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Article Cite This: ACS Omega 2019, 4, 12524−12536
http://pubs.acs.org/journal/acsodf
Thermal Processing of Chloride-Contaminated Plutonium Dioxide Kevin Webb,† Robin Taylor,*,† Catherine Campbell,† Michael Carrott,† Colin Gregson,† Jeff Hobbs,‡ Francis Livens,§ Chris Maher,† Robin Orr,† Howard Sims,† Helen Steele,‡ and Sophie Sutherland-Harper§ †
Central Laboratory, National Nuclear Laboratory, Sellafield, Seascale CA20 1PG, U.K. Sellafield Ltd., Sellafield, Seascale CA20 1PG, U.K. § School of Chemistry, The University of Manchester, Manchester M13 9PL, U.K. Downloaded via 79.110.31.26 on August 7, 2019 at 11:29:09 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
‡
S Supporting Information *
ABSTRACT: 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 ∼4 wt %. The majority of the mass loss is due to adsorbed water and other atmospheric gases rather than chloride. Heating volatilizes chloride only above ∼400 °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 ∼600 °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- and 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.
1. INTRODUCTION Civil plutonium dioxide (PuO2), accumulated from over 50 years of spent nuclear fuel reprocessing in the U.K., is safely and securely stored at the Sellafield site prior to Government decisions on its disposition.1−5 A small percentage of this stockpile contains elevated levels of chlorine from the decomposition of the poly(vinyl chloride) (PVC) liners that cover the inner metal can. This has released hydrogen chloride (HCl) gas that has adsorbed onto the PuO2 powder surface. © 2019 American Chemical Society
These materials originate from reprocessing of Magnox fuel at Sellafield mainly before 1975 and were packaged in nested, nonwelded metal containers. Thus, chloride from PVC degradation during storage, relatively high levels of moisture and other atmospheric gases have been adsorbed by the Received: March 14, 2019 Accepted: June 28, 2019 Published: July 23, 2019 12524
DOI: 10.1021/acsomega.9b00719 ACS Omega 2019, 4, 12524−12536
ACS Omega
Article
Table 1. Properties of PuO2 Powders Sampled from Two Legacy High Chloride Cans Retrieved from Storagea can 1
can 2
units
error
method
SSA LOH H/Pu chloride Am-241 Pu assay nitrogen carbon
determination
3.6* 2.61 0.25 1970 3.66 × 109 82.93 854* 173
4.3 2.55
m2 g−1 % At:At ppm Bq g(OS)−1 % ppm ppm
0.9 0.05 0.01 NQ 8.02 × 107 0.23 NQ 13/36
tap density particle size (
