Assessment of the Level of Natural Circulation of VVER-1000 Coolant on the Basis of Experimental Data from the Balakovo NPP

Given the obligation to safely navigate a full de-energization condition of NPP it is important to study the natural circulation of the coolant in the primary loop of NPP with water-moderated/cooled [pressurized-water] reactors in the event of de-energization of the main circulation pump. At Balakovo NPP No. 3 with 104% load, the power unit underwent complete de-energized along with the main circulation pumps. When switching to natural circulation of the coolant, the temperature in the hot legs dropped from 320 to 300°C. This attests reliable and stable cooling of the core in the condition of natural circulation. A coefficient is proposed for assessing the level of natural circulation. It can be used to determine the flow rate of the circulating coolant on full de-energization of the main circulation pumps. Based on the measurements of the standard information- computational system of a power unit, the residual power release and the coefficient of natural circulation were determined in the entire area of work with the main circulation pumps de-energized for 45 min.