Japanese engineers battled on Friday to cool spent fuel rods and restore electric power to pumps at the stricken Fukushima Daiichi Nuclear Power Station as new challenges seemed to accumulate by the hour, with steam billowing from one reactor and damage at another apparently making it difficult to lower temperatures. As the crisis seemed to deepen, Japans nuclear safety agency raised the assessment of its severity to 5 from 4 on a 7-level international scale. Level 4 is for incidents with local consequences while level 5 denotes broader consequences. It was not immediately clear why the action had been taken. The partial meltdown at Three Mile Island in 1979 was rated 5, and the Chernobyl disaster in 1986 was rated 7. In a further sign of spreading alarm that uranium in the plant could begin to melt, Japan planned to import about 150 tons of boron from South Korea and France to mix with water to be sprayed onto damaged reactors, French and South Korean officials said Friday. Boron absorbs neutrons during a nuclear reaction and can be used in an effort to stop a meltdown if the zirconium cladding on uranium fuel rods is compromised.