How thick should bunker walls be? Radiation shielding in plain numbers

Mass stops radiation

Gamma radiation from fallout is reduced by passing through matter. The denser and thicker the material, the more it absorbs. There is no magic material: what matters is mass between the people and the fallout outside.

Halving thickness

A useful way to think about shielding is the halving thickness: the thickness of a material that cuts fallout radiation in half. Each additional halving thickness halves it again. Ten halvings reduce it about a thousandfold (2¹⁰ = 1,024).

These figures come from US civil defence data for fallout gamma radiation and are good for estimates, not for final design.

Protection factor

A shelter's protection factor is how many times less radiation reaches people inside than they would receive standing outside. A protection factor of 1,000 means one thousandth of the outside dose. If people outside would receive 5,000 mSv over two weeks, which is often fatal, people inside a PF 1,000 shelter would receive about 5 mSv, roughly two years of natural background radiation in India.

A worked example

Take a roof of 500 mm reinforced concrete under 1.2 m of compacted earth:

• Concrete: 500 ÷ 56 ≈ 8.9 halvings
• Earth: 1,200 ÷ 84 ≈ 14.3 halvings
• Total ≈ 23 halvings, a theoretical reduction of several million times

So why do shelter designers talk about protection factors of 1,000 to 10,000 rather than millions? Because radiation does not only come straight down through the roof.

Entrances matter more than walls

Gamma radiation scatters off surfaces and travels around corners. Stairways, doors, air shafts and escape hatches are paths with much less mass than the roof. A shelter is only as good as its weakest path, so the design work goes into:

• Entries that turn at right angles, often twice, so there is no straight line from outside to the occupied room.
• Baffled air intakes and exhausts.
• Heavy doors and hatch covers, and earth cover over entry passages.

Our design targets, from a protection factor of 1,000 for a Haven to 10,000 or more for an Estate, account for these openings. You can explore the roof numbers with the shielding calculator.

What an ordinary building gives you

Ordinary buildings help more than people expect, but much less than a shelter. A typical house roof slab of about 150 mm concrete gives under three halvings on its own. The centre of a large concrete building or a basement does much better, which is why official advice is to get inside and stay inside. A purpose-built shelter adds earth cover, sealed air and shielded entrances.

Blast thickness is a separate question

Radiation shielding sets a minimum mass. Blast resistance sets the structural design: wall and slab thickness, reinforcement, and the shape of the shell. In practice, a shelter designed to carry earth cover and resist 1 to 3 bar of overpressure ends up with walls of 300 to 600 mm and roofs of 400 to 750 mm reinforced concrete. See specifications by class.

Why concrete, not steel, in India

Steel is an excellent shield per centimetre, and buried steel tanks are popular in some countries. In wet Indian soils, with seasonal groundwater rise, buried steel corrodes. Reinforced concrete, properly waterproofed, lasts far longer underground, carries earth and blast loads well, and is built by a mature local industry to IS 456. That is why every Qilyx shelter is built in reinforced concrete.