reservoir along with liquid from the pool. The sample—carried by the liquid—is passed into a filtration chamber that retains the sample. The liquid can then be passed back into the pool. 
     Prior art devices for collection of CRUD suffer from a number of shortcomings including, for instance, sample contamination, relatively low sample volume, and cladding scoring. Contamination occurs because pool water is used as the transportation media for the sample. The chemical composition of pool water can vary, and the water can also contain high levels of certain elements that can result in reduced accuracy and resolution of the sample. Contamination has also been shown to occur as a result of the crumbling of the consumable scraping stone during its use. The stone—which itself may include components of interest during analysis—can cause further contamination if it mixes with the deposit sample. Moreover, scraping stones often fail to cover a relatively sizeable area of the rod and become clogged. Each of these attributes limit the scraping efficiency of the stone and necessarily reduces the amount of CRUD removed from the rod. Stones are also known to score the surface of the cladding. 
     Thus, there remains a need for a CRUD collection system that overcomes the limitations of prior art devices. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of this invention to provide a collection system that allows for the collection of a relatively uncontaminated sample of CRUD from an irradiated fuel rod. 
     It is another object of this invention to provide a collection system that allows for the collection of a relatively large sample of CRUD. 
     It is yet another object of this invention to provide a collection system that scrapes fuel rods to obtain CRUD samples without doing damage to the fuel rods. 
     To this end, the present invention provides for a CRUD collection system for removing deposits from a nuclear reactor fuel rod submerged in pool water. The system includes a sealable collection head for containing a portion of the fuel rod. The collection head includes at least one seal positioned to receive the fuel rod. Also provided is a liquid inlet communicating with an interior portion of the collection head, the inlet is configured to inject a liquid into the interior to displace the pool water therefrom. At least one blade is provided that is positionable in scraping engagement with the fuel rod. Also included is a liquid outlet for eliminating a CRUD sample and the liquid to a filter. 
     The system as described above, has been found to provide an exemplary means for collecting CRUD wherein the CRUD sample is uncontaminated by pool water. The system further provides a sealable chamber for enclosing part of a rod so that the rod can be surrounded by deionized water and subsequently sampled in a substantially contaminant-free environment. 
     Also provided is a method for collecting a CRUD sample that begins by removing a fuel rod from a fuel assembly. Next the fuel rod is inserted into a collection head. The collection head is closed and is then flushed with deionized water to remove pool water. The rod is then scraped with at least one scraping blade to remove the CRUD sample. The deionized water is filtered to remove the CRUD sample. The filter membrane is then removed so that analysis can be performed on the trapped CRUD sample. 
     Other objects, features and advantages of the present invention will become more apparent from the following description of the preferred embodiment of the present invention when considered in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a simplified perspective view of a CRUD collection system, according to one embodiment of the invention; 
     FIG. 2 a  is a side cross section view of the system shown in FIG. 1, taken along line II—II of FIG. 1, according to one embodiment of the invention; 
     FIG. 2 b  is a side cross section view of the system shown in FIG. 2 a,  wherein the cover plate is in an open position, according to one embodiment of the invention; 
     FIG. 3 is a front cross section view of the system shown in FIG. 1, taken along line III—III of FIG. 1, according to one embodiment of the invention; 
     FIG. 4 is a simplified perspective view of a scraper plate of the system shown in FIG. 1, with mounted scraper blades, according to one embodiment of the invention; and 
     FIG. 5 is a perspective view of a control console for use in connection with the system shown in FIG. 1, according to one embodiment of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As shown in FIG. 1, a CRUD collection system  1  of the present invention includes, for example, a shoe-box-sized sealable collection head  70  for enclosing part of a nuclear reactor fuel rod  90  while submerged in a water-filled pool (not shown). Collection system  1  also includes a mounting bracket  10  and a funnel weldment  11 . Mounting bracket  10  can be used to help to maneuver system  1  into position in the pool. Additional equipment can be mounted to head  70  such as cameras and light sources (not shown). Otherwise, stand-alone cameras and light sources can be used. A remotely controlled vehicle (not shown) for maneuvering system  1  can also be attached to head  70 . As described in more detail below, in one embodiment, funnel  11  can be used to receive rod  90  for eventual sampling within head  70 . In the preferred embodiment, six cylinders  18  are disposed on head  70  to allow a slidably engaged cover plate  4  to move in an opened and closed position. 
     Also provided on the external portion of system  1  is a deionized water outlet  74  and an air outlet  76 . Likewise, on the opposite side of system  1 , a deionized water inlet  78  is provided along with an air inlet  80 . These components can also be clearly seen in FIG.  3 . Hosing (not shown) can be attached to inlets  78  and  80  for respectively providing a supply of deionized water and air to collection head  70 . Outlets  74  and  76  may vent directly into the pool, or alternatively, they may also be attached to hosing should it be desired that outputted water and air is released at another location. For instance, in one embodiment, outputted deionized water flows, along with a CRUD sample to a control console  100  (FIG. 5) for filtration. As will be further discussed, a supply of air and deionized water is necessary to fully realize the benefits of the present invention. 
     As shown in FIG. 2 a,  collection head  70  is further divided into a sealed collection chamber  73  which provides a closed environment for removing deposits from rod  90 . The closed environment makes it possible to collect CRUD samples on a relatively contaminant-free basis. As discussed below in further detail, collection head  70  contains devices that scrape CRUD from rod  90  while rod  90  is enclosed in chamber  73  of head  70 . The scraped samples are ultimately trapped in a filter for analysis as is necessary. 
     Still considering FIG. 2 a,  the cross section of collection head  70  exposes the internal devices mentioned earlier. In the preferred embodiment, collection head  70  is assembled from a slide housing  3  that is enclosed at its front and the rear respectively by a mounting bracket weldment  12  and a seal plate  2 . Slide plate  5  seals the top of slide housing  3 . As for collection chamber  73 , an openable cover plate  4  is also provided to enclose its top portion. In one embodiment, cover plate  4  is slidably connected to head  70  by means of cylinders  18  so that it can move to an open position as shown in FIG. 2 b.  Such a configuration is desirable because rod  90  can be engaged with head  4  before blades  7  are in scraping engagement with rod  90 . Alternatively, cover plate  90  may be securely sealed to head  70 . The various components can be held together by screws such as cap screw  31  shown holding seal plate  2  to other components of head  70 . Additional cap screws can be seen in use throughout the device. However, other means known in the art may be used to securely fasten together components of head  70 . 
     Slide housing  3  includes a pneumatic cylinder  72  that is matingly engaged to a corresponding base slide  17  that is configured to slide along the length of cylinder  72 . In one embodiment, slide housing  3  includes a self-contained locomotion means for moving slide  17 . For example, in this embodiment, pneumatic cylinder  72  is supplied with air from inlet  80  to allow base slide  17  to move along cylinder  72 . The speed of movement can be restricted by a hydraulic damper (not shown). A slide plate  5  is mounted to slide  17  allowing it to also move laterally back and forth. As shown in this embodiment, o-rings  37  are provided to seal collection chamber  73  from slide housing  3 . Thus, while slide plate  5  moves laterally, a seal prevents pool water from entering chamber  73  from slide housing  3  A scraper plate  6 —on which blades  7  are attached—is spring-mounted to plate  5  (FIG.  4 ). Springs  19  serve to urge blades  7  against rod  90 . Thus, by moving base slide  17 , which in turn moves base plate  5 , which in turn moves scraper plate  6  and blade  7 , blade  7  can scrape rod  90 . As will be described in more detail, blade  7  may constitute multiple blades. 
     In the preferred embodiment, multiple blades  7 , fabricated from zirconium alloy, are secured to scraper plate  6 . Referring to FIG. 4, it is noted that blades  7  have a semicircular region which corresponds to rod  90 . Preferably, blades  7  are relatively flexible. It has been found that a CRUD sample scraped from about six inches of rod  90  provides a quality sample for purposes of analysis. Thus, three blades  7  are provided in a specific embodiment where the blades are separated by two inches and are each designed to travel a distance of two inches. In this way, a total of six inches of CRUD is scraped from rod  90 . Regarding the material from which blades  7  are fabricated, a zirconium alloy—which can be similar or identical to the material used to fabricate rod  90 —is used because it has been found to prevent scratching of the cladding surface. It is generally known that underneath the CRUD layer there is an oxide layer that is harder than the cladding below. Because the scraping blades are made of the same or similar material as is the softer cladding, the CRUD can be removed without breaking through the oxide layer and damaging the cladding. Also, blades made from zirconium alloy are not drastically worn during use and therefore do not contaminate the sample. 
     As mentioned, in one embodiment, funnel  11  is provided for receiving rod  90 . Funnel  11  is mounted to cover plate  4  by means of bracket weldment  13 . This allows funnel  11  to move with cover plate  4  to the open position as shown in FIG. 2 b.  Funnel  11  may includes rod seal  8  for creating a water tight seal around rod  90  once rod  90  is received into funnel  11 . In one embodiment, as shown in FIG. 3, rod seal  8  is separable and include first part  9   a  and second part  9   b.  First part  9   a  is retained in cover plate  4  (or funnel  11 ) and is movable therewith. Second part  9   b  can be retained by seal plate  2  (FIG. 2 a ). When cover plate  4  is in the closed position as shown in FIG. 3, parts  9   a  and  9   b  engage one another to form a complete seal surrounding rod  90 . An identical sealing device  15  (FIG. 2 a ) is provided to seal the opposite end of rod  90 . Alternatively, a one-piece construction rod seal  8  may be disposed in cover plate  4 . 
     one embodiment a control console  100  can be provided as shown, for example, in FIG.  5 . In an upper portion  120  a bank of switches  122  is provided so that a user can change the system from automatic to manual for valves  74 ,  76 ,  78 , and  80  and a pump (not shown) enclosed within console  100 . A radiation monitor  124  is also provided. Monitor  124  is designed to shut off system  1  if a predetermined radiation level is reached at a filter housing (not shown) enclosed within console  100 . A control panel  126  is provided for automatic control of system  1 . For example, these controls may include input buttons for starting and stopping system  1  and a small LCD display that provides instructions and status of system  1 . 
     A lower part  140  of console  100  is provided with connections  142  for water and air lines from valves  74 ,  76 ,  78 , and  80 . For example, these lines can be for water in, sample lines, water out, and pneumatic lines. In center portion  144  of lower part  140  there are provided water and air gauges, and pressure regulators. A filter housing  146  is configured to receive deionized water from system  1  for filtration of the CRUD sample. 
     In use, CRUD collector  1  is deployed into a deactivation pool (not shown) and can be maneuvered, for example, by remote control. For ease of operation, collector  1  may include a camera and illumination source (not shown). This would allow an operator to more easily steer collector  1  into position for making a deposit collection. In one embodiment, a pole (not shown) is attached to bracket  10  of collector  1 . The pole can then be manually maneuvered in order to position collector  1 . Once collector  1  is positioned in close vicinity to a fuel rod, funnel  11  can be positioned over the rod so that the two are radially aligned. At this time, cover plate  4  is in the open position as shown in FIG. 2 b.  By moving funnel  11  towards rod  90 , rod  90  is held by rod seal  8  (FIG.  3 ). Now cover plate  4  is lowered by pneumatic or other means to seal a part of rod  90  in collector  1 . This allows rod  90  to contact blades  7  and seals collection chamber  73 . Springs  19  urge blades  7  against rod  90 . 
     To prevent contamination of the sample, it is desirable to transport deposit samples in deionized water or another inert fluid rather than pool water. In one embodiment, deionized water is injected into collector  1  through inlet  78 , while the contents of chamber  73  are purged to the pool through outlet  74 . Over time, this has the effect of removing most of the pool water from chamber  73  and replacing it with deionized water. The procedure of collecting a sample of CRUD can then continue as discussed below. 
     After deionized water is in place in chamber  73 , blades  7  are moved laterally to scrape a CRUD sample from rod  90 . The CRUD sample—now suspended in the deionized water—may be separated from the deionized water by filtering through a membrane (not shown) and stored for eventual analysis. In one embodiment the aforesaid filtration takes place in filter housing  146  of console  100 . However, it is understood that head  70  may itself include a filtration device eliminating the need for a separate console  100 . Likewise, all components of console  100  may be combined into head  70 . 
     In this way, a method is provided whereby a CRUD sample is obtained in a relatively contaminant-free manner. 
     While the foregoing description and drawings represent the preferred embodiments of the present invention, it will be apparent to whose skilled in the art that various changes and modifications may be made therein without departing from the true spirit and scope of the present invention.