SCIENCE BEHIND FUKUSHIMA NUCLEAR ACCIDENT

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1 SCIENCE BEHIND FUKUSHIMA NUCLEAR ACCIDENT Shared by W. H. Chan, Department of Chemistry, Hong Kong Baptist University Introduction Comparing with the traditional fossil-fueled power plants, the nuclear power plants produce no carbon dioxide to add to the greenhouse effect and they add no soot, fly ash, sulfur oxides, or nitrogen oxides to the atmosphere. global warming, air pollution, or acid rain. They contribute almost nothing to They also substantially reduce the national dependence on foreign oil or coal and lower the country s trade deficit. It was estimated by the US that if the 104 nuclear power plants in the US were replaced by coal-fired power plants, airborne pollutants would increase by 18,000 tons/day. Understanding of the nuclear power technology Over the last half century, basically two types of Fission Reactors have been deployed for generating electricity by many countries worldwide: 1. Pressurized light-water reactor The fuels rods of this reactor contain pellets of uranium (or uranium oxide) shielded with metal shells, which are used to generate power in each reactor core. In a case of meltdown, the pellets become molten, burning their way through the containment vessels. Naturally occurring uranium is made up mostly of 238 U, which does not fission. fissionable 235 U is only 0.7%. On the other hand, the natural abundance of the An expensive process is required to enrich the nuclear fuels to at least to 2.1% in 235 U minimally needed for light-water reactors. As shown in the Figure 1, fissile atomic nucleus like 235 U absorbs a neutron, fission of the atom can take place. Fission splits 235 U atom into two smaller nuclei such as 90 Sr and 143 Xe with kinetic energy and at the same time also releases γ radiation and three neutrons. radioactive iodine-131 or cesium-137, respectively. Another common mode of fission of 235 U affords A portion of these neutrons may later be absorbed by other 235 U atoms to trigger the chain reaction. This nuclear chain reaction can be controlled by using neutron moderators to change the portion of neutrons that will go on to cause more fissions. Both graphite (used 1

2 in Chernobyl Nuclear Power Plant) and water serve as the moderator for regulating the fission process. = neutron 235 U Other common types of splitting: 235 U + 1 n Y I n 0 90 Sr Xe Rb Cs n 0 X 238 U 235 U X Radioactive 131 I released in great quantity in the nuclear fission process. 90 Sr Xe U 235 U X Huge energy released by neutron triggering fission process : 1 kg uranium fuel = 17,000 kg of coal Figure 1. Controlled nuclear chain reaction Important features of the chain reaction Scheme 1 Pressurized light-water reactor (PWR) Power Plant The huge energy released by the nuclear fission converts water into steam which powers the turbine for generating electricity as shown in Scheme 1. The reaction is also controlled by the insertion of boron steel or cadmium control rods. Both 2

3 boron and cadmium can absorb neutrons readily, removing them starts the chain reaction and pushing them back stops the reaction. 2. Breeder reactors (Making more fuel than they burn!!) Although 235 U processes an extremely concentrated form of energy, its reserve in the world is quite limited. The major isotope 238 U, if it can be used would be sufficient to run nuclear reactors for hundreds of years, however does not fission. The Breeder Reactor was developed to use 238 U as the nuclear fuel to generate electricity. This can be achieved by irradiating non-fissionable 238 U with FAST neutrons via the nuclear reaction diagrammed in Figure 2 shown below. A breeder reactor is built with a core of fissionable 239 Pu. The plutonium 239 core is surrounded by a layer of 238 U. As 239 Pu undergoes spontaneous fission, it releases neutrons and huge amount of energy. The neutrons released in turn can convert more 239 U to 239 Pu. In other words, this reactor breeds its own fuel as it operates. Efficient production of 239 Pu requires a breeder reactor, not the PWR reactor, operating with FAST neutrons. In this way, the reactor breeds more fuel than it consumes. With this nuclear technology, there is enough uranium-238 to last several centuries. t 1/2 = 24 min 238 U 239 U 239 Np 93 Gamma emission Beta emission 238 U chain reaction with another uranium atom + fission products HUGE ENERGY t 1/2 = 2.3 day 239 Pu 94 (Bred plutonium) Beta emission Figure 2 Production of 239 Pu from 238 U bombarded with fast neutrons 3

4 Scheme 2 Breeder Reactors Nuclear Power Plant Intrinsic problems associated with the Breeder Reactors Plutonium is highly toxic and has a half-life of about 25,000 years. It emits α-particles, making it extremely dangerous if ingested. It is estimated 1µg in the lungs of a human is enough to induce lung cancer. Its long life time also create an almost impossible disposal problem if large amounts of this material are produced in nuclear power plants. Plutonium has a fairly low melt point (i.e. 640 o C), water cannot be used as the coolant if Plutonium is used as the fuel. Instead, liquid sodium must be used (see below table listing the specific heat of common elements). Sodium possesses the highest specific heat capacity among all elements. In the event of an accident a catastrophe could develop because sodium reacts violently with water and air. water 1.00 kj/kg sodium 1.21 kj/kg uranium 0.12 kj/kg potassium 0.75 kj/kg plutonium 0.13 kj/kg lead 0.13 kj/kg mercury 0.14 kj/kg silver 0.23 J/kg A further hazard is that a nation or a terrorist group could readily convert reactor-grade plutonium, unlike the uranium used in nuclear reactor, to a nuclear bomb. No breeder reactors are operating in the US, but they are used in France and other countries. 4

5 The nature of radiation Understanding radiation and how it is measured is essential in determining its health impact on the body. The radiation scale and the effects of dosage on cell and tissue damage facilitate our awareness of the major health effects of radiation. Radiation consists of particle rays such as alpha (α) and beta (β) rays, electromagnetic rays such as gamma (γ) and x-rays, and neutron particles. α- Rays are produced after spontaneous decay of radioactive atoms including radon, uranium, and plutonium. α-rays pass less than one millimeter in water, and because a single piece of paper can stop an alpha ray, health effects only appear when α-emitting particles are ingested via food supply. The β-rays consist of a fast electron with a mass of 1/2000 of the mass of a proton or neutron. These rays are produced by the spontaneous decay of 14 C, 90 Sr, 32 P, and 3 H (tritium). Like α-rays, βrays only cause health problems through internal exposure. γ -Rays, which are similar to visible light, have a shorter wavelength and higher energy than that of UV rays. Gamma rays are produced following the decay of radioactive materials such as cobalt-60, which because of its ability to penetrate deeply into our bodies, is often used for cancer radiotherapy. X rays are similar to γ-rays, but they consist of an array of different wavelengths, whereas γ-rays have a fixed value specific to the radioactive material. Neutron particles, the last component of radiation, are released after nuclear fission of plutonium or uranium as shown above. Scales and Dosages Radiation is measured in many different ways. The first people to devise a way to measure radiation were Marie and Pierre Curie. The unit of measurement they used was called the curie, and it describes the intensity of a sample of radioactive material in terms of atoms of the material that decay each second. The basis for this measurement is the rate of 37 x 10 9 atoms per second for one gram of radium. The other common units for showing the intensity of radiation are: rems and sieverts. The US uses the unit of rems and the SI unit is sievert (Sv). They can be interconverted by the following relation: 1 rem = 0.01 Sv = 10 milli-sv (or 1 Sv = 100 rem). Common way to measure radiation is through using a Geiger counter. A Geiger counter contains a special gas-filled tube that separates two electrodes. When radiation passes through the tube, it interacts with the gas, causing an electrical pulse that can be measured on a meter or by audible clicks. The number of pulses in a given time is a measure of the intensity of radiation. The background radiation of our environment can be measured by a Geiger counter. The working principle of a Geiger counter is shown in the figure below. 5

6 1. Radiation (dark blue) is moving about randomly outside the detector tube. 2. Some of the radiation enters the window (gray) at the end of the tube. 3. When radiation (dark blue) collides with gas molecules in the tube (orange), it causes ionization: some of the gas molecules are turned into positive ions (red) and electrons (yellow). 4. The positive ions are attracted to the outside of the tube (light blue). 5. The electrons are attracted to a metal wire (red) running down the inside of the tube maintained at a high positive voltage. 6. Many electrons travel down the wire making a burst of current in a circuit connected to it. 7. The electrons make a meter needle deflect and, if a loudspeaker is connected, you can hear a loud click every time particles are detected. With a Geiger counter in our disposal, our risk to radiation can be detected. Table 1 shows the risks on exposure to radioactive materials. 6

7 Table 1. Risks on exposure to radioactive materials Single dosage examples Exposure in millisieverts (msv) Dental radiography People living within 16 km of Three Mile Island accident Brain CT scan Chest CT scan International Commission on Radiological Protection From average dose of 0.08 msv to maximum dose of 1 msv msv 6 18 msv 500 msv recommended limit for volunteers averting major nuclear escalation International Commission on Radiological Protection 1000 msv recommended limit for volunteers rescuing lives or preventing serious injuries Yearly dosage examples: es: Living near a nuclear power station: msv/year Living near a coal power station: msv/year Cosmic radiation (from sky) at sea level: 0.24 msv/year Terrestrial radiation (from ground): 0.28 msv/year Average individual background radiation dose: 2 msv/year; 1.5 msv/year for Australians, 3.0 msv/year for Americans New York-Tokyo flights for airline crew: 9 msv/year Smoking 1.5 packs/day: msv/year Current average limit for nuclear workers: 20 msv/year Background radiation in parts of Iran, India and Europe: 50 msv/year Elevated limit for workers during Fukushima emergency: 250 msv/year Radiation Health Effects of Dosages The different particles in radiation don t always directly damage cells. Because neutrons don t have an electrical charge, they rarely damage cells, but often cause ionization (loss of an electron resulting in the disruption of chemical bonds) in the body after interacting with hydrogen nuclei. Although neutrons don t directly damage cells, they cause more severe damage to the body than γrays. Ionization can also cause oxidation (addition of oxygen atoms), which results in chromosome aberrations, mutations, or cell death. Radioactive materials upon nuclei decay emit detrimental radiations (i.e. α, β, or γ rays). On encountering molecules in their path (including molecules in cell tissues), they knock electrons out of the atomic shells and generate reactive species like radicals. 7

8 Table 2 summarizes the penetration depth for these three radiations and their relative effectiveness in producing damage. Table 2. Paths of energetic particles in biological tissue Type of Can be stopped by Range in Relative biological radiation biological tissue effectiveness Alpha α A sheet of paper cm Beta β 1- mm sheet of Aluminum 3 cm 1 Gamma γ Few inches of concrete wall ~20 cm 1 When living tissue is affected by ionizing radiation, there is always a chance that the cells are harmed or destroyed. However, the damage depends on what kind of radiation is received, how the dose is absorbed, how quickly it is absorbed, and how strong the tissue is. The nucleic acids (in DNA), which make up each cell s genetic materials, are particularly vulnerable. If the nuclei of reproductive cells are damaged, the result may be genetic mutations and, thus, the transmission of hereditary disorders to succeeding generations. At high levels, radiation damages all cells, particularly those which divide rapidly like white blood cells and the cells of the intestinal linings. The doses of radiation absorbed are related to the energy, and high doses of radiation causes major health effects, such as burns, cell damage, and death. Table 3 summarizes the symptoms of affected human subject on exposure of different extent of radiation. Apparently, the effects of dosage on the health effects of radiation are directly proportional the higher the dosage, the more damage occurs. Table 3. Symptoms of affected human subject on exposure of radiation Dosage exposure Symptoms of acute radiation (within one day) Sv None Sv Some people feel nausea and loss of appetite; bone marrow, lymph nodes, spleen damaged. 1 3 Sv Mild to severe nausea, loss of appetite, infection; more severe bone marrow, lymph node, spleen damage; recovery probable, not assured. 3 6 Sv Severe nausea, loss of appetite; hemorrhaging, infection, diarrhea, peeling of skin, sterility; death if untreated Sv Above symptoms plus central nervous system impairment; death expected. Above 10 Sv Incapacitation and death. 8

9 Hazards of nuclear power Reactor safety: Under normal operating conditions, the radiation released by nuclear reactors is very low, but the potential risk of radioactive fallout is a serious concern. The average reactor contains as much radioactive materials as that released by the Hiroshima atomic bomb. Discount the recent accident in Fukushima caused by earthquake and tsunami, two major historical serious accidents were triggered by both equipment malfunction and human errors. Three Mile Island: In March 1979, the uranium fuel rods in the nuclear power plant at Three Mile Island near Harrisburg, Pennsylvania, melted down when cooling water was lost. The malfunction of the primary cooling system coupled with the failure of the auxiliary pumps triggered the accident. The worse thing was that, due to operator s error, the emergency cooling system was momentarily turned off. A few minutes after the pump failure, by the time it was turned back on, a large gas bubble had formed as a result of the thermal decomposition of water to hydrogen and oxygen. The bubble cut off the cooling water from reaching the fuel rods, which partially melted. Since then, the US has stopped the construction of new nuclear power plant. 140,000 population of the town were evacuated and fortunately no fatal case was recorded. The accident was classified by the International Atomic Energy Agency (IAEA) as a level 5 nuclear plant accident. Chernobyl disastrous nuclear accident: Due to a series of operating errors, at 1:23 am on April 26, 1986 in the Ukrainian town of Chernobyl, a graphite regulated reactor Unit 4 in the gigantic nuclear power plant exploded. The fuel tubes, red hot from lack of coolant water, had broken through into the water steam system, causing a colossal explosion. The core of each reactor was made up of more than 1500 fuel tubes filled with enriched uranium fuel. In the accident, the containment walls around the reactor were blasted apart, and a deadly plume of radioactive material shot into the night sky. A second fiery explosion occurred a few seconds later when hydrogen (produced by steam mixing with hot graphite) reacted violently with oxygen in air. The fire in the reactor core proved very difficult to extinguish. The fire continued for several days. Nine days after the explosion, the wind veered to the south, and radiation levels almost 100 times the normal level were recorded in Kiev, a city of 2.4 million people 60 miles south of Chernobyl. By the end of June, the entire reactor was encased in 300,000 tons of concrete. 31 people died in the immediate aftermath of the accident. Most of those who died were heroic firemen who, well aware of the dangers, give their lives to prevent fires from spreading to the nearby Unit 3. In the years to come, it will be impossible to estimate how many cases of cancer will be directly linked to the Chernobyl accident. The accident was by far the worst of its kind and was classified as level 7 by IAEA. 9

10 Biological effects of ionizing radiation caused by radioactive materials The potential for damage is crucially dependent on the location of the isotope emitting the radiation, relative to target molecules. When a nuclear explosion occurs in the open atmosphere, radioactive materials can rain down on parts of Earth thousands of miles away, days and weeks later. In such radioactive fallout, not only the air is polluted with radioactive isotope, but also vegetables/fruits and milk products will be contaminated. If the radioactive isotope material is inhaled or ingested with contaminated food, then our body may take up these toxic substances. The most worrisome isotopes are those that lodge in particular tissues and are long-lived enough to do significant damage. Table 4 lists four isotopes of particular human concern. 239 Pu is part of the nuclear fuel cycle in Breeder reactor, while 90 Sr, 131 I, 137 Cs are fission products. They all live long enough to cause significant damage, and they all concentrate in particular tissues as shown in the Table. Table 4. Some hazardous radioactive isotopes resulting from nuclear fallout Element Type of radiation Half-life Site of concentration 239 Pu Sr 131 I Cs 55 Alpha 24,360 years Bone, lung Beta 28.8 years Bone, teeth Beta, alpha 8 days Thyroid Beta, alpha 30 years Whole body The penetrating power of α-emitter is less than the thickness of the skin s protective outer layer of dead cells. In contrast, when α-particle is ingested into the body, it produces a high density of localized damage, with an appreciable potential for cancer induction. A β-ray, being an energetic electron, is much lighter than an αray and is only singly charged. It ionizes only about 1 in 1,200 atoms in its path and thus the damage density is lower. The travel range in biological tissue is however larger, external β radiation is therefore hazardous and must be shielded. A γ ray is a high-energy photon and its probability of hitting an atom in its path is quite low. On the other hand, it can travel around 20 cm in water or biological tissue and 10

11 transfer a large amount of energy, causing damage in human body. Energetic γ emitters like neutrons require heavy shielding such as several inches of concrete wall. As mentioned above, the science of nuclear fallout is exceedingly complex. We could only focus on two of the more worrisome isotopes. Of all the isotopes, strontium-90 presents the greatest hazard to people. The isotope has a half-life of 28.8 years and it reaches humankind primarily through milk and vegetables. Because of its similarity to calcium (both are group 2A elements), 90 Sr is readily up-taken and incorporated into bone. There it remains a source of internal radiation for many years. 131 I may present a greater threat immediately after a nuclear explosion. Its half-life is only 8 days, but it is produced in relatively large quantity. Iodine-131 is efficiently carried through the food chain. In the body, it is concentrated in the thyroid gland and causing damage. Medical doctors may prescribe people living in the vicinity of the nuclear accident area with iodide tablets. Measures taken to minimizing the damaging effect from radioactive fallout Evacuating from the emission source with a safe distance, farer the better Postponing any travel plan to Fukushima and its vicinity Avoiding direct contact with materials contaminated with radioactive materials Ingesting iodide tablets to prevent the intake or accumulation of radioactive iodine-131 for those living very close to the source of nuclear emission Refraining from consuming food products produced near the nuclear accident site Aftermath of Fukushima nuclear accident the case for nuclear power In the 25 years since the Chernobyl disaster, the nuclear industry s safety record has been generally quite good. Other forms of energy generation have exacted a much higher human toll over this period of time. Fukushima accident should not prompt a hasty retreat from the use of nuclear power in solving our energy problems. Nuclear power regulation must be reviewed in the wake of Fukushima. Perhaps the International Atomic Energy Agency s safety guidelines should be made compulsory. As a scientist, by taking the lesson of Fukushima seriously, we have the faith to believe that next generation nuclear reactors will reduce risks further. The notion of completely abandon the nuclear power as an important energy production option is totally infeasible. The technology represents for roughly 13 per cent of global electricity generation. This slice of capacity cannot easily be replaced. 11

12 A more prudent attitude must be imposed for many Asian developing countries including China on the planning of construction of new nuclear power plants. The highest safety measures must be upheld for the planning, construction and operation of nuclear power plants. Safety issues cannot be compromised with the cost consideration. References: 1. P. Buell and J. Girard, Chemistry: An environmental perspective, Prentice-Hall, New Jersey 1994, P T. G. Spiro and W. M. Stigliani, Chemistry of the Environment, Prentice-Hall, New Jersey 1996, P J. W. Hill and D. K. Kolb, Chemistry for Changing Times, 10 th Edition., Pearson, 2004, New Jersey 2004, Editorial, Financial Times, March 20, 2011 issue. 12

Radioactivity. L 38 Modern Physics [4] Hazards of radiation. Nuclear Reactions and E = mc 2 Einstein: a little mass goes a long way

Radioactivity. L 38 Modern Physics [4] Hazards of radiation. Nuclear Reactions and E = mc 2 Einstein: a little mass goes a long way L 38 Modern Physics [4] Nuclear physics what s inside the nucleus and what holds it together what is radioactivity, halflife carbon dating Nuclear energy nuclear fission nuclear fusion nuclear reactors