1. Convergence in infinite rod Extrapolation length influence
NEA/NSC/WPNCS EGAMCT
Convergence in infinite rod
Extrapolation length influence
Paris 28 June 2017
Dennis Mennerdahl, EMS

2. “Long” infinite rod Fissions not converged?
2015 and 2016 meeting presentations suggest better convergence for “short infinite rod” than for a “long infinite rod”.
EMS SCALE/KENO V.a calculations (several billion histories):
Unreliable KENO V.a restart results for fission densities make conclusions difficult
Planned presentation of results cancelled

3. No problem! Infinite length rod: Only one relevant zone
Long rod, subdivided into multiple zones, gives more information on neutron transport
All information from short rod calculation available from long rod results
General: EG should express what is meant by under-sampling, etc. Some regions have no relevance for requested results

4. Dominance ratio
Several EG AMCT meetings and Monte Carlo papers on Dominance Ratio:
Extrapolation length ignored!
Symmetry: Zero flux at symmetry line!
Different dominance ratios for different rod lengths
Green’s function: Extrapolated length required:
Symmetry line: Total extrapolated length = 2*(L/2+l)
Infinite rod: L + 2*l is infinite

5. Symmetry – Added tallies
The draft final report and earlier presentations:
Symmetry -> tally of individual position is not calculated, only the sum of symmetric tallies
“True uncertainties” based on repeated runs appear to be based on non-converged tallies
¼ symmetry: Away from centre -> Sum of 4 well separated tallies. Near centre -> 4 close tallies, essentially only one (larger variations)