Types of Nuclear Waste
Introduction:One of the greatest problems with nuclear energy is the waste produced. The waste is generally radioactive, and thus toxic. There are also a few different kinds of waste, depending on how it was produced. Nuclear waste is produced in many different ways. There are wastes produced in the reactor core, wastes created as a result of radioactive contamination, and wastes produced as a bi-product of uranium mining, refining, and enrichment. The vast majority (99%) of radiation in nuclear waste is given off from spent fuel rods. However, fuel rods make up a relatively small percentage of the volume of waste. The largest volume of nuclear waste is composed of the leftovers from the mining process. This waste, however, doesn't give off much radiation. Some of the nuclear waste is extremely long-lived, meaning that it lasts a long time without its toxicity decreasing all that much, and some of it is very short-lived. Some types of nuclear waste are considered high-level and some are considered low level. The difference is in the amount of radioactive nuclei in relation to the mass of the waste. If there are a large amount of radioactive nuclei relative to the amount of waste, it is considered high level nuclear waste.
Fission Bi-Products:When a 235U atom splits, it can produce a number of different products. Many of these are radioactive elements. For example, the following reaction produces 90Sr, which has a half-life of about 29 years.
1 neutron + 235U 2 neutrons + 90Sr + 144Xe
Although its half-life is 29 years, a quantity of 90Sr is not considered safe for 290 years. After 290 years, 10 half-lives would have passed. So, if we started out with half a ton (1000 lbs.) of 90Sr, after 290 years there would be 1000 x (1/2)10 left. This is about a pound. The rest of the 90Sr would have undergone �- decay, producing 90Y. 90Y is also radioactive, but is has a very short half-life of about 2.67 days. The 90Y undergoes �- decay, forming 90Zr, which is a stable, non-radioactive isotope. 90Sr is particularly dangerous because it shares many of the same chemical properties as calcium (Ca), and, if ingested, can take calcium's place in your bones. Then, when 90Sr decays, the radiation released in your body can cause cancer.
This is just one example of a radioactive isotope that is produced from fission. There are hundreds of other fission products, many of which are radioactive. Their half-lives, however, vary greatly from less than a second to many, many years.
The fission products, or fragments, usually remain within the fuel rods of the reactor. When most of the 235U in a fuel rod is spent, the rod must be removed. The radioactive fragments are what make the spent rods toxic. The fission products can be long-lived or short-lived.
Transuranics:In previous texts we talked about how the 238U in a fuel rod is not fissile and is a neutron absorber. We made the point that because it absorbs neutrons, it stops the chain reaction in a nuclear power plant from running away (and producing a nuclear bomb effect). This is a good thing. However, think about what it means when we say "238U is a neutron absorber". The following reaction expresses that statement:
1 neutron + 238U 239U
When 238U "captures" a neutron, it is added to the original uranium nucleus, producing the radioactive isotope of uranium, 239U. This isotope has a half-life of 23.45 months. It decays, through �-, into 239Np. 239Np is also radioactive and decays into 239Pu. 239Np has a short half-life of about 2 days. This sequence of decays can be expressed like this:
1 neutron + 238U 239U
239Pu is also radioactive, and has a half-life of approximately 24,000 years. That's a long time!! A lot of 238U is turned into 239Pu through this sequence of decays. 239Pu is called a transuranic element. Any element with a higher atomic number (and thus more protons) than uranium is considered to be transuranic. This applies to all of the elements to the right of uranium in the Periodic Table. In the equations above we showed how a neutron can be captured by a nucleus and, through a series of �- decays, can produce an isotope with a higher atomic number than the original atom. More than one neutron can be captured. So, for instance, a neutron can be captured again by 239U. This produces 240U, which decays into 240Np. If 240Np captures another neutron, it becomes 241Np, which then decays into 241Pu and then into 241Am, which has a half-life of about 400 years. This sequence of decays and neutron additions can be expressed in the following reactions:
1 neutron + 238U 239U
This is only one example of how higher-atomic number transuranic elements can be produced. There are many other pathways involving �- decay and neutron capture/addition that can produce transuranic elements besides neptunium (Np), plutonium (Pu), and americium (Am).
The transuranic neutron addition products usually remain in the fuel rods, where the original 238U from which they were produced was located. This adds to the rods' toxicity, and makes it harder for them to be disposed. In general, transuranic wastes are long-lived. However, this depends on the isotope produced. The biggest transuranic waste produced is 239Pu. This is an extremely toxic and extremely long-lived compound. 239Pu is fissile. In fact, when a nuclear reactor's fuel rods are almost spent, as much as 30% of the reactor output can come from the fissioning of 239Pu. Thus, the plutonium transuranic "waste" produced in a nuclear reactor can actually be used as fuel. We will discuss more on this later.
Waste from Uranium Mining and Enrichment:When uranium is mined, it has to be separated from rock. This produces pure uranium ore and "tailings", essentially leftover rock that has had the uranium stripped from it. This rock often still contains radioactive nuclides and is somewhat dangerous. The tailings are generally long-lived, but are considered to be low-level waste. That is, the concentration of radioactive nuclei in them is small, and thus they are not extremely radioactive.
As we explained previously, uranium ore is only about .7% 235U. It must be enriched to bring the percentage of 235U up to about 4%. The enrichment process produces a lot of waste. This is because for every gram of enriched uranium fuel produced, there are about 4 grams of 238U waste. 238U is radioactive and has a half-life of 4,468,000,000 years. This means that it is long-lived, but not extremely dangerous. However, some of its "daughter products" are radioactive. Thus, wastes produced as a result of enrichment must be kept in storage. By the way, a "daughter product" is an isotope that results from a decay of another, "parent", isotope. For example, when 238U decays, it produces 234Th, which is very radioactive and has a half-life of about 24 days. The decay can be expressed in the following equation:
238U ( decay) 234Th + particle
Contaminated Stuff:A major portion of nuclear waste is comprised of spent fuel rods. These contain the fission products and transuranic wastes we mentioned above. However, a lot of other waste is produced in the reactor besides the fuel rods. This occurs as a result of radioactive contamination.
A nuclear reactor is extremely hot. This means that the particles inside the reactor are very energetic and are flying around at incredible speeds. Occasionally, an atom that is in a fuel rod can get knocked out. These atoms that get knocked out can be many different types, ranging from fission products to uranium to transuranic elements. Most are radioactive. Atoms that escape the fuel rod careen all over the inside of the reactor core. Eventually these atoms can strike something solid. This is a lot like a bullet hitting a wall. If the wall is small, it might pass through. However, if the wall is big enough, the bullet will smash into the wall and "stick" there. So it is with a nuclear reactor. Occasionally an atom can smash into a structural component of the reactor, implanting itself into it. Because many of the nuclides (fancy term for atomic nucleus) careening about the core of a fission reactor are radioactive, when they smash into a structure and "stick", they make that structure appear to be radioactive. This is because there are many radioactive nuclides embedded in it, which give off radiation. Thus, many of the structural components of a reactor become radioactive over time, as they absorb radioactive nuclei into themselves. Also, many of the pipes and other components of a reactor become radioactive. These must be replaced eventually because over time the extreme radiation inside the reactor weakens them. The biggest problem, however, arises when a nuclear reactor is turned off for good, or "decommissioned". Disposal of the reactor core is a huge problem because it is extremely radioactive.
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