Workers at the Fukushima Daiichi nuclear power station returned to the plant Friday after a 7.1 magnitude aftershock forced them to leave lateThursday night.

They returned to the plant Friday morning after the tsunami alert was lifted. The plant's operator Tokyo Electric Power Company (TEPCO) reported no new damage to the crippled facility.

Before the quake, engineers had been injecting non-flammable nitrogen into reactor 1 containment shell to counter a buildup of hydrogen in the chamber. The nitrogen injections are aimed at displacing oxygen in the reactor shell to avoid explosion. Hydrogen buildup is a symptom of overheated fuel rods in the cores of the reactors. Three explosions had been recorded since the magnitude 9 earthquake, followed by a massive tsunami, hit the eastern coast of Japan on March 11.

Hidehiko Nishiyama, spokesman for the Nuclear and Industrial Safety Agency said nitrogen injection and pumping of fresh water into reactors 2 and 3 continued while the plant was evacuated.

Workers at the damaged plant have been struggling to keep the reactors from overheating for four weeks, pouring tons of fresh water an hour into the reactors and trying to contain the spread of radioactive contamination that has been pouring out.

Thursday's quake was centered in the Pacific Ocean about 90 kilometers (56 miles) north of the plant near the epicenter of the magnitude-9 earthquake and tsunami four weeks ago. The latest aftershock caused 3 deaths as of the latest count.

Meanwhile, at the Onagawa nuclear plant, about 140 kilometers to the north, the latest aftershock caused several small leaks of radioactive water that totalled about 15 liters.

The Sendai-based Tohoku Electric Company reported that the leaks came from pools housing spent fuel from the plant's three reactors which have been shut down since the March 11 earthquake. Their radioactivity was far below the threshold that posed a threat to human health, according to data released by the company.#30