Chemistry Nuclear Chemistry Nuclear Chemistry. Dec 31, Explanation: the aim of nuclear reactors is to generate energy through nuclear fission reactions. Related questions What are radioactive isotopes? What is the difference between fission and fusion? What does nuclear chemistry involve?
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Active Oldest Votes. Fissioning any fissionable isotope inherently releases a probabilistic number of neutrons, the average of which ranges from per fission bottom table v bar , and this number is not a major criterion for choosing an isotope. Contrary to some comments here, U can fission and nuclear stability is not solely related to high neutron counts. In fissile isotopes, like U, the critical energy is at or below the excitation of the nucleus when it has absorbed a neutron.
So any absorbed neutron can cause fission in these nuclei, though they still have a capture probability U can absorb a neutron and become U instead of fissioning. In fissionable and non-fissile isotopes, like U, critical energy is greater than the excitation of the nucleus when it has absorbed a neutron, so the neutron must bring additional energy to cause fission. Low critical energy and being fissile is generally linked to odd neutron counts. Energy dependent neutron cross-sections The plot in the link shows that only very high energy neutrons can cause fission in U far right brown line note the logarithmic axes , which correlates the critical energy concept; only neutrons above a certain energy are bringing enough to the U nucleus to get it above its critical energy.
It also shows that any neutron energy can cause fission in U Answer Since too few neutrons are born from fission at the energy required to fission U and other non-fissile isotopes , a reaction with only U is not sustainable. Natural uranium will have a U concentration of approximately 0. The capacity of enrichment plants is measured in terms of 'separative work units' or SWU. The SWU is a complex unit which indicates the energy input relative to the amount of uranium processed, the degree to which it is enriched i.
The unit is strictly: kilogram separative work unit, and it measures the quantity of separative work performed to enrich a given amount of uranium a certain amount when feed and product quantities are expressed in kilograms.
The unit 'tonnes SWU' is also used. There is always a trade-off between the cost of enrichment SWU and the cost of uranium. However, especially in relation to new small reactor designs, there is increasing interest in higher enrichment levels.
Some small demand already exists for research reactors. The first graph shows enrichment effort SWU per unit of product. The second shows how one tonne of natural uranium feed might end up: as kg of uranium for power reactor fuel, as 26 kg of typical research reactor fuel, or conceivably as 5.
The curve flattens out so much because the mass of material being enriched progressively diminishes to these amounts, from the original one tonne, so requires less effort relative to what has already been applied to progress a lot further in percentage enrichment.
The relatively small increment of effort needed to achieve the increase from normal levels is the reason why enrichment plants are considered a sensitive technology in relation to preventing weapons proliferation, and are very tightly supervised under international agreements.
Where this safeguards supervision is compromised or obstructed, as in Iran, concerns arise. About , SWU is required to enrich the annual fuel loading for a typical MWe light water reactor at today's higher enrichment levels. Enrichment costs are substantially related to electrical energy used. In the past it has also accounted for the main greenhouse gas impact from the nuclear fuel cycle where the electricity used for enrichment is generated from coal. However, it still only amounts to 0.
The utilities which buy uranium from the mines need a fixed quantity of enriched uranium in order to fabricate the fuel to be loaded into their reactors. This is the contracted or transactional tails assay, and determines how much natural uranium must be supplied to create a quantity of Enriched Uranium Product EUP — a lower tails assay means that more enrichment services notably energy are to be applied.
The enricher, however, has some flexibility in respect to the operational tails assay at the plant. This is known as underfeeding. In respect to underfeeding or overfeeding , the enricher will base its decision on the plant economics together with uranium and energy prices.
With reduced demand for enriched uranium following the Fukushima accident, enrichment plants have continued running, since it is costly to shut down and re-start centrifuges. The surplus SWU output can be sold, or the plants can be underfed so that the enricher ends up with excess uranium for sale, or with enriched product for its own inventory and later sale.
With forecast overcapacity, it is likely that some older cascades will be retired. Obsolete diffusion plants have been retired, the last being some belated activity at Paducah in Natural uranium is usually shipped to enrichment plants in type 48Y cylinders, each holding about These cylinders are then used for long-term storage of DU, typically at the enrichment site.
Enriched uranium is shipped in type 30B cylinders, each holding 2. The three enrichment processes described below have different characteristics. Diffusion is flexible in response to demand variations and power costs but is very energy-intensive. With centrifuge technology it is easy to add capacity with modular expansion, but it is inflexible and best run at full capacity with low operating cost. Laser technology can strip down to very low level tails assay, and is also capable of modular plant expansion.
The gas centrifuge process was first demonstrated in the s but was shelved in favour of the simpler diffusion process. It was then developed and brought on stream in the s as the second-generation enrichment technology.
It is economic on a smaller scale, e. It is much more energy efficient than diffusion, requiring only about kWh per SWU. China has two small centrifuge plants imported from Russia. China has several centrifuge plants, the first at Hanzhun with 6th generation centrifuges imported from Russia. The Lanzhou plant is operating at 3. Others are under construction. Brazil has a small plant which is being developed to 0.
Pakistan has developed centrifuge enrichment technology, and this appears to have been sold to North Korea. In both France and the USA plants with late-generation Urenco centrifuge technology have been built to replace ageing diffusion plants, not least because they are more economical to operate. Full initial capacity of 3. In it applied for doubling in capacity to 6. It is now cancelled, and in Orano requested the NRC to terminate the licence. It was designed to have an initial annual capacity of 3.
A demonstration cascade started up in September with about 20 prototype machines, and a lead cascade of commercial centrifuges started operation in March Like the diffusion process, the centrifuge process uses UF 6 gas as its feed and makes use of the slight difference in mass between U and U The gas is fed into a series of vacuum tubes, each containing a rotor 3 to 5 metres tall and 20 cm diameter.
When the rotors are spun rapidly, at 50, to 70, rpm, the heavier molecules with U increase in concentration towards the cylinder's outer edge. There is a corresponding increase in concentration of U molecules near the centre. The countercurrent flow set up by a thermal gradient enables enriched product to be drawn off axially, heavier molecules at one end and lighter ones at the other. The Russian centrifuges are less than one metre tall.
Chinese ones are larger, but shorter than Urenco's. The enriched gas forms part of the feed for the next stages while the depleted UF 6 gas goes back to the previous stage. Eventually enriched and depleted uranium are drawn from the cascade at the desired assays. To obtain efficient separation of the two isotopes, centrifuges rotate at very high speeds, with the outer wall of the spinning cylinder moving at between and metres per second to give a million times the acceleration of gravity.
Although the volume capacity of a single centrifuge is much smaller than that of a single diffusion stage, its capability to separate isotopes is much greater. Centrifuge stages normally consist of a large number of centrifuges in parallel. Such stages are then arranged in cascade similarly to those for diffusion.
In the centrifuge process, however, the number of stages may only be 10 to 20, instead of a thousand or more for diffusion. Centrifuges are designed to run for about 25 years continuously, and cannot simply be slowed or shut down and restarted according to demand.
Western cascades are designed for 0. Laser enrichment processes have been the focus of interest for some time. They are a possible third-generation technology promising lower energy inputs, lower capital costs and lower tails assays, hence significant economic advantages. One of these processes is almost ready for commercial use. Laser processes are in two categories: atomic and molecular.
In the US Government backed it as the new technology to replace its gaseous diffusion plants as they reached the end of their economic lives early in the 21st century. French work on SILVA ceased following a 4-year program to to prove the scientific and technical feasibility of the process.
Some kg of 2. Atomic vapour processes work on the principle of photo-ionisation, whereby a powerful laser is used to ionise particular atoms present in a vapour of uranium metal.
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