Category: chernobyl

 Why Chernobyl Exploded— The Physics Behind the Reactor

from Scott Manley

The Windscale Nuclear Accident

With the popularity of the HBO series “Chernobyl”, the Chernobyl disaster is once again entering the public consciousness, along with other infamous accidents such as Three Mile Island and Fukushima.  However ever since the beginning of the atomic age there have been a number of dangerous accidents and close calls. A brief perusal of wikipedia shows exactly how many have occurred.

In 1957 a less known accident occurred in the UK at the Windscale Nuclear Reactor in Cumbria. Throughout the 1950′s, the British nuclear program was put into overdrive in order to keep up with other nuclear powers such as the Soviet Union and the United States. Ever increasing demands and shortening deadlines resulted in a dangerous habit of corner cutting and half-assery which nearly led to a horrific disaster.

The Windscale reactor was not a power plant, but a site used for the production of plutonium and later tritium for Britain’s nuclear weapon’s program. The reactor was a graphite reactor designed to enrich uranium into weapon grade materials. Essentially it was a large block of graphite with channels drilled into it which would house uranium fuel rods. The rods would emit neutrons which interact with other rods, converting the uranium into small amounts of plutonium. Since this process results in a lot of heat, each rod was contained within an aluminum capsule with vanes designed to disperse heat and prevent the rods from overheating.


Once the uranium rod was depleted, the capsule would be pushed out of the reactor, where it would be dropped into a pool of water where it would cool off and later be collected to harvest plutonium.  The reactor was air cooled, with a system of fans that a constant stream of air through the reactor and out a chimney in order to prevent the reactor from overheating.

As the Cold War progressed higher demands were placed on the reactor to produce more and more plutonium, and with the invention of the hydrogen bomb, to produce tritium. In order to produce more weapons grade material, more heat needed to be generated. A simple solution was to shave off more and more aluminum from the fuel rod casings, you know, the things which were designed to help keep the rods from overheating. By 1957 most of the casing’s vanes had been removed and the casing was just a thin aluminum shell.

In the early operations of the reactor scientists and engineers discovered that the crystalline matrix of the graphite reactor, when bombarded with neutrons, could actually store kinetic energy (called the Wigner Effect), later releasing the energy in the form of a massive and unexpected heat increase which could be dangerous to the reactor. In order to counterattack this operators would initiate something called a Wigner Release, which involved purposely heating the reactor to release the stored energy. On October 7th, 1957 operators noticed the temperature of the core was rising, and thus conducted a Wigner Release. The reactor cooled with the exception of one fuel rod which was continuing to heat up. Another Wigner release was ordered. Little did they know that the thin aluminum casing of the fuel rod had fractured and caught fire. Over the next few days the temperature of the reactor slowly increased. On October 10th, the cooling fans were set to maximum in an attempt to cool the reactor. Instead the fans stoked the fire, causing the entire reactor to erupt in flames.

The first attempt to put out the fire involved removing the fuel rods, however the aluminum and uranium rods were orange hot and had fused with the graphite reactor. Next operators attempted to pump carbon dioxide into the reactor, but not enough CO2 could be pumped quickly enough to put out the flames. In a desperate decision, it was decided to put out the fire with water, which was an extremely risky option since the reactor was 1300 degrees centigrade, around 2,400 F. At that temperature, oxygen and hydrogen can split. With hydrogen being an extremely combustible element, it was feared that using water could lead to a massive explosion. Without any other options, the reactor was sprayed with a dozen fire hoses, but it was not enough.

It was only a matter of time before the reactor would melt down, causing the uranium and graphite to turn into molten magma which would melt through the floor, into the ground, and contaminate ground water with radioactive material. In the meantime large amounts of radioactive smoke was billowing out of the smokestack. Finally the lead manager, Tom Touhy, struck upon a simple yet brilliant solution. Fire needs oxygen, why not just shut off the air, seal up the reactor, and let the fire smother? The fans were turned off, the reactor was sealed shut, and operators watched with relief as the flames died out and the reactor cooled.

Windscale was lucky that day. Some of the radioactive material was removed by the smokestack’s air scrubbers, however most of the radioactive material was released into the air. Fortunately, there was an easterly wind that caused most of the radioactive material to be blown out into the Atlantic Ocean rather than over the neighboring town and countryside. As a safety precaution milk produced from around a 500 square kilometer area was dumped. Otherwise no people were evacuated, and no clean up efforts were made outside of the plant. The plant itself went back into operation shortly afterwards. Windscale was damn lucky. A study in 2010 concluded that plant workers suffered no health effects due to the disaster. Health data on nearby residents seems sparse, but it’s clear that Windscale avoided a Chernobyl level disaster. The Windscale accident was a close call that would demonstrate that manipulating the atom is a dangerous business, with no tolerance for corner cutting and half-assery.

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