The powder they contain appears to be yellowcake uranium from neighbouring Niger.
From the telegraph.co.uk
The resulting mixture, called yellowcake, contains at least 75% uranium oxides.
From the en.wikipedia.org
A rebel fighter among the yellowcake drums in the warehouse near the city of Sabha.
From the telegraph.co.uk
Friends had joked that meals would be borscht and cabbage, plus yellowcake dessert.
From the au.news.yahoo.com
At the conversion stage, yellowcake is turned into uranium hexafluoride gas.
From the bloomberg.com
Yellowcake is the lifeline of any civilian and military nuclear program.
From the edition.cnn.com
After all, conversations about uranium and yellowcake often devolve into horror stories.
From the denverpost.com
It appears that the yellowcake escaped when a seal on the dryer broke.
From the scienceblogs.com
The aforementioned four companies account for almost 60% of world yellowcake production.
From the forbes.com
More examples
An impure mixture of uranium oxides obtained during the processing of uranium ore
Yellowcake (also called urania) is a kind of uranium concentrate powder obtained from leach solutions, in an intermediate step in the processing of uranium ores. Yellowcake concentrates are prepared by various extraction and refining methods, depending on the types of ores. ...
The concentrate of 70 to 90 percent uranium oxide extracted from crushed ore.
A natural uranium concentrate that takes its name from its color and texture. Yellowcake typically contains 70 to 90 percent U3O8 (uranium oxide) by weight. It is used as feedstock for uranium fuel enrichment and fuel pellet fabrication.
A yellow powder that is mostly uranium. Yellowcake is produced by pouring crushed uranium ore into an acid which dissolves the uranium. The acid is drained from the crushed ore and dried, leaving a yellow powder called yellowcake.
A processed oxide of uranium #U3O8, being the raw material for fuel elements in nuclear reactors; see CBR, HAZMAT, COCKTAIL, SOUP, HOT GREASE, HEAVY WATER, SCRAM, TOMB, NUKE, GROUND ZERO.
Ammonium diuranate, the penultimate uranium compound in U3O8 production, but the form in which mine product was sold until about 1970. See also Uranium oxide concentrate.