Thorium Incidents (Smith, 1956)

Spontaneous Heating and Pyrophoricity

DOE-HDBK-1081-94

ACCIDENT CASE STUDIES

During subsequent rolling, molten Fe-Zr eutectic alloy within the steel jacket was forced to

one end of the strip where it burst into flames as it sprayed out over an area approximately

10 feet wide, 10 feet high, and 25 feet long. One employee was seriously injured.

In the early program for the large-scale manufacture of metallic U, fine powder was allowed

to collect under roughly 25 feet of water. At approximately 1-month intervals, and without

prior warning, a geyser about 30 feet high would suddenly develop over the powder and

then immediately subside.

A series of cases is known in which massive pieces of metallic U, Pu, and Th have displayed

unusual pyrophoricity, e.g., spontaneously igniting at room temperature. Spontaneous fires in

U chips are, however, much more common and in one case ignition occurred 6 months after

the chips had been placed in storage. One investigator of spontaneous fires in briquette U

chips opened a drum filled with briquettes that had been in outside storage for several weeks.

After noting that the drum contents were normal and at approximately room temperature, he

was warned by an operator to stand back. A few seconds later, a flame shot to a height of

about 25 feet and then immediately subsided. Upon reinspecting the drum interior, he noted

that all of the briquettes were at an incandescent temperature.

A series of incidents have been experienced in which U and Ti alloys have displayed

explosive surface films following acid treatment. 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 described metal-surface explosions

of this type as involving a brilliant flash of white light, accompanied by a sound similar to that

of a 22-caliber rifle shot.

Thorium Incidents (Smith, 1956)

For several years scrap thorium (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 containing 30/40 pounds, replaced the

pail lid, and placed the piece on a small Th fire. An immediate sharp explosion blew the

employee 20 feet across the room. Almost immediately, a second blast involving the Th in

the pail was accompanied by a jet of orange fire and big cloud of dust. A third explosion

occurred in a nearby vacuum dryer containing about 7 pounds of moist Th powder. One

employee suffered fatal burns, while three others suffered serious injuries.

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 the 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, followed by a sudden burst of flame

covering a 45 angle and extending parallel to the floor for 40 feet. Of the eight persons

Rev. 0

Page 53

Pyrophoricity