Civil Engineering Reference
In-Depth Information
four circulation pumps each. One pump is always on standby. The reactor core is
controlled by raising and lowering 211 control rods. This combination of
low-enriched fuel, graphite as the moderator, and boiling water as the coolant
resulted in a positive coefficient of coolant temperature (see also Chap.
2
)[
4
-
7
].
The operators were preparing an experiment in which—after shut down of the
reactor- the energy of rotation of the turbine during coastdown was to be used to
produce emergency electrical power. This was considered necessary in case of
reactor shut down with subsequent failure of the external electrical grid (station
black out). The three emergency diesel generators needed about 1 min after their
start up to reach full speed and power to feed one primary coolant pump required for
cooling of the afterheat generated in the core. This existing lack of emergency
power during roughly 1 min was to be provided by the energy of rotation of the
turbine during its cast down. Three such experiments had already been carried out
4, 2 and 1 year before the accident, but they had been unsuccessful. They had shown
that the excitation voltage of the turbine- generator system was too low. This had
been modified in the meantime and the new experiment was to test the new voltage
regulation system The experiment was set to begin at a power level at a power level
of about 700 MW(th).
The experimental procedures began with a power reduction from full reactor
power. However, this had to be interrupted at 1,000 MW(th) because the electrical
grid coordinator (load dispatcher) suddenly requested power again. At this point in
time, the emergency core cooling systems had already been shut down in prepara-
tion of the test. When the load dispatcher permitted again a further drop in power,
the envisaged power level was not reached by the operators. During the period of
power production at reduced power level the production of the fission products
Xe-135 (neutron absorber) had began. It decreased the effective neutron multipli-
cation factor keff and caused the power level to drop to about 500 MW(th). A
following operator error (control rods were inserted too far) led to a further drop of
power level to about 30 MW(th) [
8
]. As a consequence, there was additional
buildup of the xenon-135 fission product (neutron absorber) with the associated
decrease of the effective multiplication factor, k
eff
. (The amount of Xenon poison-
ing and its influence on the effective multiplication factor k
eff
was not known to the
operators at that point in time.)
At this very low power level of 30 MW(th) the operators made the decision to
restore power by shutting off the automatic control system and to extract the
majority of the control rods by manual control to their upper limits. The power
started to rise and could be increased to about 200 MW(th), a value smaller than the
planned 700 MW(th).
The positive coefficient of coolant temperature of the RBMK1000 reactors was
known to cause instabilities in this low power range. When the eight primary
coolant pumps were activated during startup, this gave rise to instabilities in
power production, coolant flow and temperatures. Various alarms started going
off at this time. The operators received emergency signals regarding the levels in
the steam/water drums and large variations in the feed water flow as well as from
the neutron flux or power monitors, respectively (Fig.
9.3
).
