Treatment and conditioning of radwaste in nuclear power plants

Processing and treatment of liquid radwaste

   
The main objective of the management of liquid radwaste is to reduce the content of radionuclides in such a way that a significant part of the waste can be safely released into the environment, a certain volume of it can be reused, and only a small part can be conditioned into a form suitable for storage. Liquid radwaste is usually processed on the site of the nuclear power plant in special facilities. During the first phase, contaminated water is processed, and conditioned concentrates from the first phase are then treated in the second phase.

PHASE 1 OF LIQUID RADWASTE MANAGEMENT - PROCESSING OF CONTAMINATED WATER
For the processing of contaminated water, essentially four main methods are used: evaporation, separation of solid particles, ion exchange, and chemical treatment.    

  
Evaporation
The most universal method for processing of low- and medium-level active liquid radwaste is evaporation. Out of all methods for processing of liquid radwaste, this method achieves the highest reduction of volume and the most effective decontamination. Except a small amount of radionuclides bound in organic compounds and of volatile radionuclides, all other radionuclides can be separated. Following water evaporation, solid residues remain in the form of concentrated solutions that contain most of the radionuclides. During the evaporation, high decontamination effectiveness (app. 3 to 6 orders of magnitude) is achieved and frequently even more that the reduction by the factor of a hundred. This means that only a thousandth up to a millionth of the original amount of radionuclides is in the steam condensed.    

   
The advantage of this method is a significant independence on the composition of liquid radwaste and its decontamination is mostly possible even without other technology modifications. Waste steam can be used way in nuclear power plants for heating the evaporator in an economic.
   

  
Prior to decontamination of liquid radwaste, it is possible to use all types of evaporators. A certain problem with the evaporation is the foaming of evaporation liquid. Anti-foaming agents are added, or various structural modifications of evaporators are performed, to remove the foaming.
The evaporation of liquid radwaste should be completed at such a concentration when there is enough liquid and the radwaste can be transported from the evaporator. The thickening with the content of salts, mainly with regard to the subsequent processing of the concentrates generated, is recommended up to about 300-400 g/dm3.
Clean condensate is after-cleaned at ionexes and removed into collection tanks where it can be used as reverse water at the plant, or it is possible to release it into the environment.
    

Separation of solid particles
Separation and removal of solid particles from liquid radwaste is carried out by means of various types of filters and centrifuges. Sometimes a biological purification method is used as an additional means, which utilizes the abilities of certain microorganisms to accumulate radionuclides in them.

Ion exchange
Ion exchange filters are used for the purification of water from the reactor primary and secondary syste, of water from spent fuel storage pools and of condensate from evaporators. Ion exchangers are insoluble multi-molecular substances with a ionization functional group. These substances can be both organic and inorganic ones. Upstream the ion exchanger, liquid radwaste is cleaned at mechanical filter and subsequently it passes through ion filters where certain types of radionuclides are captured. Filters can be regenerated following their saturation, or are processed as solid radwaste. Slovak nuclear power plants with VVER-440 reactors have the annual consumption of beds of catex and anex filters in the amount of app. 60 m3.

Chemical methods
Chemical methods for processing of liquid radwaste are based on the experience from the chemical water treatment in conventional power plants and heating plants. The principle is that coagulants (aluminum and ferrous salts) are added to liquid radwaste, by means of which radionuclides coagulate and deposit on the tank bottom in the form of insoluble salts (precipitation). The other possibility is to add chemicals that create a coagulant in the form of flakes (flocculation) which bind radionuclides.
On the contrary to the evaporation methods, the decontamination factor is low and the degree of separation of radionuclides for chemical methods is not sufficient. This requires to combine this method with other more effective procedures. The advantage of chemical methods is the low costs for implementation, and they are thus used when only a small reduction of radionuclide content is required and when the volume of waste water is small.    

PHASE 2 OF LIQUID RADWASTE MANAGEMENT - CONDITIONING OF CONCENTRATES
Concentrates from evaporation of contaminated water need to be conditioned into a chemically and physically stable form reducing the possibility that radionuclides will be released during waste transport, storage, or disposal. For the conditioning of radwaste, solidification techniques such as cementation, bitumination and polymerization are used. For the conditioning of certain, mainly high-level active liquid radwaste, the method of vitrification is used..   

  
Cementation is likely the most widespread method that has been used for a number of years. The method is based on binding the water and salts dissolved in it by cement in liquid radwaste. The liquid radwaste is used as a mixing liquid for the production of cement modules. The advantages of this method are inflammability of the resulting product, low energy demands of the process and a simple technology. The disadvantage is a considerable increase of the resulting product (the reduction factor is only app. 0.25) and a higher leachability in comparison with the bitumination (by a factor of 10 to 100).

   
The method of bitumination consists in a simultaneous evaporation of liquid and mixing of the dry residue with molten asphalt. The advantages of this method are a higher volumetric reduction of the resulting product in comparison with cementation (the reduction factor is app. 0.5) and its low leachability in water. The disadvantage is flammability of the asphalt and bitumen product, higher investment costs on bitumination facility and higher demands on the serviceability of the facility.
    
Polymerization is a relatively recent method based on the incorporation of waste into artificial masses such as polyester or epoxy bitumen and irreversible hard materials are thus produced. The advantage of this method is a low leachability of radionuclides and chemical stability of the resulting product. The disadvantages are high costs and a relatively complex technology.   

  
Vitrification consists in the production of a special glass (silicate or boron silicate glass) or of a glass-ceramic from liquid radioactive waste and glass materials at the temperature of 900 up to 1200 °C. The advantage of the method is a very low leachability, high volumetric reduction and mechanical strength of the resulting product. The disadvantage is the need to use a relatively complex technology in comparison with the bitumination or cementation. It is used mainly for medium and high-level active liquid radwaste.