Monday, November 28, 2011
Sunday, November 27, 2011
Nuclear Fuel Cycle
These are the various steps that together make up the entire Nuclear Fuel Cycle:
1. Mining and milling
Uranium is usually mined by either surface (open cut) or underground mining techniques, depending on the depth at which the ore body is found. In Australia the Ranger mine in the Northern Territory is open cut, while Olympic Dam in South Australia is an underground mine (which also produces copper, with some gold and silver). The newest Canadian mines are underground.
From these, the mined uranium ore is sent to a mill which is usually located close to the mine. At the mill the ore is crushed and ground to a fine slurry which is leached in sulfuric acid to allow the separation of uranium from the waste rock. It is then recovered from solution and precipitated as uranium oxide (U308) concentrate.*
* Sometimes this is known as "yellowcake", though it is finally khaki in colour.
Some mines, notably in the USA and Kazakhstan, use in situ leaching (ISL) to extract the uranium from the ore body underground and bring it to the surface in solution. It is recovered in much the same fashion.
U308 is the uranium product which is sold. About 200 tonnes is required to keep a large (1000 MWe) nuclear power reactor generating electricity for one year.
2. Conversion
Because uranium needs to be in the form of a gas before it can be enriched, the U308 is converted into the gas uranium hexafluoride (UF6) at a conversion plant.
3. Enrichment
The vast majority of all nuclear power reactors in operation and under construction require 'enriched' uranium fuel in which the proportion of the U-235 isotope has been raised from the natural level of 0.7% to about 3.5% to 5%. The enrichment process removes about 85% of the U-238 by separating gaseous uranium hexafluoride into two streams: One stream is enriched to the required level and then passes to the next stage of the fuel cycle. The other stream is depleted in U-235 and is called 'tails'. It is mostly U-238*.
* Figures in the diagram assume enrichment to 3.5% U-235 and a tails assay of 0.25%. The 220t figure should be 172t (146 tU)
So little U-235 remains in the tails (usually less than 0.25%) that it is of no further use for energy, though such 'depleted uranium' is used in metal form in yacht keels, as counterweights, and as radiation shielding, since it is 1.7 times denser than lead.
Today's enrichment plants use the centrifuge process, with thousands of rapidly-spinning vertical tubes. A few older plants also continue. Research is being conducted into laser enrichment, which appears to be a promising new technology.
A small number of reactors, notably the Canadian CANDU reactors, do not require uranium to be enriched.
4. Fuel fabrication
Enriched UF6 is transported to a fuel fabrication plant where it is converted to uranium dioxide (UO2) powder and pressed into small pellets. These pellets are inserted into thin tubes, usually of a zirconium alloy (zircalloy) or stainless steel, to form fuel rods. The rods are then sealed and assembled in clusters to form fuel assemblies for use in the core of the nuclear reactor.
Some 27 tonnes of fresh fuel is required each year by a 1000 MWe reactor.
5. The nuclear reactor
Several hundred fuel assemblies make up the core of a reactor. For a reactor with an output of 1000 megawatts (MWe), the core would contain about 75 tonnes of low-enriched uranium. In the reactor core the U-235 isotope fissions or splits, producing heat in a continuous process called a chain reaction. The process depends on the presence of a moderator such as water or graphite, and is fully controlled.
Some of the U-238 in the reactor core is turned into plutonium and about half of this is also fissioned, providing about one third of the reactor's energy output.
As in fossil-fuel burning electricity generating plants, the heat is used to produce steam to drive a turbine and an electric generator, in this case producing about 7 billion kilowatt hours of electricity in one year.
To maintain efficient reactor performance, about one-third of the spent fuel is removed every year or 18 months, to be replaced with fresh fuel.
6. Used fuel storage
Storage pond for used fuel at UK reprocessing plant
Used fuel assemblies taken from the reactor core are highly radioactive and give off a lot of heat. They are therefore stored in special ponds which are usually located at the reactor site, to allow both their heat and radioactivity to decrease. The water in the ponds serves the dual purpose of acting as a barrier against radiation and dispersing the heat from the spent fuel.
Used fuel can be stored safely in these ponds for long periods. It can also be dry stored in engineered facilities, cooled by air. However, both kinds of storage are intended only as an interim step before the used fuel is either reprocessed or sent to final disposal. The longer it is stored, the easier it is to handle, due to decay of radioactivity.
There are two alternatives for used fuel:
- reprocessing to recover the usable portion of it
- long-term storage and final disposal without reprocessing.
7. Reprocessing
Used fuel still contains approximately 96% of its original uranium, of which the fissionable U-235 content has been reduced to less than 1%. About 3% of used fuel comprises waste products and the remaining 1% is plutonium (Pu) produced while the fuel was in the reactor and not "burned" then.
Reprocessing separates uranium and plutonium from waste products (and from the fuel assembly cladding) by chopping up the fuel rods and dissolving them in acid to separate the various materials. Recovered uranium can be returned to the conversion plant for conversion to uranium hexafluoride and subsequent re-enrichment. The reactor-grade plutonium can be blended with enriched uranium to produce a mixed oxide (MOX) fuel*, in a fuel fabrication plant.
* MOX fuel fabrication occurs at facilities in Belgium, France, Germany, UK, Russia and Japan, with more under construction. There have been 25 years of experience in this, and the first large-scale plant, Melox, commenced operation in France in 1995. Across Europe about 30 reactors are licensed to load 20-50% of their cores with MOX fuel and Japan plans to have one third of its 54 reactors using MOX by 2010.
The remaining 3% of high-level radioactive wastes (some 750 kg per year from a 1000 MWe reactor) can be stored in liquid form and subsequently solidified.
Reprocessing of used fuel occurs at facilities in Europe and Russia with capacity over 5000 tonnes per year and cumulative civilian experience of 90,000 tonnes over almost 40 years.
8. Vitrification
After reprocessing, the liquid high-level waste can be calcined (heated strongly) to produce a dry powder which is incorporated into borosilicate (Pyrex) glass to immobilise the waste. The glass is then poured into stainless steel canisters, each holding 400 kg of glass. A year's waste from a 1000 MWe reactor is contained in 5 tonnes of such glass, or about 12 canisters 1.3 metres high and 0.4 metres in diameter. These can be readily transported and stored, with appropriate shielding.
This is as far as the nuclear fuel cycle goes at present. The final disposal of vitrified high-level wastes, or the final disposal of spent fuel which has not been reprocessed used fuel, has not yet taken place.
Resource from World Nuclear Association
Friday, November 25, 2011
Fun Facts #4
Did you know that?
Discovery of Uranium
Uranium was discovered in 1789 by Martin Klaproth, a German chemist, who isolated an oxide of uranium while analyzing pitchblende samples from the Joachimsal silver mines in the former Kingdom of Bohemia located in the present day Czech Republic.
Discovery of Uranium Fissionability
It took until 1938 to discover that uranium could be split to release energy, that is fission. This was accomplished by Otto Hahn and Fritz Strassman.
Discovery of Uranium Radioactivity
Henri Antoine Becquerel discovered that uranium was radioactive in 1896.
Replies to Tun Dr Mahathir from UPM student
Tun Mahathir's View about Nuclear Power Plant for Malaysia
1. With the price of oil going up higher and higher, many in this country are thinking about power generation. At one time the Malaysian Government had decided on a four fuel policy for the generation of electric power. We wanted power plants to use either fuel oil, gas, coal or hydro power. We had excluded the use of nuclear power.
2. Why did we reject nuclear power?
3. I am not a nuclear scientist but I believe I know enough of the dangers of using nuclear (fissionable) material.
4. When Hiroshima and Nagasaki were atom-bombed, the scientists who invented the bombs thought that the destructive effect would be only from the huge explosion due to fissionable material. So did their victims - the Japanese.
5. As a result the Japanese entered the destroyed cities to carry out rescue work and to clean up.
6. It was only later that they realised that the residual radiation would cause a variety of radiation sickness and diseases. The radiation remained harmful for a long period after explosion. Even today there are people who had entered the bombed area in those days who are dying of a variety of diseases, including cancer, contracted through exposure to radiation from the Hiroshima and Nagasaki bombs.
7. I think we all know about the Chernobyl disaster in Russia. Despite thousands of tons of concrete being poured into the site, the power plant is still emitting dangerous radiation.
8. Besides this we should know that radioactive material used as fuel for power generation remain radioactive and dangerous to health after the fuel has been exhausted. The waste cannot be disposed anywhere, not by burial in the ground nor dumping in the sea. It can be reprocessed by certain countries only. This requires the dangerous material to be transported in special lead containers and carried by special ships. Most ports do not allow such ships to be berthed at their facilities. Reprocessing means that the nuclear material again becomes active and harmful to health.
9. The fact is that we do not know enough about radioactive nuclear material. Once it is processed it remains a source of danger forever.
10. We have some experience dealing with radioactive material. In Perak we have a site where we had buried by-products of tin mining (amang) which had been processed to become radioactive and which was used to colour television. We had poured tons of cement on the buried material. More than one square mile of the burial site is barred to humans. The site is still radioactive and dangerous.
11. If we have a nuclear plant, besides not being able to get rid of nuclear waste, we may have accidents which can endanger people living even far away because of the material being carried by water (ground water) and wind.
12. I think the authorities should rethink the idea of nuclear power plants. Scientists do not know enough about dealing with nuclear waste. They do not know enough about nuclear accidents and how to deal with them.
13. Until we do, it is far better if Malaysia avoids using nuclear power for electrical generation.
The History of Nuclear Energy - The Calder Hall Nuclear Power Station
Calder Hall Nuclear Power Station |
Storage pond for spent nuclear fuel |
Germany Goes Nuclear Free
Germany has voted to shut down all nuclear power reactors by 2022, making it the first major industrial nation to completely reject the technology since the Fukishima disaster in Japan. They will be powered by renewables, putting massive pressure on infrastructure development in Europe’s biggest economy. While other nations, including Britain and France, plan to build more nuclear reactors, Germany will have to scale up wind and solar power in order to keep the lights on and meet climate change targets. The decision to reject nuclear came after the fall out from the Fukishima disaster and protests on the streets against nuclear.
Angela Merkel, the German Chancellor, was forced to do a u-turn on nuclear, having previously said the technology was safe, in order to retain public and political support. Despite the fact that Germany gets 23 per cent of its power from its 17 nuclear reactors, nine of which are currently running at full capacity, she claimed that wind and solar energy could meet the shortfall. The announcement is a shot in the arm for the renewables industry as it will provide certainty and funding. "This is more than consensus for a nuclear exit, this is consensus for a switch to renewable energy," she said. "We want to remain an industrial nation and sustain growth. But we want to organise that growth so that we guarantee quality of life for coming generations as well.”
But industry is angry at her change of heart, claiming it could raise energy costs across Europe because of demand on gas and existing renewables. In particular manufacturers, the source of Germany's wealth and status have warned the decision will increase energy costs and could lead to electricity shortages.
Source: http://www.telegraph.co.uk/news/worldnews/europe/germany
Wednesday, November 23, 2011
The Way of the Nuclear, or The Way of Renewables?
Renewable energy has the uncanny fact to be as its name says, renewable! Which means it is a resource which is undepletable! As a bonus, its operation is friendly to the environment with no adverse side-effects.
However, is it practical for renewable energy to replace all other forms of energy production as a whole? Lets take a look at the table below:
From the table, we can examine that renewable energy sources (green) uses massive amounts of resources! Imagine constructing 100 km square of land area just to produce 1 MWe for 1 year when the same 1MWe can be produced by just 30 tonnes of uranium? From here, it is just not feasible to sustain the earth's resources by pooling all our investments in renewable energy. Although renewable energy has very good prospects for future use and when the technology allows it, right now, it is not able to compete competitively with nuclear energy.
Furthermore, the resources used from developing extensive renewable energy projects can be used to alleviate current world crises such as famine, homeless citizens and no jobs.
So now, nuclear energy or renewable energy?
note: Picture taken from;
http://www.clipartof.com/portfolio/a-papantoniou/illustration/collection-of-green-energy-icons-of-renewable-energy-solar-power-biofuel-water-factory-wind-turbine-green-home-electricity-recycling-and-environment-21602.html
Tuesday, November 22, 2011
Why is nuclear energy bad for the world?
Nuclear energy produces long-lived radioactive waste. There is also a possibility of accidents that would release radioactive material into the environment. Exposure to the Radioactive Material Can Be Deadly, Causing Health Problems and Cancer: Through the history of nuclear disasters we have had a living lab to see the numbers of deaths caused by nuclear power plants along with infertility, health problems, and deadly cancers among people in communities even far away from the original site.
Monday, November 21, 2011
Nuclear power plants world-wide
As of 15 September 2011 there are 433 nuclear power plant units with an installed electric net capacity of about 367 GW are in operation in 31 countries . 65 plants with an installed capacity of 63 GW are still under construction .
As of end 2009 the total electricity production since 1951 amounts to 64,600 billion kWh. The cumulative operating experience amounted to 14,570 years by August 2011
Number of reactors in operation, worldwide, 2011-09-15 (IAEA 2011, modified)
Number of nuclear reactors worldwide by age as of 2011-09-15 (IAEA 2011)
Sunday, November 20, 2011
The History of Nuclear Energy - The Very First Nuclear Power Plant
Nuclear reactor hall at Obninsk Power Station |
Fun Facts #3
Saturday, November 19, 2011
Being Clear about Nuclear!
I am Lamni, an engineering student in Malaysia,
NUCLEAR ENERGY!
- More than 55% of Malaysia's electricity is generated from gas, which supply is getting ever depleted and prices ever higher?
- Coal that Malaysia uses for electricity generation are 100% imported from other countries?
- Malaysia's energy demand is ever rising and supply is probarbly unable to keep up?