Early this morning Japan time, the Tokyo Electric Power Company (TEPCO) began bleeding nitrogen gas into the heavily damaged reactor 1 at Fukushima Daiichi. As we discussed yesterday, the purpose of the gas is to reduce the chances of another massive explosion at the reactor. The inert nitrogen will, in theory, keep volatile hydrogen and oxygen from mixing inside the containment vessel and reactor and prevent another explosion.
The threat of a hydrogen blast is among the most serious facing the reactors. After a tsunami struck the complex on 11 March, the reactors heated up and their fuel began melting. Zirconium cladding around the fuel reacted with steam inside the reactor, creating hydrogen gas. That gas exploded when it came in contact with oxygen from the atmosphere, blowing apart the buildings around units 1 and 3 and crippling the containment vessel of unit 2.
There’s good reason to believe that there’s still a fair bit of hydrogen inside the reactors, and it could explode again. TEPCO has offered an explanation that isn’t entirely clear, but here’s my take: Initially the reactor cores were quite hot, but they’ve gradually been cooling over the past week or so. As they’ve done so, the steam in the heavy containment vessels surrounding the reactors has condensed, possibly creating a hydrogen-filled area of negative pressure at the top of the containment vessel. Depending on how airtight the reactors are, oxygen could leak in and trigger another explosion. The US Nuclear Regulatory Commission was apparently deeply concerned about this threat ten days ago, and recommended injecting nitrogen, to fill the void left by the steam.
It’s no surprise that the delicate operation is proceeding slowly. According to Kyodo news, TEPCO injected just 200 cubic meters of nitrogen between 1:30 and 9:50 local time. Over the next six days they hope to inject a total of 6,000 cubic meters into the reactor. Depending on how it goes, they may try something similar for reactors 2 and 3.
For full coverage of the Fukushima disaster, go to Nature’s news special.
Credit: AP Photo/AIR PHOTO SERVICE