As northeastern Japan coped with Thursday's magnitude 7.1* aftershock, the largest since the disastrous March 11 magnitude 9.0 earthquake and tsunami, the injection of nitrogen gas into one of the crippled reactors at the Fukushima Daiichi nuclear power plant was interrupted as Tokyo Electric Power Company (TEPCo) workers evacuated to a safer site, according to the Japan Broadcasting Corp (JBP). A tsunami warning had been issued briefly but was later canceled.

Nitrogen is being introduced into the containment vessel of reactor No. 1 to counteract a buildup of hydrogen and oxygen caused by partially exposed fuel rods and to prevent another explosion. The fuel rods are nearly half exposed after a loss of cooling water, JBP reports. TEPCo says this latest aftershock did not affect the plant's power supply and no one was injured.

The nitrogen injection is expected to continue for the next six days and may also be performed at reactors No. 2 and No. 3 to keep hydrogen levels under control. Japanese officials say the series of hydrogen explosions at the plant days after the earthquake was the primary cause of the widespread radiation, The Wall Street Journal reports.

The hydrogen buildup is only one of several challenges TEPCo faces at the Daiichi site. The level of highly radioactive water in a concrete tunnel of reactor No. 2 has risen 5 centimeters over a 24-hour period, possibly the result of work on Wednesday to stop highly radioactive water leaking into the sea from a cracked concrete pit, according to TEPCo, which has so far dumped about 7,300 tons of low-level radioactive wastewater into the sea from a storage facility to make room for more highly contaminated water from the reactors. The company says the last 700 tons of water will be discharged by Friday.

Even as TEPCo deals with the troublesome Fukushima Daiichi site, the company has begun work at its Kashiwazaki Kariwa nuclear power station. After tsunami waters caused a loss of power at the Fukushima Daiichi site that touched off that plant's current crisis, TEPCo says it has installed facilities on higher ground at the Kashiwazaki Kariwa site for back-up power supply, injection of water in reactors and spent fuel pools. Located on Japan's west coast facing the Sea of Japan, Kashiwazaki Kariwa features seven reactors and was shut down for 21 months following a magnitude 6.6 earthquake in July 2007.

*Editor's Note (4/7/11): The aftershock's magnitude was originally reported as magnitude 7.4 but later downgraded

First image of Fukushima Daiichi reactors No.s 1 through 4 courtesy of the Japan Ministry of Land, Infrastructure, Transport and Tourism, via Wikimedia Commons

Second image of Tech. Sgt. John Obermuller and a Japan Ground Self-Defense Force member joining two sections of hose on March 26, 2011, at Yokota Air Base, Japan, courtesy of U.S. Air Force photo/Airman 1st Class Krystal M. Garrett. The hoses are part of water pump donated by the U.S. government to help the Japanese government stabilize the Fukushima nuclear power plant.