Patent Publication Number: US-2009238322-A1

Title: Fuel rod and assembly containing an internal hydrogen/tritium getter structure

Description:
FIELD OF THE INVENTION 
     This invention relates to fuel rod assemblies and fuel rods within such assemblies, where fuel rods contain an internal hydrogen/tritium “getter” structure located within the fuel rod, for example near the bottom. The “getter” or “absorber” structure is effective to absorb and retain any hydrogen (H) or tritium (gaseous hydrogen isotope having an atomic weight of 3, H 3  or H3—emits beta rays) and retain such inside the fuel rod, to reduce potential release of hydrogen or tritium and deterioration of the fuel rod. The tritium is a gas until absorbed by the “getter” structure, and forms a hydride. 
     BACKGROUND OF THE INVENTION 
     Various parts of nuclear reactors are subjected to attacks such as hydration and/or exidation by various gases, isotopes or the like generated during operation. In some instances there can be damage to the interior of fuel rods. This can, in rare instances, cause potential deterioration in the fuel rods. 
     In a typical nuclear reactor, such as a pressurized water (PWR), as shown for example in the Westinghouse Electric Co. brochure,  Ready to Meet Tomorrow&#39;s Power Generation Requirements Today,  2007, the reactor core includes a large number of fuel assemblies, each of which is composed of a plurality of elongated fuel rods. The fuel rods each contain fissile material such as uranium dioxide (UO 2 ) or plutonium dioxide (PUO 2 ), or mixtures, usually in the form of a stack of solid, pressed nuclear fuel pellets The fuel rods are grouped together in an array which is organized to provide a neutron flux in the core sufficient to support a high rate of nuclear fission and thus the release of a large amount of energy in the form of heat. A coolant, such as water, is pumped, generally from a lower coolant plenum, through the core in order to extract some of the heat generated in the core for the production of useful work. Fuel assemblies vary in size and design depending on the desired size of the core and the size of the reactor. 
     Nuclear reactors are provided with sufficient excess reactivity at the beginning of a fuel cycle to allow operation for a specified time period, usually between about six to eighteen months. Since a reactor operates only slightly supercritical, the excess reactivity supplied at the beginning of a cycle must be counteracted. Various methods to counteract the initial excess reactivity have been devised, including insertion of control rods in the reactor core and the addition of neutron absorbing elements to, for example, the fuel. Such neutron absorbers, known as “burnable poisons”, include, for example, boron, gadolinium, cadmium, samarium, erbium and europium compounds. These burnable poisons absorb the initial excess amount of neutrons while (in the best case) producing no new or additional neutrons or changing into new poisons as a result of neutron absorption. 
     Sintered pellets of nuclear fuel having an admixture of a boron-containing compound or other burnable poison are known. See, for example, U.S. Pat. Nos. 3,349,152 and 3,520,958 (Watanabe et al and Versteeg et al, respectively). More recently, in U.S. Pat. No. 7,139,360 B2 (Lahoda), the fuel rods contained a number of fuel pellets, where, more than one half of the rods had at least one fuel pellet of a metal oxide, metal carbide or metal nitride, plus a boron containing burnable poison to contain excess neutrons. Burnable poison coatings were addressed in U.S. Pat. No. 4,587,087 (Radford et al.) where the solid nuclear fuel substrate was coated by a burnable poison layer and enclosed within an end plugged fuel rod. 
     Addressing protection against oxidation, corrosion and hydration, Rudling et al. in U.S. Pat. No. 6,512,806 B2 taught coating the fuel rod itself, a zirconium alloy, with at least one of zirconium dioxide (ZrO 2 ) and zirconium nitride (ZrN). Also Davies in the U.S. Pat. No. 5,434,897 utilized fuel rods with a three layer cladding, where the layers facilitated mixing of gases in the cladding interior, so that any steam entry would promote mixing of steam and hydrogen. The fuel rods were sealed at their ends by end plugs with a helical member at the top to prevent axial movement of a pellet column. 
     Any hydrogen present near the cladding can cause mechanical damage by forming hydrides. If tritium (H 3 ) is present it can form a “dose” problem by passing through the cladding tube and into the coolant water. Once in the coolant, it is difficult to remove/separate since it is chemically and physically near-identical to regular hydrogen, that is, in water molecules. It will follow any reactor water/steam waste stream to the outside. The structure at risk from hydrogen or H 3  is the fuel rod cladding if the gas is in the interior of the rod. While H 3  is always generated inside the fuel rod, hydrogen can be formed in the fuel rod or introduced and enclosed during manufacture of the fuel rod. Thus, there is a need for a solution to these problems. 
     It is a main object of this invention to provide an assembly of fuel rods and individual fuel rods which address potential problems of hydrogen and tritium contamination and degradation. 
     It is also an object of this invention to find absorbers/adsorbers for tritium and hydrogen in a nuclear power plant environment. 
     SUMMARY OF THE INVENTION 
     The above needs are accomplished and objects met by providing a fuel rod having a top and bottom, wherein the fuel rod is made of a cladding material and contains a plurality of nuclear fuel pellets, including a top fuel pellet and a bottom fuel pellet, the fuel rod containing top and bottom end plugs sealing the fuel rod and nuclear fuel pellets; where a separation means, usually a spring, is disposed between the top end plug and the top fuel pellet, and where at least one hollow gas absorber structure is disposed within the fuel rod, said at least one hollow gas absorber structure having an outside surface spaced apart from the interior surface of the fuel rod, and having at least an interior surface coated with a catalyst that absorbs and retains at least one of hydrogen and tritium. 
     The above needs are also accomplished and objects met by providing a fuel assembly comprising a plurality of fuel rods each having a top and bottom, each fuel rod made of a cladding material and containing within them a plurality of nuclear fuel pellets, including a top fuel pellet and a bottom fuel pellet, the fuel rods containing top and bottom end plugs sealing the plurality of fuel rods and nuclear fuel pellets, where a spring means is disposed between the top end plug and the top fuel pellet, and a hollow gas absorber structure is disposed between the end plug and the fuel pellet, at the opposite end of the fuel rod from the spring means, said hollow gas absorber structure having an outside surface spaced apart from the interior surface of the at least one fuel rods, and having at least an interior surface coated with a catalyst that absorbs and retains at least one of hydrogen and tritium. 
     Preferably, the gas absorber structure will comprise a ceramic or a temperature resistant metal such as zirconium, and the catalyst coating will be selected from the group consisting of Ni, Pd, Cu, metallic U and mixtures thereof. The fuel assembly and included fuel rod and gas absorber structure will be disposed in a volume generally above an associated bottom reactor cooling plenum. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the course of the following detailed description reference will be made to the attached non-limiting drawings in which: 
         FIG. 1  is a longitudinal view partly in section and partly in elevation of one embodiment of a prior art PWR nuclear reactor, including a bottom reactor coolant plenum with a plurality of fuel assemblies disposed in a volume above the coolant plenum; 
         FIG. 2  is a longitudinal view, with parts sectioned and parts broken away for clarity, of one of the prior art nuclear fuel assemblies in the reactor of  FIG. 1 ; 
         FIG. 3 , which best shows the invention, is an enlarged foreshortened longitudinal axial sectioned view of one embodiment of the fuel rods which can be used in the assemblies of  FIG. 2 , showing a bottom gas absorber structure and top spring means, where the fuel rod can be used in reverse, to place the gas absorber structure on the top; and 
         FIGS. 4(A) and 4(B)  show different non-limiting embodiments of the gas absorber structure, one having five sides and entry/exit gas holes and one having a simple cylindrical structure. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings,  FIG. 1 , shows by way of example only, one of many suitable prior art reactor types, a pressurized water nuclear reactor (PWR), generally designated by the numeral  10 . The PWR  10  includes a reactor pressure vessel  12  which houses a nuclear reactor core  14  composed of a plurality of elongated fuel assemblies  16 . Relatively few fuel assemblies  16  are shown in  FIG. 1  for purposes of simplicity. In reality, the core  14  is composed of a great number of fuel assemblies. 
     Spaced radially inwardly from the reactor vessel  12  is a generally cylindrical core barrel  18  and within the barrel  18  is a baffle structure  20 . The baffle structure  20  surrounds the fuel assemblies  16  of the reactor core  14 . Typically, the baffle structure  20  is made of baffle plates  22  joined together by bolts (not shown). The reactor core  14  and the baffle structure  20  are disposed between upper and lower core plates  24 ,  26  which, in turn, are supported by the core barrel  18 . 
     The upper end of the reactor pressure vessel  12  is hermetically sealed by a removable closure head  28  upon which are mounted a plurality of control rod drive mechanisms  30 . Again, for simplicity, only a few of the many control rod drive mechanisms  30  are shown. Each drive mechanism  30  selectively positions a rod cluster control assembly mechanism  32  above and within some of the fuel assemblies  16 . 
     A nuclear fission process carried out in the fuel assemblies  16  of the reactor core  14  produces heat which is removed during operation of the PWR by circulating a coolant fluid, such as light water with soluble boron, through the core  14 . More specifically, the coolant fluid is typically pumped into the reactor pressure vessel  12  through a plurality of coolant inlet nozzles  34  (only one of which is shown in  FIG. 1 ). 
     The coolant fluid enters the reactor vessel through coolant inlet nozzles  34 , and passes downward through an annular region  36  defined between the reactor vessel  12  and core barrel  18  (and a thermal shield  38  on the core barrel) until it reaches the bottom reactor cooling plenum  27  of the reactor vessel  12  where the coolant turns 180 degrees prior to following up through the lower core plate  26  and then up through the reactor core  14 . On flowing upwardly through the fuel assemblies  16  of the reactor core  14 . The coolant then passes through the upper core plate  24  and exits the reactor vessel through outlet nozzles  40 . 
     The coolant fluid is heated to reactor operating temperatures by the transfer of heat energy from the fuel assemblies  16  to the fluid. The hot coolant fluid then exits the reactor vessel  12  through a plurality of outlet nozzles  40  (only one being shown in  FIG. 1 ) extending through the core barrel  18 . Thus, heat energy which the fuel assemblies  16  impart to the coolant fluid is carried off by the fluid from the pressure vessel  12 . 
     As briefly mentioned above, the reactor core  14  is composed of a large number of elongated fuel assemblies  16 . Turning to  FIG. 2 , each fuel assembly  16 , being of the type used in a PWR, basically includes a lower end structure or bottom nozzle  42  which supports the assembly on the lower core plate  26  (shown in  FIG. 1 ) and a number of longitudinally extending guide tubes or thimbles  44  which project upwardly from the bottom nozzle  42 . The assembly  16  further includes a plurality of transverse support grids  46  axially spaced along the lengths of the guide thimbles  44  and attached thereto. The grids  46  transversely space and support a plurality of fuel rods  48  in an organized array thereof. Also, the assembly  16  has an instrumentation tube  50  located in the center thereof and an upper end structure or top nozzle  52  attached to the upper ends of the guide thimbles  44 . With such an arrangement of parts, the fuel assembly  16  forms a integral unit capable of being conveniently handled without damaging the assembly parts. 
     As seen in  FIG. 3 , each of the fuel rods  48  of the fuel assembly  16  (shown in  FIG. 2 ) has generally identical construction insofar as each includes an elongated hollow cladding tube  54  with a top end plug  56  and a bottom end plug  58  attached to and sealing opposite ends of the tube  54  defining a sealed chamber shown generally as  60  therein. A plurality of nuclear fuel pellets  62 , usually in solid round flat puck form, are usually placed in an end-to-end abutting arrangement or stack within the chamber  60  and held in place by the action of a spring or like device/means  64  placed in the chamber  60  usually between the top pellet  70  and the top end plug  56 . A bottom pellet  72  is shown near the bottom  74  of the fuel rod  48 . Referring back to  FIG. 2  prior art structure, the new interior absorber structure will be at location generally shown as  81 . 
     As can be seen in  FIG. 3 , top end plug  56  is shown at the top  73  of the fuel rod and bottom end plug  58  is shown at the bottom  74  of the fuel rod, where a hollow gas absorber structure  80  can be disposed between the bottom end plug  58  and the bottom fuel pellet  72 . The outside surface  82  of the gas absorber structure  80  is spaced apart a distance  84  from the interior surface  86  of the fuel rod  48 , cladding tube  54 . The hollow gas absorber structure will contain, be impregnated with, or coated with a catalytic material, shown ideally as dots  83  in  FIGS. 4A-4B , effective to “get”/absorb hydrogen and tritium. Of course the catalyst would cover all the surfaces—not shown for simplicity. The inside structure  80  is kept hollow but may also contain catalyst on the inside surface. 
     Hydrogen is generated in the fuel rod interior gas primarily by reactions of retained moisture in the rod with the zirconium alloy inner cladding tube  54  surface: 
       2H 2 O+Zr=2H 2 +ZrO 2    
     Another potential source of hydrogen is internal anomalous hydrogenous-material contamination. Potential tritium sources are neutron absorption reactions with Boron and ternary fission reactions in the nuclear fuel. Tritium is a gas prior to being absorbed by the getter structure where it forms a hydride. If it escapes the fuel rod the tritium can exist as a gas in solution, or due to radiolysis can become tied in a water molecule as titrated water. 
     The hollow gas absorber structure  80  will preferably comprise zirconium, preferably a zirconium alloy but could also be any metal or metal alloy resistant to temperatures over about 300° C. or a high temperature ceramic, such as one comprising silicon. The catalyst “getter”/absorber is selected from a catalytic material that allows hydrogen and tritium to react with the gas absorber structure, preferably, it is selected from the group consisting of Ni, Pd, Cu, metallic U and mixtures thereof. The distance  84  from the outside surface  82  of the gas absorber structure and the interior surface  86  of the fuel rod, for good gas flow contact with the catalyst will range from about 0.01 inch to 0.003 inch (0.254 cm to 0.076 cm). The wall thickness  88  (shown in  FIG. 3(A) ) of the gas absorber structure will range from about 0.013 inch to 0.023 inch (0.330 cm to 0.584 cm). 
     The porosity of the gas absorber structure to assure good gas contact with the catalyst and access to the interior of the structure will preferably be at least about 99 vol. % dense or even higher. This will provide sufficient strength to resist the weight of the fuel pellets. 
     Several embodiments of the gas absorber structure  80  are shown, as illustrations only and not meant to be limiting, in  FIG. 4(A)  and  FIG. 4(B) . These structures  80  can be of various shapes to provide optional surface area, such, for example, as the pentagon shaped structure of  FIG. 4(A) , or they can be the easy to manufacture tubular structure of  FIG. 4(B) . They can have small openings  89  of various shapes in/through its sides therethrough, penetrating the sides to allow gas  90 , shown as arrows, to enter the interior volume which may be of larger volume than the annular volume  91  around the structures. In other instances of higher porosity structures the gas  90  may be allowed to permeate/diffuse through the sides of the structure to the interior. 
     The internal hydrogen/tritium “getter” structure  80  is designed to absorb and retain gases  90  such as hydrogen gas and tritium isotope gas to prevent for example any potential fine cracks or fissures, shown as  92 , at the bottom wall  74  of the cladding tube  54 . Preferably, the catalyst will also be inside the structure  80 , to avoid possible contaminating material in contact with the interior cladding and because of less restricted communication with the gases  90 . However, in some instances it may be desirable to have the catalytic material on the exterior or on both the interior and exterior surfaces of the structure  80 . 
     As an Example: Starting with a small hollow “getter” tube of Zircaloy 4 or ZIRLO, with a porosity of less than about 0.01 vol. porous %, (99 vol. % dense), a coating is applied to the tube surface (both ID and OD) that will act as a catalyst to enhance the absorption of hydrogen gas into the “getter” tube. The hydrogen contacted was found to diffuse into the solid metal matrix and chemically react with the zirconium in the tube to form zirconium hydride that precipitates as a solid phase within the metal matrix of the “getter” tube. Effectively, the hydrogen gas is converted to and is retained as a solid phase. It is not a gas phase retention mechanism. The ends of the tube are open and provide a flow path for the rod internal gas to flow into the tube ID. The seating contact with an end plug and fuel pellets is not sufficient to preclude gas flow, but in some designs there can be slots or holes that provide less hindered ingress with the same expected result. 
     Whereas particular embodiments of this invention have been described above for purposes of illustration, it will be evident to those skilled in the art that numerous variations of the details of the present invention may be made without departing from the invention as defined in the appended claims.