Energy Technology Division
Within the ASME framework. an effort has been underway since 1978 to codify a set of rules for "construction" of casks for spent fuel and for the radiologically similar material referred to as high-level waste. In ASME usage "construction" covers materials, design, fabrication. examination, testing and documentation. Casks are the containment vessels and the pressure vessels for radioactive material transportation packages. Because the component type that is closest to casks in structural characteristics and design service, as well as in importance to safety, is the reactor vessel of a nuclear power plant, the same rules appear to be a good starting point for developing rules for casks. Therefore, ASME Boiler and Pressure Vessel Code, Section Ill, Division 1, Class 1; that is, Subsection NB, was selected in 1978 as the basis for nuclear packaging (NuPack) rules. Eventually, the ASME Board of Nuclear Codes and Standards decided to give NuPack its own Division designation, resulting in a three-way split of Section III into Division 1 for metal construction, Division 2 for concrete construction, and Division 3 for transportation packaging. The new paragraph designator corresponding to NB is WB, and a general requirements subsection corresponding to NCA is included, with designator WA. At present, the exact title of the NuPack subcommittee and of Division 3 has not been definitively established, nor has the exact scope. Some of the options under consideration and the associated rationale are presented in the paper.
The service requirements for casks as containment vessels would appear to be much less severe than for reactor vessels. After all, the events taking place inside the casks are less dynamic, less energetic and more predictable. However, it must be recognized that a reactor vessel is always under a state of controlled operation and is in an enclosed environment that is highly engineered for safety. On the other hand, a cask traveling down the highway is in an uncontrolled and unpredictable environment, and is the only barrier between the radioactive material and a public that is not even aware of the cask's presence. On this basis, the cask is considered to be in the same class of importance to safety (Class 1 in ASME Section III terminology) as a reactor vessel. Another basic concept accepted in the writing of NuPack was that the new Code paragraphs would reflect today's routinely accepted regulatory practice by carefully applying existing technologies from Section III to transportation packaging. Even though writing a code that would advance the state of the art of transportation packaging with respect to analytical methods, design criteria, materials of construction, or other aspects could be desirable, NuPack does not attempt to do so.
This paper presents a brief progression of activities in the development of the draft NuPack, the policy decisions as to what is and is not covered in terms of analytical methods, criteria, scope, and other aspects. Without going into detailed technical explanations and justifications, the paper discusses some of the specific provisions presently incorporated in the draft NuPack (for example the distinction between packaging owners and design owners) and some of the significant differences between the NB paragraphs that apply to reactor vessels and the corresponding WB paragraphs that apply to "nuclear packaging" vessels (for example the differences in allowable stress intensities and the treatment of thermal stresses).
*Work supported by the U.S. Department of Energy, under Contract W-31-109-ENG-38.
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