Acid decontamination and recycling technology for decommissioning

Decommissioning related decontamination technology have been developed since 1992 to reduce radiation doses during dismantling of nuclear power plants. Early fundamental studies extracted three decontamination reagents with their operation mode and temperature, that is NP/dilute hydrochloric acid with reducing agent (circulation, 95°C), NP/dilute oxalic acid with reducing agent (circulation, 95°C), and concentrated hydrochloric acid with reducing agent (fill and drain, 60°C), where NP is nitric acid and potassium permanganate. These reagents were then tested against simulated reactor component samples with oxide layer on their surface to predict the removal property of the actual oxide layer which includes most of the radioactivity of the reactor components. The decontamination ability was finally confirmed by the actual sample tests, which revealed that decontamination factors (DF) of radioactivity were over 100 and enough for dismantling. The main decontamination reagent is hydrochloric acid (0.1-2.0 wt%) and its recycling system has been developed to reduce the liquid waste volume. The system adopted electrochemical separation of liquid waste components and consisted of one or two electrolysis cells with ion selective membranes. The second cell is needed for relatively concentrated hydrochloric acid (>1 wt%) to increase the recovery ratio. There are two kinds of membrane settled alternately in both cells, that is hydrogen ion selective membrane for the recovery of H + and anion exchange one for Cl - . Impurities in the liquid waste are vanadium (reductant), inhibitor (organic reagents) and corrosion products (CP) which includes Co-60. Two kinds of membranes do not pass these impurities and can recover almost pure HCl. Nitric acid can be also recovered by this method from NP liquid waste with impurities of potassium, manganese and CP. The experiments to confirm the performance of the recycling system were carried out by a small size equipment using simulated liquid waste (500 ml/batch), which showed the recovery ratios of HCl were 90% for concentrated HCl waste and 75% for dilute one. The DF of Co also evaluated in the tracer experiments was 100 by the recycling system. These results show that this system can be implemented for recycling acid from decontamination liquid waste.