*3.1.3 Zircalloy sheath conductivity and temperature drop across the coolant*

Zircalloy sheath heat transfer coefficient hsis calculated based on Eq. (9) [12].

$$\mathbf{h}\_s = \left[ 7.5\mathbf{1} + 2.09 \times 10^{-2} - 1.45 \times 10^{-5} \mathbf{T}^2 + 7.67 \times 10^{-9} \mathbf{T}^3 \right] \tag{9}$$

Where, hs is in W/mK and T is in K. Coolant heat transfer coefficient hcf is calculated by the Dittus-Boelter correlation [10] as shown in Eq. (10).

$$\mathrm{Nu} = \left(\frac{\mathrm{hDe}}{\mathrm{k}}\right)\_{\mathrm{b}} = \mathrm{C} \left(\frac{\mathrm{DeG}}{\mu}\right)\_{\mathrm{b}}^{0.8} \left(\frac{\mathrm{c\_{p}}\mu}{\mathrm{k}}\right)\_{\mathrm{b}}^{\mathrm{n}} \tag{10}$$

Where,

h = heat transfer coefficient, Btu/(hrft<sup>3</sup> °F); De = equivalent diameter, ft.; k = thermal conductivity of fluid, Btu/hr. ft. °*F*; cp = specific heat of fluid, Btu/lb.; G = mass velocity, lb./hr. ft<sup>2</sup> ; μ = fluid viscosity, lb./hr. ft.; De G <sup>μ</sup> > 10, 000 and <sup>L</sup> De >60; B = bulk conditions. All other terms have their standard notational convention.
