In the simulations the main goal was to extrapolate the behavior of the terminated VOLLEY tests until steady state (Pikkarainen, 2006). Various correlations of heat transfer coefficient in the pool boiling conditions were examined. Three different correlations were found that were applicable when only few measured parameters are available. All of them gave very similar results.
The simulation method was first validated by calculating such tests, which could be continued to thermal hydraulic steady state and there was available video recorded information of the coolant level in the cooling channel. The simulations gave good results.
An interesting observation was that in the situation, in which the cooling water oscillated strongly back and forth, the results corresponded with the case when the cooling channel is full of water. Temperatures in the simulations and in the tests were within a few degrees.
Results were good even when the heat transfer coefficient between cast iron and steam/air
was set to zero. This is because the heat transfer coefficient between iron and steam is very low compared to the coefficient between iron and water.
Figure 16. Temperature contours of the flow channels from Fluent CFD code (Pikkarainen, 2006).
In general the simulation results of VOLLEY tests which were successfully run until the steady state conditions are very close to the measured results. Simulations of terminated tests without visual information do not give comprehensive results; additional information of the height of the dry part of the channel wall would have been needed. From the results the magnitude of the steady state temperatures in the measuring points can be deduced. This is evidently below the cast iron melting point. It can be concluded, that if there is even a small water flow inside the cooling channels or even if a limited number of adjacent cooling channels are totally dry, the core catcher cannot melt with heat fluxes up to 95 kW/m2 used in the tests. Figure 16 presents a situation of a simulation, where one of the two channels in the VOLLEY test section is totally dry. Analyses have predicted a peak value of 80 kW/m2 for the heat flux into the cooling structure in the EPR plant (Fischer, 2004); thus melting is not predicted.
The use of the observation windows in the test facility created a risk of window breakage and subsequent test termination, Figure 17. However, the use of the windows has a benefit for the simulations even when the video recording data are not directly used in the calculations. The possibility to compare the water level seen in the video recordings in the cooling channel with the calculated results gives more confidence in the simulation.
Figure 17. Broken test section window in a VOLLEY test.
7.5 Discussion
The thermal hydraulic behavior of the cooling of the EPR core catcher construction was examined using a test rig simulating a two channel section of the cooling system.
Performing supplementing tests with realistic modeling of feeding the cooling water by gravity (not by pumps) revealed the possibility of water hammers and dry-outs in the cooling channels. Inclination of the test section by 1o had a remarkable effect on the behavior of the flow, water hammers practically disappeared. CFD codes are not yet capable to simulate two-phase conditions and condensation of steam accurately enough. Thus the experiments were essential for ensuring the proper behavior of the cooling system of the core melt spreading area in the EPR and a design improvement was discovered. Boron acid or insulating material or combination of both in the cooling water did not affect on the system performance.
Based on the results the system works as planned. The physical restrictions of the test rig were covered using computational methods. The method could be verified using the tests where physical restrictions did not affect the behavior of the test rig.
8 BOUNDARY CONDITIONS OF EXPERIMENTAL MODELING
Ensuring the safe use of the NPPs is the final goal of the safety related thermal hydraulic experiments. Thermal hydraulic data on NPPs exists to some extent. This originates mainly from tests carried out during plant start-up or during extensive plant modernizations. Some data is also available from unplanned transients that have taken place during plant operation.
It is good to use data measured in real NPPs in exercises and code comparisons. However, the instrumentation and the control systems are designed for running the plant, not for gathering experimental data. Time delays of the detectors might be large, thus preventing the observation of the fastest phenomena. The detectors are in many cases also in remote position and only some averages of the process can be seen.
The principal source of thermal hydraulic data used in code validation are the experiments carried out in test facilities, where the instrumentation and gathering of data can be optimized and all the boundary conditions, time delays and heat losses affecting the result are known to better accuracy than in the real plant.
The decisions on design, scaling and instrumentation of a test facility are always difficult in experimental work. Choosing the right scaling principle and design that does not destroy the original physical behavior of the system is essential. Especially for instrumentation the resources are often sparse; the number of channels in a data acquisition system or the number of available transducers is limited. Experience gained from earlier studies helps to make the right decisions.