Saturday, March 19, 2011

Radiation: What is Happening in Japan?

As I watched the horrific events unfold in Japan, I noticed that the concept of radiation was not well understood among much of the media. Most of us are not experts in this field but with the safety of nuclear energy now being openly questioned in many countries, perhaps it is time for us to gain a basic understanding so we can make informed choices for ourselves.


What is Radiation and What Does it Do To Our Cells?

Radiation consists of energetic particles. These particles range from photons of visible light to X-rays to very energetic electrons that can damage both our cells and the DNA inside our cells. The kind of radiation that people worry about most is often called ionizing radiation. These are very energetic particles that have enough energy to ionize an atom or molecule and this is what damages our cells. Ionization means that the energetic particle knocks out an electron. This actually happens in your cells at a low but chronic rate because of background radiation. Usually cells can detect and repair the damage or they can program themselves to die off and eliminate the potential genetic damage. However, sometimes cells can undergo a DNA mutation that is passed on and this can occasionally contribute to future cancer (if the cells are sperm or eggs, DNA damage can lead to birth defects). This is why up to 4000 people exposed to radiation during the Chernobyl nuclear accident later developed thyroid cancer and other genetic anomalies such as neural tube defects, Down syndrome and other chromosomal aberrations.

Radiation Poisoning

In addition to chronic exposure, people can also experience a single large dose of radiation, called acute radiation poisoning, for example, from nuclear explosions or handling a highly radioactive source in which a brief high exposure occurs. The amount of time between exposure and the development of initial symptoms of radiation poisoning often indicates how much radiation was absorbed. The first symptoms include nausea and vomiting and then diarrhea, usually within 24 to 48 hours. The cells of the gastrointestinal tract are rapidly dividing cells so they are destroyed first. These symptoms can occur when exposure is as low as 1 Sv (a Sievert is an SI unit of dose equivalent and is meant to quantify the biological effects of ionizing radiation). Blood cells, reproductive cells and hair cells also divide rapidly so they are also affected soon after exposure. Blood cell damage, if severe enough, can quickly lead to sepsis and death. Sepsis usually occurs when the bloodstream is overwhelmed by bacteria, but it can also occur when the bloodstream is full of dead and damaged cells. Either case stimulates a body-wide immune response so severe that organ systems can be damaged or shut down altogether. Rapid development of fever and vomiting within minutes of exposure is a sign of severe exposure. The good news is that even in severe cases, about half of all exposure victims survive (with severe sepsis being the leading cause of death).

From Ordinary to Dangerous Exposure Levels

For a comparison between exposure levels and physiological symptoms see this online chart. For a comparison between acute exposure levels and more ordinary radiation levels associated with X-rays, high altitude flights, etc., see this online chart.


Radioactive Elements

Our universe is made up of 118 known elements, many of which are radioactive. Radioactive elements have unstable atomic nuclei. These atoms emit radiation as they spontaneously transform from a high-energy unstable state into a lower energy stable state, which represents either a new isotope of the same element or a new element altogether. Many of these reactions cascade, creating different intermediate, and often also radioactive, elements along the way, eventually stopping at atoms with stable nuclei. Many elements come in more than one isotope, each of which may vary in nuclear stability. This means that an element, carbon for example, can come in isotopes that, although they all have the same number of protons in their nuclei, differ in their number of neutrons. Carbon-12 is the most abundant carbon isotope, with 6 protons and 6 neutrons, creating the most stable nuclear arrangement. Carbon-13 is also stable but less abundant. It has 6 protons and 7 neutrons. Carbon-14, used in radiocarbon dating, has a slightly unwieldy nucleus of 6 protons and 8 neutrons, which is unstable. Each carbon-14 nucleus will eventually decay into a nitrogen nucleus; the carbon atom will become a nitrogen atom. 

Both atoms have the same mass number, the same total number of protons and neutrons, but carbon-14 has an unstable arrangement of 6 protons and 8 neutrons and nitrogen has a very stable arrangement of 7 each of protons and neutrons. Carbon-14 has a half-life of about 5700 years. This is the time it takes for half of any given sample of carbon-14 to decay into nitrogen-14. In this process, a neutron is converted into a proton. When it does so, a high-energy electron and a massless particle called a neutrino are emitted. The emission of high-energy electrons is called beta decay. Neutrino radiation passes through us everyday (it comes from the Sun). In fact, neutrinos pass right through the Earth as though it is invisible to them. These particles are harmless to us, but beta radiation is not. Luckily, carbon-14 only occurs in trace amounts and it has a very slow decay rate, so it contributes very little to the background radiation to which we are exposed. Still, our bodies at any given time contain a few atoms of this radioactive element. It's part of our proteins, carbohydrates and our fat and it's part of the carbon dioxide in our lungs and tissues. An occasional high-energy electron blasts a cell or two and the damage is quickly repaired.

There are three kinds of radioactive decay: beta, gamma and alpha, and they are all emitted as part of different decay processes. Beta radiation is the emission of an electron. Alpha radiation is the emission of a helium nucleus, also called an alpha particle. Gamma radiation is the emission of a very high-energy photon. A particular radioactive element can emit different kinds of radiation as it decays through different decay modes. Different kinds of radiation have different dangers associated with them.

Beta Radiation

In this kind of radiation, or decay as it is also called, either an electron (or a positron, the electron's antimatter counterpart) is emitted. These beta particles can have energies ranging from a few MeV (mega-electron volts) up to a hundred MeV and their speed can vary, in some cases approaching the speed of light. All beta radiation is ionizing radiation.*  That means that beta particles have enough energy to knock an electron out of an atom or molecule and, as a result, damage living tissue by either changing the structure of biological molecules and altering their functions or making them nonfunctional. Beta particles can also strike molecules of DNA and create spontaneous mutations which can lead to cancer, or birth defects if the DNA is in an ovum or sperm cell. Although the energy of beta particles varies, most beta particles can be blocked by an aluminum sheet. Beta radiation can penetrate living tissue but not very far. However, this kind of radiation can be especially dangerous because, as it is made up of charged particles, electrons, it is strongly ionizing.

Iodine-131 and The Need for Iodine Pills

The beta radiation from iodine-131, a common and hazardous product of nuclear fission, can only penetrate between 0.6 to 2 mm of tissue. This isn't very far. The hazard with I-131 is not its penetration but how ionizing its beta particles are. 90% of tissue damage from this isotope comes from this ionization. Only 10% tissue damage comes from the far more penetrating gamma radiation that is also emitted as it decays (this damage occurs further away from contact). Iodine-131 becomes dangerous when it becomes airborne. This can happen when a nuclear plant or bomb explodes and sends radioactive dust into the atmosphere. It can be breathed in and it can land on water, plants and animals, which are ingested. This radioactive dust then accumulates in the thyroid where the iodine we consume is concentrated and used for thyroid functioning. As radioactive iodine decays inside the thyroid it damages it and this damage can later on lead to thyroid cancer. The risk of developing thyroid cancer is highest among the youngest, so fetuses and children are at high risk. Fortunately, people at risk of I-131 exposure can take iodine pills. This reduces the thyroid's retention of the radioactive iodine. It's then flushed out of the body in the urine before it can do much damage. Thousands of cases of thyroid cancer could have been prevented if these simple inexpensive pills had been available to the people living near the Chernobyl nuclear plant just before or immediately after the explosion there. The half-life of iodine-131 is 8 days so only a short pill regimen is needed. Within weeks, food and water exposed to iodine-131 are no longer hazardous.

*Non-ionizing radiation, in contrast, does not carry enough energy per particle or photon to ionize atoms or cells but it can nonetheless be dangerous. UV radiation from the Sun is an example of nonionizing radiation that can damage skin cells enough to cause cancer.

Gamma Radiation

Gamma radiation is different from beta radiation because it is made up of photons of electromagnetic radiation rather than particles with mass such as electrons. But, like beta radiation, gamma ray photons are emitted during some kinds of radioactive decay and they are a form of ionizing (tissue-damaging) radiation. Gamma rays have a very short wavelength and very high energy.

Health Risks of Gamma Radiation

To measure the biological effect of gamma rays, the SI unit of equivalent dose, the sievert, can be used, much like beta radiation. You might also see it described in units of absorbed dose, in grays, and for gamma rays this measurement is numerically equivalent to the sievert.

The highly energized electrons of beta radiation are rapidly slowed to non-dangerous levels in the body because the charged electrons interact electromagnetically with the atoms in the body's tissues. Gamma rays, being photons, are not appreciably slowed down as they pass through materials. However, they can be effectively blocked by materials with a high atomic number or by materials of high density. Passing through such materials, some photons are bounced off the relatively large nuclei of these barrier atoms, scattering and losing energy. Lead is very dense and it is the most effective gamma ray barrier but aluminum, concrete and soil can also be effective. Like beta radiation protection, the thickness of the barrier required depends on the energy of the radiation, in this case the energy of the particular gamma photons.

Beta particles can only penetrate a millimeter or so of the body, so they tend to cause radiation burns on the skin if someone is in direct contact with a radioactive source such as iodine-131, gamma rays easily penetrate the whole body, causing diffuse damage and, as a result, radiation sickness and increased cancer risk, rather than burns. However, please don't confuse external exposure with internal exposure. For example, iodine-131-laced dust, if inhaled or taken internally through contaminated food or water, can cause much of the same kinds of diffuse damage and radiation sickness as gamma radiation exposure even though the penetrating ability of the beta particles is very minimal.

Gamma rays are not particles so they cannot by themselves contaminate anything. This is why irradiated food poses no radiation health risk and medical equipment sterilized by gamma rays pose no threat. The radiation in these cases kills all bacteria. As well, very high-energy gamma rays do not tend to be as damaging to the body as those that are lower energy because at very high energy they simply pass right through the body as though it was invisible to them, without interacting with any of the body's atoms and molecules.

Where Gamma Rays Come From

Gamma rays tend to be emitted along with other forms of radiation from a radioactive source immediately following other kinds of decay reactions. The reason for this is that, when an atomic nucleus emits an alpha (you will learn about these particles shortly) or beta particle, the daughter nucleus is often left in an excited state. It will spontaneously move to a lower energy state by emitting a gamma photon. This is the same kind of process that happens in a light bulb. When electricity passes through the tungsten filament in an old-fashioned light bulb, the tungsten atoms emit photons in the visible spectrum (light) as they are excited and move to a lower energy state (the atoms also emit infrared photons - longer wavelength photons - and that's why the bulb gets hot and it's not very efficient).

Alpha Radiation

Alpha radioactive decay happens when an atomic nucleus emits an alpha particle. An alpha particle is the same thing as a free helium nucleus; it has 2 protons and 2 neutrons bound tightly together. For example, uranium-235 decays into thorium-231, emitting an alpha particle in the process. Alpha particles are always emitted at a relatively low velocity. They have a narrow range of possible kinetic energies. Because they are highly charged, having a charge of +2, and because they have a relatively high mass, they can't travel far from their source, only about 2 centimeters through air. Even when in direct contact with skin, alpha particles cause little damage because they can't penetrate tough skin cells. However, if atoms of an alpha-emitting radioisotope are ingested, they can cause extreme cellular damage and possibly cancer because they are extremely ionizing. Radon, for example, is a naturally occurring radioactive gas found in some soils and rocks. If it is inhaled it can damage lung tissue and lead to lung cancer. You might have heard of the famous 2006 radiation poisoning case of Russian dissident Alexander Litvinenko. He died of radiation poisoning caused by the ingestion of polonium-210, an important ingredient in nuclear weapon triggers (these will be discussed below). One theory is that a small sample of polonium-210 was smuggled into Great Britain by an unknown Russian official and placed in his tea. The radiation in his body was difficult to detect because hospitals are equipped only with gamma ray detectors and polonium-210 emits only alpha particles.


The Uranium-235 Chain Reaction

An example of a gamma ray source is uranium-235, a common radioactive isotope used in nuclear reactor fuel rods. It's not a scary green glowing liquid like you see in the movies or on The Simpson's. You can actually hold it in your hands with minimal protection because it has a very long half-life of 700 million years. So how is it used as fuel and bombs?

Uranium-235 is fissile, which means it can undergo a fission chain reaction - the kind of reaction that is sustained inside nuclear reactors.  This is what its fission chain reaction looks like. The fission chain reaction of uranium-235 is often called the actinium series, and if you take a look at the diagram here, you will see that many elements, which are themselves radioactive, are created and decay along with the uranium isotope. As the process unfolds, alpha, beta and gamma radiation are all emitted from various decay reactions, along with a sustained creation of free neutrons. For example, lead-211, an alpha decay product of polonium-215, decays through beta- decay, emitting a positron, into bismuth-211. As I mentioned, Uranium-235 has a very long half-life. This refers to its spontaneous fission rate. This decay happens all by itself, given enough time. Uranium-235 is special in that it is one of few materials that can also undergo induced fission. This means that it can be induced to decay very rapidly. This decay reaction is governed not by the uranium's very stable decay constant but by bombardment mechanics. What it needs is to be struck by a free neutron and a nuclear fission reaction is started. The uranium-235 atom readily absorbs the extra neutron and immediately becomes so unstable it splits, creating products, which are themselves unstable, and they rapidly split too, until stable nuclei are eventually formed. The mixture of daughter elements formed can take fractions of a second to months to days up to many thousands of years to decay and that is why spent nuclear fuel is so hazardous. Each daughter atom, when it has just been created, is in an excited state so a gamma photon is emitted. That is why nuclear power plants must be shielded by water and then by thick concrete. A great deal of gamma radiation is continuously emitted in every direction, just as light is emitted from a light bulb. Plant workers must also be protected from the alpha and beta radiation that is also emitted. Water is very effective in stopping both of these particles.

Free neutrons are not found under ordinary circumstances. They are all tightly bound up inside atoms, squeezed together by the strong force (even the strong force is not enough to hold the nuclei of highly unstable elements together and that is why they decay). A slow neutron bombards a uranium-235 nucleus and a self-sustaining nuclear fission reaction ensues.

Free Neutrons - A Fourth Kind of Radiation

Concentrated beams of free neutrons are used in many physics experiments and are sometimes used in medicine to treat cancer. Neutron radiation is indirectly ionizing radiation. Neutrons themselves aren't electrically charged, but when they are absorbed by atoms in the body, gamma photons are emitted and these high-energy photons in turn ionize nearby atoms, stripping an electron off each one and that is when biological molecules, of which atoms are part, can be damaged. Neutron radiation is extremely dangerous. Free neutrons have very high, but variable, energy, so they easily pass through most materials but they interact enough with most atoms in cells to cause damage. In reactors, neutron radiation contributes to the overall gamma radiation that is emitted. Fortunately, water and concrete are effective barriers. Because water molecules are small, they are particularly effective in scattering the neutrons and slowing them down, their energy being absorbed by the water as it is ionized. This special interaction with water, however, makes neutron radiation particularly effective in causing cancer or death compared to an equivalent exposure to gamma ray or beta radiation, because our bodies are composed mostly of water. Free neutrons are also dangerous because they can induce radioactivity in other atoms. When free neutrons are captured by other atomic nuclei, unstable isotopes are created which then become radiation hazards, themselves. This public safety article lists some of the most common isotopes created by this process, called neutron activation. Neutron activation is the principle behind the neutron bomb. This nuclear weapon, if detonated, leaves buildings and infrastructure intact while killing people and animals.

How An Atomic Bomb Works

The atomic bomb that was dropped on Hiroshima in 1945 was made of highly enriched uranium-235. It consisted of two subcritical masses of uraunium-235. To detonate the bomb, one of the pieces was fired at the other down a gun barrel inside the bomb casing, to create a critical mass. The critical mass of a fissionable material depends on many factors including density, shape, enrichment, purity and temperature. Once a piece of material reaches critical mass it can sustain a nuclear fission reaction by itself.

The bullet also struck a small polonium/beryllium generator. This is what creates the free neutron to kick-start the fission explosion.  The generator is kept separate and enclosed by a layer of foil. When the foil is broken by the bullet, the polonium-210, which spontaneously emits alpha particles, is able to provide free alpha particles to collide with beryllium-9 to produce beryllium-8 and free neutrons. The uranium-235 now had enough critical mass to sustain a reaction long enough to discharge explosive energy and it had the necessary free neutron trigger to set the explosion in motion.

How A Nuclear Reactor Works

Nuclear reactors aren't all that different from coal power plants. They both heat water into pressurized steam and the steam drives a turbine generator to make electricity. The difference is in how the water is heated.

First of all, you might be wondering where uranium-235 comes from. It is the most common fissionable material used in both nuclear plants and nuclear bombs. It is a common element and has been part of Earth since it was born. Its atoms were created in supernovae and eventually became part of the Solar system. Because uranium is unstable, it has been decaying, very slowly, ever since its stellar birth. The Earth had much more uranium-235 when it first formed billions of years ago.

Uranium ore only needs to be enriched by about 2-3% before it can be used as fuel, compared to weapons-grade uranium, which needs to be at least 90% pure Uranium-235. This is because a bomb is small and it needs to be very pure to achieve critical mass.

The enriched uranium is formed into pellets that in turn are formed into rods and assembled into bundles. The bundles are submerged in water. The water acts as a coolant (as well as a radiation shield). Left to its own devices, the uranium, once it has begun fission, would heat up and eventually lead to a meltdown. In addition to water, the rods are kept from overheating by control rods made of neutron-absorbing material. These can be inserted into the bundles to a varying degree to control the rate of reaction. The absorption of free neutrons slows the reaction. The control rods can be completely lowered into the bundles. When they are completely lowered they can't completely stop the reaction but they can slow it down to about 6%. The water, while acting as a coolant and radiation shield, also transfers the heat energy in the form of pressurized steam to drive the turbine and spin the generator.

What's Happening in Japan?

For a continuously updated reactor-by-reactor status  report on the 6 Fukushima reactors refer to this link. Three of these reactors are at  INES level 5 (out of a possible 7 with Chernobyl being a 7) as of March 21, 2011. Some nuclear authorities have labeled them level 6. Click on the INES link to find out what this means in terms of current threat level.

Before I discuss what is known so far about the Fukushima nuclear accident, I recommend that you take a look at what happened at Three Mile Island. You will find some of what happened there eerily similar to what is happening now. In the Japanese nuclear reactors at the Fukushima nuclear plant, the control rods automatically lowered into the fuel bundles to stop the reaction soon after the earthquake struck, a process called an emergency shutdown. The problem is that the rods were extremely hot when the reaction was stopped and the residual 6% reaction rate continued to heat the water. The fuel rods are housed in zirconium. This metal is used because it doesn't normally react with water and it doesn't absorb neutrons so it doesn't impede the reactions. The plant lost electrical power when the tsunami swamped it several minutes later. And, somehow, backup power was also lost. Without power, the plant couldn't continue to automatically keep the water tanks in the core filled. In the core, water is continuously lost to steam, from the heat given off by the decay reactions. As the core heated up to about 2200 F, the zirconium housing began to chemically react with the water that was left to produce zirconium oxide and hydrogen gas. This reaction is exothermic so it contributed even more heat to the core. The pressure of hydrogen gas increased until it explosively blew the reactor container apart, further destabilizing the reactor and making it even more difficult to keep the fuel rods under water and preventing them from melting. This CNN video explains that three separate safety systems unexpectedly failed at these reactors which resulted in catastrophe.

As the zirconium housing deteriorates, the fuel rods can leak. The pellets in these rods are not metal, they are a compressed powder of uranium oxide, so the rods per say don't melt. This is not good news though. Some of the gas products of the fission reaction are radioactive xenon and radioactive krypton. These gases normally accumulate inside the rods, but once the rods are compromised, they can escape into the reactor building. They are heavier than air so they will sink to the bottom of the building and if it's airtight they won't get out. Some uranium compounds will also escape but these are also very heavy and they will not disperse much unless there is an explosion or fire. The most dangerous by-products are the radioactive isotopes iodine-131, cesium-137 and strontium-90. They are lighter and can disperse into the atmosphere, with strontium-90 being slightly heavier and less dispersible than the other two. As mentioned, in the beta radiation section of this article, iodine-131 can be taken up and concentrated in the thyroid gland where it can damage cells and possibly lead to thyroid cancer. Cesium-137, with a half-life of around 30 years, can be mistaken for potassium inside living organisms and passed up the food chain and concentrated, all the while emitting ionizing radiation. Cesium detectors are usually used to assess how much radiation is leaking from a reactor. Strontium-90, also with a half-life of about 30 years, is less volatile than the other two isotopes but it is especially dangerous because it mimics calcium in people and animals, so it is taken up by and deposited in bones, destroying rapidly dividing bone marrow cells and potentially causing cancer. The hydrogen explosions serve to disperse the radioactive materials into the air. Technicians have attempted to add boron to the coolant water because it absorbs free neutrons and slows the reaction rate. If the core can't be cooled down, eventually, the powdery fuel and its reaction products, along with what is left of the zirconium housing, will melt into a lava-like soup called corium. Very thick steel on the reactor bottom will dissipate much of the heat from the mixture, but if nothing is done to cool the mixture, the steel will melt and it will all come into contact with the thick concrete housing below. The concrete might hold. If it doesn't the mass will eventually disperse in the ground below, the heat will slowly dissipate, and the nuclear reaction will slow to a stop.

No one can predict how much radioactive material will be dispersed from the Japanese reactors. It's also very difficult to predict exactly what materials will be dispersed because there are so many possible chemical reaction products and fission by-products, each of the latter possibly emitting beta, gamma and/or alpha radiation. Neutron radiation is another significant hazard from a reactor that is no longer properly shielded, along with the various radioactive isotopes this radiation can induce.

Health risks are also very difficult to assess in the population nearby in Fukushima prefecture because air-born radiation will travel along prevailing winds with radioactive particles deposited via raindrops, snow or dust along the way onto water reservoirs, crops, animals and people. The Japanese earthquake and tsunami disaster is already documented and being constantly updated here at Wikipedia.

I am horrified, as is everyone, by the unfolding disaster in Japan. I believe we are deeply indebted to the people there who are, unfortunately, teaching us how to endure and how to prevent future disasters, perhaps through better education, more active public involvement and better nuclear plant design.

Finally, I have two videos so share with you that are frightening to watch but valuable as an informative guide to the dangers of nuclear energy that we should all be aware of.

The first one is 6 minutes long, from RT News Station and it aired on March 17, 2011. It's an assessment of the current state of the 6 nuclear reactors in the Fukushima power plant and what might be happening right now:

This next video is called Inside Chernobyl's Sarcophagus. It's a followup on the 1991 NOVA documentary called Suicide Mission To Chernobyl and it's 46 minutes long. Well worth a viewing to compare what is going on how in Japan with what is considered the worst nuclear accident in history.


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