Application of Denitration/Oxalate Precipitation in the Petra Hot-Cell Facility

The PETRA hot-cell facility has been developed to study HAW conditioning alternatives. The first strategy to be assessed includes the denitration of high active waste (HAW) and the contemporaneous oxalate precipitation of the rare earths and actinides. A “cold” pilot plant was constructed to establish design and operating conditions which could be successfully applied within the facility. All experiments were based on feeding simulated HAW into the denitrator containing formic acid. The problem of induction period may be overcome by feeding the HAW locally, rather than well distributed, into the reactor. However, the presence of strong nitrous acid scavengers must be eliminated as these could block the denitration reaction. In this context, iron plays an intermediate role, partially protecting the oxalic acid from nitrous acid, while allowing the formic/nitric acid destruction to proceed. Off-gas and pressure measurements reliably detect the commencement and continuation of the process while electrical conductivity indicates the prevailing nitric acid concentration. After HAW addition, a reflux period of several hours is necessary to attain sufficiently low nitric/formic acid concentrations. Following supernatant removal, the recovery and destruction of the oxalate precipitate entail its dissolution in dilute nitric acid and subsequent concentration.