The design initially called for the core to be cooled like the B Reactor, which used a constant supply of water that poured through the channels in the graphite. There was considerable concern that such a system was subject to catastrophic failure in the event of a loss-of-coolant accident. This would cause the reactor to run out of control in seconds, potentially exploding. At Hanford, this possibility was dealt with by constructing a 30-mile (48-km) escape road to evacuate the staff were this to occur, abandoning the site. Lacking any location where a 30-mile area could be abandoned if a similar event were to occur in the UK, the designers desired a passively safe cooling system. In place of water, they used air cooling driven by convection through a 400-foot (120-m) tall chimney, which could create enough airflow to cool the reactor under normal operating conditions. The chimney was arranged so it pulled air through the channels in the core, cooling the fuel via fins on the cartridges. For additional cooling, huge fans were positioned in front of the core, which could greatly increase the airflow rate.