Once it has been established that the desired nuclear reaction is feasible in a candidate fuel
material, the effect of other material properties on reactor performance must be considered. For
the reactor to perform its function of producing usable energy, the energy must be removed. It
is desirable for thermal conductivity to be as high as possible throughout the temperature range
of operations and working life of the reactor. High thermal conductivity allows high power
density and high specific power without excessive fuel temperature gradients. The selection of
a ceramic fuel represents a compromise. Though it is known that thermal conductivities
comparable to those of metals cannot be expected, chemical and dimensional stability at high
temperature are obtained.
Because the thermal conductivity of a ceramic fuel is not high, it is necessary to generate
relatively high temperatures at the centers of ceramic fuel elements. A high melting point
enables more energy to be extracted, all other things being equal. In all cases, the fuel must
remain well below the melting point in normal operation, but a higher melting point results in
a higher permissible operating temperature.
The dimensional stability of the fuel under conditions of high temperature and high burnup is of
primary importance in determining the usable lifetime. The dimensional stability is compromised
by swelling, which constricts the coolant channels and may lead to rupture of the metal cladding
and escape of highly radioactive fission products into the coolant. The various other factors
leading to the degradation of fuel performance as reactor life proceeds (the exhaustion of
fissionable material, the accumulation of nonfissionable products, the accumulation of radiation
effects on associated nonfuel materials) are all of secondary importance in comparison to
dimensional stability of the fuel elements.
The main cause of fuel element swelling is the accumulation of two fission product atoms for
each atom fissioned. This is aggravated by the fact that some of the fission products are gases.
The ability of a ceramic fuel to retain and accommodate fission gases is therefore of primary
importance in determining core lifetime.
The chemical properties of a fuel are also important considerations. A fuel should be able to
resist the wholesale change in its properties, or the destruction of its mechanical integrity, that
might take place if it is exposed to superheated coolant water through a cladding failure. On the
other hand, certain chemical reactions are desirable.
Other materials such as zirconium and niobium in solid solution may be deliberately incorporated
in the fuel to alter the properties to those needed for the reactor design. Also, it is generally
advantageous for some of the products of the nuclear reaction to remain in solid solution in the
fuel, rather than accumulating as separate phases.