Fukushima plantThe nuke plant in Fukushima, Japan.Photo: Beacon RadioThe Daiichi nuclear power plant in Fukushima, Japan, currently has three boiling water reactors in danger of “meltdown.” Here are the basics on what’s happening and what that means.

How does a boiling water reactor work?

  • The nuclear core transfers energy to water by heating it.
  • This cools the core and lets the water carry the energy away to be turned into electricity.
  • If there’s not enough water for cooling, the fuel rods can be exposed and the core can overheat.
  • If the core heats up enough, it can start to melt — that’s what people mean when they talk about a “nuclear meltdown.”

What went wrong?

  • The Fukushima plants were built to withstand a 7.9 magnitude earthquake. Friday’s quake has been upgraded to 9.0.
  • The reactors have multiple backup generators to provide cooling power in an emergency. Any one system could prevent an accident, even if the others failed. But this quake took out all the backups. (For at least one of the reactors, the backup worked at first, then cut out — it’s not clear why.)
  • Because of the loss of cooling power, nuclear cores at three plants have been exposed and there’s some evidence they’ve already started to melt.
  • There have also been hydrogen explosions, from vented hydrogen gas. Those damaged buildings, but not, officials say, the reactors themselves. So the nuclear material is still contained.

What’s the worst-case scenario?

  • Some people worry that in the case of a full meltdown, radiation could seep into the groundwater.
  • There’s no evidence — yet — that that can happen. At Three Mile Island the containment systems held out even though there was a meltdown. Even at Chernobyl, melted radioactive material made it to the basement but no further. The radiation there came from gas from an explosion.
  • There’s a first for everything, though. There are concerns that the containment structures in a boiling water reactor, which are small, are not strong enough to hold out against melted radioactive material.
  • In that case, the material would seep out, and could do a huge amount of damage to the surrounding area.

What are they doing now?

  • The most important thing now is getting water to the core. Tokyo Electric is pumping in seawater and boric acid, which can slow down nuclear fission.
  • The effort has been described as a “Hail Mary pass” that’s unprecedented in the industry.
  • Restoring power to the site could allow the regular cooling systems to start functioning again, but it’s not clear when that will happen.
  • The nuclear reaction has reportedly been shut down completely, but there’s still some residual heat from radioactive decay that could damage the core.

Read more:

How Black is the Japanese Nuclear Swan?,” The Oil Drum
Uncertainty surrounds Japan’s nuclear picture,” BBC News
Nuclear Experts Explain Worst-Case Scenario at Fukushima Power Plant,” Scientific American
Desperate Attempts to Save 3 Fukushima Reactors from Meltdown,” ClimateWire