UPDATE: TEPCO has released more details about conditions inside of the unit 1 reactor. The company also says it will be providing analyses of units 2 and 3 soon.
Last week, workers entered the stricken unit 1 reactor at the Fukushima Daiichi nuclear plant and began work to further stabilize it. One of their first tasks was to recalibrate some of the sensors on the reactor, so that engineers had a better sense of how it was doing. That recalibration has led to a startling revelation: virtually all of the fuel inside the unit 1 reactor appears to have “melted down”.
Press reports on the meltdown have variously described it as a setback and or admission by the Tokyo Electric Power Company (TEPCO) that things are worse than they thought. There’s more about what it all means below the fold (for a broader overview, check out our video on Fukushima).
During normal operation, the core of a nuclear power reactor like unit 1 consists of long narrow tubes of a zirconium alloy filled with uranium fuel pellets. Tubes are bundled together into “assemblies” which in turn make up the core of the reactor. When it’s humming along at full tilt, the core boils water that is used to turn turbines in the adjacent building. After the earthquake and tsunami on 11 March, the water stopped circulating and the core heated up.
At some point, it got hot enough that the zirconium tubes began to split and warp, the beginning of the “meltdown” at unit 1. Nobody can know for sure what happened inside the core, but it appears that the uranium pellets fell out of their assemblies and began gathering at the bottom of the reactor pressure vessel (see diagram).
This much was already known, and TEPCO had suggested that only about 70% of the core had melted down. But after recalibrating its instruments for measuring water levels inside the reactor, the company now believes that the core has entirely melted down.
Some have theorized that with all the fuel at the bottom of the vessel, unit 1 may have actually restarted its nuclear reactions. If that had happened, the fuel would be pumping out some portion of its normal 1380 megawatts of thermal power—probably enough to melt through the thick steel reactor pressure vessel. It would have dropped onto a concrete slab below (the basemat), where it would have hopefully been spread out, effectively diffusing the chain reaction.
There’s some reason to think that this “China syndrome”, as it is informally known, didn’t happen. Nuclear engineers I’ve spoken to say that reactors like unit 1 are finicky beasts. Their fuel needs to be carefully configured to work, and they won’t restart if the stuff is just a gloop on the bottom of the vessel. In addition, workers injected boric acid into the reactor just before the restart. Boron is a neutron absorber and would spoil any nuclear reactions. Moreover, temperature sensors at the bottom of the reactor vessel are continuing to function, suggesting it wasn’t completely destroyed.
That doesn’t mean that portions of the fuel weren’t briefly producing power during the accident. Nor does it mean that the reactor’s fuel has remained wholly inside the pressure vessel. In fact, a note from the Japan Atomic Industrial Forum (JAIF) quotes Banri Kaieda, the nation’s Economy, Trade and Industry Minister, as saying that it is “a fact” that there were holes created by the meltdown. That would likely mean at least some of the uranium fuel is now lying on the basemat below, or perhaps even outside the concrete containment.
Clean up and beyond
Whatever happened inside unit 1, it happened weeks ago. The temperature inside the core is currently around 100C, according to the latest data from the nation’s nuclear regulator—far less than it would be if nuclear reactions were continuing inside the core. But there are still some serious implications for cleanup.
The most immediate problem is for a planned recirculation system to cool the core. The plan was to feed water through two emergency systems, the core spray system and the primary containment’s cooling spray system. Both are normally designed to funnel water into the core in the case of an emergency. Water exits the core through the AC piping system, normally used to inject nitrogen gas, according to Margaret Harding, an independent nuclear consultant who has been going over the plans in detail. From there, the water would flow to an improvised heat exchanger that would cool it before returning it to the core.
The recirculation system would have two big advantages. First, it would create less radioactive wastewater than is currently generated by just dumping water onto the reactor. Second, it would be more efficient, allowing the reactor to reach cold shutdown—a state where it is more-or-less safe in a matter of months.
Unfortunately, despite TEPCO continuously pumping fresh water into the containment vessel, water levels are too low for the system to work – presumably because the water’s surface is below the AC outlet. Nobody really know where all the water is going – but it can’t be anywhere good. If workers can’t find a way to patch up unit 1, then they’ll probably have give up on the plan.
In the longer term, the meltdown makes removing the fuel much more complicated. Normally, the fuel can only be accessed using a massive overhead crane. The crane has already likely been damaged by an earlier explosion, and now, if the fuel is damaged too, it’s unclear whether a replacement crane can easily remove it.
Despite all these setbacks, clean up plans continue. Today, TEPCO announced that it had started initial construction of a cover for the unit 1 reactor. The temporary cover will prevent the spread of radiation, and protect the damaged reactor. Proper construction will begin on 6 June.
By the way, does this mean our article titled “The meltdown that wasn’t” was mistitled? Pretty much, yes. But the point of the article stands: it could have been a lot worse.
For full coverage of the Fukushima disaster, go to Nature’s news special.
For a selection of our coverage in Japanese, see Nature Asia Pacific.
Images: TEPCO/US NRC