Blackstart (and Advanced Reactors) to the Rescue

An Underappreciated Feature of Advanced Reactors will Strengthen the Grid

“Blackstart” sounds like a character from a swashbuckling Errol Flynn flick, or maybe a horse that came in last at the Preakness. But blackstart generators are a critical part of grid resilience, and we are in serious need of more of them.

And now, for the first time, they can be zero-carbon nuclear.

Blackstart is an attribute of a generator. It means the generator can start up without outside help. This is a non-obvious and non-trivial virtue. As a rough analogy, a gasoline-powered car with a working battery is a blackstart machine; one with a dead battery isn’t, and will need a jump. Today’s nuclear reactors are especially reliant on the grid to supply power, as well as receive it as it is made.

But tomorrow’s reactors are designed to do something unimaginable for the ones running today: operate without fear of losing offsite power. This is a side benefit of a passively safe design; with reliance on natural forces like passive heat dissipation, natural circulation and gravity, and with fewer power-hungry internal systems, they can operate more independently. And that can be critical if human error or natural disaster knocks the grid down and engineers have to scramble to get it running again. Plants that can operate in abnormal conditions are always helpful, but these have the added benefit of being carbon-free.

Blackstart-capable advanced nuclear reactors are crucial for the challenges of a carbon-free grid, because most of the blackstart capability today is fossil-based. They are crucial as well for a decarbonized grid made fragile by less-reliable variable renewables. Advanced nuclear can underwrite decarbonization by getting the grid back online.

Advanced Reactors can be More Self-Reliant

The reason that reactor operators have always been concerned about outside power supplies is the production of “residual heat.” With fission, the energy doesn’t come all at once.

When fission stops, the fragments of atoms that have been split are unstable, and thus are radioactive. They seek to return to stability by giving off a particle (alpha or beta radiation) or a packet of energy (gamma radiation). And the atoms that were hit by a neutron but did not split, and instead absorbed the extra particle, are also unstable and give off radiation. The best known of these is plutonium, some of which becomes a fuel.

The radioactive materials created in the reactor will decay, and give off their particles or energy packets, on different schedules. Decay is measured in “half lives,” the time it takes for half the atomic nuclei to give off their radioactivity. The half life of these materials varies from fractions of a second to a few seconds, to months, years, decades or centuries.

But an instant after the reactor shuts down, it is giving off 6 or 7 percent as much energy as it did when it was running. That’s why it’s possible to have fuel damage that begins hours after fission stops.

Current-model reactors and the advanced reactors now moving towards commercial deployment take two different approaches to this problem. The current models maintain extensive systems to open valves, run pumps, and assure that cool water flows into the core, to prevent the water from boiling away and the fuel overheating and melting. The power needed for these tasks doesn’t come from the reactor itself, because if the reactor trips, because of any of dozens of different failures, the power would be cut off. So designers run those components using electricity drawn from the grid, not from the plant’s own generator.

This introduces three difficulties. One is that if the grid goes down, or even suffers a brief upset, the plant goes down too. The second is that the plant can’t restart until the grid is working again. It needs outside power to align the valves, run the pumps, etc. And the third is that the plant operators have to maintain a sophisticated and expensive system of diesel generators that will provide emergency power. They are not big enough to run reactor coolant pumps, though.

Advanced reactors are also set up to have adequate cooling after shut-down, but without requiring electricity. And this turns out to be a crucial difference when it comes to blackstart.

The events in which blackstart generators are called on vary. Earthquakes can shut down broad areas of the grid, as can hurricanes. Some experts postulate that electromagnetic pulses or solar storms could cause widespread blackouts. Some occur because of human error. Whatever the cause of a blackout, blackstart generators are an element of resiliency.

The North American Electric Reliability Corporation, which sets rules for the grid, has recently identified a shortage of blackstart resources as a threat to recovery. The problem is especially acute in Texas. In Storm Uri, Texas grid operators found that some “blackstart” units wouldn’t actually start. That storm killed 246 people in Texas, some from lack of electricity.

Hydroelectric plants make good blackstart generators because they don’t need much electricity to open the gates that let water flow to the turbines, and to excite the generators. But hydroelectric plants can’t be everywhere.

Wind farms typically cannot start up without outside support, because they have pitch and yaw actuators that need power, as well as other equipment. Researchers have proposed diesel generators to allow them to do so, but this is uncommon, and in crisis conditions, sometimes the wind is not blowing. Solar farms can also be blackstart generators, but they could only work when the sun is shining, and they typically only produce electricity for an average of 10 hours a day, even when it’s sunny. At some times of year, they produce considerably less.

Rooftop solar is sometimes marketed as back-up generation if the grid goes down, but it isn’t unless it has a fairly complicated and expensive battery system. And even then, it’s not set up to export power in a way that would jump-start other generators.

And all the nuclear power reactors running in the United States today need grid power to get themselves started up.

A New Capability

Enter advanced reactor designs.

The ones that are water-based, like today’s reactors, have smaller cores, and these leak heat from their outer surfaces. If the core is small enough and designed to allow “passive” heat removal, with no moving parts, then there is no need for an emergency core cooling system like the ones in today’s reactors. NuScale has a design licensed by the Nuclear Regulatory Commission with more water per unit than a typical reactor, but is only about 7 percent as powerful as a Westinghouse AP1000—the design of the reactors recently completed in Georgia. The core sits in a containment shell and the shell is immersed in a huge pool of water. During normal operations, a vacuum layer inside the shell keeps heat in; if cooling is interrupted, the vacuum is broken, and heat flows passively to the pool.

The key point of these details is that the reactor does not need emergency pumps or electricity to power those pumps. If facing a complete blackout, as happened at Fukushima after the earthquake in March 2011, the core stays safe.

But there is a side benefit: Without big in-house electric loads it can restart without grid power. It would need a diesel generator, but that generator could be a simple off-the-shelf model. It would not need to be safety-rated. And the reactor could be allowed to operate if the generator was offline for maintenance or repair. In contrast, current-model reactors have twin diesels for each unit, and if one breaks down, plant managers have only a certain number of hours to fix it or they have to shut the reactor down because of the risk that the surviving diesel might also go bad. Rules like that make advanced reactors that don’t need diesels less prone to unplanned shutdowns.

In the NuScale configuration, a cluster of small reactors, one could be restarted independent of the grid and its energy could be used to let the others restart too.

The newest GE-Hitachi design, the BWRX, does not have blackstart capability in the standard design, but can start up without grid power if a small gas turbine is attached, the company says. The Westinghouse AP300, which is a step behind in the commercial race, does not advertise blackstart capability but does have a robust ability to withstand total loss of power, according to the company.

The Natrium project in Kemmerer, Wyoming, will also have blackstart capability. Natrium and other advanced designs have an additional advantage: Their fuel form tolerates very high heat. In some cases, the fuel form requires high heat.

Natrium, for example, has a normal operating temperature of 500 to 550 degrees C (932 to 1,022 degrees F), but the sodium coolant, already melted in normal operation, won’t boil until it hits about 900 degrees C (1,652 degrees F). Water-based reactors like the ones operating now have a smaller margin, because if they depressurize the water will boil away fairly quickly.

Other reactor designs don’t advertise a blackstart capability but do stress that they don’t need power from the grid or emergency generators to stay safe. Kairos Power uses fuel “pebbles” that are uranium encased in multiple layers of heat-resistant materials, floating in a bath of molten salt that is heated up to 650 degrees C (1,202 degrees F). But the coolant salt can’t boil below 1,430 degrees C (2,606 degrees F).

Thus, emergency cooling is “driven by fundamental physics rather than engineered systems,” the company says.

Building a clean, resilient grid is going to mean picking generators with an eye to what the system needs. When the lights go out (or, more realistically, the home heating system, the air conditioning, the well pump, the power for the router and the wi-fi, etc) the first question everybody asks is, when will it come back on? If the outage is widespread, the answer is that recovery will start at the edges and slowly work its way inward, unless there is adequate blackstart capability. Providing blackstart capability is another way that advanced nuclear will help meet the grid’s needs.