Environmental behavior of hafnium : the impact on the disposition of weapons-grade plutonium

Abstract

Experimental and analytical studies were performed to examine the environmental behavior of hafnium and its utility as a neutron poison for the disposition of weapons-grade plutonium in Yucca Mountain. The hydrolysis of hafnium was investigated by potentiometric titration in solutions of varying ionic strength to determine the stability constants for the first four monomeric hydrolysis products. The specific ion interaction theory is used to extrapolate these results to infinite dilution. The solubility of hafnium hydroxide and a meta-stable hafnium carbonate solid phase are studied via solubility experiments using ICP-AES. An upper bound for the stability constant of the first carbonate complex is determined. The solubility of hafnium oxide is investigated via solubility experiments using neutron activation analysis, which is also used to investigate the complexation of hafnium by silicates.

(cont.) The potential for a near field criticality incident resulting from the disposition of weapons-grade plutonium at Yucca Mountain is examined using two integrated chemistry and transport models, which are then fed into an MCNP model of the near field at the Yucca Mountain repository. These models are used to predict the effective neutron multiplication factor for the system as the waste package degrades over time. Using the integrated degradation and criticality models, the long term criticality behavior of the proposed WGPu host phase ceramic is examined, as well as the utility of hafnium as a criticality control element for the disposition of weapons-grade plutonium.