Rising Temperatures Could Cause More Unplanned Outages at Nuclear Plants

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Written by Jesse Broehl, Editor-in-Chief, Breaking Energy News (BEN), for IIR News Intelligence (Sugar Land, Texas)

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The 1,202-megawatt (MW) Prairie Island Nuclear Power Station in Minnesota sits near the Mississippi River. On Sunday, the facility was temporarily forced to de-rate its capacity by 367 MW this week due to the high temperature of the river that is used for the station's cooling needs. It is very rare that nuclear plants are forced to reduce capacity unplanned for this reason, but it may occur more often as temperatures rise and heat waves are more prolonged.

The 593-MW Unit 02 was temporarily de-rated on July 19 by around 62%. The station has since reported it was back fully online, according to IIR Energy's Breaking Energy News (BEN). Regional utility Xcel Energy is the owner/operator via its local Northern States Power Company.

Nuclear Plant Cooling Design Differences

This is not a common reason for an unplanned outage at U.S. nuclear plants, but some plants are more susceptible than others to high river or lake temperatures reducing the effectiveness of plant cooling systems, according to Bill Hoyer, Senior Account Executive for IIR Energy.

Older power plants in Northern states like Minnesota and Wisconsin frequently rely on once-through (open-loop) cooling, drawing water directly from the river, circulating it through a steam condenser, and discharging warm effluent back into the river.

Plants built later or situated in warmer southern regions are far more likely to feature closed-loop recirculating cooling (cooling towers) or dry cooling (air-cooled condensers). Closed-loop systems evaporate heat into the atmosphere using cooling towers rather than relying on massive river heat sinks, reducing raw water intake and direct river thermal discharge by up to ~95%.

Additionally, there are federal environmental limits for maximum discharge water temperature (often around 86°F-90°F). When ambient air temperatures spike and river flow drops, intake temperatures approach these legal thresholds before the plant even adds its heat load, thereby triggering plant operators to reduce plant capacity.

Over 45% of U.S. Nuclear Plants Use Open-Loop Cooling

According to the World Nuclear Association, approximately 46% of U.S. nuclear reactors use once-through (open-loop) cooling systems, which are more susceptible to high temperatures. Details on all nuclear plants and units operating in North America are tracked here by IIR Energy.

Nuclear plants being de-rated unplanned due to heat waves could become more common as temperatures have risen over recent decades. According to the U.S. Environmental Protection Agency (EPA), across the contiguous U.S., average temperatures have already risen about 60% more than the global average since 1970. This is expected to continue as global temperatures rise due to climate change.

Multiday heat waves are expected to be longer, affect more of the country, and become more severe (EPA). In recent decades, heat waves have already become hotter, occur more often, and are larger and longer-lasting (EPA).

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