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HERACLES-CP: Towards the Conversion of High Performance Research Reactors in Europe

Periodic Reporting for period 3 - HERACLES-CP (HERACLES-CP: Towards the Conversion of High Performance Research Reactors in Europe)

Reporting period: 2018-06-01 to 2020-02-29

To minimize the use of highly enriched uranium (HEU) fuel in the civil nuclear fuel cycle, the operators of research reactors worldwide have joined in an effort to reduce the enrichment of their fuels. To support these international non-proliferation efforts, the European High-Performance Research Reactors (HPRRs) aim to convert their fuels to low enriched uranium (LEU). The reactors involved in this program are BR2 (Belgium), FRM II (Germany), RHF and JHR (both France). Further the European fuel fabricator Framatome-CERCA participates in this effort.

The development of a new fuel for this conversion is based on a metallic uranium-molybdenum alloy (U-Mo) with very high density, investigated as both a dispersed and monolithic fuel. In the years since the start of this development, much progress has been made towards a qualifiable fuel. The final goal of establishing U-Mo as an accepted and approved standard fuel for HPRRs is much nearer, but still a couple of steps away. The HERACLES-CP project continues the efforts made in the past and is paving the way for the following projects LEU-FOREvER and EU-QUALIFY. To optimize resource utilization, over recent years the considerable work-load involved in developing the new U-Mo plate-type fuel has been split between Europe and the US: The American partners have been focusing on the development of monolithic U-Mo, while Europe has dedicated larger efforts towards dispersed U-Mo solutions. Of course, contributions have been made vice-versa and also by other countries. Accordingly, HERACLES-CP puts more weight on the development of the dispersed branch, but also covers important aspects of monolithic fuel development.

Even though there is extensive data available on U-Mo, there have been several fundamental knowledge gaps to be filled before the qualification of the fuel itself. Therefore, experiments and measurements necessary to fill these knowledge gaps were identified and conducted during the so-called Comprehension Phase within the HERACLES-CP project.

Furthermore, R&D is required for the fabrication technology for both development branches: U-Mo powder production, powder coating and plate fabrication for dispersed fuel, and foil coating and plate production for the monolithic fuel branch. While in-pile experiments are indispensable to fill the knowledge gaps, many questions related to the metallurgical aspects of radiation damage in the fuel assembly can be well or even better studied by out-of-pile heavy ion irradiation experiments.

The objectives of the HERACLES-CP project were:

For dispersed fuel:
- To fill the knowledge gaps identified by performing the necessary experiments and measurements,
- To conclude on the most promising fuel design,
- To develop the necessary fabrication techniques,
- To prepare and launch the SEMPER-FIDELIS irradiation test to verify the theory and to fill the gaps,
- To perform the Post-Irradiation Examinations (PIE) of SEMPER-FIDELIS.

For monolithic fuel:
- To develop the technology and knowledge necessary for fabrication,
- To prepare test samples for the EMPIrE irradiation test (originally foreseen for SEMPER-FIDELIS).

The project has been successful in meeting these objectives. Based on this knowledge and the results of the in-pile irradiation tests SEMPER-FIDELIS and EMPIrE, two further irradiation tests, currently titled “E-FUTURE-3” for dispersed and “FUTURE-MONO-1” for monolithic fuel, will be designed to advance towards the successful qualification of U-Mo as an HPRR fuel. Even though these two irradiation tests themselves are not in the scope of HERACLES-CP, a number of items required for the preparation of the tests in terms of fabrication and irradiation performance have been accomplished within HERACLES-CP.

The general project objective which has been completed can be summarized as: the establishment of the technical and scientific foundations to enable the successful qualification of U-Mo fuel.

Following this stage, the fuel will be qualified in a series of generic and reactor-specific irradiations and examinations, then finally used in the facilities according to their specific conversion plans.
The governing objective of HERACLES-CP was to lay the technical and scientific foundations for the successful qualification of U-Mo fuel. By the end of the project, this was fully achieved by the following individual progresses.

Irradiation experiments:
- The irradiation matrix for the SEMPER-FIDELIS irradiation experiment was defined.
- The SEMPER-FIDELIS test was finished.
- Post-Irradiation Examinations (PIE) were performed.
- Fresh fuel characterization experiments were performed.
- Ion experiments were performed.

Fuel comprehension:
- It was demonstrated that dispersed U-Mo with powder heat treatment and ZrN coating can be irradiated successfully at high power up to the targeted fission densities.
- The fuel swelling was found to be stable and predictable up to about 5.9 × 10^21 f/cc.
- The addition of Si into the Al matrix was shown to be a promising solution for improving the performance of the fuel system.

Manufacturing technology:
- The U-Mo electrode casting and powder atomization processes were studied in detail and the understanding of both processes was greatly improved.
- A pilot induction furnace and a pilot arc melter were designed, built and tested.
- The modelling of U-Mo dispersion plate fabrication by rolling was advanced very far.
- PVD coating of monolithic U-Mo foils and the subsequent processing into plates (C2TWP) successfully developed.
- A pilot PVD coater for U-Mo powder was designed and built by SCK CEN in collaboration with University Ghent.

Measurement technology:
- The PERSEUS channel gap measurement device prototype was completed, experiments were performed in the ILL reactor.
- Based on the prototype, an advanced device was designed, built and fully tested.
- The potential use of the device in BR2 was assessed.

- Meetings were organized and held to ensure the best possible scientific knowledge exchange across the partners.
- Publications were organized and collected to ensure the best possible dissemination of the project's results.
- Dedicated communication plans and media were developed and set up.
- A uranium transport study was conducted.
HERACLES-CP has enabled the consortium to advance the state of the art in many aspects:

- For the design of the SEMPER-FIDELIS irradiation matrix, dozens of experts from the EU and the US have (re-)measured, collected and evaluated data from more than one dozen prior irradiation experiments to ensure that SEMPER-FIDELIS delivers the maximum relevant information for the further development of U-Mo. Both SEMPER-FIDLIS and its sister experiment EMPIrE have finished irradiation. With the complete Post-Irradiation Examination (PIE) of SEMPER-FIDELIS, the understanding of U-Mo irradiation behavior has reached a new level.
- The technique of U-Mo powder atomization is now understood to an extent that enables the consortium to build the next stage of manufacturing equipment.
- Monolithic U-Mo foils can now be coated with PVD and turned into plates with a very high yield.
- The PERSEUS measurement device is a unique tool for measuring the thickness of fuel plates in-pile. Once it is advanced and qualified for an actual material test reactor, it will enable high-quality intercycle inspections during test irradiations.