Nuclear reprocessing is one of the biggests myths proponed by nuclear advocates.
Only the plutonium, which is less than 1% of the spent fuel rod, can be really used again as MOX. However the process to seperate the plutonium is a extremely expensive and dirty one, involving pumping low level nuclear waste into the sea.
The rest of the uranium in the used fuel rod is uneconomical to reprocess because of contaminated with U232 and U-236:
"No use of reprocessed uranium in French reactors in the near future
The uranium recovered from reprocessing of spent fuel in France is not expected to be used for the manufacture of nuclear fuel in the near future. French utility EdF rather has made provisions for long-term storage of the reprocessed uranium for 250 years. This was revealed in a report of the French Court of Auditors on the decommissioning of nuclear facilities and the management of radioactive wastes.
Usage of the reprocessed uranium (REPU) is problematic for several reasons: since the REPU is contaminated with the artificial uranium isotopes U-232 and U-236, special precautions are necessary during processing: the U-232 and its decay products cause elevated radiation doses for the plant personnel, and the U-236 as a neutron absorber requires higher enrichment levels to achieve the same reactivity. In consequence, use of the REPU is not very attractive at present market conditions: conversion is three times more expensive than conversion of natural uranium, and enrichment cannot be done in France's sole enrichment plant (Eurodif gazeous diffusion plant), since the REPU would contaminate the plant's circuits. "