This tech tip is based on a podcast episode with Bill Nowicki about the latest DHS guidance on HVAC use in nuclear emergencies. You can listen to that podcast HERE. Bill has 45 years of experience in the nuclear industry, starting with his Naval career, and he also has a few podcasts of his own. His podcast about nuclear technology and leadership, “The Nuclear Leader,” can be found on his website HERE. His podcast about his Navy submarine stories, “Submarine Sea Stories,” can also be found on Apple Podcasts HERE or your preferred podcast streaming platform.

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Those of you who grew up during the Cold War might remember doing duck-and-cover drills during school to rehearse for possible nuclear attacks. Or maybe you’ve watched The Simpsons and seen the mutant three-eyed fish in the ponds near the nuclear power plant. In any case, it’s probably safe to say that most of us are aware that nuclear radiation is dangerous and should be avoided (even if it doesn’t really make fish grow an extra eye). With a resurgence in discussions of nuclear energy and global tensions, it’s natural to wonder what nuclear energy and its risks would mean for HVAC professionals—and anyone who relies on HVAC.

The U.S. Department of Homeland Security, the Lawrence Berkeley National Laboratory, the National Urban Security Technology Laboratory (NUSTL), and the U.S. Nuclear Regulatory Commission published an article with advice about HVAC use in nuclear emergencies in August 2024. It breaks down several relevant studies and summarizes the findings to update the long-standing “shelter in place, seal windows, and turn off the HVAC” advice.

The full publication is HERE, but we’re going to cover the highlights and show how the advice for dealing with nuclear radiation has changed from what we might’ve heard or expected in decades past. First, though, we need to define a few key terms and give a brief intro to nuclear power plants and their defenses.

Nuclear Contamination vs. Radiation

Contamination and radiation refer to two different things.

Nuclear contamination is the actual hazardous radioactive material. This material contains unstable atoms that are at risk of decaying. Nuclear fuel is usually isotopes of large atoms (like uranium), which means they easily come apart and decay when there is an imbalance between the number of protons (positively charged particles) and neutrons (which have mass but no charge) in the nucleus.  

On the other hand, radiation is the actual decay. Unstable atoms emit high-energy particles and electromagnetic waves, which can damage the tissues in our bodies. They disrupt molecular structures, including DNA in our cells, which is why there’s a strong link between radiation and cancer.

How Does Nuclear Energy Work?

Everything around us has mass. Even the air that we can’t physically see is made up of atoms, has weight, and takes up space. There is a ton of energy packed into mass. The atoms that make up the stuff around us each have a nucleus filled with positively charged subatomic particles (protons) and neutral ones (neutrons). There is a massive release of energy when the subatomic particles in the nuclei either come apart (fission) or come together (fusion). 

In nuclear power plants, the release of energy via nuclear fission is controlled. Nuclear reactors contain uranium-235 (the nuclear fuel) in fuel cladding, which is surrounded by water that removes the heat from the reaction. That cladding and the coolant system provide two protective layers, but the heat also interacts with the water to create steam. This steam turns a turbine connected to a generator, and the spinning turbine generates power. 

Fission occurs when a neutron is added to the uranium and makes it unstable; the nuclei split apart, releasing fission products (including neutrons). The released neutrons then get added to more uranium nuclei, creating a chain reaction. During the fission process, the contamination is kept inside the cell because there are several barriers inside a controlled nuclear reactor:

• Fuel cladding
• Coolant system (water)
• Containment layer

What Happens When Barriers Fail?

Nevertheless, even with these safeguards in place, there have been accidents. Most of these accidents were caused by some degree of human error (Chernobyl) or natural disasters (Fukushima). The most well-known nuclear power plant accident in U.S. history was the 1979 Three Mile Island (TMI) accident in Pennsylvania.

In this particular accident, some valves malfunctioned and prevented the coolant system (water) from removing heat. It was unclear to the plant workers whether the pilot-operated relief valve (PORV) was open or closed, and they assumed it was closed when it was actually open and releasing coolant. The uranium fuel overheated in its cladding and melted. The melted zirconium cladding reacted with steam to create hydrogen gas. 

Since the cladding came apart and there was no longer an adequate coolant system due to the open valve, some contamination escaped the containment system, forcing local evacuations for vulnerable groups. On top of that, the hydrogen gas presented an explosion risk (however, due to a lack of oxygen inside the reactor, there was no explosion). 

If such an accident were to happen today, the options would be to evacuate or shelter in place, depending on local resources and response times. Sheltering in place would typically require you to seal up windows and doors, but what should we do about the HVAC? We all know that buildings and HVAC systems have come a long way since the 70s, and even since the first federal nuclear emergency advice was issued in the early 2000s.

Don’t Run the HVAC! Or Should We…?

HVAC systems circulate the air in our homes (and our customers’ homes and businesses). Just as they circulate nasty particles like VOCs and viruses, it would stand to reason they’d do the same with nuclear contamination. On top of that, mechanical ventilation (like exhaust fans) can drive infiltration. 

For the majority of the 21st century so far, the advice for using an HVAC system during a shelter-in-place response to a nuclear emergency has been to seal openings to the exterior (like doors, windows, and some vents) and then shut off the HVAC entirely. This advice to turn off the air conditioning makes sense, as we wouldn’t want radioactive particles circulating throughout a building. For the most part, that advice is still recommended for the beginning of a nuclear emergency. 

However, we’ve learned a lot more about building science, particle behavior, and HVAC system functions (including filtration) since the original advice was established. That first set of advice wasn’t necessarily bad, but it doesn’t account for the knowledge we obtained about biochemical particles and filtration during the COVID-19 pandemic. HVAC system design and building codes have also come a long way in the last 20 years. 

We need a more nuanced approach to keep ourselves and our customers safe during a nuclear emergency—namely, we need to understand what we’re really keeping out and how that applies to each home and its HVAC system on a case-by-case basis.

What Are We Really Trying to Keep Out?

You may have rolled your eyes at the mention of filtration. How are filters supposed to help? Atoms are tiny, and neutrons are even smaller. How could we possibly expect buildings and filters to protect us from radiation?

We’re actually not trying to stop radiation from entering. We’re trying to stop the contamination from circulating.

Contamination is deposited on objects, especially those downwind of a nuclear event. These objects include man-made structures, plants, dirt, and airborne particulates. We want to prevent contaminated particulates from getting inside. Contamination that gets into a building will then produce radiation inside it, so our main goal is contamination source control, not filtering out radiation. 

Most Current Advice

Most updates to the established advice deal with source control and nuance between building and HVAC system designs to account for tight vs. loose building envelopes and filtration. 

Immediate Response to a Nuclear Alert

Evacuation is usually the safest move in a nuclear emergency, but it’s not always the most feasible. Sudden failures at nuclear power plants or nuclear attacks can happen within minutes, and authorities may not be able to communicate evacuation routes or advisories. In those cases, sheltering in place (SIP) is the next best solution, but it’s only effective if we shut off the HVAC system and close all windows, doors, and outdoor air vents or dampers. 

Sealing windows, doors, and vents to the outdoors is also recommended. The most common sealing methods consist of fastening plastic sheeting over doors and windows with duct tape.

The local radio should provide updates, so we want to make sure we can tune in to our local channels. We want to make sure the HVAC is off and all penetrations are sealed while the plume passes over. Once you receive official word from the radio that the plume has passed, we can adjust our protections a bit.

Tight Buildings and Recommended Actions

Construction standards have changed immensely since the beginning of the century, when the first set of SIP advice was published. We have seen buildings get tighter over time, and mechanical fresh-air ventilation is a feature in many newer residential buildings.

When we have tight buildings with controlled fresh air pathways, we don’t have to worry as much about fans or exhaust systems driving infiltration. Instead, the bigger issue is closing the fresh air pathway. A manual damper that can be closed, much like a fire damper, would lessen the risk of contamination.

Fresh Air Dilution? Maybe…

We talked about cutting off paths where outdoor air can get in, either via controlled or uncontrolled pathways. However, the COVID-19 pandemic also taught us how we can use fresh air to our advantage when faced with health threats that can’t be seen with the naked eye (albeit biological rather than chemical). 

In the case of COVID-19, a virus that binds to one cell can be easily fought by your body’s immune system before it makes too many copies of its DNA and makes you sick. When many viruses infect your cells at the same time, it becomes a lot harder for your immune system to fight off the rapidly reproducing cells with the viruses’ genetics. Bringing in fresh air dilutes the concentration of viruses, making us less likely to be exposed to a high enough concentration to get sick. 

Recent research suggests that this area of study is of interest for nuclear contamination. A similar approach could potentially be taken with any outdoor-origin aerosols. If pollen or other airborne particles become contaminated, we have ways to filter those particles out in ventilation systems. However, most studies about fresh air dilution have focused on infectious diseases, not nuclear emergencies, and we can’t definitively say whether dilution would be effective. 

Loose Buildings

It’s no secret that buildings used to be built a lot looser in the past. Looseness or a building’s ability to “breathe” was also a planned design feature in older Florida homes. 

These buildings are already at a higher risk of bringing in contamination because more natural ventilation would allow contaminants to deposit on surfaces more readily (especially on rough or rugged floors). Circulating those contaminants by running the central HVAC or a whole-house fan is one of the worst things we could do.

Your best bet for keeping the air clean is to use standalone, portable HVAC units and filtration systems. (Or, if you live downwind of a nuclear power plant and have a leaky house, you may as well consider sealing the building envelope. Tighter building envelopes have been studied to produce better SIP outcomes. The chart below speaks for itself.)

Filtration and Particle Capture

We’ve already established that no filter is going to capture neutrons. When it comes to intercepting contaminated particles, however, HEPA filtration is very promising. HEPA filtration is too restrictive to be used as a primary filter, but we can use it as a secondary filter or as part of a standalone filtration system with promising results. 

HEPA filters have been proven to remove 99.97% of particles equal to or greater than 0.3 microns. For context, after the Fukushima accident, Martin et al. (2019) found that following the Great East Japan Earthquake (2011), fission products bound to earth metals widely varied in size, from 150 nanometers (0.15 microns) to >10 microns. That’s not airborne contamination, but we can still see how HEPA filtration could be effective against all but the smallest particles in that size range. 

One of the studies reviewed in the DHS article (Ward et al., 2005) concluded that HVAC systems may reduce indoor contaminant concentrations by up to 90% when allowed to circulate air in a building with 1–3 portable HEPA filters (as opposed to remaining off). HEPA filters are most effective against smaller particles (<1 micron), as larger particles are more likely to deposit on other surfaces before the HEPA filters can intercept them (including MERV 8–13 HVAC filters). 

Other studies have shown that HVAC systems with filters of lower efficiency rates (e.g., MERV 7) are also more effective at intercepting contaminants and keep the ratio of indoor/outdoor pollution lower than when the HVAC doesn’t run at all. Of course, these filters become less effective as particle size decreases (particularly as size dips below 2 microns).

The Verdict on Whether to Run the HVAC

Here’s the unsatisfying answer: it depends!

Generally, though, it’s much more likely that running the HVAC after the initial nuclear event does more good than harm, especially with a tight building envelope and a mix of standard and standalone HEPA filters. (Those of you who live in older, leakier homes, invest in some HEPA air cleaners or building envelope sealing.) 

Overall, the DHS paper concluded that running the HVAC is detrimental if:

• The chemical release is shorter than 2.5 hours long
• Contaminants are below 0.1 microns in size
• A home is very leaky by design

On the other hand, running the HVAC is most likely to be beneficial if:

• The chemical release is longer than 2.5 hours
• You have quality filtration (including HEPA filters)
• The home was built tightly and sealed 
• You’re trying to reduce particulate matter and fungal spores

The full article has a more in-depth breakdown of studies in similar fields, including bioterrorism, that offer valuable insights into HVAC, infiltration, and air quality control for nuclear emergencies. While studies that touch precisely on all of these at once are simply few and far between, all of the reviewed literature offers something interesting. I highly recommend giving it a look, even if it means slogging through some technical jargon.