by: Donald Jones, P.Eng., retired nuclear industry engineer, 2016 August 17.
Nuclear power plants do not like to operate at anything less than 100 percent full power. The main reason is that capital costs for nuclear are high and fuel costs are low so fuel cost savings are negligible at reduced power while revenue losses are appreciable. Another reason is that when reactor power is reduced relatively quickly there is an increase of Xenon-135 in the fuel, a fission product, that tends to reduce reactivity and sets a limit on the rate and depth of any power reduction that can be achieved before the reactor shuts itself down, the so called “poison out”. On a CANDU this is about a 40 percent reactor power reduction to a reactor power of 60 percent after a fast power reduction. Xenon also slows the return to full reactor power. The xenon transient means that frequent power changes, down and up, in support of load following dispatches, would be difficult. Indeed CANDU was not designed to load follow although it was designed to load cycle, that is, reduce reactor power overnight and return to full power in the morning, without bypassing steam around the turbine to the condenser. Light water reactors use enriched fuel so are better able to respond to the xenon transient, at least with a fresh core.
In the past some domestic units and off-shore units (CANDU 6) did accumulate considerable good experience with load cycling, with some deep reactor power reductions, but not on a continuous daily basis. For example back in the 1980s several of the Bruce B units experienced nine months of load-cycling including deep (down to 60 percent full power, or lower) and shallow power reductions. All done without steam bypass. Analytical studies based on results of in-reactor testing at the Chalk River Laboratories showed that the reactor fuel could withstand daily and weekly load-cycling. However this load cycling capability has been configured out of the Ontario CANDUs and they presently operate continuously at 100 percent reactor power. Note that the eight units at the Bruce Nuclear Power Station load cycle when required to do so by bypassing steam around the turbine to the condenser but the reactor remains at full 100 percent power. With certain restrictions station electrical output can be reduced to around 60 percent of the full electrical output (reference 1).
Read the rest of this entry »