LAUSR

Barium nitrate is one of the least soluble metal nitrate phases normally present in the nitric acid based highly active liquor, which will require removal from highly active storage tanks during post-operational clean-out of the Sellafield nuclear site. Laboratory experiments have been carried out to determine the dissolution rate of barium nitrate in water and weak acid solutions. The influence of agitation (solutions being stirred by a paddle at a rate of 0, 40 and 60 revolutions per minute (rpm)), dissolution temperature (20–40 °C), nitric acid concentration (0–1 mol·L−1) and additional solution nitrate {from Al(NO3)3} concentration (0–0.5 mol·L−1) were investigated. An inscribed central composite experimental design was used for each level of stirred agitation (including unstirred), which allowed non-linear trends and interactions among variables to be statistically determined without carrying out a full factorial experimental array. It was found that the dissolution rate was faster for higher temperature, lower nitric acid concentrations and lower nitrate concentrations. Comparing experiments under like conditions, the barium nitrate dissolution rates at an agitation rate of 40 rpm were on average 56% of those at 60 rpm, while the dissolution rate in the unstirred experiments were on average 0.8% of those at 60 rpm, i.e. two orders of magnitude lower. Hence, the transition from stagnant solution conditions with diffusion controlled transfer of solute from the solid interface to agitated solution conditions was found to be significant. Statistical analysis of the data allowed the derivation of a predictive dissolution rate equation.