The Wall Street Journal said the purpose of the re-design was to assure that the bomb could borrow deeply into the mountain in which the Islamic Republic has built a new centrifuge enrichment plant at Fordo.
Much attention has been given to this bomb since the Islamic Republic announced years ago it was burying its sensitive installations to make them safe from US attack. However, Air Force officials say they can still put a buried site like Fordo out of operation simply by attacking the access points with ordinary weapons.
The bunker-buster has a certain Hollywood appeal, however, and everything about it generates much media attention. It also gives a military advantage by completely destroying any centrifuge site. But military planners emphasize that the absence of a big enough bunker-buster would not stop them from attacking Fordo and shutting down its operations with just a handful of guided munitions targeted at entry and exit points and ventilation shafts.
The centrifuges might survive such an attack intact, but no would be able to reach them until the tunnels to the plant were dug out. In the meantime, everyone inside would be asphyxiated. And a second attack would again close the site in just seconds.
The Journal article conceded that ordinary munitions could shut down Fordo, but only stated that deep into its long article.
The giant 30,000-pound bomb including 5,300 pounds of explosive is formally called the Massive Ordnance Penetrator or MOP. It is specifically designed to attack buried nuclear targets, such as those in Iran and North Korea.
The MOP is designed not to explode on hitting the earth, but rather to penetrate deep into the earth before finally exploding. How far it can penetrate depends on whether the bomb must go through packed dirt, concrete or granite. The Post said tests convinced the US military the existing MOP could not penetrate deeply enough into the mountain at Fordo to get into the centrifuge chamber.
The Journal said the Air Force last month sent Congress a secret request for $82 million to enable the MOP to borrow farther. The Air Force currently has about 20 MOPs.