Nuclear Electrical Power Generation – Why The Fuss? (Part 10)

This chart is courtesy of the Australian Government's Umpner Report.

Much is made of the two most high profile nuclear power plant accidents, and there is a plethora of information about them both. Here at our site, we included a post on exactly these two accidents, and this is the link to that post. I will again mention these two accidents here, but mainly in the form of a brief overview.

However, what I would direct you to is this chart above. Click on the chart and it will open in a new and larger window. It does make for interesting reading when doing comparisons between accidents at power plants of any form, especially with reference to nuclear power plants.

As you can see, accidents at nuclear power plants account for what can only be called the tiniest fraction of fatalities. This is not meant to trivialise those persons who have died, but to serve as an indicator as to the inherent safety of these types of plants, without doubt the most safety regulated type of power plant currently in use anywhere.

OVERVIEW OF THE THREE MILE ISLAND ACCIDENT.

This plant is in Harrisburg in Pennsylvania. The accident occurred at the larger of the two generator complexes. This is a Pressurised Water Reactor, (PWR) and for a schematic diagram of that type of plant take this link to this post and it is the middle of the three images there.

There was a relatively minor malfunction in the secondary cooling system and the reactor was immediately ‘scrammed’ driving the control rods back into the piles, and all reaction ceased in a matter of around 4 seconds.

During the scram process, a relief valve circulating water in the reactor vessel failed to close allowing water to escape from the vessel, and the failure of this valve was not indicated correctly in the control room. The water is used to keep the reactor rod assemblies covered, and can aid in cooling following the scram process and shutdown procedure. By the time it was noticed, some water had escaped from the vessel, exposing the tops of the rod assemblies inside the reactor vessel. Now with no water covering them, they were exposed to the high heat level which then continued to rise.

You may think it is a matter of just introducing more cooling water into the vessel to cool the temperature, but the problem with that was that the temperature was so high that any introduced water would immediately flash off, turning to steam immediately. Hence, the temperature rose even further inside the now partially exposed reactor. That temperature melted the exposed tops of those assemblies, rods, pellets, the lot, and the reactor was destroyed, nothing to do with a nuclear reaction, but from melting due to the incredible heat inside the vessel. However the Containment Structure was not breached, and was never at any time close to being breached. The main damage was during the first three hours, during which time the operators used all the procedures and ingenuity to control the situation. The now existing heating problem introduced other minor problems as well, mainly with the vibration of pumps to get water to the reactor vessel, so this exacerbated the problem, but not to what might be considered ‘out of control’, and further procedures were then instigated, and as is necessary in this case, time was the limiting factor. It took 15 hours to bring back under control, not because it was out of control, but that some procedures cannot be put in place until earlier steps have run their course. The build up of gases was also contained, and two days later some of those gases were exhausted to the Atmosphere, but at no time did exposure levels rise above those of normal background radiation. The operators worked for a further 4 weeks until cold shutdown had been achieved, when normal flow of water had taken over from pumped coolant. Only the upper portion of the assemblies that were in the air were melted to destruction, while that part of all the assemblies still in the water were not affected. The containment structure did the job it was supposed to do, and that whole half of the two reactor, two generator/turbine complexes at the plant was then safely removed from the site.

No significant radiation effects on humans, animals, or plants were found. In fact, thorough investigation and sample testing of air, water, milk, vegetation, and soil found that there were negligible effects and concluded that the radiation was safely contained. The most recent and comprehensive study was a 13-year evaluation of 32,000 people living in the area that found no adverse health effects or links to cancer.

In June of 1996 Judge Sylvia Rambo dismissed a class action lawsuit that alleged the accident had caused  adverse health effects. She said in her findings that the plaintiffs had nearly 20 years to find evidence to support their case and they found none at all. The court even found no evidence in their searches. There was an appeal against the finding and finally, in 2002, the Circuit Court declined to hear the second appeal of Judge Rambo’s findings, and the plaintiffs then decided to take no further action.

OVERVIEW OF THE CHERNOBYL ACCIDENT.

This was an entirely different situation altogether.

This plant had 4 reactors, and the problem occurred with one of them. Unlike plants in the U.S. and in other Countries, this Russian designed plant had a reactor vessel, naturally, but no containment structure. These type of reactors look nothing like the PWR and BWR reactors in use in those Countries, and the accident was made so much worse because of the inherent design flaws in these reactors.

The accident was not during normal operation, but during a test to see what would happen if … then if … and then …

Operators and some ‘political apparatchiks’ with no idea of how the plant operated proceeded with the test, under direction. They then proceeded to bypass, disconnect, and also ignore every important safety system in the plant. Consequently, it was an accident just bound to happen, and at the first minor problem, the reaction then ran out of control in a very short time, and with all safety measures shut down, there was no possible way to contain the reaction. The rapid runaway in temperature caused a massive steam explosion which blew the top off the structure, that top weighing 1,000 Tonnes. With no water covering the piles, and now exposed to the atmosphere, around 50 tonnes of nuclear fuel just evaporated, almost immediately, into the air. This was not a nuclear explosion of any sort but one of steam and then evaporation due to a temperature approaching 2,000C. The plant burned for ten days, not the nuclear fuel but the graphite used for the reaction moderation process. The reactor is now enclosed a concrete sarcophagus and there are plans now for an even larger concrete structure to be constructed around the remains of this reactor. Two workers died in the steam explosion, another died from a heart attack, and 28 of the staff and firefighting heroes who selflessly worked to try and contain the resultant fire died from radiation burns and thermal burns within four months of the accident, those firefighters not having it explained to them that they were being exposed to massive levels of radiation as well as thermal radiation from the fire itself.

There have however been illnesses associated with proximity to the plant in the general population, mainly thyroid cancers, and there are expected further deaths associate with this cancer expected over the years.

This accident was due to flagrant ignorance at a plant with significant design flaws, and even though some of these types of plant are still in existence, lessons learned from this accident now see more stringent safety measures in place, safety measures never to be ignored as they were in this case.

Contrary to what happens in the U.S. there were at this time no reporting measures in place in the Ukraine, which at the time was one of the 15 Countries under the Soviet Bloc as the USSR, so this accident went unreported for nearly 2 days, when all the damage had already been done, so there was no mass evacuation of people in the nearby city of 14,000 people until 4 days after the accident, and none have returned.

In the U.S. all accidents at nuclear power plants, no matter how minor in nature, are immediately reported, both to the Nuclear Regulatory Commission (NRC) and also publicly reported as well.

So, even though an accident of this sort at Chernobyl had widespread ramifications, it is glaringly obvious that the U.S. accident showed that safety measures and procedures worked as designed in the event of what may be considered a catastrophe.

Since that time of the U.S. accident, nuclear power plants have come a long way, evolved into third generation reactors, with Gen 4 reactors already proposed and in planning. With each new generation, they become more safe, training becomes better, and regulations even stronger. They are more inherently safe than for any other power plant in existence.

So, even though an accident at a nuclear power plant is often reported as a catastrophe, mostly by people who are ill informed, even major accidents are controlled better than they are at other power plants, mainly again due to that one word ….. Nuclear.

NukeSteamElec

Tagged: Chernobyl Power Plant, Kyoto, Nuclear Electrical Power Generation, Nuclear Electrical Series, Nuclear Power Plant Accidents, Three Mile Island Power Plant

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