Japanese workers measured radiation inside one reactor building of the crippled Fukushima No. 1 nuclear plant early Monday, paving way for full-scale work to stabilize the country's worst-ever nuclear emergency after the twin disasters of earthquake and tsunami.

Junichi Matsumoto, an official from Tokyo Electric Power Co. (TEPCO), shows a picture showing a worker working at the Reactor Building of Unit 1 of Fukushima Daiichi Nuclear Power Station in Fukushima prefecture at the TEPCO headquarters in Tokyo, Japan, May 9, 2011. Japanese workers measured radiation inside one reactor building of the crippled Fukushima No. 1 nuclear plant early Monday, paving way for full-scale work to stabilize the country's worst-ever nuclear emergency after the twin disasters of earthquake and tsunami. [Kenichiro Seki/Xinhua]

The move came after the plant operator Tokyo Electric Power Co. (TEPCO) opened the doors linking the No. 1 reactor building to its adjacent turbine building Sunday evening, and confirmed that the resultant release of radioactive materials into the air had not raised radiation levels on the premises, according to the firm.

Kyodo News quoted TEPCO as saying nine workers went into the reactor building around 4:20 a.m. and measured radiation and other conditions inside for about 30 minutes.

If the radiation level is confirmed to be safe for workers to operate inside, they will start building a new cooling system for the reactor -- the most severely damaged of the six at the plant -- which lost cooling functions in the March 11 quake and ensuing tsunami.

Restoration work at the reactor has been hampered by a hydrogen explosion on March 12 and high radiation levels since.

In the envisaged next step, workers will install and adjust equipment including a heat exchanger and instrument to measure the water levels in the reactor's pressure vessel containing nuclear fuel and the containment vessel shrouding it, Kyodo said.

No rises in the radiation level have been seen at nine observation points in the plant, located some 220 kilometers northeast of Tokyo since the double-entry doors were opened shortly past 8 p.m. Sunday, TEPCO said.