Sitting in the Dark.

Winter Storm Finn is hammering the East Coast. We lost power at my house almost as soon as the wind kicked up. The NWS is calling for 40 mph winds with 60 mph gusts, so it’s going to be a long night. This got me thinking about how a nuclear plant handles power outages.

First off, federal law requires nuclear plants to have at least 2 separate high voltage lines coming into the plant from the grid. Nuclear plants use grid power just like your house does. If that power goes away, the plant will trip due to the loss of various important pumps (like reactor coolant pumps that push water through the core, or circulating water pumps that pull water from rivers or oceans).

If the grid goes down, all nuclear plants have 2 (or more) back up emergency diesel generators (EDGs). These automatically start if the safety busses (again, usually 2 fully redundant power supplies) lose power and supply the things that are necessary to protect the core (things like shutdown cooling pumps & auxiliary feedwater pumps). These aren’t your typical diesel engines. They’re huge, each one supplying enough power to light up your neighborhood. They’re the size of a bus.

But, losing power isn’t normal. If the power from the grid is lost for 15 minutes, the station will declare an emergency. This is the lowest level, called a Notification of Unusual Event (NUE). This means the level of safety at the site has POTENTIALLY been effected. If both EDGs are working, everything that needs power has it, but the plant is operating without the normal redundant power supplies.

If one of the EDGs were to fail, the plant would be operating with only 1 of its 2 safety systems having power. This means that if the other EDG fails, the station wouldn’t have power to its safety systems. This would require the station to declare an Alert, the 2nd highest emergency classification. An Alert means that events are in progress which have (or could) SUBSTANTIALLY degrade plant safety. Nothing really bad has actually happened yet, but the station is 1 bad thing away from possibly not being able to cool the core.

If that 2nd EDG doesn’t work, the plant enters a condition known as a station blackout (SBO). The only power available is the batteries. These aren’t big enough to run the pumps, they just go to indications & controls. If AC power isn’t restored within the coping time (legal requirement for operating without AC power), the ability to protect the core can be lost. Nuclear plants have an auxiliary feedwater system that uses decay heat to make steam & spin a pump, so the core is OK for several hours even in this condition. But, you have to get power back ASAP.

An SBO condition requires the station to declare the 3rd level of emergency, a Site Area Emergency (SAE). An SAE means that events are in progress which have (or likely will) caused major failures of plant functions that protect the public. This rarely happens. Only 5 SAEs have been declared at US nuclear plants. No one was injured or exposed to radiation in those events.

If the station does not restore AC power within 4 hours, or there is indication that core cooling has been lost, the station declares a General Emergency (GE). A GE means that events are in progress that have caused (or will shortly do so) substantial core damage with the potential for uncontrolled release of radioactive material.

A GE has happened once in the US, at Three Mile Island in 1979. Despite melting half the core & performing a release by venting the containment to atmosphere, no one received any significant dose from TMI.

So, what should you take from this? It takes multiple levels of failures of redundant systems designed to protect the core before an event can threaten the core. It takes even more failures before that damage threatens the public. Our job as licensed operators is to protect the health & safety of the public. We take that job extremely seriously. Even with the wind blowing like it is tonight, your local nuclear plant will be fine.

Heres the link 

https://www.nrc.gov/about-nrc/emerg-preparedness/about-emerg-preparedness/emerg-classification.html