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.

The water in the spent fuel pool does two things: through a heat exchange mechanism, it keeps the fuel assemblies at reasonable temperatures; and, it captures the radiation streaming out of the fuel rods. There's a lot of this spent fuel about, some 260,000 tons, most still in storage ponds and growing by 8-10,000 tons per year (the US has some 65,000 tons, Japan about 19,000 tons).

It's seriously nasty stuff. Spent fuel from light water reactors (like Fukushima's) is composed of 93.4% uranium (with only ~0.8% U-235, the fissile isotope), 5.2% fission products, 1.2% plutonium and 0.2 % other transuranic elements. It's the middle two that are the potential killers. Some of the fission products break down very quickly to more stable (i.e. less harmful) elements, but two hang around for a long time and are particularly dangerous: strontium-90 and caesium-137. Both have half lives of about 30 years and they mimic potassium and calcium respectively. They're therefore rapidly absorbed into the food chain and become concentrated in higher-order creatures, like us. Iodine-131 is similarly lethal; it mimics iodine and concentrates quickly in the thyroid. The good news is its half life is only eight days and so it's only a factor in nuclear explosions or reactor accidents.

All this radiation also generates heat. Once the spent fuel is removed from the reactor, the radiation (and heat generation) tails off dramatically; after one year, it's down by a factor of 10 and by 10 years it's reduced to about 1% of its starting level. For much of the first 100 years, the radioactivity comes principally from the fission products with strontium-90 and caesium-137 dominant after the first 10 years. After a few hundred years only the transuranics are still going strong: plutonium, americium, neptunium and curium.

Still, the radiotoxicity of spent fuels remains a potentially lethal hazard for hundreds of thousands of years. And, given the plutonium content, the fuel also of course has to be safeguarded for all that time.

What about the heat generation, though? This was a question I wanted to get to the bottom of. With radiation tailing off so quickly, maybe after a year or two even exposure of spent fuel rods to the air wouldn't be catastrophic, in which case Fukushima might be through the most dangerous phase.

Not so, unfortunately. Gundersen and co were not exaggerating. In "Technical Study of Spent Fuel Pool Accident Risk" (2001), the US Nuclear Regulatory Commission wrote:

In summary, 60 days after reactor shutdown for boildown type events, there is considerable time (> 100 hours) to take action to preclude the fission product release or zirconium fire before uncovering the top of the fuel. However, if the fuel is uncovered, heatup to the zirconium ignition temperature [900°C] during the first years after shutdown would take less than 10 hours even with unobstructed air flow. After five years, the heat up would take at least 24 hours even with obstructed air flow cases. [PWR is a Pressurised Water Reactor; BWR a Boiling Water Reactor. Both are light water reactors]

So, however one gets there (whether by leakage, a crack in the pool or, heaven forbid, another earthquake, or even a slow boiling off if the heat exchange mechanisms were to fail), even after five years those fuel assemblies would burn.

Before such a fire starts, the cladding around the fuel rods would almost certainly swell and burst, releasing "radioactive gases present in the gap between the fuel and clad." It's the next stage that's truly catastrophic.

If the fuel continues to heat up, the zirconium clad will reach the point of rapid oxidation in air. This reaction of the zirconium and air, or zirconium and steam is exothermic (i.e. produces heat). The energy released from the reaction, combined with the fuel's decay energy, can cause the reaction to become self-sustaining and ignite the zirconium. The increase in heat from the oxidation reaction can also raise the temperature in adjacent fuel assemblies and propagate the oxidation reaction. The zirconium fire would result in a significant release of the spent fuel fission products which would be dispersed from the reactor site in the thermal plume from the zirconium fire.

Unlikely as it is, this is the nightmare scenario at Fukushima.

Lethal radioactive particles, notably strontium-90 and caesium-137, would stream into the atmosphere and be dispersed according to the vagaries of the weather, winds and currents. Even worse, were this process to unfold at Reactor No 4 (or at any of the others, of course, it's just that No 4 is the most vulnerable), it might become impossible to prevent similar processes eventually unfolding in all the other spent fuel repositories at the site (substantial as it is, the load at No 4 is little more than 10% of the total).

It has been a standard practice in the nuclear industry to avoid consideration of all of these possibilities, based on the assumption that there will be "lots of time" to react to any emergency involving the spent fuel pool, as it will normally take days for the spent fuel to reach the melting point and it will be a "simple matter" to refill the pools with water if necessary.

This ignores the fact that major structural damage may make it impossible to approach the spent fuel pool due to the lethal levels of gamma radiation emanating from the spent fuel once the protective shielding of the water is gone.

To judge by various estimates found here and there on the net, the damage from such an ultimately uncontrolled spent fuel fire hardly bears thinking about.

Based on U.S. Energy Department data, assuming a total of 11,138 spent fuel assemblies are being stored at the Dai-Ichi site, nearly all, which is in pools. They contain roughly 336 million curies (~1.2 E+19 Bq) of long-lived radioactivity. About 134 million curies is Cesium-137 - roughly 85 times the amount of Cs-137 released at the Chernobyl accident as estimated by the U.S. National Council on Radiation Protection (NCRP).[author's emphasis]

Viewed in this light, the light of what might have been, March 2011 starts to look like a win. For days (perhaps weeks) after the 11th, things teetered on the brink. Had just one spent fuel pool seriously cracked, had some pumps failed for an hour or two too long, it might have been all over; there may then have been no way to indefinitely keep the fuel assemblies covered with water and the whole thing would have fed on itself.

As it was, all that escaped into the air was a comparative smidgen. Enough, mind you, to cause the evacuation of 160,000 people and create a 20 km exclusion zone which is still mostly in force. Thankfully, the evacuations were ordered early enough to avoid any significant radiation damage to residents surrounding the plant.

The curious thing is, in one sense radioactive materials are more manageable than I'd thought. Particle emissions, for example, are for the most part easily blocked, sometimes with as little as a sheet of paper. Gamma rays are altogether more vicious, although even they can be shielded against with sufficient depth of water, concrete, lead or steel. The real trouble starts when radioactive particles get out into the environment. Then, they very quickly end up in living creatures where they wreak their damage directly. Once they've escaped and been scattered by wind, water and rain, the deed is done, much of it irrevocable.

And the effects, well, they go on, and on. Thousands, tens of thousands, hundreds of thousands of years. Much further into the future than Neanderthal man lies in the past. Have we taken leave of our senses, perhaps, to take such risks, however slight, for so little gain?