Nuclear Preparedness Guide: What to Do Before, During, and After a Nuclear Event

The Bulletin of the Atomic Scientists moved the Doomsday Clock to 89 seconds to midnight in January 2025, the closest it has ever been. Since then, the situation has not improved. The New START treaty expired in February 2026 with no replacement framework. U.S.-Iran tensions escalated through joint strikes on Iranian nuclear sites and the subsequent closure of the Strait of Hormuz. The NPT Review Conference underway in New York this month has produced little consensus.

None of this means a nuclear event is imminent. It means the margin for error is thinner than it has been in decades, and the window for preparation is now, not after a crisis alert hits your phone.

This guide covers the practical steps for nuclear preparedness: understanding the threat, building a shelter plan, assembling supplies, and knowing what to do in the first minutes and hours of an event.

Understanding the Threat Spectrum

Nuclear preparedness is not a single scenario. The threat ranges across several categories, each with different preparation requirements:

• A nuclear detonation over a city (the scenario most people imagine)
• A high-altitude electromagnetic pulse (EMP) designed to disable infrastructure over a wide area
• A radiological dispersal device, commonly called a dirty bomb, which spreads radioactive material without a nuclear explosion
• A nuclear facility accident, such as Chernobyl or Fukushima

According to FEMA's 2024 Nuclear Detonation Planning Guide, a single 10-kiloton device (the size most likely in a terrorist scenario, roughly two-thirds the yield of the Hiroshima bomb) would produce severe damage within a half-mile radius and dangerous fallout extending 10 to 20 miles downwind. A state-level attack would involve much larger warheads, but the core survival principles remain the same: distance, shielding, and time.

Radiation Basics You Need to Know

You do not need a physics degree, but you need to understand three numbers.

Radiation exposure is measured in roentgen equivalent man (rem) or sieverts (Sv). The conversion: 1 Sv equals 100 rem. According to the CDC, acute exposure above 200 rem causes radiation sickness. Exposure above 450 rem is lethal for roughly 50% of people without medical treatment. For context, the average American receives about 0.62 rem per year from natural background radiation.

The critical concept is the 7:10 rule of fallout decay. For every sevenfold increase in time after detonation, radiation intensity drops by a factor of 10. Fallout that measures 1,000 roentgens per hour (R/hr) at 1 hour post-detonation drops to 100 R/hr at 7 hours, 10 R/hr at 49 hours (roughly 2 days), and 1 R/hr at 343 hours (about 2 weeks). This is why the standard shelter-in-place guidance is a minimum of 24 hours and ideally 48 to 72 hours.

Three factors reduce your exposure:

• Distance: radiation intensity decreases with the square of the distance from the source
• Shielding: dense materials between you and the radiation source absorb particles
• Time: minimize duration of exposure, and let the 7:10 rule work in your favor

Shelter: Your Most Important Asset

FEMA's guidance is straightforward: get inside, stay inside, stay tuned. But "inside" varies enormously in protection value.

Protection Factor (PF) measures how much a shelter reduces radiation exposure. A PF of 10 means you receive one-tenth the outside dose. The Department of Homeland Security provides these approximate values:

• Wood-frame house: PF 2-3
• Typical apartment or office building (upper floors): PF 10
• Concrete or brick building (center rooms, away from windows): PF 20-40
• Basement of a concrete building: PF 100-200
• Purpose-built fallout shelter with 2-3 feet of packed earth: PF 200-1,000

A wood-frame house on a slab offers almost no fallout protection. If you live in one, identify the nearest concrete or brick structure within a 5-minute travel radius. Schools, parking garages, commercial buildings, and government offices are common options.

If you are sheltering in your home, the best position is a basement or interior room on the lowest floor, away from exterior walls and windows. Every layer of material between you and the outside counts. Stacking furniture, books, or bags of soil against exterior basement walls can meaningfully increase your protection factor.

Expedient Shelter Improvements

If you have 24 to 48 hours of warning (which is plausible in a geopolitical escalation scenario), you can significantly improve a basement shelter:

1. Cover basement windows with soil, sandbags, or stacked concrete blocks. Each inch of concrete provides roughly a PF increase of 1.5.
2. Build an inner shelter using doors or plywood laid across furniture as a frame, then pile 12 to 18 inches of soil, sand, or water containers on top.
3. Seal gaps and ventilation with plastic sheeting and duct tape to reduce particulate infiltration.

The original U.S. government reference for expedient shelter construction is Cresson Kearny's Nuclear War Survival Skills, published by Oak Ridge National Laboratory. The full text, including step-by-step shelter designs, is one of the 19 source documents included on the Deadnet USB drive and can be referenced offline without internet access.

The Nuclear Preparedness Supply List

Your supplies break into two categories: what you need for the initial 72-hour shelter period, and what you need for the extended disruption that follows.

72-Hour Shelter Supplies (Per Person)

• Water: 1 gallon per person per day, minimum 3-day supply. FEMA recommends 1 gallon, but if you are in a hot climate or have high exertion, plan for 1.5.
• Food: non-perishable, requiring no cooking. Canned goods, energy bars, peanut butter, dried fruit. Minimum 2,000 calories per day per adult.
• Potassium iodide (KI) tablets: 130mg dose for adults, 65mg for children 3-18. The FDA recommends KI to block thyroid uptake of radioactive iodine within the first 24 hours of exposure. It does not protect against other radioactive isotopes. Acquire through a pharmacy or FEMA distribution point.
• Battery-powered or hand-crank AM/FM radio with NOAA Weather Radio capability
• Flashlights and extra batteries (LED types last significantly longer)
• Plastic sheeting (6 mil thickness) and duct tape for sealing rooms
• N95 or P100 respirators: these filter particulates including fallout dust. The CDC confirms N95 masks are effective against radioactive particulate inhalation.
• First aid kit with a 14-day supply of any prescription medications
• Sanitation supplies: heavy-duty garbage bags, portable toilet or 5-gallon bucket with lid, toilet paper, hand sanitizer
• Copies of personal documents in a waterproof bag: IDs, insurance policies, medical records

Extended Duration Supplies

If infrastructure is damaged, expect disruptions lasting weeks to months. The 2011 Fukushima disaster displaced over 150,000 people, and many evacuation zones remained restricted for years. For a direct nuclear event on U.S. soil, cascading effects on power, water treatment, supply chains, and communications would compound the initial damage.

• Water purification: filter rated for 0.2 microns or smaller, purification tablets, and knowledge of boiling protocols (rolling boil for 1 minute, 3 minutes above 6,500 feet elevation per EPA guidance)
• Extended food supply: freeze-dried meals, bulk rice and beans, canned proteins. A 30-day supply for a family of four runs approximately 60,000 to 90,000 calories.
• Dosimeter or radiation detector: a personal dosimeter allows you to track cumulative exposure. Survey meters (CDV-715, CDV-717) are available surplus from the Cold War-era civil defense program and remain functional.
• Comprehensive reference materials: field medicine guides, water treatment procedures, shelter construction details. Having these in a format that works without electricity or internet is the gap most people overlook. The Deadnet drive addresses this with its offline AI assistant and source documents covering medical, military, and survival references.

What to Do in the First 15 Minutes

If you receive a ballistic missile alert or see a flash:

1. Do not look at the fireball. Temporary or permanent blindness can occur at distances of 10 miles or more from a 1-megaton detonation, according to the Department of Defense.
2. If outdoors, take cover immediately. Get behind or inside any solid structure. Lie flat and cover exposed skin if no structure is available.
3. The blast wave from a 10-kiloton detonation reaches 1 mile in about 5 seconds. If you see the flash and are close enough to feel heat, you have seconds, not minutes. Drop to the ground, face down, arms covering your head and neck.
4. After the blast wave passes, you have 10 to 15 minutes before fallout begins to settle. Use this window to move to the best available shelter.
5. Once inside shelter, remove outer clothing and bag it. Wipe or wash exposed skin with soap and water. Do not use conditioner, as it binds radioactive particles to hair. This decontamination step can remove up to 90% of external radioactive material according to the CDC.
6. Tune to emergency broadcasts on AM/FM or NOAA Weather Radio. Follow instructions on whether to shelter in place or evacuate.

Evacuation vs. Shelter in Place

The default answer is shelter in place for the first 24 to 72 hours unless authorities direct evacuation. The reason is straightforward: fallout is most dangerous in the first hours, and traveling through it exposes you to far more radiation than staying in even a moderate shelter.

FEMA's Planning Guidance for Response to a Nuclear Detonation (3rd edition) models show that for people within the dangerous fallout zone, sheltering in a PF-10 or better structure for 12 to 24 hours before evacuating reduces casualties significantly compared to immediate evacuation through the fallout plume.

Exceptions to shelter in place:

• Your structure offers minimal protection (PF under 5) and a much better shelter is within a few minutes' travel
• Authorities issue an evacuation order based on wind patterns and fallout projections
• Your shelter is structurally damaged and unsafe

If you do evacuate, travel perpendicular to the wind direction if possible. Fallout plumes follow prevailing wind patterns and can extend 10 to 20 miles in an elongated oval. Moving crosswind gets you out of the plume faster than moving upwind or downwind.

Communications and Information

Assume cell networks will be overwhelmed or down. In Maui during the 2023 wildfire, cell service failed within the first hours. A nuclear event would produce far greater disruption.

Your communication layers, in order of reliability:

1. NOAA Weather Radio: these transmitters are hardened and have backup power. A hand-crank radio tuned to the correct frequency (162.400 to 162.550 MHz) is your most reliable one-way information source.
2. AM radio: AM signals travel farther than FM, especially at night, and AM broadcast infrastructure is more widely distributed.
3. Two-way radios (FRS/GMRS): useful for family coordination within a 1 to 2 mile radius. No license required for FRS channels.
4. HAM radio: if you hold a license and have equipment, HF bands can reach hundreds or thousands of miles.

If an EMP component is involved, unshielded electronics may be damaged. Keeping a backup radio and flashlight in a Faraday bag or a sealed metal container (a galvanized trash can with a tight-fitting lid and foil tape on the seam works as a basic Faraday cage) provides a layer of protection. Testing by the EMP Commission showed that while vehicle electronics are more resilient than commonly feared, communication devices and sensitive electronics are genuinely vulnerable.

Long-Term Considerations

The acute phase of a nuclear event is measured in days. The recovery phase is measured in months to years. Preparing for the long tail means thinking about:

• Water: municipal water treatment depends on electricity and chemical supply chains. Have purification capability independent of the grid.
• Food production: stored food buys time. Seeds, basic gardening knowledge, and understanding of your local growing season provide a path toward sustainability. The USDA Plant Hardiness Zone Map and FM 21-76 (the Army Survival Manual) cover food procurement in various environments.
• Medical care: hospitals will be overwhelmed. A working knowledge of wound care, infection prevention, and common medication uses becomes a force multiplier for your household and community.
• Community: isolated households are vulnerable. The neighborhoods and communities that recovered fastest from Hurricane Katrina, the Joplin tornado, and other disasters were the ones with pre-existing social networks and mutual aid agreements. FEMA's Community Emergency Response Team (CERT) program has trained over 600,000 volunteers nationwide and is free to join.

Your Action Step for Today

Do one thing right now: identify your shelter location. Walk through your home or apartment and find the room with the most mass between you and the outside. Basement, interior bathroom, interior closet. If your home offers a protection factor under 5 (wood frame, no basement), identify the nearest concrete or brick building you can reach in under 10 minutes. Write down that location. Tell your household members. That single decision, made calmly today, is worth more than any amount of gear purchased in a panic.