What is polymorphism? Give at least three examples, b) Comment on why polymorphi

Question

What is polymorphism? Give at least three examples, b) Comment on why polymorphism of Zr may hinder the use of Zr alloys as a cladding material for advanced high temperature nuclear reactors, c) List (making a table) with a neat sketch of the unit cell, the various parameters (no. of atoms per unit cell, coordination number, relation between lattice constant, a, with atomic diameter, D, atomic packing fraction) for the cubic crystals (i.e. simple cubic, BCC and FCC) along with two examples for each. (3 + 3 + 6)

Explanation / Answer

Polymorphism is the ability of a crystal to exist in multiple crystal structures. Examples of crystals which show this property are glycine, silica and calcite.

In a coolant less case in a nuclear reactor due to an accident, Zr cladding will rapidly react with water steam at high temperature. Hydrogen gas is released when Zr is oxidized, this oxidation is accelerated at high temperatures and hence Zr alloys at high temperature cannot be used for cladding.

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