Page 18
Site Navigation Links : Please click on SITE MAP: Next Page
Page 1 made a brief reference to Nuclear Power mentioning such words as fissile material, breeder reactors and fusion to introduce this topic. A good web site on this topic has been prepared for pupils at schools and colleges with graphics and quizzes : www.darvill.clara.net - from the home page click on sitemap and then nuclear energy
Here, only brief details are given so as to have some understanding of nuclear processes.
First, do we know what heat is all about? If an object is cold the atoms or molecules inside it will joggle about rather slowly but when it is hot they joggle about very vigorously. So when a piece of ice is heated the molecules of water, initially neatly arranged, break free and wander about randomly in a liquid. As more heat is added the motion becomes extremely agitated and the molecules break free from the liquid and migrate about the atmosphere as steam. Therefore any action that produces agitation is capable of giving out heat.
Combustion is known to everyone and page 1 tells us that it has been known for a long time - a fuel plus the gas oxygen gives lots of agitated carbon dioxide, CO2 , and water molecules.
A nuclear reaction, discovered very recently compared with combustion, looks deceptively simple:
A neutron hits a uranuim nucleus which shatters into two parts and gives two or three neutrons (blue dots at the boundary of the picture). For more details see www.howstuffworks.com . At the video centre one is directed to Physical Science: Nuclear Energy for a 20 min video.
These neutrons may then go away to strike other nuclei which will be shattered in the same manner. This so called fission process produces many agitated products and therefore is a heat source. ( there is a mass difference between the original nucleus and the fission fragments and by the celebrated E = mc2 we have mass converted to energy ) Note - fission does not require oxygen as part of the process, it does not therefore produce CO2 and, although not obvious from the diagram, it is a very concentrated energy source. In the early 1950's it was suggested that a car could be made with fuel for the whole life of the car. Hence calling at petrol stations to replenish fuel would be a thing of the past. Well, we know this did not happen and may wonder why?
In the illustration the two parts (or nuclear fragments ) are elements - typically strontium ,and ceasium but they are in an unusual state not found in nature. They are isotopes of naturally occuring elements but give off intense gamma rays or alpha or beta particles which would severely damage body tissue . So, our car could be fuelled for life but would have to be encased in a huge concrete shield to protect the driver and passengers.
Thus far we have a fission process capable of producing heat - now for the detail.
Not all of the Uranium atoms are fissile. Most of the atoms in a piece of Uranuim metal are U238 and these will not break into fragments. Only 0.7% of the atoms are U235 and these atoms are fissile. ( note - in breeder reactors U238 is converted into fissile Plutonium but the technology for this type of reactor is not well advanced)
The neutrons must be rather special, too. The best chance of getting fragmentation is when a rather slow neutron impinges on the U235 atom. Thus, within the Uranium metal there is an embedded material to act as a moderator ( it slows down the neutrons ) and in some cases this will be graphite but in other cases it will be water or even heavy water ( this is H2O but with the hydrogen atom being replaced by deuterium.) Now this hot source has a good chance of going out of control since a slight imbalance in the neutron flux will cause too many fission events followed by many more fissions. Infact a nuclear bomb will have been created rather than a controllable nuclear heat source. To prevent this happening control rods, made from Boron ( Boron is a good absorber of neutrons and therefore capable of eliminating excess neutrons), are inserted into the nuclear pile to keep the reactor at criticallity.
Finally heat needs to be taken away from the reactor core and some fluid is circulated either with a one or two loop process to take the heat away and generate electricity in a conventional manner. The following illustration gives a simplified layout of a nuclear plant.
In the UK we had the WORLD'S FIRST NUCLEAR POWER STATION to deliver electricity in commercial quantities to the grid. It was connected to the grid on 27th August 1956 and the plant was opened officially by Queen Elizabeth II on 17th October 1956. For several decades nuclear power was an important energy source to augment the electricity derived from coal power stations and then there was a rapid switch to gas as this resulted in lower cost power stations.
To continue the story; there are many variations on the theme of nuclear reactors and, in the UK, only two types have been manufactured. The original Magnox reactors operated with natural Uranium, had a graphite moderator and used carbon dioxide gas as a heat exchange medium. The Uranium was encased in a special magnesium alloy ( in the trade, this is called cladding ) which does not capture neutrons and therefore allowed natural Uranuim ( 0.7 % U235 99.3% U238 ) to be used. The severe penalty for using this alloy , which had a low melting point, was that the operating temperature was low and, as our Thermodynamics section 9 tells us, the thermal efficiency is low also. The Advanced Gas cooled Reactors (AGR's) followed on from the Magnox program and, for these reactors, stainless steel was used to encase the Uranium. Stainless steel does absorb neutrons to a much larger extent than the magnesium alloy used in the Magnox reactors so it was no longer possible to use natural Uranium ( 0.7% U235) and the Uranium had to be enriched to about 3 % - that is 3% U235 and 97% U238. Graphite was again used as a moderator and the gas, carbon dioxide, was again used to transport heat from the nuclear reactor to the steam generating unit. The main outcome from the AGR's were that a higher operating temperature could be used and hence the thermal efficiency was improved. It must be noted, however, that when a country develops enrichment facilities it has the capability to produce nuclear weapons as enrichment for a bomb (about 90% U235) is just an extension of enrichment to 3%. So, in countries such as Iran, enrichment facilities could legitimately be used as part of a reactor program. It must be further noted that, whereas UK was at the forefront of the nuclear industry in the 1950/ 60/ 70/ 80's, it has greatly reduced its capability to build reactors. Any resurgence of this industry would be behoven to the French, Areva or US/Japanese, Westinghouse industrial groups.
Just as a domestic fire requires attention to keep it burning brightly so a nuclear reactor must be monitored all the time. The fuel is normally encased in long rods and loaded from a chamber above the core C. As the reactor continue to "burn" , U235 atoms are turned into fragments which are worthless for generating heat so a stage is reached when the whole rod has to be replaced. This "spent" rod is now very radioactive and is stored in water tanks till the radiation level is acceptable and then reprocessed into another rod . This reprocessing gives rise to large quantities of waste material and, so far, no nation in the world has developed a satisfactory method of containing nuclear waste. The intense radiation takes several centuries to decay to an acceptable level. The last procedure which must be carried out is site decommissioning - that is turning a nuclear generating site back to its natural state in the same way as mining districts have been landscaped to parks and woodlands. Now, all the pipework, all the concrete encasement, in fact, all the core area is a seething volume of radiation and one can only envisage that countries will take a short-cut and encase the whole core area in a concrete sarcophagus with an inscription " keep out till the next millennium". This, of course, is what has happened in Chernobyl - not only has the site been left but a whole region round the site in uninhabitable for years to come.
J H Fremlin wrote an article on Nuclear risks (see https://sth-se.diino.com/f.thompson/migrated_data/EandH and much of what was said in 1978 would apply today.Operating Nuclear Reactors on a day to day basis is not a dangerous occupation and not one person in the whole world has died from working in the Nuclear industry. Natural (not from nuclear power stations ) radiation causes Leukaemia and this gives a risk of 1/20000 whereas a smoker of ten cigarettes per day has a 1/400 risk. Professor Fremlin must not have guessed how life would change in thirty years. The indiscrimimate use of of Depleted Uranium (DU) for armaments in today's world www.newint.org (Nov 2007) is reckless beyond belief. The health risks are well known:
So the disturbing feature of nuclear waste management and site decommissioning is that our generation is bequething a resposibility to our grandchildren's grandchildren to guard a cocktail of most unpleasant materials. This is counter to intuitive thought that, we, in our generation, should aspire to leave the world, to the next generation, in a better state than it was handed down to us.
A list of nuclear power stations is given below and a full history of the UK nuclear industry is given at www.uic.com.au/nip84.htm
The geographical location is given on the map below
As one can see the location of all reactors are close to the sea so that sea water can be used to condense the steam from the turbines rather than the normal large cooling towers used in conventional power stations. The disadvantage here, of course, is that any notion of district heating is out of the question so our 2/3 heat due to Carnot ( in ) efficiency (fully discussed on page 9 ) is lost to the atmosphere.
If UK had had the wisdom of Denmark in the 1970's to pursue sustainable energy policies then it would be easy to advise against the nuclear energy route. However, to maintain our present life-style under the cloud of diminishing fossil fuel supplies and "renewables" still in an embryonic state, I see no other alternative but to re-introduce a nuclear energy program. The cost will be enormous and, if the fiasco of Areva's attempt to build a reactor in Finland is any guide, the time scale will be 15 to 20 years before the reactors start producing electricity.
| Index Page | Top of Page | Next Page |