Patent Number: 039882586
Section: summary

This invention relates to disposal of radwaste associated with nuclear power production processes. Conventional nuclear power producing facilities employ coolants which acquire radioactively-contaminated dissolved and particulate impurities through neutron interactions or from defective fuel elements. Other sources of contaminated liquids or solids are also present in such facilities. Such contaminants are removed from liquids by filters and ion-exchange beds. The radioactively-contaminated solids and liquids generated from such treatments are termed "radwaste," and most commonly comprise spent demineralizer resins, waste evaporator concentrates, disposable filter cartridges, filter sludges, and neutralized chemical wastes. Safety considerations require a process for handling radwaste for temporary storage, and for shipment to licensed burial grounds for long term storage. Among the processes currently in use are incorporation of the radwaste into a free-standing, solid matrix using a binding agent, such as cement or urea formaldehyde. Cement is the preferred binding agent over organic materials because cement is non-combustable and resistant to chemical and biological attack, offering important safety advantages over the resin binding agent, and because cement is inexpensive and available in insured supply. However, a number of problems have been encountered in radwaste disposal by incorporation in a matrix using a cement-type binding agent. One problem is the narrow range of liquid tolerence permitted to ensure hardening of the cement, requiring large volumes of cement relative to the volume of waste treated, which leads to poor shipping efficiency (defined as the number of pounds of liquid waste per cubic foot of solid shipped) and high cost. Another problem is that long mixing times of the waste-cement mixture are necessary to ensure uniform distribution of aggregate waste-solids until the cement sets, which also increases cost. Still a further problem is that boric acid wastes common to pressurized water reactors (the primary coolant in the typical PWR contains boric acid as a chemical shim for reactivity control) cannot be directly solidified using cement, but must first be absorbed in drying materials such as vermiculite, which also increases cost and reduces shipping efficiency. Another problem encountered from time to time is contraction during hardening of the waste-cement mixture causing undesirable fissures in the solid and release of contaminated liquid. The principal object of the present invention is a process for radwaste disposal by incorporation in a matrix employing a cement-type binding agent in which the problems enumerated above are overcome or mitigated. It has been found, in accordance with the process of the present invention, that the addition of alkali or alkaline earth silicate to the radwaste-cementing material mixture produces a number of unexpected benefits and important advantages over the known radwaste disposal process. These include: 1. direct solidification of all common nuclear power industry radioactive wastes, including boric acid solutions; 2. rapid hardening to a gel in less than 2 minutes, eliminating requirements for continuous mixing to insure homogeniety; 3. solidification of maximum hardness in less than 7 days, compared to 28 days for cement alone without the alkali silicate additive; 4. increased water retention over non-silicated processes due to the high capacity of silicates for water fixation by hydration; 5. production of more fluid mixes causing ready adaptability to batch or continuous processing of radioactive wastes, and 7. minimum operator training and control required to obtain solidified wastes in a form suitable for safe handling and shipment.