Workers in Japan are still struggling to limit environmental contamination at the Fukushima plant. The latest problem has been a break that is allowing heavily contaminated water flow directly into the ocean, a leak that has continued despite two attempts to plug it. Meanwhile, worries persist about the state of the reactor cores on the site.

Update: TEPCO, which runs the reactors, has announced the leak has been stopped. The New York Times has also obtained an assessment of the Fukushima reactors performed by the Nuclear Regulatory Commission that highlights some of the challenges facing the cleanup effort.

We'll start with the ocean. As we said in our previous coverage, all indications are that the cooling system for at least some of the reactors is no longer a closed loop. In order to keep the reactors cool, workers at the site are pumping prodigious amounts of water into the cooling system of some of the reactors, where it's either being vaporized into steam, or leaking back out; in either case, chances are good that it's coming in contact with the reactor core first, and will pick up radioactive isotopes then. (CNN indicates that reactor 2 has been receiving 200 tons of water a day.)

Currently, it seems that a lot of it is leaking out, creating highly radioactive pools of water on-site and having made its way into the ocean through a damaged area of concrete. The high levels of radioactivity mean that humans can't work directly on the crack, making what would otherwise be a routine repair about as difficult as fixing an oil well on the bottom of the Gulf of Mexico. Workers have tried to patch the concrete twice and, initially, only managed to reduce the flow of water slightly; the flow has since come to a halt. The extended leakage has meant that, in the area around the damage, radiation levels have reached the point where exposure would be fatal in less than a day, and are millions of times the legal limit.

Fortunately, indications are that the contamination is largely a short-lived isotope of iodine that will decay to background within a matter of months. The sheer volume of the Pacific will also ensure that it will be diluted out to low levels relatively rapidly.

The ability of the Pacific to dilute any radioactivity is also providing a solution to one of the other problems we noted last time: a limited capacity to store contaminated water on site. The plan now is to dump some of the water that only has low levels of contamination. This will free up capacity that will be used to store some of the high-level waste that has been found in the reactor buildings.

While efforts continue to focus on keeping most of the contamination on-site, the question of how to clean up the site itself remains largely unanswered. It's not clear whether there is also significant contamination of groundwater being caused by the same (or other) leaks, or the extent to which a longer-lived cesium isotope, 137Cs, is being released at the same time.

Bigger questions remain about the state of the reactors themselves. As evidenced by a Nature News report, experts appear divided about the degree to which the reactor structure melted down, the shape that the fuel is in, and whether fission chain reactions may be restarting due to the damage. The problem is a general lack of information—nobody has seen the reactors, and people are trying to interpret indirect measures of what has gone on inside. Nevertheless, the consensus is that the cleanup on-site will take years; a manager at the utility has been quoted as saying, "I don’t know if we can ever enter the No. 3 reactor building again."

There has been no doubt for weeks that the Fukushima reactors would never be used again. Our ability to limit long-term contamination in the areas around the site, however, is still an open question. Once that crisis passes, however, Japan will face difficult decisions on how best to handle all the damaged nuclear fuel that, at the moment, remains poorly contained in a seismically active area.

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