Change in hydraulic resistance of reactor of unit 1 of Bohunice power plant

On the first V-230 unit of nuclear power plant V-1 in Jaslovske Bohunice a fluctuation of the reactor neutron output was found after the 1987 overhaul, i.e., before the 8th fuel cycle, within the range of 2% of rated power. It was found that this was caused by a non-periodical change in hydraulic resistance in the lower part of the reactor, in the area of the bottom of the reactor support cylinder. During the overhaul no interventions against the project design were made in the reactor hydraulics. The internals of the reactor were not removed and only the replaceable parts of two main circulating pumps were replaced. In the following repair in 1988 the replaceable parts of the main circulating pumps were replaced in four loops but the pulsations did not disappear. It was found that all components of the hydraulics of the primary circuit contributed to the development of coolant flow pulsations, i.e., the characteristics of the main circulating pumps, the resistance characteristics of the loops, the speed of the main circulating pumps, etc. In the first unit the throughflow pulsates during the operation of six main circulating pumps and in four cases in the operation of five main curculating pumps. In the second unit the throughflow pulsates in three cases during the operation of five main circulating pumps, during the operation of six pumps there was not pulsation. Additional dynamic stress of the pressure vessel and reactor internals did not prove to be significant. (Z.M.). 2 figs., 1 tab., 2 refs