Ontario Power Generation

Update on the Darlington Water Release

What happened on Dec. 21?
On the afternoon of Dec. 21, 2009, Ontario Power Generation’s (OPG) Darlington Nuclear station had an operational event that resulted in the release of about 210,000 litres of purified (demineralized) water from a storage tank.

The release happened when operators inadvertently added lake water to a tank that already contained demineralized water, resulting in an overflow from the tank. The event occurred outside the reactor building and station operation was not affected. All four units at the Darlington station continued to operate at normal power levels.

What is demineralized water?
Demineralized water is completely free (or almost) of dissolved minerals as a result of processing. The water is filtered prior to use in station equipment to avoid debris and mineral build-up that is naturally occurring in lake water.

What is the water in the tanks used for?
Darlington stores demineralized water in many different tanks for cooling equipment during normal plant operation.

How did the overflowed water get into the lake?
The water overflowed from the tank and spilled onto the ground. Some of the water was recovered but some found its way through storm drains into the lake.

Why was the volume of water so large?
The flow of lake water being added to the tank was stopped within minutes. However because the water is transferred to the tank at a high rate through large pipes, a large amount accumulated and overflowed.

Why did the water contain tritium and hydrazine?
Although the tank contains demineralized water, a small amount of the water is circulated to cool and maintain station equipment during routine operation.

Over time the water in the tank takes on small trace amounts of elements that are by-products of normal station operations, including tritium. In addition, hydrazine is added to the tank to prevent corrosion in the associated pipes.

What actions were taken by OPG as a result of the release?
Following OPG Nuclear procedures, the Ontario Ministry of the Environment, the Canadian Nuclear Safety Commission, and Region of Durham water supply plants, other appropriate agencies, and surrounding municipalities were notified and kept informed throughout the event. OPG also posted regular updates to our website opg.com to keep the public and media informed.

Concurrent to communication activities, OPG staff began immediate efforts to contain and recover the water and began investigating the event.

What actions were taken by external agencies?
As a precaution, the frequency of water sampling at the three closest Region of Durham water supply plants was increased to hourly testing (usually sampled twice per day). The affected water supply plants were Bowmanville, Oshawa, and Newcastle. As a further precaution, increased sampling and testing took place at the Port Hope and Toronto water supply plants.

All results from water sampling indicated normal levels of tritium in the water, consistent with ongoing operations. These tritium levels are a very small fraction of the Ontario regulatory limit (see charts below).

Representatives from the Ontario Ministry of the Environment and the Canadian Nuclear Safety Commission toured the area at the Darlington site. We continue to provide them with updates as the investigation unfolds.

What was the impact on the drinking water?
Extensive monitoring, sampling and testing determined that the amount of tritium released to the environment did not impact public drinking water. The results were less than one per cent of the Ontario regulatory limit of 7,000 Becquerels/litre (Bq/l) and consistent with normal operational activities.

What were the results of the water sample testing?
The average test results from Durham Region water supply plants from Dec. 21 (following the release) to Dec. 31, 2009 are shown in the chart. Test results taken since then have continued to remain at normal levels.

Bowmanville Water Supply Plant	9.5 Bq/l
Oshawa Water Supply Plant	10.2 Bq/l
Newcastle Water Supply Plant	7.4 Bq/l

How do the test results compare to normal levels of tritium in drinking water in Durham Region?
The 2008 annual average tritium concentrations at the Bowmanville, Oshawa and Newcastle water supply plants ranged from 6.4 to 9.2 Bq/l. The weekly averages throughout 2008 sometimes ranged slightly above or below this annual average.

The Ontario regulatory limit is an annual average of 7,000 Bq/l. OPG’s nuclear plants maintain annual averages that are consistently far lower than any regulatory limit worldwide, including Ontario’s.

How does this compare with regulatory limits for tritium in drinking water in other parts of the world?
Regulatory Limits for Tritium by Country/Organization

Australia	76,103 Bq/l
World Health Organization (WHO)	10,000 Bq/l
Ontario	7,000 Bq/l
USA (Safe Drinking Water Act)	740 Bq/l
European Union “investigative” value (not a limit but a screening value)	100 Bq/l
OPG internal commitment level	100 Bq/l
OPG normal emissions (annual rolling average)	5-7 Bq/l

What is the impact of such a large volume of water containing tritium when it enters the lake?
The water released from the tank mixed with lake water and became further diluted. This resulted in no measurable impact on drinking water when tested at water supply plants.

What about the hydrazine?
Monitoring and sampling of the released water confirmed the level of hydrazine was at less than minimum detectable levels in both the water and air. There is no expected impact to human health or environment resulting from the hydrazine.

What other emissions come from Darlington Nuclear station operations?
OPG has an extensive Radiological Environmental Monitoring Program which is conducted annually in the vicinity of the Darlington Nuclear and Pickering Nuclear stations. The program determines the radiological impact on the public resulting from the operation of the nuclear stations.

Each year an annual report of the program is posted to our website and includes concentrations of radionuclides in the air, water, milk, soil, sediments, vegetation and fish samples taken in the vicinity of the nuclear stations, and the associated radiation dose assessments.

Visit our website for more information about emissions from the Darlington station and to view the 2008 Radiological Environmental Monitoring Program report.

Updated Jan. 6, 2010

Our public information line is available to answer questions on the Dec. 21 Darlington water release.

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