Why Ontario’s CANDU nuclear reactors don’t load-follow

By: Donald Jones, retired nuclear industry engineer, 2018 July 21
Ontario`s electric power grid has a surplus of power. On windy, sunny, low demand days, a large surplus. Contractually other generators on the grid have to reduce power to accommodate the generation from wind and solar even though it makes no environmental, economic or technical sense to do so.  Since nuclear and hydro supply up to 90 percent of the electricity in Ontario this duty mainly falls on them. Hydro manoeuvring is relatively straightforward, nuclear not so.

For example, an item from Ontario Power Generation’s (OPG) 2009 Annual Report confirms that Darlington and Pickering are, “not designed for fluctuating production levels to meet peaking demand“. Under the definition of Nuclear Unit Capability Factor, page 26, it states, “OPG’s nuclear stations are baseload facilities as they have low marginal cost and are not designed for fluctuating production levels to meet peaking demand……“.  However this is not totally correct. Although not designed to respond to frequent IESO (Independent Electricity System Operator) load-following dispatches the design of the CANDU plants did offer the potential for load-cycling (power reduced overnight/weekends) and this has been demonstrated. In the past some domestic units and off-shore units (CANDU 6) did accumulate considerable good experience with load-cycling by manoeuvring the reactor, with some deep 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 reactor power reductions. 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 since then, as will be shown below, regulatory concern has restricted any manoeuvring of the nuclear reactor in order to suit market conditions on the power grid.

Reactor manoeuvring requires, amongst other things, the use of the adjuster rods that are arranged in several banks. The adjuster rods are normally all inserted in the core. Their functions are to flatten the neutron flux shape in order to improve fuel utilization, to provide a sufficient margin against Xenon transients for reactor power manoeuvring and to permit operation of the core for prolonged periods (two to three weeks) when refuelling is interrupted.

In the early days of CANDU operation the adjusters were used to “beat a poison-out” from Xenon after a reactor trip with the reactor shutoff rods in the core. After the trip the reason for the trip would be determined and then the shutoff rods could be withdrawn. The adjusters would be withdrawn to increase reactivity and reactor power would be quickly raised to around 60 percent of full power before the reactor could poison-out from Xenon, with the steam bypassing the turbine and going to the condenser or the atmosphere in case of Pickering. The hot turbine-generator could then be loaded to 60 percent of full power by reducing the amount of steam bypass and full power achieved by increasing reactor power over several hours while reinserting the adjuster rods to avoid a neutron over power reactor trip as power is increased.

However operating experience convinced the AECB (Atomic Energy Control Board, now the Canadian Nuclear Safety Commission) that it was not prudent to have operators rushing the decision and action process to power up after a trip, without a full review by both the unit operators and the shift manager of the cause, and to just let the unit poison out. So trip means poison-out and no adjuster operation needed. If it were the second shutdown system that tripped the reactor the reactor would be poisoned out anyway. But adjusters would still be needed for reactor manoeuvring, or would they.

The 1998 speech, below, by the then President of the then AECB was probably the last straw for any ideas the old Ontario Hydro might have had about load-following/cycling by using the adjusters for reactor manoeuvring.

(Reference: Part of a speech to, The Canadian Nuclear Association, on, Recent Developments In The Canadian Nuclear Power Sector: A Regulatory Perspective, By Dr. Agnes J Bishop, President of the Atomic Energy Control Board, Winter Seminar Canadian Nuclear Association, February 9 – 10, 1998.)

The Ontario Government White Paper

Let me turn now to the White Paper on Ontario Hydro restructuring, published by the Ontario Government in November 1997. In order to successfully introduce competition into Ontario’s electricity system, this plan proposes, among other things, to create two new commercial electricity companies, responsible respectively for generation and transmission of electricity. An Independent Market Operator would also be established to dispatch power based on least cost bids and to arrange financial settlements between buyers and sellers.

Why are these proposed arrangements of interest to the AECB? Let me be very clear on this point. It is not the mandate of the AECB to dictate to a provincial government how it should organize its electric utility industry. It is the mandate of the AECB, however, to carefully review all factors which can affect the safety of nuclear installations. Consequently, from the AECB perspective, it is important that structural changes in the electric industry sector be made with full recognition of the specific safety needs of the nuclear power stations. In this regard, grid stability and reliability of off-site power are examples of considerations which can have a direct impact on the safety of nuclear stations. From an operational point of view, the AECB will expect, for example, power manoeuvres on the grid to be governed by rules which will minimize the probability of power interruptions to nuclear stations or minimize their duration should they occur. We would also expect that nuclear stations will not be required to change their power output frequently just because cheaper power can be imported from elsewhere as market conditions change throughout the week or month. As a minimum, the safety implications of operating in such a mode would have to be properly assessed and found acceptable before it could be authorized.

The proposed restructuring of Ontario Hydro could also affect nuclear safety in other ways. In a market-driven environment, care must be taken to ensure that decisions made in the nuclear sector are not unduly influenced by the pressure to compete successfully against other energy producers or to make short term gains at the expense of longer term safety objectives. Safety margins in design, plant maintenance, staff training, safety-related research and size of the workforce are examples of areas where cost concerns may affect nuclear safety.

At around this time, the late 1990s, the adjuster rods in the reactor cores at Darlington nuclear generating station were reduced in number, by moving out of core, and reconfigured to give better neutron economy and neutron flux control for baseload operation. (Aside -The addition of several thousand megawatts of intermittent and varying wind power and solar power to the Ontario grid over the last few years should also be a concern to the CNSC – Canadian Nuclear Safety Commission – as it could affect reliability of off-site power).

Development of wind generation in the area around the Bruce nuclear station and the inadequate, at the time, transmission links out of the area made it necessary for the station to have the capability to reduce output. Reducing output on several nuclear units is preferable to a reactor shutdown. The AECB/CNSC restrictive position on reactor manoeuvring prompted Bruce Power to use turbine steam bypass, steam dumped to the condenser bypassing the turbine, as a way of varying unit electrical output while keeping reactor power constant. This has now proved very useful in accommodating the Surplus Baseload Generation on the Ontario grid, even without wind. On a dispatch instruction from the IESO Bruce units perform a gradual reduction in electrical output of up to 300 MW per unit, with some restrictions, over a one to two hour period then hold at reduced output for the required length of time, usually several hours, before gradually increasing output over one to two hours. The slow power change is to reduce the thermal stresses on the thick walled components of the steam turbine. Much faster power changes can be made if demanded. Energy is lost when using turbine steam bypass but since CANDU reactors use natural uranium, and not the enriched uranium of light water reactors, fuel costs are low.

The steam bypass system on the Bruce units was originally designed to cater for the infrequent loss of grid or turbine-generator trip events, allowing the reactor to operate at around 60 percent full power without poisoning out and to allow rapid reconnection to the grid when the problem was fixed. Steam bypass was not meant to vary unit electrical output. Since the steam bypass system was not meant for frequent use it would have likely had to have been beefed up for its new duty on the Bruce units. Darlington does not use steam bypass for changing unit electrical output and Pickering does not have a bypass to the condenser.

Although the reactor cannot be manoeuvred without its adjuster rods and reactor manoeuvring is problematic it does appear that they can be used in certain circumstances. The following excerpts were taken from “U.S. – Canada Power System Outage Task Force” final report of 2004 April on the 2003 August 14 blackout. It mentions the part adjuster rods did or did not play in unit recovery. Only three have been excerpted as examples.

Page 122 – Existing OP&Ps place constraints on the use of adjuster rods to respond to events involving rapid reductions in reactor power. While greater flexibility with respect to use of adjuster rods would not have prevented the shutdown, some units, particularly those at Darlington, might have been able to return to service less than 1 hour after the initiating event.

Darlington – page 126

Unit 1 automatically stepped back to the 60% reactor power state upon load rejection at 16:12 EDT. Approval by the shift supervisor to automatically withdraw the adjuster rods could not be provided due to the brief period of time for the shift supervisor to complete the verification of systems as per procedure. The decreasing steam pressure and turbine frequency then required the reactor to be manually tripped on SDS1, as per procedure for loss of Class IV power. The trip occurred at 16:24 EDT, followed by a manual turbine trip due to under-frequency concerns.

Unit 3 experienced a load rejection at 16:12 EDT, and during the stepback Unit 3 was able to sustain operation with steam directed to the condensers. After system verifications were complete, approval to place the adjuster rods on automatic was obtained in time to recover, at 59% reactor power. The unit was available to resynchronize to the grid.

Bruce B – page 127

Although initially surviving the loss of grid event, Unit 6 experienced an SDS1 trip on insufficient Neutron Over Power (NOP) margin. This occurred while withdrawing Bank 3 of the adjusters in an attempt to offset the xenon transient, resulting in a loss of Class IV power.

The following equipment problems were noted:

An adjuster rod on Unit 6 had been identified on August 13, 2003, as not working correctly………

The Ontario power grid presently compensates for its lack of reactor manoeuvrability in its nuclear units by making use of the load-following, frequency response and AGC (Automatic Generation Control) attributes of its significant hydro-electric generation and to a limited extent its combined cycle gas turbine generating units, and the load-cycling (using steam bypass) at Bruce. Wind generation can also be dispatched. There are a couple of ways that dispatchable load-following and frequency control including AGC could be provided by the CANDU nuclear units.

The units are presently operated in the turbine-following-reactor mode (reference 1) so one approach would be to operate the units in the reactor-following-turbine mode but this would lead to the kind of reactor power changes that the CNSC do not like even if not exactly being prohibited. To satisfy the CNSC would take engineering and testing to show that there would be no impact on reactor safety from load-following and frequency control. Load-following, but not frequency control, might also be able to be done in the turbine-following-reactor mode if the adjuster rods could be used, except on Bruce A which does not have adjusters. Without use of the adjusters to compensate for the Xenon-135 changes manoeuvring would be very slow and responding to frequent load-following dispatches likely impractical. Even with the adjusters it might be difficult.

The other and maybe better approach would be to change the control system of the present CANDUs to one similar to the Enhanced CANDU 6 (EC6) (reference 2) that would modulate steam bypass flow around the turbine to the condenser on an IESO signal or dispatch and avoid the changes to reactor power that the CNSC do not like. This is easier said than done and if the IESO is comfortable with the present and foreseeable future situation on the grid then it is not worth the effort. Bruce and Darlington certainly would not do this without adequate compensation and prodding from the IESO. Phasing out gas-fired generation in the future would ideally (i.e. no wind) result in a nuclear-hydro grid and then more flexibility would be required from the nuclear units, to accommodate more nuclear units.



So what is the present situation with our neighbour, the United States. Just like in Canada. In the U.S. the NRC (Nuclear Regulatory Commission) prohibits load-following, primary frequency response, and AGC even though the units are likely capable of it (reference 3). They can perform a planned load-cycle (called load-shaping) by changing reactor power under the control of a licensed operatorfollowing procedures approved by the NRC for that unit but load-following, primary frequency response and AGC are prohibited.

In France (because of large share of nuclear electricity on its grid) and in Germany (because of large amount of variable wind and solar generation on its grid) some nuclear units, basically similar to units in the U.S., do load-following, that is, make changes in power as the grid demand changes during the day and week. Units also provide primary frequency response, and AGC. Not all units are needed for load-following and a significant number operate baseload.

The United Kingdom is considering the use of Canada’s EC6 to dispose of its surplus plutonium. New build in Europe has to at least meet the flexibility regulations of the European Utilities Requirements (EUR) that are based on the requirements of the grid operators, subject to licensing approval by the specific nuclear regulator. The EC6 would have to comply with the manoeuvrability specifications of the EUR if it wants to be part of new build in Europe.



1. A quick primer on how CANDUs fit into Ontario’s windy power grid, Don Jones, 2013 July, https://thedonjonesarticles. wordpress.com/2013/07/06/a- quick-primer-on-how-candus- fit-into-ontarios-windy-power- grid-2013-july/

2. Enhanced CANDU 6 and NuScale SMR have capability to easily integrate wind and solar, Don Jones, 2016 August 17,  https://thedonjonesarticles. wordpress.com/2016/08/17/ enhanced-candu-6-and-nuscale- smr-have-capability-to-easily- integrate-wind-and-solar/

3.  Can nuclear power plants deliver on all the attributes U.S. energy secretary Rick Perry claims, Don Jones, 2017 October 11, https://thedonjonesarticles.wordpress.com/2017/10/11/can- nuclear-power-plants-deliver- on-all-the-attributes-u-s- energy-secretary-rick-perry- claims/


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