ova 1700-7 17.2 17.2.1 THERMAL RADIATION Introduction General Characteristics of Thermal Radiation Immediately after it forms, the fireball of a nuclear detonation starte to emit the infrared, ultraviolet, and visible light known as thermal radiation. This emission occurs in two pulses, shown in idealized form in Figurel7-l. During the first pulee of extremely short duration (0.1 sec or less), temperatures in the fireball are very high, and energy emission rapidly rises to a maximum and rapidly declines to a minimum. The second pulee may last for several seconds, temperatures are lower, and there is a less rapid rise in energy emission to the second or final maximum, followed by a comparatively Blow decline to zero. Since temperatures during the first pulse are very high, most of the emitted radiation is in the ultraviolet region, which is attenuated rapidly in air. Furthermore, only about 1% of the total thermal radiation appears in the first pulse because it has such @ short duration and because the radiating area is still relatively small. Thus, the radiant exposure from the first pulse, at some dis- tance from the burst is insignificant. During the second pulse, most of the radiation falls in the infrared and vieible regions, and can cause fires to start when combustible materials are directly exposed to the fireball at sufficiently close range. The thermal radiation from nearly all underwater bursts will be absorbed through vaporization and dissociation of the water, and thus is of no concern as a weapons effect. However, thermal radiation from surface or extremely shallow underwater bursts is of concern, although such radiation can affect only the exposed topside personnel and materiel of e surface ship. Any opaque object along the firebdall-to- target line of sight will furnish full protection from thermal radiation; thus, topside personnel or materiel in the shadow of the ship's superatructure or topside gear would be shielded from thermal radiation. Such radiation probably will not start shipboard fires, since normally there is insufficient combustible materiel topside on combatant ships to sus- tain fire. (However, carge ships may carry combustible deck loads, and in special wartime conditions, even combatant ships might have com- bustibles topside.) The most probable thermal-radiation effects are incapacitating flash burns or flash blindness among topside personnel directly exposed to the fireball of surface bursts, topics which will be considered in detail in Chapter 18. Topics Considered The free-field data and criteria necessary for assessing thermal-rad- lation damave, and the procedure for evaluating topside thermal exposures 17-4 ee ee me a ee ee ee ee eee