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Wednesday, July 14, 2010

Re: [ALOCHONA] Nuclear waste management at Rooppur



Do not know enough to discuss any technical details discussed in this article. I am also aware that, we have to embrace this technology. We have no other option. However asking us not to worry about nuclear waste management worries me. I know USA and many other countries that use this technology are still working on this issue and often fighting (State of Nevada with the US federal government for example) to pick the best way to store nuclear waste.
 
Our government agencies still do not know how to manage a national electric grid, waste from private and public hospitals or operate a railway system. As a non-technical person we know what happened in Russia in the 80's. Based on our efficiency we should worry.
 
Government should work with IAEA and other agencies to find "Best practices" in nuclear waste management in our tiny nation. This time failure is NOT an option.....[ As both political parties are willing to give away our Talpotti island to India, we do not have anywhere to go either].
 
Only thing left is to pray to Almighty God for protection............

-----Original Message-----
From: Isha Khan <bdmailer@gmail.com>
Sent: Sat, Jul 10, 2010 5:36 am
Subject: [ALOCHONA] Nuclear waste management at Rooppur

 
Nuclear waste management at Rooppur
 
No reason to worry about
 
Abdul Matin

A nuclear power plant
NUCLEAR fuel that undergoes fission in nuclear reactors consists of heavy elements like uranium-235, available in nature, and uranium 233 and plutonium-239, both produced artificially from thorium-232 and uranium-238, respectively, in nuclear reactors by interaction with neutrons.

The concentration of uranium-235 in natural uranium is very low, only 0.71%. The rest is uranium-238, a heavier isotope of uranium which does not fission in normal reactors. For use in light water reactors, it is necessary to enrich, i.e. increase the concentration of, uranium-235 to 3% to 4%.

Nuclear fuel requires several stages of processing before it can be put inside nuclear reactors in the form of fuel assemblies or bundles. The uranium ore undergoes milling, conversion to uranium hexafluoride, enrichment and again conversion to its final form -- uranium dioxide.

This oxide looks like a black powder, which is compacted to form small pellets about one centimeter in diameter and of the same height. These pellets are put inside metallic tubes called claddings, which are later sealed to prevent leakage of any substance from inside. The tubes form what are known as fuel bundles or assemblies.

Nuclear fuel does not burn like normal fossil fuels, which produce ash and flue gases. Instead, it undergoes fission inside a nuclear reactor and produces energy and fission products, which may be either solids or gases. The fission products are highly radioactive. The energy is carried away by the coolant to produce electricity. The fission products stay inside the fuel or the sealed cladding.

The fuel assemblies can stay inside a reactor for several years before they are replaced with new ones. The used fuel assemblies, after being taken out of the reactors, are called spent fuel. They produce heat even after they are taken out of the reactor, because of the intense gamma radiation, and need cooling. They are, therefore, stored in a pool of circulating water.

The spent fuel contains some unused uranium-235 and plutonium-239 plus uranium-238, which can be recovered by reprocessing the spent fuel, and can be reused in nuclear reactors. After reprocessing, the recovered uranium is sent for further enrichment.

Mixed with uranium dioxide, the plutonium-239 can be used as fuel after conversion into oxides. The highly radioactive fission products are separated from the spent fuel and stored in sealed and shielded canisters in safe depositories.

Nuclear fuel produces waste at different stages of processing. Some of the waste are radioactive and require special treatment for protecting human health and minimising their impact on the environment.

Radioactive waste is normally classified into three categories, namely low-level, medium-level and high-level waste depending on the amount, types and half-lives of radioactivity.

Half-life is the time taken by a radioactive substance or isotope to lose half of its activity. Half-lives vary from a fraction of a second to millions of years. Radioactivity decreases with time as the isotopes decay into stable and non-radioactive ones. Short-lived isotopes decay quickly whereas long-lived ones decay slowly.

Low-level waste contains small amounts of short-lived radioactivity. It is not dangerous to handle but needs to be disposed of carefully. Usually, it is buried underground, only a few feet deep. If necessary, it can be compacted or incinerated in a closed container to reduce the volume before final disposal. By volume, it consists of 90% of all radioactive waste worldwide, but contains only 1% of the total radioactivity.

The residue after mining and milling of uranium contains very low-level radioactivity and is generally buried in the mines. Depleted uranium (the left-over after enrichment), containing mostly uranium-238, is less radioactive than natural uranium.

Medium-level waste contains higher amounts of radioactivity and may require special shielding before disposal. Worldwide, it consists of 7% of the volume and contains 4% of the radioactivity of all radioactive waste. If necessary, it can be solidified through mixing with concrete or bitumen before disposal.

The short-lived waste from nuclear reactors can be buried like low-level waste, but the long-lived waste is disposed of deep underground. Because of shorter half-lives, both low-level and medium-level waste lose their radioactivity in time and become harmless after disposal.

High-level waste includes spent fuel and the main waste from reprocessed fuel. It consists of 3% of the volume of all radioactive waste and contains 95% of the radioactivity. It is comprised of highly radioactive fission products and some heavy elements with long-lived radioactivity. It generates a significant amount of heat and requires cooling and special shielding during handling and transportation.

If the spent fuel is reprocessed, the separated waste is vitrified, i.e. turned into glass by mixing it with borosilicate glass which is sealed inside stainless steel canisters for storage or disposal deep underground. Liquid high-level waste is evaporated to solids, mixed with glass-forming materials, melted, and poured into stainless steel canisters which are sealed by welding, again for storage or disposal.

If the spent fuel is not reprocessed, all the highly radioactive isotopes stay inside the fuel assemblies, which become high-level waste. Used fuel assemblies occupy about nine times the volume of equivalent vitrified high-level waste which results from reprocessing.

Whether reprocessed or not, the volume of high-level waste is modest. A typical large reactor produces about 3 cubic meters of vitrified waste or 25-30 tonnes of spent fuel per year. Because of its small volume, it is not difficult to store in isolated locations.

The transuranium elements (heavier than uranium) formed inside the nuclear reactors by absorption of neutrons have longer half-lives (thousands of years) compared with those of fission products (about 30 years or less).

It is possible to solve the problem of management of long-lived heavy elements by converting them into fission products with shorter half-lives by transmutation, i. e. irradiating them with fast neutrons in reactors causing fissions. The resulting fission products have shorter half-lives. Disposal of such short-lived fission products will be much easier than the tranuranium elements with long half-lives.

Final disposal of high-level waste is delayed for 40-50 years to allow its radioactivity to decay, after which less than one-thousandth of its initial radioactivity remains, and it is much easier to handle. Hence, canisters of vitrified waste, or used fuel assemblies, are stored under water in special ponds, or in dry concrete structures or casks for at least this length of time.

The final disposal of vitrified wastes, or of used fuel assemblies without reprocessing, requires their isolation from the environment for long periods. The most acceptable method is to bury them in dry, stable, geological formations deep underground, preferably inside abandoned salt mines.

According to available information, Russia is likely to supply nuclear fuel in the form of fuel assemblies for the proposed nuclear power plant at Rooppur, and take the spent fuel back. If this arrangement can be successfully implemented, Bangladesh will not have to worry about disposal of any high-level nuclear waste as all high-level waste, together with the spent fuel, will be transported back to Russia in properly shielded containers.

Bangladesh has to ensure that it gets back full credit for the unused uranium and plutonium that will be transported to Russia with the spent fuel. Russia may, however, charge Bangladesh the cost of handling, transportation and reprocessing of the spent fuel and that of safe storage of the high-level nuclear waste.

In case the spent fuel is not taken back by Russia, Bangladesh will store the spent fuel in a safe location close to the nuclear power plant for the time being. It is now widely believed that the present share of electricity generation through nuclear power will increase manifold in the near future, mainly because of environmental considerations, since nuclear power is the only alternative to fossil fuels now available in large commercial quantities that does not produce harmful greenhouse gases.

As a result, the demand for nuclear fuel will also increase and, consequently, there will be a market for spent nuclear fuel since it contains valuable unused uranium and fissile plutonium which can be recycled in nuclear reactors. Moreover, several new designs of nuclear reactors, which can be fueled by spent fuel without reprocessing, are now under development. This may further increase the demand for spent fuel in the international market.

Handling, storage and disposal of low and medium-level nuclear waste pose no serious problem. Bangladesh Atomic Energy Commission has the necessary expertise and experience to manage such waste and, in fact, has been doing so since the construction of its research reactor in 1986 at the Atomic Energy Research Establishment (AERE) at Savar. There is, therefore, no reason to worry about the management of nuclear wastes from the Rooppur nuclear power plant.
Dr. Abdul Matin is a former Chief Engineer of Bangladesh Atomic Energy Commission

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