Assessment of the uncertainties of COBRA sub-channel calculations by using a PWR type rod bundle and the OECD NEA UAM and PSBT benchmarks data

István Panka, András Keresztúri, Centre for Energy Research, Hungarian Academy of Sciences, Hungary

23rd Symposium of AER on VVER Reactor Physics and Reactor Safety (2013, Štrbské Pleso, Slovakia)
Reactor dynamics and safety analysis


Nowadays, there is a tendency to use best estimate plus uncertainty methods in the field of
nuclear energy. This implies the application of best estimate code systems and the
determination of the corresponding uncertainties. For the latter one an OECD NEA
benchmark was set up [1, 2]. The objective of the OECD NEA Uncertainty Analysis in BestEstimate Modeling (UAM) LWR benchmark is to determine the uncertainties of the coupled
reactor physics/thermal hydraulics LWR calculations at all stages.
In this paper the focus is on the determination of the uncertainties of the thermal-hydraulic
analyses of rod bundles. For this purpose a 5*5 bundle with electrically heated rods
representing a 17*17 PWR bundle was selected for these investigations.
In the first part of the paper the modeling uncertainties are evaluated using the OECD
NEA/NRC 3 PSBT benchmark data. The main goal of this section is to determine an
appropriate turbulent mixing factor within its uncertainty for this type of bundle.
After that the uncertainties of the COBRA calculations are discussed using Monte-Carlo type
statistical analyses taking into account the modeling uncertainties and other uncertainties
prescribed in the UAM benchmark specification: e.g. geometrical (diameters, rod pitch)
uncertainties, boundary condition uncertainties like uncertainties of pressure, power, etc. Both
steady-state and transient cases were investigated. The target quantities are the uncertainties
of the void distribution, the moderator density, the moderator temperature and the DNBR. In
many cases both bundle averaged and single sub-channel results are presented.
We will see that – beyond the uncertainties of the geometry and the boundary conditions – it is
very important to take into account the modeling uncertainties in case of bundle or subchannel thermo-hydraulic calculations. Another important and interesting conclusion is the
strong direction-dependence of the target uncertainties in the axial direction.

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