The MIMAS production process researched and implemented by Belgonucleaire comprises three separate steps
Due to the presence of plutonium, the whole production
process
is carried out in glove-boxes.
1. Mixing the uranium and plutonium powders
Mixing uranium and plutonium oxide powders is done in two steps. The first step is
“micronisation” to grind the powders. The composition is then refined in the
secondary mixture in order to obtain the final enrichment level of 5% of fissile material.
2. Fabrication of MOX pellets
The mixture thus obtained is transferred to the press. Pelletising is carried out by
hydraulic pressing. On leaving the press, the pellets of 1 cm in diameter by 2 cm in
lenght are stored in molybdenum boats.
They are then sintered in a special furnace at 1700 to 1800 °C for about six hours.
The sintered pellets are dry ground to the specified diameter.
The pellets are examined by visual inspection. Samples are taken from each batch to control quality.
3. The pellet stacks are placed in zircaloy claddings
After acceptance, the pellets are lined up in stacks on grooved trays.
The stacks are adjusted to obtain the correct lengths.
Zircaloy cladding tubes, closed at one end by a welded plug, are placed inside the glove-box for rod filling.
The pellet stacks placed in grooved trays are inserted one at a time.
Once the rod has been filled, a spring and a plug are added. The plug is sealed and the rod cleaned.
The rod is transferred to a work station where it is filled with helium at a pressure of about 25 bars and sealed. Thus packed, it is no longer necessary to operate in glove-boxes.
In the assembly plant, the rods are assembled in a square lattice. The assemblies are about 4.5 m in length, they weigh 450 kg and comprise 264 rods. They are ready for transport to the plant in leak-proof transport casks for insertion into the reactor core.
