Page 390 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

(U)RANS pool thermal hydraulics 353

An alternative simplified method, used in STAR-CCM+, takes for granted the for-
mer approach but with a constant heat transfer coefficient. The local heat exchange
volumetric source term is rewritten as S HX ¼(ρC p ) LBE (T T 0 )/τ, where τ is thought
as a characteristic residence time for the LBE to reach about temperature T 0 . Under
nominal condition, T 0 ¼T water , and τ is adjusted to have the nominal overall heat
exchange. This setting is particularly useful for the initial transient from LBE at rest
to the nominal condition as the heat exchanger outflow can be easily forced to nominal
cold plenum temperature by setting T 0 at this temperature and τ small in confront with
the real LBE residence time in the heat exchanger.
Omitting the geometric complexity allows to construct a significantly smaller mesh
and therefore a faster simulation, though the level of approximation is increasing sig-
nificantly. It is always up to the user to check which level of simplification does not
hinder yet the quantities of interest within a predefined margin. In the next section, the
consequences of these choices will be demonstrated through the results obtained.

6.2.4.3 A look behind the curtains

One of the greatest advantages of numerical simulations is that they provide an almost
continuous representation of the physics considered upon the system under investiga-
tion. This is even more an advantage in the case of liquid-metal flows, where measure-
ments are very difficult to obtain due to the opaque nature of the material and the high
operating temperature. Moreover, in the case of MYRRHA, for a reactor that does not
yet exist in reality, numerical simulations allow to analyze the system before its actual
construction.
For a reactor under design, besides getting an insight on the general flow and ther-
mal field, most simulations are targeting safety issues. Some of these aspects will be
shown next, highlighting the advantages and limitations of the applied numerical
models.

6.2.4.3.1 General flow field

The first objective of the full power simulation is to get an insight on the large flow
patterns that develop within the reactor. These ones are essential to get a general
understanding of the flow field and to identify possible areas where low LBE refresh
rates may occur. Indeed, such stagnant zones need to be avoided since oxygen control
has to be kept within narrow limits. The second objective of this simulation is to eval-
uate the temperature distribution on the different components for stress evaluations.
The OpenFOAM steady-state model was developed for these objectives.
The overall pressure drop through the primary coolant loop is 2.57bar. One of the
most important parameters of the reactor is the pressure drop through the core. We
recall that a pressure drop of 1.7bar is expected in the core region while about
2bar should be obtained between the cold plenum and hot plenum, respectively.
The resulting values from the simulation indicate very small differences of less than