The Nuclear Regulatory Commission (NRC) addresses the problem in 10CFR 40.13.  NRC regulations mandate that people are exempt from the regulations only when quantities of Thorium in lantern gas mantles, vacuum tubes, welding rods, electric lamps, germicidal or sun lamps contain less than 2 grams or less than 0.25 percent by weight Thorium.

For more, see:

• National Institute of Health: Annual Report on Carcinogens (report home page) with the chapter on Thorium Dioxide here (PDF); Public Perception of Thorium
• RASANET provides information on Radiation and Waste Safety from the International Atomic Energy Agency. One of many references on their site: Ingestion and dosage limits.
• Dept. of Health and Human Services, Thorium
• US Nuclear Regulatory Commission (NRC): Health effects: Thorium in welding rods (exemptions); Fact Sheet on Radioactive Waste. Many other resources are available at www.nrc.gov; try the Search and the Electronic Reading Room.

Perma-Fix was called on to assist for the Defense Logistics Agency, the logistics arm of the Federal government.  A quantity of Thorium, in the form of Thorium Nitrate, ThNO₃, was being held by the Agency for potential use as reactor breeder fuel, using the Thorium Fuel Cycle.

Because of its corrosive and hygroscopic nature, the Agency wanted to convert Thorium Nitrate into a more stable and less corrosive form.  Using a jacketed Plowshare Reactor, Perma-Fix changed the Nitrate compound into the innocuous Thorium Hydroxide, Th(OH), which also resulted in a significant volume reduction of material as an added benefit.

Not only was the form changed from a solid to a safer, more dense powder, Perma-Fix repackaged the Thorium Hydroxide more efficiently, resulting in fewer drums of material to be stored.