CHIP experimental facility to study fission product transport and deposition in the primary circuit. Photo: ASNR

Following several years of experimental research on source term analysis conducted under the NEA LOFT, BIP, STEM, and THAI joint projects—as well as complementary efforts such as the 2015 Iodine Workshop (2015 Iodine Workshop)—the NEA Working Group on the Analysis and Management of Accidents (WGAMA) convened a Source Term Workshop in January 2019 (2019 Source Term Workshop). The outcome of this workshop formed the basis for the NEA Experiments on Source Term for Delayed Releases (ESTER) project, jointly proposed by the French Institute for Radiological Protection and Nuclear Safety (IRSN)—now the French Nuclear Safety and Radioprotection Authority (ASNR)—together with the French Alternative Energies and Atomic Energy Commission (CEA) and other stakeholders in the scientific community.

The ESTER project focused on two of the four thematic areas identified during the 2019 workshop. The first topic addressed the remobilisation of fission product (FP) deposits that can significantly contribute to delayed releases during a severe nuclear accident. The importance of understanding FP remobilisation, especially that of caesium (Cs), was underscored by post-Fukushima analyses carried out in the NEA BSAF and ARC-F projects. ESTER included separate-effect tests, semi-integral experiments in the CHIP facility at ASNR, and confirmatory tests using deposit samples obtained from the VERDON facility at CEA, which involved release testing from actual fuel.

The results of the ESTER experiments are to be used to support evaluations of source term and of accident management strategies. The information already served as a basis for the improvement and validation of source term predictive models. A part of this work was done in an analytical working group (AWG)  where experience of the application and enhancement of simulation tools was shared. The ESTER project had strong links with other concluded joint projects in this field: the LOFT, BIP, STEM, THAI and THEMIS projects. It was also related to post-Fukushima Daiichi projects addressing analysis of fission product behaviour such as BSAF, ARC-F, TCOFF and FACE projects. 

The second thematic focus was the formation of organic iodides within containment. The ESTER experiments extended the work initiated in the STEM project, with a series of targeted experiments performed in the EPICUR facility at ASNR. These studies provided an improved understanding of containment chemistry, particularly regarding interactions involving methane and other reactive species. The ESTER investigations were also complementary to those undertaken in the NEA THEMIS project by Becker Technologies, which concluded in 2024.

The results from ESTER contribute directly to improved source term evaluations and accident management strategies. They have already informed the development and validation of predictive models, including work conducted within an Analytical Working Group (AWG) established to share experience in simulation tool application and enhancement. ESTER maintained strong links with past joint projects—LOFT, BIP, STEM, THAI, and THEMIS—and with post-Fukushima initiatives such as BSAF, ARC-F, TCOFF, and FACE that focus on fission product behaviour.

By 2024, all planned ESTER experiments had been successfully completed. This included work at ASNR's EPICUR and CHIP facilities and experiments at CEA using prototypical radionuclide deposit samples derived from VERDON release tests involving actual fuel pellets. ASNR completed key studies in EPICUR examining the influence of methane (CH₄) on organic iodide formation and conducted small-scale remobilisation tests using the ATMIRE experimental setup. In CHIP, three semi-integral tests investigated the remobilisation of representative deposits involving major fission products such as caesium, iodine, and molybdenum. CEA completed two additional tests with real-fuel-derived deposit samples. The project concluded successfully in March 2025.

As ESTER drew to a close, planning began for a follow-up initiative, FORESEEN, aimed at reducing uncertainties in source term evaluation for accident-tolerant fuels (ATFs), including Cr-doped fuels and Cr-coated or Fe-Cr-Al claddings. Led by ASNR and Idaho National Laboratory (INL), this five-year project will focus on the effects of chromium dopants on fission product release kinetics and on the behaviour of key fission products (Cs, Mo, I) during transport and containment interactions. The FORESEEN proposal was endorsed by the CSNI at its December 2024 meeting and is expected to launch by late 2025 or early 2026.

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