A comparison of purification methods to quantify Pd-107 in radioactive wastes
Résumé
Introduction
Pd-107 is one of the U-235 fission products, which needs to be taken into account for a safe management of radioactive waste in France. As Pd-107 has a half-life of 6.500.000 years, this isotope mainly
contributes to the radioactivity for long-term storage. Currently, waste containing more than 10-4 Bq/g of
Pd-107 is declared to the French National Radioactive Waste Management Agency (ANDRA) as “low
and intermediate level radwaste” and stored in a dedicated surface facility. However, when the Pd-107
activity exceeds 3x105 Bq/g, ANDRA will classify the waste as “long-lived waste” with an envisioned
storage at a depth of 500 m in another specific facility [1]. In 2022, the French government announced
the relaunch of nuclear power plants in parallel of the increase of decommissioning and dismantling
activities of nuclear sites. Therefore, Pd-107 quantification with good accuracy is required to follow ANDRA guidelines for future radwaste. Besides, Pd is a rare and expensive metal used in organic chemistry
and automotive industry so the challenging quantification of this isotope also matters those fields of
application [2]. Developing an efficient purification method to quantify Pd-107 in nuclear waste is of high
interest. It will also provide additional information to improve Pd recycling in other fields [3].
Overview on Pd-107 separation existing methods and future needs
Before performing a quantification of Pd-107 by liquid scintillation counting (LSC) or ICP-MS, a radiochemical procedure is required to remove all interfering radionuclides contained in radwaste. Few separation and quantification methods already exist for Pd-107 determination in such matrices [4]. One of
these separation methods for Pd-107 was developed two decades ago in the LASE laboratory at CEA.
It requires a digestion of the sample with nitric and hydrofluoric acid, a liquid-liquid extraction step with
chloroform as well as a separation on an anionic exchange resin before measurements by ICP-MS. The
liquid-liquid extraction step (L-Ls), exposes the operator to radiations and potential contaminations and
implements chloroform which is within REACH restrictions. To isolate and purify Pd-107 from radwastes
containing many possible interfering elements while respecting REACH regulation, a new radiochemical
method is needed. However, most recent studies performed to separate Pd from the other platinum
group elements are conducted without taking into account these chemical constraints nor common analytes encountered in radioactive waste such as Cs-137, Co-60, Fe-55, Ni-63 or mass-interfering elements like Ag or Zr. Therefore, a new separation protocol for Pd-107 including precipitations (NaOH,
NH4OH, HCl, ascorbic acid) and chromatographic Ni-resin® [4] is being developed and will be optimized
with Design of Experiment method before validating the separation protocol on real radioactive samples.
Conclusion
This paper provides an update on the Pd-107 separation protocol currently under development for its
purification in radwastes. It shows the investigation of selectivity of common precipitations towards Pd
and various interfering elements. It also compares the decontamination factors and Pd recovery yields
obtained from both protocols: the one existing in the LASE laboratory and the newly developed one.
Those results will help to find the best purification method to characterize Pd-107 in real radioactive
waste samples.