FEASIBILITY AND EXPEDIENCE TO VITRIFY NPP OPERATIONAL WASTE

Operational radioactive waste is generated during routine operation of NPP. Process waste is mainly generated by treatment of water from reactor or ancillaries including spent fuel storage pools and some decontamination operations. Typical process wastes of pressurized water reactors (PWR or WWER) are borated water concentrates, whereas typical process wastes of boiling and RBMK type reactors are water concentrates with no boron content. NPP operational wastes are classified as low and intermediate level waste (LILW). NPP operational waste must be solidified in order to ensure safe conditions of storage and disposal. Currently the most promising solidification method for this waste is the vitrification technology. Vitrification of NPP operational waste is a relative new option being developed for last years. Nevertheless there is already accumulated operational experience on vitrifying low and intermediate level waste in Russian Federation at Moscow SIA ''Radon'' vitrification plant. This plant uses the most advanced type induction high frequency melters that facilitate the melting process and significantly reduce the generation of secondary waste and henceforth the overall cost. The plant was put into operation by the end of 1999. It has three operating cold crucible melters with the overall capacity up to 75 kg/h. The vitrification technology comprises a few stages, starting with evaporation of excess water from liquid radioactive waste, followed by batch preparation, glass melting, and ending with vitrified waste blocks and some relative small amounts of secondary waste. First of all since the original waste contain as main component water, this water is removed from waste through evaporation. Then the remaining salt concentrate is mixed with necessary technological additives, thus a glass-forming batch is formed. The batch is fed into melters where the glass melting occurs. From here there are two streams: one is the glass melt containing the most part of radioactivity and second is the off gas flow, which contains off gaseous and aerosol airborne. The melt glass is fed into containers, which are slowly cooled in an annealing tunnel furnace to avoid accumulation of mechanical stresses in the glass. Containers with glass are the final processing product containing the overwhelming part of waste contaminants. The second stream from melter is directed to gas purification system, which is a rather complex system taking into account the necessity to remove from off gas not only radionuclides but also the chemical contaminants. Operation of this purification system leads to generation of a small amount of secondary waste. This waste stream slightly contaminated with volatilized radionuclides is recycled in the same technological scheme. As a result only non-radioactive materials are produced. They are either discharged into environment or reused. Based on the experience gained during operation of vitrification plant one can conclude on high efficiency achieved through vitrification method. Another significant argument on vitrifying NPP operational waste is the minimal impact of vitrified radioactive waste onto environment. Solidified waste shall be disposed of into a near surface disposal facility. Waste forms disposed of in a near-surface wet repository eventually come into contact with groundwater. Engineered structures used or designed to prevent or postpone such contact and the subsequent radionuclide release are complex and often too expensive. Vitrification technologies provide waste forms with excellent resistance to corrosion and gave the basic possibility of maximal simplification of engineered barrier systems. The most simple disposal option is to locate the vitrified waste form packages directly into earthen trenches provided the host rock has the necessary sorption and confinement properties. Such an approach will significantly make simpler the disposal facilities thus contributing both to enhancing safety and economic al efficiency.