EMERALD1: A Systematic Study of Cross Section Library Based Discrepancies in LWR Criticality Calculations

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EMERALD1 A Systematic Study of Cross Section
Library Based Discrepancies in LWR Criticality
Calculations

• Jaakko Leppänen
• Technical Research Centre of Finland
• VTT PROCESSES / Nuclear Energy

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Outline

• Background
• Calculations, tools and methods
• Example results

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Background

• The MCNP Monte Carlo transport calculation code
  is routinely used at VTT for various reactor
  physics calculations.
• A few years ago, some validation calculations on
  newly acquired cross section libraries showed
  relatively large cross section library based
  differences in criticality calculations.
• It was decided to look into these differences
  more thoroughly, in order to find out what kind
  of uncertainties can be expected in common LWR
  calculations.
• The main conclusions of a literature survey and
  preliminary comparison calculations were that the
  differences can be 1) large, 2) systematic.
• The study was continued and the differences
  investigated in a systematic manner.

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Calculations

• Instead of using realistic geometry models, an
  infinite LWR pin-cell lattice was chosen as the
  starting point for various reasons
• The simplicity allows straightforward and
  systematic study of the most significant sources
  of discrepancies.
• The system parameters can be varied easily and
  the impacts on the results assessed.
• Most of the comparison calculations encountered
  in the literature used realistic models there
  was no need to duplicate the results.
• Since the differences seemed to be strongly
  dependent on flux spectrum, fuel-to-moderator
  ratio was chosen as the main free parameter in
  the system.
• Various modifications of the basic lattice were
  also studied (finite lattice with leakage,
  burnable and control absorber pins, high-burnup
  fuel, etc...).

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Calculation tools

• The comparison calculations were carried out
  using MCNP version 4C.
• Burnup calculations were carried out using
  Monteburns 1.0 (MCNP-ORIGEN coupling).
• The cross section data libraries were generated
  using the NJOY-99 nuclear data processing system
  from five evaluated nuclear data files
• ENDF/B-VI.8 (USA 2001)
• JEFF-3.0 (NEA Databank 2002)
• JENDL-3.3 (Japan 2002)
• JEF-2.2 (NEA Databank 1993)
• JENDL-3.2 (Japan 1994)

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Calculation methods

• In addition to a direct comparison of
  multiplication factors, an analytic method based
  on the detailed neutron balance of the system was
  used (next slide).
• Only the most significant isotopes in the fuel
  were included in the study
• U-235, U-238 and O-16 in the regular lattice.
• Gd isotopes in the modified lattice with burnable
  absorber pins.
• Boron, Ag, In and Cd isotopes in the modified
  lattice with control rods.
• U-236, the most significant plutonium and minor
  actinide isotopes and fission products in the
  regular lattices with high- and low-burnup fuel.
• Various sensitivity studies were also performed
  on the results (fuel enrichment, fuel and
  moderator temperature, chemical shim, level of
  heterogeneity, unresolved resonance probability
  treatment).

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Calculation methods

• The multiplication factor can be written as a
  function of simple source and sink terms.
• Each term consists of group-wisereaction rates
  that can be easilycalculated using MCNP.
• The impact of small differences in thegroup-wise
  reaction rates can beassessed by linearising k.
• Four energy groups were used in the calculations
  (thermal, resonance, unresolved/slowing-down,
  fast fission).
• Various significant contributions were separated
  from the overall differences.
• The total differences were predicted correctly by
  summing up the individual terms.

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Example results
Figure 1 Total reactivity differences between
the libraries. Comparison to ENDF/B-VI.8.
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Example results
Figure 2 The most significantly contributing
terms. Results of JEF-2.2 compared to ENDF/B-VI.8.
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Example results, summary

• The essential results can be summarised as
  follows
• There are significant cross section library based
  discrepancies in LWR criticality calculations.
• These discrepancies are systematic and strongly
  dependent on the level of neutron moderation.
• The most significant contributors are the fission
  and the radiative capture rates of U-235 and
  U-238, especially in the resonance region.
• The compatibility between two libraries depends
  mostly on how well the individual discrepancies
  cancel each other out.
• The differences are not particularly dependent on
  small deviations in system parameters (except
  fuel enrichment).
• These discrepancies should be taken into account
  in all calculations as an additional source of
  uncertainty.

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Thank you for your attention !