Chemical properties:

A dimensional stability of the fuel under conditions of high temperature and high burnup is of main importance in determining the usable lifetime.  A dimensional stability is compromised through swelling, that constricts the coolant channels and might lead to rupture of the metal cladding and escape of highly radioactive fission products into the coolant. The several other factors leading  to  the  degradation  of  fuel  performance  as  reactor  life  proceeds  (the  exhaustion  of fissionable material, an accumulation of nonfissionable products and the accumulation of radiation effects on the relatively nonfuel materials) are all of secondary importance in comparison to dimensional stability of the fuel components.

The primary cause of fuel component swelling is the accumulation of two fission product atoms for every atom fissioned. This is aggravated through the reality in which a few of the fission products are gases. An ability of a ceramic fuel to retain and accommodate fission gases is thus of primary importance in determining core lifetime.

The chemical properties of a fuel are also significant considerations. The fuel should be able to resist the wholesale change in its properties, or the destruction of its mechanical integrity and which may take place if it is exposed to superheated coolant water by a cladding failure. Alternatively, certain chemical reactions are desirable.

Other materials such as zirconium and niobium in solid solution might be deliberately incorporated in the fuel to alter the properties to those required for the reactor design. Also, it is commonly benefits for a few of the products of the nuclear reaction to remain in solid solution within the fuel, rather than accumulating as separate phases.