Two years ago I wrote a blog on some of the medical effects of radiation. In view of the recent problems with nuclear reactors in Japan, I thought I should add several comments, including a discussion of the lethality of different kinds of radiation, the half-life of a radioactive element, and possible means of protection. This blog will therefore discuss the chemistry, physics and medical effects of radioactivity. I have had some direct experience with this, since in the past I have worked at Oak Ridge National Labs as well as Columbia's Nevis Cyclotron, and I have done medical research for two summers in the radiation physics department of a teaching hospital. I also have had experience with patients exposed to radiation, but I am still not permitted to talk about it.
Let's begin with a few simple facts: Every atom is composed of a nucleus containing neutrons and protons, surrounded by concentric shells of electrons. The chemistry of an atom depends on the number of electrons, and the physics of the atom depends on the number of neutrons and protons in the nucleus. In free space, a neutron will decay in less than 15 minutes into a proton, an electron, and an anti-neutrino, but inside the nucleus quantum rules often forbid this. The atomic number of an element is equal to the number of protons in the nucleus (and hence the number of external electrons), and the atomic weight is approximately equal to the number of protons plus the number of neutrons. An element, which by definition has a defined number of protons in the nucleus, can have various numbers of neutrons. The different nuclei with the same number of protons but differing numbers of neutrons are called isotopes of each other.
Some isotopes of elements are naturally radioactive, such as Carbon-14, or Oxygen 17. But since Carbon-12 (which comprises over 99.9% of natural carbon) and Carbon-14 both have the same number of electrons, they appear chemically identical to living biochemical systems and are chemically incorporated into their cells and tissues. Therefore every living animal with carbon has some natural radioactivity. We know the natural C14/C12 ratio in the world, and this, coupled with the known half-life of Carbon-14 of 5600 years, allows us to carbon-date ancient bones and tools. We can apply the same principles to date the age of rocks and the earth, using uranium/lead ratios, since all uranium eventually decays into lead. However, ALL isotopes of the elements above a certain atomic number are naturally radioactive, so that there is no harmless form of thorium,radium,uranium, plutonium, radon, etc., and if you are in close enough proximity to one of these elements for a sufficient length of time, it will be fatal.
Ordinary water contains two atoms of hydrogen, and one of oxygen: H2O. Heavy water contains two atoms of deuterium (a hydrogen isotope with one proton and one neutron in the nucleus) and one of oxygen: D2O. Although they are similar chemically, if you drink enough heavy water you will die, since water passes through cell membranes by a process called diffusion, which is physical, and not chemical. Because heavy water absorbs neutrons more efficiently than ordinary water, it is often used in a nuclear reactor.
There are three types of radiation that can harm you. The first is free electrons, also called beta rays (for historical reasons). Electrons have very little penetrating power because they are charged, and interact rapidly with your skin. Therefore usually the worst that beta rays can do to you is to give you a burn, and simple clothing will generally stop them. However, your thyroid gland takes up iodine avidly and rapidly, to make thyroid hormone. Most reactors will, if breached release radioactive iodine (I-131) into the atmosphere, such as happened at Chernobyl as well as Japan. Unfortunately, if I-131 falls on food and you ingest it, it will be concentrated in your thyroid gland. Although I-131 is a low-energy beta ray emitter, you can easily swallow enough to make you gradually hypothyroid, or, especially in children, cause the development of cancer of the thyroid in a few years. This is why we recommend daily doses of KI, or potassium iodide, to children who are exposed to such radioactivity, to block the uptake of I-131. (We first realized the danger of radiation to children's thyroid glands when children and adolescents who received radiation with gamma rays to treat their acne developed thyroid cancer 10 to 20 years later.) Similarly, breached reactors release radioactive strontium, which is chemically similar to calcium in that they are both divalent cations. Therefore, if there is radioactive strontium on the grass, and the cows eat it, it will chemically replace some of the calcium in the cow's milk, and when you drink this milk the radioactive strontium will be taken up and concentrated in your bones, and then it can damage your bone marrow, and/or cause leukemia.
The second "radiation" that can cause damage is generally not thought of as radiation, but it is released in large numbers and high energies in runaway reactors, and that is neutrons. They are electrically neutral, so they have incredible penetrating power. They are preferentially absorbed and give up their energy to hydrogen atoms, which is why water is a doubly useful substance for cooling reactors. Cadmium is also an excellent neutron absorber, and cadmium rods can be used to SCRAM a runaway reactor. Unfortunately, over 60% of our body is water, and our brains are especially sensitive. There is no portable way to measure neutrons, (such as a Geiger counter for gamma rays), and there is no way to measure or detect how much damage the neutrons have done to your body, or what your total neutron exposure was. When I worked at the cyclotron lab, we all had radiation badges to measure our monthly exposure to gamma rays. But there was only a long string of yellow plastic rope stretching from one side of the cyclotron to the lab wall, with a floor sign underneath:"Do Not Approach Closer Than 10 Feet When the Cyclotron is in Operation".We all respected that sign.
The third radiation, and usually the most lethal, is gamma rays. These are photons, or "pieces of light". Their individual energy has a huge range, and the shorter the wavelength, the greater the energy. Thus radiowaves with wavelengths of kilometers are harmless, microwaves can be used to heat water, infrared waves can be used to heat rooms, ultraviolet rays will give you a sunburn or worse, and shorter wavelength rays such as X-rays can be lethal or give you radiation poisoning (which is probably why the dentist or hospital X-ray technician goes into the other room when he/she takes your x-rays---they don't want the exposure), and you aren't even given a radiation badge to measure the exposure to gamma rays you just received.
We have no way to measure the amount of radiation your body absorbed, or how much energy particular tissues (heart, brain, bone marrow) absorbed. We can only guess, since we just don't know. We think we know the safety limits for hospital radiation treatment of cancer, and we know that the old treatment of polycythemia vera with oral radioactive phosphorus caused bone marrow cancers and leukemias later, but we don't know precise doses. We really don't even know if radiation exposure increases your risk of getting cancer linearly, in that each exposure adds to the risk of the previous one, or of there is a threshold dose of radiation below which there is no risk. We do know that in acute radiation exposure you can die in 48 hours or 48 weeks. We do know that the most rapidly dividing cells are preferentially killed by gamma rays, so that the bone marrow and the lining of your gut are especially sensitive. We do know that there can be delayed burn-scar tissue results, such as fibrosis of the lungs, or constrictive pericarditis (of the heart).We know that radiation sickness can affect every organ system of the body, but we can't reverse or treat it, except to support the patient's vital signs, and hope that death from overwhelming infection or cancer does not ensue. We also know that developing embryos are exquisitely sensitive to the damaging effects of radiation.
Now as to radioactivity and reactors themselves. The natural radioactivity of the trans-thorium elements provides the heat in the earth that keeps the core liquid and the earth's surface warm. Without natural radioactivity, the energy from sunlight would not be enough to keep us from freezing to death. Now since uranium emits heat when it radioactively decays, it needs to be constantly cooled in a reactor, which is why the naked exposure of uranium-embedded zirconium rods created so much heat that it boiled the surrounding water, energized the freeing of hydrogen from water, and then exploded the free hydrogen which then burned in the presence of the free oxygen also liberated. So a naked core is an extreme heat emergency, as well as a radioactive one.
You can design a nuclear reactor that can never go critical. This is the type that is installed in nuclear subs. In a reactor, free neutrons are allowed to bombard uranium atoms. These then fission,. releasing on the average 2.5 neutrons per uranium atoms struck. If each neutron then splits a uranium atom, then two more atoms are split, and 5 more neutrons released. If this happens rapidly enough, it is a self-sustaining generator of energy (and then converted to heat via giant water boilers)., and we say that the reactor has "gone critical", and we have a chain reaction. We adjust the depth of cadmium rods in the reactor to control the rate of reaction, and if the reaction goes supercritical, we rapidly lower the cadmium rods into the rector to dampen it. I should mention that this reaction occurred naturally at least once in the past, about 2,000,000,000 years ago in Oklo, Gabon, West Africa, where the uranium isotope ratios demonstrate that there was a critical mass of uranium that started to run away, and was stopped only by its own production of radioactive xenon, which is a excellent neutron absorber.
The half-life of a radioactive isotope is the amount of time it takes half of a given number of radioactive isotopes to decay (and, hopefully into a non-radioactive product, which is the case for Carbon-14, but not for Uranium-235, Uranium -238, or Plutonium-239). I should mention that radioactive cesium, which has a very long half life and is used in hospitals as a source of radiation, as also produced by runaway reactors. The half-life is not related to the radiation danger of the isotope, but the number of radioactive daughter products certainly is, like the production of the radioactive gas radon from the decay of uranium. BTW, did you know that all marble is naturally slightly radioactive?
I think that you can see that the only safe act is total avoidance, and perhaps the carrying of personal radiation badges, to be checked once a month.If you suspect a radioactive fallout, get rid of your clothes, and shower immediately, and carefully wash all food before eating. I would assume that all government warnings minimize the danger, just as they told the terminal cancer patients in 1947 in whom they injected radioactive phosphorous and other substances that it would cause them no danger (since they were dying anyway, and then the government autopsied their bodies to find out what it could about the deposition in and effects of radiation on the body). Or when they permitted sailors to stand on the decks of their ships and have radioactive debris be showered on them after the H-bomb test at Bikini and Einewetok atolls.
BTW, a reactor is scrammed by releasing the clamps holding cadmium rods out of the nuclear core of the reactor, and let gravity pull the rods in as rapidly as possible. For one midwest reactor, the blueprints were distributed upside down, and because of atomic secrecy, the workmen knew very little of the ultimate purpose of their construction. Luckily, before the reactor was run up to full power to test the SCRAM operation, someone noticed that the cadmium rods were inserted from the bottom!
But at least we are avoiding 4 stupidities caused by insufficient knowledge of the dangers of radiation:
a) we no longer handle radioactive samples with our bare hands
b) we no longer use radioactive thorium salts to remove hair in the armpits
c) we no longer paint the numbers on watches that glow in the dark with radium-based paint, and have the painters moisten and sharpen the brush tips with their lips and thereby deposit radium salts into their mouths
d) we no longer rush over as children to the shoe store fluoroscope to stand and put our feet in the xray-fluoroscope to see how well our feet fit into our shoes
How much natural or man-made radioactive exposure is safe? No one really knows. We can sometimes, for a given energy level of gamma rays, estimate the LD50, which is the total time exposure after which 50% of exposed humans will die. But this is only an estimate, and only for one energy level. We also have no way of predicting how much exposure to Xrays and natural radioactivity is necessary to increase your risk of cancer in a given organ by 1%. There are estimates of the risk of inducing breast cancer after 30 years of annual mammograms, but these are only estimates, as is the stated "safe" level of radon, a radioactive gas. In fact all radioactive safety levels are guesstimates arrived at by compromise by a committee, and have never been verified, except for accidental exposure to a runaway chain reaction, such as happened to one scientist in Los Alamos who was working with two clumps of a radioactive element and let them approach each other too closely.
In case you think any of the above is an exaggeration, let me tell you about one incident. Shortly after 9/11, many of the upscale shopping malls were equipped with radiation detectors at their entrances. Several years ago, a male cardiac patient had a (radioactive) stress-thallium test, which involves injecting a small dose of radioactive thallium into your bloodstream when you are near peak exercise, and then scanning you with a radiation detector to see the distribution of blood to your heart when you are exercising. You then sit there for four hours, and they re-scan you to see the distribution of blood to your heart when you are at rest. An hour after this test was finished, so we are talking about what happened 5 hours after then man was injected, he went to meet his wife at the shopping mall in Short Hills, New Jersey. The residual radiation in his body triggered the radiation detectors, and the entire mall was shut down and evacuated, and everyone examined with a hand-held Geiger counter, which is how the authorities discovered what had happened.