Nuclear Power Engineering
The atomic mass of an element is fractional, because

It may have isotopes
It may have isobars
Of uncertainty principle
It contains neutrons

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Nuclear Power Engineering
Secondary cooling circuit is a must in molten sodium cooled fast breeder reactor for

Avoiding the mixing of water with radioactive sodium, as it may cause explosion
Lowering the coolant circulation pressure
Faster heat removal rate from the core
Achieving high degree of superheat in the steam

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