[en] A novel process to treat used heavy water moderator (D2O) contaminated with high concentrations of the neutron poison, gadolinium nitrate,Gd(NO3)3, is described. Gadolinium is removed by precipitation. The resultant precipitate (GdPO4. 6H2O) represents an extremely rare compound of considerable potential value. The resultant supernatant consisting of residual nitrate (NaNO3 or KNO3) is less toxic and easier to process than the original waste. Thus, the alkali metal waste handling can be done with considerably less environmental concern. This waste can potentially be treated by a combination of electrochemical and biological methods