DU catches fire = dust particles smaller than 5 µm

http://fhp.osd.mil/du/environmentalEffects.jsp -- Journal of Environmental Radioactivity; 64:93-112 (2003)
Properties, use and health effects of depleted uranium (DU): a general overview, Bleise, Denesi and Burkart,
A. Bleise et al. / J. Environ. Radioactivity 64 (2003) 93–112

In the early 1970s, the US Army began researching the use of depleted uranium
metal in kinetic energy penetrators and tank armor. High-density materials such as
tungsten (density 19.3 g/cm3) and DU (density 19 g/cm3) were considered. DU was
ultimately selected due to its availability, price and pyrophoricity (Danesi, 1990;
Anderson et al., 1997). Tungsten has a much higher melting point (3410 °C) than
uranium (1132 °C) and lacks pyrophoricity. Therefore, a tungsten projectile becomes
blunt on impact and is less effective in piercing armor (Peterson, 1999).

The DU dust which may be formed during impact can be dispersed and contaminate the environment. It is estimated that normally 10–35% (and a maximum of 70%) of the DU penetrator becomes an aerosol on impact or when the DU catches fire (Harley et al., 1999). Most of the dust particles have been reported to be smaller than 5 µm in size which keeps them airborne for an extended time, and will spread according to wind direction.

DU dust is black and a target that has been hit by DU ammunition can be often recognized by the black dust cover in and around the target (US AEPI, 1995). After an attack with DU ammunition this will be deposited on the ground and other surfaces as partially oxidized DU fragments of different size, and as uranium oxide dust. According to investigations conducted at US test sites, most of the DU dust is deposited within a distance of 100 m of the hit target (CHPPM, 2000). Claims have also been made that the DU dust can travel up to 40 km and remain airborne for a considerable time (Belgrade, 2000).