Abstract:
A high energy absorption top nozzle for a nuclear fuel assembly that employs an elongated upper tubular housing and an elongated lower tubular housing slidable within the upper tubular housing. The upper and lower housings are biased away from each other by a plurality of longitudinally extending springs that are restrained by a longitudinally moveable piston whose upward travel is limited within the upper housing. The energy imparted to the nozzle by a control rod scram is mostly absorbed by the springs and the hydraulic affect of the piston within the nozzle.

Description:
GOVERNMENT RIGHTS 
     This invention was conceived, at least in part, under a subcontract under DOE Prime Contract No. DE-AC06-76RL01830, identified as Basic Ordering Agreement No. 32850-A-R5. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to top nozzles for nuclear fuel assemblies which accommodate for differences in thermal expansion and irradiation growth of the fuel assemblies and other reactor components and, in particular, to a retrofit expandable top nozzle for use in reactors previously having components composed of essentially the same materials. 
     2. Related Art 
     In nuclear reactors of the type designed in the former Soviet Union, the reactor core is comprised of a large number of elongated fuel assemblies, each having a plurality of fuel rods held in an organized hexagonal array by a plurality of grids spaced longitudinally along the fuel rods and secured to stainless steel control rod guide thimbles. The stainless steel control rod guide thimbles extend above and below the ends of the fuel rods and are attached to the top and bottom nozzles, respectively. The fuel assemblies are arranged in the reactor vessel with the bottom nozzles resting on a lower core plate. An upper core plate rests on the top nozzles. 
     The top nozzles in the Soviet design are non-removably fixed to the stainless steel control rod guide thimbles of the fuel assembly. These complex nozzles perform several functions. First, they position the rod control cluster assembly (RCCA) relative to the guide tubes within the core so that the position of the RCCA relative to the upper core plate is fixed. The RCCA positions the control rods, which are inserted into the fuel assembly as a group or cluster. 
     The Soviet nozzle also dampens the velocity of the control rods using springs to remove energy when the RCCA rods are dropped into the reactor core during an emergency shutdown, commonly known as a “scram”. The nozzle also supplies spring loads for supporting the internals. When the upper core plate is lowered onto the nozzles, it compresses the nozzle spring. In addition, the Soviet nozzle is designed to protect the control rods when the fuel assembly is removed from the reactor vessel. Under these conditions, the RCCA is at or below the top edge of the nozzle. Finally, the Soviet design of the top nozzle allows the fuel assembly to be handled when lifted out of the core by transferring the loads through the nozzle. 
     Thus, the Soviet nozzle is designed to function in two positions, free and compressed. As stainless steel is used for the thimbles of the Soviet fuel assembly, the relative separation between the interior of the reactor vessel and the fuel assemblies remains constant once the assembly is in position. Spring loads are such that the nozzles can support the internals, and the spring loads as well as the RCCA positions are fixed so that all functions are static. As a result, the nozzle has built-in references around which the internals are designed. The stainless steel thimbles used in the Soviet design impose higher reactivity cost on the fuel assemblies due to their neutron absorption rate, and they are more difficult to attach to the grids of the fuel assemblies. Non-Soviet fuel assemblies utilize zircalloy for the thimbles which imposes less reactivity cost. However, zircalloy has a different constant of thermal expansion than the stainless steel reactor vessel, and grows during irradiation. Expandable top nozzles, which accommodate for these variations in the dimensions of the different components within the reactor are disclosed in, for example, U.S. Pat. Nos. 4,534,933; 4,687,619; 4,702,882 and 4,986,959. Such nozzles, however, are used in reactors in which the top core plate rests on a core support in the form of a circumferential ledge within the reactor vessel. In the Soviet-type reactor, the core plate rests on and is supported by the top nozzles. 
     As mentioned, the Soviet design top nozzle is permanently attached to the thimble tubes of the fuel assembly. The above-mentioned patents disclose removable top nozzles and U.S. Pat. No. 5,479,464 took that technology to another step in applying the removable top nozzles to the Soviet-type reactor nozzle design. However, the substitution of zircalloy for stainless steel in some of the fuel assembly components, such as the thimble tubes in which the control rods move, requires further modifications to assure that impact loads experienced by the assemblies can be absorbed without damaging the assemblies or other core components. For example, in the VVER 1000-type Soviet designed reactor, when the control rods scram, they freefall and impact the top nozzle at a very high velocity. This fuel design does not use a dashpot or any other hydraulic mechanical device to minimize these high impacts. In the design described in U.S. Pat. No. 5,479,464, springs are employed to absorb some of this load. However, further means are desired to absorb the shock of the load as well as the load itself. During a scram in a VVER 1000-type Soviet designed reactor, the control rod assembly and its driveline freefall into the fuel assembly. In a standard western fuel assembly design, approximately two feet before full insertion of the control rods into the fuel assembly, the tips of the control rods enter a small diameter portion of the thimble tube called the dashpot. This dashpot is approximately one (1) millimeter larger than the control rods. Because the control rods are moving very fast at this point in the scram, there is a large volume of water which has to be accelerated up past the falling control rods to make room for them in the dashpot. This process causes the control rods to decelerate rapidly, thus lessening the impact velocity of the control rod assembly at the top nozzle adapter plate. The standard VVER 1000 style fuel assemblies do not have a dashpot and therefore the control rod assembly impacts the top nozzle at a much higher velocity. As the kinetic energy is equal to the mass×the velocity 2 , if the velocity at impact on the VVER 1000 fuel design is four times that of the standard western pressurized water reactor design, then the total energy which has to be absorbed after impact is sixteen (16) times as much. 
     Accordingly, a new high energy absorption top nozzle is desired that will assure that the impact loads expected during scram events will be absorbed without damaging the nozzle, fuel assembly and/or control rod assembly. 
     Furthermore, there is a need for an expandable high-energy absorption top nozzle that can accommodate expansion and growth of the zircalloy components of the fuel assembly while supporting the upper core plate in a fixed position. 
     In addition, there is a need for such an expandable high-energy absorption top nozzle that can absorb the impact of a control rod scram while continuing to support the upper core plate substantially in its fixed location. 
     SUMMARY OF THE INVENTION 
     These and other needs are satisfied by the invention which is directed to an expandable top nozzle for a nuclear fuel assembly which includes a tubular barrel having a first end on which the upper core plate seats, and a second end. The tubular barrel further includes a hold-down plate circumferentially affixed to the interior wall of the tubular barrel intermediate the first and second ends and substantially spanning the central opening within the tubular barrel. The hold-down plate has a central opening through which an upper hub plunger assembly can pass and a plurality of peripheral secondary openings within which support tubes can move. The expandable nozzle further includes a subassembly comprising a tubular hub having a closed end and an open end, the open end being slidably positioned in the second end of the barrel. The subassembly further comprises a rod ejection plate and support tubes rigidly securing the rod ejection plate to the hub in fixed axially aligned space relation. The ejection plate is provided with apertures aligned with the support tubes that are affixed within the apertures. 
     An upper hub plunger assembly surrounds, and is coupled to, an upper portion of the central tube and extends through the central opening in the hold-down plate. The upper hub plunger assembly includes a reaction plate that substantially spans the cross section of the tubular barrel and has apertures sized and aligned to slidably receive the support tubes. In the reaction plate&#39;s uppermost position adjacent the hold-down plate, the reaction plate substantially covers the adjacent surface of the hold-down plate. Springs bias the upper hub plunger within the central opening of the hold-down plate, the reaction plate against the lower surface of the hold-down plate and the hold-down plate a predetermined distance from the open end of the tubular hub. 
     Upon a scram, the RCCA impacts the upper hub plunger assembly, driving it in a direction towards the closed end of the hub. The hydraulic attraction between the hold-down plate and the reaction plate, the hydraulic resistance caused by the displacement of water below the reaction plate as the reaction plate moves down and compression of the springs as the upper hub plunger assembly moves toward the closed end of the hub, absorbs a substantial amount of the energy of the RCCA as the control rods approach the lower portions of the thimble tubes within the fuel assembly. 
     Preferably, the movement of the tubular barrel in an expanded direction away from the hub is restrained at a given distance from the hub to assure the tubular barrel assembly does not move off the tubular hub. 
     In the preferred embodiment, the springs surround the support tubes and central tube and substantially extend between the closed end of the hub and the reaction plate. Preferably, the springs are centered and spaced from the exterior wall of the corresponding support tube and central tube that it surrounds so that movement of the spring does not damage the tube&#39;s surface. Desirably, some of the springs extend through openings in the reaction plate and rest against the hold-down plate to support the upper core plate in position during a scram. The support tubes extend to the rod ejection plate. The thimble tubes are removably coupled to the rod ejection plate within the fuel assembly. The central tube is adapted to slidably mount within a corresponding instrument tube within the fuel assembly. In this way, an integral assembly is formed with a removable, expandable nozzle capable of absorbing the large impact loads of an RCCA scram. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A further understanding of the invention can be gained from the following description of the preferred embodiments when read in conjunction with the accompanying drawings in which: 
     FIG. 1 is a cross-sectional view of an expandable, high energy absorption top nozzle of this invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, the expandable, removable top nozzle  10  of the invention comprises a tubular barrel assembly  18  having a first upper end  19  and a second lower end  21 . The upper core plate  12  is supported by the top end  19  of the tubular barrel  10 . A key  17  in the upper end of the tubular barrel  19 , in combination with a corresponding key on the diametrically opposite side of the barrel, is used as a gripping point for lifting the nozzle  10 . The key  17  fits into a corresponding keyway on the upper core plate to fix the orientation of the fuel assembly  16 . The fuel assembly  16  comprises a fuel element array  14  which is captured between the expandable upper nozzle  10  and a bottom nozzle (not shown). The upper tubular barrel assembly  18  further includes a hold-down plate  22  positioned diametrically across the interior of the tubular barrel  18  approximately intermediate the upper end  19  and the lower end  21  of the tubular barrel  18 . The hold-down plate  22  has a number of apertures that extend therethrough, including an enlarged central aperture  24  through which an upper hub plunger assembly  30  passes and peripheral angularly-spaced secondary apertures  26  through which support tubes  50  are slidably positioned. Connecting pins  48  extend from the closed end of the lower hub  40  and are anchored at their ends by connecting pin nuts  58 . In this example, there are six such connecting pin locations. The hold-down plate  22  of the upper tubular barrel assembly  18  is slidably moveable over the support tubes  50  and connection pins  48 , in the corresponding secondary apertures  26 , but is retained by connecting shoulder pin  49  and connecting pin nuts  58 . 
     An assembly comprising a tubular hub casting  38  has a lower closed end  40  and an upper open end  42 . The open end  42  is slidably received within the second, lower end  21  of the upper tubular barrel assembly  18 . A rod ejection plate  46  is rigidly secured to the lower end  40  of the lower hub casting assembly  38  by the support tubes  50 , which extend through the closed end  40  of the lower hub and are secured in corresponding apertures within the rod ejection plate  46 . The support tubes  50  are secured in both the rod ejection plate and the corresponding openings  62  in the closed end  40  of the hub by brazing or welding. The support tubes  50  then extend from the rod ejection plate  46  through the lower end  40  of the hub casting and through openings in the reaction plate  34  where they slidably terminate in the corresponding apertures  26  in the hold-down plate  22 . The connecting pins  48 , previously mentioned, that function as a stop that prevents the support tubes from being withdrawn from the apertures  26  and are secured to the closed end of the lower hub  40 . The retaining pins do not prevent the support tubes  50  from sliding within the apertures  26  in the hold-down plate  22  when the hold-down plate is compressed downward under the weight of the top core plate  12  as will be explained hereafter. The rod ejection plate  46  are designed to couple to thimble tubes in the fuel assembly through an intermediate locking sleeve. The peripheral portion of the rod ejection plate is further supported by a plurality of legs  60  that extend between, and are affixed at one end to the lower end of the hub casting  38  and at the other end to the rod ejection plate  46 . 
     An upper hub plunger assembly  30  surrounds, and is attached to, the central tube  28  and may be formed as an integral part thereof. The upper hub plunger assembly  30  includes a reaction plate  32  that extends peripherally out to the interior walls of the upper tubular barrel assembly, in its upper position, substantially adjacent to the lower surface of the hold-down plate  22 . The reaction plate  32  includes openings  34  through which the support tubes  50  slidably pass. Some of the openings  34  in the reaction plate are smaller than other openings  36  in the reaction plate to provide clearance for springs that will be described hereafter. 
     Coil springs  52  and  54  surround a number, if not all, of the support tubes  50  and central tube  28  and extend from a position proximate the closed end  40  of the lower hub casting assembly  38  up to the vicinity of the reaction plate  32  in the case of the springs  52  and the vicinity of the underside of the hold-down plate  22  in the case of the springs  54 . The enlarged openings  36  in the reaction plate  32  enable the reaction plate to move downward without compressing the springs  54 . For convenience of manufacture, a spring standoff  56  is provided for the springs  54  so that all of the springs  52  and  54  are approximately the same length. In addition, a spring-centering collar  64  is provided around the support tubes  50  to center the springs about the collars and prevent the springs from scarring the exterior walls of the support tubes  50 . It should be appreciated that similar collars can be provided for the central tube  28 . In addition, it should be appreciated that the springs may be provided to surround some or all of the support tubes and the number and placement of the springs is determined from the load that will be experienced and the balance to be achieved so that the tubular barrel assembly  18  moves smoothly over the lower hub assembly  38  when the upper core plate is placed in position and maintains that position during a scram while absorbing a portion of the added load imposed by the scram. In this preferred embodiment, there are eighteen support tubes that carry springs in addition to the central tube. Three of those springs extend through the reaction plate  32  to rest up against the lower surface of the hold-down plate  22 . 
     When the fuel assembly  16  is loaded into the core of the reactor and the upper core plate  12  is lowered, the upper barrel assembly  18  is forced to move downward. The upper barrel assembly  18  through the integral hold-down plate  22  pushes down on the reaction plate  32  and the three springs  54  which, depending on the spring constant of the three springs  54 , may in total deflect all nineteen springs at least partially toward the bottom of the assembly. This action, in combination with the preload on the springs, imparts a hold-down force to the fuel assembly  16 , which forces the fuel assembly down on the lower core plate during operation. The connecting pin shoulders  49  function as a travel stop as the reaction plate  32  is forced down and contacts the connecting pin shoulder  49 . 
     During a scram, the rod control cluster assembly (RCCA) falls until it impacts the upper hub on the plunger assembly  30  and then forces the whole reaction plate  32  downward. Three things happen when this occurs. First, sixteen springs  52  of the nineteen hold-down springs begin to deflect toward the bottom of the assembly, which counteracts some of the downward momentum of the RCCA. Secondly, as the top surface of the reaction plate  32  moves away from the bottom plate surface of the hold-down plate  22 , a large force is required to hydraulically separate the two plates. This hydraulic force also absorbs a significant amount of energy and helps to slow down the RCCA travel. Thirdly, as the bottom surface of the reaction plate  32  begins to move downward, the volume between that plate and the lower hub casting assembly  38  is reduced and water has to escape out of that area. Although there are several leak paths for the water to get through, when the RCCA velocity is high, there is a relatively large pressure buildup, which again helps to slow down the RCCA. These three conditions, in combination, result in satisfactorily slowing the RCCA prior to any solid impact without damaging any of the individual components of the top nozzle or the control rod assembly. 
     Thus, the high energy absorption top nozzle of this invention absorbs the high energy of an RCCA and control rod driveline and stops the downward travel of the control rod assembly within the space allowed without damaging either the top nozzle or control rod assembly. The energy absorption comes from a combination of mechanical spring deflections which occur after impact and hydraulic damping from the separation of two plates which are internal to the top nozzle as well as the hydraulic damping from the pressure buildup in the middle chamber of the top nozzle. 
     While specific embodiments of the invention have been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular embodiments disclosed are meant to be illustrative only and not limiting as to the scope of the invention which is to be given the full breadth of the appended claims and any and all equivalents thereof.