Treatment and conditioning of radwaste in nuclear power plants

Treatment and conditioning of solid radwaste

   
During nuclear power plant operation, various sorts of solid, liquid and gaseous radwaste are generated. The characteristic, amount and estimated activity of this radwaste are shown in section 2 "Sorts of radwaste from nuclear power plants". In order to reduce costs of storage and disposal of radwaste, and also to provide protection against radwaste from the point of view of radiation safety, it is necessary to treat and condition the waste in question.      

  
Treatment of radioactive waste represents activities resulting in a change of its physical and chemical properties and in a creation of a form suitable for its safe handling during storage and disposal. Sorting according to the sort and possible form of processing, reduction of volume and creation of a form preventing non-required physical, chemical and other processes precede such treatment.
    

   
SPENT NUCLEAR FUEL
Spent nuclear fuel is managed in our nuclear power plants in such a way that following its discharge from reactor a short-time storage in spent fuel pools (three years as a minimum) follows, then a medium-time storage in the Interim Spent Fuel Storage Facility on the plant site (up to 50 years), and an ultimate disposal in a deep underground repository (see section 4 "Storage of radwaste").     

  
No other treatment of spent nuclear fuel is carried out at the plant. Reprocessing of spent fuel, as dealt with in section 5 "Future of radwaste", is carried out in some countries.
    

   
SOLID RADWASTE, EXCEPT SPENT NUCLEAR FUEL
Fragmentation and compaction
The main objective of fragmentation and compaction of solid radwaste is to reduce its volume. A part of solid radwaste generated during plant operation can be compacted up to various levels. Among easily compressible materials belong filtration paper, foam polystyrene, fabrics, cardboard, cotton, and so on. Among worse compressible materials belong equipment from plastic, laboratory glass, objects from light plates, and so on.
   

  
The technology for easily compresible waste most frequently used is low-pressure compaction (using pressure in the range of 0.3 to 5 MPa) at which the volume can be reduced by the factor of five to fifteen. High-pressure compaction (using pressure in the range of 5 to 35 MPa) is used for hardly compressible materials and the reduction of volume by the factor of three to eight is reached. The method of compaction belongs to less demanding on equipment and energy and its use has the least negative impact on the environment from all the methods known. By compaction, volume of solid radwaste is reduced, but its physical and chemical properties are not changed.     

   
Incineration of solid radwaste
Incineration represents a very effective method for the reduction of both volume and mass of solid radwaste and of certain liquid ones. Approximately 50 up to 80% of solid radwaste is combustible, which emphasizes the importance of this way of solid radwaste treatment. By incineration, a homogenous ultimate product in the form of ash is produced in certain degree, besides a significant reduction of volume. Based on experience, reduction of the original volume by means of radwaste incineration, the factor of 50 to 100 is achieved. Approximately 97% of radioactivity from burnt radwaste is transferred into ash and the rest goes on filters of the purification system of flue gas. This residue is then further processed by cementation into barrels that are inserted into containers and then filled by cement paste. By incineration also oils, lubricants, or dissolvents are processed the treatment of which would be very difficult by other means.   

  
The operation of incineration furnace is controlled depending on the changing quality of material burning. The objective is to achieve a perfect incineration with a minimum amount of releases in flue gases. The flue gases are removed from the furnace via a purification system with various arrangements where the gases are cleaned from active components and then discharged into the stack of the incineration facility.
    

   
Processing of non-combustible and non-compressible solid radwaste
Some solid radwaste is very hardly combustible and essentially incompressible. They are parts of contaminated metal materials and relatively large amounts of plastics based on PVC. Incineration of these materials is difficult as they melt in the incineration process and cause technologic problems. Flue gases themselves contain a number of substances the release of which into atmosphere is undesirable (chlorine, sulfur, and so on). By appropriate combination of heating up the radwaste and subsequent compression, its treatment into a suitable form can be carried out. Some waste, mainly organic ones, can be disposed by microbiologic disintegration. An operating unit for such a treatment has been built close to the Loviisa nuclear power plant in Finland.
    
Processing of metal radwaste
For processing of metal radwaste, remelting is considered resulting in a categorization of radionuclides based on their chemical properties. Volatile substances are captured in slag and in the purification system of flue gases of the remelting facility. Radionuclides from metals are distributed in the metal melt. Using this method, it is possible to condition the volume of metals with a maximum reduction.