TECHNOLOGY PROGRAMME – Novel gamma-ray spectroscopy for radioactive materials testing

Project leader: René Bès    Personnel

Securing sufficient and environmentally sustainable electricity production is one of the main challenges for the future. To this aim, fission and fusion reactors do not suffer from intermittencies observed in solar or wind resources and allow an efficient and flexible basis for the highly demanding electricity network, especially around big cities and industries. Fission is exploited since more than half a century and fusion will allow energy production during the last quarter of the 21th century. In the meantime, future generation of reactors are under studies worldwide to enhance their efficiency and safety, following as most as possible sustainable ways. In this context, specific and innovative materials need to be developed as both type of reactors and their waste management rely on the precise control and design of complex material with tailored properties.

In the case of fission and fusion reactors, materials are hold under extreme environments, so that maintaining their properties during their lifetime requires building a perfect knowledge on the correlation between the microscopic and macroscopic scales. Our understanding of the local structure of atoms, as well as the impact of defects in the material is a key knowledge to assess such correlation and to increase the time required between regular replacements of structural pieces, before their potential failure. In addition, a large amount of required resources and nuclear waste to manage could be reduced by developing new materials with finely tuned properties. Therefore, one need to assess the structural composition behavior of materials at atomic scale to ensure an efficient control and design of tailored material properties for specific applications. The transfer of fundamental physics technology to innovative material characterization represents a unique opportunity to develop new answers to societal issues such as energy production.

An efficient way to characterize material behavior at atomic scale is to evaluate the elemental local structure, i.e. the nature(s), number(s) and distance(s) of neighboring atoms in a given material in correlation with the defects in presence. Two efficient, complementary and nondestructive approaches are X-ray Absorption Spectroscopy (XAS) and Positron Annihilation Lifetime Spectroscopy (PALS).

XAS is a well-established nondestructive method to determine both the oxidation state of and the local environment around one given element in materials. However, XAS experiments need a monochromatic, tunable over a wide range of energy and high flux photon beam. These needs have strongly limited its development to only synchrotron radiation facilities. Consequently, the finite and low success rate of synchrotron beamtime access, the high costs of radioactive sample shipment, and the low number of dedicated beamline accepting highly radioactive samples have strongly limited the experimental opportunities. Therefore, the usage of XAS on radioactive materials has been also quite limited despite its evident interest, excluding a large amount of potentially important scientific breakthrough to be at least consider.

PALS is a well-known method to probe the nature of vacancy type defects, i.e. the absence of atom(s), in a given  material structure, and it is widely applied on semiconductors and metals since decades. However, its application on nuclear materials have been limited due to their natural or irradiation induced high radioactivity. Indeed, positron lifetime experiments can be performed only for low-activity materials (activity below 100 kBq) when using the state of the art devices, due to the random coincidence event probability to occur. This is very low considering the activity of several MBq usually observed in typical nuclear fuels or neutron irradiated steels. Therefore, the usage of PALS on radioactive materials has been quite limited despite its evident interest, leading to important remaining gaps in the nature of defects in irradiated nuclear materials.

Through its fundamental research, CERN has acquired unique knowledge and technique on radiation detection, X-ray optics and high flux beam productions. For example, the Inverse Compton Scattering (ICS) source as developed within the Smart*light project in Netherlands aims to create tabletop hard X-ray radiation source as an efficient and intermediate alternative to X-ray tube and synchrotron radiation facilities [CERN]. The development of tabletop XAS devices will then benefits from these new X-ray sources, offering a more convenient alternative to Finnish industrial and academic synchrotron users in the near future, especially in the field of non-destructive radioactive material testing.

The project aims to overcome the two limiting factor described above by providing new XAS and PALS devices that allow to study high radioactive matter at the laboratory scale, which is most appropriate when dealing with highly radioactive materials requesting dedicated facilities.

1. PALS on radioactive materials.

By coupling three scintillator detectors and a new coincidence trigger by detecting simultaneously the two 511 keV gammas, one can significantly reduce the background arising from random coincidence event due to sample radioactivity [Heikinheimo]. In 2018 and 2019, the present project has seen the setting up of such a setup in Aalto University (now transferred into University of Helsinki), and the first demonstration of feasibility has been fulfilled. The setup commissioning and qualification phase took place in 2019 with the realization of test experiments to evaluate the performance for highly radioactive materials using well-known samples and radioactive sources as well as experiments on low activity samples. This phase is planned to end in December 2019. After that, the device is supposed to be deployed and installed at VTT Center of Nuclear Safety in 2020, depending on the on-going agreement discussed between VTT and HIP.

2. XAS at laboratory-scale: Evaluation of performance on actinide bearing materials.

By using the spherically bent crystal analyzer (SBCA) developed at ESRF as monochromator with a laboratory X-ray source, one can collect XAS data at laboratory with synchrotron quality and in reasonable time. In addition, we have recently demonstrated that this technique is highly adequate to be used on uranium compounds at L-edges. Thus, the current project aims to build a benchtop XAS device dedicated to radioactive nuclear materials in order to open new horizons on nuclear material research, and complement the synchrotron radiation studies.

The demonstration of feasibility was made in 2018 on a device at University of Helsinki with close geometry. Benefiting from such experience, 2019 was dedicated to design and acquire X-ray detector, electronics, SBCA, X-ray tube and other equipment needed for quickly setup a XAS device dedicated to nuclear energy relevant materials. The development of the laboratory-scale X-ray absorption device is then planned to be fulfilled during 2020. The first tests on nonradioactive references samples are expected during the commissioning phase in spring 2020, and the first experiments on actinide’s compounds are then foreseen to take place in the second half of 2020. This new setup will then open the way to a systematic evaluation of nuclear fuels behavior under, for example final repository conditions, as well as accurate determination of the influence of fission product on the oxidation state of UO2.

The current project aims also to strengthen the collaborations between Finland and France for the development of laboratory-scale XAS device to be used on highly radioactive samples. Project collaboration is on-going, and a device copy installation and commissioning at the CEA Marcoules hot laboratory in France is expected during the 2020-2021 period. This new device would be the first of this kind installed in a hot laboratory and would allow for experimental analysis of highly radioactive materials such as Am and Pu bearing nuclear fuels.

Collaboration

HZDR / ROBL beamline at ESRF: Dr K. Kvashnina.

CEA Marcoules: Dr. P. Martin.

VTT Center for Nuclear Safety