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Nuclear power reactors use a reaction called nuclear fission. The fission is a source of energy for the generation of power. Two isotopes in common use as nuclear fuels are uranium-235 and plutonium-239.
Splitting the atom
Fission is another word for splitting. The process of splitting a nucleus is called nuclear fission. Uranium or plutonium isotopes are normally used as the fuel in nuclear reactors. Their atoms have relatively large nuclei that are easy to split, especially when hit by neutrons.
When a uranium-235 or plutonium-239 nucleus is hit by a neutron, the following happens:
The nucleus splits into two smaller nuclei – daughter nuclei, which are radioactive
Two or three neutrons are released (2,88 on average)
Energy is released
The additional neutrons released may also hit other uranium or plutonium nuclei and cause them to split. This means that you now have nine released neutrons. They can hit other nuclei and you’ll have 27, then 81, then 243 etc. This is called a chain reaction. The chain reaction in nuclear reactors is controlled to stop it moving too quickly.
The power plant
The energy that is released by a reaction, whether they’re chemical of nuclear, can’t just be caught in a basket. It takes a power plant to convert the energy into a useful form, most of the time that’s electrical energy. Funny enough, even the most advanced nuclear installation is still using the age-old principle of the steam-engine.
Steam
Fossil fuel (coal, oil and gas) power stations and nuclear power stations both use the same processes to make electricity. These are:
· Fuel produces heat and heat is used to boil water and is turned into steam
· Steam turns a turbine
· Turbine turns a generator and the generator makes electricity
· Electricity goes to the transformers to produce the correct voltage
The only difference between fossil fuel and nuclear power stations is how the water is heated. Fossil fuel power stations burn their fuel while a nuclear power station uses the fission of uranium or plutonium to generate heat. Uranium and plutonium are non-renewable energy resources.
The reactor
The nuclear reactor is designed to allow a controlled chain reaction to take place. Each time a uranium nucleus splits up it releases energy and three neutrons. If all the neutrons are allowed to be absorbed by other uranium nuclei the chain reaction will spiral out of control causing a meltdown. To control the energy released in the reactor moveable control rods are placed between the fuel rods. These control rods are made of boron which absorbs some of the neutrons so fewer neutrons are available to split uranium nuclei. The control rods are raised to increase and lowered to decrease the number of free neutrons.
You can see this in this simulation
This simulation uses a program to play java-programs in a browser. This does mean loading can take a bit longer. When you've pressed the "play" button, you'll be prompted by a question, answer "Run Cheerpj browser-compatible version".
Opdrachten
Use the tab called “Fission – One Nucleus” to answer these questions:
95. U-235 is quite stable, what is done to destabilise it?
96. Under normal circumstances, U-236 is stable as well, what’s different here?
97. In your own words, what does “unstable” mean when used to describe Uranium?
Explore the features of the “Chain Reaction” tab.
98. If you wanted to explain nuclear chain reactions to someone, what would you tell them? Briefly, explain your ideas using appropriate vocabulary and drawings. Make certain that your answer explains why the reaction occurs AND what affects the speed of the reaction.
99. Why is Uranium-235 and its nuclear reaction perfect for creating chain reactions?
Explore the features of the “Nuclear Reactor” tab.
100. What is the purpose of the control rods within a nuclear reactor?
Other products
As well as producing heat the nuclear reactor can be used to make other materials radioactive. The chain reaction inside the reactor releases neutrons. If a material is put into the reactor some of these neutrons will be absorbed by the nuclei of its atoms. This will make the atom's nucleus unstable which means it has become radioactive. These man-made radioisotopes are often then used as tracers in hospitals to diagnose patients. This is not done in reactors in a power plant but in specifically designed reactors. The largest producer of medical isotopes is located in the dunes of the Dutch town of Petten.
Research
A more scientific use of nuclear fission is of course research. Some research focusses on improving reactors to get more energy out of nuclear power stations but there's also a lot of fundamental research. Research that doesn't focus on "How can we improve a certain device?" or "How can we make use of this?" but "How does this actually work?" The video below is made in a research reactor. The video is quite long but the captioned explanation is quite good.
Radioactive waste
When the uranium in the nuclear reactor is used, you are left with plutonium. This can be used in another reactor to undergo fission again but after that, it’s waste. The biggest problem with this waste is that it is still really active, a person standing near would receive a lethal dose within minutes.
The other half of this problem is that the half-live is quite long as well. Depending on the concentration, the waste will remain dangerous for something between 1000 and 10 000 years.
Waste with a low activity can be put in a regular garbage dump or burned in an incinerator. When the activity is too high it obviously can’t. The use fuel rods from a reactor can be recycled first. All the uranium is taken out an put in new fuel rods. The rest is poured in glass, put in steel containers and stored deep underground.
Pouring the waste in glass seems odd because we know glass to be a brittle and breakable material but that’s not an issue here, the glass is put in steel containers.The reason glass is used is that it has a tight molecular structure. The radioactive atoms could leak through the metal grid of steel and the glass prevents this. If you’ve ever seen the white stuff on batteries that you’ve kept for too long you’ll know what we’re talking about. That white stuff are chemicals that didn’t come through crack in the metal casing of the battery, it came through the metal itself.
The video below shows you how the Dutch waste is stored at the moment.
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