The NAC-LWT transportation cask is certified by the Nuclear Regulatory Commission (NRC) as a Type B package for the shipment of spent nuclear fuel. The LWT cask is cylindrical in shape, with a diameter of approximately 3.5 feet and a length of approximately 16.5 feet. It weighs approximately 24 tons (21.8 metric tons) when empty, or 25.6 tons (23.3 metric tons) when loaded. The cask body has approximately 8-inch-thick walls, consisting of a 0.75-inch-thick XM 19 high-strength stainless steel inner shell, a 5.75-inch-thick lead gamma shield, a 1.2-inch-thick XM-19 high-strength stainless steel outer shell and a neutron shield tank.

The NRC specifies physical requirements that Type B packages (casks plus radioactive contents) must meet, or exceed, in order to be certified to safely transport spent fuel. These requirements are identified as standards in NRC regulations (10 CFR 71, Packaging and Transportation of Radioactive Materials) and address issues such as design, construction materials, welding criteria, external radiation restrictions, allowable heat content, nuclear criticality and physical integrity of the package. The NRC specifies how compliance with the regulatory requirements will be demonstrated and identifies performance standards for packages under normal transport conditions and hypothetical accident conditions. Conformance with these performance standards is evaluated by the NRC before a certificate or license is approved. To demonstrate that the package meets the performance requirements, a cask owner may use scale-model tests or other methods of demonstration, such as computer modeling, that are acceptable to the NRC to show compliance with the regulatory requirements (10 CFR 71.41). The accident conditions that the NRC uses to evaluate package performance are a 30-foot drop onto an unyielding surface at various orientations, a drop of 40 inches onto a six-inch-diameter bar, a cask and its contents fully engulfed in a 1,475 degree Fahrenheit fire for 30 minutes and immersion in water.

The NRC issued the original Certificate of Compliance, Number 71-9225, for the NAC-LWT transportation cask on December 6, 1989. The cask is designed to transport a variety of spent nuclear fuel types, including commercial power reactor and research reactor fuels. For any new spent fuel proposed to be transported, a modification to the original certificate may be required, depending on how the characteristics of that spent fuel compare with those previously certified for transport. The most recent modification for the NAC-LWT was issued on May 27, 1998, for transport of the specific fuel from South Korea. Each cask in the shipment from South Korea will contain approximately one-tenth or less of the amount of radiological activity allowed under the modified certification.

As part of the analysis and certification effort for the original certificate, drop and puncture tests were performed on a quarter-scale model of a NAC-LWT cask. Additionally, full-scale quasi static compression tests in three orientations were performed on the impact limiter energy absorption material which is attached to the ends of the cask to absorb energy associated with an accident. Testing and analysis to demonstrate compliance with the water immersion and fire standards were verified using standard computer modeling techniques.

In addition, on June 1, 1998, the Department of Transportation issued its Competent Authority Certification for the NAC-LWT, certifying that the package design meets the U.S. and International Atomic Energy Agency (IAEA) requirements for a Type B package. The IAEA requirements include a standard for immersion of the cask at 200 meters for no less than one hour.

All design testing and analyses were based on materials, methods, codes and standards developed by industry, national laboratories and the Nuclear Regulatory Commission. The analysis and testing of the NAC-LWT Cask demonstrated its compliance with all NRC requirements of 10 CFR 71, Packaging and Transportation of Radioactive Materials and DOT regulations of 49 CFR 173, Shippers - General Requirements for Shipments and Packaging.