Archive for March, 2011

Fukushima: the struggle continues

Posted in Environment on March 26, 2011 by max

The effort to contain the damage at the Fukushima Daiichi nuclear plant underwent a setback yesterday, when high levels of radiation forced workers to vacate reactor building #3. Two workers suffered radiation burns from wading through contaminated water; reportedly, the workers were not wearing protective boots. The BBC reported that turbine halls at reactors 1 and 3 had radiation levels 10,000 times higher than normal. Japan’s Nuclear Safety Agency denied that the reactor cores were breached, but admitted that some sort of leakage may have occurred.

Levels of radiation measured in sea water 300m off the coast were eight times higher than a week ago. An NSA spokesman stated that sea life should not be affected, as the radioactive iodine isotope responsible has a half-life of only eight days, but many Japanese remain concerned. It is not clear whether the radioactive material was swept to sea as runoff, or was borne by offshore winds.

Considering the amount of water that workers have been spraying around the plant over the last week in an attempt to keep temperatures in the spent fuel rod pools under control, elevated radiation levels in runoff would hardly be surprising. This could also explain the concentration of radioactive contaminants at low points in the reactor buildings. It is not necessary to presume a breach in the core containment. However, so long as the fuel rod temperatures remain above normal, the danger of meltdown and core breach remains. It is critical that technicians are able to keep working in the reactor buildings to bring control and cooling systems back online. This is the major concern raised by yesterday’s developments: if workers are unable to safely approach the reactor buildings, the status of the damaged reactors, already unstable,  will rapidly deteriorate. Hopefully, as in the case of previous radiation alerts, the workers will be able to resume their valiant efforts after a brief hiatus.

Japan nuclear plant: Radioactivity rises in sea nearby

Fukushima update

Posted in Environment on March 23, 2011 by max

Here is an updated status from BBC News:

  • Reactor 1: Damage to the core from cooling problems. Building holed by gas explosion. Power lines attached.
  • Reactor 2: Damage to the core from cooling problems. Building holed by gas blast; containment damage suspected. Power lines attached.
  • Reactor 3: Damage to the core from cooling problems. Building holed by gas blast; containment damage possible. Spent fuel pond partly refilled with water after running low. Power lines attached.
  • Reactor 4: Reactor shut down prior to earthquake. Fires and explosion in spent fuel pond; water level partly restored. Power lines attached.
  • Reactors 5 & 6: Reactors shut down. Temperature of spent fuel pools now lowered after rising high. Power lines attached.

Note that power lines have been hooked up to each building. In reactors 5 and 6, they are reported to have successfully brought the cooling systems online. The have brought up power and lighting in the control room for reactor 3, and are planning to try to start the cooling pumps tomorrow.

All of this is good news. The fact that they have accomplished this much is an indication that radiation in the immediate area is not increasing, and gives hope that progress will continue at this pace. It may still be weeks or even months before all of the reactors are stabilized, due to the physical damage to systems and structures; but I would go so far as to say that the possibility of a core meltdown is greatly diminished. Good job.

The Fukushima disaster (part 2)

Posted in Environment on March 19, 2011 by max

I ended part one of this story by describing the destruction caused by the hydrogen explosions in the reactor buildings, and how this must complicate efforts to get the cooling systems operational. I just found this status report on the BBC News site:

  • Reactor 1: Was first to be rocked by an explosion on Saturday; fuel rods reportedly 70% damaged
  • Reactor 2: There are fears a blast on Tuesday breached a containment system; fuel rods reportedly 33% damaged
  • Reactor 3: Explosion on Monday; smoke or steam seen rising on Wednesday; damage to roof and possibly also to a containment system
  • Reactor 4: Hit by a major blaze (possible blast) on Tuesday and another fire on Wednesday

According to the BBC, all four reactor buildings have experienced explosions or fires or both. There are two more reactor buildings on site (total of six), but the other two were offline at the time of the earthquake. These two are also reported to have elevated fuel rod temperatures, but like many of the reports, this is difficult to evaluate — who is measuring these temperatures, and with what instruments?

Desperate measures

Most of us have seen the video of the helicopters attempting to dump water into the spent fuel rod storage pool, located on the upper level of a reactor building. This brings up another dangerous situation, which I haven’t addressed yet. The Fukushima plant stores ”spent” fuel rods in water tanks on the upper floors of the reactor buildings, as well as some in a common pool. The number of these rods dwarfs the number of rods in the actual reactors — over 600,000 rods are stored on site. Each rod consists of around 5kg of uranium oxide pellets, sealed in a zirconium tube — that’s about 3 million kilograms of hot radioactive material outside of the reactor vessels, that must also be submerged in continuously circulating water. Needless to say, the pumps responsible for this circulation are not functioning, either. One of the serious flaws in this reactor design, that has been heavily criticized in the past, is the fact that these storage pools and their support systems are not nearly as robust as the main reactor systems. They are not “hardened” against natural disasters or protected by multiple redundancies. In the case of  Fukushima 1 through 4, it is entirely possible, perhaps even likely, that some or all of these systems were irreparably damaged by the hydrogen explosions. The fact that you have a hole in the roof of the containment building large enough to attempt a helicopter water drop is a pretty good indication that the interior is seriously messed up. And when the official spokesman is reduced to expressing hope that there might possibly be some water left in the storage pool, I think we can assume that if it is not dry now, it is going to be very soon.

What prompted the managers to attempt such low-probability stunts as helicopter drops and water cannons (which would seem to have the same chance of success as trying to fill your neighbor’s backyard pool by blindly tossing water balloons over the fence)? Well, they probably felt they had to do something, considering the alternative. Left to themselves, the spent rods would eventually boil off the remaining water, becoming exposed to air, at which time the temperature would increase enormously. The zirconium casings might even catch on fire (it was speculated that this may have been the source of the fire in building 4); eventually the fuel would melt through the tank and drop through to the lower levels of the building (where it would continue to do damage), all the while releasing high levels of radioactivity, preventing workers from approaching the area and essentially forestalling any mitigation measures. All things considered, squirting water from a distance may have been the only viable option.

What happens next?

The latest reports state that TEPCO workers have succeeded in bringing grid power to Building 2. This is great news, but it is not in itself a solution. The suppression chamber at reactor 2 is reported to have been breached; if true, this will need to be repaired before water flow can be restored to the reactor. Then there is the question of the building utilities — considering all the various types of damage that have been inflicted, starting the pumps is not going to be a simple matter of flipping a switch or resetting breakers. Circuits and pipelines will need to be tested for integrity, computers will need to be rebooted, safety systems reset or even overridden. In the other buildings, which have gross visible structural damage, this task will be even more difficult. One encouraging sign is that the situation has not become obviously worse over the last two days — there have been no new explosions or columns of smoke, or sudden spikes in radiation (although the effective capacity for monitoring radiation at the site has been questioned). My guess is that right now the outcome is still in the balance — they may be starting to get a grip on events, but there is still potential for matters to spiral out of control on several fronts.

Meanwhile, I would like to send a deep bow in the direction of the heroic workers.

Related stories:

Japan steps up moves to cool stricken nuclear reactors

Fukushima Coverup: 40 Years of Spent Nuclear Rods Blown Sky High

Nuclear fuel (Wikipedia)

The Fukushima disaster (part 1)

Posted in Environment on March 18, 2011 by max

Like many people, I have been absorbed in the news reports about the horrifying triple-tragedy in Japan — first, an enormous earthquake, followed by a devastating tsunami, and now this continuing crisis with the damaged nuclear power plant. The available information on this latter topic has been sketchy, at best. Some people are blaming the Japanese government or the power company for a “lack of transparency”. I don’t think it’s appropriate to be pointing fingers, considering the overwhelming problems they are facing, but for my own satisfaction I have been trying to piece together a clearer picture of what went wrong, and how these incidents might be prevented in future. After all, the nearest nuclear plant to my location is also situated on the edge of the ocean, near an active fault, so the question is all too germane.

Keeping cool

All the issues at the Fukushima Daiichi plant can be related to a single factor: cooling, or rather, lack of same. A fission plant generates electricity in pretty much the same way as a natural gas or coal plant — it boils water, and the steam is used to turn a turbine, which spins a generator. The difference is,  in the fossil fuel plants, if you shut off the gas valve, or stop shoveling coal, the fire goes out, the water cools down, and you can pretty much walk away. But nuclear fuel rods generate heat via their natural radioactivity — you can dampen the reactor core to shut down the chain reaction, but the fuel stays hot. In the reactors used at Fukushima, the core will be boiling off 60 gallons of water per minute one week after shutdown. So it is imperative that a continuous supply of cool water is circulated through the reactor in order to draw off this heat, for more-or-less the entire life of the plant. In the absence of cooling water, bad things start to happen (can you say meltdown?), which I will get into later. For now, let’s examine this idea that the cooling system cannot be allowed to fail, ever.

Redundancy and more redundancy

First, we have to realize that the power for running the cooling pumps cannot come from the reactor itself. If it did, and you had to shut down the reactor, the turbine, or the generator for either maintenance or an emergency, then you’d immediately be into meltdown mode (no coolant circulation). So that doesn’t work. In our example, the power for the pumps comes from the national electrical grid. Unsurprisingly, highly-developed countries such as Japan or the US consider the grid to be a reliable source of power — except, of course, in the case of a major natural disaster. But TEPCO thought they had that covered, with sufficient diesel-powered generator capacity to run the pumps on all the reactors (and presumably the plant lighting and control functions). To back up the back-up, they also had battery capacity for 8 hours of operation. So far, so good.

An unanticipated sequence of events

It is unclear whether the earthquake itself knocked out power to the plant. Assuming it did, the pumps would have been running on power from the diesel generators when the tsunami hit. Leaving the tsunami aside, the “by the book” scenario would have had TEPCO emergency workers restoring power to the plant as a matter of priority. Given the scale of this particular earthquake, that may have taken days, but in the meantime trucks could deliver diesel fuel to keep the generators running. Potentially there could have been some delays involved with clearing the roads, but there was a certain amount of fuel storage on-site (I haven’t seen a figure for this, yet), and, in the worst case, they could go to batteries overnight. But, as we know, the earthquake was followed within an hour by a massive tsunami which not only drowned the diesel generators but probably severely damaged or destroyed the related infrastructure (power lines, control systems, fuel pipelines and storage tanks). By the following morning the battery power was exhausted, and the water in the reactors began to boil away.

Bad things start to happen

So, what are the adverse consequences of failing to circulate water through the reactor core? The first thing is the water starts to boil, creating steam. But the steam isn’t going anywhere (for instance, to power the turbine) because you can’t afford to let it flow out, since you don’t have any way of replacing the water (since the pumps are down). But that creates pressure, and eventually you have to release some of it to avoid blowing up the reactor vessel. So you are forced to vent some steam into the secondary containment structure. There are a few problems with this. One is that the steam contains traces of radioactive elements, but it’s not like you’re releasing it into the environment (not yet). The second is that the steam also contains hydrogen gas (the heat of the nuclear fuel breaks water vapor into hydrogen and oxygen; apparently this process is catalyzed or otherwise exacerbated by the zirconium casings on the fuel rods). Hydrogen gas is explosive (think Hindenburg disaster). Now the secondary containment is designed to deal with this situation — it is equipped with air pumps, filters and scrubbers that can capture the radioactive trace elements and the hydrogen gas — except for the fact that the power is out and none of this equipment is operational. Oops. The monitoring instruments that would inform the operators of a hydrogen build-up are also off-line.

Out of control

Up to this point nothing irreversible has occurred. If the power suddenly came back up, we’d soon be back to normal, with no permanent harm done. But unfortunately, events at Fukushima took a different turn. The first clue that something was seriously wrong at the complex occurred when one of the reactor buildings suffered a hydrogen explosion. The news reports stressed that this was not a nuclear explosion, and that the reactor containment was not breached. But what had happened was bad enough. The images clearly showed severe damage to the upper portion of the reactor building — gaping holes in the walls and roof, piles of twisted metal debris, and an ominous cloud of smoke. In my opinion, this was the critical event in the sequence of failures.

Prior to the explosion, full plant operation could have been restored by the flick of a switch. Now, it is impossible to say — it is conceivable that nothing can be done to prevent a full meltdown. This is a more pessimistic position than we have been hearing in the media, but I am finding it difficult to imagine how the situation can be quickly turned around. Look at the wreckage of the reactor building — even if TEPCO manages to hook up grid power, as they are attempting today, it seems unlikely that many of the building utility systems could remain functional. Lighting, control, monitoring and plumbing have all been massively damaged, at least insofar as the secondary containment and spent fuel storage functions are concerned (the reactor vessel and its systems can’t be seen). Debris removal, damage control, structural repair and systems replacement will have to be performed before full function can be restored, and all of this will necessarily be attempted in the dark (no lighting), in a structurally unstable environment choked with debris and subject to radiation leaks and further explosions, by exhausted workers hampered by rad suits and the absence of heavy equipment and power tools. My understanding is that this description applies to three of the reactor buildings as of Friday morning.

The situation of the plant workers is equivalent to that of a smoke jumper whose pumper has just been overrun by a fast-moving forest fire, leaving him standing downwind with a shovel and a bucket. He’s game, and he knows his business, but he doesn’t have anything to work with.

(to be continued)