Gheorghe Vieru, Ph.D
Institute for Nuclear Research
Pitesti, Romania 0300 Pitesti, P.O. Box 78, Romania

The radiological risks of transporting radioactive materials can be determined using probability safety assessment (PSA) techniques [1] and has been evaluated in terms of expected additional latent cancer fatalities, by using generated data on accident severities and frequencies. To generate input data, a large body of information available with national authorities were analysed. Two risks were estimated: the resulting from normal (accident-free) transport and that resulting from transportation accidents involving radioactive shipments [2]. The accident risk calculation incorporated accident probabilities and package release fraction estimates.

Safety in the transport of radioactive material is dependent on packaging appropriate for the contents being shipped, rather than operational and/or administrative actions required on the package. The IAEA Regulations [3] ensure safety in the transport of radioactive material by laying down detailed requirement which appropriate to the degree of hazard represented by the material taking into account its form [4] and the quantity of it being carried. It is assumed that a package may be damaged in a severe accident and a proportion of the contents may be released. The paper presents, in the first part, a review of the qualification tests performed for the type A packages used for transport of radioactive materials known either low specific activity (LSA) and, in the second part, the risk assessment activities aiming the evaluation of risk categories that may arise either during normal (accident - free) transport, and those resulting from transportation accidents involving radioactive shipments. In order to evaluate the dose resulting from accidents involving radioactive shipments the code INTERTRAN II, has been used. Thus, the dose assessment has been based on the frequency of occurrence of accidents of specified severities.

The type A package [4, 10] tested is a standard industrial drum, made in INR Pitesti, of 1 mm thick mild steel (having a volume of about 220 1). The type A packages are required to be capable of resisting normal and routine conditions of transport without loss of their contents or without allowing more than a specified increase in external surface radiation level. Qualifications tests requirements constitute the compulsory minimum specifications for the manufacturer. The following tests were performed by the Reliability and Testing Laboratory of INR Pitesti in accordance to the Romanian and IAEA's Regulations [4, 7, 9, 10]:

• The mechanical test (free drop test): the drum was dropped so as to suffer maximum damage; the drop height being 1.2 m. After the test [3, 6], no damage or defects were observed;
• Stacking test: to ensure that effectiveness of containment, shielding and any spacers is maintained when package is stacked in such a way which is likely to occur normally during loading, unloading, transport and intermediate storage. The requirements to whistand for a period of 24 h at 5 times (2.5 t) the package weight were fulfilled [3, 4,10] and no damage were observed in the end of test.
• The penetration test: the damage conditions were: no rupture of the outer shield and no pulling out the sealing lid. The limits of the release fraction of the package contents were in the range of 0.1% to 1% (approx. 35%). The drum shield indented about 0.1 mm. The sealing lid was not affected and no other damages were observed.
• The 9 m free drop test: the 9m free drop test [3, 4, 6] was performed for packages used for transport and storage of liquids radwaste. The loss of shielding integrity was no more than a 20% in crease in the radiation level at any external surface point of the package. No other severe damages were observed. The simulated contents was water.

Radwastes generated by Romanian nuclear facilities (INR Pitesti, IAP Bucharest, NPS Cernavoda, hospitals, etc.) are transported and stored in the national disposal site, Baita, by road and by rail. The transportation of radwaste is performed under the authority of the Romanian National Commission for Nuclear Activities Control (NCNAC), [11]. The quantities of wastes transported, in the last four years, are given in Figure 1:

The frequency distribution i.e. surface dose rate (mSv/h) against package number is shown in Figure 2:

The frequency distribution of the activity is presented in Figure 3:

The knowledge of isotopes and their distribution contained by each package is useful for the assessment of the expected radiological consequences [5, 8, 12] and accident risk of transporting radioactive material, of a given accidental release. In Figure 4 is presented the distribution of Co-60 isotope for those 140 packages:

The 220 1 standard drums [4] have been proved to comply with the Romanian and IAEA's Safe Nuclear Transportation Regulation impact test requirements. The road route (608 km), from Bucharest to Baita, consist of: 110 km of motorway, 37 km of unclassified road and 461 km of national roads and the population was considered to be distributed among three population density zones [14]: urban (5%), intermediate(45%) and rural (50%). These data form the input to estimate collective doses for the population [8, 12] from normal transportation and the probability of an accidental release occurring in a particular categories of population. For radiological accident consequence calculations [12], three sites having representative population densities were selected along the route. The associated population densities/km² are shown in Figure 5:

The typical population densities (people/km²) chosen for each zone are: rural (40), intermediate (45), urban (330). These data form the input to the normal transport dose calculations [8, 12, 13]. During incident or routine free transport, the radwaste package external dose field might result in small radiation doses to workers and general public, such as: a) a member of the public located alongside the route; b) a member of the public using roads in the same time with trucks; c) a member of the public being accidentally near the package; d) transport workers from only the off-site transport (Romanian Regulation provide 2mSv/h, in every point from the vehicle cab).

To determine the probabilities and collective doses for normal transportation, the following input data presented in

Table. I were used:

The IAEA Regulation limit of 0.l mSv/h at 2 m from the vehicle, assumed to be effectively 2m from the package surface, is applied to all package movements. The collective doses assessed, are shown in the Figure 6:

Dose to public alongside route:	0.75 ´ 10-3 man-Sv/y
Dose to public during stops:	1.12 ´ 10-6 man-Sv/y
Dose to package truck crew1:	1 ´ 10-3 man-Sv y
Dose to public sharing route:	0.3 ´ 10-4 man-Sv/y

The total annual collective dose [8, 12, 13] to members of the public is 0.58 ´ 10^-3 man-Sv and can be compared with naturally occurring sources cosmic of radiation which in Romania is 5mSv/d or 1.825 mSv/year. The additional collective dose due to package movements is an insignificant percentage over natural background level. The annual collective dose to a member of the public corresponds to 0.34 ´ 10^-4 expected fatalities per year due to routine transport. The individual dose is 0,25 mS/y and the associated latent cancer fatality risk is 1.2 ´ 10^-8/y. For a person exposed in a traffic jam, the individual dose is 10 mS/y corresponding to a latent cancer fatality risk of l ´ 10^-7/y.

It is possible to postulate accidents which could compromise the containment (package) or shielding performance of the package (proved by mechanical testing). The accident risk analysis for transportation of radioactive wastes was made following the model given below, in Figure 7:

A risk assessment of radwaste transport hazards for the route Bucharest - Baita was carried -out. Hazards were divided into impact and fire hazards

1. Underbridges (i.e. bridges the package vehicle passes across)
2. Overbridges ( i. e. bridges carrying roads, etc. over the package vehicle)
3. Roadside Objects / Overturns/Embankments

Drops with the potential to result in impacts of greater severity than IAEA 9m drop test are considered in this assessment. It is assumed that at the speeds (max. 40 km/h) associated with road transport, impacts with roadside objects other than those identified above will not threaten the integrity of package [7, 8, 12, 13].

1. Collision with second road vehicle (truck or bus, tank carrying flammable);
2. Collision with train at level crossing
3. Collision with train on railway line adjacent to route

• Impact with bridge support
• Collision with second road vehicle (head-to-head or head-to-tail):

1. With other Truck vehicle or bus
2. With a vehicle carrying flammable material load

• Collision with train at level crossing
• Collision with train on railway line adjacent to route

For the estimation of potential accident probabilities with radiological consequences road transport accident information have been used. The accident probabilities are: motor way (3.5 ´ 10^-6/(vehicle km), national roads (2.32 ´ 10^-5/(vehicle km), other roads (4.35 ´ 10^-6/(vehicle/km).

For fire accidents, the probability of ignition is 0.033, and the probability per year is 0.017; the probability that a truck to be involved in a collision with a tanker carrying petroleum was estimated at 2¸ 5.4 x 10^-11/truck km. Rail level crossing accidents, in Romania [14] there were 75 in 1996 and the average probability was estimated at 1.83 x l0^-9/(vehicle km).

The hazards identified from observations whilst travelling the route were classified in the following categories, such as: Overbridges (123) underbridges); Roadside hazards; Other hazards, such as: No. of Overbridges: 123; No. of Underbridges: 16; Other hazards: Level crossings: 13, Railway along side: 127 Km; Brickwall and rocks faces alongside: 0.2 km/2m from road; Factory/Industrial enterprises: 10 m from road

It is assumed that the package will be breached in any impact with impact velocity exceeding than that experienced in the IAEA drop test (13 m/s). The associated probability calculations are:

• for national road (498 Km): 0. 0244 (hazard size probability);
• for M = way (110 Km): 0. 091 (hazard size probability

For a collision with a train alongside route the hazard length is 0.0027 and the conditional

• probability is 1.73 ´ 10^-3. The road accident probabilities are:
• impact with bridge support: 7.01´ 10^-7 (M-way) and 1.79 ´ 10^-6 (national road);
• collision with a truck/bus: 6.83x10^-6 (M-way) and 1.77x10^-6 (national road)
• collision with a tank carrying flammable: 8.3 ´ 10^-12 (M-way) and 8.43 ´ 10^-12 (national road)
• collision with a train at level crossing: 7.3 ´ l0^-2 (national road)

• probability of impact only: O. 537 ´ 10^-5/(package journey)
• probability of impact and fire: 1.43 ´ 10^-11/(package journey)

For 10 shipments per year, the accident frequencies are:

• probability of impact only: 5. 37 ´ 10^-5/year
• probability of impact and fire: 1.43 ´ 10^-10/year

It is also assumed that, following packaging failure, the content may become available for dispersion in the air. Two impact release possibility were taken into consideration:

• for low wind speed condition;
• for high wind speed condition

The fraction of solid material released from the package is 10^-3. For an impact in low speed conditions, the package release fraction is 4´ 10^-6 and for an impact in high speed conditions, the fraction is 10^-4 . [12, 13].

A preliminary risk assessment of the rail route for radwaste transportation was carried out. This route covers 764 Km from Bucharest to Stei. There is a single wagon with a capacity of 72 standard package of 220 l in volume. The calculated radiological risks include:

• radwaste exposure of the public and transport personnel from routine (incident free)
  transportation of radwaste material (public and occupational exposure);
• transport accidents resulting in radiation exposure of the population and contamination of environment and its probabilities of such events.

In the period of 1994 - 1997, 1699 packages were transported, by rail, which means an average of 425 packages/year (6 shipments/year). The average population density calculated along the route is 93 population density/Km² (National Commission for Statistics, 1995). From the Romanian Railways available database, rail accidents are classified on the basis of the rail gauge and the type of train (passenger or goods). It was assumed that the train has in its composition about 12 wagons and the result accident rate (the overall accident rate) for rail transport is determined to be 3´ 10^-7 per wagon km (in comparison with, for example: German rail: 0.5 per 1 million train.km or 0,026´ 10^-6 rail-car km or Indian Railways, which is at the average of 5´ 10^-8 per wagon km). Transport and handling may occur for a number of reasons posing a risk to man and environment. As a consequence, members of the public may eventually be exposed due to a number of pathways to radiation from material that might be released during transportation. Thus, is necessary to establish the magnitude of such a release and the related frequency of occurrence which depends on a number of factors, such as: type and volume of wastes transported, the severity and the frequency of accidental events (collision, derailment, striking an object, etc.). The Risk Assessment method adopted in order to quantify the potential radiological consequences and the expected probability of such accidental sequence, includes 5 steps:

• Characterisation of the type and quantity of waste shipments;
• Determination, selection and description of the type, severity and probability of occurrence of transport and handling accidents;
• Assessment of transport packaging and waste to specific mechanical impact and release fraction;
• Estimation of radioactive release and frequency of occurrence taking into account the shipping patterns and the accident severities;
• Assessment of potential radiological consequences for the spectrum of wealth condition encountered along the rail transport route.

Based on this adopted method, were taken into consideration 9 severity categories representing the accidental load conditions to a type A package: 3 mechanical and 6 combined-mechanical and thermal. Rail accident involves: impact between train and road vehicles, derailment, collision between trains and fire. For the mechanical impact load conditions experienced by a package based on the train velocity prior to the accident event, three severity levels were defined to describe this events: < 40Km/h, 40 ¸ 80Km/h, > 80Km/h. The potential thermal impacts includes: a fully engulfing 30 minute at approx. 800° C, a fully engulfing 60 minute at approx. 800° C fire. Based on the Romanian Railway database, the relative frequency of mechanical-only accident was determined to be 93%, of combined mechanical 5% and of fire engulfing 2%.

Summary of the Waste product (Bituminous Waste) and transport packages:

There are different kinds of operation contributing to the overall risk, such as: rail transport, rail road transfer activities (from Stei to Baita, the packages are transferred into a truck and transported to Baita), handling and misoperation activities, etc. The potential radiological consequences have been calculated using intertran code II. The relevant exposure patway considered in the estimation of dose include: cloudshine, groundshine, inhalation and ingestion. This process of calculation for potential radiological consequences are under progress within our scientific research contract concluded with IAEA Vienna, and the results presented are premininary.

The probability of a waste waggon for damage in an railway accident is estimated at 0.020, i.e. 1 chance to 50 for the total volume of waste transport. Based on a conservative approach concerning the structural package response (impact on a rigid target surface), it has been found that 1 to 10 railway accidents resulting in material damage to a waste waggon give rise to a package release of radioactive package release of radioactive material for the total transport volume of bituminous wastes. The quantity of radionuclides escaping from the waste transport container is low, if not insignificant. The effective dose to members of the public has been predicted of not excedeeng a value of lmSv even in the vicinity (250 m) to the accident site. Radioactive releases are not expected to occur in close proximity of the accident site (250m) at a probability level as low as 10^-6, i.e. 1 chance to 1 milion for the total volume of bituminous wastes.If expressed as a probability per year, the corresponding value might be below 10^-7 per year. The potential radiological consequences will decline rapidly with distance from the accident site. Other results, i.e. radiological risks, complete potential radiological consequences, etc. will be presented in other paper.

This safety assessment gives the accident probabilities, the frequency of accidents in case of different scenarii assumed. The radiological risk in radioactive materials transportation in Romania results primarily from routine exposure associated with the normal transport process. The routine transport collective dose to member of the public along the route is an insignificant increase over their natural background dose.

It is concluded, on the basis of the best estimation of these accident probabilities, that the proposed radwaste road and rail transport operation would have acceptably low societal, individual and expected risk values.

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