Iran has fortified the sites containing its highly enriched uranium (HEU) stockpile by collapsing tunnels and mining entrances, according to sources familiar with U.S. intelligence. The U.S. military contemplated an operation to seize the HEU in mid-May, but concluded that the risk was too high. Iran's fortification efforts subsequently continued and increased the time and risk that would be required to retrieve the uranium, even if the retrieval were to be carried out under the terms of a negotiated settlement or by Iran itself. U.S. President Donald Trump said in early June that removal of the HEU would take at least two weeks to complete. A former head of the U.S. National Nuclear Security Administration voiced concern that Iran could claim some of the uranium is irretrievable, thereby obfuscating compliance with any potential agreement to remove it.