A wasted journey

In late April 1998, the Federal Environmental Ministry (Bundesumweltministerium, BMU) was informed by the French supervisory authority DSIN of contamination of the shipping casks and vehicles that were used in the transport of spent fuel elements from German nuclear plants to the Cogema reprocessing facility in La Hague. Similar contamination was discovered in casks belonging to French and Swiss nuclear plants as well as in unloaded casks. Further investigations found evidence of contamination during fuel element transports from German nuclear plants intended for reprocessing in the UK, and in shipping and storage casks in the interim storage site at Ahaus. These contamination findings led to the temporary ban on the transport of spent fuel elements.

Potential exposure

Two different kinds of contamination were determined for the shipping casks and railroad wagons. Surface contamination was found whose activity rarely exceeded 2000Bq/cm2. This surface contamination is caused by the transformation of adherent contamination into non-adherent ("wipe-off") contamination as a result of environmental conditions during transportation such as temperature fluctuations, rain or dust deposits. The physical-chemical processes and main parameters responsible for this phenomenon, known as "weeping" in the literature, are not fully known. Also, on the cask surface and within the protective guard, so-called crud particles were found that derived from residues on the fuel elements. From a radiological perspective, these crud particles are of special significance since they can, among other things, detach themselves from the surface of a shipping cask or railroad wagon and can have a relatively high activity of up to around 100kBq.

Next to direct radiation, potential radiation exposure of escort staff and population can occur via the crud particles of the loaded shipping cask. Three relevant scenarios for determining a possible exposure with reference to the aerodynamic characteristics of crud particles have been identified:

• Inhalation of particles by breathing.

• Ingestion of particles by swallowing.

• Skin exposure through particle deposits.

The aerodynamic equivalent diameter (AED) of crud particles describes the characteristics of particles during airborne transport and inhalation. The AED is the calculated diameter of a particle that shows the same sedimentation rate as a comparable spherical particle for an assumed density of 1g/cm3.

Depending on the size and sedimentation rate, particles are either deposited within the transport vehicle (for example in the drip tray of the railroad wagon) or are transported outside by the air current. Contamination of the shipping casks and railroad wagons is mainly caused by crud particles with an AED below 100mm. Larger crud particles with correspondingly higher activity up to 100kBq have only been found in isolation in the cooling fins of the TN shipping casks or the drip tray of the railroad wagon. Particles reaching the size of 100mm AED are deposited on surfaces such as clothing or the ground as a result of being transported in an air current because such large particles, due to their inertia, do not follow the air current exactly. The selection of a crud particle with 100mm AED as a reference particle represents a conservative approach in terms of the potential radiation exposure. Smaller particles are less active and thus lead to lower doses. Larger particles are so heavy and so un-airworthy that once they detach from the shipping casks they fall downwards, most often into the drip tray of the wagon, and stay there.

Aerosol particles of 100mm AED have an airborne transport capability of approximately 100m. Thus particles having a diameter of 100mm AED can be regarded as particles with the maximum activity that need to be considered for ingestion and skin exposure scenarios in the immediate vicinity of the shipping casks. The size of the particle relevant to the inhalation path is limited to a maximum diameter of 10mm AED due to restricted lung permeability.

Apart from the aerodynamic characteristics, the nuclide-specific composition of crud particles also determines the potential radiation exposure. Nuclide vectors for crud from BWRs and PWRs can be derived and used for calculating the potential radiation exposure. On the basis of these nuclide vectors, a crud particle with 100mm AED has a total activity of about 1800Bq.

The Table shows the effective subsequent dose for small children and adults after inhalation of 10mm AED particles or ingestion of 100mm AED particles. The partial body dose of the skin after exposure to a 100mm AED particle is also given. Among these ex-posure scenarios, the major proportion of exposure is caused by high-energy beta/gamma emitters (for example Co-60). The Table also gives the limit values for radiation exposure of the general population according to Euratom guideline 96/29. As the Table shows, the maximum permissible dose rates for the general population are not reached in any of the exposure paths considered.

Other studies on this topic that partly rest on different assumptions arrive at similar conclusions in terms of potential radiation exposure and probability. For example, calculations on radiation exposure conducted by the Öko-Institute used extremely conservative assumptions and did not consider the probability of exposure. These studies therefore overestimated the potential radiation exposure of the population.

In order to determine the actual radiation exposure through inhalation or ingestion of crud particles, whole-body-measurements of plant staff, railroad staff and police were carried out in France, Switzerland and Germany. The evaluation of these findings showed that no additional radiation exposures were evident in any of the cases.

Direct exposure

Next to the three exposure paths already indicated, direct radiation by the loaded shipping cask can lead to potential radiation exposure of the general population and especially of the escort staff. A dose limit value of 1mSv per year was determined for escort staff such as police officers in accordance with the Euratom guideline 96/29. This limit was not exceeded.

Catalogue of improvements

The operators of German nuclear plants cooperated with the reprocess-ing plants in France and the UK to develop a comprehensive catalogue of technical, organisational and administrative improvement measures. This catalogue consists of:

• Technical measures for improving contamination protection during loading in the power plant and unloading in the reprocessing plant.

• Systematic and uniform collection and evaluation of radiological measurement data.

• A detailed information and reporting system for transportation of spent fuel elements as well as vitrified high level waste.

• Organisational improvements for the institutes participating in the transports.

The technical improvement measures mainly affect the transports using the TN casks of the Cogema reprocessing plant. These casks have a zone with cooling fins that cannot be sufficiently safely decontaminated to the required level, should contamination occur. In order to avoid contamination — especially in the cooling fin area — these casks are usually wrapped in a contamination protection skirt made from plastic fibres, or additional protective flaps for the upper and lower cask area during loading. Additionally, before being reused, the unloaded casks at Cogema are cleaned from the outside and inside.

For transports to BNFL's reprocessing facility in the UK, containers of the type NTL 11, CASTOR S1 and Excellox are used.

Agreements were reached between plant operators and Cogema and BNFL for adopting a common approach in terms of incoming and outgoing radiological measurements at the nuclear plants, the reprocessing plants and the reloading stations when changing mode of transport. The contamination of empty shipping casks is examined by integrated direct measurement methods that cover the entire cask surface.

For improving information flow, the operators installed the "Transport-, Kontroll- und Informationssystem TKI" for transport of spent nuclear fuels and unloaded casks. This information and reporting system captures the information requirements and paths between all those involved in the transport on a national and international basis as well as the reporting requirements and reporting paths for the national supervisory authorities. The computer-aided documentation system captures all relevant transport data for all German nuclear plants in a uniform way. The data is stored in a central database to which all nuclear plants and the relevant supervisory authorities have access. Additionally, the reporting system has inbuilt deadlines and limit values that determine further proceedings according to the extent to which the limit values have been exceeded.

Resuming transport

In France, fuel transports were resumed as early as July 1998 after various improvements had been introduced. These included technical improvements in loading, standardisation of the methods of measurement, an increase in the number of meas-uring points, and crosschecks by an organisation that was independent of the operators. In 1999 (up until 15 August), limits were exceeded on eight occasions. 15 out of 20 French nuclear plants involved carried out their shipments without any findings of contamination. In order to inform the public more fully, the French authorities decided to issue official reports of any limits exceeded on transports on the basis of the international evaluation scale for nuclear events, INES.

In Switzerland, according to the responsible supervisory authority Central Department for the Safety of Nuclear Installations (Hauptabteilung für die Sicherheit der Kernanlagen, HSK), the frequency and extent of limit value transgressions in 1988 to 1998 were roughly comparable, in quantitative terms, with those found in German transports. The HSK concluded that shipments could be resumed since the frequency and extent of limit value transgressions could clearly be reduced by additional measures, even if absolute prevention was unattainable. The measures introduced were technical improvements such as the use of a new kind of protective film during loading, an extended programme of measurement, and administrative and organisational steps. The first Swiss shipment after the freeze on transports took place in September 1999.

Fuel transports within the United Kingdom and Sweden were never interrupted. As all Swedish power plants and the central repository for fuel elements (CLAB) are situated on the coast, roughly 100 transports per year using TN17 casks have taken place since 1983. Additionally, there were some isolated cases in which limits were exceeded when casks arrived at the interim store (some up to several thousand Bq/cm2).

In Germany, the BMU outlined in May 1998 conditions for resumption of the transports in a so-called 10-point-plan. Shipments were resumed early last year.