Rod Group and Individual Control System - Nuclear Power Plant Instrumentation and Control Systems for Safety and Security - page 322

example of specific realization of the general
properties of safety important I&C systems and
their components, provided in Chapter 3, which
take into account the purpose, categories of the
performed functions and safety class of SHC
RG&IC-R.

then - fifth and tenth; the most distant are CR,
which form third, fourth and ninth groups.
Each CR has its own controlled electromechan-
ical step-motor, with the help of, which it can move
vertically in the reactor core independently of the
other CR (“individual control”) or synchronically
with a few other CR, which form one of ten fixed
control rods groups (“group control”).
In individual control the selection of one of
CR and control of its movement is performed via
operating personnel commands. Group control
provides simultaneous lift (or lowering) of all
CR, which are the parts of the corresponding
group, however, the selection and movement
control of any group (except the fifth one) can
be performed via operating personnel commands,
and also automatically. Movement control of the
fifth group intended for aligning of power density
axial distribution (vertically) in the reactor core
and for xenon oscillation suppression (see Goro-
chov, 2004) is performed only via the operating
personnel commands.
While downwards movement (lowering) of
control rod, neutron absorber, which is in it, is
put into the, in this case, multiplication factor
decreases and, as well as neutron reactor power;
while upwards movement (lift) of control rod
reactor power increases.
Control rods move in the reactor core vertically
to the fuel assembly's (for WWER-1000 reactors
operational range is 3500 mm). The limits of op-
erational range are usually interpreted as a position
of the virtual upper limit switch (ULS) and lower
limit switch (LLS): CR movement is stopped by
the sensor of CR position signals corresponding
to the positions of ULS and LLS. All performance
range between ULS and LLS is figuratively divided
on 10 equal sections, which borders are defined
by the sensor of CR position signals.
In the reactor subcritical state all CR are in
lower limit switch position. In order to bring
the reactor into critical state and into minimal
controlled power level a successive lift of CR
groups (except the fifth one) into the upper core

PURPOSE

During the power unit operation the reactivity
change in the nuclear reactor takes place, which
is caused by the continuous fuel burn, slagging
and poisoning, which influence reactor thermal
power and speed of its change. In order to maintain
normal power unit exploitation in the operational
modes this change must be compensated. The slow
reactivity change is compensated by the change
of liquid/fluid poison (boric acid) concentration
in the coolant, which is usually clean demineral-
ized water, at the same time, serving as a neutron
absorber (Boron regulation system). Fast change of
reactivity is performed by the mechanical control
rods movement with a solid neutron absorbers -
absorbing material made of boron carbide and
titanic dysprosium powder put into a enclosure
vessel (Gorochov, 2004).
In the WWER-1000 reactors control rod (CR)
is a construction of 18 vertical thin absorbing ele-
ments, which are located at some distance from one
another, and hard-secured at the top. Each control
rod can move in special guiding channel inside the
fuel assembly. Power unit 1 of South-Ukrainian
NPP has 49 CR, on other Ukrainian power units
with WWER-1000 reactors the number of CR
was increased to 61. Each control rod is identified
by two coordinates - numbers of horizontal and
vertical row of CR, which are represented on the
fuel assembly's positioning map in the reactor
core (core map - see Figure 1).
CR of each group is located symmetrically to
the center of reactor core and at proximately equal
distance from it. Closer to the center than all oth-
ers, are located control rods of the sixth group,