N-Fuel to Melt again 38 Hrs. after Coolant Loss: TEPCO

Tokyo (Jiji Press) - Tokyo Electric Power Co. <9501> has estimated that the interruption for about 38 hours of water injection into the cores of its crippled nuclear reactors would prompt their nuclear fuels to melt again.
Unless water injection is restarted about 18 hours after being stopped, a massive amount of radioactive substances would be released into the environment, TEPCO estimated.
In the estimate for the No. 1 to No. 3 reactors at the March disaster-ravaged Fukushima No. 1 nuclear power plant, TEPCO assumed that their pressure vessels would have no water to cool nuclear fuels when water injection stops.
The temperate of the nuclear fuels would rise by about 50 degrees Celsius every hour from 300 degrees at the time of the coolant loss and reach 2,200 degrees about 38 hours later, the power utility estimated.
At that time, the nuclear fuel would start melting, and some would break through the pressure vessel to fall into the containment structure, according to the company.
TEPCO, however, estimated that the nuclear fuel would be cooled by the concrete of the containment structure and be kept from reaching the ground or going into the recriticality state.
Some 18 hours after the water loss, the nuclear fuel temperature would reach 1,200 degrees, the level to cause zirconium, used in cladding of nuclear fuel rods, to react with water.
It is necessary to resume water injection before this stage to avert a huge release of radioactive substances, TEPCO said.
The company said it can resume supplying water into the reactors 30 minutes after a single pump failure or a single external power supply cut.
Even in multiple troubles, water injection can be resumed in three hours, according to the company.