Gas Release

Purpose

This routine computes released gas inventories and standard release metrics from the current state of the fuel microstructure.

It does not solve transport equations; instead, it post-processes the gas inventories after diffusion, trapping, and grain-boundary models have been applied.

Physical meaning

At each time step, fission gases are partitioned into:

  • produced

  • decayed (radioactive gases)

  • retained in the grain

  • retained at grain boundaries

  • released to the free volume

The released fraction is obtained by mass conservation.

Algorithm

For each gas species i:

  1. Compute released inventory by mass balance:

    \[N_i^{rel} = N_i^{prod} - N_i^{dec} - N_i^{grain} - N_i^{GB}\]

  2. Enforce non-negativity:

    \[N_i^{rel} \ge 0\]

  3. Compute intergranular gaseous swelling:

    \[S_{IG} = \frac{3}{R_g} N_b V_b\]

  4. Compute integral release metrics (FGR, R/B).

Outputs

  • Xe released, Kr released

  • Fission gas release

  • Xe133 R/B, Kr85m R/B

  • Helium fractional release

  • Helium release rate

Numerical notes

  • Release is computed only for non-restructured matrix systems.

  • Division-by-zero conditions are explicitly guarded.

  • Negative released inventories are clipped to zero.

Relation to other models

This routine depends on:

  • Intragranular diffusion

  • Grain-boundary bubble evolution