Nuclear Preparedness Guide: What to Do in the First 24 Hours

Nuclear threat planning has moved from Cold War museum piece to active civil defense concern. The Stockholm International Peace Research Institute reported roughly 12,121 nuclear warheads in global inventories as of January 2024, with approximately 9,585 in active military stockpiles. The United States holds about 5,044 and Russia about 5,580, according to SIPRI's 2024 yearbook. The Bulletin of the Atomic Scientists set the Doomsday Clock at 89 seconds to midnight in January 2025, the closest it has ever been.

This guide covers what happens in the first 24 hours after a nuclear detonation, what the science says about survivable distances, and the specific actions that change outcomes. It assumes you are not at ground zero. If you are within the fireball radius of a modern weapon, no preparation matters. If you are outside it, preparation matters more than almost any other variable.

What a Nuclear Detonation Actually Does

A nuclear weapon produces four effects in sequence: blast wave, thermal pulse, initial radiation, and delayed fallout.

The U.S. Department of Homeland Security Planning Guidance for Response to a Nuclear Detonation (3rd edition, 2022) divides the affected area into three zones:

• Severe Damage Zone: buildings collapsed, fires widespread, survival unlikely
• Moderate Damage Zone: significant structural damage, many survivors, many injured
• Light Damage Zone: broken windows, some injuries, infrastructure intact

For a 10-kiloton improvised nuclear device — the yield FEMA typically plans against for terrorist scenarios — the Severe Damage Zone extends roughly 0.5 miles from ground zero, the Moderate Damage Zone reaches about 1 mile, and the Light Damage Zone extends to roughly 3 miles. Strategic warheads in the 100-kiloton to 1-megaton range scale these distances significantly.

Thermal pulse from a 1-megaton airburst can cause third-degree burns at distances up to 8 miles on clear days, according to the National Academies' 2014 report on lessons from Fukushima. Looking at the flash causes retinal burns at far greater distances. If you see a sudden bright light, do not turn toward it.

The 15-Minute Window

FEMA, CDC, and the National Council on Radiation Protection converge on a single piece of tactical guidance: after a detonation, you have approximately 15 minutes before significant fallout begins arriving in areas outside the blast zone. That 15 minutes is the most valuable interval in the entire event.

The rule, published by FEMA and Ready.gov, is three words: Get Inside. Stay Inside. Stay Tuned.

Getting inside means moving to the center of the most substantial building available. CDC guidance documents rank shelter quality by dose reduction factor (DRF), which measures how much radiation is blocked:

• Outdoors or in a vehicle: DRF of 1 (no protection)
• Wood-frame house, above ground: DRF of 2 to 3
• Wood-frame house, basement: DRF of 10
• Brick or concrete building, center floor: DRF of 10 to 20
• Large office building, central core: DRF of 100 to 200
• Sub-basement of a large concrete structure: DRF of 200 or more

A DRF of 200 means you receive 1/200th of the external radiation dose. That is the difference between a lethal exposure and a survivable one.

During the 15-minute window, the correct action is to move to the best available shelter, not the shelter at your home. If you are at work in a concrete building and your house is wood-frame two miles away, stay at work.

Fallout Timing and the 7-10 Rule

Fallout consists of debris from the blast site that has been made radioactive by the detonation, lifted into the atmosphere, and returned to earth. It looks like gray sand or ash. It can arrive in large pieces early and fine dust later.

The 7-10 rule describes how fallout radiation decays. The Oak Ridge National Laboratory and FEMA's Protective Action Guide both cite this approximation: for every sevenfold increase in time, radiation intensity drops by a factor of ten.

Practical implications:

• At 1 hour after detonation: baseline intensity
• At 7 hours: 10% of initial intensity
• At 49 hours (roughly 2 days): 1% of initial intensity
• At 14 days: well below 0.1%

This is why staying inside for 48 to 72 hours is the standard recommendation. The radiation dose you would receive going outside at hour 6 is 80% lower than going out at hour 1. Waiting two days reduces it by 99%.

The CDC's Radiation Emergencies guidance states that unless instructed otherwise by emergency broadcasts, civilians should remain in shelter for at least 24 hours and ideally 48 to 72 hours after a detonation.

What Counts as Shelter

Not every indoor space provides meaningful protection. The variables are mass between you and the fallout, and distance from exterior walls and the roof.

Best options, ranked:

1. Basement or sub-basement of a multi-story concrete or brick building
2. Center of the middle floors of a large concrete building (floors 3 through n-2)
3. Basement of a single-family home
4. Interior room of a brick home, no exterior walls
5. Interior room of a wood-frame home

Avoid:

• Top floor of any building (roof provides minimal shielding)
• Rooms with exterior walls or large windows
• Mobile homes (DRF approaching 1)
• Vehicles (essentially no protection)

If you are in a single-family home with no basement, the best available position is typically a central bathroom or closet on the ground floor, using mass from walls and furniture on all sides. Fill bathtubs with water before the fallout arrives if time permits.

Decontamination After Exposure

If you were outside when fallout began arriving or arrived at shelter with fallout on your person, you must decontaminate before settling in.

CDC protocol:

1. Remove outer clothing before entering the shelter's living area. This alone removes up to 90% of contamination, per the CDC Radiation Emergencies protocol.
2. Place contaminated clothing in a plastic bag and move it away from people, ideally outside the living area.
3. Wash exposed skin with soap and warm (not hot) water. Hot water opens pores and can increase absorption.
4. Do not scrub hard enough to damage skin. Broken skin absorbs radioactive material more readily.
5. Blow nose, wipe eyelids and ears with a damp cloth.
6. Wash hair with shampoo but not conditioner. Conditioner binds radioactive particles to hair.

Pets that were outside need similar treatment. Wipe down fur with a damp cloth and wash feet. Contaminated pets bring fallout into the shelter.

Potassium Iodide: What It Does and Does Not Do

Potassium iodide (KI) is one of the most misunderstood items in preparedness discussions. It is useful, narrow, and time-sensitive.

What it does: Saturates the thyroid gland with stable iodine, preventing uptake of radioactive iodine-131 (I-131), which is released by nuclear reactor accidents and some weapon detonations. The FDA-approved adult dose is 130 mg taken once per 24 hours during exposure. Pediatric doses are lower: 65 mg for children 3 to 18 years old weighing over 150 pounds, 32 mg for children 3 through 12, 16 mg for ages 1 month through 3 years, per FDA guidance.

What it does not do:

• Does not protect against any other radioactive material
• Does not protect against external radiation
• Does not prevent acute radiation sickness
• Does not reverse exposure that has already occurred

Timing: KI is most effective when taken within 1 hour before exposure and up to 4 hours after. Taking it 24 hours after exposure provides minimal benefit.

Risks: People with thyroid conditions, iodine allergies, or dermatitis herpetiformis should consult physicians before taking KI. Pregnant and breastfeeding women should take only one dose unless otherwise directed by health authorities, per CDC guidance.

Do not take KI unless instructed by emergency authorities or unless you are certain a radiological release has occurred in your vicinity.

Supplies for 72 Hours of Shelter

The following list assumes a family of four sheltering for 3 days. Adjust as needed.

Water:

• 1 gallon per person per day for drinking and minimal hygiene, per Red Cross standards
• Family of 4 for 3 days: 12 gallons minimum
• Store in food-grade containers, rotate every 6 months

Food:

• 2,000 calories per person per day minimum
• Ready-to-eat items requiring no cooking
• Canned goods, energy bars, peanut butter, dried fruit
• Manual can opener

Medical:

• Potassium iodide tablets (one course per family member)
• Prescription medications (7-day supply minimum)
• First aid kit with trauma capability
• N95 or P100 respirators for each family member

Communication:

• Battery or hand-crank AM/FM/NOAA weather radio
• Spare batteries
• Written list of emergency frequencies

Sanitation:

• Heavy-duty plastic bags for waste
• Bucket with tight-fitting lid (improvised toilet)
• Bleach (unscented, 5.25% sodium hypochlorite)
• Feminine hygiene, toilet paper, soap

Light and power:

• Flashlights, headlamps
• Extra batteries
• Chemical light sticks (no heat, no fire risk)

Information:

• Printed local maps
• Written contact lists (cell towers may be down)
• Important documents in waterproof container

Communications in a Post-Detonation Environment

Assume cell networks are degraded or offline. A nuclear detonation produces electromagnetic pulse (EMP) effects. A high-altitude detonation, specifically designed for EMP, can affect electronics across continental scale. A surface or low-altitude detonation produces a smaller EMP footprint but still damages nearby electronics and stresses power grids.

The 2008 EMP Commission report to Congress assessed that a high-altitude EMP attack could damage unhardened electronics across most of the continental United States. Civilian infrastructure, including cellular networks, commercial power, and most vehicles manufactured after the 1980s, is not hardened.

Planning implications:

• AM radio has the longest range and is least affected by most interference
• NOAA weather radio frequencies (162.400 to 162.550 MHz) carry emergency alerts
• Ham radio operators maintain independent networks and are often first to restore local communication
• Keep a paper list of contacts, addresses, and meeting points

For households without ham operators, a $30 handheld scanner radio and a $40 NOAA weather radio together cover the essential inbound information channels.

Information Access When Infrastructure Fails

The weakness in most nuclear preparedness plans is not supplies. It is knowledge. The specific dose reduction factor of your bathroom, the correct dilution for bleach as a surface decontaminant, the pediatric KI dose by weight, the expected wind pattern for your region — this information lives online for most households and becomes inaccessible the moment networks fail.

This is a specific gap that offline reference materials address. Printed copies of FEMA's Planning Guidance for Response to a Nuclear Detonation, CDC's Radiation Emergencies guidance, and the Oak Ridge Nuclear War Survival Skills manual together cover most tactical questions. A paper three-ring binder with these documents weighs about four pounds.

Deadnet packages these same documents on a USB drive along with 16 other military and government survival manuals and a local AI assistant trained on their content. When the query "pediatric KI dose for a 45-pound child" needs an answer and the internet is down, the documents and query system exist on the drive. That is the relevant function. It does not replace printed references — both belong in the kit.

Regional Risk Variables

Nuclear risk is not uniform across geography. The National Resources Defense Council and various academic studies on target modeling identify higher-risk regions:

• Washington D.C. metropolitan area (political leadership)
• New York City (finance, symbolism)
• Major military installations: Omaha (STRATCOM), Colorado Springs (NORAD), Minot ND, Cheyenne WY, and Great Falls MT (ICBM fields)
• Major naval bases: San Diego, Norfolk, Bremerton, Kings Bay
• Major ports: Los Angeles/Long Beach, Houston, Savannah

Counterforce targeting — weapons targeting military assets — concentrates risk around ICBM silo fields in Wyoming, Montana, North Dakota, and Colorado. Residents within 50 miles of these facilities face elevated fallout risk even if their own locations are not direct targets.

Wind patterns matter. The prevailing winds in the continental United States move west to east. A detonation in Los Angeles produces fallout that drifts eastward across the southwest. A detonation in Minot drifts toward Minnesota and the Great Lakes. NOAA historical wind data provides a first approximation. The HYSPLIT model, maintained by NOAA Air Resources Laboratory, allows more detailed plume modeling and is free to access.

Mental Preparation

The psychological component of nuclear preparedness is often omitted from tactical guides. It matters.

The RAND Corporation's 2006 report on psychological impacts of radiological terrorism documented that in the Goiânia radiological accident in Brazil in 1987, where only 4 people died from radiation exposure, approximately 112,000 people sought medical evaluation out of fear of contamination. The medical system was overwhelmed by psychological casualties, not radiation casualties.

Panic degrades decision-making at exactly the moment when decisions matter most. The countermeasures are procedural knowledge and rehearsal. A family that has walked through the "if you hear the alert, you go here" conversation responds faster and makes fewer fatal mistakes than a family encountering the question for the first time during the event.

Children benefit from age-appropriate discussion framed around existing emergency drills. "We know what to do for a fire. We also know what to do for a radiation emergency. It is the same kind of plan."

One Thing to Do Today

Pick up the phone and look up, right now, the closest large concrete or brick building to your home, to your workplace, and to your children's school. For each of those three locations, identify the route to that building that takes less than 10 minutes on foot.

Write those three routes down on an index card. Put the card in your wallet.

That single action — identifying three shelter-quality buildings within walking distance of where you spend your time — is the highest-leverage nuclear preparedness step most households can take, and it requires no purchases, no permits, and no special knowledge beyond being able to recognize concrete construction. Every other preparation, from potassium iodide to water storage to communications gear, assumes you have already answered the first question: where do I go.

Answer that today.