Preliminary Report for Comment
The Sea Transport of Vitrified High-Level Radioactive Wastes: Unresolved Safety IssuesEdwin S. Lyman, PhD
Nuclear Control Institute
In February of 1995, the ship Pacific Pintail set sail from Cherbourg, France for the port of Mutsu-Ogawara in northern Japan, carrying a shipment of 28 canisters of vitrified (glassified) high-level radioactive wastes (VHLW). This shipment of extremely hazardous material, the first of dozens being planned to return VHLW generated during the overseas reprocessing of Japanese-origin spent fuel, ignited considerable controversy at many points along its sea route. In the face of united opposition by Caribbean states, the ship avoided the Panama Canal and instead sailed around Cape Horn, the tip of South America, where it antagonized Brazil, Argentina and Chile.
When the Pintail reached Japan in late April of 1995, the governor of Aomori Prefecture initially refused to permit the ship to dock, employing a resolution which granted him the authority to refuse entry into port of any vessel if he felt that the assurance of safety and the provision of information to be inadequate. The standoff was resolved a day later, after he had extracted a promise from the Science and Technology Agency (STA) that it would not authorize a final disposal site for HLW in Aomori without the governor's consent.
In my view, the worldwide unpopularity of the Pintail and its cargo was entirely justified. In late 1994, when I was a postdoctoral research associate at the Center for Energy and Environmental Studies at Princeton University, I was commissioned by three environmental and nuclear non-proliferation advocacy organizations (Greenpeace International, Nuclear Control Institute and Citizens' Nuclear Information Center) to examine safety issues associated with the upcoming sea transport of VHLW. Initially, I was skeptical that these shipments posed significant environmental risks. However, as I began to analyze the technical and regulatory basis for the safety of shipping VHLW by sea as it was being practiced, I found it seriously deficient in many respects. In particular, I found that the degree of uncertainty surrounding the potential performance of each component of the shipping system was so large as to make assurances of safety practically worthless.
The report of my findings 1 was released in December 1994. The public response of the nuclear industry to it was to attack my qualifications, my integrity and my independence.2 However, the industry was unable to refute the principal technical points in my report, and in retrospect it has stood up quite well. Also, additional information has since come to light that raises new questions.
Now, nearly two years later, the industry is about to launch another shipment of VHLW. This shipment which will consist of 40 VHLW canisters, is a bridge between the initial, pilot shipment of 28 canisters and subsequent full-scale shipments of 150 canisters each. This shipment will proceed despite the fact that the industry has not provided satisfactory responses to the serious safety concerns raised in my 1994 report. These issues, which include the use in the TN 28 VT VHLW shipping cask of elastomer seals which can lose their function in a fire of moderate severity, and the use by COGEMA and BNFL of a type of stainless steel for packaging the VHLW which loses its corrosion resistance during VHLW processing, are reviewed below.
Details of the Second VHLW Shipment
Japan has released a description of the 40 VHLW canisters which will be transported in the second sea shipment, including an inventory of radionuclides. The composition of the VHLW canister labeled "1021C" is typical (although below average for the shipment); some of the more significant radionuclides are:
Americium-241 (Am-241): 101 Terabequerels (TBq)
Curium-244 (Cm-244): 95.6 TBq
Strontium-90 (Sr-90): 3220 TBq
Cesium-134 (Cs-134): 121 TBq
Cesium-137 (Cs-137): 5000 TBq
Total alpha emitters: 210 TBq
Total beta-gamma emitters: 18,000 TBq
Total heat generation: 1.5 kilowatts (kW)
The second shipment will consist of two casks carrying 20 VHLW canisters each, while the first shipment consisted of one cask carrying 28 VHLW canisters. One explanation for this is that the average heat generation of the second shipment will be 1.61 kW per canister, which is slightly higher than the average for the first shipment, which according to measurements made by Japanese authorities after delivery, was 1.54 kW/canister.
The Consequences of a Loss of VHLW Cargo at Sea
One type of accident that was not considered in detail in the 1994 report was one in which the ship and its VHLW cargo is lost at sea. A closer examination of this accident scenario indicates that it can potentially result in severe health consequences for humans and for marine biota. However, the large uncertainties inherent in calculations of this type, especially for accidents in which the cargo is lost in coastal waters, result in extremely wide ranges for predictions of potential consequences. This is clear from a survey of studies which attempt to evaluate the consequences of the loss of a VHLW cask at sea, and obtain results which differ by as much as six orders of magnitude (a factor of 1,000,000). The predictions of doses to the public, which range from very low to highly significant, are sensitive to numerous assumptions, such as the location of the accident, the condition of the VHLW packages, the leach rate of the glass in seawater, the local current patterns, food consumption, the extent to which radionuclides would be sorbed on sediments and the likelihood of salvage.
However, it is important to note that in the worst case, the loss of a damaged VHLW cargo in coastal waters can cause levels of chronic exposure to the public far in excess of those permitted by international standards. This fact was pointed out in an 1987 Organization for Economic Cooperation and Development/Nuclear Energy Agency (OECD/NEA) study of the feasibility of disposing of VHLW under the ocean floor, an undertaking that would require by necessity the large-scale transport of VHLW by sea. The analysis contained in this study led to the conclusion that "coastal [VHLW] transportation accidents are ... unacceptable."3
The most credible accident scenario that could lead to damage and loss of VHLW cargo is a collision in which the VHLW transport ship is struck in the side, where the cargo holds are most vulnerable, by the bow of another vessel. A ship with sufficient kinetic energy could penetrate the cargo holds and cause the VHLW casks to be crushed. The OECD study recognized that because the consequences of this accident would violate its own safety criteria, it was necessary to design a transport system in which the probability of occurrence of such an accident would be "extremely small." To meet this goal, the study proposed the use of purpose-built ships designed so that their holds could not be penetrated by ships of any displacement (mass) travelling at a speed of 24 knots (12.3 m/s) or below.
The Pacific Nuclear Transport Limited (PNTL) ships that transport VHLW today are supposedly designed to be resistant to collisions. This type of ship is designed so that if it is struck by a ship of 24,000 tonnes displacement travelling at 15 knots then the cargo area will not be penetrated. However, the protection is afforded by this design criterion is limited, since there are ships in service which have much greater displacements and travel at much greater speeds. Some ships travel at speeds up to 25-30 knots, and displacements of tankers can exceed 100,000 tonnes. A ship with a 50,000 tonne displacement traveling at 30 knots would have a kinetic energy eight times greater than the ship modeled in the design basis collision with a PNTL vessel.
If a damaged VHLW cask were lost at sea in shallow waters, contact of the stainless steel VHLW canisters with highly corrosive seawater would begin almost immediately. Because the Type 309 stainless steel has been extensively sensitized (see below) it will undergo pitting and stress-corrosion cracking at an accelerated rate, exposing the glass underneath within a couple of months.
A number of studies have identified the radionuclides cesium-137 (Cs-137), americium-241 (Am-241) and curium-244 (Cm-244) as those responsible for the largest contributions to both individual and collective doses for a loss of VHLW in coastal waters. In order to calculate these doses, one must know two pieces of information: first, what are the rates of release of these radionuclides under the appropriate conditions, and second, how these radionuclides are transported from the release point through the marine food chain and finally to humans.
a) Release of radionuclides from VHLW
The leach rates of radionuclides from VHLW in contact with deionized water are fairly well-characterized. According to a recent French study,4 the room temperature dynamic leach rate (e.g. water replenished daily) of Cs- 137 from R7T7 reference glass is 2 10-7 g/(cm2-d) (grams per square centimeter of surface area per day), equivalent to 6.410-7 terabequerels (TBq) per square centimeter per day. For Am-241, the leach rate for R7T7 glass doped with 1.1 GBq/g Am-241 is 310- 7 g/(cm2-d);5 for the composition 1021C, which contains about 0.22 GBq/g Am-241, the corresponding leach rate would be 6.710-8 g/(cm2-d), or 8.510-9 TBq/(cm2-d). Similarly, the leach rate of Cm-244 from 1021C is found to be 1.910-8 g/(cm2-d), or 5.810-8 TBq/(cm2-d).
Less information is available for VHLW leach rates in contact with highly saline seawater. However, the presence of salt generally accelerates the dissolution of glass in water.6 For static leaching of R7T7 glass at 90C, the presence of salt enhances the release of glass constituents such as boron and plutonium (therefore indicating a general increase in the glass alteration rate) by a factor of approximately 4.
Previous studies of the loss of a VHLW cask have assumed these or similar values. One flaw of this assumption is that it does not take into account that these values are steady-state average release rates. However, when water first comes in contact with VHLW, the initial leach rates are higher by a factor of around 100 (prior to the formation of a less permeable surface layer) and then decrease steadily over the course of about a month until they reach the steady- state values.7 Integration of the exponential transient leach rate versus time curve shows that the average Cs-137 release rate for the first month is 2.210-6 g/(cm2-d), eleven times higher than the steady-state value. The same increase is true for actinide release rates. This means that as much Cs-137 would be released in the first month following contact of the VHLW surface with water as would be released in the following eleven months. The initial leaching behavior is important for determining how soon salvage of the cargo would have to occur to avoid significant release of radiation into the environment.
Using the above room-temperature leach rates in pure water (room temperature is valid, because even though the ambient water temperature on the continental shelf is around 10C, the VHLW itself is a significant heat source and will raise the temperature of the glass-water interface by several degrees at a minimum), a VHLW geometric canister surface area of 19,000 cm2 and an effective surface area ten times greater (because the VHLW is extensively fractured), the radionuclide release rates per VHLW canister for the first few years after the accident are approximately:
Cs-137: 1.4 TBq/d (first 30 days); 0.12 TBq/d (subsequently)
Am-241: 0.018 TBq/d (first 30 days); 0.0016 TBq/d (subsequently)
Cm-244: 0.12 TBq/d (first 30 days); 0.011 TBq/d (subsequently)
Leach rates in seawater may be as much as four times greater. Over the course of a fifty year period, radioactive decay will attenuate the rate of emission of Cs-137 (half-life = 30 y) and Cm-244 (half-life = 18.1 y) by factors of two to three. The Am-241 rate (half-life = 432 y) will decrease much more slowly.
b) Conversion of releases to exposures
Calculation of the radiation exposures to the public that would result from the release of radionuclides from a damaged VHLW cask lost in coastal waters is a complicated and uncertain undertaking. One issue is the complex and site-specific nature of the ocean current flows in these regions. A review of four studies shows that there can be large variations in estimates of the contributions of different radionuclides and pathways to final exposures.
i) Individual doses
The following table illustrates this disparity by comparing the coefficients which relate unit releases of the above three radionuclides to the 50-year committed effective doses (CED) to individuals that can result. These coefficients are usually not explicitly given and have to be extracted from other data provided in the reports; they are therefore approximate. It is important to note that it has been shown that the models used to compute doses are linear with respect to the radionuclide leach rate; therefore, these results can be used for the R7T7 leach rates given above. It should also be noted that the dose computations in all of these studies (with the exception of Nielsen ) were originally based on ICRP 30 data. ICRP 30 has since been replaced with ICRP 68, in which the coefficients expressing dose per unit intake of ingested actinides have been reduced (e.g. Am-241 by a factor of 3 and Cm-244 by a factor of 2.5); the data in Table I has been adjusted accordingly.
Committed Effective Doses to Individuals Resulting from Radionuclide Releases in Shallow Waters (mSv/TBq)
Radionuclide Klett (1986)8
CRIEPI (1995) Nielsen (1996)9 Cs-137 0.002 0.002 1 x 10-8 0.02 Am-241 0.001 0.44 2.3 x 10-7 0.4 Cm-244 0.0004 0.24 8 x 10-7 not given
On the low end of the scale are the results of a study done in 1995 by the Central Research Institute of [the] Electric Power Industry of Japan (CRIEPI).10 The largest disparities in the table exist between the CRIEPI results and the others, although there is a significant difference between the OECD and Klett (1986) results for the actinides (which is odd because they are both based on the same model).
The CRIEPI results differ so dramatically from the others because they make a number of assumptions which are unreasonable. CRIEPI's "damaged cask" assumes that the shipping cask remains intact except for the O-ring seal, so that water can only enter the cask through the narrow gap between the cask lid and body. CRIEPI also assumes that the only mechanism for release of radionuclides from the cask is by natural convection and diffusion. These assumptions lead to a very slow leak rate from the cask. However, there are appreciable currents near the sea bottom in coastal waters (up to 2 cm/s), and depending on the orientation of the cask, these could facilitate flushing of the cask contents. The only way to explain the anomalously low dose rates obtained by CRIEPI is to assume that even even if the O-ring fails, the cask will inhibit the release of radionuclides by a factor of as much as one million. The level of conservatism inherent in such an assumption is questionable, to say the least.
Excluding the CRIEPI results, the studies seem to agree to within a factor of 10 on the individual CEDs resulting from Cs-137 releases.
If the OECD values in Table I are used, one finds the following results for the peak individual CEDs, summing over all three radionuclides: 2.3 mSv/canister after the first year, 1.3 mSv/canister-year for subsequent years. For 20 canisters, this corresponds to 46 mSv after the first year, and 26 mSv/y for the first few years afterward. Alpha-emitting radionuclides (Am-241 and Cm-244) are responsible for 97% of the total CED. These are huge dose rates, exceeding not only the ICRP 60 limit for exposure of the general public (1 mSv/y), but also the limit for those occupationally exposed (20 mSv/y). Enhancement of the leach rate by the presence of salt could increase these doses by as much as a factor of four. An average individual receiving the above (decay-corrected) radiation exposure for a fifty-year period would have a 7% risk of contracting a fatal cancer as a result of this exposure.11 In the U.S., this would correspond to an increase in lifetime risk of cancer mortality of more than 25%.
ii) Collective doses
Of the four studies reviewed above, only two of them, OECD (1988) and Nielsen (1996), calculated collective dose commitments resulting from radionuclide releases in shallow waters. It is not as straightforward to adjust these results for the 1021C source term. However, it is possible for a few radionuclides. According to the OECD results, the 50-year integrated collective dose resulting from an initial Am-241 leach rate of 0.038 TBq/yr is 1.84 person-Sv (again adjusting for ICRP 68), and for Cs-137, it is 21.8 person-Sv for an initial leach rate of 1.65 TBq/yr. Scaling these results for 20 canisters of R7T7 VHLW gives a 50-year collective dose of 566 person-Sv from Am-241 and 1.2104 person-Sv from release of Cs-137. This collective dose from these two radionuclides alone would cause around 650 fatal cancers in the 50-year period.
It is clear from these results why the OECD recognized that the loss of a VHLW cask in shallow waters would indeed have "unacceptable consequences." Frankly, it is unknown whether the current shipping system operates so that the risk of such an accident would be acceptably low.
PNTL assumes that if a VHLW cask were lost in shallow waters, it would be immediately salvaged. However, they have provided no evidence to the public that they would be capable of carrying out such a hazardous operation, especially if the cask were damaged. In fact, at a March 1996 meeting at the International Maritime Organization, the question of salvaging potentially damaged, highly radioactive cargoes was discussed. It became clear in the course of the discussion that issues specific to this cargo that may affect the feasibility of salvage operations have not been carefully thought out.
As shown above, salvage of a damaged VHLW cargo would have to be carried out within a few months of the accident to prevent a substantial release of contamination. Once that has occurred, salvage operations would become immensely difficult, posing great health risks for the salvage crew. Raising the entire ship would probably be the only tenable option, but such an operation would likely facilitate the spread of contamination to surface waters. Until the industry provides a credible salvage plan that addresses these contingencies, the public cannot give much credence to industry assurances that salvage will be carried out.
Safety Issues Previously Raised that Remain Unresolved
In the 1994 report, a number of issues with implications for the safety of VHLW sea transport were raised. Industry has never provided to the public a satisfactory resolution of them.
a) Elastomer cask seals
The TN 28 VT VHLW transport cask utilizes a lid seal made of an elastomeric (rubber-like) material. The lid seals play an essential role in preventing the escape of radioactive gases and fine particulates from the cask should an accident occur. 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.12
In 1994, my report pointed out that the thermal power of a loaded TN 28 VT cask was so great that the temperature of the cask seals during normal transport was dangerously close to the 250C failure threshold; I estimated that it would lie in the range 120-170C. In December 1995, a paper by CRIEPI on thermal studies of the TN 28 VT cask was presented at an international meeting which found the seal temperature to be 148C and thus verified my estimate.13
(One should also note that the CRIEPI report found that the centerline temperature of the VHLW canisters in the fully loaded cask was 390C. The actual temperature of the 1995 transport was higher (probably greater than 400C) because the average heat loading of the VHLW canisters shipped was 1.54 kW, with some as high as 1.65 kW, greater than the 1.46 kW assumed in the Japanese test. When my report pointed out in 1994 that the cask was designed so that the centerline temperature could be as high as 510C, it was ridiculed. Calling the report "dubious science," Gavin Carter of BNFL told the Journal of Commerce that the temperature "...will be 200 to 250 degrees Centigrade."14 Of course, my report never stated that the temperature was 510C, only that such high temperatures were permissible. In any event, we now know that the actual temperature was closer to 510C than to 250C.)
Transport casks with elastomer seals are able to be qualified as IAEA 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, even with a high initial seal temperature (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 to 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. While this result was judged to provide a "sufficient safety margin" of around 70-100C below 300C, which CRIEPI used as the maximum acceptable temperature, 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 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.15 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.
b) Sensitization of stainless steel VHLW canisters
The stainless steel canister that encases VHLW plays an important role in ensuring the safety of transport, handling and storage of the material. It should be obvious that the chosen canister material should provide a high degree of confidence in its integrity. However, at the R7-T7 vitrification plant at La Hague and at the Waste Vitrification Plant (WVP) at Sellafield, the VHLW canister material being used does not provide such confidence. In fact, one can show that this material, known as Type SUH 309 austenitic stainless steel, undergoes a phase transformation while the VHLW canisters are being cooled. This phenomenon, known as sensitization, greatly reduces its resistance both to certain types of corrosion and to mechanical impact.
Sensitization of austenitic stainless steels occurs when the steel is held for a certain period of time at a temperature in the range of approximately 400-850C. The time necessary to cause extensive sensitization depends on the composition of the steel, and in particular will decrease as the carbon content increases. For instance, one experiment found that while a 3 millimeter (mm) sample of stainless steel with a carbon content of 0.03% underwent 100% sensitization after 10 hours, one with a carbon content of 0.08% sensitized completely after only 30 minutes.16 Type SUH 309 has an even higher carbon content (0.15 weight-percent) and therefore will be completely sensitized in less time.17 Furthermore, it has been observed that stressed materials (such as the VHLW canisters) undergo sensitization more rapidly than unstressed ones.
COGEMA data shows that as the VHLW canisters are being cooled after being filled with glass, the canister temperature remains within the sensitization range for about 7 hours. Thus there is little doubt that the canisters being produced at La Hague are extensively, if not completely, sensitized.
A number of stainless steels have been developed which are resistant to sensitization. Partly for this reason, one of these steels, Type 304L, is being used as the canister material at the HLW vitrification plant now operating in the United States. Type 304L is also the steel that was chosen for use at the domestic HLW vitrification plants in Japan. However, one should note that even if Type 304L is used, sensitization may occur to a limited extent during VHLW production. (There are materials which are somewhat more resistant to sensitization, such as the high-nickel Alloy 825). Why COGEMA and BNFL initially chose (and continues to use) a type of steel highly susceptible to rapid sensitization, apparently without prior consultation with Japanese authorities, is by no means clear.
Austenitic stainless steels exhibit a high degree of resistance to uniform corrosion (corrosion that takes place uniformly across a surface). However, when they are exposed to certain chemical and thermal environments, they can undergo localized corrosion processes such as intergranular corrosion (IC) and intergranular stress-corrosion cracking (IGSCC).18 Localized corrosion can typically be two or three orders of magnitude more severe than uniform corrosion, and has often led to unexpected, catastrophic failures of materials.
Some contaminants, such as chloride salts and hydroxyl (OH- ions), can initiate intergranular corrosion of stainless steels at very low concentrations, if water is also present. This is clearly a concern with regard to the integrity of VHLW canisters: for instance, when reporters were recently shown empty stainless steel canisters at the U.S. vitrification plant, they were warned not to touch them to prevent exposure of the steel to potentially corrosive salts in their sweat.19
Because the VHLW canisters being returned to Japan are produced, stored and shipped in marine environments (all facilities are located near oceans), the ambient air concentrations of chlorides from sea salt are always high and extreme care should be taken to prevent excessive salt contamination of the canisters. It is not clear that such care is taken, however.
Stainless steel that has become sensitized is much more vulnerable to localized, intergranular corrosion than the same type of steel in the unsensitized condition. Because localized corrosion is a much more unpredictable phenomenon than uniform corrosion (e.g. it is more susceptible to small changes in environmental conditions) it is less accurate to apply the results of laboratory tests to predictions of long-term performance, or to extrapolate data from a single sample to an entire lot. Thus one consequence of using sensitized stainless steels in VHLW canisters is that uncertainties in predictions of canister performance will be greatly increased. This can only reduce confidence in the results of safety analyses.
In the event of an accident in which a VHLW cask is damaged and then lost at sea, as described above, the accelerated corrosion of sensitized Type 309 stainless steel in seawater will virtually guarantee that the canisters will fail within a few months' time, exposing the radioactive glass matrix inside to seawater.
The shipment of VHLW by sea is a practice with potentially catastrophic consequences for the inhabitants and economies of coastal states along the shipping route. In particular, the loss of a VHLW cargo ship in coastal waters, a credible event, can result in extensive radioactive contamination of the environment. However, the industry responsible for the shipments continues to refuse to provide to the public the technical basis for its assurances that it has reduced the probability of accidents to an acceptable level. In many cases, the technical basis does not exist. Given this situation, en-route states would be justified in not permitting the passage of these cargos through their Exclusive Economic Zones.
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End Notes1. E. Lyman, "Safety Issues in the Sea Transport of High-Level Radioactive Wastes from France to Japan," report commissioned by the Nuclear Control Institute, Greenpeace International and Citizens' International Information Center Tokyo, Center for Energy and Environmental Studies, Princeton University, December, 1994. Back to document
2. See, for example, "Vitrified Waste on the High Seas," SpentFuel, January 30, 1995, p.3; "Shipment of Nuclear Materials from France to Japan," British Nuclear Fuels plc. media brief (undated). Back to document
3. Nuclear Energy Agency (NEA), Feasibility of Disposal of High-Level Radioactive Waste into the Seabed, Volume 2: Radiological Assessment, Organization for Economic Cooperation and Development (OECD), Paris, 1988, p. 144. Back to document
4. E. Vernaz and N. Godon, "Leaching of Actinides from Nuclear Waste Glass: French Experience," in Scientific Basis for Nuclear Waste Management XV (C. Sombret, ed.), Materials Research Society, Pittsburgh, Pennsylvania, 1992, p. 37. Back to document
5. Vernaz and Godon, op cit. Back to document
6. H. Scholze, Glass: Nature, Structure and Properties, Springer-Verlag, New York, 1991, p. 336.Back to document
7. Vernaz and Godon, op cit., Fig. 2, p. 39. Back to document
8. R. Klett, 1985 Subseabed Disposal Project Annual Report, SAND86-0244, Sandia National Laboratories, May 1986, p. 77.Back to document
9. S. Nielson, "Ecological and Public Health Implications from Flasks Lost at Sea," presentation to IMO Special Consultative Meeting, International Maritime Organization, London, 4-6 March 1996.Back to document
10. Central Research Institute of [the] Electric Power Industry, Japan, "Environmental Impact Assessment of Radioactive Materials During Sea Transportation: Case Study of Vitrified Wastes Released in the Ocean," presentation at the Special Consultative Meeting of Entities Involved in the Maritime Transport of Nuclear Materials Covered by the INF Code, International Maritime Organization, London, 4-6 March 1996. Back to document
11. The cumulative (decay-corrected) exposure over the 50-year period from the three radionuclides is 0.73 Sv, of which 93% results from ingestion of alpha-emitting radionuclides. The appropriate risk factor to apply is therefore (0.1 fatal cancer/Sv) (0.93 + 0.5 0.07) = 0.097 fatal cancers/Sv. This is because the risk associated with high linear energy transfer (LET) radionuclides, such as alpha-emitters, is about 0.1/Sv, while for low-LET radionuclides (such as Cs-137, a beta-gamma emitter), it is about 0.05/Sv. The fact that there is no evidence for applying a dose-rate effectiveness factor for high-LET radiation, as pointed out by the United States BEIR Committee, generally is ignored in risk calculations [National Research Council, U.S. National Academy of Sciences, Health Effects of Exposure to Low Levels of Ionizing Radiation (BEIR V), National Academy Press, Washington, D.C., 1990, p.6). Back to document
12. The industry has denied that elastomer seals are a bad choice. However, they d