12 ORIGIN AND NATURE OF RADIOACTIVE DEBRIS WORLDWIDE EFFECTS OF ATOMIC WEAPONS To From the data summarized by Brewer, we may arrange the oxides in 1000 the order of increasing volatility as follows: Vaporization Temperature (°K) (Pressure == 1 atm) Oxide = 10° (scole right) noe e 100 3 THO, 2 eee eee eee 0 & Bed eee eects Al,Og . 0. ec eee eee 4300 (decomposes) 3800 (decomposes) SO ween tea eae 3500 : E io* : \ ‘ i? ioe? 7© & re se ys? “ <a, Surf se tg? 2 va “ “ aoa oO! a8 "19° t a Oo € "I0! Fig. 1—-Time-temperature relations FejOy occ eee molecules is formed, (Oxygen gas at 1 atm pressure is 50 per cent dissociated at 3800°K. ) The relative stability of gaseous oxide molecules is given roughly by the D, value, which is the heat of dissociation of the molecule into the component atoms in the ground state at O°K. Values of D, have been summarized by Brewer."’ The following are some gaseous oxides in the order of decreasing stability: UO, ZrO,, SiO, AlO, StO, and FeO. Although most of the fission products exist as oxides, some are present with very electropositive elements such as cesium and rubidium from the fireball and sodium from the dust in the air; silver, palladium, and rhodium are very likely to be in metallic form; whereas xenon and krypton remain in the atomic gaseous state. . As the materials cool, condensed phases will be formed from the gaseous oxide molecules.he order of condensation of the various oxides will eT Te eee CTT en +t 3800 ~ the relative volatility of the oxides. The volatility of the oxides is not necessarily related directly to the stability of the gaseous molecules, but may best be related to the temperature at which the vapor pressure has a specified value. 2060 (decomposes) Condensed phases will form at some time when the temperature ts between 5000°K and 2000°K, which on a time scale for a 20-KT bomb means between 1 and 5 sec after detonation. The actual condensation may occur in a very short time. At 3.5 sec the temperature will have reached 2500°K and the fireball will have attained its maximum radius and will contain approximately 2000 tons of air. If we assume that it contains 100 Ib of unused plutonium or uranium,’ a total of 1 kg of fission products, and 1 ton of bombstructure, the material in the fireball will then have the following composition: Ait... 2... ee eee ee 2X 108 gm (99.99%) Bombstructure ......... 108 gm (0.1%) Fission products ........ 10% gm (0.00005%) PuorU............... 5 X 104 gm (0.0025%) in other forms: iodine and bromineexist as halide ions, probably combined os 4570 MgO 2... cect ee ee 33590 |© 3400 (decomposes) BaO ow. eee ees 3000 SID, cece teens ~ 2800 (decomposes) Time (sec) 2 4670 (PuO) «0... ccc eee (~4500) ©) rar (~4500) La,O, occ cece cece eee nan eas 4470 CaO occ ccc e eee z 13 With this composition and the relative volatilities of the oxides in mind, let us now consider which oxides are likely to condense first and the probable form of the condensed particles. It should be noted that the condensable material is extremely dilute compared with that used in the 4 TART OLY ra RMVDIAIUEUT SS 3 LEEERIUE ret TITySe COTES oTT experiments are carried out. If we consider ZrO,, which has the highest vaporization temperature of any of the oxides of the fission products, then, assuming a maximum fission yield of 5 per cent and a total pressure of t atm, the partial pzessure