Nuclear Waste Managment

It has been quite a while since I last posted. Kudos to my members for their well done job! I was pondering on what to talk about today, and when I saw rubbish around, (lights up) there it goes, we are going to talk about rubbish today! Of course, in nuclear world sense. :)

One of the major concerns of the public going against nuclear is regarding the waste or rubbish from nuclear industry. It is indeed a very worrying and worth worrying matter. Logically, people will start imagining having nuclear waste all around us, in the rivers, at the roadside etc. What a havoc it will be if thing is really going to be so. Nuclear waste are generally materials that are no longer useful or productive for the industry. However, this does not means that these materials are not harmful, some of them are still highly radioactive. They are classified as low-level, medium-level or high-level waste according to the amount and types of radioactivity in them.

Generally, there are three principles that are employed in the management of radioactive wastes:

• concentrate-and-contain
• dilute-and-disperse
• delay-and-decay

The first two are widely used in today's non-radioactive wastes management while the third one is specially for nuclear waste only. Concentrate-and-contain basically means the waste is concentrated and then isolated. Dilute-and-disperse meanwhile is the exact opposite of concentrate-and-contain, waste is diluted to a level acceptable by the surrounding or environment(non-hazardous) and ejected to the environment. Delay-and-decay method however is unique for radioactive waste management only, the waste is stored and radioactivity is allowed to decrease naturally by decay of the radioisotopes in the waste.

Radioactive wastes are produced in all stages of the nuclear fuel cycle-the process of producing electricity from nuclear materials. The fuel cycle is often divided into 2 parts- the "front end" which stretches frm mining through to the ue of uranium in the reactor and the "back end" which covers the removal of used fuel from the reactor and its subsequent treatment and disposal.

Below is a clearly and well explained extract from world-nuclear.org website:

Residual materials from the "front end" of the fuel cycle
The annual fuel requirement for a l000 MWe light water reactor is about 25 tonnes of enriched uranium oxide. This requires the mining and milling of perhaps 50,000 tonnes of ore to provide about 200 tonnes of uranium oxide concentrate (U3O8) from the mine.
At uranium mines, dust is controlled to minimise inhalation of radioactive minerals, while radon gas concentrations are kept to a minimum by good ventilation and dispersion in large volumes of air. At the mill, dust is collected and fed back into the process, while radon gas is diluted and dispersed to the atmosphere in large volumes of air.
At the mine, residual ground rock from the milling operation contain most of the radioactive materials from the ore, such as radium. This material is discharged into tailings dams which retain the remaining solids and prevent any seepage of the liquid. The tailings contain about 70% of the radioactivity in the original ore.
Eventually these tailings may be put back into the mine or they may be covered with rock and clay, then revegetated. In this case considerable care is taken to ensure their long-term stability and to avoid any environmental impact (which would be more from acid leaching or dust than from radioactivity as such).
The tailings are usually around ten times more radioactive than typical granites, such as used on city buildings. If someone were to live continuously on top of the Ranger tailings they would receive about double their normal radiation dose from the actual tailings (ie they would triple their received dose).
With in situ leach (ISL) mining, dissolved materials other than uranium are simply returned underground from where they came, as the water is recirculated.
Uranium oxide (U3O8) produced from the mining and milling of uranium ore is only mildly radioactive - most of the radioactivity in the original ore remains at the mine site in the tailings.
Turning uranium oxide concentrate into a useable fuel has no effect on levels of radioactivity and does not produce significant waste.
First, the uranium oxide is converted into a gas, uranium hexafluoride (UF6), as feedstock for the enrichment process.
Then, during enrichment, every tonne of uranium hexafluoride becomes separated into about 130 kg of enriched UF6 (about 3.5% U-235) and 870 kg of 'depleted' UF6 (mostly U-238). The enriched UF6 is finally converted into uranium dioxide (UO2) powder and pressed into fuel pellets which are encased in zirconium alloy tubes to form fuel rods.
Depleted uranium has few uses, though with a high density (specific gravity of 18.7) it has found uses in the keels of yachts, aircraft control surface counterweights, anti-tank ammunition and radiation shielding. It is also a potential energy source for particular (fast neutron) reactors.
Wastes from the "back end" of the fuel cycle
It is when uranium is used in the reactor that significant quantities of highly radioactive wastes are created. When the uranium-235 atom is split it forms fission products, which are very radioactive and make up the main portion of nuclear wastes retained in the fuel rods. There is also a relatively small amount of radioactivity induced in the reactor components by neutron irradiation.
About 25 tonnes of used fuel is taken each year from the core of a l000 MWe nuclear reactor. This fuel can be regarded entirely as waste (as, for 40% of the world's output, in USA and Canada), or it can be reprocessed (as in Europe and Japan). Whichever option is chosen, the used fuel is first stored for several years under water in cooling ponds at the reactor site. The concrete ponds and the water covering the fuel assemblies provide radiation protection, while removing the heat generated during radioactive decay.


A few methods to deal with the wastes are as below:



Reprocessing

The used fuel is reprocessed, dissolved and separated chemcially into uranium, plotonium and high-level waste solutions. About 97% of the used fuel can be recycled, leaving only 3% as high-level waste. The recycled portion contains uranium depleted to less tha 1% U-235 and some precious plutonium.

Immobilising high-level waste

The liquid high-level wastes are evaporated to solids, mixed with galss-forming materials, melted and poured into robust stainless steel canisters which are then sealed by welding.

Borosilicate glass from the first aste vitrification plant in UK in the 1960s. This block contains material chemically identical to high-level waste from reprocessing. A piece this size would contain the total high-level waste arising from nuclear electricity generation for one person throughout a normal lifetime.

Waste Disposal

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 caniters of vitrified waste, or used fuel assemblies, are stored under water in special ponds, or in dry concrete structure or casks for at least this length of time.

The ultimate disposal of vitrified wastes, or of used fuel assemblies without reprocessing, requires their isolation from the environment for long periods. The most favoured method is burial in dry, stable geological formations some 500 metres deep. Several countries are investigating sites that would be technically and publicly acceptable. The USA is pushing ahead with a repository site in Nevada for all the nation's used fuel.
One purpose-built deep geological repository for long-lived nuclear waste (though only from defence applications) is already operating in New Mexico.
After being buried for about 1000 years most of the radioactivity will have decayed. The amount of radioactivity then remaining would be similar to that of the naturally-occurring uranium ore from which it originated, though it would be more concentrated.

Layers of protection

To ensure that no significant environmental releases occur over a long perio after disposal, a 'multiple barrier' disposal concept is used to immobilise the radioactive elements in high-level (and some intermediate-level) wastes and to isolate them from the biosphere. The principal barriers are:

• Immobilise waste in an insoluble matrix, eg borosilicate glass, Synroc(or leave them as uranium oxide fuel pellets - a ceramic).
• seal inside a corrosion-resistant container, eg stainless steel.
• surround containers with bentonite clay to inhibit any groundwater movement if the repository is likely to be wet.
• Locate deep underground in a stable rock structure.

For any of the radioactivity to reach human populations or the environment, all of these barriers would need to be breached before the radioactivity decayed.

Hence, finally, I would like to say one thought of mine, nothing comes perfectly. For even we humans are not perfect how can anything we build or design be perfect. Do not let this nuclear waste problem be a hinder for us to march into the nuclear era since it is so well taken care of. Nuclear is safe and well organized! :)