Control rod position indication system for a nuclear reactor

The invention relates a to control rod position indication system for a nuclear reac tor and to an according nuclear reactor.

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 para mount 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 sen sors are required. Usually, there is only limited installation space for the sensors. Furthermore, physical working conditions with respect to, for example, tempera ture 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 num ber 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 perma nent magnet when passing by, thereby allowing determination of the control rod position.

US 4 064 451 A discloses a position indication system according to the preamble of claim 1 . Here, the reed switches S1 to S72 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).

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 condi tions 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 invention to provide a control rod position indication system of the above-mentioned kind with improved reliability and/or detection ac curacy. In particular, position ranges wherein the corresponding reed switch is nei ther surely open nor surely closed shall be as small as possible.

According to the invention, this objective is achieved by a control rod position indi cation system with the features of claim 1.

Correspondingly, a key element of the invention 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 in clination 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 ac cording to the invention 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 interfer ing magnetic fields (like stray fields) from other sources than the permanent mag net. 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 infor mation. 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 control ling 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 Pressur ized Water Reactor’ (EPR).

Exemplary embodiments of the invention and related advantages are subsequent ly 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.

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 indicat ed 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 re tracted 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. Flowever, such details may vary with different reac tor types and are not integral to the working of the invention.

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 con trol 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. S1T12C017) magnet or made of similar rare earth materials. The perma nent 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 perma nent 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 gener ated 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 longitudi nal 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 con cluded 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 re sistor voltage divider circuit (with individual resistors 32), such that depending on the individual switching states a first resistance circuit (with total resistance R1 ) and/or a second resistance circuit (with total resistance R2) are formed. For re- sistance measurement a potentiometric measurement device may be used. There fore, 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 nec essary 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 meas ured sensor signals and thus the position indication dubious. There can be situa tions, 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 posi tions 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 sys tem 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 ob tained 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 an gle of inclination W of said size, as indicated in FIG. 2. A reversed tilting corre sponding 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 posi tive 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 direc tions 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, sure ly 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 indica tion 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

5 south pole

B distance

R distance

W angle of inclination