Meltdown at Fukushima I Nuclear Power Plant

This is the Fukushima I Nuclear Power Plant, a 6-reactor commercial nuclear electricity-generating facility owned and operated by TEPCO (Tokyo Electric Power Company). Though it cannot be seen at this angle, the Pacific Ocean is just down the bluff immediately to the right. The nuclear reactors are housed in the large, free-standing box-shaped outer containment structures. From front-left to rear-right: Reactor No. 4; Reactor No. 3; Reactor No. 2; Reactor No. 1; Reactor No. 5; Reactor No. 6.

Actually, we have probably seen a quadruple partial meltdown at Fukushima I (or “Fukushima Daichi,” or “Fukushima Dai Ichi,” or just “Daichi” to those who don’t know that it’s their way of designating “I”) Nuclear Power Plant in coastal Fukushima Prefecture, Japan.  It isn’t clear whether a full-core meltdown has occurred at any of these reactors–though Reactor No. 3, a mixed uranium and plutonium design that was recently breached, is the best contender for this status.

This will be the longest blog post I’ve written–not a habit I intend to continue, but I wanted to fix a sequence of events in my mind (and for the convenience of readers to whom these events might be unclear) as I write more about this incident in coming days.

The automatic power-down during Friday’s catastrophic magnitude-9.0 earthquake also shut off the coolant feed for the plant.  Coolant–in this as in most cases water–is most-crucial by far for the physical regulation of a nuclear power plant.  The water that cools a water-boiling reactor such as those at Fukushima I vents out from the top of it as steam, quickly running down a pipe and spinning a turbine before cooling in the large tanks with which nuclear power plants are most-often associated in our country.  Nuclear power plants are attractive in the sense that this process is incredibly efficient–it involves the release of energy from a pure metallic substance at the atomic level–and it is carbon-neutral.  In fact, breeder reactors, which start with uranium and actually produce plutonium as a by-product, are the most-efficient power source yet conceived by humankind.  But the Liberal Ironist doesn’t think any of that is going to matter now, because the coolant that moderates a nuclear power plant cannot be removed from fuel rods or they will start to melt-down.  This is true even if there is no chain reaction going on within them, or if they are spent.

If nuclear reactors power their own water coolant feed and a significant seismic reading (and the March 11th Sendai Earthquake, as it is now known, was definitely significant as it knocked northern Japan 8 feet to the east) triggers an automatic shut-off of the nuclear chain reaction, what is the big deal?  Hasn’t the plant automatically canceled the problem?  Well, no, because even when the insertion of the control rods prevents a chain reaction, the uranium or plutonium fuel rods in the reactor still emit an ambient level of radiation–at each other.  At Fukushima I’s Reactor No. 1, for example, this natural radioactivity is about a mere 3% of the reactors normal top operating capacity–but the unaided process of radioactive decay is enough to slowly heat the uranium or plutonium in a nuclear reactor until it begins to melt–they are metals, after all.  While a water-cooled reactor running at full capacity can maintain a temperature of about 250 degrees Celsius, a fully-moderated reactor without coolant can heat up through naturally-determined radioactive decay to around 3,000 degrees Celsius–the point at which the uranium or plutonium fuel will melt away its zirconium alloy casing and pile on the floor of the reactor container as molten slag.  When this process visibly begins it’s called a partial meltdown; when this transition to a liquefied state is more-or-less completed and this slag begins to eat away at the floor of the reactor, it is called a full-core meltdown.

Yes, the water that flows through a reactor is that important: Some reactors slowly begin to melt-down as soon as it is removed–regardless of whether the nuclear chain reaction continues.  When the reactors powered-down, they also stopped powering their own coolant loops.  The coolant systems for these reactors could have drawn power from the rest of Japan’s grid; but the primary transmission lines from which Fukushima I provides power for the Tokyo area apparently have been damaged, and electricity was unavailable from these.  So the coolant systems for the plant’s reactors had to depend on the gas-powered backup generators.  These switched on and powered the coolant feed without much trouble…until they were destroyed when the massive tsunami washed over Fukushima I’s protective sea wall about an hour later.

This westward view from the Pacific Ocean shows the plant's seaside situation. The backup generators that powered the plant's coolant circulation system were located between the main structures of the plant and the sea wall visible a short distance into the water. While the well-built nuclear plant survived the force of a magnitude-9.0 earthquake quite intact, the tsunami jumped the sea wall and destroyed those generators. Plant workers have desperately worked to keep Reactors 1-3 and spent fuel rods stored in the upper levels of Reactor 4 cool over the past 5 days. As of this writing, they're clearly losing that battle. Reactors 4 through 1 are housed from left to right in the large cubic structures in this picture. Photo courtesy of Agence France Presse and JIJI Press.

Plant workers had one last recourse for keeping their deactivated but excited reactors cool: Battery-powered backup generators.  These worked perfectly–for about 8 hours, at which time the batteries died.  (Batteries do that.)  By the wee hours of Saturday morning, Fukushima’s reactors were starting to boil-off their remaining water and get hot.  This situation led to the active transport of any freshwater available into the coolant intake pipes at recently-active Reactors 1, 2 and 3.

On Saturday afternoon, Japan’s time, a now-famous spectacle occurred at Reactor No. 1:

Fukushima I's Reactor No. 1 sustained an explosion in the upper levels of its outer containment structure. All day emergency cooling had fed water into a reactor that had slowly been heating up. The reactor had become hotter than plant workers realized, as flooding it with water didn't just evaporate the water now; it actually broke the water down into its base components, which they reacted with metlging zirconium alloy in the fuel rods. This case drifted up into the outer containment area of the reactor, where the volatile chemicals ignited a large explosion.

This explosion, of course, got a lot of media coverage (and makes for spectacular video), but mainstream media generally didn’t jump to any conclusions about what they were looking at, and many nuclear engineers brought on to comment said the same thing: “I know this is going to be hard to believe, but that probably isn’t a big deal in the scheme of things.”  Though it took a few hours to determine this, the explosion on Saturday afternoon was caused by hydrogen buildup in the cube-shaped outer containment structure surrounding Reactor No. 1.  The fact that the explosion didn’t involve inner containment or the nuclear fuel in any way was the good news; the bad news was the reason it had happened: Plant workers (numbering about 800 at this point in the crisis, or about 11 times its personnel on a “slow day”) had to let lightly-irradiated steam from the reactor core gather in outer containment before venting it out into the atmosphere, as it couldn’t be directed into the non-functioning coolant system.  The fuel in the core had grown so hot that water being piped in wasn’t just evaporating but actually being broken down into its base components of oxygen and hydrogen.  Oxygen is very reactive in large quantities, and hydrogen is extremely reactive–especially with the oxygen from which it was just separated.   As this built up in the upper levels of Reactor No. 1, it exploded, blowing those levels up.  The upper part of Fukushima I’s outer containment structure were designed to blow outwards rather than downwards in the event of an internal explosion, directing explosive force away from the inner containment structure.

As fresh water for manual supply became scarce, parent utility TEPCO decided to start pumping seawater and a boron compound into Reactor No. 1.  Seawater ruins a reactor, corroding a number of metal components inside it–but Fukushima I is next to the Pacific Ocean, the largest liquid water supply we’ve found.  Boron helps absorb radiation in the reactor–but its introduction to the core would ruin the chemistry that sustains the nuclear chain reaction.  Having been completed in 1969, this reactor was well towards the end of its useful lifespan and was scheduled to be decommissioned in a few weeks; this seemed like a small price to pay to get the situation there under control.  Pumping enough seawater into Reactor No. 1 would be a challenge, but it was definitely doable.

Then Reactor No. 3 began to heat-up to unacceptable levels.  This became clear on their Sunday afternoon–our wee hours of Sunday.  Suddenly plant workers had to double their efforts to keep water flowing into these 2 reactors, eventually giving up on Reactor No. 3 and using seawater there as well.  A second, more-dramatic and fiery hydrogen explosion at Reactor No. 3 Monday morning (our Sunday night) confirmed that the seawater was pumping into the reactor reliably, but that it was very hot.  While the event injured some workers and indicated how urgent it was to continue cooling the reactor, as with Reactor No. 1 the explosion itself posed a problem.  In a few hours, it would become quite clear that it did.

These 4 images show the larger, more-fiery hydrogen explosion at Fukushima I's Reactor No. 3 Monday morning. In terms of the impact on plant operations it was thought to be as innocuous as the hydrogen explosion at Reactor No. 1; it was not. Photo courtesy of the Associated Press/NTV.

Things really started to get out of hand at Fukushima I on Japan’s Tuesday morning: A third explosion was heard at Reactor 2, this one more-muffled than those that destroyed the outer containment structure at Reactors 1 and 3.  The outer structure remained intact, but the pressure gauge for the reactor core plummeted, and the radiation level within the reactor increased seriously.  It appeared that the inner containment structure had been cracked and the core, now partly-melted down, was exposed to the interior of the structure.

Earlier it had been discovered that the hydrogen explosion at Reactor No. 3 on Monday morning had wrecked four of Reactor No. 2’s five coolant intake pipes.  This was discovered after both temperature and pressure were found to have risen sharply.  Plant workers soon realized that Reactor No. 2, which had sent water lost through the broken pipes, had been high and dry for about 2 hours and 40 minutes.  This created a frantic race to pump seawater, the mark of long-term abandonment of Reactors 1 and 3, into Reactor 2 through the one surviving containment pipe.  This didn’t result in any drop in temperature or pressure to the reactor core, making one plant worker desperate.  He opened a steam release valve at the top of the reactor and increased the flow of water into the reactor.  This cause the exposion–a steam explosion inside the reactor.  The steam relief valve at the top of the reactor had jammed; water levels hadn’t risen inside the reactor and pressure was increasing because steam couldn’t be vented-off.

This was the first time a reactor encasement had cracked, exposing those inside a reactor structure to sustained significant radiation.  While such concerns had already been raised about Reactors 1 and 3, Reactor No. 2 now seemed very likely to have experienced partial meltdown.  With the damage to inner containment it wasn’t clear that it could be cooled, and radioactive steam could now spread from the reactor core to the rest of the facility.  It was at this point that TEPCO told 750 of the plant’s 800 on-hand workers to just go home.  From here on, a skeleton crew (under the circumstances) of 50 workers would try to keep the reactors cool.

Shortly after the reactor breach at Reactor No. led to an increase in radiation levels, the outer containment structure of Reactor No. 4–which doesn’t even have any fuel rods in its core right now–burst into flames.  At this point there were more hazards than there were anticipated causes.  In a situation that had either received inadequate attention or simply couldn’t be addressed because of a desperately-stretched manpower and water supply, the spent uranium fuel rods deposited in a “pool” (a standard means of on-site storage for spent fuel rods) had boiled off the top of their coolant water supply and begun to melt at their exposed tip, causing the fire.

This is a fact which actually makes me skeptical of nuclear power: For weeks after being removed from the chain-reaction fission process in a nuclear reactor, spent nuclear fuel rods are sufficiently-concentrated with unstable radioisotope byproducts of the fission process that they will still melt down under their own radiation unless they are also kept water-cooled.

At foreground-left is the mangled heap of Reactor No. 3 after a hydrogen explosion ripped through its outer containment structure Monday morning; in the background is the damaged Reactor No. 4 structure following the Tuesday fire. Reactor No. 4 didn't even have any fuel in its core at the time of Friday's earthquake and tsunami; however, on Tuesday the worst ambient radiation was apparently coming from the overheating spent fuel rods stored in a pool in the upper levels of that structure. The fire exposed those spent fuel rods, raising the first prospect of a significant health hazard to those on the plant grounds since the disaster began 4 days before. Photo courtesy Kyodo News Agency.

If not hopeless, at this point things began to look rather bad.  Radiation levels around the plant rose to around 400 millisieverts per hour, about 160 times Japan’s regulation yearly radiation limit and 40% of the dosage that will cause radiation sickness.  This was because the spent nuclear fuel rods stored atop Reactor No. 4 were exposed to the open air following this fire.  Shortly after this, radiation levels 20 times the normal background exposure are detected in Tokyo.  This increase in radiation exposure in itself is not threatening or even cause for alarm, but it is significant: At the time of this reading in Tokyo, a strong prevailing easterly wind is still blowing most radioactive steam straight out into the Pacific Ocean.

On Wednesday conditions really took a turn for the worse: Reactor No. 3, which contains a uranium-plutonium mix and which previously lost the upper levels of its outer containment structure to a hydrogen explosion during steam release Monday morning, suffered a second explosion which apparently tore open its inner containment structure, releasing higher quantities of radiation than any previously experienced in the vicinity of the plant.  Reactor No. 3 began venting large quantities of steam, and after a while Reactor No. 1 began smoking.  Attempts to keep spent nuclear fuel rods at Reactor No. 4 submerged again failed, and the rods again ignited a fire there; 2 plant workers were apparently lost when this fire broke out, though details are not yet available.  In a report on “the faceless 50” taking most of the risk to control rising temperatures and prevent radiation leaks at Fukushima, the New York Times reports that a total of 5 TEPCO plant workers have died at Fukushima since the trouble began on Friday.  Radiation levels in the air in the vicinity of the plant reached about 1,000 millisieverts per hour.  Exposure to 1,000 millisieverts of radiation in an hour will produce radiation sickness.  The Japanese government, which had overtaken operations at Fukushima I from TEPCO, ordered the remaining 48 plant workers to come back to avoid prolonged exposure to radioactive vapors.

During the hours of human inactivity at the plant, Reactor No. 3 steamed and Reactor No. 1 smoked.  Reactor No. 2, site of the most-troubling deterioration until a reactor with plutonium fuel suffered an interior breach, continued heating up (and probably melting-down) unchecked.  As they left the increasingly-dangerous plant after a long day’s struggle on their own, the plant workers were unable even to confirm that their second attempt to put out the fire by the spent fuel rods in Reactor No. 4 was successful.  Shortly before they left, there was evidence that Reactors 5 and 6, powered-down before the earthquake, were heating up without functioning coolant loops.

During these hours when Fukushima I was abandoned, the whole plant filled with steam until it was almost impossible to see anything there.

The Japanese government’s desperate plan to use a helicopter to airdrop water onto the overheating and exposed spent fuel rods at Reactor No. 4 was canceled on Wednesday–though it was tried to little effect on Thursday; the 48 plant workers returned to the plant late Wednesday after the radioactive steam cleared somewhat.  Overall radiation levels have fallen again to 1.5 millisieverts–far-above background radiation but no longer dangerous.  So, this is where the situation stands on Wednesday in Japan, 5 days after the most-powerful earthquake to strike Japan in over 1,000 years, as 4 dozen nuclear plant workers try to flood 3 increasingly hot and damaged nuclear reactors with seawater and boron in an attempt to cool their fuel rods and further-inhibit their process of radiative heating.  The Liberal Ironist isn’t optimistic about their prospects for getting this situation under control.  It is no longer clear to me that writing these reactors off while there is a prevailing easterly wind wouldn’t be a better strategy.  But in increasingly-unsafe conditions, these dedicated and diligent plant workers confront the worst nuclear incident at Chernobyl–trying, with depressingly-little success, to stop it from getting much worse each day.  The New York Times now makes regular updates on the status of each reactor at Fukushima I on its website.

Fukushima I on Tuesday, March 16, 2011 in aerial view looking west. From left to right: Reactor No. 4, where there was no fuel in the core but spent fuel rods were overheating and releasing radiation; Reactor No. 3, which contains some plutonium but at this time had only suffered a hydrogen explosion in the upper decks; Reactor No. 2, which that morning experienced a core breach that exposed the structure to radiation; Reactor No. 1, the oldest and the first to heat to the point of a hydrogen explosion. The visible damage wasn't caused by the earthquake or the tsunami, but by the inability of the plant's many desperate workers to find an efficient means of cooling these reactors over the previous 4 days. By this time all 4 reactors were overheating, and TEPCO didn't have a long-term plan besides dousing them continually in the seawater-boron mix. Photo by DigitalGlobe-Imagery.

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3 thoughts on “Meltdown at Fukushima I Nuclear Power Plant

  1. Pingback: With the World Watching, a Pot Boils | The Liberal Ironist

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