On Monday at 15:18, TEPCO workers began work removing spent fuel from the crippled Fukushima Daiichi Unit 4 spent fuel pool, which holds 1,533 assemblies. Of the 1,533 assemblies, 1,311 are spent fuel, the other 222 are unused.
This project has been labeled by some as one of the most dangerous nuclear operations in human history. Experts all agree that the engineering challenges are on a scale unseen to date.
Workers are removing the unused assemblies first, as they do not emit as much radiation and heat as the spent fuel assemblies.
Each spent fuel assembly is made up of about 80 fuel rods, which by assembly can contain up to 7,500 trillion becquerels of radioactivity and around 1% plutonium by weight.
The 2013 World Nuclear Industry Status Report claims that the “full release from the Unit-4 spent fuel pool, without any containment or control, could cause by far the most serious radiological disaster to date.”
Work will be challenging, due to the amount of debris in the spent fuel pool from the March 2011 explosion, which ripped the building asunder.
Workers will discover new damage to fuel units they were unaware of previously. The pieces of debris from the explosion were ejected with enough force to potentially damage fuel rods or jam multiple assemblies together.
The assemblies could also be moved too close together. The spent fuel rods could break, or be exposed to air and potentially ignite; both scenarios would potentially release a massive amount of radiation, which could necessitate the evacuation of workers from the plant.
According to Bloomberg, if the rods were to overheat or break, a self-sustained nuclear chain reaction similar to the meltdowns in the crippled reactors could be prompted.
The spent fuel assemblies are gripped by a huge remote-controlled crane and transferred into dry cask storage containers which each can hold up to 22 assemblies. Moving the first assembly into the cask took workers more than an hour.
Once the storage cask is full, workers will lift the container out, load it onto a trailer, and move it to the central repository. Work is largely carried out at a slow crawl. The movement of one cast out of the pool takes 12 hours over a 48-hour period, and 8 to 10 days to complete transfer to the new storage pool.
Tokyo Electric plans to finish removing all the spent fuel from the Unit 4 reactor building by the end of 2014.
Many experts have brought to light their concerns related to another major earthquake striking Japan near the Fukushima Daiichi site while workers are conducting removal work.