Fukushima

A recent Reuters article ("After disaster, the deadliest part of Japan's nuclear clean-up") proved something of a rabbit hole. Having ventured down into this unfamiliar terrain, new tunnels kept opening up.

The 2011 earthquake and tsunami left 400 tons of "highly irradiated spent fuel" more or less hanging in the sky 30 metres up in Reactor Building No 4. Its roof, and much else, was pulverised by a hydrogen explosion so there's no containment structure left. Only desperate efforts in the immediate aftermath when all power sources were knocked out kept the pool in which the fuel rods are stored covered with water.

It wasn't alone in suffering severe damage. Reactor Nos 1, 2 and 3 (which were all online when disaster struck) are now in permanent shutdown with their reactor cores largely or entirely melted down and sitting in intensely hot lumps at the bottom of their containment chambers. Vast quantities of water keep their temperature within tolerable bounds but much of it is leaking into the groundwater and, eventually, the Pacific.

What sets No 4 apart is three things. First, it has far more spent fuel in its cooling pond then any of the others because for maintenance purposes the entire fuel contents of its reactor had been transferred to the pond only four months previously. Second, because of that transfer, some 550 of the 1231 used fuel rod assemblies were much more radioactive than normal. And, finally, the building itself is structurally unsound. Tokyo Electric Power Co (TEPCO) have done some shoring up, but it wouldn't take too much of a shake to crack it, or maybe even tip it over.

D-Day for TEPCO's plan to move this spent fuel to a safer location is nigh. Since the infrastructure to handle spent fuel was destroyed, they've had to recreate that capacity from scratch. Handling fuel rod assemblies is a delicate business and no one can know if they'll succeed. The plan is to start in November and finish within a year. It's just one (particularly important) piece of the winddown of Fukushima, estimated by a spokesman "to take about 40 years and cost $11 billion." The total cost for Japan may range up to $100 billion.

There were some alarming scenarios raised in the article.

No one knows how bad it can get, but independent consultants Mycle Schneider and Antony Froggatt said recently in their World Nuclear Industry Status Report 2013: "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."

And Arnie Gunderson talked about a few ways that sort of release could happen.

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

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

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

The rods are also vulnerable to fire should they be exposed to air, Gundersen said.

Fascinating, so much so I badly wanted to get a better handle on the processes at work. Was all this unduly alarmist, or not?

It seems not. Let me share some of the fruits of my trip down the rabbit hole.

When reactor fuel is used up, no longer useful for fission purposes, it's replaced. The spent fuel is still intensely radioactive, however, which simply means some of the materials created through the fission process are unstable and constantly emit a stream of particles and gamma rays until they attain a more stable structure. It must therefore be stored under water for at least a year and usually much longer.