Nuclear Preparedness Guide: What to Do in the First 72 Hours

The week of April 27, 2026, FEMA will run a biennial Radiological Emergency Preparedness exercise at the Peach Bottom Atomic Power Station in Pennsylvania, evaluating state and county response to a simulated reactor incident. It is one of dozens of REP exercises FEMA runs every two years across the 54 commercial nuclear power sites in the United States. The fact that these exercises exist, and that the federal government publishes 300-plus pages of response doctrine for a nuclear detonation, tells you something: the agencies tasked with getting you through the first 72 hours assume you will spend those hours on your own.

This guide covers what to actually do — before, during, and after — if a nuclear event occurs near you. It is drawn from FEMA's Planning Guidance for Response to a Nuclear Detonation, the CDC's radiation emergency protocols, and the Department of Homeland Security's Nuclear Detonation Response Guidance for the First 72 Hours.

Understanding the Threat

Nuclear events fall into three categories, each with different physics and different response windows:

• Improvised nuclear device (IND): a yield typically in the 0.1 to 10 kiloton range, ground burst, maximum fallout. The FEMA planning scenario uses a 10 kT ground burst in a dense urban core.
• State-actor warhead: yields from roughly 100 kT to several megatons. Larger blast radius, potentially airburst (less local fallout) or ground burst (more).
• Reactor release or radiological dispersal device (dirty bomb): no nuclear yield, but airborne radioactive material. Consequence is contamination, not blast.

FEMA's planning guidance states that a 10 kT ground burst would create three damage zones: a severe damage zone with near-total destruction out to roughly 0.5 miles, a moderate damage zone out to about 1 mile, and a light damage zone out to about 3 miles. The fallout plume — driven by wind — can deposit dangerous levels of radiation tens of miles downwind within the first hour. This matters because outside the severe zone, the majority of preventable casualties come from fallout, not from the blast itself.

The Department of Homeland Security estimates that sheltering and evacuation decisions made in the first 60 minutes determine the survival outcomes for a significant share of the population within the fallout footprint.

The Three-Word Doctrine: Get Inside, Stay Inside, Stay Tuned

FEMA's public messaging for a nuclear detonation compresses to three commands. They are simple because the physiological clock is short and because panic kills more reliably than radiation.

Get Inside. Stay Inside. Stay Tuned.

The logic:

• Get Inside: fallout begins arriving within 10 to 20 minutes of a ground burst. Mass and distance from the source are the only two variables that reduce dose. Any building is better than no building. A basement is better than the ground floor. The center of a large multi-story building is better than either.
• Stay Inside: radiation from early fallout decays rapidly. Every factor of seven in time corresponds to a factor of ten in dose rate. One hour becomes seven hours becomes 49 hours. Sheltering for 24 to 72 hours cuts total exposure by roughly 90 percent compared to evacuating through the plume.
• Stay Tuned: fallout patterns depend entirely on wind. You cannot guess correctly from inside a shelter. Authorities need 30 to 60 minutes to model the plume. Leaving before instructions are issued means guessing in the direction of the wind.

The First 10 Minutes

If you see a bright flash:

1. Do not look at the flash. A nuclear flash can cause temporary flashblindness at distances of up to 13 miles in daylight and 53 miles at night, according to DoD references cited in FEMA training material.
2. Drop behind cover. Any solid object reduces thermal burn exposure. A ground burst produces a thermal pulse lasting several seconds.
3. Count. Blast waves travel at roughly the speed of sound: 1 mile every 5 seconds. A 10-second count before the shockwave means you are about 2 miles from the detonation — inside the light damage zone, outside the moderate zone.
4. Move. You have somewhere between 10 and 20 minutes before fallout arrives. Get to the most substantial structure you can reach on foot. Do not drive through glass-strewn streets toward a "better" shelter if a concrete parking garage is 300 feet away.

The CDC recommends removing outer clothing on entering your shelter. This single act removes up to 90 percent of external contamination. Bag the clothing, seal it, place it as far from occupied space as possible. Shower with soap if water is available. Do not scrub hard enough to break skin.

The First Hour: Choosing a Shelter

Not all shelters are equal. Shelters are rated by protection factor (PF) — the ratio of outside dose to inside dose.

• A wood-frame house center room: PF of 2 to 3
• A brick or masonry home with basement: PF of 10 to 20
• The center of a large multi-story office or apartment building, interior rooms on middle floors: PF of 100 or higher
• A dedicated fallout shelter or subway tunnel: PF of 200 or more

FEMA guidance: if a PF-100 shelter is within 15 minutes of your current PF-10 location, do not move. The exposure during the move exceeds what you avoid. If you are in a car, get out and get into a building. Cars provide a PF of roughly 1.5.

In the shelter, move to the lowest, most central space. Seal windows and external vents with tape, plastic sheeting, or wet towels. Turn off HVAC. Bring water. Bring a radio.

The First 24 to 72 Hours: Staying Put

Dose rate after a ground burst follows the 7:10 rule of thumb. At H+1 (one hour post-detonation), radiation is at baseline X. At H+7, it is X/10. At H+49 (roughly two days), it is X/100. At H+14 days, it is X/1000.

This means the single highest-leverage action you can take is not moving. A 24-hour wait reduces external radiation exposure during evacuation by approximately 80 percent compared to immediate flight. A 72-hour wait reduces it by approximately 95 percent, according to dose modeling in the FEMA Planning Guidance for Response to a Nuclear Detonation.

What you need to shelter for 72 hours:

• Water: 1 gallon per person per day, minimum. For a family of four, that is 12 gallons, or two cases of bottled water, for 72 hours.
• Food: shelf-stable, no-cook. Peanut butter, crackers, canned goods with a manual opener, energy bars.
• Radio: battery or hand-crank AM/FM/NOAA Weather Radio. Cell networks will be overloaded or offline.
• Flashlights and batteries. One per person. LED headlamps free the hands.
• First aid kit with trauma supplies. Blast glass injuries are the most common post-detonation presentation outside the severe zone.
• Prescription medications: 7-day supply minimum.
• N95 or P100 respirators. Fallout is particulate. A properly fitted respirator reduces inhaled dose substantially.
• Sanitation: heavy-duty trash bags, kitty litter, bleach. You will not be leaving to use a functional bathroom.
• Documents: ID, insurance, medical records in a waterproof bag.

Potassium Iodide: The Specific Tool for the Specific Problem

Potassium iodide (KI) is frequently misunderstood. It does one thing: it saturates the thyroid gland with stable iodine, preventing uptake of radioactive iodine-131. It does not protect against external radiation. It does not protect against cesium, strontium, or any other radionuclide.

The FDA and the CDC recommend KI for anyone under 40 in the projected plume of a reactor release or detonation where iodine-131 is a confirmed threat. Dosing, per FDA guidance:

• Adults 18 to 40: 130 mg
• Pregnant or breastfeeding: 130 mg
• Children 3 to 18: 65 mg
• Infants 1 month to 3 years: 32 mg
• Newborns under 1 month: 16 mg

Do not take KI speculatively. Take it within four hours before or up to eight hours after exposure — its window is short. Over 40, the thyroid cancer risk from I-131 is low enough that KI's side effects typically outweigh its benefits unless exposure is high.

State governments in the 10-mile Emergency Planning Zones around nuclear power plants are required to pre-distribute or stockpile KI. If you live within 50 miles of a reactor, buy your own. A bottle of 14 tablets costs roughly $10 and has a shelf life of seven years.

Communications During the Blackout

An electromagnetic pulse from a high-altitude nuclear detonation can damage unshielded electronics across a wide area. A ground-level detonation produces a localized EMP effect but does not generate the continental-scale pulse often depicted in fiction. Either way, expect cell networks to fail from physical damage, power loss, and congestion.

A hand-crank radio with NOAA Weather Band receives Emergency Alert System messages directly. Keep one in a metal box or Faraday bag. A basic AM/FM/SW receiver lets you pick up regional broadcasts if local infrastructure is degraded.

Written reference material matters when you cannot reach the internet. FEMA publishes dozens of relevant documents — the Planning Guidance for Response to a Nuclear Detonation, the CDC's Radiation Emergencies guidance, Army field manuals on radiological operations — and the CDC's Radiation Emergency Medical Management site (remm.hhs.gov) aggregates them. Download and store them offline.

The Deadnet USB drive carries 19 military and government source documents — including FEMA's nuclear response planning documents and Army field manuals on radiological operations — along with an offline AI that can query them without a network connection. If the grid goes down or cell service collapses, the information does not go with it.

After 72 Hours: Decontamination and Evacuation

Authorities will direct evacuation when radiation levels along the planned route drop below exposure thresholds. Typical thresholds are expressed in terms of projected dose: less than 5 rem (50 mSv) for the general public, less than 10 rem for emergency workers performing protective actions, per EPA Protective Action Guides.

On leaving the shelter:

1. Wear long sleeves, long pants, sturdy shoes, and a respirator.
2. Tape gaps at wrists and ankles.
3. Avoid standing water, mud, and dust.
4. Move perpendicular to the wind direction when possible, not upwind or downwind along the plume axis.
5. Report to a designated Community Reception Center for radiation screening and, if needed, external decontamination.

Do not eat food or drink water that was uncovered during the event unless it has been screened. Sealed containers (canned food, sealed bottled water) are safe; their contents could not be contaminated. Root vegetables and leafy greens from outdoor gardens in the fallout zone are not safe until lab-screened.

Building a Nuclear-Specific Kit

Your general emergency kit probably covers 80 percent of what you need. The nuclear-specific add-ons:

• Potassium iodide tablets (dosed for every member of the household)
• N95 or P100 respirators with P100 cartridges for adults, appropriately sized child masks
• Heavy-duty plastic sheeting and duct tape (minimum 10 ft x 25 ft roll, two rolls)
• A dosimeter or radiation survey meter if you can afford one; a basic personal dosimeter runs $150 to $300
• A dedicated hand-crank radio kept in a Faraday bag or metal ammo can
• A printed copy of FEMA's "Are You Ready?" nuclear chapter and the CDC's KI dosing chart

Cost for the nuclear-specific add-ons, excluding the dosimeter: under $150 for a family of four. Shelf life on everything except KI is effectively indefinite with proper storage.

Mental Model for the Family

Run a tabletop drill with whoever shares your household. Pick a weekday afternoon and a Saturday night. For each, answer these six questions out loud:

1. Where is the best shelter you can reach on foot in 10 minutes?
2. Where is the next-best option if the primary is cut off?
3. Who picks up the children, and from where?
4. What is your out-of-state contact number? Local calls will fail; long-distance often works.
5. Where is the KI stored, and who is allowed to administer it?
6. What do you listen to, and on what device, for official instructions?

If you cannot answer any of these in under 15 seconds, that is the question your preparation should address first. The FEMA Community Emergency Response Team (CERT) program, free in most counties, runs 20 hours of training covering exactly this material.

Your Next Step Today

Before you close this page: open a browser, go to epa.gov/radiation/radnet-near-real-time-air-monitoring, and locate the nearest RadNet monitoring station to your home. RadNet is the EPA's nationwide radiation monitoring network — 140 fixed stations plus deployable monitors — and it publishes hourly background radiation readings. Bookmark the page. If you ever need to assess whether levels are elevated, you need to know what normal looks like at your specific location. Normal background in the continental US is typically 5 to 15 counts per minute on the gamma gross count channel, depending on elevation and geology. If you do not know your baseline, you cannot recognize a deviation.

Then, if you do not already own them: buy a bottle of potassium iodide tablets, one 14-pack per adult, one per child, and write the dosing chart on the inside of the cap with a permanent marker. Total cost: under $40. Total time: under 10 minutes. It is the single cheapest, highest-leverage nuclear-specific purchase you can make today, and it is the one most people never get around to.