The performance of these reactors varies up to 190MWs, used in bigger flattops. British, American and Russian fleets use steam turbine driving, while Chinese and French ones prefer the turbines connected to an electric generator.
The use of these reactors on merchant ships, railway engines and airplanes is still in experimental stadium because their application is held up by problems with the economical insurance and other permits.
Old type reactors
There are three obsolescent reactors to be mentioned.
*The Magnox type reactors use highly pressurized carbon dioxide as coolant, lead moderators and natural uranium as fuel. First people used it for plutonium production to make nuclear weapons out of it, but afterwards switched to generating electricity. The name Magnox refers to a special alloy (Magnesium Non-Oxidising), because special magnesium alloys were used (with a bit of aluminium) as cladding to protect the unenriched uranium fuel. The disadvantage of this material was that it limited the maximum temperature, and hence the thermal efficiency of the plant, and besides that, it reacted with water, preventing long-term storage of spent fuel in the coolant. It is a disused reactor type because of its unsafety, low efficiency and its being carbon dioxide cooled.
*The AGR (Advanced Gas-Cooled Reactor) type reactors are the second generation of British reactors, which – as it is shown by their name – were gas cooled (carbon dioxide) and had lead as moderator. However, they didn’t use natural uranium as fuel, but uranium dioxide enriched by 2,5-3.5% , stored in stainless steel tubes. They worked with good thermal efficiency (41%) and were this way more effective than modern PWR reactors with the thermal efficiency of 32%. This is largely due to the higher coolant outlet temperature of about 640°C with gas cooling, compared to about 325°C for PWRs. Their disadvantage is that they are not safe enough and the reactor core has to be larger. They were developed form the Magnox reactor.
*The RBMK (reaktor bolshoy moshchnosti kanalniy ) type reactors are the most developed Soviet nuclear reactors. Water was used as coolant along with lead moderators, while the fuel was natural uranium, later replaced with low-enriched uranium oxide. These plants were firstly used for plutonium production. The first reactor of the kind which served friendly purposes generated 5 MW electricity. It was situated in Obninsk, between 1954 and 1959 (the world’s first plant not used for destructive intentions)
What happens to the nuclear waste?
During the operation of nuclear power stations, a great quantity of gaseous, liquid and solid radioactive waste is produced. The highly radioactive waste is perilous and doesn’t dissolve in thousands of years’ time, which means that it remains life threatening. The spent fuel is a deadly mixture of radioactive products: it contains plutonium, strontium and caesium.
Luckily, the volume of highly radioactive waste is small. A plant with the typical efficiency of 1000MWs produces annually 2 m3 of waste.
Nuclear wastes can be classified according to:
Their activity: *highly active( >4000 Bq/cm3)
*moderately active(40-4000 Bq/cm3)
*slightly active( <40 Bq/cm3)
*long half-life (> 30 years)
*medium half-life (30 years – 30 days)
*short half-life (< 30 days)
These wastes are very dangerous to humans and animals as well, so they have to be neutralized with the proper treatment. The treatment of waste takes into account three basic aspects. Those materials that do not reach the level of radiation defined by the radiation protection regulations, after dilution, are released into the nature. Those having medium or short half-lives are stored in transitory containers until their radiation falls under the permissible level. Materials having long half-lives, after decreasing their volume and giving them the proper treatment, are placed into checked definite containers.
There are more methods to store these harmful and dangerous nuclear wastes:
- In the United States of America one part of the treated radioactive waste is stored in double-walled, stainless steel containers surrounded with a 1m thick concrete facing. The major part however, is made up from used heating element rods in their original concrete shields, sunk into special pools near the reactor. This unfortunately doesn’t represent a definite solution.
-In Great Britain the waste is stored as a fluid similar to strong tea. It is placed in steel containers bedded into concrete. The decay of the waste produces heat, therefore the container has to be cooled, because if the liquid solidifies that could lead to radioactive leak. Spiral tubes are put into the containers in which cold water flows. Although these containers have been in use for almost 40 years, they are not a definite solution!
-At the moment the best solution is to melt the waste into glass rolls which are to be buried deep under the ground. In the French Marcoule this method has been used since 1978. The waste is heated in a whirling drum until it dries and only the solid ingredients remain. After that, it is mixed with silicon dioxide, boron and other materials of which glass is made. Next the mixture is locked into a vertical chamber where it is heated to 1500 °C. The liquid glass flows directly into stainless steel containers, twice the size of a regular milk-can. A power station with the annual performance of 1000MWs fills 15 of these containers. After the glass consolidates, the containers are soldered.
In Mercoule, these containers are stored in special shafts in a neighbouring building. One by one, the cans produce 1.5 kW heat so they need to be cooled with air. The British and Americans start to use this solution too. The radioactive waste does not represent any danger if its status is followed with attention, but eventually has to get into a place where human interventions are not necessary.
There is a suggestion that the metal containers should be surrounded with cast iron or copper smocks and held in underground caves. Namely, the containers should be placed into hollows or ditches and covered with concrete or betonite which entrap the leaking radioactive materials. The metal containers must not corrode and let out the radioactive radiation in 1000 years time. In 500 years the radioactivity of the waste drops to about the radioactivity of uranium ore. According to specialists, if the caves are in a good place and deep enough (a few hundred meters), 1 million years could pass until the materials reach the surface, while by that time the slightest remains of the materials would decay. The selected dumpsite must not hide valuable minerals, for fear of later civilizations might bump into them while mining.
It is hard to find a place where inhabitants would agree to stock nuclear wastes. Nobody wants a nuclear dump near their home. Eventually authorities handling radioactive waste were forced to dig caves under existing rehashing institutions or under the sea and not to try to find new locations.
We don’t think that these options are definitive and safe solutions to store the waste, because polluting the earth with such maleficent materials for a long time just won’t do. If something goes wrong, they can easily get into the ground or ground water. We think that the best solution would be to send them out into the space which, however, is very costly but safe and it would not pollute the Earth.