Gap Partial Pressure

This model computes the partial pressure in the fuel–cladding gap based on thermodynamic equilibrium relations. The intent is to represent oxidizing conditions in the gap atmosphere when stoichiometry deviation effects are activated.

Reference

Lewis et al., Journal of Nuclear Materials 227 (1995) 83–109.

D.R. Olander, Nuclear Technology 74 (1986) 215.

Inputs

The model uses:

  • Temperature (history variable)

  • Steam pressure (history variable)

  • iStoichiometryDeviation (input flag)

Outputs

  • Gap oxygen partial pressure (sciantix variable, atm)

Model activation

The model is executed only if:

iStoichiometryDeviation = 1

If the flag is not active, the calculation is skipped.

Model formulation

An equilibrium constant is defined as:

\[K = \exp\left( -\frac{25300}{T} + 4.64 + 1.04\left(0.0007\,T - 0.2\right) \right)\]

where \(T\) is the temperature (K).

The oxygen partial pressure in the gap is computed as:

\[p_{\mathrm{O}_2}^{\mathrm{gap}} = \left( \frac{K^2 \, p_{\mathrm{H_2O}}^2}{4} \right)^{1/3}\]

where \(p_{\mathrm{H_2O}}\) is the steam pressure (atm).

Implementation

The computed value is assigned to the variable Gap oxygen partial pressure (atm) at each time step.