Konditionierung von Borsäure mit Natronwasserglas und AIPO$_{4}$-Härter

Nuclear power stations whose reactors operate on the principle of a pressurized water reactor have water containing boric acid circulating in their primary loop. The annual inspection has to deal with considerable quantities of boric acid containing fission products. According to current practice, this boric acid is evaporated to dryness by various drying processes and then disposed of in a repository. Since boric acid is readily soluible in water with 39.9 g/1 at 20°C and 290 g/l at 102°C and in the case of an ingress of water into the interim store or final repository the boric acid together with its fission product inventory is thus dissolved, the present report studies the conditionability and the product properties of boric acid concentrates fixed with sodium silicate and AIPO$_{4}$ hardener. Solutions of any boric acid concentration as well as powdery boric acid can be used to fix boric acid with sodium silicate and AIPO$_{4}$ hardener. Both hot and also cold solutions can beused to condition boric acid concentrate solutions. With both the hot and cold conditioned boric acid concentrate solutions it became apparent that during water storage the elements boron, strontium and cobalt were much more easily leached out of the water glass monoliths than during storage in quinary alkaline solution so that a salt dome can be recommended for the final storage of conditioned boric acid residues. The leaching rates from hot conditioned boric acid concentrate are in the region of 7.3 wt % for boron and 0.06 and 0.03 wt % for the main activity carriers of the boric acid solution, namely cobalt and strontium respectively, after 176 days of leaching in pinery alkaline solution. Studies of hot and cold conditioned boric acid concentrate solutions by X-ray diffractometry indicated that in the hot conditioned specimens only Na$_{2}$B$_{4}$O$_{7}$ $\cdot$ 10H$_{2}$O can be identified as the major component, as well as traces of the compounds Al$_{2}$Si$_{2}$O$_{5}$ (OH)$_{4}$ and NaAl$_{2}$(AlSi$_{3}$ O$_{10}$) $\cdot$(OH)$_{2}$. For cold conditioned boric acid concentrate solution, containing48 % less fixative, Na$_{2}$B$_{4}$O$_{7}$ $\cdot$10H$_{2}$O and Al(HP$_{2}$O$_{7}$) $\cdot$ 2,5H$_{2}$O can be detected as reaction products