Abstract:
A thermal actuator for a rotating shaft shutdown seal that has a piston with a portion of its axial length enclosed within a chamber shell with a material that expands upon a rise in temperature. The portion of the actual length of the piston within the chamber has at least two different diameters with the larger diameter leading in the direction of travel of the piston. Upon a rise in temperature, expansion of the material surrounding the piston within the chamber creates a force on the piston in the desired direction of travel. Below a preselected temperature the piston is positively locked with a passive release when the preselected temperature is reached.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a continuation-in-part application of and claims priority of U.S. patent application Ser. No. 14/867,048, filed Sep. 28, 2015, which application is a divisional application of and claims priority of U.S. patent application Ser. No. 13/970,899, filed Aug. 20, 2013, which application is a continuation-in-part of parent application Ser. No. 13/798,632, filed Mar. 13, 2013, entitled “Pump Seal With Thermal Retracting Actuator,” and claims priority to U.S. Provisional Patent Application Ser. No. 61/862,304, filed Aug. 5, 2013, entitled “Reactor Coolant Pump Shut Down Seal Thermal Retracting Actuator With Thermal Safety Lock.” This application also claims priority of U.S. patent application Ser. No. 14/072,891, filed Nov. 6, 2013, entitled “Pump Seal With Thermal Retracting Actuator.” 
     
    
     BACKGROUND 
       [0002]    1. Field 
         [0003]    This invention pertains generally to rotary shaft seals and, more particularly to a thermally actuated seal for a centrifugal pump and in particular to a new thermal actuator for such a seal. 
         [0004]    2. Related Art 
         [0005]    In pressurized water nuclear power plants a reactor coolant system is used to transport heat from the reactor core to steam generators for the production of steam. The steam is then used to drive a turbine generator for the production of useful work. The reactor coolant system includes a plurality of separate cooling loops, each connected to the reactor core and containing a steam generator and a reactor coolant pump. 
         [0006]    The reactor coolant pump typically is a vertical, single stage, centrifugal pump designed to move large volumes of reactor coolant at high temperatures and pressures, for example, 550° F. (280° C.) and at pressures of approximately 2,250 psia (155 bar). The pump basically includes three general sections from bottom to top; hydraulic, shaft seal and motor sections. The lower hydraulic section includes an impeller mounted on a lower end of the pump shaft which is operable within the pump casing to pump reactor coolant about the respective loop. The upper motor section includes a motor which is coupled to drive the pump shaft. The middle shaft seal section includes three tandem seal assemblies; lower primary (number 1 seal), middle secondary, and upper tertiary seal assemblies. The seal assemblies are located concentric to, and near the top end of, the pump shaft and their combined purpose is to provide for minimal reactor coolant leakage along the pump shaft to the containment atmosphere during normal operating conditions. Representative examples of pump shaft seal assemblies known in the prior art are described in U.S. Pat. Nos. 3,522,948; 3,529,838; 3,632,117; 3,720,222 and 4,275,891. 
         [0007]    The pump shaft seal assemblies which mechanically seal the interface between the stationary pump pressure boundary and the rotating shaft, must be capable of containing the high system pressure (approximately 2,250 psi (155 bar)) without excessive leakage. The tandem arrangement of three seal assemblies is used to break down the pressure in stages. These three mechanical pump seal assemblies are controlled leakage seals which in operation, allow a minimal amount of controlled leakage at each stage while preventing excessive leakage of the reactor coolant from the primary coolant system to the respective seal leakoff ports. 
         [0008]    The pump seal assemblies are normally maintained at temperatures well below those of the primary coolant system, either through the injection of cool fluid at the seal assemblies or through the use of a heat exchanger which cools the primary fluid before it reaches the seal assemblies. Theorized failure of these systems may expose the seal assemblies to high temperatures which will likely cause the controlled leakage of the seal assemblies to increase dramatically. When the cause of the loss of all nuclear fuel cooling in the reactor core is due to losing all AC power, the seal leakoff has no means of returning to the coolant system without electricity to power the makeup pumps. Controlled leakage without the means of makeup could hypothetically lead to reactor coolant uncovering the reactor core and subsequent core damage. 
         [0009]    Consequently, a need exists for an effective way to back up the standard seal assemblies in the event of a coincidental loss of all fuel cooling and loss of makeup pumping. Preferably, such a back up seal should be operable upon loss of power or other cause for the loss of makeup pumping capacity to substantially seal the shaft from leakage. 
       SUMMARY 
       [0010]    The foregoing objectives are achieved, in accordance with this invention, by a thermally actuated shutdown seal for a shaft of reduced speed or stopped rotating equipment such as a pump, compressor or the like, that is designed to restrict the normal leakage of coolant through a shaft seal. The shutdown seal claimed hereafter is useful for sealing any equipment having a narrow flow annulus between its shaft and housing. 
         [0011]    The shutdown seal is characterized by a “split ring” that is designed (i) to surround the shaft with an annulus therebetween during normal operation and (ii) to constrict against the shaft when the shaft slows below a predetermined speed or stops rotating. The split ring has confronting ends that are maintained in spaced relationship by a spacer when the shaft is rotating during normal online operation. When the shaft slows or stops rotating and the temperature in the housing rises, the spacer is removed from the confronting ends of the split ring and the split ring constricts against the shaft as the confronting ends of the split ring approach each other, which blocks a substantial portion of the leakage of coolant through the annulus. 
         [0012]    Preferably, the shutdown seal also has a pliable polymer seal ring which is urged against the shaft by an increase in pressure in the housing when the split ring blocks the leakage of coolant through the annulus. 
         [0013]    In particular, this invention provides such a seal with an improved actuator for removing the spacer from between the confronting ends of the split ring when the liquid in the annulus rises above a preselected temperature so the split ring can constrict to narrow or substantially seal the portion of the annulus covered by the split ring. The actuator includes a cylinder having an axial dimension with a piston axially moveable within the cylinder with the cylinder having an upper and lower end which is sealed around the piston. A piston rod is connected at one end to the piston and at another end to the spacer. A cavity occupies a space within the cylinder between the upper and lower ends, through which space the piston travels. An axial dimension of the piston extends through the space within the cavity when the spacer is disposed between the confronting ends of the split ring. The axial dimension of the piston has at least two separate diameters with a largest of the diameters leading a smaller of the diameters in a direction of travel of the piston to remove the spacer from the confronting ends of the split ring. A material occupies at least a portion of the space within the cavity. The material expands upon an increase in temperature to exert a force on the piston that causes the piston to move in a direction to remove the spacer from between the confronting ends when the material rises above a preselected temperature. Preferably, the force is exerted over an area around a circumference of the piston wherein at least a portion of the at least two diameters of the piston extend. 
         [0014]    In one embodiment, the actuator includes a first seal supported between the cavity and the piston at a lower end of the cavity and a second seal supported between the cavity and the piston at an upper end of the cavity with the first and second seals being operable to confine the material to the cavity. Preferably, the first and second seals are cup seals and are constructed of PEEK. In this embodiment, the actuator may also include backup seals for either or both of the first and second seals. Preferably, the backup seals are O-ring seals and desirably, the O-ring seals are formed from EPDM or HNBR. In another embodiment, the support for the first seal or a support for the second seal is designed to relieve a pressure within the cavity when the pressure exceeds a predetermined value and, desirably, the material is in thermal communication with the liquid. 
         [0015]    In another embodiment the actuator includes a thermally activated safety lock configured to prevent the piston from moving in the cylinder in a direction that will remove the spacer from between the confronting ends of the split ring when the material is below the preselected temperature and free the piston to move and remove the spacer from confronting ends of the split ring when the material rises above the preselected temperature. Preferably, the thermal safety lock is configured to passively unlock the piston when the fluid is above the preselected temperature. In one embodiment the thermally activated safety lock comprises a pin that is suspended from one end of the cylinder and extends in a direction that the piston moves to remove the spacer from the confronting ends of the split ring. The pin extends at least partially within a recess in an end of the piston. A substantial remainder of the recess is substantially filled with a thermally activated material, wherein the thermally activated material has a viscosity at temperatures below the preselected temperature that prevents the thermally activated material from flowing alongside a side of the pin and out of the recess. At temperatures substantially at or above the preselected temperature the thermally activated material has a reduced viscosity that enables it to flow alongside the side of the pin and out of the recess. The resulting displacement of the thermally activated material enables the piston to move in a direction to remove the spacer from between the confronting ends of the split ring. The thermally activated material may, for example be a polymer such as polyethylene. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    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: 
           [0017]      FIG. 1  is a schematic representation of one cooling loop of a conventional nuclear reactor cooling system which includes a steam generator and reactor coolant pump connected in series in a closed loop system with the reactor; 
           [0018]      FIG. 2  is a cutaway perspective view of the shaft seal section of a reactor coolant pump, illustrating in cross section the seal housing and the lower primary, middle secondary, and upper tertiary seal assemblies which are disposed within the seal housing and surround the pump shaft; 
           [0019]      FIG. 3  is an enlarged cross sectional view of a portion of the seal housing and seal assemblies of the reactor coolant pump of  FIG. 2 ; 
           [0020]      FIG. 4  is a sectional view of the shaft seal arrangement showing an enlarged view of the lower primary seal shown in  FIGS. 2 and 3 , to which this invention may be applied; 
           [0021]      FIG. 5  is an enlarged portion of the insert of a primary seal shown in  FIG. 4  with a portion of the pump shaft and the shutdown seal of this invention hatched with the shutdown seal shown as employing a thermally actuated mechanical piston to remove the spacer from the split ring; 
           [0022]      FIG. 6  is an enlarged view of the piston arrangement shown schematically in  FIG. 5  with the piston in the fully extended position with the spacer inserted between the opposing ends of the split ring of the shutdown seal that can benefit from this invention; 
           [0023]      FIG. 7  is a sectional view that shows the piston arrangement of  FIG. 8  employed by the prior art showing the piston in a state before an actuation event in which the spacer is removed from between the opposing ends of the split ring; 
           [0024]      FIG. 8  is a sectional view of an improved actuation mechanism in accordance with this invention which can be applied to remove the spacer of the shutdown seal shown in  FIG. 7 ; 
           [0025]      FIG. 9  is a sectional view that shows the piston arrangement of a second embodiment of this invention; 
           [0026]      FIG. 10  is a cross-sectional view of the embodiment shown in  FIG. 9  taken along the lines A-A thereof; 
           [0027]      FIG. 11  is an enlarged portion of the insert of the primary seal incorporating the shutdown seal embodiment illustrated in  FIGS. 9 and 10 ;  FIG. 12  is a sectional view that shows the piston arrangement of a third embodiment of this invention; 
           [0028]      FIG. 13  is a sectional view of an alternate embodiment for locking the piston prior to thermal actuation of the shutdown mechanism; 
           [0029]      FIG. 14  is an enlarged sectional side view of one embodiment of an actuator that employs the principles claimed hereafter that replaces the actuator shown in  FIG. 5 ; 
           [0030]      FIG. 15  is a longitudinal cross-sectional view of the embodiment shown in  FIG. 14  supported within a seal housing; and 
           [0031]      FIG. 16  is a longitudinal cross-sectional view of another embodiment of the thermally actuated mechanical piston claimed hereafter. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0032]    In the following description, like reference characters designate like or corresponding parts throughout the several view. Also, in the following description, it should be understood that such terms of direction as “forward,” “rearward,” “left,” “right,” “upwardly,” “downwardly,” and the like, are words of convenience and are not to be construed as limiting terms. 
       Prior Art Reactor Cooling Pump 
       [0033]    To understand the invention, it is helpful to understand one environment in which the invention will operate. However, it should be appreciated that the invention has many other applications. Referring to  FIG. 1 , there is shown a schematic representation of one of a plurality of reactor coolant loops  10  of a conventional nuclear reactor coolant system. The coolant loop  10  includes a steam generator  12  and reactor coolant pump  14  connected in series in a closed loop coolant system with the nuclear reactor  16 . The steam generator  12  includes primary heat exchange tubes  18  communicating with inlet and outlet plenums  20 ,  22  of the steam generator  12 . The inlet plenum  20  of the steam generator  12  is connected in flow communication with the outlet of the reactor core  16  for receiving hot coolant therefrom along flow path  24 , commonly referred to as the hot leg of the closed loop system. The outlet plenum  22  of the steam generator  12  is connected in flow communication with an inlet section side of the reactor coolant pump  14  along flow paths  26  of the closed loop system. The outlet pressure side of the reactor coolant pump  14  is connected in flow communication with the inlet of the reactor core  16  for feeding relatively cold coolant thereto along flow path  28  of the cold leg of the closed loop system. 
         [0034]    The coolant pump  14  pumps the coolant under high pressure about the closed loop system. Particularly, hot coolant emanating from the reactor  16  is conducted to the inlet plenum  20  of the steam generator  12  and through the heat exchange tubes  18  in communication therewith. While in the heat exchange tubes  18 , the hot coolant flows in heat exchange relationship with cool feedwater supplied to the steam generator  12  via a conventional means (not shown). The feedwater is heated and portions thereof is changed to steam for use in driving a turbine generator (not shown). The coolant, whose temperature has been reduced by the heat exchange, is then recirculated to the reactor  16  via the coolant pump  14 . 
         [0035]    The reactor coolant pump  14  must be capable of moving large volumes of reactor coolant at high temperatures and pressures about the closed loop system. Although, the temperature of the coolant flowing from the steam generator  12  through the pump  14  as a result of the heat exchange has been cooled substantially below the temperature of the coolant flowing to the steam generator  12  from the reactor  16  before heat exchange, its temperature is still relatively high being typically about 550° F. (288° C.). To maintain the coolant in a liquid state at these relatively high temperatures, the system is pressurized by injection pumps (not shown) and operates at pressures that are approximately 2,250 psia (155 bar). 
         [0036]    As seen in  FIGS. 2 and 3 , the prior art reactor coolant pump  14  generally includes a pump housing  30  which terminates at one end in a seal housing  32 . The pump also includes a pump shaft  34  extending centrally of the pump housing  30  and being sealed and rotatably mounted within the seal housing  32 . Although not shown, the bottom portion of the pump shaft  34  is connected to an impeller, while a top portion thereof is connected to a high horsepower, induction type electric motor. When the motor rotates the shaft  34 , the impeller within the interior  36  of the pump housing  30  causes the pressurized reactor coolant to flow through the reactor coolant system. This pressurized coolant applies an upwardly directed hydrostatic load upon the shaft  34  since the outer portion of the seal housing  32  is surrounded by the ambient atmosphere. 
         [0037]    In order that the pump shaft  34  might rotate freely within the seal housing  32  while maintaining the 2,250 psia (155 bar) pressure boundary between the pump housing interior  36  and the outside of the seal housing  32 , tandemly arranged lower primary, middle secondary and upper tertiary seal assemblies  38 ,  40 ,  42  are provided in the positions illustrated in  FIGS. 2 and 3  about the pump shaft  34  within the seal housing  32 . The lower primary seal  38  which performs most of the pressure sealing (approximately 2,200 psi (152 bar)) is of the noncontacting hydrostatic type, whereas the middle secondary and upper tertiary seal assemblies  40 ,  42  are of the contacting or rubbing mechanical type. 
         [0038]    Each of the seal assemblies  38 ,  40 ,  42  of the pump  14  generally includes a respective annular runner  44 ,  46 ,  48  which is mounted to the pump shaft  34  for rotation therewith and a respective annular seal ring  50 ,  52 ,  54  which is stationarally mounted within the seal housing  32 . The respective runners  44 ,  46 ,  48  and the seal rings  50 ,  52 ,  54  have top and bottom surfaces  56 ,  58 ,  60  and  62 ,  64 ,  66  which face one another. The facing surfaces  56 ,  62  of the runner  44  and seal ring  50  of the lower primary sealing assembly  38  normally do not contact one another but instead a film of fluid normally flows between them. On the other hand, the face surfaces  58 ,  64  and  60 ,  66  of the runners and seal rings  46 ,  52  and  48 ,  54  of the middle secondary and upper tertiary seal assemblies  40  and  42  normally contact or rub against one another. 
         [0039]    Because the primary sealing assembly  38  normally operates in a film-riding mode, some provision must be made for handling cooling fluid which “leaks off” in the annular space between the seal housing  32  and the shaft  34  rotatably mounted thereto. Accordingly, the seal housing  32  includes a primary leakoff port  69 , whereas leakoff ports  71  accommodate coolant fluid leakoff from the secondary and tertiary seal assemblies  40 ,  42 . 
         [0040]      FIG. 4  is a cross section of the seal housing in the area of the number  1  or primary lower seal of the type illustrated in  FIGS. 2 and 3  and provides a better understanding of the operation of the number  1  seal and how it will interface with this invention. The structure shown comprises a housing  32  having annular wall  33  adapted to form a pressure chamber  35  within the housing  32 ; a shaft  34  rotatably mounted within the housing  32 ; a seal runner assembly  44  and a seal ring assembly  50  disposed within the housing  32 . As previously mentioned, the shaft  34  may be driven by a suitable motor (not shown) and utilized to drive the impeller of a centrifugal pump (not shown) which circulates the coolant in the pressurized system. Injection water may be supplied to the chamber  35  at a higher pressure than that developed by the pump. The runner assembly  44  comprises an annular holder  70  and an annular seal plate  72 . Similarly, the seal ring assembly  50  comprises a holder  74  and an annular face plate  76 . 
         [0041]    The holder  70  rotates with the shaft  34  since it is mounted on an annular support  78  which engages a shoulder  80  on the shaft  34  and is secured to the shaft by means of a sleeve  82  which is assembled onto the shaft  34  between the shaft and an upwardly extending leg  84  of the support  78  which is generally L-shaped in cross section. It should be appreciated that although this embodiment of the invention is being described as applied to a pump that employs a sleeve over the pump shaft, the invention can be employed equally as well on pump shafts that do not employ sleeves. A shoulder  86  on the holder  70  rests on the upper end of the leg  84 , and a shoulder  88  on the sleeve  82  retains the holder  70  on the support  84 . A pin  90  is pressed into a recess  92  in the sleeve  82  and engages an axial slot  94  in the holder  70 . An axial clamping force is exerted on the sleeve  82  and the support  78  from a nut (not shown) which causes the sleeve  82  and the support  78  to rotate with the shaft  34 . The pin  90 , in turn, causes the holder  70  to rotate with the sleeve  82  which rotates with the shaft  34 . O-ring seals  96  and  98  are provided between the support  78  and the shaft  34  and the holder  70 , respectively. An O-ring seal  100  is also provided in the interface  102  between the holder  70  and the face plate  72 . 
         [0042]    The face plate  72  is composed of a corrosion and erosion resistant material having substantially the same coefficient of thermal expansion as the material of which the holder  70  is composed, and the holder  70  has a high elastic modulus. Similarly, the face plate  76  is composed of a corrosion and erosion resistant material having substantially the same coefficient of thermal expansion as the material of the holder  74  which has a high elastic modulus. Examples of suitable materials are carbides and ceramics. An O-ring seal  104  is provided in the interface  106  between the holder  74  and the face plate  76 . 
         [0043]    The holder  74  is movably mounted on a downwardly extending leg  108  of an annular seal ring insert  110  which is generally L-shaped in cross section. The insert  110  is retained in the housing  32  by cap screws  112 . An O-ring seal  114  is provided in the interface between the insert  110  and the housing  32 . Similarly, O-ring seal  118  is provided in the interface  120  between the holder  74  and the leg  108  of the insert  110 . Rotative movement of the holder  74  is prevented by the pin  122  which is pressed into the insert  110 . The pin  122  extends into a well  124  in the holder  74  with sufficient clearance between the wall of the well  126  and the pin  122  to permit axial movement of the holder  74  but limit rotative movement of the holder  74 . 
         [0044]    The face plate  76  is attached to the holder  74  by clamping means  128  which includes a retainer ring  130 , a clamp ring  132 , a lock ring  134 , a plurality of cap screws  136  and belleville springs  138  mounted on the cap screw  136  between the lock ring  134  and the clamp ring  132 . The cap screws  136  extend through the retainer ring  130 , the clamp ring  132 , the belleville springs  138  and are threaded into the lock rings  134 . The interface  106  of the holder  74  is recessed at  140  to provide an annular fulcrum  142  on the interface at an outside diameter which is less than the outside diameter of the interface of the face plate  76 . The retainer ring  130  has an inwardly extending flange with a ridge  144  which engages the portion  146  of the face plate  76  extending beyond the fulcrum  142 . The clamp ring  132  has an inwardly extending flange with a ridge  148  which engages a face plate  150  on the holder  74 . Thus, when the cap screws  136  are tightened to draw the clamp  132  and the retainer ring  130  towards each other, a force is produced which exerts a cantilever effect on the face plate  76  about the fulcrum  142 . During the clamping action, the belleville springs  138  are partly compressed and the face plate  76  is deformed by the clamping force. 
         [0045]    The face plate  72  is attached to the holder  70  by a clamping means  151  in a manner similar to that described with reference to the face plate  76 . However, the fulcrum  152  on the interface  102  of the holder  70  is located closer to the outside diameter of the face plate  72  than is the fulcrum  142  on the holder  74 . Thus, the clamping force on the face plate  72  does not produce as much deformation of the face plate about the fulcrum  152  as is produced on the face plate  76 . If desired, the fulcrums  142  and  152  may be placed at the same locations with respect to their corresponding face plates. 
         [0046]    As previously described, the seal ring  50  is mounted for limited axial movement relative the shaft  34  and the seal runner assembly  44 . Also, rotative movement of the seal ring assembly  50  is limited by the anti-rotational pin  122  which fits loosely in the well  124  in the seal ring holder  74 . A seal face  154  on the face plate  76  is biased toward the confronting seal face  156  on the face plate  72  by gravity. 
         [0047]    In operation of the pump driven by the shaft  34 , surfaces  158  and  160  of the seal ring holder  174  are subjected to the full pressure in the high pressure chamber  35 . It is desirable to provide a pressure barrier between the high pressure chamber  35  and an annular low pressure region  162  adjacent the sleeve  82 . The seal ring assembly is utilized as the pressure barrier means, but permits a controlled amount of fluid leakage flow to the region  162  from the pressure chamber  35  through a seal gap  164  provided between the confronting seal surfaces  154  and  156  on the seal plate  76  and  72 , respectively. 
         [0048]    During operation, a balanced or equilibrium position of the axially moveable seal ring assembly  50  is maintained in accordance with the pressure on opposing faces of the seal ring assembly. The thickness of the fluid in the gap  164  and, consequently, the amount of leakage flow through the gap  164  is determined by the configuration of the gap  164 . 
         [0049]    In order to obtain a self-restoration of the relative position of the seal ring assembly  50  and the runner assembly  44  upon a variation in the seal gap  164 , a fluid flow path of decreasing thickness is provided from a high pressure edge or extremity  166  to a position between the seal faced extremities. More specifically, in the structure illustrated, the fluid flow path of decreasing thickness extends between the outer edge  166  and an intermediate concentric circle located at  168  on the sealing face  154 . 
         [0050]    As shown in the present structure, the decreasing flow path thickness is formed by tapering the surface  154  slightly away from the confronting surface  156  of the face plate  72  between the circle  168  and the outer edge  166  of the face plate  76 . The angle between the surfaces  154  and  156  shown in the drawing is exaggerated. This configuration or structure is known as a tapered-face seal. The operation of a seal of this type is fully described in U.S. Pat. No. 3,347,552, issued Oct. 17, 1967 to Erling Frisch. 
         [0051]    The current shutdown seal is fully described in U.S. Pat. No. 8,356,972, issued Jan. 22, 2013 and assigned to the Assignee of this invention. The shutdown seal, described in that patent is illustrated in  FIGS. 5-7  and provides an additional seal  170  in the pump  14  as a backup safety or shutdown device which is actuatable to prevent excessive leakage along the shaft  34  between it and the seal assemblies  38 ,  40 ,  42  of the pump in the event of a loss of seal cooling. As shown in  FIG. 5 , the shutdown seal  170  is situated in a machined groove in the annular opening in the insert  110  of the primary number 1 seal  38 . The shutdown seal is characterized by a “split ring”  172  that is designed (i) to surround the shaft  34  with an annulus  174  therebetween during normal operation and (ii) to constrict against the shaft  34  when the shaft significantly slows or stops rotating after a loss of seal cooling. The split ring  172  is a single piece discontinuous ring member that is split axially and the confronting ends are maintained in a spaced relationship by a spacer  176  during normal pump operation. In  FIG. 5 , the opposing ends of the split ring  172  are machined in a tongue-and-groove configuration so that the tongue can ride in the groove as the ends of the split ring overlap. In another embodiment, the opposing ends may be butt ended or have a mitered half lap joint so the ends overlap. The spacer  176  is shown in the gap to keep the opposing ends of the split ring  172  from closing on the shaft  34  to maintain the annulus  174  opened for controlled leakage during operation. In accordance with the embodiment illustrated in  FIG. 5 , the shutdown seal is activated when the temperature of the seal rises as a result of a loss of cooling and preferably rotation as the pump shaft is slowed or stopped. The spacer is responsive to the rise in temperature (either because the shaft has significantly slowed or stopped rotating or for any other reason) to be removed from the confronting ends of the split ring  172 . This causes the confronting ends of the split ring to constrict against the shaft  34  as the confronting ends of the split ring approach each other, which blocks the leakage of coolant through the flow annulus  174 . Preferably, the split ring and shaft (or shaft sleeve where a sleeve is employed over the shaft) are constructed from gall resistant materials, so that if actuated on a rotating shaft gall balls will not be created which would otherwise serve as a wedge to open a leak path between the sealing surfaces. Materials such as 17-4 stainless for both the split ring and the shaft have proven to work well. A pliable polymer seal ring  178  is preferably situated around the shaft  34  against the split ring  172  between the split ring and a solid retaining seat ring  180 . The pliable polymer seal ring  178  is urged against the shaft by an increase in pressure in the housing when the split ring restricts the leakage of coolant through the annulus  174 , thus forming a tight seal. 
         [0052]      FIG. 5  schematically depicts a shutdown seal  170  of the type described above installed in the reactor coolant pump of  FIG. 4 . The shutdown seal of  FIG. 5  is designed to activate after a loss of seal cooling when the pump shaft  34  slows or is not rotating. The shutdown seal is located within the pump housing, encircling the shaft  34 . In the case of the type of reactor coolant pump illustrated in  FIGS. 2-4 , the number 1 seal insert may be modified to accommodate the shutdown seal by machining out a portion of the inner diameter at the top flange. Until activated, the shutdown seal  170  is substantially completely contained within the space once taken up by the number 1 insert prior to modification, thus substantially unaltering the annulus  174  between it and the shaft  34 . In this way, coolant flow through the annulus  174  along the shaft  34  is not substantially impeded during normal operation of the rotating equipment. 
         [0053]      FIG. 5  shows a shutdown seal  170  made up of a retractable spacer  176  holding the confronting ends of the split ring  172  open. The retractable spacer  172  is activated by a thermally responsive mechanical device  184 , such as the piston  186  described hereafter with regard to  FIG. 6 . When the spacer  176  is retracted from the ends of the split ring  172 , the split ring  172  snaps shut, constricting around the shaft  34 , while also remaining retained in the modified number  1  seal insert  110 . The split ring  172  sits on a wave spring  182  that forces the split ring  172  up against the seal  178  which pushes against the retaining ring  180 . The pressure drop caused by the interruption of the flow through the annulus  174  also forces the split  172  and seal ring  178  upwards, ensuring a tight seal between all of the sealing surfaces. The split ring  172  sits on a wave spring  182  that forces the split ring  172  up against the primary sealing ring  178  to ensure an initial sealing contact so the pressure drop across the split ring  172  is also acting on the primary sealing ring  178 . 
         [0054]      FIGS. 6 and 7  depict the spacer  176  and actuator assembly  184  before an actuation event. The actuator  184  as shown in  FIGS. 6 and 7 , is comprised of a canned piston  186  for restraining a spring loaded spacer  176 . Within the can is a wax  188  that changes phase at the desired activation temperature, e.g., 280° F. (138° C.) for reactor coolant pumps, as further explained herein. This change in phase results in a substantial increase in volume of the wax  188 . For example, a wax such as Octacosane will increase about 17% in volume. When the wax  188  changes phase and expands, it pushes a piston head  190  away from the pump shaft  34 . When the piston head  190  moves, balls  192 , that were once held in place by the piston  190 , will drop out of the way and allow a compressed spring  194  to expand which pushes back the plunger  196  that is connected to the spacer  176 . As the spring  194  expands it pushes the plunger which pulls the spacer  176  with it, thus retracting the spacer  176  from between the split ring ends. 
         [0055]    Thus, thermal activation is achieved as follows: As temperature rises, the wax  188  changes state and expands. Two HNBR (Hydrogenated Nitrile Butadiene Rubber) O-ring seals  198  are used to contain the wax with the upper O-ring providing a sliding interface for the cam  190 . Expansion of the wax translates cam  190  permitting ball bearings in the race  192  to disengage plunger  196  from the housing  200 . With the ball bearings disengaged, compression spring  194  translates plunger  196  upward along with spacer  176  thus releasing the piston ring and activating the shutdown seal. 
       Improved Pump Shutdown Seal Actuator 
       [0056]      FIG. 8  shows an improved thermal retracting actuator. As described earlier, translation of the spacer  176  permits closure of a split ring enabling activation of the shutdown seal. When the temperature rises and reaches the phase transition point of the wax  188 , the wax volume may increase up to approximately 17%. If the volume is held constant, the wax pressure will increase and can exceed 10,000 pounds per square inch (68,947.6 kpa). Piston  196  has a larger diameter D 1  and a smaller diameter D 2  with corresponding cross sectional areas A 1  and A 2 . As the pressure (P) increases, a translational force (F) is applied to the piston  196  equal to the product of wax pressure and the difference in cross sectional areas, i.e., F=P×(A 1 −A 2 ). With such an arrangement shown in  FIG. 8 , a typical piston force may be in the range of 50 to 100 pounds (22.7-45.4 kg) while achieving adequate piston travel to remove the spacer  176  from the split ring. This is a significant increase over the approximate force of 15 pounds (6.8 kg) available from the compression spring  194  shown in  FIG. 7 . Cup seals  204  and  206  provide a pressure boundary for containment of the wax  188 . They may be constructed from PEEK (polyetheretherketone) and have sufficient strength to contain the wax  188  at high pressure and are chemically compatible with both the wax and the surrounding reactor coolant. O-ring seals  208 ,  210  and  212  are made of EPDM (ethylene-propylene diene M-class rubber) or HNBR which are compatible with the reactor coolant. In the event the PEEK seal should fail during activation, the EPDM or HNBR seal can act as a redundant pressure boundary. The EPDM seal can withstand short term exposure to the wax. End cap  214  is free to slide within the housing  216  and is secured in place with multiple shear pins  218 . In the event that the piston  196  travels full stroke and the wax pressure continues to rise, pins  218  shear to release end cap  214  permitting seal  206  to decouple from the housing  216  thereby releasing excess wax volume and reducing the pressure to a safe state. 
         [0057]    Since the entire retracting assembly  202  can be subjected to higher than atmospheric pressure, several radial openings  220  are oriented about the upper flange of piston  196 . Without the radial openings  220  it may be possible that the head of the piston  196  could seal against the mating end cap  214 . The external pressure (without radial openings present) could induce an undesirable axial force to the piston  196 . 
         [0058]    While it may not be required, sleeve  222  is placed over the exposed diameter of piston  196  to maintain the piston free from contaminants which may be present in the surrounding environment. The sleeve may be constructed of polypropylene which may melt when the activation temperature is reached. Alternately, a small wiper may be placed in the end of housing  216  to remove unwanted debris during translation of the piston. 
         [0059]      FIG. 9  shows an alternative embodiment of the invention with a different configuration. Alternative end cap  214  is secured in alternative housing with a spiral retaining ring  224 . Spring  226  provides a small force to maintain the piston  196  in the extended position prior to actuation. 
         [0060]    Housing  216  contains at least two pockets  228  where the wall thickness of the chamber containing the wax is reduced in thickness T 1 . In the event that the piston  196  travels full stroke and the wax  188  pressure continues to rise, the housing wall can bulge at the pockets  228  thereby releasing excess wax volume and reducing the pressure to a safe state. Section A-A of  FIG. 9  is shown in  FIG. 10  and illustrates a cross-section of the housing  216  at the location of the pockets  228 . The thinner wall (T 1 ) can bulge when the wax pressure becomes excessive. Heavier wall (T 2 ) helps maintain structural integrity of the housing  216 . 
         [0061]    Another configuration is to have a housing where the thin wall section (T 1 ) is continuous for 360°. Since the actuator will have performed its function at the time when the wax pressure can become excessive, structural integrity from a thicker section (T 2 ) is not necessary. 
         [0062]      FIG. 11  shows a cross-section of the alternative retracting actuator as applied to the insert of the primary seal shutdown seal. 
         [0063]    While the foregoing embodiment has a spring to prevent inadvertent movement of the piston, it is highly desirable to provide a more robust mechanism as inadvertent actuation would be extremely costly due to shutdown of the power plant.  FIG. 12  illustrates such a robust mechanism. 
         [0064]      FIG. 12  shows a third embodiment of the retracting actuator used in the shut down seal. Like reference characters are used among the several figures to designate corresponding components. Thermal activation is achieved when temperature rises causing the wax  188  to change state and expand which gives rise to pressure, as explained above. As the wax pressure increases, a translational force is applied to the piston  196  equal to the product of wax pressure and the difference in cross sectional areas of the piston. Without pin  232  in place, piston  196  translates spacer  172  towards the housing  200 . Removal of spacer  176  permits closure of a split piston ring  172  enabling operation of the reactor coolant pump shaft shutdown seal. 
         [0065]    A typical piston force may be in the range of 50 to 100 lbs while achieving adequate piston travel to remove the spacer  176  from the split ring  172 . O-ring  204  and cup seal  206  provide a pressure boundary for containment of the wax  188 . O-ring seals  210  and  212  act as a redundant pressure boundary for the wax and provide isolation from the surrounding fluid from entering the wax chamber. Wiper  208  in conjunction with cup seal  206  are semi-rigid and act as dual bushings keeping piston  196  centered within housing  200 . The primary function of wiper  208  is to exclude foreign material from entering into the housing  200 . The wiper is also a seal to minimize surrounding fluid from entering new O-ring  204 . 
         [0066]    In one embodiment, in order to keep piston  196  from translation prior to thermal activation, plug  230  encapsulated in the piston  196  is sufficiently rigid to hold metal pin  232  in place. As can be seen in  FIG. 12 , plug  230  has a thin wall at location  234  that provides a flexible boundary which allows for a slight press fit between the outside diameter of pin  232  and the inside diameter of a recess  236  in piston  196 . The press fit minimizes any motion between the piston  196  and the pin  232 . The preferred material for the plug  230  is a polymer such as polyethylene. It is both flexible which makes a good press fit and has a melt temperature very close to the phase change temperature of the wax. Polyethylene is also compatible with the surrounding reactor coolant pump environment. To avoid potential creep, it is important that plug  230  is encapsulated. If a load is applied to pin  232  in the direction of the plug  230 , without the plug being contained by the piston  196 , the increased force on the plug  230  would cause the plug to creep radially outward and/or buckle. As configured, with an increasing load on the plug  230 , the plug would have to extrude along the thin boundary  234  in order to escape. Due to the nature of the polymer bonds, extrusion through the small gap between the pin  232  and the piston  196  is extremely difficult until a higher temperature is obtained. The holding force from the pin/plug combination can easily exceed 100 lbs prior to activation. 
         [0067]    During thermal activation, above normal operating temperature, plug  230  becomes soft before the wax begins to change state. As the temperature further increases, the plug can reach melt temperature either prior to or as the piston begins to move. As the plug  230  melts, the plug material becomes viscous and freely flows around pin  232  and subsequently permits translation of pin  232  within the piston recess  236  such that the piston is free to activate the shut down seal. 
         [0068]      FIG. 13  shows an alternate embodiment for keeping the piston  196  from translation prior to thermal activation, To prevent the piston  196  from moving prior to actuation, pin  232  is slidably fixed within bore or recess  236 . The leafs  244  between slots  238  in pin  232  are elastically biased outward in the radial direction (like a leaf spring) to keep the outside diameter of the pin  232  in intimate contact with the inside diameter of the piston bore  196  at the interface  240 . Shear pin projections  242 , that extend radially outward from the pin  232  prevent the pin from actuating until the piston  196  exerts sufficient force to shear or break off the projections  242 , thereby permitting full travel of the piston  196 . 
         [0069]    Thus, this improved actuator has a simplified thermal retracting design that has a higher output force and fewer components than the previous design described above. The previous design of the actuator uses HNBR O-rings with the life expectancy which may be less than the desired twelve years of operation. The seal arrangement in the preferred design uses long life EPDM O-rings and PEEK seals to provide separate and redundant boundaries for the thermal retracting actuator components. 
         [0070]    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.