Rod Group and Individual Control System - Nuclear Power Plant Instrumentation and Control Systems for Safety and Security - page 338

coil block is inside of the moving bar, to which
fastens control rod.
Magnetic shunt in form of magnetic soft steel
pipe is embedded inside the bar (in its upper part)
and covers sensor case along with coil block.
Shunt length is a bit more than half of the core
height. Coils and shunt are placed to each other
in such way that when CR is positioned on lower
hard support shunt covers (“overlaps”) one of
the base coils (lower one), and when lift in lower
limit switch position - two. Then the quantity of
coils covered by shunt increases by one while
upwards movement of CR on each 350 mm. In
CR position in the middle of lower and upper limit
switch shunt covers all seven main coils. Further
lift leads to decrease by one of quantity of coils
covered by shunt while upwards movement of CR
on each 350 mm up to upper limit switch position
in, which shunt does not cover any of the coils.
Cover protects coil outputs and places of sol-
dered connections on electrical connector sensor
from environmental influence (under contain-
ment). The base coils are series-connected and
supplied by alternating current. In case of coil
coverage by magnetic shunt its inductance rises,
with that total resistance of series-connected base
coils (and sensor output signal - voltage decrease
from flowing current) depend on quantity of oils
covered by magnetic shunt. It allows to define
the CR position within the borders of each of 10
sections, which all the CR movement range is
divided into, as well as CR position on upper and
lower limit switches (accuracy to ± 20 mm at the
end and ± 30 mm at the intermediate sections).
Reserve coils allow to find out CR fall in case of
base group coils failure.
Signal from each sensor of CR position is
transformed into staircase (multilevel) signal,
which information parameter is direct current,
which identify number of section within the
borders of which CR is at the moment, as well
as CR position on upper and lower limit switch.
Two indication devices are used for imaging with
single digit indicators gathered in 10 groups in

accordance with CR allocation in groups. This
means of displayed has name rough indication
of CR position.
For operators' convenience, which got used
to decimal system, each section is divided into
10 fixed stages (35 mm each), which makes 1%
of whole upwards CR movement in the core. In
such a way, movement of CR on each stage is
performed in the result of one or two steps, move-
ment on one section - 175 steps (nominal step of
CR drive equal to 20 mm). CR position within
each section is determined by count of quantity
of steps made by the CR drive after the border of
respective section was crossed. In such a way, CR
position displayed of two digital characters, the
first of, which corresponds to section number, the
second - to stage number within the section where
CR is at the moment. Such means of displayed has
name precise position indication. On the control
and monitoring panel are three blocks with two-
digit indicators for precise indication of middle
positions of main and additional CR groups and
position CR, controlled manually.
Another type of linear sensors of control rod
position differ from the described sensor only in
quantity, connection and placement of inductance
coils (reserve coils are not provided). Six coils
are divided in two groups (upper and lower),
each of them is formed by three series-connected
coils. Coils supply is performed from CR posi-
tion cabinet in form of sum of alternating (100 ±
1 mА, 120 ± 1.2 Hz) and direct (40 ± 0.4 mА)
current components. Output signal is formed as
components of alternating and direct voltage,
which are defined by complex resistance of the
upper and lower coil groups, dependable of shunt
position. On the results of its processing, discrete
signals are formed for rough CR position indica-
tion (section number, position CR on upper and
lower limit switch). Precise CR position indication
is determined by the section number and quantity
of steps made by the CR drive on each section.
Stepwise sensor allows to define control rod
position with 20 mm discretion, i.е. within one