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WP4: Fuel Coolant Interactions

Work package number: 4
Start date or starting event: M1
Work Package title: Fuel Coolant Interactions
Activity Type: RTD
Participant number: 10 - 12 - 4
Participant short name: JRC-ITU - CHALMERS - NRG
Person-months per participant: 22.10 - 13.6 - 12

Objectives
In this work package, the goal is to study the LBE-fuel interaction and characterize the reaction products and their potential effect on the fuel element in order to qualify the fuel for MYRRHA. In addition, a more fundamental approach is taken to understand the phase relations Pb-U-O, Bi-U-O, Pb-Pu-O and Bi-Pu-O for which there is no published available data.
The MOX fuel used in the experiments will be as close as possible to the MYRRHA specifications. In particular, the Pu fraction is set to 30% if possible, the O/M ratio representative of fresh fuel is set to 1.97 and the O/M ratio representative of irradiated fuel surface is set to 2.00.

Experiments will be done on solid pellets (or disks) to study the effect of LBE-fuel interaction on the surface. Additional experiments will be done with powders which have much higher specific surface area and show a different behaviour that the pellet (e.g. dissolution)

A temperatures range 500-800°C is chosen as representative of the fuel surface temperature in nominal operation condition. Experiment will be carried out at the limiting temperature 500°C and 800°C and possible at an intermediate temperature.
The oxygen concentration in LBE is set at [O]=1.10-6 wt.% in accordance with MYRRHA operating condition.
Additionally, two MOX manufacturing route will be investigated. One based on wet chemistry (sol-gel technique) that produces a very homogeneous solid solution of (U, Pu)O2 at JRC-ITU and one based on a dry fabrication route that consists in mixing powder of UO2 and PuO2 followed by ball milling and sintering at NRG. The latter produces a heterogeneous microstructure with Pu or U-rich agglomerates. The reason behind using two different preparation routes is to study the effect of the microstructure homogeneity on the interaction with LBE.

Complementary experiments on UO2/PuO2 pellets/powder will also be done.
Another objective of the work package is to acquire knowledge on phase relation between the different species involved in order to extent the current knowledge the phase diagrams. This work will be supported by dedicated experiments. A CALPHAD modelling of the phase diagrams is also foreseen but outside of this project.

Description of work (JRC-ITU, NRG, CHALMERS)
Work package 4 is divided in 3 tasks.
T4.1: Fuel preparation (JRC-ITU, NRG, CHALMERS)
The objective of this task is to obtain the right materials and experimental conditions to study the LBE-fuel interactions. The manufacturing of MOX is a complex operation that can strongly influence the fuel behaviour under irradiation and also possibly, its interaction with LBE. In order to take into account the variations in microstructure in the LBE-fuel experiments, three different kind of MOX pellets/powder will be investigated. A homogeneous MOX will be prepared via the 'sol-gel' method, a heterogeneous MOX will be prepared via the powder route and a commercial MOX pellet (also heterogeneous) prepared for fast reactor by Belgonucléaire (BN) will also be studied. Existing pellets/powder of UO2 and PuO2 will also be used for additional experiments with LBE. The experimental set-up to control the oxygen potential and temperature in LBE is also described.

T4.1.1: Manufacturing of homogeneous MOX (JRC-ITU)
This manufacturing route is based on wet-chemistry 'sol-gel' method. The constituent elements are dissolved in a nitric acid solution and mixed in the appropriate ratios. Then, a polymer is added to increase the viscosity and the mixture is dispersed using a rotating cup atomiser. The resulting droplets are collected in an ammonia bath where a droplet-to-particle conversion occurred via a gel supported precipitation. After ageing, the beads are washed with water, calcined, compacted into pellets and finally sintered. The 'sol-gel' technique give a very homogeneous solid solution of (U, Pu)O2.
The obtained product is (U0.7Pu0.3)O2.00. To obtain a lower O/M ratio, it is necessary to do a heat treat the MOX in a reducing atmosphere. Part of the original (U0.7Pu0.3)O2.00 is therefore reduced to obtain (U0.7Pu0.3)O1.97 .

T4.1.2: Manufacturing of heterogeneous MOX (NRG)
This manufacturing route is based on direct blending of PuO2 and UO2 powders, combined with ball milling. It is followed by forced sieving to decrease the agglomerate size, pressing and sintering. Alternatively, UO2 and PuO2 could be sieved independently before blending. An intimate mixing of U and Pu on the (sub-) micron scale is obtained. However, compared to the homogeneous MOX, heterogeneous MOX still have Pu or U-rich agglomerates.
Many variants of this direct blending process have been developed and implemented on an industrial scale. It represents today the main production route for commercial fuel MOX pellets.
The obtained product is (U0.7Pu0.3)O2.00. To obtain a lower O/M ratio, it is necessary to heat treat the MOX in a reducing atmosphere. Part of the original (U0.7Pu0.3)O2.00 is therefore reduced to obtain (U0.7Pu0.3)O1.97 .

T4.1.3: Shipping of BN MOX (Chalmers)
A commercial MOX pellet (or pellet slice) prepared for fast reactors at Belgonucléaire (BN) with a Pu fraction of at least 20% and O/M ratio of 2.00 will be shipped from SCK•CEN to Chalmers University to study its interactions with LBE. A slice of the pellet will be heat treated in reducing atmosphere to obtain (U0.7Pu0.3)O1.97 .

T4.2: LBE/fuel interaction experiment and analysis (JRC-ITU, NRG, CHALMERS)
T4.2.1: LBE experimental set-up
The test apparatus is sketched in the figure. The oxygen concentration in LBE is controlled by adjusting the oxygen partial pressure in the atmosphere of the furnace in which the LBE-fuel containing crucible are located, through control of the Ar-H2/H2O mixture fed to the furnace.

Oxygen concentration for MYRRHA in nominal conditions is CO2=1.10-6 wt.%. Experiments will be done at this concentration. Temperature will be constantly monitored.

Analyses

Several types of analyses will be performed during or after the LBE/fuel interaction:

  • Thermal analysis (DTA) to follow the reaction kinetics and its energy release
  • Radiography after to check for sample integrity
  • XRD to identify phase change(s)
  • Ceramography/SEM-WDS/TEM/EPMA to characterize the interactions LBE/pellet
  • Inductively coupled plasma mass spectroscopy (ICP-MS) for dissolved products in LBE
 
  Example of experimental set-up to control the oxygen concentration in LBE 

For each of the following tasks, 4.2.2-4.2.5 (U0.7Pu0.3)O2.00 and (U0.7Pu0.3)O1.97 pellet (or disk) and powder will be immerged in LBE under controlled atmosphere (CO2=1.10-6 wt % in LBE) using the experimental setup described above at 500°C and 800°C for minimum 50h. Analyses will be carried out to identify the properties of the reaction, its products and its effect on pellet microstructure. Similar experiment will be carried out with UO2 and PuO2.

T4.2.2: LBE - homogeneous MOX interaction
T4.2.3: LBE - heterogeneous MOX interaction
T4.2.4: LBE - BN MOX interaction
T4.2.5: LBE – UO2/PuO2 interaction

T4.3: Phase relations study (JRC-ITU)
T4.3.1: Study of Pb-U-O and Bi-U-O phase relations
The study of the phase relations in the Pb-U-O and Bi-U-O to obtain basic knowledge on the possible phases that could be formed in the system. Solid state synthesis, thermogravimetry and X-ray powder diffraction will be the main techniques used, but additional characterization techniques (e.g. MASNMR, Raman) will be employed when needed.

T4.3.2: Study of Pb-Pu-O and Bi-Pu-O phase relations
The study of the phase relations in the Pb-Pu-O and Bi-Pu-O to obtain basic knowledge on the possible phases that could be formed in the system. Solid state synthesis, thermogravimetry and X-ray powder diffraction will be the main techniques used, but additional characterization techniques (e.g. MASNMR, Raman) will be employed when needed.

Deliverables

D4.1 Study of Pb-U-O and Bi-U-O phase relations (JRC-ITU) (M18)
D4.2 Analysis of LBE-fuel interaction (M36)
- Homogeneous MOX (JRC-ITU)
- Heterogeneous MOX (JRC-ITU)
- BN MOX and UO2/PuO2 (CHALMERS)
D4.3 Study of Pb-Pu-O and Bi-Pu-O phase relations (JRC-ITU) (M36)