This IAEA Co-ordinated Research Project (CRP) has re-examined the safety of shipping large quantities of packaged radioactive material by sea. First, the frequencies of ship fires, ship collisions, and ship collisions that initiate fires were developed from maritime casualty data.
Because these data provided little information about the severity of the collisions and fires, the chance that such events could subject a radioactive material package to conditions that might cause it to fail was examined. Finally, given that flask failure is assumed, the amount of radioactive material that might be released from the flask, its rate of release, and the radiological consequences that might result were estimated for several hypothetical accidents.
Because highly radioactive material, such as spent power reactor fuel and vitrified high level wastes (VHLW), are only transported in very strong, thick walled, heavily shielded flasks called Type B packages, only extremely severe accidents have any chance of subjecting such packages to conditions that might cause the packages to fail and allow radioactive material to escape and enter the environment (the atmosphere or the ocean). The possibility of flask failure during ship fires was examined by performing shipboard fire tests, by modelling those tests and by using the test and modelling results to estimate the likelihood of ship fires severe enough to cause the failure of an irradiated nuclear fuel or VHLW Type B package. The possibility of flask failure during ship collisions was examined by performing finite element calculations which estimated the magnitude of the forces that might be applied to a Type B irradiated nuclear fuel package or a VHLW package during ship collisions. These ship collision forces were then compared to the forces that characterize the regulatory tests that the package must survive in order to be certified for transport.
The radiological consequences were estimated for several hypothetical ship accidents involving radioactive material cargo. The calculations considered accidents that were assumed to lead to the loss into the ocean of Type B packages carrying irradiated nuclear fuel, VHLW, and plutonium as well as accidents that were assumed to cause fission products in spent power
and its incorporation into marine food chains was considered for accidents that occur in coastal waters and in the open ocean. Release of fission products from irradiated nuclear fuel to the atmosphere was estimated for two accident locations, in a port and while sailing a coastal route, and for two types of accidents, a severe ship collision that does not initiate a fire and one that does.
8.2. Regulations
The shipment of highly radioactive material (irradiated nuclear fuel, VHLW, plutonium) is very carefully regulated. The IAEA transport regulations provide the bases for consistent international modal and national regulations governing package design, certification, marking, labelling, placarding, and stowage, for contents that can be carried in different types of packages. Other IAEA documents address physical protection and radiation safety measures.
The IMO’s International Materials Dangerous Goods (IMDG) code establishes standards for the safe stowage, handling and segregation of radioactive material on ships. The IMO’s Irradiated Nuclear Fuel (INF) code specifies requirements for three classes of ships (INF Class 1, 2, and 3 ships) that carry irradiated nuclear fuel, high level waste, and plutonium in packages and the amounts of radioactive material that can be carried by each class of ship.
The specifications cover ship damage stability, electrical systems, fire protection, cargo stowage, cargo space temperatures, radiological protection equipment, emergency planning, and crew training. INF Class 1 ships can carry radioactive material that contain at most 4 × 103 TBq of radioactivity. Because of their increased requirements for damage stability, fire protection, and electrical supply, INF Class 2 ships can carry material that contain in aggregate as much as 2 × 106 TBq of radioactivity and up to 2 × 105 TBq of plutonium. Larger quantities of radioactive material or plutonium can only be transported on INF Class 3 ships.
Existing INF Class 3 ships have double hulls and redundant propulsion, fire protection, and navigation systems, and ship’s officers certified for at least one position higher than the position in which they serve.
8.3. Packages
The Type B packages used to transport irradiated nuclear fuel or VHLW typically weigh about 10–20 tonnes if designed for transport by truck or 100 tons if designed for transport by rail.
The bodies of these flasks are usually sandwich structures consisting of outer and inner steel shells which encase a thick layer of lead or depleted uranium that functions as a radiation shield. The thick lid of the flask is secured to the flask body by an array of bolts. Lid sealing is provided by elastomer and/or metallic O-ring seals that are set deep within the lid well.
Although metal seals are failed by small deformations of the seal region, they retain their sealing function when exposed to the high temperature fires. Conversely, elastomer seals fail if heated to about 400°C but lose sealing function during collisions only if the flask seal region is significantly distorted by very large impact or crush forces.
8.4. Ship accidents
Lloyd’s ship casualty data divides ship accidents into the following categories: collision, contact, foundered, wrecked/stranded, fire/explosion, and missing, where contact means striking of the sea bottom or a fixed object, foundered means sunk due to heavy weather,
springing of leaks, or breaking apart, and wrecked/stranded means beached on the sea bottom, a sand bank, the seashore, or an underwater wreck. Because ships have double bottoms and a bow compartment in front of the first ship hold, contact accidents cannot damage a RAM flask, although they may cause the ship to founder or to become wrecked or stranded, which might lead to the loss of the flask into the ocean if the ship breaks up or sinks. Thus, only severe ship collisions or ship fires can directly cause the failure of a Type B irradiated nuclear fuel or VHLW flask.
8.5. Ship collision and ship fire frequencies
Per year of sailing, ship collisions frequencies range from about 4 × 10–2 for collisions of any severity to about 4 × 10–4 for collisions that lead to total loss of the ship. Since a typical ship sails about 60 000 nmi/a, this means that the chance of any collision is about 7 × 10–7 per nmi sailed, a result in good agreement with ship collision frequencies per nmi developed for specific ocean regions which range from 7 × 10–9 in the open ocean to 2 × 10–6 in the most heavily sailed regions of the world’s oceans, about 2 × 10–7 per nmi sailed in general coastal waters and 4 × 10–5 per port call for collisions in ports, irrespective of port traffic density.
Thus, for a 1000 nmi voyage from a departure port across open ocean to a destination port, the chance of a collision is about 4 × 10–5 + 100 (2 × 10–7) + 900 (7 × 10–9) ~= 1 × 10–4, where the first term represents the chance of a collision while leaving the departure port and entering the destination port, the second term represents the chance of a collision while sailing out to or back from the open ocean through coastal waters, and the third term represents the chance of a collision while traversing the 900 nmi of open ocean that separates the two ports. This simple analysis shows that for a typical voyage, the chance of a collision is about equal while sailing in port, through coastal waters, and in the open ocean.
Per year of sailing, ship fire frequencies range from 10–2 for fires of any severity to 2 × 10–3 for fires that start in or spread to cargo holds to 8 × 10–4 for fires that lead to the total loss of the ship. Given that a typical ship sails 60 000 nmi per year, the frequency of fires that lead to the total loss of the ship is about 10–8 per nmi, The frequency of fires of any severity is about 2 × 10–7 per nmi which agrees well with the value of 10–7 per nmi sailed developed by examination of fire data by sailing region. The examination showed that fire frequencies depend very little on sailing region or traffic density and found that port fire frequencies were about 5 × 10–5 per port call.
Thus, for a 1000 nmi voyage from a departure port across open ocean to a destination port, the chance of a fire of any type is about 2 (5 × 10–5) + 1000 (2 × 10–7) = 3 × 10–4 where the first term represents the chance of a fire while leaving the departure port and entering the destination port, and the second term represents the chance of a fire while sailing out to or back from the open ocean through coastal waters and while traversing the open ocean that separates the two ports.
8.6. Ship collision and ship fire severity
Because of their massive and robust designs, Type B irradiated nuclear fuel and VHLW packages can fail only as a result of unusually severe collisions or fires. Since the casualty data provide little information about accident severity, estimates of the fraction of all
were developed by modelling ship collisions using Minorsky’s correlation and finite element methods and by performing shipboard fire tests, modelling those tests, and developing a simple bulkhead fire spread model and a probabilistic multi-hold fire spread model.
Revalidation and extension of Minorsky’s correlation of collision penetration depth with collision energy allowed an estimate to be made of the fraction of all collisions that are severe enough to allow the bow of the striking ship to penetrate a hold to the location where a RAM flask would normally be stowed. For moderately large break-bulk freighters carrying other cargo in addition to the RAM flask, given that the RAM hold has been struck, the chance that the striking ship bow will overrun or compress cargo around the flask thus subjecting the flask to impact or crush forces is about 0.25 to 0.5 per collision, and for smaller freighters chartered to carry only the RAM flask the chance is smaller, about 0.15 per collision, because more of the collision energy is spent pushing a small ship sideways through the water than a large ship.
If the bow of the striking ship overruns or compresses cargo around the flask, impact or crush forces will be applied to the flask. Whether the flask fails depends on how those forces are relieved. Relief of impact or crush forces was examined by finite element calculations that divided the flask and the hull, decks, and bulkheads of the striking and struck ships into many small regions and then modelled the displacement and deformation of these regions due to the applied forces. These calculations showed that the largest crush force that might be applied to a RAM flask during a collision is comparable to the inertial forces experienced by RAM flasks during the regulatory impact test. They also showed that if impact or crush forces are applied to the flask, the forces will be relieved by compression of cargo behind the flask, if other cargo is present in the RAM hold, or by collapse of ship structures after the flask is pushed up against the far hull of the ship or a ship bulkhead. The forces will be relieved by cargo compression and ultimately by collapse of ship structures because the massive and robust nature of flask designs means that RAM flasks are much harder to deform than cargo or ship structures. Because flask structures are so difficult to deform, the probable outcome of severe ship collisions where the RAM hold is struck and deeply penetrated is the pushing of the flask across the struck hold and out through the far hull into the ocean, probably without compromising the integrity of the flask.
For a flask to fail during a ship collision, it must be caught between the bow of the striking ship and some set of structures in the struck ship that are stronger than the flask and thus able to function as a barrier that hinders the flask from being pushed through the far hull of the RAM hold into the sea. The finite element calculations performed for this study never predicted that collapsing ship structures would form a barrier that would prevent the flask from being pushed out of the hold into the ocean and thus allow the flask to be crushed, thereby causing the flask seal to fail. Despite this outcome, the probability of flask crush was qualitatively and conservatively estimated to occur no more frequently than once in every one hundred collisions (<10–2) given that crush or impact forces have been applied to the flask.
And given that flask crush and seal failure have occurred, the chance that the collision would also lead to a second flask failure by puncture or shearing of the flask body by, for example, a beam torn from some collapsing ship structure, was estimated to be no larger than once in every ten collisions (<10–1).
Performance of shipboard fire tests, that did not engulf the test hold and modelling of those tests using a fluid dynamics fire code showed (a) that heat transfer to the flask and to hold
bulkheads was dominated by radiation, and (b) that the fire heat fluxes were generally smaller than those developed by the regulatory flask certification fire test. Modelling of fire-spread through a bulkhead by radiative heat transfer to the bulkhead and from the bulkhead to highly combustible cargo in the next hold suggested that small fires located close to a bulkhead can ignite combustible cargo located not far from the other side of the bulkhead. This suggests that some fires on cargo ships (break-bulk freighters and container ships) may creep through holds and from hold to hold. This means that, if ship fires are not extinguished by fire fighting, they may burn for lengthy periods of time even though they are not likely to burn at very high temperatures or for very long periods of time in any one location.
Thus, even if a ship fire reaches the hold where the RAM flask is stowed, it is unlikely to cause the flask to fail and significant quantities of radioactive material to be released from its contents, as only a hot, prolonged fire can heat an object as massive as a Type B irradiated nuclear fuel or VHLW flask to temperatures which not only cause the flask seal to fail but also the irradiated nuclear fuel rods to fail by burst rupture. Furthermore, only a fire fuelled by a large amount of a material that burns with an unusually high flame temperature (considerably greater than 1000°C) can raise the temperature of a VHLW flask and of the glass matrix of the vitrified wastes being carried in the flask to temperatures where the matrix glass might soften or melt, thereby allowing fission products to escape by vaporization from the glass matrix.
Finally, the chance that a ship fire that starts at a random location on the ship while docked in a port with hold covers removed will spread to the RAM hold and there burn at sufficiently high temperatures and long enough to cause the RAM flask to fail and radioactive material to be released was calculated using conservative estimates for the probability that the ship holds contain significant quantities of combustible materials to support fire spread and that the fire is not oxygen starved or extinguished by the operation of shipboard fire suppression systems.
Although quite approximate, this analysis suggests that, given that a fire has started, the probability of a fire spreading to the RAM hold and there burning at sufficiently high temperatures and long enough to lead a Type B irradiated nuclear fuel or VHLW flask to fail is of order 10–3 for both medium sized break-bulk freighters carrying other cargo and for smaller break-bulk freighters chartered to carry only a RAM flask. Accordingly, for a purpose-built ship which carries no combustible cargo and is equipped with redundant fire suppression systems, given that a fire has started, its chance of spreading to the RAM hold should be even smaller, certainly less than 10–4, an estimate that is consistent with a much more detailed estimate of 10–5 for the chance that an engine room fire on a purpose-built ship will spread to a RAM hold.
8.7. Severe accident probabilities
The reviews of casualty data and the modelling of ship collisions and ship fires allows estimates of event probabilities to be made for the events that enter severe ship accident scenarios. Table XXVII lists these events and their probabilities of occurrence for a small break-bulk freighter chartered to carry only the RAM flask.
The data in this table allow estimates to be made for two severe accidents, a collision that leads to the sinking of the struck ship, and a collision that causes a double failure of the RAM flask and also initiates a severe fire that spreads to the RAM hold and there burns at sufficiently high temperatures and long enough to enhance the release of fission products from
used to show that the probability of these two accidents are respectively approximately 4 × 10–7 and 4 × 10—14 respectively for a collision followed by a sinking and for a collision with a release of radioactive material. Thus, for a voyage of about two thousand nautical miles, the probability that a collision will lead to a sinking and the loss of a RAM flask into the ocean is of order 10–6; and the probability that a severe collision will lead to a double flask failure, uneven heating of the flask by a severe fire, burst rupture of irradiated nuclear fuel rods, and release to the atmosphere of all fission products released to the flask interior by rod failure due to a buoyant flow of combustion gases through the flask is in the order of 10–13.
TABLE XXVII. SEVERE SHIP ACCIDENT EVENT PROBABILITIES
Event Probability Value
A ship collision occurs while making a 1000 nmi voyage Pcollision 1 × 10–4 The RAM ship is the struck ship PRAM ship struck 0.5 The strike location is midship Pstrike/midship 0.33 The RAM flask location is midship Pflask/midship 1.0 Crush forces are applied to the flask Pcrush forces 0.15 Flask crush causes the flask seal to fail Pcrush <10–2 Flask puncture or shear occurs Ppuncture/shear <10–1 The collision initiates a fire Pfire start/collision 0.016 The fire spreads to the RAM hold Pfire spread ~ 10–3
The ship sinks Psink 3.6 × 10–3
The data in this table allow estimates to be made for two severe accidents, a collision that leads to the sinking of the struck ship, and a collision that causes a double failure of the RAM flask and also initiates a severe fire that spreads to the RAM hold and there burns at sufficiently high temperatures and long enough to enhance the release of fission products from an irradiated nuclear fuel flask. For a 1000 nmi voyage, the values from Table XXVII can be used to show that the probability of these two accidents are respectively approximately 4 × 10–7 and 4 × 10—14 respectively for a collision followed by a sinking and for a collision with a release of radioactive material. Thus, for a voyage of about two thousand nautical miles, the probability that a collision will lead to a sinking and the loss of a RAM flask into the ocean is of order 10–6; and the probability that a severe collision will lead to a double flask failure,
The data in this table allow estimates to be made for two severe accidents, a collision that leads to the sinking of the struck ship, and a collision that causes a double failure of the RAM flask and also initiates a severe fire that spreads to the RAM hold and there burns at sufficiently high temperatures and long enough to enhance the release of fission products from an irradiated nuclear fuel flask. For a 1000 nmi voyage, the values from Table XXVII can be used to show that the probability of these two accidents are respectively approximately 4 × 10–7 and 4 × 10—14 respectively for a collision followed by a sinking and for a collision with a release of radioactive material. Thus, for a voyage of about two thousand nautical miles, the probability that a collision will lead to a sinking and the loss of a RAM flask into the ocean is of order 10–6; and the probability that a severe collision will lead to a double flask failure,