Edwin S. Lyman
Center for Energy and Environmental Studies
School of Engineering and Applied Science
Princeton University
Princeton, NJ 08544May 8, 1995
Questions/Comments Concerning the Air Transport of "Very Low Dispersibility" (VLD) Material
1. Given the current definition of VLD material, it is absolutely unclear what level of protection is provided by the standards for air transport of fresh MOX fuel and whether it is consistent with that for ground- and sea-based modes. To determine this with a reasonable degree of confidence, knowledge of the residual containment ability of a generic Type B package following exposure to the more severe conditions expected during an aviation accident would be essential. Yet there is no provision in the revised standards to determine this by testing. While the IAEA expects the packages to "fail gracefully" as accident severity increases, this is a sufficiently important point to warrant more comprehensive experimental confirmation.The IAEA apparently also neglects the fact that the elastomer seals in use in many Type B package designs will not fail gracefully under thermal conditions only slightly more severe than the Type B thermal test, but may fail abruptly, resulting in increases in leak rates of many orders of magnitude. Also, the fact that material requirements for shielding often exceed mechanical requirements is obviously not relevant for the shipment of the very low gamma-power fresh MOX fuel assemblies, so it is not clear a priori how much excess mechanical protection is built into current Type B shipping casks for fresh MOX.
In the absence of quantitative information on the behavior of Type B casks under Type C conditions, the 100A2 release limit from VLD material seems completely arbitrary and under plausible circumstances could lead to an exceedance of the dose limits permitted elsewhere in SS No.6.
2. It is also unclear whether basing release limits solely on an individual dose limitation criterion is appropriate in this case. For instance, although particles with aerodynamic diameters greater than 100 m will not be dispersed easily as aerosols, emission of larger particles that remain near the accident will still pose environmental hazards (e.g ingestion by wildlife, external hazard from Am-241 buildup) and will contribute to the costs of site decontamination. Thus it is imprudent not to impose any activity limits on the release of particles greater than 100 m. This is a loophole that should be closed.
3. There appears to be a great deal of controversy regarding the non-sequential nature of the impact and thermal tests required for Type C packages. Since it is proposed that VLD material will be defined by its performance under the same test regimen, it is crucial that the justification for non-sequential tests be demonstrated in a quantitative and statistically convincing way before the VLD exception can be permitted. This is especially pertinent for a material such as fresh MOX fuel assemblies, for which the expected dispersibility would be dramatically different depending on whether or not the tests are sequential. As the Germans point out, it is highly doubtful that a one-hour 800C fire test alone on a fresh MOX assembly would cause cladding rupture (unless it were grossly defective). However, if the mechanical test were to cause rupture of the cladding and fracture of the pellets, the likelihood of significant dispersal during a subsequent fire would be greatly increased. Thus a non-sequential test would grossly underestimate the consequences of an accident in which these events did occur sequentially.
4. In the thermal test definition of VLD material, there is no attempt to quantify variables that may affect the outcome significantly, such as the availability of oxygen to the fuel meat during heating. Data such as the total mass of generated particles and the particle size distribution should be collected for a range of different oxygen potentials to to determine their limits of uncertainty.
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