WASHINGTON (AP) -- The House voted Wednesday to bar the U.S. government from future purchases of heavy water from Iran, undercutting the controversial nuclear pact with that nation and earning a certain veto threat on a key government funding bill.


Wednesday night's 251-168 vote came on an amendment by Florida GOP Rep. Ron DeSantis to a funding bill for the Energy Department.


A similar amendment died in the Senate after a major dust-up earlier this year, when Democrats filibustered a companion proposal by freshman Sen. Tom Cotton, R-Ark.


Last month, the Obama administration completed an $8.6 million deal to buy 32 tons of heavy water from Iran. The amendment wouldn't affect that deal but would thwart purchases next year. Nonetheless, the White House has weighed in strongly with a veto promise that may get the proposal removed during House-Senate negotiations.


The sale will help Iran meet the terms of last year's landmark deal, in which Iran agreed to curb its atomic program in exchange for billions of dollars in sanctions relief.


Heavy water, formed with a hydrogen isotope, is a key component for one kind of nuclear reactor. It is not radioactive but has research and medical applications and can also be used to produce weapons-grade plutonium. Under the nuclear deal, Iran is allowed to use heavy water in its modified Arak nuclear reactor, but must sell any excess supply of both heavy water and enriched uranium on the international market.


There are no current plans for further U.S. purchases of heavy water. The pending deal calls for the Energy Department's Isotope Program to purchase the heavy water from a subsidiary of the Atomic Energy Organization of Iran. The heavy water will be stored at the Oak Ridge National Laboratory in Tennessee and then resold on the commercial market for research purposes.


By AP