Contenders for nuclear flexibility at Ontario’s Darlington B, AP1000 and EC6, and the winner is …..
By: Donald Jones, P.Eng., retired nuclear industry engineer – 2013 January
A condensed version of this article will appear in the upcoming 2013 March edition of the Canadian Nuclear Society’s BULLETIN journal.
Although Ontario Power Generation (OPG) has a site preparation licence from the Canadian Nuclear Safety Commission for new nuclear build at Darlington it has yet to select a vendor. Two vendors are in the running. Westinghouse with the AP1000 and Candu Energy Inc. with the EC6 were asked by OPG to prepare detailed construction plans, schedules and cost estimates for two reactors. The reports must be ready by June of this year and will be submitted to the Ontario government for selection and go-ahead, or not. With Ontario’s unique generation mix of baseload/intermediate/peak hydro (8,000 MW), nuclear (13,000 MW), frackgas/oil (10,000 MW), coal (3,300 MW until shutdown in 2014), and intermittent wind (2,000 MW) and solar generators (482 MW), the operating characteristics of the new nuclear units will need to be carefully considered.
By 2015 wind will be over 5,800 MW, nameplate, and solar over 2,000 MW, nameplate, based on Ontario Power Authority approved contracts. Ontario’s Long-Term Energy Plan calls for 10,700 MW of wind/solar/bioenergy by 2018 with around 8,000 MW of this likely to be wind. Although not contributing much to the annual MWh electricity supply these intermittent suppliers become a problem during periods of low demand and/or significant wind production since their output has to be accommodated on the grid. This means other generators on the grid need to have operating characteristics flexible enough to cater to several thousand MW of wind that come and go over short periods of time (reference 1). Periods of surplus baseload generation are already a major concern on the Ontario grid and result in nuclear being dispatched (manoeuvred and/or shutdown) before wind/solar. In the future the Independent Electricity System Operator (IESO) is proposing that “flexible nuclear” (presently only Bruce A and B) be manoeuvred, but not shutdown, before wind/solar. With very flexible coal being removed from the mix by 2014, and with the limited flexibility of the hydro and gas-fired units and the present Darlington and Pickering A/B nuclear units (references 2 and 3), there is an opportunity for the new nuclear units at Darlington B to replace the missing flexibility. Indeed this has been a regular requirement in the “18-Month Outlook” series from the IESO, for example,
“The existing coal fleet, though running at vastly reduced levels from previous years, provides the IESO with desirable flexibility, such as quick ramping and operating reserve, under all market conditions. As Ontario’s coal-fired generation is shut down over the next two years, its associated flexibility will be lost. Therefore, future capacity additions should also possess this flexibility to help facilitate the management of maintenance outages, provide effective ramp capability, supply of operating reserve and even provide regulation when necessary”.
The AP1000, at around 1117 MWe net, operates in the reactor following plant load mode and does not use steam bypass for normal at power manoeuvring although steam bypass would operate for significant load reductions. The plant is designed to accept a full load rejection without reactor trip and then operate at house load power. This infers that In the event of a loss of grid connection or grid blackout steam bypass together with a reduced reactor power will supply unit house load until the connection is re-established or the unit is needed to help grid restoration. It can also accept a turbine trip from full power operation without reactor trip. In the 15 to 100 percent full power range it can ramp at +/-5 percent power per minute subject to reactor core power distribution limits not being exceeded. For 90 percent of its 18 month fuel cycle it is designed to perform a 100-50-100 percent full power daily load cycle with between 2 and 10 hours spent at 50 percent power and with two hour linear load ramps. The unit has the capability of handling grid frequency changes equivalent to 10 percent peak-to-peak power changes at +/-2 percent per minute (regulation). This capability is provided over a 15 to 100 percent power range through the plant operating life. A total of 35 peak-to-peak swings per day is allowed. The unit can satisfy a 20 percent power increase or decrease within 10 minutes. It is capable of a 10 percent step load decrease between 100 and 25 percent full power and a 10 percent step load increase when between 15 and 90 percent full power. There are slight differences in Westinghouse operating data depending on the source, and indeed it may have all been revised, but these data are felt to be representative.
The EC6 has a net output of around 700 MWe. Unlike Westinghouse, Candu Energy Inc., like AECL before it, does not list manoeuvring details of the EC6. It only says that it has condenser steam discharge valves designed to discharge up to 100 percent of the steam flow directly to the condenser, bypassing the turbine, and that this provides operational flexibility in support of load-following operation in conjunction with overall reactor control. It also says that in the event of a loss of connection to grid bypass capability would allow a quick return to full power from house load power. From this it can be inferred that the turbine steam bypass system can accommodate 100 percent bypass with the reactor kept at 100 percent full power and would be capable of dispatchable load following from zero to 100 percent power with the rate of manoeuvring set by the turbine metal temperatures and not by the reactor. This would typically be a ramp of up to +/-10 percent of full power per minute, with relatively low temperature nuclear steam. The reactor would be manoeuvred to follow the turbine at a slower rate to approximately match the turbine power to reduce wear and tear on the reactor systems and on the fuel as well as reducing the amount of steam bypass. Monitoring systems can detect the location of failed fuel, if any, and the fuel can be replaced while at full power. If the reactor power cannot be changed because of operational reasons this will not affect unit manoeuvring in response to dispatches. Steam bypass operation would also allow the rapid and significant power changes needed for the automatic generation control of grid frequency (regulation). Since the reactor can remain at 100 percent full power, if necessary, after a loss of grid connection or grid blackout the unit can return to full power very quickly from unit house load operation after the connection is restored or when the unit is needed to help restore the grid. Although at the present time energy would be wasted bypassing steam CANDU fuel costs are very low. Unlike the AP1000 the EC6 does not have to shutdown every 18 months to replace part of the fuel load since it is fuelled on line at full power. It is designed to shutdown for an average of one month every 36 months for maintenance that cannot be done during normal operation. At the mid-point of its 60 year design life the EC6 will require an outage to replace fuel channels and feeders and for other replacements/repairs to maintain or improve safety and reliability.
The team reviewing the proposals should ensure that the IESO flexibility requirement is met. A design having power changes done by steam bypass would be inherently more flexible than one depending on reactor power changes since there would be times when the reactor power would need to be held constant, say for calibration of power measuring systems, during on-line refueling in the case of EC6, and near end of fuel cycle in case of AP1000. Thus it would seem that the EC6, with a unit output that can be varied from 100 percent to zero and is always available to follow dispatches, would be more suitable for the wind/solar heavy Ontario grid than the AP1000, or any other light water reactor for that matter. When Ontario’s infatuation with wind comes to an end the flexible nuclear stations that are built or refurbished now will enable more and more nuclear to be part of the future grid (reference 4).
1. “More wind means more risk to the Ontario electricity grid”, Don Jones, 2011 January 22,