Without water, your life expectancy is measured in days—and not many days at that. After three days, dehydration will be a huge problem, and death is almost certain after a week. Making sure you can stay supplied with water is a top survival priority. Unfortunately, the only water you can rely on is what you already have stored in your bunker. Even if the bunker has plumbing and your system draws water from a well, that’s likely to get contaminated by fallout within a few days of a nuclear attack. You need to have enough water stockpiled to get you through the initial period of high radiation.

1. Water Containers 

Aim to store, as a bare minimum, a gallon of water per day for each person who’ll be in the shelter. You need enough for at least two weeks, but if there’s room for more, that’s ideal. Water needs to be in sealed containers to keep contamination out. For normal storage, opaque or blue containers are best as a lot of microorganisms rely on sunlight to grow. If you’re storing your water in a bunker, that’s less important; sunlight won’t be reaching it.

Portable containers are better than large barrels. Plastic army-issue five-gallon jerrycans are perfect; they’re tough, but they can be carried easily for short distances, and their rectangular shape is good for efficient storage. Plastic gasoline canisters are also good, but only use new ones that haven’t held fuel before.

Bottled water works as an alternative, but it has some limits. On the positive side, it’s probably going to be cheaper than buying containers and filling them with tap water. As a baseline, assume that containers will cost you around $5 a gallon; you can get bottled water for less than half of that. It’s also easy to control and account for. Instead of measuring out daily drinking water rations, you can just hand everyone a bottle every morning.

On the other hand, the bottles aren’t as robust as a military jerrycan. They’re not as tough as a canteen either. You can pick one up and take it with you when you go out, but it’s more likely to split than a one-quart GI canteen. Finally, heavy duty containers can be reused many times. A jerrycan will last for years—probably decades—unless you actually drop it off a cliff. That makes them valuable when you start collecting and purifying water.

2. Filters

No matter how much water you stockpile, eventually it’s going to run out. After spending two weeks in the bunker, you’re going to have to find a water supply, and then you’re going to have to purify it.

Even if you have your own well, any water supply is potentially contaminated with nuclear fallout. Fallout can contain many different radioactive substances. Most of them are in the form of tiny particles that a filter will remove from water, and they won’t turn the water itself radioactive. However, at least one of them can dissolve in water—and that