The energy distribution in a nuclear explosion at or near ground level is split up as below
45% as Blast and Shock Waves
35% as Light and Heat radiation – thermal radiation
5% as Initial nuclear radiation
15% as Residual Nuclear Radiation from fission products
At the instance a nuclear explosion occurs, an intense blinding light occurs which is brighter that the sun, and lasts a number of seconds. The duration depends on the yield or size of the weapon. Looking at the flash can cause temporary or even permanent blindness. The effect of the flash will be greater at night or in clear weather conditions.
At the moment of the explosion a large fireball is created, its size can vary from a few hundred feet to several miles in diameter depending on weapon size (see tables 1, 2, 3). The fireball is as hot as the sun, that is to say several million degrees.
Lasting some 1 to 20 seconds is a heat pulse that travels at the speed of light in straight lines away from the explosion. The heat pulse can cause severe injury to people unprotected in the open at a considerable distance from Ground Zero. For example, a 20MT could cause heat damage up to 15 miles if a ground burst and up to 25 miles if air burst.