Tokyo Electric Power Company (Tepco) said analysis of the state of melted fuel in unit 1 had shown that melted fuel burned not only through the reactor pressure vessel (RPV), but also through the PCV and then through concrete in the lower part of the PCV.

According to the Japan Atomic Industrial Forum the fuel analysis was carried out by Tepco and several research institutes. The analysis was based on temperatures inside the reactors, the amounts of cooling water and other data.

Tepco said that in the “worst case”, all fuel rods in unit 1 may have melted and dropped through the bottom of the RPV into the PCV.

Tepco estimates the fuel then eroded about 65 centimetres into the 2.6-metre concrete base of the RPV in the lower section of the PCV. A government model estimated the erosion at up to two metres.

The molten core stopped short of reaching the PCV’s steel casing, under which lies an additional 7.6 metres of concrete foundation, Tepco said.

Tepco also said as much as 57 percent of the fuel in unit 2 and 63 percent of the fuel in unit 3 may have melted, and that some of the melted fuel may have dropped through the bottom of the RPVs into the PCVs.

In May 2011, Tepco said most of the fuel rods in unit 1 had melted and dropped to the bottom of the RPV within 16 hours of the earthquake and tsunami that struck the plant.

The company said that about four-and-a-half hours after the scram – the automatic emergency shutdown of the plant due to the earthquake – the level of water in the RPV fell below the top of fuel rods and fuel started to overheat.