The U.S. Department of Energy (DOE) facility storing melted fuel from the Three Mile Island nuclear plant has not done enough to address crumbling concrete modules encasing the radioactive material, the U.S. Nuclear Regulatory Commission (NRC) said in a letter made public April 15. The 30 dry shielded canisters at the DOE facility at Idaho National Laboratory contain melted fuel from the Three Mile Island-2 reactor core. The concrete modules are ―showing significant cracking and degradation,‖ even though they were built in 1999 to last for 50 years, NRC said in the April 7 letter. DOE analyzed the structural integrity of the modules, which have walls 2 feet thick, and determined the problem is getting progressively worse, NRC said. Since the inspection, DOE has identified funding to pay for repairs and will begin the work this construction season, a spokeswoman said in an email April 15. ―These cracks have no impact on the storage modules‘ ability to safely store spent nuclear fuel,‖ she said. NRC asked DOE to provide information about corrective measures, a schedule for their implementation, and a plan for monitoring the effectiveness of actions taken. The degradation of the modules was likely due to ―water intrusion and the annual thawing and freezing cycle,‖ NRC said. Chunks of concrete have fallen from areas of the modules and there are signs they are no longer water-tight, NRC said. Cracking was first recognized in 2000 but considered to be ―cosmetic,‖ NRC said. In 2008, DOE recognized that continued cracking called into question the ability of the modules to protect the fuel canisters inside from natural phenomena and shield people from the radiation of the fuel. A recent study determined that protective caps should be installed, damaged concrete replaced, and a sealant applied. NRC ordered DOE to respond within 30 days.