With the cooling system down, the water that surrounds the nuclear fuel rods began to burn off.

在冷却系统失灵的情况下，核燃料棒周围的水开始蒸发。

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But the cooling water has not been restored to its original level, it was judged nuclear fuel rods are still exposed.

但冷却水位尚未恢复到原有水平，据判断核燃料棒仍然暴露。

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A meltdown occurs when nuclear fuel rods cannot be cooled, thus melting the reactor core and causing a release of radioactivity.

如果核燃料管无法降温，就会发生熔毁，到时反应器的内核也会熔化，从而引起核泄漏。

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Iran also reported this month that it had begun installing domestically made nuclear fuel rods into the Tehran research reactor.

WSJ: Iran Is Ramping Up Nuclear-Fuel Output

The nuclear industry has about doubled the potency of nuclear fuel rods since 1970, allowing plants to stay online longer between refueling outages and to produce less waste.

FORBES: Fancy New Lids for Nuclear Waste Casks, As Contents Get Hotter

The system cooling water which contained spent - but still highly radioactive - nuclear fuel rods failed and it took engineers some 30 hours to repair the damage.

BBC: Fukushima: Rat linked to outage at Japan nuclear plant