Numerical analysis of Monju upper plenum thermal-hydraulics in 40 by rma97348


									   Numerical analysis of Monju upper plenum thermal-hydraulics in 40% rated power
                                operational condition

                                   Kei HONDA and Hiroaki OHIRA

       Monju startup test is preparing in JAEA and scheduled to start from the end of this Japanese fiscal
year. In this experiment, many kinds of plant and core data, which include abnormal transient and
accident test data, will be measured in order to verify the design specifications and the safety assessment
criteria. We are also going to validate the plant dynamics analysis code and detail thermal hydraulic
analysis codes in order to apply to the design and the safety assessment of future FBRs, i.e. Japanese
Sodium Fast Reactor (JSFR), etc. In these code validations, thermal hydraulic behavior in the upper
plenum is considered to be one of the most important factors, because the behavior mainly affects the
plant dynamics and the safety margins of the reactor vessel. Hence, the evaluation of the
thermal-hydraulics by numerical simulations is required to construct the proper numerical modeling of
the plant dynamics codes.
        The upper plenum of Monju from the support plate to the dip plate is approximately 7.0 m high,
the inner diameter of the inner barrel is approximately 6.5 m which has the upper and the lower flow
holes, and the inner diameter of the vessel wall is approximately 7.0 m. In this study, we calculated the
thermal-hydraulics by the detail analysis model using commercial FVM code, FrontFlow/Red. The
present analysis model has high resolution mesh simulated all structures which are guide tubes,
thermocouple (TC) plug, Fuel Handling Machine, etc. and the computational mesh consists of
approximately 10 million cells. In this mesh, however, the structure of the upper core structure was not
modeled since the inside and the outward velocity of the UCS was estimated to be very small and not to
affect the thermal-hydraulics in the upper plenum in the present steady state conditions. We applied the
Monotone Advection Reconstruction Scheme (MARS) and the central difference scheme to the
advection and the diffusion terms in the government equations, respectively. The RANS approach to
turbulence modeling (RNG k-ε model) was also applied to the turbulent model. The temperature and
flow rate of the outlet of the fuel subassembly were constant values measured in the previous startup test
condition which was 40% rated power operational one as the temperature boundary conditions. The
uniform pressures on the outlet nozzle surfaces were also prescribed as the pressure boundary conditions.
        The calculated temperature agreed well with measured temperature on the TC plug. In the region
of TC tubes and flow guides below the UCS, we also evaluated the pressure drops and the equivalent
mesh model, which has the porosity and was available to practical small mesh models with
approximately 100,000 cells, and calculated by 3 dimensional thermal hydraulic analysis code, AQUA.
The comparison with the fine mesh results indicated the pressure equations and the equivalent mesh
below the UCS showed close flow distribution and pressure drops. Hence, it was estimated from this
study that the practical small mesh model was available to evaluate the thermal-hydraulics in the upper
plenum in this steady state condition. We also schedule to calculate the plant trip conditions from these
results and propose the equivalent mesh model.

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