What’s preventing Ontario’s Darlington Nuclear Generating Station from going flexible?

By: Donald Jones, P.Eng., retired nuclear industry engineer – 2013 November.

A condensed version of this article will appear in the 2013 December edition of the Canadian Nuclear Society’s BULLETIN.

The frequent periods of Surplus Baseload Generation (SBG) on Ontario’s electricity system arise when baseload supply exceeds demand.  The Independent Electricity System Operator (IESO) takes steps to minimize the amount of SBG when it is forecast by increasing positive price out of province exports, reducing hydro and gas generation to the technical/contractual minimum, and reducing output of the Bruce A/B nuclear stations to the technical minimum by bypassing steam around the turbine-generators. Further reductions in SBG then come from curtailing output from wind generation and finally from nuclear unit shutdown. The major contributor to SBG is the 2,000 or so nameplate MW of unreliable intermittent wind on the system, soon to increase to around 8,500 MW, and not helped by low demand.

Bruce Power now takes output changes in its stride, quickly responding to IESO dispatches day and night to reduce or increase output.  Bruce Power should be congratulated for its initiative in providing the IESO with 2,400 MW of nuclear flexibility to help the IESO handle SBG (reference 1), even though it is in relatively large MW chunks. This is not theoretical, the full 2,400 MW amount of curtailment has been provided on occasion. 

This 2,400 MW is equivalent to the full output of three Bruce B units. Without this flexibility nuclear units would have to be shutdown and come offline during periods of SBG and when shutdown they take take two to three days to come back online.  This means that frackgas (methane)-fired generation, producing greenhouse gases (GHGs) that contribute to climate change, would be used in the interim as demand picks up. The flexibility provided by Bruce significantly reduces the number of times nuclear units need to be shutdown and reduces GHG emissions and wear and tear on the nuclear systems.

The four nuclear units at Darlington do not provide flexibility but operate baseload, and very well one may add. There would appear to be no technical reason why Darlington could not provide 1,200 MW of flexibility by using turbine steam bypass similar to Bruce A/B.  This would reduce even more the need for a nuclear unit to shutdown and come offline during SBG periods. Like Bruce the steam bypass system was originally designed to keep the reactor from poisoning out (becoming unavailable for two or three days) after very infrequent loss of load events, e.g. a grid collapse – it was not designed for frequent use.  It does this by rapidly reducing reactor power to around 60 percent of full power once the loss of load is detected and bypassing steam around the turbine-generator via the condenser steam discharge valves to the condenser while allowing enough steam through the turbine-generator to maintain unit house loads, indefinitely.  Once the grid is reestablished the unit is available right away to increase output to the grid on instructions from the IESO.  Obviously Bruce Power took steps to examine the original as-built steam bypass system and made changes to enable it to be used more frequently.

Bruce, quite rightly, gets paid for the output lost from complying with the IESO SBG dispatches, deemed generation – there is a price for flexibility.  If Bruce were not paid for deemed generation it would have to increase its generation costs to cover the times spent operating at reduced output – the capacity factor curse.  The publicly owned Darlington generating station operates under a regulated generating price set by the Ministry of Energy (MOE) and would not get paid for any lost generation from curtailment of output to mitigate SBG.  The IESO has to get on the case and convince the MOE that more nuclear flexibility is essential for the inflexible Ontario grid and the first step is to pay Darlington for deemed generation, then Ontario Power Generation can go ahead with the investment to make the turbine steam bypass systems more robust to handle more frequent use, just like Bruce.

Of course the IESO may have already asked the MOE about getting Darlington to contribute to SBG abatement and may have been rebuffed. The MOE may not want a lot of favourable publicity about nuclear flexibility since that would negate the premise in the 2010 Long-Term Energy Plan (LTEP) that nuclear generation must be limited to 50 percent (a capacity of around 12,000 MW) because it is an inflexible baseload supply. This is the foundation of the government’s case for its diversified energy supply mix policy of nuclear, hydro, wind/solar and frackgas, a view unfortunately supported by the province’s pro nuclear organizations.  Nuclear, especially the new Enhanced CANDU 6 (EC6), is far from inflexible (references 1 and 2) –  the EC6, that was being considered for new build at Darlington, has full output flexibility.

The province’s Professional Engineers support nuclear flexibility but only because it can be used to curtail nuclear output to accommodate wind during periods of SBG, something that makes no environmental, economic or technical sense. Instead, flexible nuclear must be used to accommodate more flexible nuclear on the grid. Wind is completely unnecessary on the Ontario grid. Nuclear flexibility capability can be used to make Ontario’s electricity grid completely free from the GHG emissions (reference 3) that contribute to climate change, using just nuclear and hydro.

Despite the need for more flexibility on the inflexible Ontario grid, and the need to reduce GHG emissions, nuclear flexibility must remain a government secret, hidden from public scrutiny, otherwise the public may start querying the premise of Ontario’s soon to be released revised LTEP! Let’s wait and see if the government mentions anything about the flexibility capability of Bruce A/B in its 2013 LTEP.


1. Ontario’s already “flexible nuclear” CANDU even better by satisfying IESO requirements to replace flexible coal, Don Jones, 2012 October, https://thedonjonesarticles.wordpress.com/2012/10/20/ontarios-already-flexible-nuclear-candu-even-better-by-satisfying-ieso-requirements-to-replace-flexible-coal/

2. Contenders for nuclear flexibility at Ontario’s Darlington B, AP1000 and EC6, and the winner is…., Don Jones, 2013 January, https://thedonjonesarticles.wordpress.com/2013/01/10/contenders-for-nuclear-flexibility-at-ontarios-darlington-b-ap1000-and-ec6-and-the-winner-is/#more-71

3. An alternative Long-Term Energy Plan for Ontario – Greenhouse gas-free electricity by 2045, Don Jones, 2011 May,  http://coldaircurrents.blogspot.ca/2011/05/alternative-long-term-energy-plan-for.html


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