We would not want pure U235 in a power plant, this would make it too much like a bomb.[from that link]
George W. Bush will be reelected by a margin of at least ten per cent
Long answer follows for those who are interested:
Whether you're making bombs or electricity, you need enriched uranium—uranium in which the ratio of U235 to U238 is higher than the naturally-occurring 0.7%. The question is a) how enriched it needs to be and b) how much you need.
The gas centrifuge enrichment process consists of turning the uranium into gas and running it through a centrifuge. Since U238 is slightly heavier than U236, the gas pulled out from the center of the centrifuge will be slightly more enriched than the gas pulled from the rim. The slightly-more-enriched gas is simply fed into a new centrifuge to be enriched even more. To create reactor fuel, you repeat this process several times. To create bomb material, you repeat this process many more times still.
So up to this point, Iran's story holds water: The simple fact that they are enriching uranium doesn't tell us what they're enriching it for. And there's really no way, short of direct inspection of the facility, to determine the degree to which they're enriching it.
That brings us to the second question, that of quanity. The operation of the enrichment process is measured in separative work units (SWUs). Iran's enrichment facility is still under construction, but right now it can produce somewhere around 6,000 SWUs a year. Of course we don't really know, since Iran isn't entirely forthcoming about their capabilities (fancy that), so it might even be more.
Iran claims that the purpose of their enrichment program is to be self-sufficient for Iran's nuclear power industry. However, reactors require a lot of fuel—one reactor load is around 20 tons of fuel. Running Iran's planned reactors for a year would require some 160,000 SWUs of uranium. In other words, Iran's current enrichment facilities would have to run non-stop for 27 years just to power their reactors for one year. At that rate there is no point in even bothering.
By contrast, a bomb only requires about 4,000 SWUs of uranium—it needs one-thousandth the mass of metal a reactor needs, but that metal must be 50 times more enriched than the reactor's. If Iran starts now, which they appear to be doing, they can have a bomb within 9 months. Iran is either producing one nuclear weapon or nine days of reactor fuel—which do you think is more likely?
Now you can see why Kerry thinks the solution to this problem is to give Iran uranium. If we promise to provide Iran with the 160,000 SWUs' worth of uranium their reactors would need, then they would, supposedly, not need any domestic enrichment ability, and so, naturally, they'd simply dismantle their facilities.
Yeah, right.
But wait, it gets worse. It takes 4,000 SWUs to turn unprocessed uranium into enough highly-enriched uranium for a bomb. But most of the real work is involved in enriching the uranium just enough for reactor fuel. If you start with reactor fuel, turn that into UF6 and then enrich that, you only need 700 SWUs to make a bomb. So if Kerry gets elected and ships a huge quantity of reactor fuel to Iran, Iran will be able to make a bomb just 42 days later. But, since Kerry drove a boat for three months in Vietnam 35 years ago, I guess he must understand this issue better than I do.