TY - JOUR
T1 - Thermal Processing of Chloride-Contaminated Plutonium Dioxide
AU - Webb, Kevin
AU - Taylor, Robin
AU - Campbell, Catherine
AU - Carrott, Michael
AU - Gregson, Colin
AU - Hobbs, Jeff
AU - Livens, Francis
AU - Maher, Chris
AU - Orr, Robin
AU - Sims, Howard
AU - Steele, Helen
AU - Sutherland-Harper, Sophie
N1 - Funding Information:
This work was funded by Sellafield Ltd. with additional funding from the Nuclear Decommissioning Authority (NDA) for SSH for user access to Central Laboratory and also NNL’s Advanced Recycle and Isotope Separations (ARIS) core science theme. Sellafield Limited Analytical Services is thanked for QAAM analyses given in Table 1. SSH was supported by the Next Generation Nuclear (NGN) Centre for Doctoral Training at the University of Manchester, funded by the Engineering and Physical Sciences Research Council.
Publisher Copyright:
© 2019 American Chemical Society.
Copyright:
Copyright 2020 Elsevier B.V., All rights reserved.
PY - 2019/7/31
Y1 - 2019/7/31
N2 - Over 80 heat treatment experiments have been made on samples of chloride-contaminated plutonium dioxide retrieved from two packages in storage at Sellafield. These packages dated from 1974 and 1980 and were produced in a batch process by conversion of plutonium oxalate in a furnace at around 550 °C. The storage package contained a poly(vinyl chloride) (PVC) bag between the screw top inner and outer metal cans. Degradation of the PVC has led to adsorption of hydrogen chloride together with other atmospheric gases onto the PuO2 surface. Analysis by caustic leaching and ion chromatography gave chloride contents of 2000 to >5000 ppm Cl (i.e., μgCl g-1 of the original sample). Although there are some subtle differences, in general, there is surprisingly good agreement in results from heat treatment experiments for all the samples from both cans. Mass loss on heating (LOH) plateaus at nearly 3 wt % above 700 °C, although samples that were long stored under an air atmosphere or preexposed to 95% relative humidity atmospheres, gave higher LOH up to â4 wt %. The majority of the mass loss is due to adsorbed water and other atmospheric gases rather than chloride. Heating volatilizes chloride only above â400 °C implying that simple physisorption of HCl is not the main cause of contamination. Interestingly, above 700 °C, >100% of the initial leachable chloride can be volatilized. Surface (leachable) chloride decreases quickly with heat treatment temperatures up to â600 °C but only slowly above this temperature. Storage in air atmosphere post-heat treatment apparently leads to a reequilibration as leachable chloride increases. The presence of a "nonleachable" form of chloride was thus inferred and subsequently confirmed in PuO2 samples (pre- A nd post-heat treatment) that were fully dissolved and analyzed for the total chloride inventory. Reheating samples in either air or argon at temperatures up to the first heat treatment temperature did not volatilize significant amounts of additional chloride. With regard to a thermal stabilization process, heat treatment in flowing air at 800 °C with cooling and packaging under dry argon appears optimal, particularly, if thinner powder beds can be maintained. From electron microscopy, heat treatment appeared to have the most effect on degrading the square platelet particles compared to those with the trapezoidal morphology.
AB - Over 80 heat treatment experiments have been made on samples of chloride-contaminated plutonium dioxide retrieved from two packages in storage at Sellafield. These packages dated from 1974 and 1980 and were produced in a batch process by conversion of plutonium oxalate in a furnace at around 550 °C. The storage package contained a poly(vinyl chloride) (PVC) bag between the screw top inner and outer metal cans. Degradation of the PVC has led to adsorption of hydrogen chloride together with other atmospheric gases onto the PuO2 surface. Analysis by caustic leaching and ion chromatography gave chloride contents of 2000 to >5000 ppm Cl (i.e., μgCl g-1 of the original sample). Although there are some subtle differences, in general, there is surprisingly good agreement in results from heat treatment experiments for all the samples from both cans. Mass loss on heating (LOH) plateaus at nearly 3 wt % above 700 °C, although samples that were long stored under an air atmosphere or preexposed to 95% relative humidity atmospheres, gave higher LOH up to â4 wt %. The majority of the mass loss is due to adsorbed water and other atmospheric gases rather than chloride. Heating volatilizes chloride only above â400 °C implying that simple physisorption of HCl is not the main cause of contamination. Interestingly, above 700 °C, >100% of the initial leachable chloride can be volatilized. Surface (leachable) chloride decreases quickly with heat treatment temperatures up to â600 °C but only slowly above this temperature. Storage in air atmosphere post-heat treatment apparently leads to a reequilibration as leachable chloride increases. The presence of a "nonleachable" form of chloride was thus inferred and subsequently confirmed in PuO2 samples (pre- A nd post-heat treatment) that were fully dissolved and analyzed for the total chloride inventory. Reheating samples in either air or argon at temperatures up to the first heat treatment temperature did not volatilize significant amounts of additional chloride. With regard to a thermal stabilization process, heat treatment in flowing air at 800 °C with cooling and packaging under dry argon appears optimal, particularly, if thinner powder beds can be maintained. From electron microscopy, heat treatment appeared to have the most effect on degrading the square platelet particles compared to those with the trapezoidal morphology.
UR - http://www.scopus.com/inward/record.url?scp=85078729564&partnerID=8YFLogxK
U2 - 10.1021/acsomega.9b00719
DO - 10.1021/acsomega.9b00719
M3 - Article
AN - SCOPUS:85078729564
SN - 2470-1343
VL - 4
SP - 12524
EP - 12536
JO - ACS Omega
JF - ACS Omega
IS - 7
ER -