Patent Number: 040081710
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT In accordance with conventional practices conditioned water supplied to various nuclear reactor systems flows over ion-exchange materials which remove minerals, metallic ions and other foreign substances. Referring to the block diagram of FIG. 1, after the exchange materials become contaminated, the nuclear power plant 10 discharges the spent radioactive materials to a storage tank 12. The ion exchange material may comprise resins for example, such as nuclear grade mixed bed and cation resins of the type manufactured by Rohm and Haas, Diamond Shamrock Co., or other manufacturers, or other materials which effectively remove these types of contaminants from water. As used herein, the term resin is intended to encompass those materials which act to condition water to the purity needed for nuclear reactor purposes. When the resin in the reactor system becomes ineffective to the point where it is considered spent, removal from the system is necessary and the resin is thus disposed of as radioactive waste. In the past, high labor and material costs were incurred for disposing of the wastes and resin together with minimizing liquid discharges from the plant, and to overcome these disadvantages, the process described herein recycles the water back to the reactor systems and reduces the volume of resin by about 50%. This is accomplished by discharging the water and resin in the form of a slurry comprising about 50% by volume free water to a fluidized bed chamber 14. The slurry desirably is needed to facilitate pumping of the resin from tank 12 to the bed chamber. The bed chamber 14 containing the slurry is equipped with electrical or other heaters 16 mounted either inside chamber 14 or on its outer surface used for heating the chamber to a specified temperature between 40.degree.-150.degree. C, preferably between 70.degree.-80.degree. C. The chamber further is evacuated to a pressure between 15-29" mercury although the preferred pressure is about 25" mercury. With the chamber at its operating condition the free water around the resin beads is removed by vacuum filtration equipment 18 through the settled resin bed and discharged for recycling to the reactor hold-up tanks 20. At this point in the process, the free water has been evacuated from chamber 14 thus leaving wet resin which typically contains at least 50% internal water content. The second step in the process comprises maintaining the vacuum in chamber 14 and then passing superheated steam from a source 22 at a temperature of 200.degree.-500.degree. F or higher through the wet solids to fluidize the bed. Heat therefore is transferred to the wet resin by both the superheated steam and the heaters on bed chamber 14. The fluidized bed is subjected to a vacuum and as the resin loses moisture, heat is continuously added to the fluid bed to maintain the bed temperature in a range consistent with the thermal stability of the resin to be dried. The exposure of resin to these drying conditions results in loss of intrinsic water and such loss is determined by established drying curves for the particular resin being dried. A typical drying curve for a mixed bed resin is shown in FIG. 2. The first part of curve A is at constant temperature indicating an initial constant drying rate for the resins. As the drying rate falls off curve B, the temperature in the bed increases. This operation is continued until the desired bed temperature is achieved. Typical residence times in the fluid bed were found to be 30-60 minutes depending on the final desired resin moisture content and the rate at which heat is added to the system. When a set bed temperature is obtained the fluidizing-drying in the bed chamber is halted and the vacuum system isolated. At the conclusion of this step, the fluid bed chamber is isolated and the resin discharged directly into an evacuated drum 24 for disposal or discharged through a two fluid spray nozzle 26 directly into the drum. To effect resin transfer through the nozzle superheated steam from source 22 transports the resin from the fluid bed chamber 16 to one inlet of the nozzle while superheated steam from source 22 is supplied through line 28 to the other inlet to the nozzle. The transporting steam as well as the superheat content of the nozzle injected steam is imparted to the resin to further remove intrinsic moisture therefrom. After mixing and drying, the resin and steam are finally sprayed into the evacuated drum 24 and the resin at this time has reached its final stage of drying. Steam from drum 24 is discharged to condenser 30 then to hold-up tanks which are connected to the reactor liquid systems. In the over-all drying sequence, most of the heat input is supplied by the hot walls of the feed chamber during fluidization, a smaller fraction by the superheated steam used to fluidize the resin beads and the remainder by the superheated steam during final spray drying. However these heat inputs can be adjusted to reduce time sequences or character of the final product. With the above resins used in this process, approximately 40-50% removal of water from the resins is most advantageous. Further removal of water from the resins is a comparatively slow operation requiring bed temperatures which cause some degradation of the material and long fluid bed residence times. The water removed from the wet resins is condensed along with the fluidizing and transporting steam and sent back to the reactor liquid waste processing system. The overall volume reductions obtained in the process are normally 45-50% of the settled bed volume with maximum reductions thus far of 56% attainable. To reduce radioactive carry over of volatile species such as iodine the fluid bed chamber residence time and temperature is kept to a minimum to prevent anion resins from decomposing and thus releasing volatile species. The above described process exhibits serveral important advantages since the process affords substantial reduction in resin disposal and operating costs while simultaneously minimizing release of radioactivity to the environment or atmosphere. The process does not generate any other contaminated waste streams, other than the water from the steam and resin which can be sent directly to the reactor liquid waste treatment system, because the process described herein and such treatment systems are all contained within closed loops. The water from the dewatering steps is of a quality that is acceptable for direct recycle to the reactor make-up tanks through a polishing demineralizer. Moreover, the components may be sized smaller than those used at commercial reactor plants and mounted on vehicles of different types and sizes. These vehicles with the installed components may then be used to service nuclear reactor plants which may desire such a disposal service. It will be apparent that many modifications and variations may be made in light of the above teachings. It therefore is to be understood that within the scope of the appended claims, the invention may be practiced other than is specifically described.