The surface temperature drop to the coolant is calculated from an empi­rical heat-transfer coefficient that depends on the channel geometry, the fuel element and its cladding, and the coolant velocity and thermodynamic properties. There are a number of such correlations for different coolants and different ranges of operation.

Water. The Dittus-Boelter correlation (7a) is

Na = 0.023(Re)»-8(Rr)»-33 (1.36)

Sodium. The main correlations all follow a general equation of the form (7b)

Na = A + 0.025(Re)os(PI)°-8 (1.37)

and in both cases the heat-transfer coefficient h can be calculated from the Nusselt number Na, given by

Na = hdJkA (1.38)

These correlations usually result in the heat transfer being proportional to flow to the 0.8 power because of the Reynolds number dependence.

Internal heat-transfer coefficients are calculated from the conductivities combined as in Eq. (1.25). The main uncertainties occur in not knowing the condition of the fuel or the fuel/cladding gap.

Alternatively heat-transfer coefficients can be calculated from the steady — state temperature distributions for a given heat input [Eqs. (1.29)—(1.32)] if they are known from experimental measurements.