Convergence Testing & Acceleration in MCNP5 Criticality Calculations
- Los Alamos National Laboratory (LANL), Los Alamos, NM (United States)
Monte Carlo criticality calculations use the power iteration method to determine the eigenvalue (keff) and eigenfunction (fission source distribution) of the fundamental mode. Determining convergence of Monte Carlo criticality problems is complicated by the statistical noise inherent in the random walks of the neutrons in each generation. The latest version of MCNP5 incorporates an important new tool for assessing convergence: the Shannon entropy of the fission source distribution, Hsrc. Shannon entropy is a well-known concept from information theory and provides a single number for each iteration to help characterize convergence trends for the fission source distribution. MCNP5 computes Hsrc for each iteration, and these values may be plotted to examine convergence trends. For criticality problems where a distribution of results is required (e.g., power density, dose rates, etc.), convergence testing should include both keff and Hsrc, since the fission distribution will converge more slowly than keff, especially when the dominance ratio is close to 1.0. A recently proposed method for accelerating convergence of the Monte Carlo power iteration using Wielandt's method has been implemented in a test version of MCNP5. The method is shown to provide dramatic improvements in convergence rates and to greatly reduce the possibility of false convergence assessment. While further testing is needed, and work on implementing it into a production version of MCNP5 is continuing.
- Research Organization:
- Los Alamos National Laboratory (LANL), Los Alamos, NM (United States)
- Sponsoring Organization:
- USDOE National Nuclear Security Administration (NNSA), Nuclear Criticality Safety Program (NCSP)
- DOE Contract Number:
- AC52-06NA25396
- OSTI ID:
- 1235226
- Report Number(s):
- LA-UR-07-1471
- Resource Relation:
- Conference: 8. International Conference on Nuclear Criticality Safety (ICNC-2007), St. Petersburg (Russia), 28 May - 1 Jun 2007; Related Information: https://www.ans.org/meetings/view-6/
- Country of Publication:
- United States
- Language:
- English
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