This creates what is known as a critical mass of nuclear material, which releases its energy instantaneously as atoms inside it split in an uncontrolled chain reaction.

这会造成核原料所谓的临界质量，当其中的原子在无法控制的连锁反应中***时，能在瞬间开释出能量。

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In Japan in 1999, an uncontrolled nuclear chain reaction at a uranium-reprocessing plant killed two employees and spewed radioactive neutrons over the countryside.

WSJ: Nuclear Issues Put Focus on Japan Regulators

Or they can be introduced with uranium 235, which in this process has a much lower risk of an uncontrolled reaction than it does in today's nuclear plants.

WSJ: After Japan's Earthquake and Fukushima Disaster, Matt Ridley on Gas-Fired Generation and Thorium | Mind & Matter