Mystery Under Vigorous Attack 
between the lower and upper explosive 
limits. It is important to note that 
actual combustion involves an air- 
flammable-vapor reaction and not an 
air-flammable-liquid reaction. Ex- 
periments indicate that free burning of 
metals in air takes place at tempera- 
tures well above the metal melting 
point, at which temperatures the metal 
vapor pressure is well within a range 
comparable with that of flammable li- 
quids undergoing combustion. 
When a small amount of flammable 
liquid is burned, the heat of combustion 
raises the temperature of the liquid to 
its boiling point where it remains con- 
stant until all of the liquid has vapor- 
ized. A very recent experiment in- 
volving free-burning tests of small 
magnesium specimens indicated that 
combustion took place while the Mg 
temperature was very close to the boil- 
ing point. Such data again suggest a 
close analogy between the mechanisms 
respectively involved in combustion of 
flammable liquid and flammable metals. 
(Recent high-speed movies of an Mg 
wire burning in oxygen showed that 
combustion rates along the wire were 
erratic and that oxidation was accom- 
panied by periodic small localized high- 
energy explosions, the causes of which 
are uncertain.) 
FIG. 3. 
When a fire occurs on the surface of 
a large amount of oil, initially at room 
temperature and contained in a tank, 
liquid temperatures at the surface are 
again at the boiling point while liquid 
at the base of the tank may remain at 
the initial temperature. Such fires 
have been extinguished through use of 
AEC’s Three-Pronged Attack on Pyrophoricity 
Followi 
two recent serious and unusual explosions, involving metallic Zr (/) and 
wing 
Th (2) (see Incidents listed on p. 30), AEC efforts to study unusual metal fires and 
explosions were intensified and metal-pyrophoricity research activities expanded. 
1. A fundamental research program covering the mechanics of metal pyrophoricity 
is being conducted at Argonne National Laboratory. 
2. A “crash” program aimed at rapidly determining specific methods to ensure 
safe handling of metallic zirconium was recently initiated by the Bureau of 
Mines at Pittsburgh (see box on p. 95' for details). 
3. Very-high-temperature metal-water reactions are the subject of classified re- 
search at a third location. 
From these three programs AEC expects to obtain fundamental information per- 
mitting increased safety in the manufacture and utilization of such heavy pyrophoric 
metals as uranium, plutonium, thorium, hafnium, and zirconium. Incidental to the 
major purposes of this research, it is probable that some fundamental information will 
also be derived relative to theoretical approaches for attaining oxidation and water- 
resistant alloys as well as affording a better understanding of the conditions under 
Masta oer Reece and explosive water-metal reactions may be prevented, controlled, 
or ite 
Close-up of damage in metallurgical laboratory after a thorium explosion 
in July 1956. Tile is part of wall of ground-floor room; above it is floor of second- 
story lab room where explosion took place; apparatus seen diagonally in lower left 
is part of a calciner that had stood on part of floor that dropped: standing next to it 
had been a drum containing major part of thorium scrap to be burned to oxide. 
Triggered by initial blast, this material exploded in its wake 
agitation, permitting replacement of 
relatively hot liquid oil at the surface 
with cool oil from subsurface locations. 
In the case of massive pyrophoric 
metals, the surface is similarly rapidly 
cooled by conduction of heat to sub- 
surface levels at a rate far greater than 
can take place between quiescent layers 
of oil. This ability to dissipate heat 
accounts in large measure for the diffi- 
culty in obtaining and sustaining com- 
bustion of such specimens without con- 
tinued external application of heat. In 
view of the extremely high tempera- 
tures required for massive metal com- 
bustion, plus the fact that heat-radi- 
ation losses take place at a rate 
proportional to the fourth power of the 
temperature, the rarity of massive- 
metal fires is not surprising. 
Massive-Metal Combustion 
Spontaneous ignition of any massive 
pyrophoric metal is very rare. How- 
119 
