INSIGHT-After disaster, the deadliest part of Japan's nuclear clean-up

August 13, 2013|Reuters

By Aaron Sheldrick and Antoni Slodkowski

TOKYO, Aug 14 (Reuters) - The operator of Japan's crippledFukushima nuclear plant is preparing to remove 400 tonnes ofhighly irradiated spent fuel from a damaged reactor building, adangerous operation that has never been attempted before on thisscale.

Containing radiation equivalent to 14,000 times the amountreleased in the atomic bomb attack on Hirosihima 68 years ago,more than 1,300 used fuel rod assemblies packed tightly togetherneed to be removed from a building that is vulnerable tocollapse, should another large earthquake hit the area.

Tokyo Electric Power Co (Tepco) is already in alosing battle to stop radioactive water overflowing from anotherpart of the facility, and experts question whether it will beable to pull off the removal of all the assemblies successfully.

"They are going to have difficulty in removing a significantnumber of the rods," said Arnie Gundersen, a veteran U.S.nuclear engineer and director of Fairewinds Energy Education,who used to build fuel assemblies.

The operation, beginning this November at the plant'sReactor No. 4, is fraught with danger, including the possibilityof a large release of radiation if a fuel assembly breaks, getsstuck or gets too close to an adjacent bundle, said Gundersenand other nuclear experts.

That could lead to a worse disaster than the March 2011nuclear crisis at the Fukushima plant, the world's most serioussince Chernobyl in 1986.

No one knows how bad it can get, but independent consultantsMycle Schneider and Antony Froggatt said recently in their WorldNuclear Industry Status Report 2013: "Full release from theUnit-4 spent fuel pool, without any containment or control,could cause by far the most serious radiological disaster todate."

Tepco has already removed two unused fuel assemblies fromthe pool in a test operation last year, but these rods are lessdangerous than the spent bundles. Extracting spent fuel is anormal part of operations at a nuclear plant, but safelyplucking them from a badly damaged reactor is unprecendented.

"To jump to the conclusion that it is going to work justfine for the rest of them is quite a leap of logic," saidGundersen.

The utility says it recognises the operation will bedifficult but believes it can carry it out safely.

Nonetheless, Tepco inspires little confidence. Sharplycriticised for failing to protect the Fukushima plant againstnatural disasters, its handling of the crisis since then hasalso been lambasted.

Last week, Prime Minister Shinzo Abe ordered the governmentto take a more active role in controlling the overflow ofradioactive water being flushed over the melted reactors inUnits 1, 2 and 3 at the plant.

GIANT FRAME

The fuel assemblies are in the cooling pool of the No. 4reactor, and Tepco has erected a giant steel frame over the topof the building after removing debris left behind by anexplosion that rocked the unit during the 2011 disaster.

The structure will house the cranes that will carry out thedelicate task of extracting fuel assemblies that may be damagedby the quake, the explosion or corrosion from salt water thatwas poured into the pool when fresh supplies ran out during thecrisis.

The process will begin in November and Tepco expects to takeabout a year removing the assemblies, spokesman Yoshikazu Nagaitold Reuters by e-mail. It's just one instalment in thedecommissioning process for the plant forecast to take about 40years and cost $11 billion.

Each fuel rod assembly weighs about 300 kilograms (660pounds) and is 4.5 metres (15 feet) long. There are 1,331 of thespent fuel assemblies and a further 202 unused assemblies arealso stored in the pool, Nagai said.

Almost 550 assemblies had been removed from the reactor corejust before the quake and tsunami set off the crisis. These arethe most dangerous because they have only been cooling in thepool for two and a half years.

"The No. 4 unit was not operating at the time of theaccident, so its fuel had been moved to the pool from thereactor, and if you calculate the amount of caesium 137 in thepool, the amount is equivalent to 14,000 Hiroshima atomicbombs," said Hiroaki Koide, assistant professor at KyotoUniversity Research Reactor Institute.

Spent fuel rods also contain plutonium, one of the mosttoxic substances in the universe, that gets formed during thelater stages of a reactor core's operation.

INADVERTENT CRITICALITY

"There is a risk of an inadvertent criticality if thebundles are distorted and get too close to each other,"Gundersen said.

He was referring to an atomic chain reaction that leftunchecked could result in a large release of radiation and heatthat the fuel pool cooling system isn't designed to absorb.

"The problem with a fuel pool criticality is that you can'tstop it. There are no control rods to control it," Gundersensaid. "The spent fuel pool cooling system is designed only toremove decay heat, not heat from an ongoing nuclear reaction."