When using thorium in modified light water reactor (LWR) problems include: the undeveloped technology for fuel fabrication; in traditional, once-through LWR designs potential problems in recycling thorium due to highly radioactive 228Th; some weapons proliferation risk due to production of 233U; and the technical problems (not yet satisfactorily solved) in reprocessing. Much development work is still required before the thorium fuel cycle can be commercialized for use in LWR, and the effort required seems unlikely while (or where) abundant uranium is available. *Thanks Wikipedia*
http://www.ga.gov.au/minerals/research/national/thorium/index.jsp
Uranium will be with us for a while yet I am afraid. Uranium-235 has a half life of about 7000 million years.