MISTRAL is loaded into a BR2 fuel element, which means that:
- its neutron flux is high: for example the neutron fast fluence of 5.x 10 24 n/m² (E>0.1 MeV) is reached in about ½ BR2 cycle.
- the space available for specimens is restricted: number of specimens and their dimension. Typically, MISTRAL was designed to irradiate mini-charpy V (4 x 3 x 27 mm³) and round tensile (5 mm diameter and 27 mm long) specimens (see figure below).
A maximum of 80 such specimens can be loaded over a 500 mm IPS length centred to the reactor mid-plane, the axial flux profile limited to 500 mm leads to a ratio maximum flux/minimum flux larger than 50%. Of course, other types of specimens may be considered if they are compatible with the MISTRAL basket.
In MISTRAL, the electrical heater can deliver 6 kW/m. There are 2 in-pile sections (MIPS1 and MIPS2) that have 2 different configurations. When the reactor does not work (0% power), the water (and then the specimens) can be brought to:
- Configuration 200 °C (MIPS1) = 100 °C
- Configuration 300 °C (MIPS2) = 160 °C
The targeted water temperatures (200 °C and 300 °C) are reached when the BR2 is at 60% nominal power due to the gamma heating (8 to 14 W/g) generated in the material of the in-pile section structures and in the specimens. These temperatures can be maintained or adjusted at higher reactor power (up to 100%) by controlling the electrical heater.
To reach higher temperature into the specimens, the specimens have to be encapsulated in matrix made of material that has a good thermal conductivity (capsule – see photo) ) and the capsule has to be put in a holder that has the typical outer shape (4 x 3 x 27 mm³ or 5mm diameter & 27 mm long). The targeted temperature (> 300 °C) is fixed by the thickness of the gas gap between the matrix and the holder.
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