The three mile island nuclear accident


The nuclear power reactor, described as a “pressurized water reactor”, moderated and cooled by water, was developed in the 1950s simultaneously by the Americans and the Russians to propel their naval vessels, submarines. aircraft carriers and ice-breakers. The Americans called it the Pressurized Water Reactor, PWR, and the Russians called it Water (moderated), Water (cooled) Energetic Reactor, WWER, which in Russian acronym, reads VVER. The PWR and WWER are virtually the same, except that, steam exchanger for PWR is in vertical position (see attached image), that for WWER is in horizontal position.

These power reactors for naval vessels use highly-enriched fuel, and so they have small cores with relatively small volume of fuel, but very high energy density for the propulsion of heavy naval vessels.  Imagine submarines which can stay submerged 250-350m below the surface of the sea for months cruising at speeds of up to 36km/h. The nuclear fuel is changed once every 5 to 7 years. For instance, the decommissioned USS Enterprise, a 93000ton aircraft carrier with a flight deck length of about 324m, could carry over 5000 people and about 60 aircrafts, cruising at 30-60km/h. The carrier’s refuelling period was well over 3 years. Nuclear-powered ice-breakers are operated by the Russians to make it possible for ships, including containers and oil-tankers, to sail to and from their destinations in the Northern Artic sea during the long harsh winter months, during which the sea is frozen.

About 6 countries now own over 100 naval vessels, primarily submarines, operated by a pair of 150-300MW nuclear power reactors. Powering such vessels by diesel generators would require several thousands of drums of diesel oil on a daily basis, which would cause significant pollution. Notably, these nuclear reactors have operated with good safety records in spite of the very harsh conditions.

Some Basic Facts About PWRs

Modern nuclear power plants are housed in massive containment buildings, robustly built to (1) prevent any inadvertent release of radiation into the environment, and (2) protect the reactor building from missiles and falling aircraft. From the attachment showing PWR of TMI-2, there are 2 sections labeled as primary and secondary sides. Note that the three most sensitive devices: the reactor, the pressurizer and the steam exchanger, are all located in the primary side, hence under the containment.

Below are some key terms:

The Reactor: (not seen), may occupy about 1/3 of the huge cylinder, known as the reactor pressure vessel, RPV. The diameter of the rector of approximately 4.5m, will be the inner diameter of the PRV.

The Pressurizer: this device is only found in PWRs and WWERs. The pressurizer makes sure that water, the coolant, is pressurized and maintained at about 15 atmospheres. Thus, the super-heated water at over 3000C, dose not boil inside the reactor.

The Heat Exchanger: this is where the super-heated coolant in the primary loop transfers its heat energy to water in the secondary loop, which is under normal pressure. Therefore, water from the secondary loop leaves the heat exchanger as steam, as shown in the figure.

Loss of Coolant Accident, LOCA: the loss of coolant is the most worrisome for reactor operators, especially those who operate LWRs, the main stream reactors. The LOCA occurrence in the Three Mile Island was due to lack of water; in Fukushima, it was the disruption to the electricity supply caused by the wave/tsunami.

When an operating nuclear reactor is suddenly stopped, the fission process that produces massive amounts of energy also stops. At that very moment, the heat coming from the core fuel, known as decay heat, is 6-7% of the generating power of the reactor.

The decay heat declines quite steeply in the first hour or so, then it declines slower as short-lived fission products decay away. Therefore, when a sudden stop of an operating reactor is followed by a LOCA, it leads to core meltdown and/or hydrogen explosion.

Three Mile Island (TMI-2) Accident

It is appropriate to discuss briefly the film China Syndrome, released in March 1979, starring Jane Fonda, Jack Lemmon and Michael Douglas.  Jane Fonda and Michael Douglas played the roles of a reporter and camera man respectively, while Jack Lemmon acted as an operator at the facility. The China Syndrome, a very catchy name for a fictional thriller film, featured a crisis in a (fictional) Ventana Nuclear Power Station, near Los Angeles in California. It was believed in the fictional story that, the core of the reactor in the crisis could burn its way through the earth’s core and appear in China, hence the fanciful name “China Syndrome”. The film portrayed that the accident at Ventana would pollute an area as large as the State of Pennsylvania. The film was released on 16 March 1979.

Eerily, the accident at TMI-2 started around 4.00am on the 28th of March 1979 – a mere 12 days after the film release. The Three Mile Island nuclear station is located in the State of Pennsylvania. About 12 hours before the accident, maintenance work was carried out on a condensate pump in the secondary loop. This pump suddenly stopped, which meant that the coolant in the primary loop was not cooled in the steam exchanger (see the attached figure). Hence, the pressure in the primary loop increased, opening the pressurizer relief valve, and that allowed a large amount of the radioactive coolant in the primary loop to escape into the containment building.

When the emergency cooling system switched on automatically, the operators turned it off, thinking that there was water in the pressurizer, and that was the main cause for the accident. The TMI-2 accident is clearly attributable to unskilled human interference – they would have been better off if the operators were asleep. The reactor would have regulated itself.

Out of many obvious recommendations made by several committees that reviewed the accident, I readily endorse the one that says: “There should be a skillful supervisor at every shift.”

The timing of the accident, so close to the release of the China Syndrome film, stoked irrational fear in the general public as well as local, state and federal authorities. They expected information about the severity of the accident as well as guidance to evacuate given the assumed disaster. Many were concerned that there was no reassurance from the TMI operators. The TMI operator reports, when they came, were dismissed as contradictory, which led to confusion and ultimately mistrust by the general public.

There was no loss of life as a result of the TMI-2 accident. There was negligible radioactivity as seen in the U.S. BEIR Report on the Biological Effects of Ionizing Radiation stating that “The collective dose equivalent resulting from the radioactivity released in the Three Mile Island nuclear accident was so low that the estimated number of excess cancer cases to be expected, if any, were to occur, would be negligible and undetectable.”

And yet, the safety of nuclear power took a blow. The main consequence from the 1979 TMI accident was the loss of credibility of nuclear power because of the way it was discredited by celebrities and state officials and distorted by the media. I hope by shining a light on these accidents, we can re-educate ourselves on the true potential of nuclear power as a safe and reliable energy option that deserves a place in our energy mix if we are to really see sustainable development in Africa.

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