The nuclear fuel cycle

 
The nuclear fuel cycle describes the progress of nuclear fuel through a series of steps in the preparation of nuclear fuel at the beginning of the process and the safe management, containment and disposal of spent fuel at the end. Here is a breakdown of that process.

Uranium and secondary ores from Nabrlek, Northern Territory

Mining and milling:
 

Uranium and secondary ores from  Nabarlek, Northern Territory Mined uranium ore is sent to a mill where it is crushed and ground into a fine slurry. This is leached in sulphuric acid to allow the separation of the uranium from the waste rock. The uranium is then recovered from the solution and precipitated as uranium oxide concentrate.

Conversion:

Uranium needs to be in the form of a gas before it can be enriched. The uranium oxide concentrate is converted into the gas uranium hexafluoride.

Enrichment:

The enrichment process separates the uranium hexafluoride into two streams. One stream is enriched to the required level and passes to the next stage of the cycle. The other stream is depleted and used in metal form in such things as yacht keels, as counter weights, and as radiation shielding since it is 1.7 times denser than lead.

Fuel fabrication:

Enriched uranium is transported to a fuel fabrication plant where it is converted to uranium dioxide powder and pressed into small pellets. These pellets are inserted into thin tubes of either a zirconium alloy or stainless steel to form fuel rods. The rods are assembled in clusters to form fuel elements for use in the core of a nuclear reactor.

Nuclear reactor:

In the reactor core the uranium 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 and is fully controlled. To maintain efficient performance, about one third of the spent fuel is removed every year, to be replaced with fresh fuel.

Spent fuel storage:

Spent fuel is highly radioactive and gives off a lot of heat. It is stored in special ponds to allow this heat and radioactivity to decrease. The water in the ponds acts as a barrier against radiation and disperses the heat from the spent fuel. This is an interim step before the spent fuel is either reprocessed, to recover the usable portion, or sent to final disposal. The longer it is stored, the easier it is to handle due to decay of radioactivity.

Reprocessing:

Spent fuel still contains a small amount of usable uranium and plutonium. Reprocessing separates the uranium and plutonium from waste products by chopping up the fuel rods and dissolving them in acid to separate the various materials. Recovered uranium is returned to the conversion stage and re-enters the fuel cycle. The plutonium can be blended with enriched uranium to produce a mixed oxide fuel. The remaining wastes are stored in liquid form and subsequently solidified.
 

Vitrification:

After reprocessing the liquid 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 that can readily be transported and stored with appropriate shielding. Another method of long term storage for nuclear waste is Synroc, which stores the waste in the same way naturally occurring radioactive material has been stored in actual rocks, essentially mimicking the natural environment. This is as far as the nuclear fuel cycle goes at present. The final disposal stage has not yet taken place.

Final disposal:

The most widely accepted plans are for the stainless steel canisters to be buried in stable rock structures deep underground. Geological formations such as granite, volcanic tuff, salt or shale will be suitable.    

 The nuclear fuel cycle describes the progress of nuclear fuel through a series of steps in the preparation of nuclear fuel at the beginning of the process and the safe management, containment and disposal of spent fuel at the end. Here is a breakdown of that process.