Sunday, December 27, 2009

Nuclear Accidents / Sources of pollution

Nuclear Accidents

a) The Chernobyl Accident

In April 1986, the Chernobyl nuclear power plant in the ­Soviet Ukraine exploded, dispersing more than 1016 Bq of ra­dioactive material into the atmosphere.

In the figure technicians are observed checking for radiation inside the damaged Chernobyl nuclear power plant.

Two peo­ple were killed in the explosion, and hundreds more died of radiation sickness. Fallout affected much of eastern Europe and Scandinavia, and tens of thousands of people are expected to develop fatal cancers as a result. Details of the Chernobyl accident illustrate many aspects of basic nuclear physics and of reactor engineering and control.

Ironically, the accident occurred during a test of the power supply for the emergency core-cooling system (ECCS), de­signed to dump water on the reactor core in the event of a loss-of-coolant accident. Mismanagement of the test, serious operator errors, and reactor design all contributed to the Cher­nobyl disaster. Sequence of events leading to the Chernobyl reactor explo­sion, shown as a plot of reactor power as a function of time. Time scale is not linear.

Preparation for the test began at 1 :00 A.M. on April 25 as operators slowly decreased the reactor output from its normal 3200 MW thermal power to 1600 MW, a process that took 12 hours. Then, following the test plan, they disconnected the turbine-generator and disabled the emergency core-cooling system. Although the plan called for shutting off the ECCS to prevent its coming on and disturbing the test, this move vio­lated the reactor's operating procedures. When the operators resumed lowering the reactor power, one of them failed to set an automatic control that would have maintained a thermal power at 700 to 1000 MW. The power level plunged to a mere 30 MW.

We know that fission products act as "poisons," ab­sorbing neutrons and thereby inhibiting fission. A particularly virulent reactor poison is xenon-135, whose cross section for neutron absorption is 4400 times that of 235U. Xenon-135 forms in the 6.7-h-half-life decay of 135I. In normal operation, the 135Xe concentration reaches a steady level in which neutron capture destroys the isotope as quickly as 135I decay creates it. But when reactor power decreases, neutron production drops and with it the destruction of 135Xe. But 135I continues to decay into 135Xe, so xenon concentration increases. The "poisoning" effect then makes it difficult to raise the reactor power until several of the xenon's 9.2-h half-lives have passed.

At Chernobyl, an operator's error had resulted in a rapid power drop, leading to high 135Xe concentration. Impatient to complete the test, operators committed another safety viola­tion: To compensate for neutron absorption in the xenon, they withdrew too many control rods. By 1: 19 A.M. on April 26, they had managed to raise the power to 200 MW, still well below the 700 MW minimum needed for the test. About the same time they turned on two additional cooling water pumps, as called for in the test procedures.

In a US. light-water reactor, the cooling water is also the moderator. But in a graphite-moderated reactor like Cher­nobyl, the dominant effect of water is to absorb neutrons. So the additional water required withdrawal of still more control rods. Now the reactor was in a dangerous situation: An increase in power would boil water, decreasing neutron absorption and thus increasing the fission rate. That would make the water boil even faster, increasing the power even more, and a runaway reaction could result. Worse, with so few control rods in place, the reaction might be sustained by prompt neutrons alone, resulting in a power increase too rapid to halt with mechanical control rods.

The Chernobyl operators realized they had too much water, and at 1 :22 A.M. they reduced the flow. But they did not imme­diately reinsert control rods. Thirty seconds later a computer warned that the reactor should be shut down. Ignoring the warning, operators continued the test by diverting steam from the turbine-generator. The decreased load caused more water to boil, again reducing neutron absorption. The reactor went supercritical from prompt neutrons alone, and the power level soared by a factor of 4000 in 5 seconds.

The power surge ruptured water pipes, causing a steam explosion that lifted the heavy concrete reactor cover. A second explosion followed, caused perhaps by hydrogen generated from steam reacting with the zirconium cladding on the fuel rods. The graphite moderator caught fire, and heavy smoke carried highly radioac­tive fission products into the atmosphere. Substantial radiation release continued for 10 days, and fallout dropped on much of Europe . The distribution of 137Cs following Chernobyl accident is shown in the map.

Could a Chernobyl accident happen in the United States?

I For commercial light-water reactors the answer is decidedly no. Loss of the water coolant/moderator in a light-water reactor immediately halts the chain reaction, making a runaway reac­tion impossible. But that doesn't mean light-water reactors are entirely safe. Even after the chain reaction stops, the immense heat generated by radioactive decay is enough to melt the core.

Chernobyl Health Effects

An explosion in a nuclear reactor at the Chernobyl nuclear power plant on 26 April 1986 caused the release of substantial quantities of radionuclides during a period of ten days. Airborne material was dispersed throughout Europe from the site in Ukraine. As the contaminated air spread throughout Europe and beyond, local weather condi­tions largely determined where the radionuclides were to fall. Rainfall caused more radionuclides to be deposited in some areas rather than others.

The accident had a catastrophic effect locally and high radiation expo­sures of emergency workers led to the deaths of 31 people, including 28 firemen. The firemen received large external doses from deposited radionuclides, between 3 and 16 Sv, and contamination on their skin led to severe erytherna, mostly due to beta emitters. A further 209 people were hospitalized of whom 106 were diagnosed as having acute radiation sick­ness. Fortunately all of these people recovered and were able to leave hospital within a few weeks or months.

In terms of doses to people in the vicinity and beyond, the most significant radionuclides were iodine-131, caesium-134 and caesium-137. Almost all the dose was caused by external irradiation from radionuclides on the ground, by inhalation of iodine-131 giving rise to thyroid doses, and by internal irradiation from radionuclides in foodstuffs.

Following the accident, over 100 000 people were moved from their homes in what are now Belarus, Ukraine and the Russian Federation, and various areas became "restricted" because of the levels of fallout on the ground. A vast clean-up operation was mounted at the Chernobyl reactor site itself involving over 750000 people. The people doing the decontamination work became known as "liquidators", and some of them received doses above the ICRP dose limit of 50 mSv. Such exposures may be justified in accident situations and ICRP recommends that exposures should not exceed 500 mSv in such circumstances. This ensures that workers could not experi­ence any deterministic effects of radiation exposure, and published data from monitor­ing teams show that the average doses were kept below 165 mSv in the first year after the accident. In subsequent years, they were gradually reduced to below 50 mSv.

There have been exhaustive studies of populations in the vicinity of Chernobyl and elsewhere, looking for possible health effects from the accident. The only significant effect that has so far been shown to be caused by radiation is in children in regions of Belarus and Ukraine, who have an increased incidence of thyroid cancer due to intakes of iodine-131 , particularly through drinking milk contaminated with iodine. Iodine-131 is a short lived radionuclide (8 days half life) known to concentrate in the thyroid, and using monitoring and other data it has been possible to estimate risk factors for this health effect in children. In 2000, UNSCEAR published a review of the effects of the Chernobyl accident. Their scientific assessments indicated that there had been about 1800 cases of thyroid cancer in children who were exposed at the time of the accident. Fortunately, in the great majority of cases, it is not a fatal condition, although it is a serious illness.

UNSCEAR found no scientific evidence of increases to date in the incidence of any other health effects that could be related to radiation exposure. This does not mean that there will not be any other effects – the most highly exposed individuals have an increased risk of suffering radiation-associated effects in the Mure - but UNSCEAR concluded that the great majority of the popula­tion are not likely to experience serious health consequences attributable to radiation from the accident.

The other serious health effects seen in local populations appear to be the result of the stress and anxiety caused by the accident, including the fear of radiation itself. Although these effects are different in kind to the thyroid disorders mentioned above, they are no less real and occurred widely throughout Europe in regions affected by the fallout. For example in Scandinavia, doses of about 0.1 mSv were received on average during the first few weeks after the accident, and many people reported to their doctors feelings of nausea. headaches, diarhea and some skin rashes. Following a century of scientific study of the effects of radiation, it can be concluded that it is not possible that such Iow doses could lead directly to the effects reported. However, a potent fear of radiation is obviously real for some people, and this was one of the lessons of the Chernobyt accident.

b) Three Mile Island Accident

A partial meltdown occurred in the 1979 Three Mile Island accident in Pennsylvania, but fortunately the reactor's containment structure held in nearly all the radioactivity. The threat of a hydrogen explosion during that accident-a possibility not previously considered in reactor accident scenarios-showed that we may not yet realize all the potentially dangerous situations possible in a system as complex as a nuclear power plant .

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