Nuclear explosion: why the cellar can become a deadly trap

Sirens, a push notification, blind panic: many people immediately think of heading down to the cellar.

Specialists warn that this exact response can become a deadly trap.

For most people, the threat posed by nuclear weapons feels abstract, almost like something from a film. Civil protection experts, however, examine very specifically where people would have the best chance of surviving in an emergency. Their clear message is surprising: a conventional cellar is often not a safe place – in many homes, the most effective protected area is somewhere else entirely.

Why the cellar can quickly become a trap

The assumption that “underground = safe” dates back to the Second World War and the Cold War. Many people subconsciously picture their own cellar as a bunker. Anyone reassured by that idea is relying on a misleading image.

A standard domestic cellar is generally:

• not reinforced or purpose-built to withstand blast waves;
• often constructed with thin walls or ageing masonry;
• separated from the ground floor by timber floors or lightweight intermediate ceilings;
• fitted with small windows positioned just above ground level.

When a powerful blast wave – for instance, from an explosion – hits a building, these features work against those inside. Ceilings may collapse, sections of masonry can break away, and heavy furniture or entire parts of rooms may crash down and bury people in the cellar.

There is also a second, less visible danger: gases and poor air quality. Heavy gases accumulate particularly easily in low-lying rooms. Smoke, toxic fumes and high CO₂ concentrations often remain close to the floor. If the cellar is then sealed tightly, it can rapidly become an airless trap. Without controlled ventilation, the risk of suffocation or poisoning rises considerably.

Specialists stress: a safe shelter in a basement must be specifically designed and constructed. An ordinary domestic cellar meets this standard only in exceptional cases.

How a nuclear explosion affects buildings

A simulation by the University of Nicosia in Cyprus demonstrated how dramatically the blast wave from a major nuclear explosion can affect buildings. The researchers modelled a weapon with an explosive yield of 750 kilotonnes, detonating several kilometres above the ground.

Close to the centre of the explosion, virtually no room offers protection. As the distance from the detonation increases, however, the situation changes: the building’s structure and a person’s precise location inside suddenly become crucial.

Windows, patio doors and open corridors act as “channels” for the blast wave. They effectively drive air, shards of glass and debris through rooms at speed. Even in sturdy homes, people in these areas can suffer serious injuries.

The picture is different in sheltered corners facing away from the façade. There, the building’s structure significantly weakens the pressure. The more walls there are between a person and the outside, the more effectively they cushion the force of the blast.

Walls as a shield against radiation

A nuclear explosion involves more than heat and pressure; radiation is also a concern. So-called gamma radiation is especially hazardous because it penetrates deeply into matter. It can nevertheless be reduced: every substantial building element acts like a filter.

Guideline values used in radiation protection show that:

• A concrete wall 15–20 centimetres thick can reduce the radiation dose inside a room by roughly a factor of ten.
• Several walls in sequence – such as a corridor, bathroom and storage room – increase this effect significantly.
• Rooms with numerous internal walls, ceilings above them and no direct external windows provide considerably better protection than a single room on the façade.

The more substantial layers there are between the body and the outside world, the lower the radiation exposure – this applies in particular to gamma radiation.

The “nuclear safety core” in the home

Disaster protection experts refer to a building’s “nuclear safety core”. This means the area furthest from windows and external walls, enclosed by as much building material as possible.

Typical options in flats and houses include:

• internal corridors;
• windowless toilets;
• small storage cupboards;
• walk-in wardrobes or larders without an external wall.

In a detached house, for example, the central ground-floor hallway may work as a shelter if it is surrounded by rooms and does not open directly towards the front door. A narrow corridor with a bathroom and storage room on either side provides more buffering than a large living room with a glazed frontage facing the garden.

Three quick rules for finding the safest room

Anyone who does not want to study every floor plan in detail can use three straightforward steps:

1. Rule out every room with windows or large glazed doors.
2. Mentally draw crossing lines across the home’s floor plan and choose the approximate centre.
3. Prioritise the point surrounded by solid walls and as close as possible to a water supply and toilet.

In multi-storey buildings, two further points apply: a middle floor is preferable to a top-floor flat or a ground-floor room directly facing the street. At the top, damage from blast pressure and heat is more likely; at the bottom, the blast wave hits the façade and windows particularly hard.

Preparing your shelter room

Even the best room is of little use if it cannot be used in an emergency. A few changes are enough to turn a corridor or small bathroom into a practical emergency room.

• Keep access clear: Do not place heavy furniture in the way, where it could obstruct the route or topple over.
• Keep emergency supplies close at hand: Store bottles of water, long-life snacks, a small first-aid kit, a torch and a battery radio nearby.
• Prepare materials for sealing: Keep towels, blankets or adhesive tape ready to seal gaps beneath doors temporarily.
• Provide somewhere to sit and sleep: A thin mattress, camping mat or stack of blankets makes a longer stay easier.

If an alarm is issued, residents should close all windows in the home, lower shutters, switch off ventilation systems and lock the front door. Inside the shelter room, the gap beneath the door can be sealed with damp towels. The room should not be made completely airtight, however, as this may cause problems with breathable air after several hours.

Planning is half the battle here: anyone who identifies the right room in advance and equips it minimally saves valuable seconds in an emergency.

How long should you remain in the shelter room?

Following a nuclear event, the nature of the danger changes over time. In the first few minutes, blast pressure and heat dominate; in the hours that follow, the main threat is radioactive fallout – radioactive particles that fall back to earth and contaminate ground, roofs and roads.

Radiation from this fallout decreases substantially over time. A rough rule of thumb from radiation protection teaching is that, after around seven hours, the dose rate has fallen to about one tenth of its initial level, and after two days to a few per cent. For this reason, authorities advise people in many scenarios to remain in the shelter room initially and wait for official announcements before leaving the building.

Key terms explained briefly

Blast wave

A major explosion creates an extremely rapid rise in air pressure that spreads out in a spherical pattern. This blast wave shatters windows, tears doors from their hinges and can even damage concrete structures. Inside a house, doors, corridors and large window fronts intensify this effect in certain areas.

Gamma radiation

Gamma radiation is among the most energetic forms of radiation. It penetrates deeply into matter but can be effectively reduced by sufficiently thick layers. In buildings, walls, ceilings and floors perform this function. The more material there is, and the denser it is, the less radiation reaches the body.

Practical examples from typical homes

In a typical three-room city flat with a living room, bedroom, child’s bedroom and an internal bathroom, the bathroom will usually be the best shelter room. If the flat also has a small, windowless storage cupboard in the centre, it may be even more suitable – especially if it is enclosed by several walls and ceilings.

In a detached house with an open-plan living and dining area and extensive glazing facing the garden, the narrow hallway in the central section is a possible option, perhaps combined with a storage room or windowless guest WC. The cellar is suitable only if it is solidly built, has no high-level windows and is structurally reinforced – in other words, if it resembles a genuine shelter rather than a hobby room with small windows.

Anyone living in a high-rise block should move away from the very top to a middle floor, if possible, and apply the same principle there: stay away from external walls and move into the building’s most compact internal core.