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Foundation protection measures

The government commission was concerned that the molten core would burn into the earth and contaminate groundwater. To reduce the likelihood of this, it was decided to freeze the earth beneath the reactor, which would also stabilize the foundations. Using oil well drilling equipment, injection of liquid nitrogen began on 4 May. It was estimated that 25 tonnes (55 thousand pounds) of liquid nitrogen per day would be required to keep the soil frozen at −100 °C (−148 °F). This idea was quickly scrapped.

As an alternative, subway builders and coal miners were deployed to excavate a tunnel below the reactor to make room for a cooling system. The final makeshift design for the cooling system was to incorporate a coiled formation of pipes cooled with water and covered on top with a thin thermally conductive graphite layer. The graphite layer would prevent the concrete above from melting. This graphite cooling plate layer was to be encapsulated between two concrete layers, each 1 metre (3 ft 3 in) thick for stabilisation. This graphite-concrete "sandwich" would be similar in concept to later core catchers now part of many nuclear reactor designs.

The graphite cooling plate and the prior nitrogen injection proposal, were not used following the drop in aerial temperatures and indicative reports that the fuel melt had stopped. It was later determined that the fuel had flowed three floors, with a few cubic meters coming to rest at ground level. The precautionary underground channel with its active cooling was deemed redundant and the excavation was filled with concrete to strengthen the foundation below the reactor.