The Sea Shipment of Radioactive Materials:

                                                Safety and Environmental Concerns

 

                                                             Edwin S. Lyman, PhD

                                                                 Scientific Director

                                                            Nuclear Control Institute

 

                                                      Presented to the Conference on

                                     Carriage of Ultrahazardous Radioactive Cargo by Sea:

                                                         Implications and Responses

 

                                                            Kuala Lumpur, Malaysia

                                                              October 18-19, 1999

 

 

                                                                    Introduction

 

            The shipment of large quantities of radioactive materials by sea is a controversial practice.  There are vast differences in the perception of the risks to the world's population and to the environment among the various stakeholders in the debate: those sectors of the nuclear industry that rely on sea shipments of highly radioactive materials, the populations of coastal states that are situated adjacent to shipping routes or who depend on fisheries that may lie on those routes, the international agencies who set the standards for shipping radioactive materials, and international environmental groups.

 

            What is lacking in the debate is an authoritative analysis of the true risks posed by these shipments, including a critical review of conventional wisdom and assumptions.  The inventories of radioactive materials that are being routinely shipped ---in the form of spent nuclear fuel, vitrified high-level nuclear waste (VHLW) and plutonium-uranium mixed-oxide (MOX) fuel ---are extremely large (on the order of tens of millions of curies for the first two types of material), and failure of the containment system can cause serious radiological releases.  The key issue is the validity of the assertion of the International Atomic Energy Agency (IAEA) that the so-called "Type B" standards it recommends for the packaging and transport of these materials provide adequate protection to the public and the environment during sea shipments, even though these regulations were originally developed for land transports.

 

            The safety case made by the nuclear shipping industry and its regulators is based largely on their belief that accidents severe enough to lead to significant radiological releases are incredible.  However, in the wake of several recent incidents, including the worst nuclear accident in Japanese history, it is becoming increasingly clear that this perception is not fully rooted in reality, especially in a time of increased economic pressure on the nuclear industry. 

 

            The severe accident at the fuel conversion plant in Tokai-mura, Japan, on September 30 has revealed a massive chain of procedural and technical violations, involving the foreman supervising the work, the plant management, the company executives and the regulatory authorities that were supposed to be overseeing the plant.  Only a few weeks before, it was revealed that a plutonium fuel fabrication plant in the United Kingdom, manufacturing mixed-oxide (MOX) fuel to be shipped to Japan by sea, had falsified crucial data for 22 lots of fuel pellets to save time.  And in the spring of 1998, the discovery of widespread excessive external contamination on spent nuclear fuel transport casks in Western Europe led to a suspension of shipments in many of the affected countries, and which remains in place today in Germany.

 

            All of these incidents stem from a single root cause:  a lack of attention to the numerous details which are essential for the safe operation of a nuclear technology infrastructure.  The general erosion in quality control, maintenance, respect for procedure and regulatory oversight which is manifest stems both from an increasing need on the part of the nuclear industry to cut costs, and from a complacency which has resulted from the excessive and unjustified confidence the industry has in its own safety record.  This is an extremely dangerous trend which has serious ramifications for the safety of the system for shipping RAM by sea around the world.  In view of this, an exhaustive reassessment of the entire safety case for these shipments is overdue, with particular attention not only to the regulations as they exist in theory but how they are applied in practice, i.e. the way they are applied in the manufacture and routine operation of shipping casks and the vessels which carry them.           

 

            The International Atomic Energy Agency (IAEA) bears a good share of the responsibility for the worldwide decline in attention to safety.  While it would like to project the image of a hard-nosed overseer that continually reassesses the effectiveness of its safety standards, it is actually an inertia-driven organization that devotes much of its energy to preserving the status quo.

 

            Recently, the IAEA has demonstrated an alarming lack of interest in the enforcement of its own regulations.  For example, the IAEA standards for external contamination of shipping casks were found last year to have been routinely violated all over Western Europe for a decade or longer, by factors of up to ten thousand.  One of the contributing factors was a design flaw that made adequate decontamination of some shipping casks very difficult.  However, instead of reviewing the standards that permitted these casks to be licensed, it took no action.  This merely reinforced the attitude which led to the problem in the first place --- a pervasive belief on the part of shippers that IAEA standards were unnecessarily stringent and could be ignored.   The public has no way of knowing how many other aspects of the existing regulations are treated in such a cavalier fashion.

       

            It is time for a comprehensive environmental assessment of the impacts of shipping RAM by sea, with full opportunity for participation by any member of the public who may potentially be affected.  Despite the public relations efforts of the industry and its supporters to belittle the risks of this practice, there is growing evidence that the long-term collective radiological consequences of the sinking of a ship transporting RAM, or a collision and fire involving a RAM ship in a congested waterway, could be as severe as those from a nuclear reactor loss-of-containment accident.  Given that the shippers of RAM do not have the credibility to demonstrate convincingly that the probability of occurrence of such accidents is indeed minimal, the international community has every right to demand that these shipments cease until such assurances can be made.   

 

 

The IAEA Transport Safety Standards

 

            The shipping packages now used to transport large quantities of radioactive material (RAM) by sea are designed to meet a set of performance requirements known as "Type B" standards, which are defined in the IAEA’s transport standards, the most recent of which are the "Regulations for the Safe Transport of Radioactive Material” (1996 edition).[1]  Most notably, the standards require that Type B packages withstand a series of drop tests from a height of 9 meters, followed by an 800 degrees C fire for thirty minutes, without significant breach of the containment.  For packages containing large inventories of RAM, an immersion test in water at 200 meters' depth for one hour is required. 

 

            These standards were originally developed for land-based modes of transport, and questions have arisen regarding their adequacy for packages used for sea shipments, which may be subject to more severe accident conditions, including more energetic collisions, long-duration, high-temperature fires and long-term immersion or immersion at greater depths.  The IAEA's response to this issue has been two-fold.  First, it argues that although accident conditions that occur aboard ships may be more severe than the Type B testing regimen, the actual accident environment experienced by a RAM package most likely would be less severe.  Second, it claims that Type B packages have substantial safety margins built into them, so that even if they experience more severe conditions than they were designed to withstand they will "fail gracefully" rather than abruptly. 

 

            There is scant evidence, however, for either of these assumptions.  The principle of "graceful failure" is largely accepted on faith, and has not been verified for any package used to ship RAM by sea.  In fact, there is good reason to believe that current RAM package designs do not exhibit “graceful failure” in important respects. 

           

            Concerning collision resistance, IAEA has referred to a recent Sandia National Laboratories (SNL)  study that finds "cask-like" structures can survive "forces greatly exceeding those imparted by the regulatory tests" without gross failure --- in particular, that they can survive impacts of 4 times the energy of the regulatory tests (an impact speed of 26.8 meters per second [m/s], about twice the 13.3 m/s regulatory speed).  However, both neglect to mention that the same study found that these casks exhibited "significant leakage from both seals" following an impact of 5 times the energy (2.25 times the velocity, or 30 m/s).[2]

 

            IAEA has often cited the fact that the Type B drop tests require the use of “unyielding” surfaces as an argument that Type B packages would automatically survive much more energetic collisions with the softer “yielding” surfaces that they would be more likely to encounter in an accident.   However, this does not necessarily follow.  According to a recent paper, under some circumstances (for instance, if it has impact limiters, which are standard for high-activity RAM packages) a collision with a yielding surface would be as damaging as one with an unyielding surface.[3] There are even situations in which a yielding surface could cause more damage than an unyielding one.  This led the paper’s author to lament that complications like these make “communicating with the public more difficult.”  A more useful conclusion would be that the IAEA should stop oversimplifying complicated technical issues when they attempt to convince the public of the adequacy of their regulations.

 

            Concerning thermal resistance, the use of elastomer seals in VHLW shipping casks is an example of how Type B casks can be designed with inadequate safety margins and potentially abrupt failure modes.  Because of the low temperature failure threshold (250-300 degrees C) of these materials, an 800 degrees C fire of a modestly greater duration than the 30-minute regulatory fire, or a lower-temperature, longer-duration fire, could cause breach of containment.  In view of the documented record of shipboard fires that last from days to weeks, elastomer seals must be regarded as a serious vulnerability of these casks.  

  

However, to evaluate the containment performance in a fire of an actual shipping cask, which presumably has grooves of the standard width, the effect of overfilling on seal performance must be taken into account.  Thus the results of the SNL test series, which did not include "specific controlled tests ... to compare performance of `standard' groove width fixtures to the modified ... fixtures," are not useful for evaluating seal performance in real situations.

           

            Concerning resistance to water immersion, the IAEA has said that Type B shipping casks must "withstand an immersion test of 200 meters without rupture of the containment."  They do not point out that this test need be carried out only for one hour, and that an undamaged specimen can be used.  This test is not relevant to a realistic accident situation in which a cask, perhaps initially damaged in a collision or fire, then remains at a depth of 200 meters for a period of weeks to months or longer.  It should be noted that COGEMA, BNFL and FEPC provided erroneous information on this point to the IMO in an paper submitted to the Special Consultative Meeting in March 1996,  which claimed that Type B packages are "after these [collision and fire] tests, put in at least 200 meters of water for 8 hours."[4]  While the IAEA mandates that the immersion test at 200 meters must be carried out for "not less than one hour," VHLW casks have only been tested to the minimum standard of one hour[5].  Also, there is no indication that the immersion test has been performed on packages previously subjected to collision and fire test.  The effect of prolonged periods of high pressure on performance of the elastomer seals, which can "creep" at low temperature, has not been determined.  Moreover, localized corrosion on the metal grooves in the seal area can cause loss of sealing in a matter of months, as is discussed below.

 

 

The IAEA Coordinated Research Program

 

            In response to concerns from environmental groups and coastal states, the IAEA decided in 1994 to undertake a Coordinated Research Program (CRP) "to assess the severity of accidents on radioactive material packages and their expected frequencies of occurrence for sea transport."[6]  One of the chief goals of this program was to determine whether the assertion that Type B standards were adequate for sea transport is indeed correct, by analyzing the mechanical and thermal loadings that a package would experience during shipboard collisions and fires, respectively.  This was to be accomplished through computer simulation and actual experiments.  To accomplish the latter, the U.S. conducted fire experiments upon a test vessel called the Mayo Lykes. 

 

            In principle, the CRP had the potential to make a significant contribution to the understanding of the risks of marine transport of RAM in Type B packages.  However, active participants in the CRP include France, Japan, and the U.K., the three states which are most heavily involved in the sea shipment of RAM and which have commercial and institutional interests in maintaining the status quo.  This was reflected in the groundrules of the study.  For instance, "in considering fires on board ships, the CRP participants agreed that data from oil tankers and LPG tankers were not relevant to vessels used to carry radioactive cargo,"[7] arguing that the probabilities of collisions between the two types of vessel would be too small.  Thus they prejudged one of their conclusions before they even got started.  Therefore, there was little hope from the outset that the review would provide an unbiased and satisfactory resolution of the outstanding issues. 

 

            The IAEA has said that "the question of whether [a] sea-mode dependent package requirement might be needed ... appears to depend upon the final results of the CRP."  Unfortunately, we are unable to analyze the document at this time, as its release, which was planned for 1998, has been delayed for more than a year.  However, we do have the benefit of access to some of the information that the U.S. has provided as part of its contribution to the CRP effort, which contains some surprises.[8]

 

 

                              The Consequences of the Loss of a RAM Package at Sea

 

            One of the more contentious issues in the debate over sea shipments of RAM has involved the assessment of the environmental consequences of the loss of a RAM package at sea.  Because of the impact such an event would have on the tourism and fishing industries in the affected region, the shippers of RAM are anxious to allay the fears of coastal states.  British Nuclear Fuels Limited (BNFL) claims in its public relations materials that even if VHLW became "directly exposed to the sea ... the effect of such a scenario would be negligible."[9]  The IAEA CRP participants maintain that "even with upper value nominal releases the impacts on the environment and health were small or negligible."[10]  Highly publicized studies by the Central Research Institute of the Electric Power Industry of Japan (CRIEPI) claimed to find that radiation doses to humans would be extremely small from losses of either VHLW packages or spent fuel packages at sea.[11]  On the other hand, the IAEA itself has stated that "if a large irradiated fuel package were to be lost on the continental shelf, some large exposures could result"[12] (a comment which applies equally well to a VHLW package).

 

            Part of the confusion stems from the large uncertainties that plague calculations of this type, and the strong dependence of the results on the assumptions that are made.                 

 

            In December 1996, the Nuclear Control Institute (NCI) released a report titled "The Sea Transport of Vitrified High-Level Radioactive Wastes (VHLW):  Unresolved Safety Issues."  The report analyzed the potential consequences of a severe accident in coastal waters that resulted in damage to the transport ship and the VHLW shipping cask, causing the cargo to sink to a depth of 200 meters (typical of the maximum depth of the continental shelf). 

 

            The report's main finding was that such an event could result in significant health consequences to consumers of marine products from the region.  This contradicted the claims made by the shippers of VHLW.  NCI showed that the CRIEPI results were not consistent with other studies, such as one by the Nuclear Energy Agency (NEA) of the OECD, that analyzed similar accidents and found that doses could be hundreds of thousands of times larger.[13]  Some of these results appear in Table I, which illustrates the wide range of variation in consequence estimates among different studies.

 

 

                       TABLE I  Committed Effective Doses to Individuals Resulting from Unit

                                      Radionuclide Releases in Shallow Waters (mSv/TBq)

 

Radionuclide	OECD (198 OECD (1998) "worst case"	CRIEPI (1995)	Nielson (1996)
Cs-137 Am-241 Cm-244	not given 3.5 1.7	1x10-7 2.0x10-6 2.5x10-6	0.02 0.4 not given

 

           

            The million-fold difference between the results of the CRIEPI study and those of the others in Table I can be attributed to two factors:  the assumptions regarding the residual containment function of the damaged package, and the details of the environmental transport model relating releases to exposures, with the first factor being the most important.    

 

            The CRIEPI study is a good example of how assumptions can be manipulated to get a desired result.  It assumes that the shipping cask remains undamaged except for failure of the O-ring seal, so that the only flow path for water is the gap between the body and the lid of the shipping cask (the "radial clearance").  The low dose rates it obtains (6x10^-4 mSv per year) can be largely attributed to the assumption of an unrealistically small value for the gap width (0.01 millimeter), which results in an extremely low flow rate of water through the cask.[14]  The low flow rate suppresses the release of radionuclides both from the cask and from the VHLW blocks themselves (because the cask cavity rapidly becomes saturated with radionuclides).  The gap width was obtained by CRIEPI on the assumption that it was due entirely to the roughness of the surface finishes of the metal grooves.     The size of the gap width is essential to the calculation because it can be shown that the flow rate depends on the cube of this parameter ---  a 10-fold variation in gap size results in a 1000-fold variation in flow rate.    

 

However, the CRIEPI report does not discuss the sensitivity of its results to variations in the inputs.  This is a serious flaw of the study, because it is questionable whether the gap width specified by CRIEPI is appropriate.  Consultation of standard dimension charts for installation of O-rings shows that the minimum radial clearance, for the smallest diameter O-rings available, is 0.025 mm.  Radial clearances for larger diameter O-rings, such as those used in the TN 28 VT, would be a minimum of 0.05 mm and could be as large as 0.13 mm.  To use the smallest clearances requires very fine and expensive metal finishes, and discussions with seal manufacturers indicate that customers rarely request minimum clearances, even for nuclear applications. 

 

            In any event, once the cask were submerged in seawater, one cannot expect that the very narrow gap assumed by CRIEPI would remain for very long.  The cask materials --- carbon steel and stainless steel --- are vulnerable to localized corrosion in seawater.  A recent analysis of the durability of RAM shipping cask materials in seawater concluded that only "some months may be enough for seawater to `wash away' the stainless steel under the seals and penetrate the packaging."[15]  In particular, the study concluded that the TN28VT, the cask used to transport VHLW, could lose its leaktightness after only 16 months of immersion."  Therefore, it is completely unreasonable to assume that the gap width would remain unchanged over the course of decades, as does CRIEPI.  Moreover, the same study finds that the cover of the TN28VT would be perforated elsewhere by localized corrosion within 6 years, creating other pathways for radionuclide release.

           

            The corrosion study also makes clear that a shipping cask need not sustain damage during the initial stages of an accident to pose a radiological hazard if it is lost at sea.  The IAEA is going to great lengths in the CRP to show that even in the event of an accident severe enough to sink a ship, RAM shipping casks would not be damaged.  However, the IAEA standards do not require demonstration that long-term leaktightness under submerged conditions is maintained, and it appears that RAM shipping casks now in service could not meet such a standard.

 

            If the radial clearance were ten times greater than the value assumed by CRIEPI, the flow rate would be on the order of 0.2 cc/sec, corresponding to a glass dissolution rate at 90 degrees C of about 3x10^-5 grams per square centimeter per day [g/(cm²-d)], which is only about one-tenth of the maximum observed rate.[16]  

 

            If it is assumed that the VHLW dissolves congruently (meaning that all glass components are released at rates proportional to their concentrations in the glass), NCI estimates that a glass dissolution rate of 3x10^-5 g/(cm²-d) could result in dose rates over 50 milliSievert (mSv), or 5 rem, per failed cask per year to consumers of marine products from the affected region, from the release of two actinides (americium-241 and curium-244) alone. This should be compared to the ICRP limit for public exposure to artificial radiation of 1 mSv per year. (According to ICRP, the 1 mSv/year limit applies to all sources of artificial radioactivity; the dose rate from any single source should be only a fraction of this limit.)

                       

            These calculations also have large uncertainties because of uncertainties in the dissolution behavior of VHLW in seawater.  Under some conditions, such as dissolution in pure water, low-solubility actinides are preferentially retained at the glass surface.  Retention of actinides at the glass surface may lower dose estimates, but there is strong evidence that retention may not occur.  If it is assumed that actinides are fully retained at the glass surface, dose rates around 0.5 mSv per year, are possible, a significant fraction of the ICRP limit.

 

            Sediments present on the sea floor can have a large impact on radionuclide releases from VHLW.  Vernaz and Godon have observed, for example, that the capability of VHLW to retain americium was reduced by two orders of magnitude if a mixture of granite, sand and clay was added to the system.  The OECD study, on the other hand, conservatively assumes that the presence of sediments will inhibit saturation and re-precipitation of solubility-limited radionuclides such as the actinides.  Given the large range of uncertainty in this calculation, this seems like a reasonable assumption.

 

            The most recent volume of the series Scientific Basis for Nuclear Waste Management contains two papers which provide further evidence that the release rates of actinides (e.g. Pu, Am and Cm) from HLW glass may be greater than have been previously assumed, and that congruent dissolution of actinides may provide a more conservative model. 

           

            A Japanese study of R7T7 glass confirmed that the actinides Pu and Cm were released primarily in the form of colloids (insoluble suspensions of large particles) during glass dissolution.[17]  When in the colloid phase, these radionuclides do not contribute to the saturation of the solution because they are not dissolved.  Consequently, total concentrations of Pu and Cm in the colloid phase were 10-100 times greater than the concentrations in the solution phase.  Also, the pattern of release of these radionuclides was not characteristic of solubility-limited dissolution but of "alteration-limited" dissolution, e.g. the rate of release was controlled only by the rate of reaction of the glass with water. 

 

            A paper from the U.S. found somewhat different but also significant results.[18]  This study found that actinides were initially retained on the glass surface, but after several years the plated-out material began to flake off in the form of large particles that formed colloids.  The result was a rapid increase in the rate of release of Pu and Am from the glass at this time.  According to the paper, "the spallation of alteration phases, some of which have incorporated Pu and Am, led to total release of these elements approaching that expected from congruent dissolution of the glass."

 

            Even accounting for thousand-fold differences in the assumed flow rate, the CRIEPI result is still considerably smaller than the estimate based on the OECD results.  Therefore, other assumptions in their model must also play a role.  Another reason for the difference in the radiological consequences of such an accident could be different assumptions for the most highly exposed groups in Japanese study, compared to those in the region considered in the OECD study (the East Coast of the U.S.).  It has been shown that the results of consequence calculations like these can vary by several orders of magnitude, depending on the critical group assumptions.

 

            IAEA, in a review of the NCI study, argued that it is misleading to directly compare different studies because of their sensitivity to local geographic patterns. [19]  However, it has not complained to BNFL about their use of the results of the CRIEPI study, specific to a sinking off the coast of Japan, to reassure en-route states all along the route.  IAEA's point only adds weight to the argument that site-specific environmental impact statements need to be carried out along the entire shipping route.

 

            The IAEA has argued that if a RAM package were lost at a depth of 200 meters or less, recovery of the package would take place.  However, there is an internal inconsistency in its logic which is extremely troublesome.

 

            Even if one accepts the unproven proposition that salvage of a severely damaged VHLW cask is an easy task, the rhetoric of IAEA and the RAM shippers provides good evidence that salvage will NOT take place.  Salvage will only be attempted if the will exists to undertake a complex and potentially dangerous operation.  However, IAEA and others claim that there is little risk, even in a worst-case accident, that significant radionuclide releases would occur in the near-term.  This raises the concern that should such an accident occur, the shippers will use arguments to justify not attempting recovery of the cargo.  A similar situation occurred late in 1997 when the MSC CARLA, carrying cesium chloride therapeutic radiation sources, sank in the mid-Atlantic.  The French safety agency IPSN quickly released a calculation which purported to show that there would be only negligible harm from the sources if left in the ocean, and therefore there was no need to recover them.      

 

            Prompted by the NCI study, the IAEA announced in 1998 that it was undertaking a literature study related to the consequences of severe maritime accidents involving radioactive materials.  Like the CRP, this study has not been released either.

 

                        A SNL report analyzing the NCI study wrote that "the scenario analyzed is so improbable that it is of very little or no concern," and that "even if, against all odds, the scenario endpoint were somehow to be reached," the consequences would be extremely minor and well below background levels. [20] 

 

            However, this statement appears puzzling when SNL's contribution to the IAEA CRP, known as the "SeaRAM" study, is considered.[21]  In this study, a calculation of the radiological consequences of the loss of a shipping cask containing 12 spent nuclear fuel assemblies (with the same radionuclide content as 4 VHLW canisters) at various positions in the North Atlantic.  In the worst case, a loss in Labrador (selected to represent a large fishery), the individual dose rates and collective doses calculated are surprisingly large.  The report estimates that individual doses can be as large as 1.8 mSv per year.  The collective dose resulting from this accident over a 50-year period is estimated to be between 540,000 and 630,000 person-Sievert, corresponding to a total of 27,000 to 31,500 latent cancer fatalities as a result of the accident.  This should be compared to the total collective dose resulting from the Chernobyl accident estimated by United Nations Scientific Committee on the Effects of Ionizing Radiation (UNSCEAR), 600,000 person-Sv.[22]  Therefore, according to SNL, RAM transport ships are indeed "floating Chernobyls," as Greenpeace refers to them.      

 

 

                                                                    Conclusions

 

Recent evidence indicates that the long-term public health consequences of a severe accident during the sea transport of highly radioactive materials could be comparable to those resulting from a loss-of-containment accident at a nuclear reactor.   On the other hand, the shippers of RAM and regulatory authorities are unable to provide convincing arguments that the risk of such an accident is negligible.  Therefore, the safety case for these shipments has not been made.