The reactor core is cooled by natural circulation in the range from full power operation to residual heat removal The hydraulic resistance along the primary circuit is dominant by the primary heat exchangers The "U" type tube bundles are adopted for PHES in order to give facilities for repairing The pitch of fuel elements is chosen to 13mm, a little tight than usual The flow resistance does not mcrease too much, but it is good for negative reactivity feedback There is a long riser on the core outlet to enhance the natural circulation capacity The high of the riser is about 6m Hence the average coolant velocity m core is 0 57m/s about 2 times larger than that in Although the core power density increases comparing with NHR — 5 the MDNBR is still larger than the limitation with a sufficient margin

A number of measurements and experiments have been earned out in NHR-5 and demonstrated that the capacity of natural circulation is sufficient to carry out the heat m the power range from full power operation down to residual heat removal Even m case of interruption of natural circulation m the primary circuit due to LOCA the residual heat of the core can be transmitted by vapor condensed at the uncovered tube surface of the primary heat exchangers [1]