OMICO: Oxide Fuels – Microstructure and Composition Variations


The OMICO project (standing for "Oxide Fuels – Microstructure and Composition Variations"), was a Fifth Framework Project (2001-2007), coordinated by SCK•CEN aiming to study the behaviour of three different fuel compositions:

  • Conventional UO2
  • Uranium-based MOX: (U,Pu)O2 
  • Thorium-based MOX: (Th,Pu)O2

The other partners in the project are EC-JRC-ITU and FRAMATOME ANP (today AREVA NP).

Its primary objective was to provide insight in the separate effects of fuel chemistry (matrix composition) on the one hand and the degree of dispersion of the fissile material (microstructure) on the other hand.


To guarantee the safe operation of nuclear reactors, the behaviour of nuclear fuel under various operating conditions needs to be assessed.  Even though well-validated codes for MOX fuel currently exist, there is still discussion on the role that is assumed by the specific microstructure of MOX, especially regarding the release of fission gas. The test matrix compares in a systematic way the behaviour of three different fuel compositions (UO2, (U,Pu)O2 and (Th,Pu)O2), and for each composition, two different microstructures are compared (homogeneous and fine dispersed ceramic-in-ceramic). This results in six different fuels which are assembled in a small experimental assembly and irradiated under representative PWR conditions.

  • Two sets of six fuel rods were produced by EC-JRC-ITU, one set instrumented for pressure and temperature, and a second, non-instrumented set. 
  • The fuels were irradiated in the BR2 reactor of SCK•CEN.
  • Fuel performance calculations were performed by the teams of FRAMATOME ANP (COPERNIC code), EC-JRC-ITU (TRANSURANUS code) and SCK•CEN (MACROS code)

SCK•CEN's contribution

SCK•CEN coordinated the OMICO project (coordinator: Marc Verwerft) and performed the design calculations, irradiation tests and post-irradiation examinations.

Results & Future work

OMICO successes

  • The experimental OMICO assembly had to be calculated in a three dimensional approach. At the time of start of the OMICO project, 3D Monte Carlo calculations were not yet established for the complex core of the BR2 reactor. The thorough cross-checks with the conventional 2D approach, comparison with reference programmes and the irradiation of a dummy assembly contributed to the confidence in the MCNP modelling of complex experiments in the BR2 reactor, and are today common practice.
  • Fuel manufacturing to tight LWR specifications demonstrated once again ITU's key role as an independent experimental fuel manufacturer in Europe, and the OMICO programme has extended the core competence of this laboratory to heterogeneous (Th,Pu)O2 and UO2 fuels.
  • The thermal diffusivity analysis was a first-of-its kind for (Th,Pu)O2.  It was performed for different Pu contents, and thereby filling a knowledge gap open for many decades and settling a number of debates on this parameter.
  • The combined use of self-powered neutron detectors with the long-established thermal balance methods improved the reliability of power determination, which is crucial to all fuel experiments.  Together with the MCNP approach and qualified gamma spectrometry methods, the accuracy and precision of the linear power determination has improved considerably.
  • In terms of fuel performance data, the OMICO experiment fully achieved the goals with respect to the acquisition of thermal data (power vs centreline temperature data set).

Further work

  • The (Th,Pu)O2 fuels, irradiated in OMICO were further investigated in the LWR-DEPUTY project (2006-2011).
  • Today, the (Th,Pu)O2 fuel is continuing its irradiation and will soon achieve a burnup of 40 GWd/tHM. With the irradiation to high burnup of thorium-based MOX fuel, SCK•CEN continues its leading role in advanced MOX technology.

SCK•CEN contact: Marc Verwerft