Today, 32 years ago, a disaster befell the
population of Pripyat. But the genesis of the disaster began the day before, on
25 April 1986. The nuclear power plant just outside of town was preparing a
test. See, in normal operation over 6% of a reactor’s power is derived from
decay heat of accumulated fission product, not the nuclear reactor itself. This
heat remains even if the reactor is stopped via emergency shutdown. As such, it
needs its cooling pumps to operate after an emergency shutdown even if the
power grid was down.
They had three backup diesel generators,
but they take about 60 seconds to start up – leaving a one minute gap that was
considered an unacceptable safety risk. However, it was theorized that the
rotation of the steam turbine as it wound down could be used to generate
electricity to cover that gap. But on 25 April, as the day-shift workers were
preparing for the test of this theory, another power station went offline, and
the test was postponed, as it would require the power station to reduce its
output from peak of 3200 MW to only 700 MW for the test.
As a result, clearance for a reduction in
power was only given by 23h04, by which time day shift had already left, and
the evening shift was preparing to leave. The test would fall to the night
shift, who had very little time to prepare and carry out the test. By 00h05 on
26 April 1986, reactor 4 had its power reduced to 700 MW, but due to a build-up
of Xenon-135, a by-product of the nuclear reaction that is usually ‘burned off’
at high operating levels, began to build up and further decreased the power.
Power dropped to 500 MW, and Leonid Toptunov mistakenly inserted the reactor
control rods too far. This put the reactor in a near-shutdown state.
The control room decided to increase the
power level by disabling automatic systems and manually removing control rods
to increase power – but it would not increase beyond 200MW due to Xenon-135
poisoning. This also resulted in unstable temperatures and coolant flow. Extra
water pumps were activated at 01h05, but the increased coolant flow rate meant
that the inlet coolant temperature rose, as the water no longer had enough time
to release heat to the turbines and cooling towers.
At this stage, only 18 of the 211 control
rods were still fully inserted into the core, when the test began at 01h23. However,
as the turbine coasted down, the power it should have supplied to the water
pumps began to decrease, lowering the flow rate. This resulted in steam bubbles
forming in the coolant, which is less effective than water at absorbing
neutrons and keeping the nuclear reaction controlled. This started a positive
feedback loop in power generation.
Greater power resulted in more steam,
resulting in greater power, etc. With power levels rapidly increasing, a manual
emergency shutdown was initiated. This resulted in all of the control rods
manually removed earlier being reinserted into the core. But the core rods had
been designed with a graphite neutron moderator tip to boost reactor output when
the rods had been fully retracted. By inserting the rods, water (neutron
absorbing) was first displaced with Graphite (neutron moderating) resulting in
another power spike. Within three seconds power output had spiked to 530 MW.
The core overheated, causing the fuel rods to fracture, blocking the control
rods at only one-third inserted.
Now nuclear fuel was mixed in with the
coolant, and the temperature rose rapidly. Power levels spiked to 33 000
MW, ten times what the reactor was rated for, and steam pressure built up. This
resulted in a steam explosion that destroyed the external cooling structure
around the reactor, and destroyed the reactor casing. With coolant lines now
broken, the remaining coolant flashed to steam and escaped from the core. Two seconds
later, the now overheating core exploded, and Reactor 4 of the Chernobyl Power
Plant was no more.
Reactor containment was breached in the
explosion, and hot lumps of the graphite moderator was ejected. Now exposed to
air, it ignited and started graphite fires. Radiation levels in the reactor was
now about 6 Roentgens per second, or more than 20 000 per hour. A lethal
dose is around 500 Roentgens over 5 hours. Reactor Chief Akomov assumed the
reactor was still intact, and sent members of his crew to try and pump water
into the reactor. Akimov and his crew died of radiation exposure within three
weeks.
The Chernobyl firefighter brigade also gave
their lives to try and stop the fire. Grigorii Khmel reported, “Misha filled a cistern
and we aimed the water at the top. Then those boys who died went up to the roof
– Vashchik, Kolya and others, and Volodya Pravik.... They went up the ladder
... and I never saw them again.” Anatoli Zakharov said, “If we'd followed
regulations, we would never have gone near the reactor. But it was a moral
obligation – our duty. We were like kamikaze.” The fire was finally extinguished
by dropping over 5000 tons of sand, lead, clay, and neutron-absorbing boron
onto the burning reactor along with liquid nitrogen using helicopters.
The city of Pripyat was not immediately
evacuated, however, until dozens of people suddenly fell ill a few hours later,
with severe headaches and metallic tastes in their mouths, coupled with
coughing and vomiting. By the evening of the 26th, two people in the
town had died and 52 were hospitalized, and an evacuation was organized. At
14h00 on 27 April 1986, Pripyat was evacuated.
The rest of the world only learned about it
when a radiation alarm was set off by airborne radiation at a Swedish power
plant on 28 April 1986, over 1 100 km from Pripyat. In total, over
160 000 square kilometres of land was affected, and over 1 million people
exposed to radiation over Westerns Europe. Today, Pripyat stands as a city
slowly being retaken by nature, in the middle of the 2 600 square
kilometre Chernobyl Exclusion Zone, with only limited human occupation allowed
for short periods of time – a landmark created by mankind’s greatest nuclear
disaster.
