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Addressing Safety Issues in the Sea Transport of Radioactive Materials

Edwin S. Lyman, PhD
Scientific Director
Nuclear Control Institute

Presentation to the
IMO Special Consultative Meeting
4-6 March 1996


Institution of the INF ("Irradiated Nuclear Fuel") Code was originally inspired by a drive in 1985 by some IMO member states to correct an inconsistency in the rules regulating the transport of dangerous goods by sea, which neglect to impose any specific requirements on the transport system when the goods in question are radioactive materials (RAM). The IAEA disputed the need for such requirements, arguing that safety is assured through its regulatory philosophy that the primary responsibility for transport safety should lie not in the details of the transport system, but in the design of the shipping package. The IAEA has the authority to regulate packages for the shipment of RAM through the specification of performance criteria in IAEA Safety Series No. 6 (SS6), "Regulations for the Safe Transport of Radioactive Material." In spite of the initial objections of the IAEA, the INF Code was ultimately recommended by a joint IMO/IAEA working group and subsequently adopted in 1993 as a standard for ships carrying INF, plutonium, and high-level reprocessing wastes (HLW), albeit only as a voluntary Code of Practice.

Whether the INF Code is necessary (or sufficient) therefore depends on the extent to which the standards for Type B package design laid out in SS6 are stringent enough to guarantee safety with respect to a broad range of sea accident environments. Many observers have pointed out that the mechanical and thermal stresses that may occur during marine accidents can far exceed those imposed by the Type B test series. Also, studies by this author have identified numerous technical uncertainties associated with the sea shipment of vitrified high-level reprocessing wastes (VHLW) that at best reduce confidence in the accuracy of risk assessments, and at worst could have serious implications for safety.

The IAEA has consistently dismissed the possibility that Type B casks may not be robust enough to be safely used for marine transport. However, because the information and analysis necessary for a definitive resolution of this issue is not now available, this position is largely based on speculation. In response to public concerns, the IAEA initiated the Coordinated Research Program (CRP) in 1994 in an attempt to establish a more rigorous quantitative basis for evaluating the risks involved in transport of RAM by sea.

The joint IMO/IAEA working group stipulated that, should new evidence demonstrate that the severity of sea accidents is indeed greater than that of the Type B test, upgrading of the package standards through the established SS6 revision process, rather than a strengthening of requirements for the other elements of the marine transport system, should be undertaken. However, the IAEA strategy appears to be designed to guarantee that neither of these measures will ever be enacted, no matter what the outcome of assessments like the CRP. The IMO must take a harder look at the IAEA approach and evaluate whether it is adequate for resolution of the legitimate safety concerns of States which are at risk today from sea shipments of RAM.

The "Type B" Package Controversy

As defined in the 1985 version of IAEA SS6, "Type B" is the designation given to packages which are designed to be partially "accident-resistant" and are permitted to carry radioactive materials in unrestricted quantities in any transport mode. (In the 1996 revision, a new, more durable package, designated Type C, will be introduced to replace Type B casks in some air transport applications, although serious questions remain, both as to the adequacy of the Type C cask and the safety of the special exemption for so-called "low dispersible materials, including fresh MOX fuel.1)

In order for a shipping cask to qualify as a Type B package, one must demonstrate (by testing actual specimens, using computer simulations, or some combination thereof) that the cask will be able to maintain significant containment function following a series of tests intended to simulate the conditions of a transport accident. The required tests include a drop test from a height of 9 m (corresponding to an impact velocity of 13.3 m/s) onto an essentially unyielding surface, a thermal test involving exposure for 30 minutes to conditions equivalent to a fire with a flame temperature of 800C, and a water immersion test (200 m depth for 1 hour for large inventory packages).

Many observers have pointed out, in submissions to the IMO, to the joint IMO/IAEA Working Group, and in the public record, that historical marine accidents have often been of a duration which greatly exceeds that of the Type B test sequence. For example, the U.S. delegation to the IMO noted in 1990 that "fires on board ships, which can burn for days, certainly have the potential to exceed the testing parameters of IAEA, and further strengthen the need for special requirements."2 There are also several examples of fires at sea that have burned for weeks.3

Another well-known fact is that typical hydrocarbon fuels can burn at temperatures several hundred degrees higher than that specified by the Type B thermal test. This discrepancy has led to unease among governments as well as environmentalists. For example, in the course of a joint U.S.-Russian development program for a container to transport plutonium weapon components, the Russians expressed concern that the IAEA thermal test was inadequate and requested that the container be tested at 1000C.4

The joint IAEA/IMO working group, which met only twice, asserted that "there was no information or data in the papers submitted to the first and second sessions that would cast doubt on the adequacy of the IAEA Regulations." Does the IAEA have the information available, however, to demonstrate the adequacy of the regulations? The answer is, quite unambiguously, no. Proposals before the IMO to study this issue quantitatively were made as far back as 1990; now, over five years later, the effort has only just begun (see the section on the Coordinated Research Program, below). The burden of proof clearly must lie with the IAEA to demonstrate why historically severe marine accidents can be safely excluded from consideration.

Regulatory Inertia and IAEA Safety Series No.6

Why is the IAEA so reluctant to reconsider its packaging standards for marine transport? A fundamental principle applied in the ongoing SS6 revision process is "the need to maintain regulatory stability."5 The Agency is under tremendous pressure from the nuclear industry to refrain from introducing more stringent transport standards which could increase costs, even if they are clearly warranted from a safety perspective. On the other hand, it has shown little reluctance to make changes that lighten the regulatory burden. This is seen in a number of different areas.

One example is IAEA's selective incorporation of new radiological protection information into the 1996 revision of the SS6. Since the 1985 edition was published, new data from the ongoing study of Japanese atomic bomb survivors has demonstrated that the carcinogenic potential of ionizing radiation is more than four times greater than was previously thought, a conclusion which has led to reductions in the international recommendations for occupational and public exposures to radiation.6 However, in spite of a professed commitment to "keeping abreast of of the latest radiation protection practices,"7 the IAEA refused to reduce the coefficients in SS6 describing permissible radiation releases accordingly. This means that the revised SS6 will permit, by default, a fourfold increase in the health consequences of a transport accident.

On the other hand, SS6 does incorporate a new dosimetric model which permits increases in the permissible releases of a significant fraction of radionuclides (only "very few" of the changes are more restrictive). For example, permissible releases of plutonium and higher actinides are increased by a factor of five in the 1996 revision. This inconsistency is a prime example of the fundamentally political nature of the SS6 revision process.

Another blatant example is the development of standards for air transport casks. After years of wrangling, the IAEA finally produced a standard which has been judged to be inadequate by a number of international associations of air transport professionals.8 To add insult to injury, it recently voted to exempt plutonium from the new standard if it is in the form of MOX fuel, on the basis of flimsy technical arguments.

This discussion makes apparent that in its quest for "regulatory stability," the IAEA is providing a level of safety through its recommendations which is increasingly divergent from that of other international organizations.

Type B Package Vulnerabilities: An Example

It is useful to examine some of the implications of the possibility that the IAEA is overestimating the safety of marine transport of RAM in Type B casks.

The following discussion of safety issues is based largely on an analysis, conducted by this author in 1994, of the transport of vitrified high-level radioactive wastes (VHLW), the first shipment of which took place from France to Japan in early 1995.9 It should be noted that, while the companies involved in the transport issued blanket condemnations of the report, they have never responded to it in a substantive way, despite repeated attempts to engage them in a dialogue. On the other hand, private conversations with experts in government and industry over the last year have confirmed the validity of many of the technical issues raised in the report; furthermore, the paper has been accepted for publication in a peer-reviewed journal. This episode is in itself an indication of the fundamental unresponsiveness of the IAEA regulatory process to external criticism.

Elastomer cask seals

The weaknesses of the Type B standards are exemplified by the fact that they have not prevented the widespread use of elastomeric materials to seal the lids on RAM shipping casks. The lid seals play an essential role in preventing the escape of radioactive gases and fine particulates from the cask following an accident. However, elastomeric materials have poor heat resistance and will fail after exposure for a couple of hours to temperatures in the vicinity of 250-300C; above this range, they will fail in under one hour. Furthermore, elastomers are damaged by exposure to high radiation fields. For these reasons, elastomer seals do not appear to be the best choice for casks transporting heat-generating, highly gamma- emitting materials like spent fuel or VHLW, especially when compared to costlier metallic seals, which offer superior heat and radiation resistance.10

Transport casks with elastomer seals are able to be qualified as Type B packages because the heat input generated by the Type B thermal test is low enough so that the seal temperature remains below the failure threshold (provided the cover protecting the seal remains intact following the impact tests). But the current regulations do not require the cask designer to determine the conditions which would cause the seal to fail and ensure that a large safety margin is present.

For instance, when a prototype VHLW transport cask was tested by Japanese authorities, the seal temperature reached 178C following exposure to Type B thermal conditions, an increase of 30C.11 While this result was judged to provide a "sufficient safety margin" of around 70-100C below the failure threshold, this conclusion is open to argument. Extrapolating from this result and assuming a linear average seal heating rate, an 800C fire would cause the seal to fail after approximately 2.5 hours. Exposure to higher temperatures would obviously reduce the time to seal failure. Seal failure could also be induced by fires of lower temperature and longer duration. Furthermore, the synergistic effect of gamma radiation damage may lower the temperature threshold or reduce the time to failure at elevated temperature, a point which the Japanese did not consider when assessing the safety margin.12 Without an understanding of the probabilities with which different fire scenarios may be encountered during marine transport, it is impossible to judge whether a particular "safety margin" is sufficiently conservative.

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