How CANDU has faired in handling small loss of coolant events in the 35 years since TMI

By: Donald Jones, P.Eng., retired nuclear industry engineer, 2014 April

It has been 35 years since the small loss of coolant accident (small LOCA) at Three Mile Island (TMI) Unit 2 caused the loss of the plant. Rod Adams has some interesting views on the event and its precursors on his informative pro-nuclear website, Atomic Insights, http://atomicinsights.com/tmi-operators-took-actions-trained-take/ including some from licensed reactor operators and shift supervisors.

CANDU has not been immune to small LOCAs but none resulted in the kind of damage and infamy that inflicted TMI Unit 2 although a pressure tube failure at Pickering A unit 2 in 1983 did result in the retubing of all four Pickering A reactors. It is worth remembering that no one was injured and that there were no long term health effects from TMI. Small LOCAs are much more likely to happen than large break LOCAs. Unlike TMI the CANDU small LOCAs reviewed below originated in the primary heat transport (PHT) system and not in the balance of plant. There have been other CANDU significant events that did not originate in the PHT system but these did not progress to small LOCAs. For those interested in a TMI comparison all CANDUs after Pickering operate with a degree of boiling at reactor core outlet at high powers (nominally 4 percent on CANDU 6 and Darlington) and onset of boiling results in a spike in pressurizer level. Pickering does not have a pressurizer.

The TMI event occurred near the end of the CANDU 6 design process in which I was involved and it was reviewed in detail and resulted in little or no design changes but did result in a continuing emphasis on ergonomics. To the best of my knowledge and memory what follows is a description of all the major CANDU small LOCAs. This does not include precursor events and events that caused acute leaks of heavy water.

Karachi Nuclear Power Plant (KANUPP), Pakistan

How wind affects Ontario’s power system dynamics and effect on CANDU refurbishment

By: Donald Jones, P.Eng., retired nuclear industry engineer, 2014 April.

This is really something that the experts at the Independent Electricity System Operator (IESO) and the Ontario Power Authority (OPA) should be looking at with Candu Energy Inc., Ontario Power Generation (OPG), and Bruce Power but while we wait I will give it my ten cents worth.

Adding variable wind and solar generation to the Ontario grid displaces gas-fired generation and some hydro generation. Wind and solar have no inherent capability to help maintain frequency control of the grid. The gas and hydro generation do have this capability but it is being eroded by wind. The substantial amount of nuclear generation, in its present mode of operation, does not contribute to frequency control of the grid. All in all this will contribute to a degradation of frequency control on the grid unless changes are made.

The Ontario power grid is not very flexible. Present nuclear units do not provide much flexibility to the grid and provide no primary frequency control (see reference 1). The improved power output flexibility of the eight nuclear units at Bruce (see reference 2) has improved the flexibility of the Ontario grid somewhat, reducing but not eliminating the need to shutdown nuclear units during periods of surplus baseload generation (SBG) caused by low demand and excess generation. However it has done nothing that helps primary frequency control on the grid to limit variations in frequency caused by wind generation and normal load changes. Grid frequency can give an indication of the system balance between generation and load and needs to be accurately controlled for the proper functioning of the grid.