Program - Brookhaven National Laboratory

PR 43
Innovative Reactors and Nuclear Data Needs
R. Jacqmin, CEA, DEN, Cadarache, 13108 Saint-Paul-lez-Durance, France..
A large spectrum of advanced nuclear reactors, ranging from evolutionary concepts to very innovative
(e.g., GEN-IV or dedicated transmutation) systems using novel fuels and materials, are being studied by
different organizations. The characteristics of such systems vary widely, depending on the design criteria
and intended applications. For the most innovative of these concepts, core physics and safety simulations,
using the latest neutron data and codes, point to the poor quality of nuclear data as the leading cause
of larger-than-desired uncertainties in key engineering parameters. As a consequence, to preserve the
targeted performance and avoid excessively conservative design margins, reactor physicists and engineers
request substantial improvements in evaluated nuclear data files, sometimes without sufficient distinction,
not realizing that these can translate into major nuclear physics measurement and modelling challenges.
Assessing the ”necessary and sufficient” improvements in evaluated nuclear data for innovative reactors
is therefore a central issue. However, this is not as straightforward as one might expect, for reasons
which are detailed in this paper, along with illustrative examples. Indeed, even though the theory and
methods for error propagation are rather well established, difficulties still arise because the assessment
process entails a number of assumptions and restrictions, the importance of which is not always fully
appreciated. These include: (i) The strong dependence on the particular system under study and its
detailed characteristics. The example of two large SFRs is given showing that, depending on the detailed
core design, the computed uncertainties in major core parameters can differ by a factor of 1.6 to 4, for
the same nuclear data file; (ii) The particular choice of design parameters, which can be complicated
functions of nuclear and other data, and the corresponding targeted maximal uncertainties. Top-priority
quantities impacting the feasibility or viability of a reactor concept are not always distinguished from
less-critical quantities relating to performance optimization; (iii) The evaluation of missing a priori nuclear
data uncertainties and associated correlations, with sufficient confidence so that they can be used reliably
in error propagation calculations. Examples show that incomplete correlation information can lead to
underestimation of some parameter uncertainty by a factor of two; (iv) The necessary compromise, or
redistribution of effort, between several possible nuclear data improvement pathways, for the same end
result; (v) The specific methods used to perform sensitivity, perturbation, and impact calculations. For
these reasons, the nuclear data improvement needs assessed independently by different groups can differ
notably, even when they relate to similar systems.
PR 44
Effect of Variations in Iron Cross Section in Thermal Region On VVER-1000 Neutronics
Michal Koˇsˇtál, Ján Milčák, Vojtěch Rypar 1, Vlastimil Juˇríček, Marie Svadlenková, Research Centre Rez,
Czech Republic. Antonín Kolros, CTU in Prague, Faculty of Nuclear Sciences and Physical Engineering,
Department of Nuclear Reactors, V Holeˇsovičkách 2, 180 00 Prague 8, Czech Republic.
The cross section of iron has an important impact on thermal neutron flux distribution in reactor pressure
vessel. The thermal neutrons affect RPV activation, thus influence the composition of RPV and the
distribution of originated nuclides. Moreover this cross section influences calculated prediction of pin power
density in the boundary row of fuel pins. The relevance of this quantity is in fact that fuel conditions are a
very important criterion reflected in limits and conditions of reactor operation, which must not be violated.
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