Burnup calculation capability in the PSG2/serpent Monte Carlo reactor physics code

Serpent is the new version of the PSG continuous-energyMonte Carlo reactor physics code, developed at VTT Technical Research Centre of Finland since 2004. The code is mainly intended for lattice physics calculations, such as group constant generation for coupled few-group nodal diffusion codes. The capabilities of Serpent have recently been extended to fuel cycle studies and the modeling of irradiated fuels by introducing built-in burnup calculation routines. This paper presents the methodology used for burnup calculation. The code has two fundamentally different options for solving the depletion equations: 1) the Transmutation Trajectory Analysis method (TTA), based on the analytical solution of linearized depletion chains and 2) the Chebyshev Rational Approximation Method (CRAM), an advanced matrix exponential solution developed at VTT. The results are compared to deterministic CASMO-4E calculations. The lack of computing power is still today a major factor limiting the practical use of the Monte Carlo method for burnup calculation. The Serpent code uses special techniques for reducing the overall calculation time enough for the method to become a viable alternative to deterministic assembly burnup codes.