Automated burnup sequence

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Homogenized group constants form the input data for core-level fuel cycle and transient simulator calculations. The data is parametrized according to discrete state-points, which are defined by the local thermal hydraulic conditions together with reactivity control. The process of group constant generation must cover the full range of operating states within the reactor core, which often requires repeating the assembly-level calculation thousands of times. Since the local operating conditions inside a fuel assembly also affect how the materials are depleted, the state-points by which the data is parametrized are not completely independent. The calculations are instead divided into:

  1. Branch variations, taking into account momentary changes in the operating conditions, such as fuel temperature, moderator density and temperature, boron concentration and insertion of control rods
  2. History variations, taking into account conditions that persist for an extended period of time, such as moderator temperature and density, boron concentration and positioning of control rods

In practice, the procedure involves burnup calculations covering all assembly types and history variations. Branch variations are accounted for by performing restart calculations for each burnup point, varying the local operating conditions accordingly.

Serpent provides an automated burnup sequence, capable of performing branch variations for a single history run. The procedure works by first running a burnup calculation, after which a number of restarts are performed for selected burnup points. For each restart the code invokes a number of user-defined variations in the input, corresponding to the branches to different state points.

The available variations in version 2.1.25 include: • Change in material temperature and density • Replacement of one material with another • Replacement of one universe with another • Application of a universe transformation The changes in material temperatures and densities can be used to account for variations in the thermal hydraulic state. The ad- justement of cross section library temperatures is handled using the built-in Doppler-broadening preprocessor routine (Viitanen, 2009),3 and moderator temperature effects by interpolation be- tween S(α, β) tables (Viitanen and Lepp ̈ anen, 2016). Coolant boron branches can be invoked by changing the entire mate- rial, and control rod branches in 2D calculations by replacing the universe, for example, an empty guide tube with a rodded tube. The capability to apply universe transformations allows moving and rotating different parts of the geometry, which is practical for positioning the control rods if the homogenization is performed in 3D.