The Savannah River Site has the majority of the United States' supply of
neptunium currently stored in an acid solution in one of their canyon
facilities. A program is being developed that could be utilized to ship this
material, as glass, to Oak Ridge National Laboratory where the Np could be
leached from the glass, purified by ion exchange and made into target
material for the production of Pu-238. Ion exchange purification dictates no
material be in the leachate making the isolation of the Np difficult. We
have developed a process using thorium as a surrogate for Np that could
immobilize the Np into a soda borosilicate glass for shipment. To achieve
recovery of the Np, the glass can be phase separated prior to leaching with
nitric acid. Phase separation would produces a Np-rich sodium-borate phase
and a Si-rich phase similar to a Vycor® glass. The nitric acid selectively
attacks the sodium-borate phase allowing high Np recovery in a solution that
contains only sodium and boron. These can be easily separated from Np by ion
exchange. Essentially all of the silicon which would interfere with ion
exchange by precipitation is retained in the Vycor®-type phase. This
technology may also be applied to other actinides stored in relatively pure
solutions.
This paper will report the optimization of variables for maximizing Th (a Np
surrogate) recovery while minimizing Si release. Th solubility in glass,
heat treatment conditions and leaching parameters will be discussed.
Transmission Electron Microscopy (TEM) with energy dispersive spectroscopy
(EDS) data will be included to show phase separation after heat
treatment.