The U.S. Nuclear Regulatory Commission (NRC) is amending its spent fuel storage regulations by revising the Holtec International, Inc. (Holtec), HI-STORM (Holtec International Storage Module) Underground Maximum Capacity (UMAX) Canister Storage System listing within the "List of approved spent fuel storage casks" to add Amendment No. 1 to Certificate of Compliance (CoC) No. 1040. Amendment No. 1 provides a seismically enhanced version of the HI-STORM UMAX Canister Storage System, identified as the "Most Severe Earthquake (MSE)" version, that could be used in areas with higher seismic demands than those analyzed previously. Amendment No. 1 also includes minor physical design changes to help ensure structural integrity of the amended system. These are the addition of a hold-down system to the closure lid; replacing the fill material in the interstitial spaces between the cavity enclosure containers (CECs) surrounding the casks with 3000 psi concrete; strengthening the multi-purpose canister (MPC) guides; and engineering the guides' nominal gap with the MPC to be tighter than the original HI-STORM UMAX Canister Storage System design.