The disclosure by Iran last week that it has had to remove the initial fuel load from its newly built Bushehr power reactor has ignited or re-ignited a storm of speculation, much of which is best ignored. Well before the latest difficulties, a controversy was raging among experts as to whether the plant had been damaged or its operations impaired by the spectacularly insidious Stuxnet malware. Now, with the news the Iranians have had to take the highly unusual step of de-loading the reactor's fuel, one well-known reactor specialist at a top organization speculated for the press that the plant might be vulnerable to a Chernobyl-type accident.
That possibility can be safely dismissed. The Bushehr reactor is a second-generation Soviet reactor of the VVER type, not an RBMK like the one that exploded at Chernobyl. The RBMK has a singular design defect, namely, that at certain power levels, if the reactor suffers a loss of cooling water, its reactivity can increase rather than decrease. In the boiling water and pressurized water reactors used exclusively in the United States and western Europe, because the chain reaction depends on the presence of water, which acts as a so-called "moderator," if there is a loss of water, the reactor automatically shuts down. (This is a very important and little appreciated passive safety feature of the light water reactor.) The RBMK on the other hand is moderated mainly by carbon, which accounts for why a loss of water can have the perverse effect of boosting reactivity. In the Chernobyl accident, an unexpected spike in power caused liquid cooling water to become steam and thus become less dense; that set off a positive feedback loop that caused the plant's reactivity to escalate by orders of magnitude in microseconds.
The VVER is a light water reactor more like the U.S. and U.S.-derived plants and cannot blow up the way the Chernobyl reactor did. The Iranians, under the deposed Shah, originally planned to have Germans build them a U.S.-type light water reactor at Bushehr. When that deal fell apart after the revolution, they persuaded the Russians to install a VVER at the site they had begun to prepare.
As for Stuxnet, all the expert analysis indicates that its payload was designed specifically to reprogram electronic controllers in Iran's Natanz uranium enrichment plant. The outer shells of Stuxnet infected many other industrial control systems around the world but generally did no damage elsewhere. It appears now that the problem at Bushehr was a defective pump that must be repaired or replaced.