USSR position paper for the specialists' meeting on thermal stratification in sodium

The problem of adequate coolant mixing and flow pattern optimisation becomes more significant while considering pool type fast reactors. Within a vessel of this reactor type there are considerable bulks of coolant at different temperatures. Because of the complex design requirements certain difficulties exist in the problem of ensuring sufficient mixing of the coolant at the outlet of the core, radial blanket, in-pile fuel storage and in-pile neutron shielding. n the framework of the R and D activities carried out to justify fast reactor design solutions a proper attention is given to investigations of the coolant flow pattern within plena and in those sections of pool type reactor, where the coolant now takes place. In this connection It should be noted that, up to now, we did not obtain any clear evidence of the presence of a pronounced thermal stratification of sodium within reactor mixing chambers under normal operating conditions. The conclusion may be drawn on the basis of our experiments and design experience that there are certain gradients of temperature and Inefficient mixing of coolant flows with different temperatures within the chambers (both 'hot' and 'cold' ones). All these factors to a certain degree complicate a reactor operation. However, from our point of view, the main point in the problem of arranging an 'optimum' coolant flow pattern within the chambers is to insure the stability of coolant motion in the chambers (Including flows with different temperatures) by means ol special design solutions based on results of experimental and calculation studies. It is considered that the instability of coolant flow could be the most important factor influencing temperature distribution within structures and components. That is why coolant flow in all reactor chambers and cavities should be stable, including natural convection flows