Concerns have been raised that spent nuclear fuel rods stored at the Number Four reactor in the Fukushima Number One nuclear power plant in Japan could generate a fission chain reaction.

The Tokyo Electric Power Company said that currently, water in the tank where the spent fuel rods are being kept is boiling and the water levels are dropping.

According to reports, the fuel rods, if exposed to air, could heat up to a point where a series of nuclear fission reactions becomes a possibility.

The company is reportedly reviewing the dropping of boric acid by helicopter to prevent such a scenario.

Unless they are cooled, spent fuel rods heat up again due to residual heat. This can melt the coating, which leads to leaks of radiation. The fuel rods then ignite and emit gas filled with radioactive substances.