Precipitation of crystalline neptunium dioxide from near-neutral aqueous solution

Abstract

We report experimental evidence that crystalline neptunium dioxide, NpO

Neptunium_Oxide_Precipitation_Kinetics_AJohnsen

“…Using XRD, SEM, TEM, and EELS, these neptunium phases were shown to be consistent with distinct crystalline NpO 2 and Np 2 O 5 phases within the larger, oxidized uranium matrix, showing similar neptunium precipitation behavior to that noted by Roberts et al [4].…”

“…The study presented here was undertaken to confirm the initial findings of Roberts et al [4] and to examine the [17]. A purified stock solution of NpO2 + chloride media was prepared by precipitation of NpO2OH(am) via NaOH addition, the precipitate rinsed three times with DI water, and the remaining solid dissolved in a minimum of 0.1 M HCl [18,19].…”

“…The first experiment used unfiltered DI Milli-Q water and was performed in triplicate in an effort to replicate and , and all three had long-term aqueous neptunium concentrations lower than those reported by Roberts et al [4].…”

“…Several studies have examined aqueous NpO 2 + (aq) systems under near neutral conditions to determine the solubility-limiting solids that will control neptunium behavior over long time scales; a detailed summary can be found in Roberts et al [4]. Briefly, two independent research groups [5][6][7][8][9] -3 M that raised estimated neptunium releases from a repository by several orders of magnitude compared to the solubilities determined from spent fuel leaching tests [10][11][12].…”

“…The authors surmised that the formation of the more thermodynamically stable solid NpO 2 (cr) might be kinetically hindered, and that the month-to year-long times scales of previous experiments had been inadequate to allow NpO 2 (cr) formation. Roberts et al [4,14] sought to test this kinetic limitation hypothesis by conducting NpO 2 + (aq) solubility experiments in dilute chloride media at elevated temperatures to accelerate the reaction kinetics. The initial neptunium concentration in the solutions was 10 −4 M neptunium, low enough to prevent the precipitation of various neptunium(V) solids at room temperature.…”