1 and 2 were shut down only some 12 hours later. The principal problems appear to have been that ventilation systems drew radioactivity into the buildings.
Many measurements around the reactor have been, and continue to be made. A 50 m square grid is used and readings are taken from both the ground and in the air. In the main, decontamination efforts seem to have been successful and the principal radiation source on the site is the damaged reactor itself.
The condition of the remains of the core are being monitored by special instrument "buoys", lowered onto the debris by helicopters on 240 m of steel rope. Ten will be installed in total, so far 7 are in place. These will provide data on temperature, heat flow and air flow required to determine how the planned concrete "tomb" should be built.
Final decisions concerning the entombment have still to be made. Some of the requirements for this structure are:
1. To protect the adjacent site, and particularly the other nuclear units.
2. To remove residual heat from the fuel and the collapsed part of the reactor.
3. To ensure proper monitoring of the important physical parameters.
4. To enable contingency plans to be made just in case something goes wrong after the entombment.
Whilst it was stated that an open ventilation system had been chosen for the design, the Russians were keen to invite any advice from other countries on how best to handle this phase of the operation.
Finally, the measures planned for other RBMK reactors were outlined again; they consist of four elements.
1. To reset the upper limit of level of control rods so they are permanently inserted 1.2 m into the core. This essentially makes the core "smaller".
2.
Reactivity margins will be increased by having a minimum of 80 effective rods as their requirement instead of the current 30.
3. In the longer term, fuel with a higher fraction (enrichment) of U 235 will be used. This will improve the situation on the positive void coefficient. Fuel of 2.4% enrichment has already been tested.
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