In fact, the top portion of the fuel units became exposed and overheated, leading to the formation of hydrogen gas in a reaction of steam with the zirconium fuel rod casing. Surrounding the reactor is a bell-shaped concrete structure, the primary containment, built to prevent radiation that escapes the reactor from reaching the atmosphere outside the plant.
As hydrogen continued to form, relief valves in the primary containment structure may have opened allowing the gas to exit the primary containment and concentrate in the secondary containment building that surrounds the primary containment, according to U.S. experts familiar with the Japanese reactors.
Compounded by hydrogen explosions
In the General Electric Mark 1 reactor design, the secondary containment structure --a large factory-like outer building --is designed to hold small amounts of gas escaping from the reactor and its primary containment, said David Lochbaum of the Union of Concerned Scientists. But the hydrogen gas concentrated at the top of Unit 1 on Saturday. Something ignited it and the explosion blew off the metal roof over unit 1 at 3:36 pm local time, in a scene viewed worldwide on news reports and social media. The same reaction is believed to have caused the second explosion, at Unit 3, on Monday.
In both cases, the concrete primary containment shell was not damaged, officials reported. However there have been no reports of what happened to metal tanks at the top of the secondary containment building that contains the spent fuel rods, which continue to give off heat and must be cooled by circulating water, Lochbaum said.
"One of the challenges is that the explosion took the roof away. If any of it fell into the [spent fuel] pool, it could either have damaged the fuel," or impaired the water circulation in the pool. "That's the concern that's in play," Lochbaum said Sunday.
Around 8 p.m. Saturday in Japan, Cabinet Secretary Edano announced a high-level decision had been made to inject seawatater into Unit 1. "We've decided to fill the reactor container with sea water. Trade Minister Kaieda has instructed us to do so. By doing this, we will use boric acid to prevent criticality," he said, according to Reuters.
Edano said it the operation would take five to 10 hours initially, and around 10 days to complete the process. Precise details of how the sea water was being piped into the reactor facility were not available Sunday, and it wasn't clear whether crews had succeeded in putting it into the reactor vessel as well as the surrounding primary containment shell.
On Sunday, the same emergency measures were required at the Fukushima Daiichi Unit 3. Toyko Electric (Tepco) said it was unable to restart the reactor's high-pressure water injection system that supplies the reactor core with coolant after it shut down. Tepco then notified the Japanese government of the second emergency at the plant, according to World Nuclear News.
Japan's Nuclear and Industrial Safety Agency (NISA) reported late on Saturday that a gauge measuring water levels inside the No. 3 reactor appeared to be malfunctioning. It was reporting that the tops of the fuel assemblies were uncovered by water and thus vulnerable to failing.
"It is unknown whether [the reading] is real or not," the Japan Atomic Industry Forum (JAIF) said, according to press reports. Other readings contradicted the gauge information, indicating that the threat to the fuel rods was not acute. The order to introduce sea water followed.
Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC. www.eenews.net, 202-628-6500