Safeguarding bulk handling nuclear material facilities, and in particular spent fuel reprocessing plants, poses challenges, due to the nature and quantity of the special fissionable material (235U and plutonium) present.
In reproecessing plants, nuclear material is handled in loose form and chemical and physical changes occur, allowing for the material to be combined or split. Verifying the inventories is more challenging and samples are typically collected for analysis.
The increase in reprocessing capacity in the 90s in La Hague (France) lead to the installation of the Euratom on-site laboratory to perform these independent analyses. The major advantages were timeliness, higher efficiency and cost effectiveness, waste reduction, reduced transport needs, assuring sample authentication. Furthermore, re-verifications could be done with minimal delay.
Larger facility throughputs contribute to larger absolute uncertainties in the material balance evaluation. It is thus of paramount importance to apply methods that deliver fit-for-purpose precision and accuracy combined with short analysis times and low resource consumption.
Analysts from the JRC Karlsruhe perform independent analyses of fissile material for nuclear safeguards. They provide the Euratom inspectors (DG ENER) with near-realtime analytical results and verification of the nuclear material accountancy.
This work is critical to maintaining the integrity of the nuclear industry and protecting the public from potential hazards.
Analytical methods
The JRC Karlsruhe developed a radiometric analytical method, HKED, as a combination of K-edge Densitometry (KED) and X ray Fluorescence (XRF), in use at the LSS in La Hague, that allows determining the uranium and plutonium concentrations above 0.5 g/l in liquid samples within a few days and with uncertainties better than 1% and well within the International Target Values.
To avoid having to dissolve powder samples, gamma spectrometry is used for the assay of plutonium isotopic composition.
The method is sensitive to matrix characteristics, such as the acidity, and needs appropriate calibration. The LSS has opted to prepare, from excess samples, reference solutions certified using the second available primary analytical method isotope dilution (ID) coupled with thermal ionization mass spectrometry (TIMS). The spike used (IRMM 1027 series) guarantees traceability to the international standard.
ID-TIMS is the method used to resolve discrepancies between the operator’s declarations and the LSS results. Measurement uncertainties in the order of 0.1% are possible with this method. It is furthermore used to determine U enrichment. High precision density measurements complete the analysis performed.
Practical Implementation
Two to three JRC analysts travel weekly to the LSS in La Hague to perform a multitude of tasks related to the analyses of the samples. These include measurements, quality control, chemical preparations, data management, maintenance and repair, and innovations. Under the constrains of working with highly radioactive materials in a small team, this requires a trained workforce with unique skill sets.
The LSS is operated by the JRC on average 45 weeks during the year (team leader weeks in the figure). Even during the Covid-19 pandemic, the lab was closed only for a few extra weeks.
Around 15000 samples have been analysed over the more than two decades of operating the LSS. This amounts to performing well over 50000 measurements under stringent quality assurance.
Fit for the future
Investments have been or are being made to replace and upgrade the existing instrumentation with newer state-of-the-art equipment. Examples are the upgrade of the MAT262 mass spectrometer and the replacement of all NIM modules with Mirion Lynx digital signal analysers.
Off the shelf solutions are seldom available so the JRC invests resources for their development and nuclearisation. Examples are the development of a semi-automatic separation unit and an external density measuring cell.
Future objectives
The Euratom on-site laboratory in the nuclear reprocessing facility in La Hague (F) is an essential part of effective safeguards assuring that nuclear material remains in peaceful use.
Continued efforts to increase efficiency and to maintain the high quality of the analyses performed are primordial. Refurbishments and renovations of the infrastructure, instrumentation and procedures, taking into account changing boundary conditions and resources, will guarantee longer-term nuclear safeguards.