PAKS, Hungary [Operating experiences and lessons learned]

Starting from 90s, power uprate of existing LWR units as well as changing strategy from annual to longer fuel cycles became a worldwide tendency. In the beginning changes caused unexpected challenges: high power peaks and non–desired consequences appeared due to highly enriched fuel. Burnable absorber application in advanced fuel solved the problems and long fuel cycles became more or less general. Nevertheless, long cycles may not be proper solution on units with uprated power and older design. In the last years PAKS NPP, Hungary, operating 4 units of WWER–440 on 500 MWe uprated power turned from 12 to 15m cycles. Preliminary estimations showed that using the existing fuel geometry, 18–month cycles cannot be evaluated with satisfactory results. The core is small, and the necessary fuel enrichment for a realistic and economic 18–month cycle would exceed 5 wt %. But the analysis also demonstrated that there was an opportunity for a 15–month cycle design. The next task was to decide how to schedule the operation of the four PAKS units in order to make use of 15–month cycles. One can assume that the refuelling outages of the PAKS units are arranged as follows (see Figure 14). The first refuelling is in February, the second in May, the third in August and the fourth in November. Supposing that reloads take approximately one month, evaluating cycles with a cycle length of 415–420 full power days, the following will happen: outages of a given unit will move forward in each year, from February to May, from May to August, etc., and one of the units will jump over to the next year.