Yet Another Explosion: Spent Fuel Overheating

According to TEPCO Press office:

Press Release (Mar 15,2011) Damage to the Unit 4 Nuclear Reactor Building at Fukushima Dai-ichi Nuclear Power Station

At approximately 6:00am, a loud explosion was heard from within the power station. Afterwards, it was confirmed that the 4th floor rooftop area of the Unit 4 Nuclear Reactor Building had sustained damage. After usage, fuel is stored in a pool designated for spent fuel. Plant conditions as well as potential outside radiation effects are currently under investigation. TEPCO, along with other involved organizations, is doing its best to contain the situation. Simultaneously, the surrounding environment is being kept under constant surveillance.

Information is sparse, but here is what I’m piecing together.

The issue at Unit 4 seems to be not the reactor, but rather “spent fuel”, older, used, but still radioactive fuel rod assemblies that are stored in the reactor housing, kept cool by water pumps, but are not contained in the reactor containment vessels. Apparently the Unit 4 spent fuel cooling water ran dry, and excess heat may have caused a fire, producing hydrogen, and leading to another explosion.

New York Times reports this morning:

Though the situation remained dangerous, there were signs that workers had, at least for the moment, contained some of the danger:  The escalated radiation levels of earlier in the day  — possibly from a fire in the No. 4 reactor — stabilized and then declined towards evening, according to Japanese authorities.

Engineers at the plant, working at tremendous personal risk, on Tuesday continued efforts to cool down the most heavily damaged unit, reactor No. 2, by pumping in seawater. According to government statements, most of the 800 workers at the plant had been withdrawn, leaving 50 or so workers in a desperate effort to keep the cores of three stricken reactors cooled with seawater pumped by firefighting equipment, while crews battled to put out the fire at the No. 4 reactor, which they claimed to have done just after noon on Tuesday.

Spent Fuel is stored at many reactor sites. In the US, it’s become the de facto “solution” to rad waste storage because long term storage of spent fuel is lacking.

The Times adds, “Worryingly, temperatures appeared to be rising in the spent fuel pools at two other reactors at the plant, No. 5 and No. 6, said Yukio Edano, the chief cabinet secretary.”

Reports on radiation levels outside the facility are conflicting, but probably it is safe to say, not immediately hazardous to local populations.  One issue that has not been addressed is what kind of isotopes are being released, and what the long term effects of uptake in the biota, soils and food chain might be.

The Times article contains the first clear, quantitative statement I’ve found about radiation levels at the site:

Radiation measurements  reported earlier on Tuesday showed a spike of radioactivity around the plant that made the leakage significantly categorically worse than it had been, with radiation levels measured at one point as high as 400 millisieverts an hour. Even 7 minutes of exposure at that level will reach the maximum annual dose that a worker at an American nuclear plant is allowed. And exposure for 75 minutes would likely lead to acute radiation sickness

In addition, The Times quantified the gravity of the situation at hard hit Unit 2:

Tokyo Electric Power said Tuesday that after the explosion at the No. 2 reactor, pressure had dropped in the “suppression pool” — a section at the bottom of the reactor that converts steam to water and is part of the critical function of keeping the nuclear fuel protected. After that occurred, radiation levels outside No. 2 were reported to have risen sharply.

“We are on the brink. We are now facing the worst-case scenario,” said Hiroaki Koide, a senior reactor engineering specialist at the Research Reactor Institute of Kyoto University. “We can assume that the containment vessel at Reactor No. 2 is already breached. If there is heavy melting inside the reactor, large amounts of radiation will most definitely be released.”