Patent Publication Number: US-2023162875-A1

Title: Control rod damping system

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application is a divisional of U.S. Pat. Application No. 17/246,251, filed on Apr. 30, 2021, and titled “CONTROL ROD DAMPING SYSTEM,” which is a continuation of U.S. Pat. Application No. 15/858,073, filed on Dec. 29, 2017, and titled “CONTROL ROD DAMPING SYSTEM,” which claims priority to U.S. Provisional Pat. Application No. 62/441,038, filed on Dec. 30, 2016, and titled “CONTROL ROD DASH POT INTEGRAL TO THE UPPER TIE PLATE,” the contents of each of which are herein incorporated by reference in their entirety. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
     This invention was made with Government support under Contract No. DE-NE0000633 awarded by the Department of Energy. The Government has certain rights in this invention. 
    
    
     TECHNICAL FIELD 
     This disclosure generally relates to a control rod damping system. 
     BACKGROUND 
     Dash pots constrict diameters near the bottom of the guide tubes slowing the fall of control rods during a scram to reduce potential impact damage. Low coolant flow through nuclear reactor guide tubes can lead to problems such as boiling, reduced fuel economy, and potential interference with control rode operations due to build-up of guide tube corrosion and precipitates. One potential cause of low coolant flow are the dash pots. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The included drawings are for illustrative purposes and serve to provide examples of possible structures and operations for the disclosed inventive systems, apparatus, methods and computer-readable storage media. These drawings in no way limit any changes in form and detail that may be made by one skilled in the art without departing from the spirit and scope of the disclosed implementations. 
         FIG.  1    shows a schematic diagram of an example power module. 
         FIG.  2    is a perspective view of a drive shaft and control rod assembly. 
         FIG.  3    is a side view of the control rod assembly partially inserted into a fuel assembly. 
         FIG.  4    is a side sectional view of the control rod assembly and fuel assembly. 
         FIG.  5    is an enlarged detail side sectional view of a control rod dampening section. 
         FIG.  6    is a top sectional view of the control rod assembly. 
         FIG.  7    is a side sectional view of the dampening section of the control rod inserting into an associated guide tube. 
         FIG.  8    is a further enlarged detail side sectional view of the control rod dampening section. 
         FIG.  9    is a side view of a dampening area located on a drive shaft. 
         FIG.  10    is a sectional view of the dampening area of  FIG.  9   . 
     
    
    
     DETAILED DESCRIPTION 
     A damping area or “dash pot” on the upper ends of control rods reduce the need to constrict the diameter of guide tubes. As a result, water can more freely flow through the guide tubes reducing boiling coolant issues. The restriction at the upper portion of the control rod assembly creates hydraulic back pressure which slows the fall and associated impact of the control rods hitting the fuel assembly during a scram procedure. 
     The control rods include a first section having a first diameter retaining an active material for inserting into the guide tube and controlling a fission rate in a nuclear reactor core. A second section of the control rods attach to a head assembly. The novel dampening section is located between the first and second section with a second larger diameter. The dampening section reduces a separation distance between an outside surface of the control rod and an inside surface of the guide tube that decelerates the control rod when entering a top end of the guide tube. 
     In one example, the control rod may have a cylindrical cladding including a bottom end retaining the active material and having a first wall thickness. A top end of the cladding may have a second continuously increasing wall thickness larger than the first wall thickness. 
     In another example, the dampening area may be located on a drive shaft. The drive shaft may slidingly extend through an opening in a support member. The drive shaft may include a dampening section having a diameter larger than the opening in the support member to decelerate the drive shaft when dropped by a rod drive mechanism. 
       FIG.  1    illustrates a cross-sectional view of an example reactor module  100  comprising reactor pressure vessel  52 . Reactor core  6  is shown located near a lower head  55  of the reactor pressure vessel  52 . The reactor core  6  may be located in a shroud  22  which surrounds reactor core  6  about its sides. A riser section  24  is located above the reactor core  6 . 
     When primary coolant  28  is heated by reactor core  6  as a result of fission events, primary coolant  28  may be directed from shroud  22  up into an annulus  23  located above reactor core  6 , and out of riser  24 . This may result in additional primary coolant  28  being drawn into shroud  22  to be heated in turn by reactor core  6 , which draws yet more primary coolant  28  into shroud  22 . The primary coolant  28  that emerges from riser  24  may be cooled down and directed towards the outside of the reactor pressure vessel  52  and then returned to the bottom of the reactor pressure vessel  52  through natural circulation. 
     Primary coolant  28  circulates past the reactor core  6  to become high-temperature coolant TH and then continues up through the riser section  24  where it is directed back down the annulus and cooled off by a heat exchanger to become low-temperature coolant TC. One or more control rod drive mechanisms (CRDM)  10  operably coupled to a number of drive shafts  20  may be configured to interface with a plurality of control rod assemblies  82  located above reactor core  6 . 
     A reactor pressure vessel baffle plate  45  may be configured to direct the primary coolant  28  towards a lower end  55  of the reactor pressure vessel  52 . A surface of the reactor pressure vessel baffle plate  45  may come into direct contact with and deflect the primary coolant  28  that exits the riser section  24 . In some examples, the reactor pressure vessel baffle plate  45  may be made of stainless steel or other materials. 
     The lower end  55  of the reactor pressure vessel  52  may comprise an ellipsoidal, domed, concave, or hemispherical portion  55 A, wherein the ellipsoidal portion  55 A directs the primary coolant  28  towards the reactor core  6 . The ellipsoidal portion  55 A may increase flow rate and promote natural circulation of the primary coolant through the reactor core  6 . Further optimization of the coolant flow  28  may be obtained by modifying a radius of curvature of the reactor pressure vessel baffle plate  45  to eliminate/minimize boundary layer separation and stagnation regions. 
     The reactor pressure vessel baffle plate  45  is illustrated as being located between the top of the riser section  24  and a pressurizer region  15 . The pressurizer region  15  is shown as comprising one or more heaters and a spray nozzle configured to control a pressure, or maintain a steam dome, within an upper end  56  or head of the reactor pressure vessel  52 . Primary coolant  28  located below the reactor pressure vessel baffle plate  45  may comprise relatively sub-cooled coolant TSUB, whereas primary coolant  28  in the pressurizer region  15  in the upper end  56  of the reactor pressure vessel  52  may comprise substantially saturated coolant TSAT. 
     A fluid level of primary coolant  28  is shown as being above the reactor pressure vessel baffle plate  45 , and within the pressurizer region  15 , such that the entire volume between the reactor pressure vessel baffle plate  45  and the lower end  55  of the reactor pressure vessel  52  may be full of primary coolant  28  during normal operation of the reactor module  100 . 
     Shroud  22  may support one or more control rod guide tubes  124 . The one or more control rod guide tubes  124  serve to guide control rod assemblies  82  that are inserted into, or removed from, reactor core  6 . In some examples, control rod drive shafts  20  may pass through reactor pressure vessel baffle plate  45  and through riser section  24  in order to control the position of control rod assemblies  82  relative to reactor core  6 . 
     Reactor pressure vessel  52  may comprise a flange by which lower head  55  may be removably attached to a vessel body  60  of reactor pressure vessel  52 . In some examples, when lower head  55  is separated from vessel body  60 , such as during a refueling operation, riser section  24 , baffle plate  45 , and other internals may be retained within vessel body  60 , whereas reactor core  6  may be retained within lower head  55 . 
     Additionally, vessel body  60  may be housed within a containment vessel  70 . Any air or other gases that reside in a containment region  74  located between containment vessel  70  and reactor pressure vessel  52  may be removed or voided prior to or during reactor startup. The gases that are voided or evacuated from the containment region  74  may comprise non-condensable gases and/or condensable gases. Condensable gases may include steam that is vented into containment region  74 . 
     During an emergency operation, vapor and/or steam may be vented into containment region  74 , only a negligible amount of non-condensable gas (such as hydrogen) may be vented or released into containment region  74 . 
     Certain gases may be considered non-condensable under operating pressures that are experienced within a nuclear reactor system. These non-condensable gases may include hydrogen and oxygen, for example. During an emergency operation, steam may chemically react with the fuel rods to produce a high level of hydrogen. When hydrogen mixes with air or oxygen, this may create a combustible mixture. By removing a substantial portion of the air or oxygen from containment vessel  54 , the amount of hydrogen and oxygen that is allowed to mix may be minimized or eliminated. 
     It may be possible to assume from a practical standpoint, that substantially no non-condensable gases are released into or otherwise housed in containment region  74  during operation of reactor module  100 . Accordingly, in some examples, substantially no hydrogen gas is present in the containment region  74 , such that the levels and/or amounts of hydrogen together with any oxygen that may exist within the containment region  74  are maintained at a non-combustible level. Additionally, this non-combustible level of oxygen-hydrogen mixture may be maintained without the use of hydrogen recombiners. In some examples, separate vent lines from the reactor pressure vessel  52  may be configured to remove non-condensable gases during start up, heat up, cool down, and/or shut down of the reactor. 
     During the emergency scram condition, drive assemblies  10  may release drive shafts  20  dropping control rod assemblies  82  into guide tubes  124 . Conventional guide tubes  124  may narrow toward bottom ends to hydraulically dampen the impact of control rod assemblies  82  dropping into reactor core  6 . As described above, the narrow bottom diameters of guide tubes  124  may reduce the flow of primary coolant  28  through reactor core  6  causing coolant  28  to boil resulting in corrosion and reduced fuel economy. 
     Control Rod Dampening System 
       FIG.  2    is a perspective view of a control rod assembly  82  that includes dampening areas  130 . Control rod assembly  82  may be held above and then inserted into reactor core  6 . As explained above in  FIG.  1   , multiple drive shafts  20  extend from rod drive mechanisms  10 , through baffle plate  45  and shroud  22  down to the top of reactor core  6 . In one example, drive shafts  20  extend through a drive shaft support  122  that may be part of baffle plate  45  described above in  FIG.  1   . However, drive shaft support  80  may be located and attached anywhere within reactor pressure vessel  52 , such on shroud  22 , annulus  23  or riser section  24 . 
     A head assembly  86  may include a cylinder  88  that attaches to the bottom end of drive shaft  20 . Head assembly  86  also may include arms  90  that extend radially out from cylinder  86  and attach at distal ends to top ends of control rods  92 . Head assembly  86  is alternatively referred to a spider machining and control rods  92  are alternatively referred to as fingers. 
     Control rods  92  extend into a fuel assembly  120  that is alternatively referred to as a fuel bundle and in  FIG.  1    forms part of reactor core  6 . Fuel assembly  120  may include a top nozzle  122  that supports multiple guide tubes  124 . Guide tubes  124  extend down from nozzle  122  and in-between nuclear fuel rods (not shown). Control rods  92  control the fission rate of uranium and plutonium fuel rods. 
     Control rods  92  are typically held by drive shaft  20  above fuel assembly  120  or held slightly inserted into fuel assembly  120 . Reactor core  6  may overheat. A nuclear scram operation is initiated where rod drive mechanisms  10  in  FIG.  1    release drive shafts  20  dropping control rods  92  down into guide tubes  124  and in-between the fuel rods. Some fuel assemblies narrow bottom ends of guide tubes  124  to reduce the impact of control rod assembly  82  slamming into fuel assembly  120 . 
     As explained above, these narrow diameters at the bottom ends of guide tubes restrict coolant flow causing steam created corrosion. Negative effects of low coolant flow can be even more detrimental in a nuclear reactor, such as nuclear reactor module  100  that may use natural circulation, instead of pumps, to circulate coolant through guide tubes  124 . 
     Dampening areas  130  are integrated into the upper ends of control rods  92  to reduce the impact of dropping control rod assembly  82  onto fuel assembly  120  during a scram operation. Instead of continuously restricting coolant flow through the bottom ends of guide tubes  124 , dampening areas  130  only restrict coolant flow at the upper ends of guide tubes  124  during the scram operation. In addition, coolant flow is only restricted after control rods  92  are mostly inserted into guide tubes  124 . In another example, dampening areas  150  are located on drive shafts  20  moving impact forces even further above control rod assembly  82  and fuel assembly  120 . 
       FIG.  3    is a side view and  FIG.  4    is a side sectional view of control rod assembly  82  and fuel assembly  120 .  FIG.  5    is a more detailed side sectional view for a portion of control rod  92  that includes dampening area  130 . Referring to  FIGS.  3 - 5   , guide tube sleeves  126  extend downward from substantially the middle of holes  128  formed in floor  123  of nozzle  122 . Guide tubes  124  extend from a top surface of floor  123  through holes  128  and sleeves  126  down in between fuel rods of the reactor core. 
     Control rods  92  each include a top plug section  136 , an intermediate section  129  that holds a spring  134 , and a bottom section  131  that holds active control rod material  132 . Active material  132  is used in reactor core  6  of  FIG.  1    to control the fission rate of uranium and plutonium. At least in some examples, active material  132  may include chemical elements such as boron, silver, indium and cadmium that are capable of absorbing neutrons without themselves fissioning. 
     Each control rod  92  extends down from head assembly  86  into the top end of an associated guide tube  124 . In a fully inserted position, control rods  92  extend through nozzle  122  and down to the bottom of guide tubes  124  in between the fuel rods. Control rods  92  are normally held by drive shaft  20  above nozzle  122  and are typically not completely inserted into fuel assembly  126  unless an overheating condition is detected. 
     Dampening area  130  is located in the top ends of intermediate sections  129  between plug  136  and above active material  132  where spring  134  is located. As explained in more detail below, dampening area  130  reduces the impact when control rods  92  are dropped into guide tubes  124  during a nuclear scram. In one example, the diameters of control rods  92  in dampening area  130  are larger than the diameters of the lower sections  131  that extend down into fuel assembly  120 . This allows substantially the entire lower section  131  carrying active material  132  to fully insert in between the fuel rods prior to dampening area  130  reaching the top ends of guide tubes  124 . 
       FIG.  6    is a top sectional view of control rod assembly  82  and guide tubes  124 .  FIG.  7    is a side sectional view of a control rod  92  partially inserted into an associated guide tube  124 .  FIG.  8    is a further enlarged detailed side sectional view of dampening area  130  formed in control rod  92 . 
     Referring first to  FIGS.  4 ,  6  and  7   , as explained above, plug  136  of control rod  92  includes a top end  137 A that inserts into the bottom end of arm  90  on head assembly  86  and a bottom end  137 B that inserts into a cylindrical cladding  140 . In one example, cladding  140  has a circular cross-sectional shape that retains spring  134  and active material  132 . In one example, cladding  140  may be made out of stainless steel. 
     A guide tube collar  142  extends up from floor  123  of nozzle  122  as shown in  FIG.  4   . Guide tube  124  extends down from collar  142  through floor  123  of nozzle  122  and down to the bottom of fuel assembly  120 . A sleeve collar  144  sits in hole  128  of nozzle  122  as shown in  FIG.  4   . Sleeve  126  extends down from collar  144  below floor  123  of nozzle  122 . A spring  146  extends around the outside surface of guide tube  124  between collar  142  and collar  144 . 
     Referring now to  FIG.  8   , a wall thickness and an associated outside diameter of cladding  140  may continuously increase from a lower dampening location  130 A to an upper dampening location  130 B. This increased wall thickness and corresponding increased diameter reduces a spacing  148  between the outside surface of cladding  140  and an inside surface of guide tube  124 . For example, space  148 A between cladding  140  and guide tube  124  at dampening location  130 A is larger than space  148 B between the outside surface of cladding  140  and the inside surface of guide tube  124  at upper dampening location  130 B. 
     Referring to  FIGS.  1 - 8   , during normal operations, drive shaft  20  may hold control rods  92  almost completely above fuel assembly  120 . During an overheating condition, rod drive mechanisms  10  in  FIG.  1    release drive shafts  20  dropping control rod assembly  82 . The lower sections  131  of controls rods  92  that contain active material  132  have a uniform smaller diameter and accordingly drop freely down into guide tubes  124 . Control rods  92  may push coolant out the top and bottom ends of guide tubes  124 . 
     Control rods  92  continue to drop freely until bottom ends  130 A of dampening area  130  reach the top ends of guide tubes  124 . The continuously increasing diameter of dampening area  130  start reducing the spacing  148  at the top ends of guide tubes  124  between the outside surface of control rods  92  and the inside surfaces of guide tubes  124 . 
     Dampening area  130  starts restricting the coolant from escaping through the top ends of guide tubes  124 . The restricted coolant creates a back hydraulic pressure that slows down and absorbs some of the energy from the control rods  92  falling inside of guide tubes  124 . As a result, the coolant in guide tubes  124  acts like a hydraulic cylinder decelerating the falling speed of control rod assembly  82 . 
     One substantial advantage of using larger diameter dampening section  130  is that guide tubes  124  may remain at a consistent diameter throughout the entire length of fuel assembly  120 . Thus, guide tubes  126  may avoid creating the boiling and corrosion problems that exist in guide tubes with narrow diameter bottom ends. 
     Wider dampening areas  130  also may be easier to manufacture compared with changing a diameter at the bottom of guide tubes  124 . Wider dampening areas  130  also may stiffen the upper ends of control rods  92  and reduce binding when control rods  92  are dropped into guide tubes  124 . 
     In one example, a bottom outside diameter  149 A at damping location  130 A may be around 9.677 millimeters (mms), lower spacing  148 A may be around 0.866 mms, upper outside diameter  149 B at dampening location  130 B may be around 10.668 mms, upper spacing  148 B may be around 0.375 mms, and the distance between lower dampening location  130 A and upper dampening location  130 B may be around 85 mms. 
     The spacings, diameters, and distances of dampening area  130  may vary based on the size and weight of control rod assembly  82 . The dimensions of dampening area  130  can also be varied to provide a more gradual deceleration of control rod assembly  82 . For example, the length between lower dampening location  130 A and upper dampening location  130 B may be increased to provide a more gradual deceleration of control rod assembly  82 . In another example, holes may be drilled through the top ends of guide tubes  124  to provide an alternative coolant escape path. 
     In another example, cladding  140  may remain at a same uniform thickness. However, outside diameter  149 A of cladding  140  still may continuously increase from lower dampening location  130 A to upper dampening location  130 B. For example, an extrusion process used for forming cladding  140  may form a continuously increasing diameter within dampening area  130 . 
     In one example, plug  136  of control rod  92  shown in  FIG.  7    may have substantially the same larger outside diameter  149 B as the upper end of cladding  140 . In another example, cladding  140  may maintain substantially the same diameter  149  and fully extend into guide tubes  124 . Dampening area  130  may be formed in plug  136  and have a continuously increasing outside diameter starting from bottom end  137 B and extending up to upper end  137 A. The diameter at upper end  137 A may be sized so arms  90  do not fall on top of nozzle  122  when control rods  92  are released during the scram. 
     In yet another example, V-shaped slots may extend up from floor  123  of nozzle  122 . The slots may receive arms  90  and decelerate and stop control rod assembly  82  before slamming into the top of fuel assembly  120 . 
       FIG.  9    is a side view of drive shaft  20  and control rod assembly  82  shown above in  FIG.  2   .  FIG.  10    is a sectional view of a portion of drive shaft  20  and drive shaft support  80 . Referring to  FIGS.  9  and  10   , drive shafts  20  may be used instead of control rods  92  to dampen the speed of control rod assembly  82  during a nuclear scram. 
     A lower portion of drive shaft  20  may have a first outside diameter  152 A. Lower dampening location  150 A may start at first outside diameter  152 A and continuously increase until reaching a second larger outside diameter  152 B at upper dampening location  150 B. Drive shaft  20  may maintain smaller outside diameter  152 A below dampening location  150 A and may maintain larger outside diameter  152 B above upper dampening location  150 B. 
     In one example, the outside diameter of drive shaft  20  is increased by increasing a thickness  156  of drive shaft wall  154 . Of course, the outside diameter  152  of drive shaft  20  also may be increased without increasing the thickness  156  of drive shaft wall  154  using known extrusion processes. Drive shaft  20  may have a cylindrical shape and dampening area  150  may have an inverted cone shape. 
     A circular opening  158  in drive shaft support  80  may be formed with an inclining inverted cone shaped inside wall  160  that receives and retains dampening area  150 . A diameter of opening  158  may continuously increase from a bottom side of support  80  to a top side of support  80 . Drive shaft  20  below dampening location  150 A can slide freely through opening  158  dropping control rod assembly  82  down into fuel assembly  120 . 
     The diameter at the bottom end of opening  158  is smaller than diameter  152 B of drive shaft  20  at upper dampening location  150 B. Accordingly, drive shaft  20  starts decelerating as the outside surface of dampening area  150  starts sitting against inside wall  160  of support  80 . 
     Dampening area  150  may stop drive shaft  20  before drive rod assembly  82  slams down against the top of nozzle  122 . For example, dampening area  150  may stop drive rod  20  just before arms  90  of head assembly  86  reach nozzle  122  as shown in  FIG.  5   . 
     Alternative dampening schemes may be used with drive shafts  20 . For example, a spring may extend up from the top surface of support  80 . A transverse bar or wider outside diameter  152 B of drive rod  20  may compress the spring to decelerate and eventually stop drive rod  20 . In another example, a cone shaped facet with upwardly inclining sides may extend up from the top surface of support  80  and operate similar to upwardly inclining wall  160  of support  80 . In another example, dampening areas  130  in control rods  92  and dampening areas  150  in drive rods  20  may be used in combination to further distribute the impact of falling control rod assembly  82 . 
     Having described and illustrated the principles of a preferred embodiment, it should be apparent that the embodiments may be modified in arrangement and detail without departing from such principles. Claim is made to all modifications and variation coming within the spirit and scope of the following claims. 
     Some of the operations described above may be implemented in software and other operations may be implemented in hardware. One or more of the operations, processes, or methods described herein may be performed by an apparatus, device, or system similar to those as described herein and with reference to the illustrated Figures. 
     It will be apparent to one skilled in the art that the disclosed implementations may be practiced without some or all of the specific details provided. In other instances, certain process or methods have not been described in detail in order to avoid unnecessarily obscuring the disclosed implementations. Other implementations and applications also are possible, and as such, the following examples should not be taken as definitive or limiting either in scope or setting. 
     References have been made to accompanying drawings, which form a part of the description and in which are shown, by way of illustration, specific implementations. Although these disclosed implementations are described in sufficient detail to enable one skilled in the art to practice the implementations, it is to be understood that these examples are not limiting, such that other implementations may be used and changes may be made to the disclosed implementations without departing from their spirit and scope. 
     Having described and illustrated the principles of a preferred embodiment, it should be apparent that the embodiments may be modified in arrangement and detail without departing from such principles. Claim is made to all modifications and variation coming within the spirit and scope of the following claims.