Workers at the crippled nuclear station north of Tokyo are pumping nitrogen into a reactor to try to prevent more explosions, as the US warned a pressure vessel containing radioactive fuel may have been damaged.
Tokyo Electric Power Co started injecting nitrogen gas into a containment vessel of the Number 1 reactor at its Fukushima Daiichi nuclear plant from 1:30 am, spokesman Yoshinori Mori said. The pumping will continue for about six days, he said.
“It takes a lot of time as they are manually injecting nitrogen through a very narrow pipe,’ Tadashi Narabayashi, a professor of nuclear engineering at Hokkaido University in northern Japan, said today by phone. ‘‘High radiation levels in the building are also making it difficult as workers have to keep rotating.’’
Almost four weeks since an earthquake and tsunami damaged the station, the company is still using emergency equipment to try to cool reactors. Tokyo Electric is trying to prevent another hydrogen explosion at the plant after three blasts damaged reactor buildings and released radiation into the air.
‘‘When the nitrogen concentration rises to a certain level, there will be simply combustion without an explosion,’’ Narabayashi said. ‘‘With higher concentrations, combustion stops. They are probably targeting this level.’’
Damage to the pressure vessel on reactor Number 2 raises the risk of further releases of radiation, according to the US Nuclear Regulatory Commission.
‘‘After you lose the vessel, then you are down to one final barrier, that’s the containment,’’ Martin Virgilio, the US agency’s deputy director for reactor and preparedness programs, told reporters after a House of Representatives hearing on the disaster yesterday. The containment unit surrounds the pressure vessel.
The commission warned that some fuel rods at the Fukushima plant, which has six reactors, had melted and that salt build-up in spray nozzles was probably impeding the flow of cooling water.
The assessment was written on March 26 by the commission’s reactor-safety team and sent to its engineers in Japan who are helping stabilize the reactors and spent-fuel pools, which emitted radiation after power was shut off by the earthquake and tsunami on March 11.
The report, labeled ‘‘For Official Use,” recommends ways engineers at the site should attempt to keep the reactors cool and minimize the risk of further explosions, such as by injecting nitrogen, an inert gas, into containment vessels.
The document also said workers should “consider the water weight on seismic capability of containment,” a reference to the possibility that an aftershock may wreck reactor containment structures that have been flooded with water.
Virgilio said the commission doesn’t think the “core has breached,” which would let radiation escape. The commission gets reports several times a day from agency staff in Japan and none mentioned a breach, he said.
