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---
layout: default
title: Chernobyl accident timeline
category: chernobyl
description: Timeline of the events in the Chernobyl nuclear accident
author: natallia
---
<h1>Chernobyl Timeline</h1>
<div class="row">
<div class="col-md-8">
<table class="table">
<tr>
<th>Date</th>
<th>Time</th>
<th>Event</th>
</tr>
<tr>
<th>April 25, 1986</th>
<td>1:06 am</td>
<td>
Normal operation: reactor was at full power. Steam power was directed
to both turbines of the power generators. The purpose of the test was
to determine if one of the turbines could supply power to feedwater
pumps until standby diesel generators came on line in the case of a
local power failure.
</td>
</tr>
<tr>
<th scope="row"></th>
<td>1:00 pm</td>
<td>
In preparation for the test prior to routine shutdown, reactor power
was powered down to 1600 MWt for a test sequence. Now only one turbine
was needed to take in the decreased amount of steam from the reduced
power and one of the turbines was switched off.
</td>
</tr>
<tr>
<th scope="row"></th>
<td></td>
<td>
The test was designed to be conducted at 22%-32% full power. However
the demand in electricity was still high at the time, and authorities
asked to keep the reactor at at 50% power for additional 9 hours.
</td>
</tr>
<tr>
<th scope="row">April 26, 1986</th>
<td>12:28 am</td>
<td>
Permission was received to resume the reactor power reduction. One of
the operators failed to reprogram computer to maintain the power at
30%, which caused the power to fall down to 1% and reactor was filled
with solid water (no boiling) - an extremely unstable situation
because of the positive void coefficient. This amount of power was too
low to conduct the test.
</td>
</tr>
<tr>
<th scope="row"></th>
<td>12:32 am</td>
<td>
To try to bring the power back to the desired level operator pulled
number of control rods out of the core (less than 26 control rods
remaining in the core), but the power only increased to 7%, still well
below the test design value. The reason the power could not be brought
back up was the "xenon poisoning" effect. Xenon is a decay product of
I-135 and is a strong neutron absorber which "poisons" the fission
reaction. It reaches an equilibrium at normal operating power levels
by being "burned away" by neutron absorption and further decay. When
the power level was decreased from the 1600 MW level, there were lots
of I-135 to decay into xenon, but a small neutron flux with which to
burn it away, so it built up rapidly.
</td>
</tr>
<tr>
<th scope="row"></th>
<td>1:15 am</td>
<td>
In order to keep reactor from automatically shutting down under these
conditions, emergency core cooling system (ECCS) and several of the
automatic scram circuits were disconnected. More control rods were
pulled out to overcome "xenon poisoning". 6 control rods
were still remaining on the core. At this point reactor could not be
shut down rapidly if needed.
</td>
</tr>
<tr>
<th scope="row"></th>
<td>1:20 am</td>
<td>
All eight cooling water pumps were running at the low power, compared
to a normal six even at full power, so there was nearly solid water
with almost no void fraction, which increased the vulnerability to any
power excursion which produced boiling.
</td>
</tr>
<tr>
<th scope="row"></th>
<td>1:22 am</td>
<td>
The turbine was tripped to initiate the test, which caused the
switching off of four of the eight recirculation pumps. This would
have scrammed the reactor if the automatic scram circuit had not been
disconnected.
</td>
</tr>
<tr>
<th scope="row"></th>
<td>1:23.35 am</td>
<td>
Reduced coolant flow caused voids to form rapidly in the pressure
tubes, increasing reactivity because of the positive void coefficient.
The higher reactivity produced more steam. As the system fed itself,
heat production soared, casing uncontrollable steam generation.
</td>
</tr>
<tr>
<th scope="row"></th>
<td>1:23.40 am</td>
<td>
Operator, recognizing an emergency, hit the button that was to lower
all control rods into the core and shut down the reactor.
</td>
</tr>
<tr>
<th scope="row"></th>
<td>1:23.44 am</td>
<td>
Ironically, this last attempt was the final straw. The reactor system
had a design problem that no one had foreseen. The control rods had
six-inch graphite tips at their lead ends. These tips enter the core
before the boron carbide section. As they enter the core, they
displace water, which is essentially displacing absorber with
moderator. Being inserted all at once they caused power to increase
for a few seconds. Under normal operating conditions, this small
temporary increase has little effect. But Chernobyl Reactor Number
Four was not operating under normal conditions. As control rod tips
entered the core, power surged to 100 times normal in four seconds.
</td>
</tr>
<tr>
<th scope="row"></th>
<td>1:24 am</td>
<td>
Under intense heat, the core began to break down. Fuel assemblies
fragmented, control rod channels warped, steam built up furiously and,
finally, steam tubes burst. Tons of steam and water shot into the
reactor, causing a tremendous steam explosion. Steam pressure blew the
1000-ton steel- and cement-filled biologic shield off the top of the
reactor, destroying the roof of the reactor building along the way and
exposing the hot core to the atmosphere.
</td>
</tr>
<tr>
<th scope="row"></th>
<td>1:28 am</td>
<td>14 firemen arrived on the scene of the accident.</td>
</tr>
<tr>
<th scope="row"></th>
<td>2:00 am</td>
<td>
The largest fires on the roof of the reactor hall were brought under
control by a group of 100 fire fighters called in from the Pripyat.
</td>
</tr>
<tr>
<th scope="row"></th>
<td>5:00 am</td>
<td>
Most of the fires were put out, but graphite fire had started.
Graphite fire caused the dispersion of radionuclides high into the
atmosphere.
</td>
</tr>
</table>
<p>
<strong>Table 1</strong> Timeline of the Chernobyl accident compiled by
Natallia Pinchuk.
</p>
<p>
<a href="{% link chernobyl-main.html %}"
>Back to main Chernobyl section</a
>
</p>
<h1>References</h1>
<ol>
<li>
<a href="http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/cherno.html"
>HyperPhysics Chernobyl section</a
>, C.R. Navy, hosted by department of Physics and Astronomy of Georgia
State University, 2005.
</li>
<li>
D. Marples, "The Social Impact of the Chernobyl Disaster."
London, 1988, pp. 12-19
</li>
<li>
Glenn Alan Cheney, "Chernobyl: The Ongoing Story of the World's
Deadliest Nuclear Disaster," Macmillan, 128pp. New York, 1993.
</li>
</ol>
</div>
</div>