Rachel Aubrey
D2 – Case Study – Chernobyl
On April 26, 1986, a test was scheduled at the Chernobyl Nuclear Power Plant to test
a method of keeping the reactors properly cooled in the event of a power grid failure.
If the test had gone as planned, the risk to the plant was very small. When things did
go wrong, though, the potential for disaster was miscalculated and the test was
continued even as serious problems arose. Meltdown occurred at 1:23 AM, starting a
fire that dispersed large quantities of radioactive materials into the atmosphere. The
amount of radioactive material released was 400 times more than the amount the
atomic bombing of Hiroshima released. The fallout was to be detected in almost all
parts of Europe.
The test was scheduled to run during the day shift of 1986, so that the night shift
would only have to maintain cooling of the radioactive decay in the shut-down plant.
However, another power generator nearby unexpectedly shut down, this meant that
the test needed to be delayed whilst the plant worked on continuing to produce
power. The experiment was then resumed at 11:04 PM, by which time the day shift
had departed and the evening shift was about to finish. This meant that the
experiment would have to be conducted in the middle of two shifts, leaving very little
time for the night shift employees to be briefed about the experiment and told what
to do.
The power reduction of reactor 4 to 700 MW was accomplished at 00:05 AM on the
26th of April. However, the natural production of a neutrino absorber led to a further
decrease in power. When the power dropped to about 500 MW, the night shift
operator committed an error and inserted the reactor control rods too far. This
caused the reactor to go into a near-shutdown state, dropping power output to
around 30 MW. This was too low for the test, the operation of the reactor at such a
low power level lead to unstable temperature and flow. Numerous alarms and
warnings were recorded regarding emergency measures taken to keep the reactor
stable. In the time between 0:35 and 0:45 AM, alarm signals regarding thermal-
hydraulic parameters were ignored to preserve the reactor's power level.
All these actions led to the reactor being in an unstable state that was clearly outside
safe operation protocol. Almost all the control rods had been removed, which
reduced the effectiveness of inserting safety rods in an emergency shutdown. The
water was very close to boiling, which meant that any power increase would cause it
to boil. If it started boiling, it would be less effective at absorbing neutrons, further
increasing the reactor's power output.
At 1:23:40, the emergency shutdown of the reactor was started and all control rods
were inserted. It is believed that this was done as a routine method to shut down the
reactor to conclude the experiment and not as an emergency measure. The process of
inserting the control rods was initiated, but it took about 20 seconds for the rods to
be completely inserted. A flawed design in the graphite-tip control rod meant that
coolant was displaced before the neutron absorbing material could be fully inserted
and slow down the reaction. This meant that the process of inserting the control rods
increased the reaction rate in the lower half of the core. A massive power spike
occurred, causing the core to overheat. Some of the fuel rods fractured, causing the
D2 – Case Study – Chernobyl
On April 26, 1986, a test was scheduled at the Chernobyl Nuclear Power Plant to test
a method of keeping the reactors properly cooled in the event of a power grid failure.
If the test had gone as planned, the risk to the plant was very small. When things did
go wrong, though, the potential for disaster was miscalculated and the test was
continued even as serious problems arose. Meltdown occurred at 1:23 AM, starting a
fire that dispersed large quantities of radioactive materials into the atmosphere. The
amount of radioactive material released was 400 times more than the amount the
atomic bombing of Hiroshima released. The fallout was to be detected in almost all
parts of Europe.
The test was scheduled to run during the day shift of 1986, so that the night shift
would only have to maintain cooling of the radioactive decay in the shut-down plant.
However, another power generator nearby unexpectedly shut down, this meant that
the test needed to be delayed whilst the plant worked on continuing to produce
power. The experiment was then resumed at 11:04 PM, by which time the day shift
had departed and the evening shift was about to finish. This meant that the
experiment would have to be conducted in the middle of two shifts, leaving very little
time for the night shift employees to be briefed about the experiment and told what
to do.
The power reduction of reactor 4 to 700 MW was accomplished at 00:05 AM on the
26th of April. However, the natural production of a neutrino absorber led to a further
decrease in power. When the power dropped to about 500 MW, the night shift
operator committed an error and inserted the reactor control rods too far. This
caused the reactor to go into a near-shutdown state, dropping power output to
around 30 MW. This was too low for the test, the operation of the reactor at such a
low power level lead to unstable temperature and flow. Numerous alarms and
warnings were recorded regarding emergency measures taken to keep the reactor
stable. In the time between 0:35 and 0:45 AM, alarm signals regarding thermal-
hydraulic parameters were ignored to preserve the reactor's power level.
All these actions led to the reactor being in an unstable state that was clearly outside
safe operation protocol. Almost all the control rods had been removed, which
reduced the effectiveness of inserting safety rods in an emergency shutdown. The
water was very close to boiling, which meant that any power increase would cause it
to boil. If it started boiling, it would be less effective at absorbing neutrons, further
increasing the reactor's power output.
At 1:23:40, the emergency shutdown of the reactor was started and all control rods
were inserted. It is believed that this was done as a routine method to shut down the
reactor to conclude the experiment and not as an emergency measure. The process of
inserting the control rods was initiated, but it took about 20 seconds for the rods to
be completely inserted. A flawed design in the graphite-tip control rod meant that
coolant was displaced before the neutron absorbing material could be fully inserted
and slow down the reaction. This meant that the process of inserting the control rods
increased the reaction rate in the lower half of the core. A massive power spike
occurred, causing the core to overheat. Some of the fuel rods fractured, causing the
