25 ft and then immediately subsided. Upon reinspecting the drum interior, he noted 
that all of the briquettes were at an incandescent temperature. 
4 A series of incidents have been experienced in which U and Ti alloys have dis- 
played explosive surface films following acid treatment (/2, 14). Studies at Argonne 
National Laboratory showed that such explosions could be averted through use of 
adequate fluoride ion concentrations in nitric acid etching baths. Witnesses have de- 
scribed metal-surface explosions of this type as involving a brilliant flash of white 
light, accompanied by a sound similar to that of a 22-gage rifle shot. 
Thorium Incidents 
4 For several years scrap Th powder had been disposed of by burning in successive 
small amounts. In July 1956, employees were engaged in burning scrap Th powder 
that had previously been washed with several aqueous solutions and vacuum-dried 
3 days earlier. Some of the Th was placed in a special hood and ignited without 
incident. An employee took a “golf-ball-size” piece of Th from a metal pail con- 
taining 30-40 Ib, replaced the pail lid, and placed the piece on a small Th fire. An 
immediate sharp explosion blew the employee 20 ft across the room. Almost imme- 
diately, a second blast involving the Th in the pail was accompanied by a jet of 
orange fire and a big cloud of dust. A third explosion occurred in a nearby vacuum 
dryer containing about 7 lb of moist Th powder. One employee suffered fatal burns, 
while three others suffered serious injuries. The extent of property damage is evi- 
dent in Fig. 3. [This took place at Sylvania Electric Company’s Metallurgical 
laboratory, Bayside, N. Y.] 
4 In preparing an experimental charge for making metallic Th in a reduction bomb, 
a mixer was being used to blend metallic calcium, dry zinc chloride, and dry thorium 
fluoride. After several revolutions of the mixer, the operator opened the mixer vent 
and, noting that dust and gas were escaping, decided to call his foreman. A second 
operator closed the vent, started the mixer, and soon heard a rumbling noise, fol- 
lowed by a sudden burst of flame covering a 45-deg angle and extending parallel to 
the floor for 40 ft. Of the eight persons injured by the blaze, two subsequently died. 
Reason for initiation of the reduction reaction in the blender is uncertain and un- 
precedented. 
It was subsequently found that the calcium used was particularly reactive. In one 
test a marked, but unexplained, temperature rise occurred when some —50 mesh cal- 
cium fines were left standing in an argon atmosphere. 
Miscellaneous Incidents 
4 On June 16, 1954, employees of a non-AEC high-energy-fuel laboratory were 
sampling 15 drums of “bag fines” Mg powder, which were opened in a special room 
that had been purged with nitrogen until the oxygen content had dropped below 
1%. During sampling of the fifth drum, the powder ignited suddenly. The flame 
shot out from the drum, immediately subsided, and the operators left the room after 
replacing the drum cover. From an external observation window, the employees 
noticed a gradual darkening of the drum’s exterior, moving down to within 2—4 in. 
of the drum bottom. The following day the drum was opened and contained a 
Lari yellow coloration, which was presumed due to formation of magnesium 
le. 
4 A massive block of metallic barium was cut into 34-in.-square pieces while sub- 
merged in kerosene. During attempts to remove residual kerosene with carbon tetra- 
chloride (an operation that had been performed many times before without incident), 
violent reaction dispersed glass fragments and burning barium over the immediate 
area. Similar explosions have also been suffered when Na, U, and Zr were treated 
with carbon tetrachloride. 
4 Trouble had been experienced in getting a Kroll process reduction of ZrCh: with 
Mg to go to completion. When the furnace was opened up, a slate grey material was 
noted on the surface, which was thought to consist of Zr, Mg, and MgCl. A sample 
of this material, roughly 14 in.-thick and 8 in.-square, was removed for test and was 
totally inert when scratched with a file or hit with a hammer. A piece of the sample 
melted under an oxyacetylene flame but showed no pyrophoric properties. Samples 
were then placed in water and slight evolution of gas noted. The following day an 
attempt was made to further wash the samples in several changes of water. While 
under 5 in. of water and without any prior evidence of reaction, an explosion -oc- 
curred that shattered the laboratory bench, threw the technician against the 
wall, and blew out a window 25 ft away. Portions of the water-washed sample 
blown to the floor ignited and “spit” when stepped upon. Small samples were sub- 
sequently tested and found to contain Mg, Zr, and 1% C. 
Crash Program Aims 
Objective of AEC’s crash program on Zr sensitivily is to determine those prop- 
erties of the various Zr forms and alloys that affect inflammability and explosiveness, 
and the reaction mechanisms involved in the sensitization of the metal. The study 
will assess the hazards involved in Zr manufacture, handling, storage, and shipping. 
Hf and Ti will be examined on a limited scale to establish relative sensitivity of the 
three metals. 
The experimental approach to the problem will be to determine flammability and 
explosive characteristics of various types of sponge, process materials, powders and 
metallic scrap to establish relative standards and regions of sensitivity for the vari- 
ous materials. To do this, tests will determine the sensitivity of the material to 
friction and impact forces, flame or radiation, temperature and high velocity shock 
waves under controlled environmental conditions. Detailed studies will weigh effect 
of particle size, surface area (and, in sponge, density and extent of agglomeration), 
impurities and process contaminants such as Fe and other metals, salts, carbon, O, 
N, H, water, etc. 
mass and with decreasing particle size. 
Several interesting observations have 
been made of the effect of powder mass 
on pyrophoricity. Dust obtained by 
filing a zirconium-titanium alloy never 
ignited when dispersed over a bench, 
but always ignited spontaneously when- 
ever small layers were accumulated. 
Zr powder dispersed over a red hot 
plate slowly oxidized, while a layer of 
the same powder thrown on the same 
plate immediately ignited and burned 
violently. Small layers of very fine U 
powder under water appear inert, while 
larger amounts tend to ‘‘ball up”’ fol- 
lowing which spontaneous ignition is 
virtually certain, 
The foregoing and other reported 
work (3, 4) suggest that for a metal 
powder of particular particle size there 
exists a definite minimum quantity of 
metal that must be exceeded for spon- 
taneous ignition. Following the same 
line of reasoning, any metal capable of 
reacting exothermically with oxygen 
should also be capable of spontaneously 
igniting in air if the metal is sufficiently 
finely divided and if a sufficient quan- 
tity is present. Experimental work 
(5) tends to confirm this suspicion for 
a. series of metals (including iron, cop- 
per, and tungsten). 
In many respects spontaneous igni- 
tion of metal powders is similar to 
spontaneous ignition of oily rags, while 
self-sustaining burning of massive metal 
is more nearly comparable to burning 
of ordinary flammable liquids and 
solids. 
Powder Explosions 
In general, the combustion in air of 
flammable vapors generates much less 
heat but a much greater volume of 
gases than does combustion of pow- 
dered pyrophoric metals. Since explo- 
sion pressure is roughly proportional to 
the product of the heat generated and 
the ultimate gas volume, an increase in 
explosion pressure is expected if the 
metals tested contain dissolved gases 
(notably hydrogen). Comparison of 
air-metal and air—metal-hydride dust 
explosion tests (6) tends to confirm this 
suspicion. Maximum explosion pres- 
sures attainable upon detonation of 
air—powdered-metal mixtures are ap- 
proximately equal to the maximum 
attainable by detonation of flammable- 
vapor-air mixtures (7). 
Vapor or powdered-metal explosion 
pressures obviously vary with the 
materials involved, the ratio of mixture 
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