Nuclear Incidents and Accidents

Nuclear Incidents and Accidents

8Nuclear and Hydropower

Nuclear Incidents and Accidents

A nuclear incident occurs when released radioactivity is contained; that is, prevented from escaping to the outside environment with no resulting loss of life and with minimal impact on the health of those exposed to radiation. Nuclear accidents involve radioactivity escaping to the outside environment with or without injuries or deaths. The history of nuclear accidents starts in 1952 with a partial meltdown of a reactor’s core at Chalk River near Ottawa, Canada, when four control rods were accidentally removed. The resulting radioactive release was contained in millions of gallons of water and no injuries resulted. In 1957, Windscale Pile No. 1 north of Liverpool, England, sustained a fire in a graphite moderated reactor and spewed radiation over a 200 square mile area. In the same year, an explosion of radioactive wastes at a Soviet nuclear weapons factory in South Ural Mountains forced evacuation of over 10,000 people from the contaminated area. In 1976, a failure of safety systems during a fire nearly caused a reactor meltdown near Greifswald in former East Germany.

Human error played a major role in both Three Mile Island incident and Chernobyl accident. Nearly all the radioactive release of the Three Mile Island incident was kept within its containment system, as it was designed to do. Soviet nuclear power plants do not have containment systems built to withstand pressure generated from a ruptured reactor system, but are housed in buildings to protect against the weather. Nor did the Soviet Union select a safe plant design. Whereas most reactors shut down when the water moderator in the core boils away (an example of a negative feedback system), the same phenomenon with the Soviet graphite moderated reactor led to a runaway power surge (an example of a positive feedback system). Fukushima was not a matter of human error in its operations. Fukushima is a victim of a tectonic plate shift and an accompanying tsunami for which the plant turned out to be extremely vulnerable. Some maintain that human error is ultimately the cause of all accidents; here it was the decision to remove land to lower the plant’s elevation with respect to the ocean surface.

Three Mile Island Incident

The Three Mile Island incident in March 28, 1979 was preceded by the release of the movie China Syndrome on March 16, 1979, a case of Hollywood prescience or fiction preceding fact. China Syndrome was about a nuclear plant with internal problems that, if unattended, could have led to a core meltdown, which, after melting through the containment system, would then burrow its way toward China. The film dealt with management’s decision to ignore and cover up the plant’s problems until exposed by a crusading journalist.

The Three Mile Island incident proved that nuclear power plants were not immune to accidents, despite claims to the contrary. In this case a malfunction of a secondary cooling circuit caused temperature in the primary coolant to rise, shutting down the reactor as expected. What was not expected was failure of a relief valve to close and stop the primary coolant from draining away. The relief valve indicator on the instrumentation panel showed the valve as being closed, making it difficult for operators to diagnose the true cause of the problem. As a result, coolant continued to drain away until the core was uncovered. Without coolant, residual decay heat in the reactor core raised its temperature sufficiently for a partial core meltdown.

Although the instrumentation panel failed to show that the relief valve was still open, blame for the accident was eventually assigned to inadequate emergency response training for the operators. In other words, despite faulty indication of the relief valve, operators should have identified the true cause of the problem and taken proper action before it was too late. The containment system performed as it was designed to do—nearly all released radioactivity was prevented from escaping to the outside environment. Contrary to the China Syndrome plot, management did not hide the plant’s problems from the public and the core would not have melted through the earth.

There were minor health impacts and no injuries from the Three Mile Island incident. Even though the nuclear power industry took remedial steps to improve training and operations to make reactors even more safe and reliable, Three Mile Island incident dealt a deathblow to the US nuclear power industry. The incident halted all further orders of nuclear power plants in the US and was blamed for the cancellation of over 40 orders for plants not yet started. Some maintain that the Three Mile Island incident just turned out to be a convenient excuse for cancelling plants that could not be built without enormous cost overruns and construction time delays. The economic benefit of nuclear power was not living up to its initial expectations. Most plants under construction were completed, although a few were converted to fossil fuel plants. Public concern over nuclear safety generated by this incident was sufficient to prevent the Shoreham plant on Long Island from becoming operational when it was completed in 1984. A study showed that if a more serious incident than that of Three Mile Island were to occur at the Shoreham plant, the few bridges and tunnels connecting Long Island with the mainland would preclude any large-scale evacuation. For this, the plant was dismantled in 1992.

Radiation

Radiation is ever present. It is defined as energy traveling through space epitomized by sunlight that delivers light, heat and, if one is not careful, sunburn. Sunglasses, shade, clothing, and sunscreen protect from discomfiting or harmful effects of the sun’s radiation. Radiation comes in different wavelengths with long wavelengths associated with radio waves, and in descending order, microwaves, infrared, visible light from red to blue, ultra violet, X-rays, gamma, and cosmic rays. Radio waves are measured in meters, microwaves in hundredths, visible light in thousandths, and on down to cosmic waves in hundred millionths of a meter. Artificial X-ray radiation identifies medical and dental problems hidden from view and treats cancer.

Most elements are stable, but some isotopes of stable elements (same number of protons, but greater number of neutrons) have excess energy that is emitted as radiation as the isotope steps down from a higher to a lower energy state before becoming stable. Some isotopes decay remaining the same element, but others change to different elements if one or more protons are ejected. Radiation is measured in becquerel (Bq), where one Bq is defined as one atomic decay per second.[1] Table CW8.1 shows radioactivity of some materials.[2]

Table CW8.1 Examples of Radioactivity in Bq

Quantity / Item / Bq
1 kg / Granite / 1,000
1 kg / Coffee / 1,000
1 kg / Coal Ash / 2,000
100 sq meters / Australian home / 3,000
100 sq meters / European home / 30,000
Adult human / 100 Bq/kg / 7,000
Smoke detector / Americium / 30,000
1 kg uranium ore / Australia (0.3 percent U) / 500,000
1 kg uranium ore / Canada (15 percent U / 26 million
1 kg / Low level nuclear waste / 1 million
Radioisotope for medical diagnosis / 70 million
Luminous Exit sign (1970s) / 1 TBq*
1 kg / High level nuclear waste / 10 TBq
Radioisotope for medical therapy / 100 TBq

*One tera-becquerel (TBq) is a million million Becquerel or 1012 atomic decays per second.

Atomic decay can take various forms. Alpha particles are helium nuclei consisting of two protons and two neutrons stripped of their two electrons and are emitted from decay of heavy elements such as uranium and radium. Emission of an alpha particle, because it contains protons, changes an isotope of one element to an isotope of another. Radon is a radioactive gas found in the earth’s surface and in building materials and collects in the atmosphere of closed spaces such as basements. Radon emits alpha particles, which at best can barely penetrate skin, but are dangerous if inhaled. In the lungs, alpha particles penetrate cell membranes and can do significant damage including causing cancer.

As isotopes decay by emitting a neutron to a more stable state, beta particles, which are fast-moving electrons, are also emitted. Neutron and beta emission do not change an element, but reflects a change to a lower and eventually stable energy state. Beta particles are more penetrating than alpha particles, but can be easily shielded by a few millimeters of wood or aluminum. Beta particles can penetrate a little way into human flesh, and, of course, can do more damage if inhaled or ingested as they can penetrate cell membranes of vital organs. Exposure to high energy beta rays produces a type of sunburn slower to heal than that from the sun while low energy beta rays are stopped by skin or cellophane. Alpha and beta radioactive substances are safe if kept in appropriately sealed containers.

Ionizing radiation with wavelengths above visible light affects living cells; if ionizing radiation is strong enough, an organism (man) can suffer from radiation sickness; or if severe enough, death. Birth defects can occur if cells associated with reproduction are affected by radiation. The most dangerous radiation are high-energy beams of gamma rays similar to X-rays, but more energetic. They are associated with many forms of radioactive decay and are very penetrable, requiring substantial shielding. Gamma rays are the main hazard to people dealing with radioactive materials. Radiation dose badges worn by workers during times of exposure detect and monitor gamma ray exposure. Geiger counters measure gamma rays. All life is exposed to low-level background gamma radiation from the sun and universe plus radioactive decay in rocks and other naturally occurring substances. X-rays are lower frequency, less energetic gamma rays while cosmic radiation from space, normally high energy protons, is the most energetic form of gamma rays. Neutrons are very damaging form of radiation that can penetrate far into living matter, but are mostly released by nuclear fission and encountered far less often outside a nuclear reactor core.

Hence a becquerel (Bq) defined as one atomic decay per second does not measure the ionizing damage to human cells. An alpha particle stopped by skin is not nearly as deleterious as a high energy gamma ray tearing apart the genetic information within cells. Thus it would be preferable to place radiation on the same scale in terms of its biological impact on living cells regardless of the form of radiation. Biological effects of radiation are measured in Sieverts, the equivalent dose of receiving one joule of X-rays per kilogram of bodily mass.[3] A Sievert (Sv) is equivalent to 100 rem, an older measure of radiation damage. A millisievert (mSv), one thousandth of a Sievert, is the normal unit of measurement. Table CW8.2 provides the level of impact of different doses of millisieverts.

Table CW8.2 Likely Effects of Whole-body Radiation Doses

Dose in mSv/Year / Likely Effect
1.5–2.5 including 0.7 from radon in air / Average global normal background
3 including 2 from radon in air / North America normal background
15 / Parts of India
40 / Parts of Brazil and Sudan
50 / Parts of Europe and Iran (260 in Ramsar)
10 / Maximum allowable dose uranium miner in Australia
20 / Radiological personnel in nuclear industry, hospitals
50 / Lowest dose with evidence of causing cancer
100 / Probability of cancer begins to increase for bodily cells; for reproductive cells, damage may be genetic in nature causing birth defects
250 / Maximum dose for workers at Fukushima
1,000 (1 sievert) / In short term whole-body dose, radiation sickness
10,000 (10 sieverts) / In short term whole-body dose, death within a few weeks; between 2–5 sieverts severe radiation sickness with increasing likelihood of being fatal

Another measure of radioactivity is the half-life of radioactive substances. Radioactive decay is the disintegration of an unstable atom with an accompanying emission of radiation and atomic particles. Various types of radiation are emitted at each step of an isotope on its pathway toward a stable configuration where radiation ceases. Emission rates of radioactive decay become less frequent with time as unstable isotopes decay to another isotope on their pathway to stability. When all atoms are stable, material is no longer radioactive.

Radioactive decay occurs at a fixed rate for a given number of isotopic atoms and the half-life of a radioisotope is time required for one half of the atoms to reach the next step in radioactive decay. At one half-life, intensity of radioactive decay is cut by 50 percent because half of the atoms have been transformed to the next lower isotopic state. At two half-lives, intensity is cut by a factor of four. After nine half-lives, less than one-thousandth of the original activity will remain. Length of a half-life differs markedly for each radioisotope. For instance, cesium 137 has a half-life of 30.2 years and strontium 90 of 28 years, both common radioisotopes released during nuclear accidents. Radioactivity associated with these two isotopes falls in half in about 30 years, thus its decay in terms of becquerel will be cut in half and sieverts, a measure of biological damage, will also decline, but not necessarily in a linear relationship with bequerels. It may take several hundreds of years for intense releases of radioactive cesium and strontium to decay to an acceptable level. Other radioactive isotopes have half-lives of hundreds of years requiring thousands of years for a significant reduction in radiation. Alpha emitting plutonium 239 (Pu239), produced as a waste product in nuclear reactors, has a half-life of 24,300 years. Once ingested, it tends to concentrate in the liver, lungs, and bones causing severe biological damage from its high energy alpha emissions. Hundreds of thousands of years may be necessary to reduce radiation from Pu239 to a safe level. On the other hand, iodine 121 and 135 have half-lives of only 8 days, which means in a few months they may not be detectable. However rate of release of radioactivity is much higher for iodine than cesium or strontium because of its shorter half-life.

Release of radioactivity at Chernobyl and Fukushima is measured in terms of 100 quadrillion becquerel, a number that defies imagination. Radiation released at Fukushima was greater than at Chernobyl primarily because Fukushima involved three reactor meltdowns, not one as at Chernobyl.[4] Moreover Chernobyl was eventually contained in a cement sarcophagus whereas Fukushima is still open to the environment with no plan to contain the ongoing release of radioactive material. But that does not necessarily make Fukushima more dangerous than Chernobyl for humans. Radioactive material from Chernobyl was spread by wind over a wide land mass in an uneven fashion affecting different populated and unpopulated areas with a wide disparity in radiation intensity. The prevalent wind pattern over Fukushima spread much of the radioactivity over the Pacific Ocean, which was eventually deposited on its surface, then spread on the surface and in depth by atmospheric winds and ocean currents. Moreover, contaminated water from the reactor buildings is spilling toward the Pacific Ocean, not toward populated areas. This does not make Fukushima less of a calamity just because the Pacific Ocean was more affected by radiation than the Japanese mainland. In investigating radioactive impact, various sources had far different assessments on the amount of radiation released by Chernobyl and Fukushima and on their effect on the general population. Government spokespeople and environmentalists generally have polar views on the radiological consequences of these two accidents. Truth is probably lost in the middle.

Chernobyl Nuclear Accident

Chernobyl nuclear accident occurred on April 26, 1986. In one respect, Three Mile Island and Chernobyl are similar: both involved human error. At Chernobyl, a runaway reactor occurred during a test, ironically one associated with reactor safety—how long could turbines supply power when cut off from reactor power? What made Chernobyl so much worse than Three Mile Island was the nature of its reactor design, actions taken by operators to defeat safety features, and absence of a containment system (the reactor housing was not built to contain a pressure buildup from a rupture of the reactor or its piping). In conducting the test, automatic reactor trip mechanisms were disabled and emergency core cooling system was shut off. With its valves locked shut, none of the operators knew who had the keys! Having disabled the reactor’s safety features, two principal operators started “doing their own thing” in running two separate tests without communicating to each other what they were doing. On site management could have acted, but management was far away in Moscow who were virtually ignorant of what was happening in the plant.

Reactor design made a bad situation worse. Soviet reactors use graphite as a moderator and water as a coolant. Graphite has several undesirable features as a moderator. At too high a temperature, graphite can burn or react violently with steam to generate hydrogen and carbon monoxide, both combustible gases. In a US reactor, water, as both moderator and coolant, shuts down the reactor when it boils in the core. Void spaces in boiling water reduce the number of neutrons being slowed to keep the reactor critical (negative feedback). In the Soviet reactor, creation of void spaces in boiling water allowed a greater number of neutrons to reach the graphite moderator, increasing the fission rate (positive feedback). From a low power condition, operators retracted more control rods than recommended and the reactor went supercritical, generating enough heat to turn coolant to steam, which further increased the number of fissions. The resulting power surge ruptured the fuel elements and blew off the reactor cover plate. When air gained access to the core, the graphite moderator burst into flames and the resulting blast, along with escaping steam, ruptured the roof of the building housing the reactor. Large chunks of reactor core and graphite moderator were scattered outside the building, releasing far more radioactivity than nuclear bombs dropped on Hiroshima and Nagasaki.