Nuclear bombs release 35 per cent of their energy as thermal radiation. A 1.2 megaton device, such as the B83 bomb in the US arsenal, will release two million gigajoules of energy. This will cause third-degree burns to exposed skin, at a distance of 10km. If you are already standing next to a wall or a car, duck behind it. Otherwise throw yourself on the floor, feet facing towards the blast, and cover your head with your arms. The thermal pulse will last around 10 seconds for a ground burst explosion. The initial gamma radiation is less dangerous than the heat pulse.
Once that ends, you have a brief lull before the blast wave hits you with 50 per cent of the bomb’s energy. It travels at 1,250km/h towards you. At 10km from the impact point, you have 20 seconds to reach safety. The overpressure of 200 kilopascals will feel like being punched all over, simultaneously, which isn’t enough to kill you, but it will knock down all buildings and send vehicles flying. The best place to be is the middle of an empty field. Failing that, crouch behind a low wall, away from tall buildings that can fall on you.