Low temperature plasma incineration of radioactive ion-exchange resin

The results of the incineration studies of radioactive ion-exchange resin in an oxygen plasma atmosphere at pressures 20...50 mbar are presented. The plasma is generated using a 27.12 MHz radio frequency (RF) generator. The chamber temperature during the incineration varies between 100...140 deg C depending on the oxygen pressure in the chamber. The virtue of the low-temperature incineration is that the radioactive substances remain in the solid ash. The average retention of 60Co and 137Cs in the incineration chamber was about 94 %. The retention of 99mTc was in the average 77 % and of 131I about 70 %. The results indicate that, within the limit of the measurement accuracy, practically all Co and Cs remains in the chamber and can be recovered and further conditioned with the ash. The rest of technetium and iodine can be easily recovered on any cooled surface right after the incineration chamber. The mass reduction achievable with this method is 95 %. The ion-exchange resins in use at the Loviisa NPP were used as test samples. The activity retention and ashing experiments were made with a 1:1 mixture of granular anion and cation resin. The same kind of mixture is used for the primary water purification at the Loviisa NPP. The test resin was labelled with 60Co, 137Cs, 99mTc and 131I. Also spent Loviisa NPP resin was incinerated. The VVER-440 reactors of the Loviisa NPP use boric acid to control the reactivity of the core. The boric acid in the resin presents an extra cumber in the incineration process tending to prevent the oxygen plasma from getting into contact with the resin. The results with the granulated spent Loviisa NPP resin were drastically inferior to the results with the labelled test resin. The average unburnt residual mass was about 20 %. By prior crushing of the resin equal incineration results could be achieved with the test resin and with the spent Loviisa NPP resin. The low-temperature plasma incineration of radioactive ion-exchange resins presents good prospects to reducing the total mass and volume of the medium-level radioactive resin waste. By drastically reducing the resin volume to be disposed of finally, it gives good prospects for a considerable reduction of the final disposal costs of the spent ion-exchange resin of the NPPs. (author)