Chernobyl unit 4 in Ukraine.
On April 26 1986, during a low-power engineering test in Chernobyl RBMK-1000 reactor 4, a power surge triggered multiple steam explosions, causing a loss of coolant, allowing a total meltdown of the fuel and starting a fire in the reactor housing, which burned for ten days, dispersing radionuclides into the atmosphere. The details of how this occured is described here.
Thousands of the victims of Chernobyl have died.
Thousands of those in the affected area have died because people do die when they reach the end of their lives. The issue is how many deaths are directly related to the accident. The United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) released a full report to the General Assembly in 2000 on the sources and effects of ionising radiation. Annex J: Exposures and Effects of the Chernobyl Accident contained a review of the effects of the fallout fourteen years on. The report identified 31 short-term deaths from plant workers and emergency workers, 28 of which were due to radiation exposure. The Chernobyl forum agrees with UNSCEAR about the 31 immediate deaths plus 16 further deaths among workers over the following two decades, attributable to the accident and 9 deaths due to complications in adolescent thyroid cancer, bringing the death toll as of 2004 to 56, just over the number of people killed on 7/7.
The Chernobyl Forum estimated in 2005 an expected maximum death rate at 9000 globally. This estimate is the subject of controversy because the means of its extrapolation. It is based upon the LNT and simply applies it to the estimated radiation dosages to the global population. In addition, it is difficult to say that these would be detectable, even if they did occur, because it is a low figure in global terms, therefore statistical significance is difficult to achieve. It is also more likely that those marked will die from other causes before the hypothetical effects of the radiation catch up with them.
Extracts from Annex J can be found here.
Among the affected populations, there is an epidemic of all sorts of cancer.
Not quite true. There is only one particular cancer that has increased in incidence and that is thyroid cancer. Some 1800 cases of adolescent thyroid cancer have been recorded in the affected population due to high doses of iodine-131 in milk, but the Chernobyl Forum recently made the number 4000 including 9 deaths from complications. Part of this increased incidence was attributed to an iodine deficiency in the affected areas, which increased the susceptibility to the radiological effects. The reports also suggest that increasing iodine intake in the population could help mitigate the effects. The economic problems of the region meant the proper preventative action was not taken at the time to reduce the thyroid cancer risk. The increased risk is about 5 to 10 times over the general population, depending on the age at the time of the accident. The effects of increased screening on the measured rise cannot be ignored either, possibly exaggerating the perception. Because iodine-131 has a half-life of just 8 days, the contamination has long since decayed and so the radioiodine threat no longer exists.
No increase in leukemia has been observed in the public or the workers, despite the expectation that it would be the most likely indicator of high radiation dosage. There is no scientific evidence of an increase in other cancers as of yet due to ionising radiation.
There has been an increase in birth defects due to the disaster.
Untrue. No scientific evidence links birth defects to the accident. Birth defects are an unfortunate fact in all populations from all radiation backgrounds. Many factors, most unrelated to Chernobyl, can cause health problems in the population. Immediately blaming every case on the fallout is a non sequitur. The worst part of this particular claim was that in the immediate aftermath, mothers terminated their pregnancies for fear of their unborn children developing genetic abnormalities. The fear was baseless, but the abortions happened nonetheless. If there has ever been a demonstration of validity of FDR's famous words, this is it.
What about secondary effects?
The UNSCEAR reports that aside from the tragic rise in incidence of thyroid cancer affecting an unfortunate minority, there have been sizeable secondary effects due to the reaction to the accident. More than 300,000 people were evacuated from the surrounding areas and resettled in less preferable environments. This destroyed communities, broke up families, leading to great economic hardship, depression and stress related disorders. This is said to have caused more problems than the radiation itself and some say that had the residents been allowed to remain in their homes, these secondary effects would have been avoided with a less significant increase in primary effects. Certainly had the efforts been dedicated to dealing with the thyroid deficiency, a large number of the thyroid cancer cases could have been avoided. Nearly a thousands residents have illegally returned to their homes within the exclusion zone and are experiencing no adverse health effects.
More seriously is the result of Western world labelling the residents of the contaminated areas and victims. Getting classed as a victim brings with it handouts from well-meaning, but overbearing Western charities, and this has created a dependency culture and a sense of fatalism. In addition, the persistence of the West to conjure up all sorts of horrible deaths the residents of the contaminated areas might indure, has resulted in increased hypochondria, the dirty bomb effect, whereby the fear of what horrible death awaits causes the would be victim to worry himself into hypertension.
The facts underlined by UNSCEAR report show that widespread detrimental health effects have no been observed and for large sections of the affected population, there is no need to worry about the fallout from the disaster. Annex J ends saying, "Lives have been disrupted by the Chernobyl accident, but from the radiological point of view and based on the assessments of this Annex, generally positive prospects for the future health of most individuals should prevail."
The Chernobyl forum study only involves six hundred thousand residents of the local area and not the millions across Europe.
That is because the millions across Europe received a greater radiation dosage from watching the doom laden news reports on their cathode ray tube television sets than they did from the event itself. It should be no mystery why the study focused only on those who received a significant dosage.
The contamination of the large swathes of the Eastern bloc and Europe represents a radiological hazard that will last for a long time.
The vast majority of the radioactive material deposited from the fallout has long since decayed, most significantly, iodine-131, which has shown to have had a signficant affect on the health of the population. The most significant isotopes now are cesium-137 and strontium-90 to a degree. Cesium is a particular worry because it is chemically similar to potassium and so is absorbed by organisms accordingly. However, over the past 20 years, exposure to the population has decreased significantly. Not only has 40% of the cesium decayed, but the material remaining has sunk deeper into the soil, where it is not as accessible by plants and so does not enter the food chain in such quanities. Further, it bind chemically with the soil making absorption even more difficult. The contamination of the local food chain is much less than it was and will continue to go down. Strontium-90 was not as widely dispersed and neither in such quantity, so it represents a secondary hazard at this point. It is also not a gamma emitter and so does not contribute to external dosage.
What is important is the effect of this longer lived contamination of the affected populations. Now that only these lower activity isotopes remain, future exposure will be significantly lower than they were before. In some areas, the radiation environment has already returned to pre-accident background levels. Even without countermeasures, the estimated lifetime dosage would be 400mSv in the more contaminated areas. Typical lifetime dosage worldwide is around 170mSv but can be as high as 700mSv. So we can see that while the dosage in the affected areas is high, it is not extraordinary high. While these isotopes will be present in quantity for some time, it will not represent a large hazard for the future. The vast majority of the damage has already been done.
The exclusion zone has been called a radioactive dead zone.
For a radioactive dead zone, it sure has a lot of flourishing wildlife. With the evacuation of the residents from the area, aside from those who have illegally returned to their homes, the area has become a thriving nature reserve.
Chernobyl is said to be the worst manmade disaster in history. Are you saying that Chernobyl was really not that bad?
Not at all. Chernobyl was a terrible disaster and the kind of mismanagement and irresponsibility that led to it amounts to criminal negligence and has been pursued as such with six people having been given prison time for their role in allowing the accident chain to lead to this. But it is important to consider the event in context. It would be fallacious to single out Chernobyl as an example of the evils of nuclear power, while painting as more benign, other events such as dam bursts, gas explosions, transport accidents, not to mention the general effects of air and water pollution from various sources, when they result in more death and suffering. Chernobyl was a terrible manmade disaster, but it was by no means the worst manmade disaster in history. It just had a better publicity agent.
A Chernobyl type event could happen at any nuclear power station.
No it could not. There are several specific design flaws in the RBMK that allowed this disaster to happen. These design flaws are not present in Western reactors and certainly not in Generation III+ reactors. First it is important to recognise the conditions under which the power surge occurred.
On the morning of April 25th, Chernobyl 4 was scheduled for use, rather ironically, in a safety demonstration test. The intention was to show that in the event of a combined loss of offsite power and reactor scram, the inertia of a turbine is sufficient to maintain power to vital systems for the few minutes it would take to startup the backup diesel generator (the test itself is fairly standard worldwide). The reactor was powered down from its nominal 3200 MWt power level to an intended low level of 1000 MWt. However, while this was being done the power down was delayed a 1600 MWt following demand from the grid.
At this low level, xenon-135, the strongest neutron absorber of all, begins to build up in the reactor. Xenon-135 is formed by the decay of fission products. The absorption of neutrons converts it to xenon-136. In a steady state operation, it will be created by decay as quickly as it is destroyed by neutrons and its own radioactive decay. When the power level is reduced, the fewer neutrons flying around does not destroy it as quickly and so it builds up poisoning the reactor. Because of this, it amplifies its instability at low power levels due to a major design flaw: the positive void coefficient.
The RBMK design is graphite moderated and light water cooled. Uranium-graphite-water is inherently unstable and from the early 50s, it was declared in the West that no such reactor would ever be licensed. The Soviets may not have known or may have not cared about this, when they designed the RBMK in the early 1970s. Light water is a good moderator and is used in the vast majority of reactors around the world, but graphite is a much better moderator. In the RBMK, because of the vast moderating power of the graphite, the moderation due to light water is negligible. But because light water is also an absorber of neutrons, in the unique setup of the RMBK, it is overall a minor poison; at least when it is liquid. Steam is not as good a poison and neither is it as good a coolant. If the temperature was to rise in this design of reactor, voids of steam would form in the coolant, not only reducing its cooling capability, but also reducing the neutron absorption. The result is more neutrons able to cause fissions than before so reactivity increases. This produces more heat, which forms more steam, leading to more reactivity and so on. This is the positive void coefficient.
Now add the xenon-135 to the mix, which accumulates at low power levels for the first few hours before it has decayed significantly without the large number of neutrons flying around to destroy it. At low power, a small increase in temperature leads to steam voids forming, which reduces absorption. There are now more neutrons than there were before. Some of these extra neutrons cause extras fission raising the power level and so the temperature. Some also are now able to convert the xenon-135 to xenon-136 reducing absorption even further still. And so a small increase in reactivity leads to a much larger increase in reactivity. The reactor becomes that much more unstable.
Due to the demand from the grid causing the operators to delay the test and hold the power level at 1600 MWt, they were now allowing xenon-135 to buildup. They should have aborted the test completely and returned the reactor to nominal power to prevent this but they did not. This was the first big mistake in operation.
Later that night on the 25th, when demand had dropped, the operators resumed the test in the xenon poisoned reactor. By this time, they had made second big mistake in operation. They had disabled the emergency cooling and shutdown systems. They wanted to control the reactor themselves for this test rather than leaving it to automation. That this could be done at all is the second major design flaw. In Western reactors, it is not physically possible to override the automatic shutdown system. They reduced the power further, but because of the xenon poisoning, it drops much more sharply than anticipated, down to 30MW.
By this time, it was the early hours of April 26th. The power was forcibly raised to 200 MWt by retracting the control rods to an extreme level, much greater than that of regulations. The fact that the control rods could physically be moved further out than the regulations allow is the third major design flaw. Because the control rods are further out, it follows it will take longer to reinsert them in the event of a manual scram. The decision to retract the control rods in this way and not abort the test despite the difficulties is the third big mistake in operation.
In order to perform this test, which is to determine how long the turbines can provide electrical power from residual momentum alone in the event of a power loss, the steam flow to the turbines was reduced. This means heat was not being carried away from the core as normal. The result was the temperature in the core rose, giving rise to more boiling and increasing reactivity due to the positive void coefficient.
Upon noticing it, a technician attempted a manual scram. The control rods began to be inserted, but the fourth major design flaw caused an increase in reactivity even further. The control rods were tipped with graphite, the material of the moderator. The concern during the design of the RBMK reactor was that when control rods are withdrawn, water would naturally move in to take their place. Water is an absorber, a poison, in this reactor design and so it would defeat the purpose of removing the control rods; to increase reactivity. So the tips of the control rods were made of graphite, which do not absorb neutrons, but moderate them.
Since in this situation, the control rods had been withdrawn so far, when the technician initiated the scram and the control rods began to be inserted, the first thing that happened was more graphite entered the core. With more moderation, higher temperature leading to more voids and less absorption, and the destruction of xenon-135, the reactivity increased so much, the reactor went prompt critical. At prompt criticality, there was a total loss of control of the reactor.
The massive heat generated distorted the reactor structure jamming the control rods in place, where they were doing more moderating than absorbing. The cladding around the fuel burst and the fuel melted. The water reacted with zirconium cladding forming hydrogen and the oxygen reacted with the graphite forming carbon monoxide. The pipes containing the coolant burst under the intense heat and pressure and the water flashed into steam blowing the cover off the reactor.
Now exposed to the air, the mix of hydrogen and carbon monoxide triggered a second chemical explosion, which blew the roof off the building. The fifth and final major design flaw became apparent. And this one is the most important of all. The RBMK design did not include the steel reinforced concrete containment structure present in all other reactor designs. Had there been one, the aerosolised fuel and fission products would have been contained.
To summarise, the Chernobyl-4 disaster happened because of five major design flaws not present in Western reactors.
- The uranium-graphite-water type is inherently unstable and this was known since the early 50s, which is why since then such a reactor would never have been certified in the West. The misoperation of the reactor allowing xenon-135 to build up only enhanced this failing.
- The automatic shutdown system could be overridden by the operators. This cannot be done in Western designs. If the reactor wants to scram, nobody will be able to stop it. After all, a reactor scramming without reason is an inconvenience. A reactor prevented from scramming when necessary is a litigation nightmare.
- The control rods could be physically retracted further than regulations allow. In Western reactors, control rods will only be able to be withdrawn as far as deemed safe. They cannot be physically withdrawn further.
- The control rods were tipped with graphite, which leads to an increase in reactivity upon first insertion, quite the opposite of what you want if the reactor is scramming.
- The RBMK design lacked proper containment, which allowed the radioactive material to escape into the environment.
There are still RBMKs operating in Russia and the Eastern block.
After the accident, the remaining RBMK reactors were backfitted to improve safety. This included introducing more control rods, adding neutron absorbers, improving the automatic safety systems and making them resistant to tampering and improvement in emergency cooling systems. Despite not meeting the criteria of Western reactors, they are now much safer and have been operating without further incident.
New Russian designs are much better and meet Western safety standard and in time the RBMK and early VVER reactors will be decommissioned to be replaced by these newer and better Generation III and IV reactors.
What are you trying to say?
The Chernobyl accident is of little relevance to nuclear power in the West and the future of nuclear power elsewhere in the world. The reactor was badly designed, irresponsibly operated and lacked basic defense-in-depth required of Western reactors. Such an accident is not credible in Western nuclear power stations.
What we did learn from Chernobyl was the worst nuclear power has to offer. The accident was a worst case scenario: a total meltdown followed by ten days unrestricted dispersal of radionuclides over large areas of Europe. The resulting detriment to health of the public was relatively limited compared to accidents in other activities. Given thousands of premature deaths each year due to air pollution from coal, Chernobyl made it clear that nuclear power is by far the least dangerous of the major energy sources, especially given that a reoccurence is not likely anyway.
The other thing we learned was how irresponsible the media and the alarmists can be. As a result of the fear mongering, 300,000 people were forcibly removed from their homes. Communities were destroyed and the social effects of this have killed more people than the fallout itself. Widespread panic across Europe led to similar problems on a smaller scale including the unncessary termination of pregnancies. Fear is a more dangerous narcotic than tobacco and the media and alarmists sell their product with more glee than any tobacco baron.
Chernobyl showed us that we need less Soviet carelessness and more level headed reporting.