Thermal stratification in liquid occurs when liquid masses with different temperatures form layers that act as barriers to mixing. These layers are arranged according to density, with the least dense liquid masses sitting above the more dense layers. Fig. 1 shows seasonal shifts in lake stratification where the highest gradients in water temperatures are reached during summer.
Similar phenomenon can also occur in the upper plenum of Sodium-cooled Fast Reactors (SFR) under low flowrate conditions. For example, during plant startup and shutdown, buoyancy forces largely affect both flow and temperature fields as shown in Fig. 2 for the specific case of the Japanese SFR demonstration plant Monju. A strong thermal stratification may be developed and result in severe thermal stress problems.
The overall objective of the CRP was to improve national analytical capabilities in participating countries in the field of fast reactor in-vessel sodium thermal-hydraulics. The CRP contributed towards achieving this objective with the help of benchmark exercises focusing on the numerical simulation of thermal stratification of sodium observed in the upper plenum of the Monju reactor vessel during a startup turbine trip test conducted in December 1995.
The CRP’s specific research objectives were:
- validation of various multi-dimensional fluid dynamics codes in use worldwide through simulation of sodium cooled fast reactor upper plenum temperature distributions and comparison with measured data; and
- identification of weaknesses in current methodologies and of the R&D needs to resolve the identified open issues.
The main benchmark results and achievements are as follows:
- Analytical capabilities of the participating organizations in the field in-vessel sodium thermal hydraulics, especially meshing and algorithm selection criteria, have been improved.
- The steady state calculations have shown that momentum driven solutions are considered as the actual flow pattern in the upper plenum of the Monju reactor vessel at the 40% rated power operational condition.
- Turbulence models based on the standard k-ε model for high Reynolds number and the thermal capacity of the upper instrumentation structure do not affect significantly the moving up rate of the thermal stratification front.
- The shape of the flow hole edge is important for the accurate prediction of the stratification front propagation.
Eight research organizations from seven countries with an active programme on SFRs - namely China, France, India, Japan, Russia, South Korea, and the USA - contributed to this CRP. The experimental data on thermal stratification of sodium observed in the upper plenum of the Monju reactor were provided by the Japan Atomic Energy Agency.
The results of the Monju-NCT CRP were presented in a special session at the NURETH-14 conference:
The full report was published in IAEA-TECDOC-1754:
For more information, please see the CRP description:
http://www.dgdingfa.net/projects/crp/i31017
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