Encyclopedia Dentin Someone recently asked me some questions about nuclear radiation, and I actually happened to know the answers. Here is a short write-up of what was discussed. I did most of it from memory so there's likely minor mistakes, but there shouldn't be any gross inconsistencies. Post back if you find anything sufficiently out of whack and I'll correct it; but keep in mind it's intended for a layman audience and precise technical details aren't particularly important. ----- There are three types of nuclear radiation that are most commonly encountered. All of these are so called 'ionizing' radiation, because they cause damage by ionizing atoms and molecules as they pass through matter. This can cause damage to living tissue by breaking the bonds between large molecules such as protein chains and DNA strands. A single particle of high energy ionizing radiation can potentially break hundreds of thousands of molecular bonds and penetrate deep into a target body. Type 1 - alpha particle radiation --------------------------------------------- This consists of a heavy, highly charged alpha particle, also known as a helium nucleus. Because it is heavily charged and massive, it causes massive disruption but is very quickly stopped. These particles typically never make it through more than the equivalent of a few sheets of paper, or the outer layer of your skin. If heavily exposed, skin burns and skin cancer would be the most likely result. Alpha emitters are particularly dangerous if inhaled or ingested; this is what makes plutonium dangerous (other than it being a chemically poisonous heavy metal, like mercury or lead.) In the case of plutonium, one of the most harmful types of exposure is inhalation of a very small amount of dust, which becomes lodged in the lungs. Over time, exposure to the resulting alpha radiation can lead to cancer in the surrounding lung tissue. Type 2 - beta particle radiation --------------------------------------------- This consists of very high energy, fast moving electrons. These electrons are small and light, and are not so easily stopped as alpha particles. They penetrate deeply, and it often takes several inches of matter to stop them. The damage done by beta particles is not as localized or disruptive as for alpha particles, but can cover a wider and deeper area. Type 3 - gamma radiation --------------------------------------------- This is the hardest to stop radiation, as it is uncharged. Gamma radiation consists of only a single, high energy photon, and as the photon collides with atoms and electrons it ionizes them and breaks molecular bonds. It is convenient to consider a gamma ray to be an X-ray with very much higher energy. Gamma rays, depending on energy level, may not be stopped even by several feet of iron, lead, or water. When considering nuclear fission reactors, there are two other types of nuclear events to consider. In nature, these occur infrequently and are very rarely seen in comparison to the three types listed above. Nuclear reactors are designed to artificially enhance both of these types so as to create a chain reaction and generate heat. Type 4 - spontaneous nuclear fission --------------------------------------------- This event occurs when a heavy nucleus such as uranium-235 or plutonium-239 spontaneously fissions. Typically, the nucleus breaks in half, with one half a bit heavier than the other; the pieces are highly charged and recoil from the site of the fission at high speed and with a substantial amount of energy. As the fission fragments recoil away from the site of the fission, the pieces create a short trail of ionizing destruction similar to that produced by alpha particles. In addition, the pieces themselves may be radioactive (more on this below.) When the fragments come to a stop, they form new atoms of krypton, barium, strontium, cesium, and other elements. In a nuclear reactor, fission is artificially induced by neutrons to generate heat. Type 5 - free neutrons --------------------------------------------- Free neutron radiation is typically a result of nuclear fission, but can also occur infrequently in other decay processes. When a nucleus fissions, in addition to the large fragments, often several neutrons are released. Neutrons are heavy, uncharged particles that can travel undisturbed through matter without causing much damage. Neutron radiation is not typically ionizing, and it does not cause damage like the other four types listed above. The real danger of neutron radiation is that when absorbed, it can cause the absorbing mass to become radioactive. Types 1 through 4 above do not spread; when a gamma, beta, or alpha ray is stopped, that is the end of its trail of destruction. A neutron however, typically travels until it is captured by some unsuspecting nucleus. Depending on what nucleus captures the neutron, the result can be an innocuous isotope, or a horrendously unstable and radioactive isotope. Fortunately, the artificially induced radiation produced by neutrons is typically short lived, with half lives on the order of days or less depending on the isotopes formed. Diversion -------------- The so called 'neutron' bomb was designed with exactly this purpose in mind: the bomb produces a large number of neutrons, which are absorbed by everything near the explosion. This tends to make the entire blast zone radioactive, above and beyond any radiation that might descend from the sky in the form of nuclear fallout. This is intense neutron-spawned radiation would kill most lifeforms in the blast area, but because the lifespan of the induced radiation is short the blast zone would be inhabitable within a few weeks or months. A particularly nasty type of neutron bomb was at one time proposed that did not have this property. For this design, a neutron bomb would be detonated inside a large, heavy sphere of the metal cobalt. The large neutron flux would be absorbed by the cobalt, turning it into highly radioactive cobalt isotope with a five year half-life. The explosion would totally vaporize all of the now radioactive cobalt, which would become part of the fallout from the weapon. The nasty aspect of this 'dirty' bomb has to do with the artificially radioactive cobalt. The five year half life is short enough that the radiation flux is very high. The five year half life is also long enough that the fallout would remain very radioactive for well over a hundred years. As if this wasn't bad enough, the cobalt produces highly penetrating beta radiation, which is not easily stopped by suits or other protective gear. In short, use of one of these weapons effectively makes several square miles of land uninhabitable for decades (if not centuries) barring a massive robotic cleanup effort. -------------- Natural uranium --------------------------------------------- Natural uranium primarily consists of two isotopes - uranium-235, and it's heavier cousin uranium-238. U-235 is the isotope typically used in nuclear reactors, as it fissions much more easily than U-238. Natural uranium is radioactive, but only slightly. Uranium is much more dangerous chemically than via radiation; eating uranium dust can give you heavy metal poisoning and kill you long before you'd ever develop cancer from it. People build houses on top of uranium deposits, and for hundreds of years used uranium-based glaze on dinner plates to get pretty orange colors. Living around uranium deposits will arguably raise your risk of cancer, but the risks should be considered similar to those of developing skin cancer from tanning, or cosmic ray exposure from flying on an airline above the safety of the atmosphere. Near Moab, Utah, you can go hiking with a geiger counter and find uranium ore lying on the ground. You can pick it up, put it in your pocket, take it home and put it on your mantle. Having it there for your entire life might take a few months off your expected lifespan. I've held uranium ore in my hand. It looks like an ordinary, if somewhat bright, yellow rock. Fuel rods for fission reactors --------------------------------------------- Typically, a fuel rod for a nuclear fission reactor consists of an enriched fuel, some nuclear stabilizers or absorbers, and other inert ingredients to structurally hold the rod together. For a uranium reactor, the rods may be 60% uranium-238, 5% uranium-235, and 35% everything else. Depending on the fuel cycle, a used/burned out fuel rod may be 59% uranium-238, 2.5% uranium-235, 3% fission residuals, 0.5% plutonium, and 35% everything else. It would not be unreasonable to use only 3% of the uranium in a fuel rod before it is considered burned out. When a rod is burned out, the 3% fission residuals are the primary source of radiation remaining in the rod. These residuals are nuclei of barium, strontium, and other miscellaneous heavy elements as described under 'Type 4' radiation above. Because the byproducts of fission are fairly random, they have widely ranging decay rates and produce many different types and energies of radiation. Once a spent fuel rod is withdrawn from a reactor, it is highly radioactive, to the point that it may be warm (or hot!) simply from the energy of the decay inside it. This initial radioactivity decreases very quickly as the short lived fission byproducts decay, and 95% of the initial radioactivity may be gone within a week. The removed rods are placed in large cooling pools or tanks, so that they are surrounded by twenty to thirty feet of water. The water has several purposes: 1) it acts as a cooling mechanism to dissipate the heat produced by the radioactive decay in the rods 2) water is immune to all types of ionizing radiation and the large depth is sufficient to stop the penetrating beta and gamma rays 3) water is a good neutron absorber, because the hydrogen in the water captures free neutrons to become the stable isotope deuterium 4) the water is easily monitored and tested for leaks or leaching of the fuel rod elements The water in the tanks is constantly circulated, cooled, filtered, tested and measured for radiation levels. This is some of the most pure water in the world, cleaner by far than the distilled water you can buy at the supermarket. The depth of the water provides a considerable safety margin. At least one nuclear engineer/physicist has offered to go swimming in a nuclear cooling tank while rods are in it, actively cooling a mere 30 feet below the surface. The radiation exposure expected in such a stunt would be less than that obtained by going for a walk outside on a sunny day. (It should be noted that the act of placing a human being into one of these tanks would set off every alarm in the place. In addition to picking up gross levels of contaminants, the naturally radioactive potassium and other salts in the washed off sweat would trip the radiation sensors.) After the fuel rods have cooled for one to three years, they are considered cool enough to be moved to mid level radiation storage. Nearly all the short-lived byproducts have decayed, and the radiation level of the rods is only a few times higher than it would be for pure, naturally occurring uranium. The rods are placed in concrete bunkers on site, or moved somewhere more permanent, like Yucca mountain. A recent realization is that it is safer and more effective to store nuclear waste on site, rather than in some distant and permanent location. Rods are placed in large concrete casks, sitting on large concrete pads, out in the open. These casks are typically surrounded by fences and security systems, and patrolled as appropriate to maintain security. Theft of fuel rods is prevented by the simple fact that it takes heavy machinery to move a fifty ton slab of concrete off the top of one of the casks. Having the rods accessible also means that as methods for dealing with nuclear waste improve, we won't have to dig it out of a collapsed mountain to retrieve it. It also makes the fuel rods available for reprocessing, should that ever become cost effective. Other miscellaneous notes --------------------------------------------- Regarding storing nuclear waste at the reactor site, the storage facilities can be surprisingly small. Fifty years of continuous power operation for a large reactor might produce enough spent fuel rods to fill one cask the size of a small house. Reprocessing fuel rods to use all of the uranium is not cost effective at this time. Reprocessing has turned out to be more difficult than expected, and the cost of natural uranium is so low that it's cheaper to just store the burned up rods and use brand new ones. For example, a new rod might be one fifth the price of recycling an old one. Lead doesn't stop all radiation. Lead isn't even a particularly good radiation shield. When shielding radiation, only two factors really come into play: how far you are from the source, and how much mass is between you and the source. A thousand pounds of water will actually shield you -better- than a thousand pounds of lead; but lead is very dense which allows you to fit more of it in a smaller space. Lead is also pretty common, and as a very soft metal it is easy to work with.