TECHNICAL BACKGROUNDER: PLUTONIUM AIR SHIPMENTS
September 4, 1996
Edwin S. Lyman, PhD
The International Atomic Energy Agency (IAEA) tries to create the impression that the safety standards which it has proposed for the air transport of radioactive materials (RAM) are supported by extensive technical analysis. However, the process by which these standards were actually set is driven by the political and economic concerns of a handful of IAEA member states with a vested interest in the outcome; in many cases technical justifications appear to have been tacked on as an afterthought. The IAEA's technical case for the standards consists largely of unsupported (and often unsupportable) claims, hand-waving arguments and sloppy back-of-the-envelope calculations that lack scientific rigor. Consequently, it is impossible to have confidence in the ability of the IAEA standards to protect the public from catastrophic releases of radiation in the event of a severe air transport accident. Below, a number of technical uncertainties concerning the proposed regulations for the transport of plutonium and other types of RAM by air are discussed.
The most glaring indication that the IAEA disregards pertinent technical evidence is that the view of IAEA officials with little expertise in the aviation field repeatedly prevailed over the objections of professional aviation organizations. For example, IAEA has ignored the criticism of the Dangerous Goods Panel of ICAO (International Civil Aviation Organization), which regards the Type C impact speed, fire temperature and lack of sequencing of impact and fire tests as insufficiently severe and uninformed by the large body of data on flight accidents that the airline industry has accumulated based on the survival of flight recorders. Information from a meeting of the Dangerous Goods Panel on this issus is included below.
Type C Package Standards:
Under present regulations, RAM can be transported by air in the same packages ("Type B") that are approved for ground transportation, even though the severity of aircraft accidents can greatly exceed those occurring on land. The IAEA has proposed new standards for packages intended for air transport of large quantities of RAM. These standards ("Type C"), while more stringent than Type B standards, are not stringent enough to guarantee that air transport of RAM will be acceptably safe.
Type C casks will be designed to survive an impact of 90 m/s (203 mph) on an "unyielding" surface. However, actual crash impacts can be much greater. According to the IAEA's own data, there is approximately a 10% chance that the impact speed experienced in a plane crash will exceed 90 m/s. This value was chosen because IAEA concluded that it would be too expensive to develop a cask which could withstand a significantly greater fraction of accidents. In contrast, airplane flight data recorders "black boxes"), which are designed to survive all but 2% of plane crashes, are impact tested at a severity comparable to a crash at 135 m/s. (Even so, 10% of the black boxes recovered after accidents show some evidence of failure).
Fire test temperature and duration
Type C casks will be designed to survive a 60-minute, fully engulfing fire with a flame temperature of 800C. Data used by IAEA indicates that 10% of fires following air crashes exceed this duration. Furthermore, jet plane fuels burn at temperatures up to 1100C. For this reason, "black boxes" are designed to survive a 60-minute fire at 1100C. Although IAEA is quick to point out that it is the total heat input to the package that is relevant, not the flame temperature and fire duration, it is known that some black boxes recovered from air crashes have experienced total heat inputs equivalent to the 1100C, 1-hour fire.
Although the crash and fire tests are designed so that a Type C package will fail in 10% of air accidents involving either a crash or a fire, the actual failure probability may be much greater because the Type C impact and fire tests do not have to be performed sequentially. Therefore, the ability of a Type C package to withstand an accident involving both a severe crash and a fire is unknown.
IAEA attempts to justify the decision to make the Type C test non- sequential by claiming that "in severe accidents, high speed impact and long duration fires are not expected to be encountered simultaneously because high velocity accidents cause fuel dispersion." This argument is invalid for two reasons. First, the impact speed above which fuel dispersal becomes a significant factor is around 130 m/s,1 which is far greater than the Type C impact speed. In fact, several accidents have been recorded involving speeds from just below to greater than 90 m/s to that were followed by severe fires, including a crash at an airport in Qatar in 1979 (impact speed 87 m/s, fire duration 3.5 hours). Second, the argument ignores the possibility of combustion from an external source of fuel, such as a severed natural gas line, as was the case in the crash of an El Al jet into an apartment building in Amsterdam.
Low Dispersible Material (LDM) Exemption:
Although the Type C standards were devised so that the economic impact on industry would be minimized, apparently they are still too burdensome for some shippers of radioactive materials. Included in the proposed Regulations is an exemption from Type C requirements for so-called "low dispersible materials" (LDM), a prime candidate of which is plutonium-containing mixed-oxide (MOX) fuel. Materials classified as LDM could continue to be transported by air in Type B casks, which only have to survive an impact speed of 13.2 m/s (30 mph) and a 30-minute, 800C fire.
Quantities of RAM below a certain threshold (around 120 grams for reactor- grade plutonium, for example) would also be exempt from Type C requirements, independent of physical form.
RAM could be certified as LDM if, after being subject to either the Type C impact or fire test, it did not release more than one hundred times the total amount of radioactivity allowed by IAEA to be released from the accident site in one week. Why one hundred times? There is evidence that this number was chosen not based on a safety analysis, but because it was anticipated that MOX fuel would not be able to meet a more stringent criterion.
"Graceful failure": fact or fiction?
The LDM and de minimis exemptions are based on the premise that Type B casks would "gracefully fail" if they were exposed to accident conditions more severe than the conditions they were designed to withstand (e.g. those encountered in a plane crash), and would only release a small fraction (around one one-hundredth) of their contents. This would therefore compensate for the elevated release permitted from the material itself.
The "graceful failure" assertion is apparently based on a single study, which consisted of "limited [impact] tests carried out on a representative Type B packaging."2 This study, which did not involve Type B cask designs actually in use, is an extremely weak basis for such a major exemption. There is simply no data on how Type B casks would perform in accidents more severe than their design basis, because cask manufacturers have no obligation to carry out such studies. The "graceful failure" concept, although called a "fact" by IAEA, is merely speculation.
Furthermore, the "graceful failure" behavior of Type B packages with respect to fire has not been examined, even in the one study used by IAEA. Because of the heavy reliance of current Type B packages on elastomeric seals, which fail abruptly at temperatures as low as 250C, it is extremely doubtful that these packages would fail "gracefully" if exposed to long-duration fires.
The Consequences of a Crash of a Plutonium Air Shipment
How much plutonium, contained in so-called "low dispersible" MOX fuel, could be released in the event of a high-velocity air crash which the shipping cask would not be capable of surviving? Impact studies of uranium fuel pellets indicate that in the relevant range of impact energies (e.g. speeds of 90-130 m/s), roughly 1-10 joules per gram of material, from 0.1 - 1 percent of the fuel will be released in the form of particles less than ten microns in diameter, which can be easily inhaled and retained deep in the lung. This corresponds to a quantity of respirable plutonium in the range of 25 to 250 grams for each MOX fuel assembly in the shipment. For a mid-sized shipment carrying 40 fuel assemblies, as much as several kilograms of plutonium in respirable form could be released as a result of the impact alone. A subsequent fire could cause further disintegration and dispersal of the material.
A rapid release of one kilogram of plutonium at ground level in dispersible, inhalable form would cause a public health emergency of the first magnitude. Plutonium air concentrations could be on the order of hundreds of micrograms per cubic meter of air at one kilometer from the release site. Individuals breathing this air would inhale enough plutonium to cause cancer with certainty within minutes. In the worst case, a crash in a densely populated area could cause tens of thousands of latent cancers.
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End Notes1. L. Fischer, J. VanSant and C. Chou, "Draft Criteria for Controlled Tests for Air Transport Packages," Lawrence Livermore National Laboratory, UCRL-ID-103684, August 1990. Back to document
2. International Atomic Energy Agency, TECDOC-702, p.30. Back to document