Electricity: measuring and testing – Magnetic – Displacement
Reexamination Certificate
1998-10-06
2002-04-30
Carone, Michael J. (Department: 3641)
Electricity: measuring and testing
Magnetic
Displacement
C324S207130, C324S207220, C322S031000, C322S032000, C073S313000, C073S314000, C376S228000, C376S258000
Reexamination Certificate
active
06380734
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to an apparatus for verifying the position of a movable member. More particularly, the present invention is directed to a control element assembly (CEA) position indicator that includes a CEA position verification apparatus.
BACKGROUND OF THE INVENTION
Nuclear power plants typically include independent shut-down and safe-operations systems that monitor plant operation and evaluate numerous safety-related parameters. In the event one or more measured parameters indicate the existence of an unsafe condition, the shut-down system designed to mitigate the effects of an anticipated transient condition and/or the safe-operation can automatically effect the appropriate remedial action. It is imperative that these safety control systems, known as plant protection systems, operate reliably, and accordingly, it is imperative that all measured and sensed parameters be valid.
In the context of nuclear plant protection systems, it is not uncommon to measure a multitude of parameters related to plant operation. These parameters include, for example, temperatures, pressures, flow rates, power density, neutron flux, fluid levels, etc. Other functions of the plant protection system include the status-monitoring of various components including valves, pumps, motors, control devices and generators.
Additionally, the plant protection system, under certain defined conditions, may initiate a reactor trip (RT), i.e., the rapid, controlled, and safe shut-down of the reactor by actuating various field systems and remote actuation devices. In the case of a pressurized light water reactor, the shut-down is often accomplished by the dropping of moderating control rods into the reactor core to cause the reactor to become sub-critical.
The rod assemblies are comprised of four or twelve rods joined by a spiderlike connecting flange. Each rod assembly is commonly referred to as a control element assembly (CEA). The CEAs are arranged in groups or sub-groups consisting of a minimum of four CEAs. The connecting flange couples the CEA to a control element assembly drive mechanism that controls the movement of the CEA in and out of the reactor core. The control element assembly drive mechanism normally includes a position indicator that senses location of the CEA. As the CEA controls the overall reactor power level and provides the principal means of quickly and safely shutting down the reactor, sensing and monitoring the CEA's position in a nuclear reactor is imperative.
There exist well known systems for sensing and monitoring CEA position. In U.S. Pat. No. 3,656,074, which is assigned to the Assignee of the present invention and incorporated herein by reference, there is described a CEA position sensing apparatus
10
. As shown in
FIG. 1
, the apparatus includes a CEA represented by a single control rod located within a control rod housing and a control rod drive motor; a permanent magnet physically positioned on the CEA drive shaft; and a position transmitter including a plurality of flux responsive devices and a voltage divider network.
As the CEA illustrated in
FIG. 1
is represented as a single control rod, only one control rod housing
12
is shown extending upwardly from the top of the reactor
11
. The control rod housing
12
will typically be a nonmagnetic stainless steel tube that is approximately five inches in diameter having a one inch thick wall. The control rod drive shaft is situated within housing
12
, and the control rod itself extends into the main portion of reactor
11
.
A control rod drive motor
14
is mounted above a cap which seals the upper end of housing
12
. Through appropriate gearing, the control rod drive motor
14
causes the control rod drive shaft to move axially relative to housing
12
, thus adjusting the position of the CEA relative to the reactor core. The control rod drive shaft of motor
14
communicates with the interior of housing
12
through specially designed seals that are available commercially.
Permanent magnet
16
is mounted on the control rod drive shaft. The magnet
16
must be constructed of material capable of withstanding the highly corrosive conditions of the environment to which it will be exposed, as the magnet will be located inside a nuclear reactor. Further, the magnet
16
must be sufficiently strong so that its flux field will bridge the thick stainless steel walls of the housing
12
.
As described below, a portion of the position sensing apparatus
10
is mounted on the exterior of the housing
12
and is contained within a separate transmitter housing
18
. The housing
18
comprises an elongated hollow member, generally of tubular form, comprised of a nonmagnetic material which preferably has a high coefficient of thermal conductivity. The upper end of housing
18
is adapted to receive an electrical connector
20
.
The connector
20
provides a means for connecting the position transmitter circuitry to external circuitry in a manner which permits removal of the connector
20
from the housing
18
. The connector
20
also electrically couples the output of the position sensing apparatus
10
to a position indicator
22
.
FIGS.
2
(
a
) and
2
(
b
) respectively show top and side views of the position transmitter circuitry located within the housing
18
through cut-away portions of said housing. Shown within the housing
18
is a terminal strip
24
which is mounted to a reed switch position transmitter
26
including a plurality of magnetic flux responsive switches and the components of an incremental potentiometer. The flux responsive devices are shown as reed switches
28
and
28
′, and the components of the incremental potentiometer are shown as resistors
30
. The reed switches
28
,
28
′ and the resistors
30
, are mounted to terminal strip
24
and electrically interconnected by means of standoff and feed through connectors
32
. The reed switches
28
,
28
′ are spaced on the terminal strip
24
at uniform incremental distances small enough to insure that at least one of the switches will be actuated from any location of the magnet.
In the preferred embodiment, the reed switches are wired in pairs as designated in
FIG. 3
by the reference numerals
28
,
28
′. The reed switches
28
,
28
′ have a length of approximately one inch, and are disposed in serial axial alignment parallel to the path of magnet
16
. The reed switches
28
,
28
′ forming the reed switch pairs are spaced apart approximately one inch to provide an arrangement wherein a small overlap of switch actuation will occur, thus reducing the possibility of a CEA position at which no switch would be actuated. As the control rod drive shaft travels axially within control rod housing
12
, switches
28
,
28
′ will be sequentially closed at the approach of the field of magnet
16
and opened after the magnetic field passes. The switches are arranged such that serially adjacent switches will be closed as the magnet
16
is intermediate the two switches.
With regard to the electrical portion of the position indicating apparatus
10
,
FIG. 3
shows a plurality of resistors
30
of the same size and type connected at end points
33
and
34
across the power supply
38
(power supply
38
shown in FIG.
1
). The resistors
30
form an incremental potentiometer or voltage divider. As discussed above, reed switches
28
′ are electrically connected in series with each of switches
28
, and are positioned in substantially the same locations as switches
28
. Each of reed switches
28
is connected to a different point or tab on the voltage divider comprising resistors
30
. All of the circuits comprising the series connected switches
28
,
28
′ are connected to a signal bus bar having a terminal point
20
. Thus, upon closing of one of the switches
28
and its serially connected back up switch
28
′, a signal from the incremental potentiometer comprising resistors
30
will be applied to bus bar terminal
20
. The amplitude of this signal indicates
Chari Deva R.
Sirica Edward G.
Carone Michael J.
Richardson John
Westinghouse Electric Company LLC
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