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Fission uranium 235 sr905/6/2023 It’s the Cs and Sr that makes nuclear waste “hot”. This is why the NRC’s recent waste confidence ruling is so important. Eventually, it can be burned in GenIV fast reactors to produce over ten times the energy obtained from the first round of burning, or it can be disposed of permanently in a deep geologic repository.Įither way, cooler is easier and safer. The fuel can be recycled to make new fuel, or it can be set aside in dry-cask storage for 90 years or more as the waste becomes much cooler. The fuel is not spent, only 5% of the fuel is actually used. Along with some other age issues, the chain reaction then fizzles out, all neutrons stop, and the fuel needs to be replaced. In fact, after only two years, so much Pu has formed in the fuel that it’s producing as much energy as the original U-235, but is being burned up just as fast.Īfter about five years, the fission products from both U and Pu build up to the point where they scarf up so many neutrons that they poison the reaction. Pu-239 is also a fissile element, even more so than U-235, and can split to produce additional energy. Similar parallel formation mechanisms exist for the other actinide elements but Pu-239 is the main product of these reactions. When U-238 captures the neutron, it becomes U-239 which is also unstable (but not fissile), throwing off some beta particles (the nucleus’ equivalent of an electron, essentially turning neutrons into protons), and becomes plutonium-239 (Pu-239). Because Sr-90 and Cs-137 produce beta and gamma radiation, this makes the waste containing them radioactive enough to require shielding for about 200 years.Īfter coming out of the reactor, spent fuel is placed in pools of water for five years as all of the short-lived nuclides decay away so just Cs and Sr are left, and the fuel cools off sufficiently to be put in dry cement casks for 90 years or more as the Cs and Sr decays through a few half-lives and gets much cooler.īecause of neutron capture, shown in the figure above, other elements produced during the nuclear reactions include the heavy actinide elements, the last row in the periodic table - plutonium, americium, neptunium, and so on. A rule of thumb is it takes seven half-lives for a radioactive element to decay away to background. They only make up about 6% of the fission products, but they are what make the waste hot. While there some long-lived fission products like Tc-99 and I-129, the two longest-lived elements that are really "hot" are strontium-90 (Sr-90) and cesium-137 (Cs-137) each with a half-life of about 30 years. Most of the elements produced are non-radioactive or not very radioactive, or have a short enough half-life that they decay away in days or months, some in seconds. The fragments produced when U-235 splits are new lighter elements, the fission products. That furnace turns water into steam, that turns a turbine and produces electricity just like any coal or gas-fired power plant. The reaction is easily controlled, it cannot explode like a bomb, and the reactor just becomes a furnace that produces a lot of heat from the released energy. Nuclear fuel used in commercial reactors is only enriched up to 5% U-235 of the total U, so that most of the neutrons released hit the other natural isotope of uranium, U-238 which does not split but captures the neutron and changes into a new element (see figure above). On the other hand, a commercial nuclear reactor is engineered to contain and control this chain reaction. The pressure and heat produced by this uncontrolled chain reaction are extreme and causes a huge explosion plus a momentary blast of neutrons. If the material is all U-235, as in weapons, then these neutrons see only more U-235, and they all split in an uncontrolled chain reaction. This three-for-one split with respect to neutrons means that those three neutrons can then split three other U-235 atoms releasing 9 more neutrons, which can split 9 more U-235 atoms, releasing 27 more neutrons, and so on. Judith Wright, The GeoPolitics of Energy ). Since they’re flying apart at near-light speed, there is an enormous amount of kinetic energy released. It is the speed at which the two fission pieces are moving away from each other. However, the real energy released is not radiation. If its nucleus is struck by a neutron at just the right speed, it will split into two unequal parts, called fission products, throwing off about three neutrons and some energy (see figure above). U-235 consists of 92 protons and 143 neutrons, a combination that is slightly unstable.
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