“Radioactive waste will last billions of years and it is unfair to gain benefit from nuclear today and leave the next generation to deal with the nuclear waste.”
Emphasize on the content of the spent nuclear fuel, reprocessing activities, fission product transmutation, and turning the waste into useful energy. We are looking for you to deliver a CLEAR MESSAGE in your blog to say “We are the young generation, we are the young Malaysians, it is our future we are talking about, and we want a sustainable, green, beautiful, secure planet that we can show our own children with our very own eyes instead of just through pictures in history books – We want a future. And let us manage the waste”.
Hi guys!!! We are back!!!
As you can read the excerpt above, i am here to give you the clarification regarding this matter of radioactive waste.
Firstly, what do you understand about nuclear waste? Ok let's started our discussion...get ready with pop corn and notepad. For sure, you will be amazed with this all nuclear thingy and terms. To remind you that, dont get confused!
1.What is nuclear waste???
Nuclear waste is the material that nuclear fuel becomes after it is used in a reactor. It looks exactly like the fuel that was loaded into the reactor --> assemblies of metal rods enclosing stacked-up ceramic pellets. But since nuclear reactions have occurred, the contents are’t quite the same. Before producing power, the fuel was mostly Uranium (or Thorium), oxygen, and steel. Afterwards, many Uranium atoms have split into various isotopes of almost all of the transition metals on your periodic table of the elements.
The waste, sometimes called spent fuel, is dangerously radioactive, and remains so for thousands of years. When it first comes out of the reactor, it is so toxic that if you stood within a few meters of it while it was unshielded, you would receive a lethal radioactive dose within a few seconds and would die of acute radiation sickness within a few days. Hence all the worry about it.
In practice, the spent fuel is never unshielded. It is kept underwater (water is an excellent shield) for a few years until the radiation decays to levels that can be shielded by concrete in large storage casks. The final disposal of this spent fuel is a hot topic, and is often an argument against the use of nuclear reactors. Options include deep geologic storage and recycling. The sun would consume it nicely if we could get into space, but since rockets are so unreliable, we can’t afford to risk atmospheric dispersal on lift-off.
2. What is inside 'em???
Nuclear reactors are typically loaded with Uranium Oxide fuel, UO2. Neutrons are introduced to the system, and many of them are absorbed by uranium atoms, causing them to become unstable and split, or fission, into two smaller atoms known as fission products. Sometimes, the uranium absorbs a neutron and does not fission, but rather transforms to a heavier isotope of uranium, such as U-239. U-239 beta-decays to Np-239, which in turn beta-decays to Pu-239.
The heavier nuclide may then absorb another neutron to become an even heavier element. These heavier atoms are known as transuranics. Nuclear waste, with regard to nuclear reactors, is the collection of nuclides left over after a reactor has extracted some energy out of nuclear fuel. Many of the isotopes are very radioactive for a very long time before they decay to stability.
The radioactivity causes the spent nuclear fuel to continue emitting heat long after it has been removed from the reactor. A few of the radioactive isotopes in the mix of spent fuel are gaseous and need to be carefully contained so that they do not escape to the environment and cause radiation damage to living things. Other types of nuclear waste exist, such as low level waste from other applications. This discussion will focus on high-level waste (HLW), the spent nuclear fuel from nuclear power reactors. The discussion become more interesting when we select the appropriate level of waste being produced. We will discussed after this. So,stay tune to us!
3. What is the composition in it???How long reactor operated??
Spent nuclear fuel composition varies depending on what was put into the reactor, how long the reactor operated, and how long the waste has been sitting out of the reactor. For simple example, take a look at a typical US reactor's waste composition is laid out in table 1.
| Charge | Discharge | |
|---|---|---|
| Uranium | 100% | 93.4% |
| Enrichment | 4.20% | 0.71% |
| Plutonium | 0.00% | 1.27% |
| Minor Actinides | 0.00% | 0.14% |
| Fission products | 0.00% | 5.15% |
Heavy metal composition of 4.2% enriched nuclear fuel before and after running for about 3 years (40,000 MWD/MT). Minor actinides include neptunium, americium, and curium. This table does not include structural material such as zirconium and stainless steel.
As we can notice that most of the Uranium is still in the fuel when it leaves the reactor, even though its enrichment has fallen significantly. This Uranium can be used in advanced fast reactor as fuel and is a valuable energy source. The minor actinides, which include Neptunium, Americium, and Curium, are very long-lived nuclides that cause serious concern when it comes to storing them for more than 100,000 years. Fortunately, these are fissionable in fast reactors and can thus be used as fuel! This still would leave us with the fission products. Too technical right?So, leave it as simple as that. Hope you all guys get the points here.
When atoms split, the smaller remaining atoms are often radioactive. There is no known way of getting rid of these atoms, and geological storage is often suggested as means of storing them until they decays to stability. Some fission products, such as Strontium-90, Cesium-137, and Iodine-131, are readily absorbed by biological systems and are capable of causing serious health problems. When the Chernobyl disaster occurred, these three isotopes caused most of the concern.
Ok, i think it is enough with that nuclear data thingy. Lets go for next topic.
4. Recycle???How the process start???
As we all know that nuclear waste is recyclable. Once Uranium fuel is used in a reactor, it can be treated and put into another reactor as fuel. This cycle can be repeated several times. Once all the energy is finally extracted from the fuel, the waste that is left over decays to harmlessness within a few hundred years, rather than a million years as with standard nuclear waste. See! My explaination is gonna get to explains how this interesting process is possible.
Before we go on, recall that Uranium exists in nature as 2 isotopes: the less common U-235, and the more common U-238. Conventional reactors mainly split U-235 to produce power, and the U-238 is often considered useless. When a standard reactor runs low on U-235, it must be refueled, even though there is a lot of U-238 still in there.
When we refer the diagram above, it is stated that the “useless” U-238 is the secret to recycling nuclear fuel. When it absorbs a single neutron, it goes through a series of nuclear reactions within a few days and turns into a very splittable isotope of Plutonium, Pu-239. The Pu-239 acts a lot like the U-235 that powers conventional reactors, so if you convert your U-238 to Pu-239 as you run your reactor, you can then use that Pu-239 to continue powering your reactor, or others!
A common type of nuclear reaction is called beta-decay. When a nucleus has more neutrons than it would like to have, it often beta-decays by breaking a neutron into a proton and an electron. The electron (called a beta-particle in this case, since it originated in the nucleus) flies off into nature, and the main result seen in the nucleus is a neutron converting to a proton (see figure).
When U-238 absorbs a neutron in a nuclear reactor, it becomes U-239, which is just the isotope of Uranium with one extra neutron than U-238. This beta-decays quickly and becomes Np-239. Then, the Np-239 beta-decays again to become Pu-239, which is a fissile isotope that can power nuclear reactors.
5. Recall back: Nuclear Fuel cycles. What is it???
In a simple word--> It is a nuclear fuel cycle is the path that nuclear fuel (Uranium, Thorium, Plutonium, etc.) takes as it is used to generate power in a nuclear reactor. They describe where the material comes from and where it ends up. Different fuel cycles range from very simple to fairly complicated. We describe several of these below.
6. Defined : Once Through Cycle
Yes!!! The simplest fuel cycle is the once-through cycle. It is the de-facto standard in most operating nuclear power plants, with a few exceptions in Europe and Asia. Uranium is mined, enriched, used in a reactor (where it becomes radioactive nuclear waste), and then stored until it is no longer dangerously radioactive. While this cycle is cheap, there are two major problems with it. Firstly, the waste is radioactive for hundreds of thousands of years. No one has been able to design a repository that is convincingly capable of storing material for that long. Secondly, Uranium is not the most abundant element on Earth, and in this kind of cycle, the global supply of cheap uranium could run low within 200 years. So much for sustainability!
7. Defined : Closed Fuel Cycle
It is defined as if spent fuel is not reprocessed, the fuel cycle is referred to as an open fuel cycle (or a once-through fuel cycle) but then if the spent fuel is reprocessed, it is referred to as a closed fuel cycle. Closing the fuel cycle involves recycling the nuclear waste as new fuel. Since the main component of nuclear waste is Uranium-238 (which can be transmuted to Plutonium), we can get significantly more energy out of the waste than in a once-through cycle. The recycling plant separates the good stuff from the bad stuff. The bad stuff is mostly fission products, the atoms that a Uranium atom becomes after it splits in the fission process.
These fission products mostly decay to safe levels within 300 years, which is significantly shorter than standard nuclear waste. So, by closing the fuel cycle, we address both the issues identified with the once-through cycle. Now, the uranium resources can last for tens of thousands of years. In this case, nuclear power can be considered sustainable. However, the reprocessing technology is expensive and separates out pure Plutonium, which could possibly be stolen, bringing a rogue entity closer to having a nuclear weapon. For these reasons,some other countries do not currently recycle. There are ways to solve these issues.
8. Malaysia should consider to use this type of reactor. You know why? Just read my point of view below!
A breeder reactors can create more fissile material (atoms that readily split) than they use. These special reactors are designed to have extra neutrons flying around, so that some can convert U-238 to Pu-239 (see above) and the others can run the reactor. Often, these special reactors are deemed "fast" reactors because the neutrons are moving through the reactor at higher speeds, on average. In a full breeder fuel cycle, we get the maximum use of the Uranium resources on Earth.
The cycle has the same two downsides as the closed cycle. Additionally, we have significantly less operational experience with breeder reactors, so we would need to train builders and operators for such a machine. Using a Thorium cycle instead of a Uranium-Plutonium cycle may allow breeding in less exotic reactors. So, we back on track after this!
9. Re-visited: Nuclear fuel cycle
Interesting! A car need fuel / juice to run the engine (combustion engine type). In simple word, a nuclear fuel cycle is the path that we put heavy atoms through in order to extract energy from them, starting at the day we find them and ending when their wastes are no longer dangerous. Fuel cycles can take on a wide variety of configurations, leading to lively debate about one particular cycle being superior to another.
All commercial power-producing reactors for example in the USA are on a once-through cycle (which is more of a line than a cycle), while some in Europe and Asia go through a single-recycle cycle (which sounds funny). The economics, politics, and long-term sustainablity of nuclear energy depend critically on fuel cycles.
10. What is Front end???
To whip up a new fuel cycle, we usually have several processes to mix and match. These typically fall into three broad categories. First, we have the front end -- steps used to prepare heavy atoms for insertion into nuclear reactors. So, you might survey the land, find uranium (or thorium) ore, dig it up, convert it to a gas so that you can enrich it, enrich it, convert it to a solid fuel form, and then fabricate it into fuel assemblies. The nuclear engineers have coined the processes of the front end of the nuclear fuel cycle as mining, conversion, enrichment, and fabrication.
The diagram above explained to us how closed fuel cycle works.
11. Steps of operational in fuel cycles
Next, the second category of fuel cycles is where we split atoms to generate energy. While we’re at it, we have an exciting list of fuel cycle processes to do that is only limited by our imaginations. We can put fertile material around the reactor core and breed new fissile fuel, we can use liquid fuel with online refueling and fission-product separation, we can strategically place transuranic targets to reduce the toxicity of nuclear waste, we can perform alchemy, converting common atoms to rare or valuable ones, etc.
As we noticed that we can reprocess and recycle and refabricate here. In reality, we are inhibited by economics, politics, licensing, and operations experience. Because of these, the most common type of fuel cycle has only one thing in this category: burn Uranium fuel rods in a reactor for about 3-5 years and remove them. For example, in France, they recycle plutonium for one loop in something called MOX fuel. MOX is Mixed Oxide.
12. What is Back end???
Enough with those info?? Ok, lets continue with next topic. This is it. Back End.
It is defined as to safely manage, contain, and either reprocess or dispose of spent nuclear fuel. In other words,
once we’ve finished getting energy out of the heavy atoms, we must dispose of the left-over ones and the dangerously radioactive minor actinides and fission products (the two smaller atoms created by splitting one large one). Processes to safely and responsibly accomplish this are the third category of our dynamic fuel cycle cast, called the back-end. The internationally accepted approach to waste disposal is to first ensure that the nuclides are immobilized (put in a material with low leachability, good mechanical strength, and the capability to hold large amounts of waste) and then placed underground.
Of course, arguments arise in how these two steps should be done. Materials used for immobilization are typically ceramics or glass. To protect against criticality accidents, these materials often have neutron-eating atoms such as boron mixed in. Finding a place to bury it is certainly a politically hot topic. The idea is to have as many barriers as is practical between the dangerous nuclides and the environment.
In addition, the back end of the of the nuclear fuel cycle mostly spent fuel rods, often contains fission products that emit beta and gamma radiation, and may contain actinides that emit alpha particles, such as 234U, 237Np, 238Pu and 241Am, and even sometimes some neutron emitters such as Cf. These isotopes are formed in nuclear reactors.It's important to distinguish the processing of uranium to make fuel from the reprocessing of used fuel. Used fuel contains the highly radioactive products of fission.
But many of these are neutron absorbers called neutron poisons in this context. These eventually build up to a level where they absorb so many neutrons that the chain reaction stops, even with the control rods completely removed. At that point the fuel has to be replaced in the reactor with fresh fuel, even though there is still a substantial quantity of 235U and plutonium present. Currently, in the USA, this used fuel is stored.
For example, in other countries (the UK, France, and Japan in particular) the fuel is reprocessed to remove the fission products, and the fuel can then be re-used. The reprocessing process involves handling highly radioactive materials, and the fission products removed from the fuel are a concentrated form of High Level Waste as are the chemicals used in the process.
No,it is not the end yet!!!
It is just the beginning!
Please continue to next post!!!
0 comments:
Post a Comment