Future of radwaste management

Fuel cycles

   
Fuel cycle means a sequence of technology processes carried out with the aim to generate energy in nuclear reactors. There are two completely different cycles - open fuel cycle and closed fuel cycle.      

  
OPEN FUEL CYCLE
The open fuel cycle is as follows: mining of uranium ore - milling of uranium ore - conversion of uranium ore into nuclear material (dioxide or hexafluoride of uranium) - enrichment - production of fuel - use of fuel - storage of spent nuclear fuel - permanent disposal of spent nuclear fuel. Such cycle is used not only in all countries with VVER reactors (including the Slovak Republic), but also in other countries with other reactor types. Following the use of fuel in nuclear reactor, its reprocessing is not considered and it is expected that the fuel will be disposed permanently following the phase of interim storage. It means that some valuable fission isotopes contained in the spent fuel will not be further used. On the other side, permanent storage of spent fuel from nuclear power plants has not been implemented up to now and still there is a possibility to reprocess it and use it later.
The main reasons that can be given in favor of the use of the open fuel cycle are high costs associated with the technology for spent nuclear fuel reprocessing, lower current rate of nuclear power plant construction and relatively low prices of natural uranium due to the reduction of its consumption and development of new resources.

CLOSED FUEL CYCLE
The closed fuel cycle can be described in the following way: mining of uranium ore - milling of uranium ore - conversion of uranium ore into nuclear material - enrichment - production of fuel - use of fuel - interim storage of spent nuclear fuel - reprocessing of fuel - conversion, or directly production of new fuel - recurrent use of fuel. The main difference in comparison with the open fuel cycle is that the usable part of the spent nuclear fuel becomes again the material for the production of new nuclear fuel and the fuel cycle is thus closed. The closed fuel cycle is generally considered very promising and methods for fuel reprocessing are further improved.
To be complete it can be said that not only spent nuclear fuel can be reprocessed or conditioned. Based on international treaties on the reduction of mainly Russian and US nuclear weapons, also the processing of military nuclear materials from these arms is carried out (program "Swords to Ploughs").
     

The result of this process is new nuclear fuel for nuclear power plants. With a little exaggeration it can be said that nuclear power thus uses for human benefit the material that was originally intended for killing the entire life.

Advantages from the reprocessing of spent nuclear fuel
Spent nuclear fuel from pressurized water reactors is a valuable nuclear material as it still has a higher content of 235U than natural uranium, and moreover new fission isotopes 239Pu and 241Pu. That is why some countries reprocess spent nuclear fuel. It results in new nuclear fuel and high-level active residues. Spent nuclear material thus returns back into fuel cycle and is effectively used for power production. One ton of reprocessed nuclear fuel represents a saving of two tons of natural uranium.

Methods of reprocessing and reprocessing plants
Reprocessing of spent nuclear fuel represents a complex chemical process designed for the separation of uranium from plutonium and for the removal of fission products. Prior to its implementation, it is necessary to store spent nuclear fuel for a few months or years in pools during which time a significant reduction of activity of fission products with short half times of decay occurs.      

  
Currently a few tens of technologic methods for reprocessing of spent nuclear fuel are being developed. The PUREX (Plutonium Uranium Extraction) is the most widespread method in practice. High radioactivity of fuel requires to perform the entire process of reprocessing by means of remotely controlled manipulators and robots. From fuel assemblies, protection cladding made out of zirconium is removed first. Spent fuel is then dissolved in nitrogen acid and the particular components are then separated from the solution. The resulting effect of the reprocessing is uranium in the form of oxide or hexafluoride, and plutonium, usually as oxide. Plutonium and uranium are then used as fuel for various reactor types. The residues from the metal cladding of fuel assemblies are processed as medium-level waste. Fission products are concentrated by evaporation, they are temporarily stored in liquid form and fixed prior to the ultimate disposal, usually by vitrification. This method can be used when processing fuel with the burn-up achieved in light water reactors, if fuel cooling following its removal from the reactor core lasts at least 100 days. From one ton of spent nuclear fuel, only 115 liters of high-level radwaste in vitrified form is thus produced.

    
Only economically strong countries can afford reprocessing. Commercial reprocessing plants are for example in La Hague and Marcoule in France, in Sellafield in UK, in Tokai-Mura in Japan, and also the reprocessing complex Majak on Ural is known. The capacity of these plants makes it possible to reprocess only a smaller part of spent nuclear fuel from operating nuclear power plants, however, other reprocessing plants are under construction. The offer of reprocessing, however, exceeds now the demand. The reprocessing is carried out based on commercial contracts while customers can be from abroad. However, in all cases the customer gets back not only the new fuel material, but also high-level active residues which he had to take care of.