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).
The Enhanced CANDU 6 (EC6) gets over the xenon hurdle for fast frequent power changes by using a combination of steam bypass and reactor power changes. Very fast power changes can be made using steam bypass and reactor power can be changed much more slowly to optimize the amount of steam bypass bearing in mind anticipated future demand of the grid. This makes the EC6 a better load follower/load cycler than a reactor that does not use steam bypass (reference 2). AECL/Candu Energy have been quite sparse in their description of EC6 flexibility, despite its importance. The EC6 Technical Summary document just says, “The condenser steam discharge valves are designed to discharge up to 100 percent of the steam flow directly to the condenser, bypassing the turbine. This feature provides for operational flexibility in support of load following operation in conjunction with overall reactor control.”
Another reactor that uses a combination of steam bypass and reactor power changes to load follow is the Small Modular Reactor (SMR) from NuScale Power. NuScale has trade marked this capability as NuFollow. This NuScale SMR is a 50 MWe (gross) pressurized water reactor and a power station would consist of up to 12 of these small reactors to provide a 570 MWe (net) output. Each NuScale power module is a 160MWt reactor core housed with other primary system components in an integral reactor pressure vessel and surrounded by a steel containment pressure vessel. The World Nuclear Association says that a preferred site has been identified for the construction of a NuScale SMR at the US Department of Energy’s Idaho National Laboratory (INL), near Idaho Falls. Earlier in 2016 NuScale confirmed it intends to participate in a UK government competition to identify the most suitable SMR design for possible future deployment.
To quote Nuclear Engineering International magazine, from a 2016 February 1 article,“Equipment in the NuScale plant is being designed for load-following operation to further reduce impacts from power cycling. For example, the module design and operating parameters allow reactor power changes using only control rod movement down to 40% reactor power, without adjusting boron concentration in the primary coolant. This improves the flexibility of the reactor without creating the additional liquid wastes associated with boron addition and dilution. The condenser is designed to accommodate full steam bypass, allowing rapid changes to system output while minimising the impact to the reactor, which can continue to run at full power. Finally, the multi-module nature of the NuScale plant and a staggered refuelling regime results in a plant configuration in which at least one module is near beginning of life. It is generally easier to perform power changes on beginning of life cores because the higher core reactivity allows better xenon override. The operator can use such modules to vary power for intermediate-term load-following, while the modules with higher burn up can be used for coarser power adjustments.”
and, “Even with the NuFollow features, load following with a nuclear plant has operational and economic impacts. Reactor operations are affected least when changes in electrical output are accomplished by closing or opening the bypass valve to redirect main steam flow from the turbine to the condenser. This can be done much more quickly than adjusting reactor power. The drawback is that energy is wasted in the form of turbine bypass flow, and extended periods of high bypass flow to the condenser will increase wear on the equipment, resulting in increased maintenance and equipment replacement. Adjusting reactor power for partial or full load-following requires a reliable wind forecast such that reactor power can be scheduled for daily or even hourly dispatch while remaining at a power level reasonably above that required to generate the expected electrical output. Turbine output is then trimmed via the turbine bypass valves for fine-tuned matching of output to demand. This option minimises the amount of wasted energy, which minimises the excess loading of the bypass equipment, including the condenser.”
So it appears that the EC6 is not the only nuclear plant taking advantage of steam bypass for fast frequent power changes. Such a load following and load cycling capability means that intermittent wind and solar generation can easily be integrated into power grids without using greenhouse gas producing fossil-fuel burning power plants. However no matter what way a nuclear power plant is manoeuvred to accommodate wind and solar it still does not make any technical, environmental or economic sense to do so.
References.
1. Ontario’s already “flexible nuclear” CANDU even better by satisfying IESO requirements to replace flexible coal, Don Jones, 2012 October 20, https://thedonjonesarticles.wordpress.com/2012/10/20/ontarios-already-flexible-nuclear-candu-even-better-by-satisfying-ieso-requirements-to-replace-flexible-coal/
2. Ontario’s IESO prefers Enhanced CANDU 6 over AP1000 for new build at Darlington, Don Jones, 2013 August 25,
https://thedonjonesarticles.wordpress.com/2013/08/25/ontarios-ieso-prefers-enhanced-candu-6-over-ap1000-for-new-build-at-darlington/
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