Page 208 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

System thermal hydraulics for liquid metals 179

25

MP101 IMF measure (Exp)
20 Heat balance (Exp)
Heater (R5)
LBE mass flow rate (kg/s) 15

10

5

0
1 2 3 4 5 6 7 8
Time (h)
Fig. 4.14 LBE mass flow rate time trend.

reproducing FPS experimental power trend in RELAP5 model is mandatory to obtain
adequate temperature trend from the code.
During the enhanced circulation regime, the measured mass flow rate (see Fig. 4.14)
reaches a mean value of about 13kg/s characterized by oscillating behavior mainly
due to the argon injection compressor system, while a heat balance gives a slightly
lower value of about 12kg/s, very close to the value estimated by the RELAP5 code.
The ULOF event takes place deactivating gas injection (t¼5.85h) inside the loop.
Inlet/outlet temperatures undergo a sudden decrease/increase of about 10°C followed
by an ascending trend up to a new equilibrium value (after <2h)of320°C and 348°C,
respectively, achieving a stationary state for this new regime. During this new steady-
state condition based on natural circulation regime, the mass flow rate drops to about
5kg/s, and a good agreement can be observed between experimental data and
RELAP5 results. FPS and HX are then shut off (at t¼7.6h) producing a decrease
of temperatures due to the heat losses toward the outer environment. After the deac-
tivation of FPS and HX, the mass flow slowly decreases to zero. LBE temperature
trends related to FPS inlet and outlet sections are plotted in Fig. 4.12, and a good
agreement between experimental values and RELAP5 results is found again.
RELAP5 data adequately reproduce the temperature profile characterizing the test
and the transition from forced to natural circulation regimes, even if minor discrep-
ancies are observed during the early ULOF transient phase. Fig. 4.13 plots measured