exotic nuclei structure and reaction noyaux exotiques ... - IPN - IN2P3

simulation of flow is based on three or four partial
differential equations: conservation of mass, energy
and momentum vector for the water liquid/
vapor mixture (optionally a fourth equation can be
added which tracks the vapor mass separately).
The heat transfer model features a full boiling
curve, comprising the basic heat transfer regimes:
single phase forced convection, sub-cooled nucleate
boiling, saturated nucleate boiling, transition
and film boiling. Heat conduction in the fuel, gap
and cladding is calculated using the balance equation.
To obtain a realistic description of a coupled neutronics/thermal-hydraulics
system, a number of
iterations with upgrades of data are
blies. Inlet pressure is also uniform 15.5 MPa. All
radial and axial powers are normalized to average
value. A uniform core flow of 82.12 kg/s/assembly
is assumed.
For each assembly, there are three types of pins:
UOX, UOX + IFBA (IFBA is burnable absorber:
necessary to obtain converged results. The number
of iterations required for a fixed number of
Monte-Carlo histories depends on the specific details
of each system. The more the system is
asymmetric the greater the number of iterations
required. For a pressurized water reactor (e.g.
PWR), such as the one used for the benchmark
(see below), less than ten iterations are needed.
For a fast reactor (e.g. SFR), fewer than five are
needed, and for a boiling water reactor (e.g. BWR)
in which the void fraction has a huge gradient with
correspondingly large changes in the flux more
than ten iterations are needed for convergence.
To validate the 3D coupling MURE-MCNP/
COBRA, a PWR MOX/UO2 NEA benchmark has
been chosen 10 . It presents a large heterogeneity
which permits a rigorous test of the coupling. Four
degrees of enrichment are present in the fuel with
seven different levels of fuel depletion. Some fuel
pins contain integrated burnable poisons. All of
these aspects strongly affect the flux profile, which
will be highly asymmetric. The MURE coupling allows
heterogeneous axial and radial fuel compositions.
However in this benchmark the core axial
material compositions are assumed to be homogeneous.
The benchmark for four assemblies is presented
in the next section. The quarter core used
for the benchmark has been cut. It correspond to
the assemblies A1 (top left – burn-up 35 GWd/t),
A2 (top right – burn-up 0.15 GWd/t), B1 (bottom
left – burn-up 0.15 GWd/t) and B2 (bottom right –
17.5 GWd/t). Consequently reflective conditions
are imposed on lateral boundaries, and void is assumed
on extreme axial planes (zero flux).
Coolant inlet temperature is uniform for all assem-
zirconium diboride ZrB 2 , cf. fig. 10), and guide
tubes. Similar pins have been grouped together to
accelerate the calculation. The choice of these cell
groups have been performed for neighboring cells,
and cells which are exposed to similar fluxes.
A sample of the benchmark results are shown in
Figs. 4 & 5. All results of axial power deposition
are in good agreement (less than 1%) with the
other simulations of the benchmark. Results of the
coupled calculation MURE-MCNP/COBRA are in
agreement with benchmark results (cf. fig. 12),
and the accuracy is within the errors of the benchmark:
2%). Only the four most peripheral pins have
a difference of 4% in our calculation. These are the
most difficult to simulate, due to the assumption of
reflective conditions.
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