TRANSPORTATION ISSUES FACING THE INTERNATIONAL COMMUNITY

C. HAUGHNEY, E. EASTON, C. CHAPPELL, N. OSGOOD, R. CUNNINGHAM

Office of Nuclear Material Safety and Safeguards, US Nuclear Regulatory Commission,

Washington, D.C., United States of America

material (LSA); the properties of materials used for shipping casks; plutonium air transport; and transport system certification. The views presented in the paper are based on the NRC's experience as the lead agency in developing and implementing Type B and fissile material package standards for the US. However, it should be emphasized that the views presented do not necessarily represent the official or final US position on these issues. The official US position will be determined by the U.S. Department of Transportation, which serves as the National Competent Authority for the United States.

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Abstract

This paper presents the concerns of the technical staff of the U.S. Nuclear Regulatory Commission on several proposals to revise the IAEA Regulations for the Safe Transport of Radioactive Material and the supporting documents.

1. INTRODUCTION

The International Atomic Energy Agency (IAEA) first published international regulations on the safe transport of radioactive material in 1961, and has revised these regulations, from time to time, as needs and experience indicated. Most member states having significant nuclear programs have adopted IAEA transport regulations as a basis for their national regulations and for application to international transportation. The packaging standards embodied in the IAEA transport regulations have provided a high level of protection for the public, and have made a significant contribution to the excellent transportation safety record achieved by IAEA member states.

Nevertheless, the public remains quite concerned about the transport of nuclear materials, particularly spent fuel, on their highways and railways and through their communities. These shipments bring large segments of the public in closer contact with large quantities of radioactive material than most other nuclear activities. As a consequence, major studies and tests have been conducted in the United States and other countries, to demonstrate that large margins of safety exist under IAEA regulations. Now, however, we see several proposals to revise IAEA regulations, that seem to be moving toward lower costs and reduced safety margins (i.e., less rigorous package standards). It is likely that the NRC staff would be unable to justify or support movement in this direction.

This paper presents the views of NRC technical staff on several important issues before the IAEA. These include: the transport of low-specific-activity

2. LOW SPECIFIC ACTIVITY MATERIAL

The shipment of LSA material is an important issue both within the United States and within the international community. We believe that there needs to be a fresh look at the rules governing shipments of LSA material. Some types of LSA materials being shipped today were not contemplated in the initial development of LSA package requirements. The transport of large volume LSA packages containing contaminated resins from nuclear reactors, for example, raises several technical questions about existing package performance criteria. The NRC staff believes that the system used to regulate the shipment of LSA materials in the United States needs to be re-examined in light of these new materials being shipped as LSA material.

The total quantity of radioactivity which can be shipped in a non-accident resistant LSA package is limited by the dose rate from the unshielded material.

The dose rate limit does not fully address potential problems in that it: a) does not prevent tons of highly dispersible contaminated resins from being shipped in a single package; b) is not an effective method to control total activity content of the package; c) bears little, if any, relationship to the radiological risk associated with large volume, highly dispersible materials in the event of a package rupture; and d) does not take into account the costs of intervention (as defined by ICRP) in the event of package rupture, particularly if it should occur on a major highway transport system (such as the accident involving fresh reactor fuel which occurred on a major US interstate highway near Springfield, Massachusetts on December 16, 1991). Intervention costs might include, for example, closing of a major highway system and the safety problems that could create, evacuation of

contaminated areas and associated costs, elevated public concern and the cost of

The potential for large intervention costs exists because of the large volume of material, and total activity level that can be shipped in a single LSA package.

-Jo For example, an LSA package used to ship contaminated reactor resins can contain up to six cubic meters of resin, and may have a total activity of greater than a terabequerel of cobalt-60. In contrast, non-LSA Type A packages typically contain much smaller quantities of material, generally only tens of cubic centimeters, and are limited, in the case of cobalt-60, to an activity of 0.4 terabequerel. Because of the larger volumes and activities, cleanup efforts in the event of a severe accident could be significantly more difficult and costly for an LSA package than for a Type A package. In fact, the magnitude of the cleanup efforts needed could lead to significant secondary safety problems and dislocations should the accident occur on a major highway. In addition, the larger volumes of material in an LSA package could become more widely dispersed than the contents of a Type A package, exposing a greater segment of the public. For these reasons, NRC staff believes that current transport regulations, which allow large volumes of contaminated reactor resins to be shipped as LSA material, may not be consistent with the level of protection afforded by other non-accident resistant, i.e, Type A, packages.

In short, NRC staff believes that a new approach is needed in regulating the shipment of LSA material - an approach that requires a much more in-depth review of the totality of accident consequences, available options and rigorous application of the optimization principle. The in-depth review should include a rigorous regulatory analysis of the potential costs versus benefits for the various options considered, as well as any necessary backfit analysis that may be required.' The review is a necessary first step at arriving at a defensible position, as to what kinds of shipments are appropriate for the current type of LSA packages, and which ones need to be shipped in an accident resistant Type B package.

While on the subject of LSA packaging, it should be noted that there are also technical problems with dose rate limit as a means to control the quantity of radioactivity in LSA packages. Dose rate measurements from a shielded package are often too low to permit a meaningful or reliable extrapolation to an unshielded configuration. This is particularly true for radionuclides with lower gamma energies. For example, a typical resin package, which has substantial lead and steel shielding, would provide an attenuation factor of seven orders of magnitude for cesium-137. Dose rate calculations can introduce additional errors, for example, incorrect buildup factors or èxposure-to-dose conversion factors.

Nonuniform source concentration and irregular source geometry also complicate calculations of unshielded dose rates. Applying the IAEA limit may also result in

1 NRC r e g u l a t i o n s in 10 CFR Part 50 require a b a c k f i t a n a l y s i s to determine if proposed regulatory changes have a s i g n i f i c a n t impact on the operational requirements for the C o m m i s s i o n ' s licensed n u c l e a r power reactors.

confusion as to whether the unshielded dose rate measurements should include secondary containers such as steel drums or liners.

In summary, our collective challenge is to develop within the IAEA safety framework, an overall strategy for regulating the shipment of LSA material that considers both the consequences and intervention costs of a severe accident.

3. MATERIAL PROPERTIES OF SHIPPING CONTAINERS

IAEA and US transportation regulations provide a high degree of confidence that transportation packages will survive the conditions expected in most transportation accidents. One of the factors that has contributed to this high degree of confidence in the United States, is that NRC has applied stringent criteria to materials used for transportation packages. This practice is justified by the potentially demanding loads and conditions to which transport packages may be subjected. It is also desirable to have large margins in material behavior, for transportation packages, because these packages are used in environments where public access cannot be controlled, and where the potential consequences of a package failure are great.

NRC has performed safety studies of spent fuel shipping casks under accident conditions that are beyond those encompassed by its regulations. ' These studies have shown that NRC-certified casks provide a high degree of safety, even for accidents that exceed the performance requirements in US regulations. A major reason is that NRC-certified transportation casks are constructed of materials that behave in a ductile, plastic manner, when subjected to high levels of stress and strain. If overloaded, the degree of failure would likely be limited, and characterized by arrested cracks and localized leaking.

In contrast, brittle failure can be characterized as a sudden fracture and potentially total rupture. Based on this potential for catastrophic failure, NRC has not approved brittle materials as structural components of shipping packages or casks. These materials include nodular cast iron, depleted uranium, borated stainless steel, and borated aluminum.

The use of one material, nodular cast iron, for spent fuel transport casks, has been a matter of controversy in the United States between the NRC staff and industry. It has been used in Europe for transportation casks for several years.

Casks constructed from this material could possibly pass US requirements in 10 CFR Part 71,~ depending on the size of potential internal material flaws and the

effectiveness of cask impact limiters in controlling stresses. However, NRC staff believes that the experience and data available for this material do not provide the assurance of sufficient margin, given the uncertainties of potential loads, uncertainties of the existence of flaws and the temperature to which the package may be subjected. Also, nodular cast iron casks do not have the large tolerance for overload that is inherent in present casks.

The material properties of nodular cast iron (and other non-ductile materials) are sensitive to the fabrication process and are difficult to reproduce. As a result, a high level of quality assurance is required to adequately control the fabrication process. Nodular cast iron is not authorized by the American Society of Mechanical Engineering (ASME) code for use in nuclear vessels, nor for use in non-nuclear vessels that contain lethal substances.

The NRC staff believes that the IAEA's recent intent to publish guidance on brittle fracture criteria, which would allow use of brittle materials in spent fuel casks, runs counter to good safety practice and is not in the public interest. The criteria being considered by the IAEA appears to represent a substantial reduction in the safety margins provided by previous IAEA regulations, and as practiced by NRC. Use of the IAEA brittle fracture criteria would introduce the possibility of catastrophic failure of casks involved in transportation accidents. In addition, the environmental and safety studies that have been conducted in the United States and elsewhere, to demonstrate the safety of spent fuel shipping containers to the public, are based upon ductile behavior of cask materials at high levels of stress and strain.³ As a consequence, existing studies cannot be used to support the safety of non-ductile materials in transport casks.

Technical criteria for preventing brittle failure in transportation casks have been developed and published in NRC regulatory guides.⁴'⁵ These guides have undergone extensive peer review and evaluation in the United States. NRC staff is currently reviewing the appropriateness of adopting these fracture criteria in 10 CFR Part 71 to exclude specifically, by regulation, non-ductile materials for use as structural components in transportation casks. The NRC staff would urge the IAEA to carefully consider the implications of its brittle fracture criteria, not only on package safety, but also on the public's perception of, and confidence in, transportation safety.

4. SYSTEM CERTIFICATION

A fundamental principle underlying both IAEA and NRC transportation regulations is that the shipping package provides the primary means of protection

for the public. The shipping package must be demonstrated to provide adequate containment, shielding, and criticality control. By requiring that shipping packages meet these conditions for a rigorous set of normal and accident conditions, the public is assured that the package will protect against a wide range of possible accidents. Other factors, such as restrictions on the way packages are transported, or required operating conditions, have been used in some package approvals, but have always been secondary in importance to package integrity.

A change that IAEA currently is considering for its 1996 regulations is system certification. System certification would permit packages to be approved that do not meet current performance standards, provided that other controls are placed on the shipment of these packages. To approve a package under system certification, it would be necessary for a package designer to show that controls placed on package shipment provide a comparable degree of safety as a package meeting the required performance criteria. Application for package approval, under system certification, would be based on a risk analysis of the package, imposed operation controls, and individual shipment plans.

It is not clear that system certification is needed or justified. There are already provisions in IAEA, and in most national regulations, that permit exemptions, under specific instances, for packages that do not meet required performance standards. For example, special arrangements are permitted in Safety Series 6, in paragraphs 141, 211, 720, and 727. Similarly, exemptions from US package regulations can be approved under 49 CFR 107, Part B (the U.S.

Department of Transportation) or 10 CFR 71.7 (NRC). The existing IAEA and US provisions are broad enough to deal with any situation that might arise, on a case-by-case basis.

Although an argument could be made that either package certification or system certification would provide a very low level of risk (risk = probability times consequence), it seems much more prudent to rely on package standards.

Package design standards are well understood. They have been the subject of numerous environmental and risk studies that show that the standards provide a wide margin of safety against the consequences of most probable accidents. Once a package has been certified as meeting regulatory standards, there is a very high degree of certainty that the packages will performed as expected. In relying on package standards, one can assume that the probability of a severe accident (i.e., one involving conditions within the performance envelope of existing standards) is unity. In system certification, the hope is that the controls in the system prevent the package from experiencing the forces of a severe accident or otherwise mitigate the consequences of a package failure.

For system certification, it would be necessary to establish an appropriate limit on risk, and then perform a probabilistic safety assessment (PSA) involving all components of the system to demonstrate that it meets the risk criteria.

Examples of special operating controls or special conditions that are often mentioned include: routing through less populated areas, limiting transit to nighttime hours, satellite tracking of shipments, prepositioning of highly trained response personnel, reduced speeds of transit, and traffic control. All of the operating controls or special conditions mentioned seek to control risk by reducing the probability of an accident occurring. The risks of relying on special operational controls are not well quantified and may be dependant on many variables that are difficult to determine. It is doubtful that a PSA could be performed with a high degree of confidence, if at all.

Simply stated, there is little assurance that performance standards can be developed to analyze system safety standards, which will be as rigorous and as well understood as those now applied to cask design. Before proceeding further, there should be a careful review of ICRP 60 and 64, as well as the IAEA-INSAG report (currently in draft), as they relate to potential exposure and the status and applicability of PSA to these situations.

5. PLUTONIUM AIR TRANSPORT

One area where the public has expressed significant concern is the shipment of plutonium, especially by air. The public has demanded, oftentimes through its legislative representatives, a very high standard of safety for plutonium air transport packages. The risk that the public is willing to accept from plutonium shipments is perhaps lower than for any other hazardous substance. The NRC staff is concerned about the discrepancy between the proposed IAEA Type C standards and existing US standards for plutonium air transport, and the effect this discrepancy might have.

The United States has had very conservative safety standards for the air transport of plutonium since 1975, when U.S. Public Law 94-79 (the Scheuer Amendment) was enacted. This law prohibited NRC from licensing the export, import, or domestic shipment of plutonium by air until a crashproof package, able to withstand the crash and explosion of a high-flying aircraft, was designed and certified to the U.S. Congress. The NRC staff believes that the new Type C standards under consideration for the 1996 Edition of Safety Series 6 (which would apply to plutonium air shipments) cannot be shown to be as stringent as the standards required under the Scheuer Amendment. Accordingly, plutonium air

transport packages designed to the proposed Type C standards would not be acceptable for import, export, or domestic shipments, within the United States.

There are several good technical reasons why the IAEA should consider adopting criteria based on the more stringent US standards. First, two package designs, PAT-1 and PAT-2, have already been developed, tested and certified as meeting the performance standards of US law. The certifications by the NRC are based upon the criteria in NUREG-0360, "Qualification Criteria to Certify a Package for Air Transport of Plutonium."6 In addition, other countries have reportedly designed packages that meet the US criteria. Second, the criteria in NUREG0360 have been reviewed and endorsed by two important peer groups -the U.S. National Academy of Sciences and NRC's Advisory Committee on Reactor Safeguards.

The US peer reviews have concluded that the qualification criteria in NUREG-0360 would assure that package survival will approach certainty in aircraft accidents occurring during take-off, landing, or ground operations, and would provide a high degree of protection against accidents that occur in other phases of flight, (e.g., mid-air collisions). Other studies have predicted that packages built to these criteria, which require a package to withstand an impact test at 130 m/sec on to an essentially unyielding surface, would survive over 99 percent of aircraft accidents.7 In contrast, the same studies predict that packages built to the proposed Type C standards, which require an impact test at 85 m/s onto an unyielding surface, would survive anywhere from 67 to 98 percent of possible aircraft accidents (ref.7).

Finally, it should be noted that the trend, at least in the United States, has been towards even more stringent standards. In 1987, the U.S. Congress passed U.S. Public Law 100-203 (the Murkowski Amendment). This law prohibits transport of plutonium through US airspace unless the NRC certifies to Congress that the package can withstand both an aircraft crash test, and a drop test from the

Finally, it should be noted that the trend, at least in the United States, has been towards even more stringent standards. In 1987, the U.S. Congress passed U.S. Public Law 100-203 (the Murkowski Amendment). This law prohibits transport of plutonium through US airspace unless the NRC certifies to Congress that the package can withstand both an aircraft crash test, and a drop test from the