Patent Publication Number: US-2022223306-A1

Title: Control rod position indication system for a nuclear reactor

Description:
The present disclosure relates to a control rod position indication system for a nuclear reactor and to an according nuclear reactor. 
     BACKGROUND 
     A nuclear reactor comprises a reactor core whose reactivity can be controlled, amongst others, by a number of control rods. Typically, the according control rod is movable along a linear path of movement into the reactor core. A reliable and precise assessment or measurement of the current control rod position is of paramount importance for safe reactor operation. This is a difficult task since a control rod is usually enclosed by a pressure housing, so that indirect or contactless sensors are required. Usually, there is only limited installation space for the sensors. Furthermore, physical working conditions with respect to, for example, temperature and radiation are demanding. These conditions may also vary considerably in time. 
     Besides Rod Position Indication (RPI) systems based on inductive sensors there are according systems based on reed switches. A reed switch is an electrical switch operated by an applied magnetic field. Typically, a reed switch has a number of reed contacts which are aligned along a centerline or longitudinal axis. In the context of an RPI system, there is a permanent magnet with a magnetic north-south axis mounted on the control rod or a corresponding drive rod connected to the control rod. A number of reed switches are arranged around the path of movement in order to be switched by the magnetic field generated by the permanent magnet when passing by, thereby allowing determination of the control rod position. 
     U.S. Pat. No. 4,064,451 A discloses a position indication system according to the preamble of claim  1 . Here, the reed switches S 1  to S 72  are arranged perpendicular to the north-south axis of the permanent magnet  10  ( FIG. 1 ). 
     DE 25 21 340 A1 shows a similar arrangement, wherein the reed switches  6  are arranged in parallel to the north-south axis of the permanent magnet  5  ( FIG. 2 ). 
     SUMMARY 
     A problem occurring with these systems is that for certain positions of the control rod no reliable position indication can be derived, since in a series of subsequent measurements for the same position—but possibly with varying physical conditions as mentioned above—the according reed switch can be either be found to be in an open or a closed state. 
     It is an objective of the present disclosure to provide a control rod position indication system of the above-mentioned kind with improved reliability and/or detection accuracy. In particular, position ranges wherein the corresponding reed switch is neither surely open nor surely closed shall be as small as possible. 
     Correspondingly, a key element of the present disclosure is that the longitudinal axis of at least one of the reed switches is inclined or slanted or tilted or skewed or oblique relative to the north-south axis of the permanent magnet, wherein the angle of inclination has an absolute value which is within a range from 15 to 65 degrees. 
     Surprisingly, it has been found by numerical simulations that an arrangement according to the present disclosure improves the detection accuracy of the position indication system. In particular, for a tilted reed switch the position region of the assigned control rod with undefined switching status is considerably smaller than for a non-tilted reed switch. This is because the tilted orientation takes into account interfering magnetic fields (like stray fields) from other sources than the permanent magnet. This has been confirmed by experimental measurements. 
     In a preferred embodiment, the absolute value of the angle of inclination is within a range around a preferred value of 35 degrees, in particular in the range from 30 to 40 degrees. 
     In another preferred embodiment, the north-south axis of the permanent magnet is arranged in parallel to the path of movement, which means that the longitudinal axis of the according reed switch is tilted relative to the axis of the control rod or drive rod. 
     In one expedient configuration, all the reed switches belonging to the same control rod or drive rod are inclined in the same manner. However, in other configurations it may be advantageous to adjust the angle of inclination individually for each of the reed switches. This allows fine-tuning of the detection accuracy depending on the local point of installation. 
     In yet another preferred embodiment, a plurality of reed switches are arranged at the same axial position of the control rod or drive rod to provide redundant information. In contrast to RPI systems based on inductive sensors this is possible due to the compact size of the reed switches. 
     In a preferred application, there is nuclear reactor comprising a reactor core and at least one control rod which is movable along a linear path of movement for controlling the reactivity of the reactor core, and further comprising a control rod position indication system of the above-described kind. For example, the nuclear reactor may be a pressurized water reactor, in particular of the type ‘European Pressurized Water Reactor’ (EPR). 
    
    
     
       BRIEF SUMMARY OF THE DRAWINGS 
       Exemplary embodiments of the present disclosure and related advantages are subsequently described with reference to the accompanying drawings. 
         FIG. 1  shows a sensor arrangement of a control rod position indication system within a nuclear reactor. 
         FIG. 2  shows a corresponding circuit diagram for the sensor arrangement shown in  FIG. 1 . 
         FIG. 3  shows a diagram illustrating a domain of uncertainty for position indications provided by the control rod position indication system of  FIG. 1  at a given set of operating parameters. 
         FIG. 4  shows a similar diagram for a different set of operating parameters. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  gives a schematic overview of relevant elements of a control rod position indication system  2  within a nuclear reactor  4 . A control rod  6 , only partially shown here, is provided for controlling the reactivity of the reactor core  8  which is indicated purely schematically in the drawing. To this end, the control rod  6  is movable back and forth along a linear path of movement, along its axis  10 , between a retracted position and an extended position reaching at least partially into the reactor core  8 . The control rod  6  is firmly connected or coupled to a drive rod  12  which effectively forms an extension of the control rod  6  along the same axis  10 . The combination or unit of control rod  6  and drive rod  12  may also be regarded as a single rod which is movable in axial direction (along the direction of the arrow  30 ). Such movement is effected by a corresponding driving motor (not shown here) acting on the drive rod  12 . The control rod  6  and the drive rod  12  are enclosed by a sealed pressure housing  14  which is magnetically permeable. In particular, the drive rod  12  is enclosed by a cylindrical drive rod housing. In the shown example of a pressurized water reactor, the control rod  6  is arranged to be lowered into the reactor core below  8 . Therefore, the drive rod  12  is above the control rod  6 , and both are aligned along the same vertical axis  10 . In other words, the path of movement is aligned vertically. However, such details may vary with different reactor types and are not integral to the working of the present disclosure. 
     To provide a reliable position indication for the current position of the control rod  6 , there is a control rod position indication  2  system based on reed switches. 
     The control rod position indication system  2  comprises a permanent magnet  16  mounted on the drive rod  12  (or, less preferred, on the control rod  6 ). Hence, the permanent magnet  16  moves together with the unit made of drive rod  12  and control rod  6  along the linear path of movement within the enclosure formed by the pressure housing  14 . The permanent magnet  16  is preferably a strong Samarium-Cobalt (e.g. Sm 2 Co 17 ) magnet or made of similar rare earth materials. The permanent magnet  16  has a magnetic north pole N and a magnetic south pole S aligned along a magnetic north-south axis  18 . Due to the rigid connection of the permanent magnet  16  to the drive rod  12 , the orientation of the north-south axis  18  is constant along its path of movement. 
     Furthermore, there is a number of reed sensors or reed switches  20  arranged around the path of movement in order to be switched by the magnetic field generated by the permanent magnet  16  when passing by. The respective reed switch  20  has a number of reed contacts  22  which are essentially aligned along a longitudinal axis  24  or reed axis. The reed switches  20  are arranged outside the pressure housing  14  with some lateral distance to it. There is preferably a multitude of reed switches  20  spread evenly across the maximum travel distance of the permanent magnet  16 , the reed switches  20  preferably being located along an installation line  26  or installation axis which is parallel to the rod axis  10 , thereby forming a reed chain. Such a mounting is achieved, for example, by virtue of a suitable fitting panel  28  or fitting tube. This way, a discretized position indication can be obtained on the basis of the sensed position of the permanent magnet  16 , as can be concluded from the exemplary circuit diagram of  FIG. 2 . 
     For example, the reed switches  20  are of the type ‘normally open’ and get closed only under the magnetic influence of the nearby permanent magnet  16 . Hence, a situation like in  FIG. 2  may occur, wherein two adjacent reed switches  20  of the reed chain are closed while the rest is in an open state. This may be detected, for example, by placing the reed switches  20  within the output lines of an electric resistor voltage divider circuit (with individual resistors  32 ), such that depending on the individual switching states a first resistance circuit (with total resistance R 1 ) and/or a second resistance circuit (with total resistance R 2 ) are formed. For resistance measurement a potentiometric measurement device may be used. Therefore, the measured resistance of the resistance circuit(s) provides an indication of the position of the permanent magnet  16 , and thus of the control rod  6 . The necessary calculations are performed in a corresponding analysis unit and the results are displayed on a corresponding display not shown here. 
     In practice, however, a few complications may arise which may render the measured sensor signals and thus the position indication dubious. There can be situations, wherein for a given position of the permanent magnet  16  the corresponding reed switches  20  are neither surely open nor surely closed, in particular under the influence of varying operating conditions such as temperature and/or radiation. 
     For example, for a given configuration and a given rod position reed switch positions no. 1, 2, 3, 8 (encircled numbers in  FIG. 1 ) correspond to reed switches  20  at such a distance from the permanent magnet  16  that the switches cannot be closed. Reed switch positions no. 5, 6 correspond to reed switches  20  at such a distance from the permanent magnet  16  that the switches are closed for sure. Reed switch positions no. 4, 7 correspond to reed switches  20  at such a distance from the permanent magnet  16  that the switches may be closed, but not for sure. 
     This makes the design of the control rod position indication system  2  and the specification of according design parameters non-trivial. 
     Firstly, there are physical design parameters such as:
         field strength of the permanent magnet  16     response threshold (=pull-in value) of the reed switches  20     hysteresis of the reed switches  20     manufacturing tolerance of the reed switches  20     temperature dependence of above parameters   influence of the periphery: in particular neighboring control rod drives and according magnets or magnetized components       

     Secondly, there are geometric parameters such as (see  FIG. 1 ):
         distance or height B between two adjacent reed switches  20     distance R between the reed switches  20  and the permanent magnet  16     tilting of the reed-switches  20         

     Surprisingly, it was found that the tilting of the reed-switches  20  has a significant impact on the operation and reliability of the control rod position indication system  2 . During simulations and experiments it was concluded that position ranges wherein the corresponding reed switch  20  is neither surely open nor surely closed are minimized by choosing its longitudinal axis  24  to be inclined relative to the north-south axis  18  of the permanent magnet  16 . In general, good results are obtained when the absolute value of the angle of inclination W is within a range from 15 to 65 degrees. A preferred sub-range lies within 30 to 40 degrees. In particular, the permanent magnet  16  is preferably mounted such that the magnetic north-south axis  18  is aligned vertically (i.e. parallel to the axis  10  of the drive rod  12 ), and the respective reed switch  20  is tilted against the vertical direction with an angle of inclination W of said size, as indicated in  FIG. 2 . A reversed tilting corresponding to negative W values shows equally strong influence on the open/close state of the reed switches  20  and may also be employed, just like a tilting in positive W direction. Preferably, the tilting is such that that the longitudinal axis  24  of the reed switch  20  remains within the plane that contains the north-south axis  18  of the permanent magnet  16 . That is, secondary tilting or twisting in other directions is preferably avoided. 
     As can be seen by the diagrams shown in  FIG. 3  and  FIG. 4 , for a tilted reed switch) (W=−35°) the position region with undefined (value zero) switching status is considerably smaller than for a non-tilted reed switch (W=0°. In these diagrams the x-coordinate (MeasuredPos 1-35) represents the vertical position (similar to the encircled position numbers in  FIG. 1 ) of the respective reed switch  20  in the reed chain, whereas the y-coordinate (RodPos 1-21) represents the rod position, specified as multiples of an elementary step (e.g. 3 cm), relative to a given home position (e.g. deepest position). The switching state (surely open, undefined, surely closed) of the reed switch is indicated by values (−1,0,1) and according textures. This is because the tilted orientation effectively takes into account radial and axial components of interfering magnetic fields (like stray fields) from other sources than the permanent magnet. This has been confirmed by experimental measurements. 
     In summary, by tilling the reed switches  20  in the above-described manner the domain of uncertainty can be considerably reduced, and thus the position indication 2 is made more reliable and accurate. 
     LIST OF REFERENCE NUMERALS 
       2  control rod position indication system
 
 4  nuclear reactor
 
 6  control rod
 
 8  reactor core
 
 10  rod axis
 
 12  drive rod
 
 14  pressure housing
 
 16  permanent magnet
 
 18  north-south axis
 
 20  reed switch
 
 22  reed contact
 
 24  longitudinal axis
 
 26  installation line
 
 28  fitting panel
 
 30  arrow
 
N north pole
 
S south pole
 
B distance
 
R distance
 
W angle of inclination