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---
layout: default
title: The Chernobyl Disaster
category: issues
description: Learn about the Chernobyl nuclear disaster
author: nick
byline: true
---
<div class="col-lg-8">
<p>On April 26th, 1986, a nuclear power plant in Chernobyl, Ukraine failed violently during a
scheduled test of the turbine system. The power of the reactor increased out of control from a
low-power state, rapidly boiling water and causing a steam explosion that blew the roof off the the
reactor. A graphite fire was ignited, spewing toxic radioactive substances into the atmosphere. 31
people were killed during the accident, and it has been estimated that the released radiation has
caused an additional 4,000 cancer deaths. </p>
<p>Here, you can find out what caused the Chernobyl accident and why we are convinced that nothing
like this could happen again. Check the links to the right for our collection of information on
Chernobyl. </p>
<div class="alert alert-success d-flex align-items-center" role="alert">
<p><strong>Featured Content Alert!</strong>
We are extremely fortunate to have some
<a href="{% link chernobyl-memories.html %}">unique first-hand stories told by two parents of young
children</a>
living near Chernobyl when it happened, as well as the reflections from
<a href="{% link chernobyl-memories-nat.html %}">one of their children</a>
(who ended up working as a nuclear engineer!)
</p>
</div>
<h3>On this page</h3>
<div class="list-group">
<a class="list-group-item list-group-item-action" href="#short">What happened at Chernobyl? (short version)</a>
<a class="list-group-item list-group-item-action" href="#sortof">What happened at Chernobyl? (longer
version)</a>
<a class="list-group-item list-group-item-action" href="#again">Can Chernobyl happen again?</a>
<a class="list-group-item list-group-item-action" href="#more">More Info</a>
</div>
<h2 id="short">What happened at Chernobyl? (short version)</h2>
<p>Because Xenon built up due to a rapid shut-down mainly caused by nearby coal plant problem,
operators at Chernobyl retracted the control rods all the way out of the core only to notice power
instabilities due to the inherent positive coolant void coefficient of the RBMK reactor and reinsert
them. But the tips of the rods were graphite, which displaced water without introducing neutron
poison, thus increasing the power rather than decreasing it and the positive void coefficient
allowed the reactor to undergo a runaway power excursion, resulting in a catastrophic steam
explosion and graphite fire that killed 30 workers and released radiation in levels that were soon
detected in Finland. </p>
<h2 id="sortof">What happened at Chernobyl? (longer version)</h2>
<p>The Soviet government ordered the operators of Chernobyl to perform a test on their reactor. The
test was supposed to measure how long the spinning generator would continue producing electricity
after the reactor was shut down before it slowed and stopped. The test was to happen with the
reactor at low power. As the operators were about to start slowly and carefully bringing the reactor
to low power, the utility company called and said that a nearby coal plant just shut down
unexpectedly and asked Chernobyl to stay at full power for a while longer to keep the nearby homes
lit. After peak hours, the operators brought the plant to low power rapidly. Nuclear engineers can
explain to you that this kind of shut-down would result in an unusually large amount of Xenon
build-up in the reactor fuel. Xenon eats up neutrons, thus making the chain reaction more difficult
to maintain. Because of this, the operators were surprised when the reactor went to a lower power
than they had expected. </p>
<p>To get the power of the reactor to its expected value, the operators pulled the control rods out
further than they would ever do in any normal situation. Finally, the reactor went up to the power
level needed for the test. Unfortunately, the Chernobyl reactor was designed in a dangerous manner,
allowing the reactor to get hotter if bubbles formed in the water coolant flowing past the fuel. As
water boiled in certain locations of the reactor, this power instability started rearing its head.
Apparently, one of the operators noticed this highly unstable situation and pressed the button to
insert the control rods and shut down the reactor. But, an even more serious design flaw was in the
control rods. The neutron poison in most of each rod had a small graphite tip on the bottom. This
graphite is not a neutron poison (and is usually beneficial to chain reactions). So with the control
rods all the way out, inserting them for the first few inches displaced some water without
introducing any neutron poison, and this actually increased the power of the reactor. This led to
more boiling of the water, which resulted in even more power and then positive feedback took the
reactor power sky-high, immediately boiling all the water to steam. The steam pressure was so great
that it blew the lid right off the reactor and through the roof of the reactor building (which was
not one of those steel containments, by the way. It was just a concrete building). </p>
<p>With no more coolant, the fuel heated up and became molten. Standing graphite rods in the reactor
ignited into a very hot graphite fire that began spewing bits of the radioactive fuel into the open
air. The fire took over a week to extinguish, costing the lives of about 30 emergency responders due
to acute radiation poisoning.
</p>
<h2 id="again">Can this happen again?</h2>
<p>Modern reactors have reinforced steel-concrete containments domes that would not be so easily
penetrated by a steam explosion. Control rods are designed such that all parts of them are neutron
poisons, including the bottom. Reactors are designed to have NEGATIVE void coefficients, meaning
boiling water would result in power decreasing automatically rather than increasing. Also, graphite
is not typically put in reactor cores. Short answer: no.</p>
<h1 id="more">More info</h1>
<ul>
<li><a href="{% link chernobyl-timeline.html %}">Detailed timeline of Chernobyl accident</a></li>
<li><a href="{% link chernobyl-memories.html %}">Memories of a mother and father living ~100 miles from the
accident</a></li>
<li><a href="http://www.unscear.org/unscear/en/chernobyl.html">United Nations Scientific Committee on the
Effects of Atomic Radiation</a> - Chernobyl section (very good)</li>
<li><a href="https://www.nrc.gov/reading-rm/doc-collections/fact-sheets/chernobyl-bg.html">NRC discussion of
effects of accident</a></li>
</ul>
</div>