What this covers
How a nuclear blast in the upper atmosphere could disable the power grid. The bundle deal with Curiosity Stream has ended, but you can still get a great discount on Nebula and support Practical Engineering here: https://go.nebula.tv/practical-engineering
Correction: 10:32 I meant to say "earth's magnetic field," not "earth's gravity." Correction: A previous version of this video included a segment where I used a doorbell transformer to demonstrate core saturation. That model did not correctly demonstrate the phenomena I was describing, so I cut it from the video.
This video is a summary of the EPRI study on the impacts that a high-altitude nuclear electromagnetic pulse (HEMP) would have on the US power grid. It’s the first in a deep-dive series of videos about large-scale threats to the grid.
EPRI Study: https://www.epri.com/research/products/3002014979
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Grady argues that while a high-altitude nuclear electromagnetic pulse (EMP) would cause significant regional power grid disruptions through E1 relay damage and E3 transformer saturation, the actual damage would likely fall short of Hollywood apocalypse scenarios but could still cause multi-week or multi-month blackouts in worst-case scenarios.
- E1 pulses can damage digital protective relays on transmission lines, with EPRI modeling suggesting ~5% of relays could be affected by a 1-megaton burst at 200km altitude
- E3 pulses induce DC current in transmission lines that saturates transformer cores, potentially causing regional blackouts across multiple states
- Combined E1 and E3 effects, plus disruption to communications and other systems, could extend recovery to multi-week or multi-month timescales
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The electromagnetic pulse from the Yucca test was recorded at around 5 times the maximum limit of the instruments at one monitoring station, which was so far beyond expectations that it was dismissed as an anomaly for years.
“What they recorded was so far beyond their expectations, that it was dismissed as an anomaly for years. All that appears in the report is a casual mention of the estimated electromagnetic field strength at one of the monitoring stations being around 5 times the maximum limit of the instruments.”
For E1 pulse damage to relays, a single relay failure is probably not a big deal because there are often redundant paths for current to flow in the transmission system due to the grid's interconnected nature, but the more equipment that goes offline and the greater the stress on remaining lines, the greater the likelihood of cascading failure or total collapse.
“For the E1 pulse damaging some relays, that's probably not a big deal. There are often redundant paths for current to flow in the transmission system. That's why it's called the grid. But the more equipment that goes offline and the greater the stress on the remaining lines, the greater the likelihood of a cascading failure or total collapse.”
On April 28, 1958, the USS Boxer aircraft carrier launched a 1.7 kiloton nuclear warhead code-named Yucca from 85,000 feet altitude off Bikini Atoll as part of operation Hardtack, making it the first test of a nuclear blast in the upper limits of earth's atmosphere.
“Late in the morning of April 28, 1958, the USS Boxer aircraft carrier ship was about 70 miles off the coast of the Bikini Atoll in the Pacific Ocean... attached to this balloon was a 1.7 kiloton nuclear warhead, code named Yucca... Yucca was the first test of a nuclear blast in the upper limits of earth's atmosphere. About an hour and a half after the balloon was launched, it reached an altitude of 85,000 feet or about 26,000 meters.”
At the height of the Yucca test (85,000 feet), the E1 pulse would affect a circle with an area larger than Texas, and a weapon at 200 kilometers in altitude could impact a significant fraction of North America.
“At the height of the Yucca test, that's a circle with an area larger than Texas. A weapon at 200 kilometers in altitude could impact a significant fraction of North America.”
Physicist Conrad Longmire proposed a theory 5 years after the Yucca test (approximately 1963) for why electromagnetic pulses from high-altitude nuclear blasts are orders of magnitude stronger than those generated from blasts on the ground, and this theory is still the widely accepted explanation.
“It wasn't until 5 years later that the US physicist Conrad Longmire would propose a theory for electromagnetic pulses from high-altitude nuclear blasts that is still the widely accepted explanation for why they are orders of magnitude stronger than those generated from blasts on the ground.”
The longer the wavelength of electromagnetic radiation, the larger an antenna has to be to couple well with it, meaning transmission and distribution lines that run for miles are ideally sized to couple with E3 pulse wavelengths.
“As you can imagine, the longer the wavelength, the larger an antenna has to be to couple well with the electromagnetic radiation. And hopefully you see what I'm getting at. Electrical transmission and distribution lines often run for miles, making them the ideal place for an E3 pulse to couple and generate current.”
Modern digital protective relays are full of circuit boards, screens, and microelectronics that are particularly susceptible to electromagnetic interference, and most countries have strict regulations about the strength and frequency of electromagnetic radiation permitted on the airwaves.
“most relays these days are digital equipment full of circuit boards, screens, and microelectronics. And all those components are particularly susceptible to electromagnetic interference. In fact, most countries have strict regulations about the strength and frequency of electromagnetic radiation you can foist upon the airwaves.”
In 2019, the Electric Power Research Institute (EPRI), a coalition of energy organizations and government entities, funded a study to understand what could happen to the power grid from a high-altitude nuclear EMP, with this study containing the most comprehensive engineering details available.
“As with most weapons, the best and most comprehensive research on EMPs is classified. But, in 2019, a coalition of energy organizations and government entities called the Electric Power Research Institute (or EPRI) funded a study to try and understand exactly what could happen to the power grid from a high altitude nuclear EMP... it has the most juicy engineering details from all the research I could find.”
When combining the effects of E1 and E3 pulses, it's not hard to imagine how the grid could be seriously disabled, and the widespread nature of an EMP plus its potential impacts on other systems like computers and telecommunications could frustrate recovery efforts.
“When you combine the effects of the E1 pulse and the E3 pulse, it's not hard to imagine how the grid could be seriously disabled. It's also easy to see how, even if the real damages to equipment aren't that significant, the widespread nature of an EMP, plus its potential impacts on other systems like computers and telecommunications, has the potential to frustrate the process of getting things back online.”
In a worst-case scenario, a nuclear EMP could cause a multi-day, multi-week, or even multi-month blackout, though it's probably not going to cause a Hollywood-style return to the stone age for humanity.
“A multi-day, multi-week, or even multi-month blackout isn't out of the question in the worst-case scenario. It's probably not going cause a hollywood-style return to the stone age for humanity, but it is certainly capable of causing a major disruption to our daily lives.”
The E2 pulse from a nuclear EMP is generated from the interaction of gamma rays and neutrons and is roughly comparable in strength to a lightning strike, often being less powerful than many lightning strikes from high-altitude nuclear detonations.
“The E2 pulse is slower than E1 because it's generated in a totally different way, this time from the interaction of gamma rays and neutrons. It turns out that an E2 pulse is roughly comparable to a lightning strike. In fact, many lightning strikes are more powerful than those that could be generated by high-altitude nuclear detonations.”
For a strong enough electromagnetic pulse spread across a huge range of frequencies, essentially any metallic object will act like an antenna, converting the pulse into massive voltage spikes that can overwhelm digital devices.
“You're probably familiar with antennas that convert radio signals into voltage and current within a conductor. Well, for a strong enough pulse spread across a huge range of frequencies, essentially any metallic object will act like an antenna, converting the pulse into massive voltage spikes that can overwhelm digital devices.”
When a nuclear detonation occurs in the upper atmosphere, Earth's magnetic field interacts with free electrons produced by gamma rays to create a significantly stronger electromagnetic pulse than if detonated in denser air at lower altitudes.
“That's generally bad on its own, but when detonated high in the atmosphere, earth's magnetic field interacts with those free electrons to produce a significantly stronger electromagnetic pulse than if detonated within the denser air at lower altitudes.”
An E3 pulse can induce a roughly DC (direct current) flow of current through transmission lines, creating a bias in the sine wave of AC current that can saturate transformer cores when current moves in one direction but not the other, distorting the output waveform.
“An E3 pulse can induce a roughly DC flow of current through transmission lines. So you have DC on top of AC, which creates a bias in the sine wave. If there's too much DC current, the transformer core might saturate when current moves in one direction but not the other, distorting the output waveform.”
The E1 pulse happens so quickly that even devices meant to protect against surges may not be effective, making it difficult to protect sensitive electronics with conventional surge protection.
“And, the E1 pulse happens so quickly that even devices meant to protect against surges may not be effective.”
The E1 pulse is spread across a huge part of the electromagnetic spectrum, referred to as DC to daylight, and reaches anywhere within line of sight of the detonation, covering an enormous area of land at high altitude.
“the energy is somewhat jokingly referred to as DC to daylight, meaning it's spread across a huge part of the electromagnetic spectrum. The E1 pulse generally reaches anywhere within a line of sight of the detonation, and for a high-altitude burst, this can cover an enormous area of land.”
Transformer core saturation from E3-induced DC current can lead to hot spots in the transformer core, damage to devices connected to the grid that expect a sinusoidal voltage pattern, and other cascading failures.
“That can lead to hot spots in the transformer core, damage to devices connected to the grid that expect a nice sinusoidal voltage pattern, and lots of other funky stuff.”
EPRI's modeling didn't lead to widespread damage to actual transformers from a 1-megaton EMP, and this is a good thing because power transformers are large, expensive devices that are hard to replace and most utilities don't keep many spares sitting around.
“Their modeling didn't lead to widespread damage to the actual transformers, and that's a good thing because power transformers are large, expensive devices that are hard to replace, and most utilities don't keep many spares sitting around.”
E3 pulses are similar to geomagnetic storms caused by solar events that disrupt Earth's magnetic field, and large solar events could potentially pose a bigger threat to the grid than nuclear EMPs because of the disturbance and movement of Earth's magnetic field.
“This is similar to what happens when a geomagnetic storm on the sun disrupts earth's gravity, and large solar events could potentially be a bigger threat than a nuclear EMP to the grid. In both cases, it's because of the disturbance and movement of earth's magnetic field.”
In transformers, there is a relationship between how much current flows and the strength of the resulting magnetic field, but this relationship breaks down at a saturation point beyond which additional current won't create much further magnetism to drive current on the secondary winding.
“relationship between how much current flows and the strength of the resulting magnetic field. But, this relationship breaks down at the saturation point, beyond which additional current won't create much further magnetism to drive current on the secondary winding.”
An E1 pulse can couple to the wires serving as inputs to electronic devices and can also radiate the equipment directly, with both coupling mechanisms presenting pathways for EMP damage.
“You can also see the two different types of E1 vulnerabilities here. An EMP can couple to the wires that serve as inputs to the device. And an EMP can radiate the equipment directly. In both cases, this little device wasn't strong enough to cause permanent damage to the electronics.”
AM radio uses frequencies between 540 kilohertz and lower, corresponding to wavelengths upwards of 1800 feet or 550 meters, and AM radio towers are the antenna themselves rather than serving as a mounting point, with the entire metal structure energized.
“For example, AM radio uses frequencies between down to 540 kilohertz. That corresponds to wavelengths that can be upwards of 1800 feet or 550 meters, big waves. Rather than serving as a place to mount antennas like FM radio or cell towers, AM radio towers are the antenna. The entire metal structure is energized!”
When a magnetic field moves through a conductor, it generates a current (coupling), and this is essentially how antennas work, with antennas working best when their size matches the size of the electromagnetic waves they are designed to receive.
“You probably know what happens when you move a magnetic field through a conductor: you generate a current. We call that coupling, and it's essentially how antennas work. And in fact, antennas work best when their size matches the size of the electromagnetic waves.”
Digital protective relays on the electrical grid are separate from circuit breakers and monitor voltage and current for problems to tell breakers when to disconnect lines under fault conditions, and these devices are located in substations to protect them from weather.
“Most folks have seen the breakers that protect circuits in your house. The electrical grid has similar equipment used to protect transmission lines and transformers in the event of a short circuit or fault. But, unlike the breakers in your house that do both the sensing for trouble and the circuit breaking all in one device, those roles are separate on the grid... the devices that monitor voltage and current for problems and tell the breakers when to fire are called relays. They're normally located in a small building in a substation to protect them from weather.”
The E3 component of a nuclear EMP is not really a pulse but a slowly changing phenomenon caused by disturbance and distortion of Earth's magnetic field following detonation, which takes a few minutes to return to its original state.
“The final component of an EMP, called E3, is, again, much different from the other two. It's really not even a pulse at all, because it's generated in an entirely different way. When a nuclear detonation happens in the upper atmosphere, earth's magnetic field is disturbed and distorted. As the blast dissipates, the magnetic field slowly returns to its original state over the course of a few minutes.”
Electrical transformers work by using one coil of wire to generate a magnetic field that passes through a core to induce current to flow through an adjacent coil, and the main reason AC is used on the grid is because it allows simple voltage step-up and step-down using transformers.
“Those transformers work using electromagnetic fields. One coil of wire generates a magnetic field that passes through a core to induce current to flow through an adjacent coil. In fact, the main reason we use alternating current on the grid is because it allows us to use these really simple devices to step voltage up or down.”
A nuclear electromagnetic pulse (EMP) has three distinct components—E1, E2, and E3—formed by different physical mechanisms that can have significantly different impacts on Earth's surface.
“An EMP actually has three distinct components all formed by different physical mechanisms that can have significantly different impacts here on Earth's surface. The first part of an EMP is called E1.”
The E1 component of a nuclear EMP is extremely fast and intense, lasting only a few nanoseconds, and is generated when gamma rays released during detonation collide with electrons, ionizing atoms and creating a burst of electromagnetic radiation.
“The first part of an EMP is called E1. This is the extremely fast and intense pulse that immediately follows detonation. The gamma rays released during any nuclear detonation collide with electrons, ionizing atoms and creating a burst of electromagnetic radiation... The E1 pulse comes and goes within a few nanoseconds.”
EPRI's report on nuclear EMP grid impacts is not without criticism, and many believe that an EMP could result in far more damage to electric power infrastructure than their study suggests.
“All that being said, their report isn't without criticism and many believe that an EMP could result in far more damage to electric power infrastructure.”
The EPRI report subjected digital relays to strong EMPs using a Marx generator (a voltage multiplying circuit) to test the effects of electromagnetic pulses on protective relays.
“The EPRI report actually subjected digital relays to strong EMPs to see what the effects would be. They used a Marx generator which is a voltage multiplying circuit.”
The grid is already hardened against high voltage pulses such that lightning strikes usually don't create much damage, so the E2 pulse from a nuclear EMP is not as threatening to power infrastructure as E1 and E3.
“the grid's not entirely immune to lightning, but we do use lots of lightning protection technology. Most equipment on the grid is already hardened against some high voltage pulses such that lightning strikes don't usually create much damage. So, the E2 pulse isn't as threatening to our power infrastructure, especially compared to E1 and E3.”
EPRI modeled a one megaton nuclear bomb detonated at 200 kilometers altitude and estimated that about 5% of transmission lines could have a relay that gets damaged or disrupted by the resulting E1 pulse.
“EPRI did tests simulating a one megaton bomb detonated at 200 kilometers in altitude. They estimated that about 5% of transmission lines could have a relay that gets damaged or disrupted by the resulting EMP.”
EPRI's 5% relay damage estimate alone probably isn't enough to cause a large-scale blackout of the power grid, but when combined with E3 effects, it could lead to regional blackouts encompassing multiple states.
“That alone probably isn't enough to cause a large-scale blackout of the power grid, but don't forget about E3. EPRI found that the third part of an EMP could lead to regional blackouts encompassing multiple states because of transformer core saturation and imbalances between supply and demand of electricity.”
On the electrical grid, E1 pulse damage concerns are primarily located in three places: control systems within power plants, communications systems used to monitor and record data for grid operators, and digital protective relays.
“But on the grid, there are really only a few places where an E1 pulse is a major concern. The first is with the control systems within power plants themselves. The second is communications systems used to monitor and record data to assist grid operators. The EPRI report focused primarily on the third hazard associated with an E1 pulse: digital protective relays.”
Due to Earth's magnetic field, the maximum amplitude of an E1 pulse occurs slightly south of ground zero in the northern hemisphere, creating a pattern called a smile diagram.
“But, because of earth's magnetic field, the maximum amplitude occurs a little bit south of ground zero (in the northern hemisphere), creating this pattern called a smile diagram.”
AM radio towers sit atop small ceramic insulators that electrically separate them from the ground to prevent grounding.
“You can often tell an AM tower by looking at the bottom because they sit atop a small ceramic insulator that electrically separates them from the ground.”