Iran is the only country so far to have built a uranium-enrichment plant before having even a single working reactor that would need its uranium as fuel for the reactor core.

At that point, the fuel at the core of the reactors would have dropped in temperature and no longer be capable of boiling the surrounding water.

By contrast, reactors have strong containment vessels that stand a better chance of bottling up radiation from a meltdown of the fuel in the reactor core.