Power Plant Engineering
A nuclear unit becoming critical means

It is generating power to rated capacity
There is danger of nuclear spread
It is capable of generating much more than rated capacity
Chain reaction that causes automatic splitting of the fuel nuclei has been established

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Power Plant Engineering
In natural uranium, the constituents of three naturally occurring isotopes are

U₂₃₄ = 0.006%, U₂₃₅ = 99.282% and U₂₃₈ = 0.712%
U₂₃₄ = 0.712%, U₂₃₅ = 0.006% and U₂₃₈ = 99.282%
U₂₃₄ = 0.006%, U₂₃₅ = 0.712% and U₂₃₈ = 99.282%
U₂₃₄ = 99.282%, U₂₃₅ = 0.006% and U₂₃₈ = 0.712%

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Power Plant Engineering
Natural uranium is made up of

99.282% U₂₃₅, 0.712% U₂₃₈, 0.06%' U₂₃₄
99.282% U₂₃₅, 0.712% U₂₃₄, 0.006% U₂₃₈
99.282% U₂₃₈, 0.712% U₂₃₅, 0.006% U₂₃₄
99.282% U₂₃₄, 0.712% U₂₃₈, 0.006% U₂₃₅

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