Chairman F. LANGE
SAFETY ANALYSIS OF THE TRANSPORTATION OF RADIOACTIVE WASTE TO THE KONRAD
WASTE DISPOSAI, SITE
F. LANGE, HJ. FETT, D. GRÜNDLER, G. SCHWARZ Gesellschaft für Anlagen- und Reaktorsicherheit,
Köln, Germany Abstract
A safety analysis has been conducted for the transports of low - to medium-level radioactive waste to the planned final repository KONRAD at Salzgitter in Germany. The expected annual transport vol-ume is about 3400 shipping units The mam objective of the study was the assessment of radiological risks from Iransport accidents in the region of the final repository. Two shipping scenarios - 100%
transportation by rail and 80% transportation by rail, 20% by road were analyzed. The availability of an extensive and detailed waste data survey of the German Federal Office for Radiation Protection (BFS) was of fundamental importance to the safety analysis. To a large extent probabilistic safety assessment techniques have been applied to take into account the vanational range of quantities and parameters which determine potential radiological consequences of transport accidents and the ex-pected frequencies of their occurrence Influencing quantities and parameters are. 217 waste categories representing different types of waste, packaging and radionuclide inventory; 9 seventy categories to classify the spectrum of accidental impacts; 8 waste package groups representing the release beha-viour of containers and waste products; different loading configurations of vehicles and varying num-bers of waste wagons within a train, expected frequencies of accident severity categories and, with respect to rail transports, of the number of waste wagons affected in an accident; variability of atmo-spheric dispersion conditions and consequently of potential radiation exposures from accidental re-leases. The results of the probabilistic nsk analysis of transport accidents in the region of the final repository KONRAD are expressed as cumulative complementary frequency distributions. These dis-tribution curves show expected frequencies of radiological consequences such as potential effective doses of individuals in the region of the repository.
1 INTRODUCTION
The former iron ore mine KONRAD situated within the city limits of Salzgitter in Germany is planned to be used as deep underground final repository for radioactive wastes with negligible heat generation About 95% of the radioactive waste volume resulting from nuclear industry including re-processing of German spent fuel abroad, from research, medicine and other applications could be dis-posed of in the KONRAD waste repository Detailed safety analyses were performed concerning both the operating and the post-operating phase of the planned repository. This work has resulted in the es-tablishment of preliminary waste acceptance criteria. Presently the licensing procedure is still underway.
Although questions concerning possible risks associated with waste transportation are not a formal part of the licensing procedure, they nevertheless play an important role in public debate, especially in the local region of the repository. On behalf of Die German Federal Ministry for Environment, Nature Protection and Nuclear Safety the GRS has conducted an extensive safety analysis of the transporta-tion of radioactive wastes to the KONRAD waste disposal site [ 11 The main objective of the study
was the assessment of radiological nsks from transport accidents in the region of the final repository.
Two shipping scenarios, which can be considered to bound the real conditions were analyzed'
• 100% transportation by rail
• 80% transportation by rail, 20% by road
The expected annual transport volume to be shipped to the final waste repository is about 3400 ship-ping units, z shipship-ping unit being either a cubical container or one or two cylindrical containers on a pool palette.
2 SAFETY REQUIREMENTS FOR WASTE PACKAGES
Requirements concerning activity contents, waste products and qualification of waste containers result from.
• the waste acceptance criteria derived from a detailed safety analysis (operating and post-operating phase) for the KONRAD repository
• the transport regulations for dangerous goods
Both, the transport regulations for the shipment of radioactive materials which are based on the Safety Senes No. 6 of UK IAEA [2] and the "KONRAD preliminary waste acceptance criteria" [3] represent the framework of the safety requirements for the waste packages
2.1 KONRAD Preliminary Waste Acceptance Criteria
The waste acceptance criteria are the result of the safely analysis for the final waste repository. They represent a set of requirements which originate from
• incident analysis (operating phase)
• thermal influence to the host rock
• cnticality safety
• limitation of releases of volatile radionuchdes from the repository (operating phase)
• limitation of dose rates of packages
The systematics of the waste acceptance criteria distinguish between two categories of waste containers.
• waste container class I, basically equivalent to strong industrial packages
• waste container class II, packages with increased qualification to withstand severe mechanical or"
thermal impacts
There are three main types of standardised transport containers accepted for disposal: Cylindrical con-crete containers, cylindrical cast iron containers and cubical containers (sheet steel, concon-crete or cast iron). In addition because of differences in release behaviour following mechanical and/or thermal (fire) impact six different waste form groups (eg bitummized, cemented, high pressure compacted waste) arc distinguished for waste container class I Also different levels of leak tightness of contain-ers arc provided for m case of high activity contents of volatile radionuclidcs With respect to the
ac-ceptable radionuclide inventories of waste containers requirements resulting from different safety domains such as incident analysis or limitation of heat generation have to be observed simultaneously.
This, of course, means that the most limiting of the parallel requirements will restrict acceptable ra-dionuclide contents of waste containers
2 2 T.-ansport Regulations
The national requirements concerning the transportation of waste containers to the final repository are essentially identical to the international IAEA Regulations for the Safe Transport of Radioactive Ma-terial. The syiiematics of these requirements is quite analogous to the waste acceptance criteria:
• limitations of dose rates
• different package categories (e g strong industrial, Type A, Type B packages)
• distinction of the physical/chemical form of activity contents (e.g. special form, LSA-1I, LS A-III, SCO-I, SCO-II)
• limitation of activity contents or of activity concentrations in relation to properties of package and physical/chemical form of radionuclides
3 WASTE DATA BASE
Both sets of requirements - the transport regulations and the waste acceptance criteria - do not provide any information on the type, quantity and properties of the radioactive waste actually produced and re-quiring disposal. Consequently, the availability of an extensive and detailed waste data survey of the German Federal Office for Radiation Protection (BFS) was of fundamental importance to the safety analysis. Completed in summer 1990, the survey was conducted with the aim of obtaining compre-hensive data on the radioactive waste produced and to be anticipated in the foreseeable future in the Federal Republic of Germany. The spectrum of radioactive wastes suitable for disposal in the KON-RAD waste repository compnses 217 reference waste types. For each reference waste the following information is available:
• origin/originator
• type of waste
• conditioning/immobilization type
• type of packaging
• radionuclide inventory
• local dose rate of the package
• mean annual number of packages
METHODS AND DATA FOR PROBABILISTIC ACCIDENT RISK ASSESSMENT
The nsk of transport accidents is determined by the frequency of accidents leading to a release of ra-dioactive substances and the potential radiological consequences, such as radiation exposure of
per-sons and contamination of the biosphere. To assess the nsk associated with transport accidents, the region in the proximity of the final repository KONRAD is considered and this is defined as the area within a radius of 25 km around the installation. This region, for which the accident risk is calculated, is chosen since it cover» all waste transports converging m the vicinity of the final repository and the rail and road transport routes representative for this region This includes the Braunschweig marshal-ling yard, through which a large proportion of the waste transport is expected to be routed
To a large extent probabilistic safety assessment 'echniques have been applied to take into account the variational range of quantities and parameters which determine potential radiological consequences of transport accidents and the expected frequencies of their occurrence. Influencing quantities and pa-rameters are:
• 217 waste categories representing different types of waste, packagings and radionuclide inventories.
• 9 seventy categories to classify the spectrum of accident impacts,
8 8 waste container groups representing the release behaviour of containers and waste products,
• different loading configurations of vehicles and varying numbers of waste wagons within a train,
• expected frequencies of accident seventy categories and, with respect to rail transports, of the number of waste wagons affected in an accident,
• variability of atmospheric dispersion conditions and consequently of potential radiation exposures from accident releases.
4.1 Seventy categories and accident frequencies
The mechanical and/or thermal impact on the waste containers caused by the accident together with the properties of the waste containers and the waste product they contain (e g. cement/concrete, bitu-men, compacted waste etc.) determine the extent to which radioactive materials are released into the environment To permit a quantitative evaluation of accident nsks, the broad spectrum of possible ac-cident impacts must be condensed into a finite number of severity categories, each of which m turn encompasses a wide range of possible effects on waste containers caused by accidents. For the pur-poses of the present study, nine severity categones (SC) were defined with the characteristics shown in Fig. 1.
Detailed analyses have been performed to determine expected accident frequencies per vehicle-km for heavy trucks (articulated lorries), pergoods-tram-km and per rail car-km and to assess the relative fre-quencies of the 9 severity categones in each case. The overall accident rate for articulated lorries (damage to vehicle > 4000 DM) on federal motorways was determined to be 3.5 • 10'1 km'1. For freight trains an accident rate (damage to rail car > 3000 DM) of 5 • 10'pertrain-km and of 25 • 104
per rail-car-km was established. Details of the analysis made for this purpose of German rail accident statistics of goods-trains covenng the 10 year penod 1979 to 1988 are given in [4]. From this accident analysis also the relative frequencies of the 9 seventy categones were determined. Taking accidents of freight trains as an example these relative frequencies are included in Fig. 1. All events where only a fire occured without prior mechanical impact were included into severity categones 2 or 3 (impact ve-locity < 35 km/h)
1E+OCH
1E-01-0.. 35 km/h 36.. 80 km/h above 80 km/h
no fire 30 min, 800°C D 60 min, 800°C Fig.l : Relative frequencies of severity categories (SQ for goods train accidents
4.2 Waste container groups and release fractions
Releases of activity from accident impacts depend on the properties of the transport container and the waste product which it contains. For this reason, the range of waste containers in use is divided into waste container groups (WCG) with the aim of categonzing waste containers with similar release characteristics in a single group. Eight waste container groups are
distinguished-WCG 1 Bituminizcd waste in sheet steel cubical containers
WCG 2 Non-immobilized and non-compactable metallic and non-metallic waste in sheet steel cubical containers
WCG 3 Metallic waste in sheet steel cubical containers WCG 4 Compacted waste in sheet steel cubical containers
WCG 5 Waste immobilized in cement in sheet steel cubical containers WCG 6 Bitumimzed waste in concrete containers
WCG 7 Waste immobilized in cement in concrete containers WCG 8 Waste m cast iron containers
Airborne fractional releases from waste packages suffering a transport accident were determined for the 8 waste container groups and 9 seventy categones defined above for particles in the following size range intervals of aerodynamic equivalent diameter (AED). 0 - 10 urn, 10 - 20 urn, 20 - 50 urn and 50 - 70 urn. For particles below 10 ^m (excluding H3, C14, halogens) the fractional releases are shown
ooo
AED0...10pm
SC1 .. SC9 Seventy Categones WCG1 .. WCG8 Waste Container Groups
SC3 SC2 SC1
Fig. 2: Release fractions from waste containers for different severity categories
in Fig. 2. In each case it is assumed that the mechanical impact to waste containers is equivalent to an impact onto an unyieliiing target with a speed equal to the upper limit of the respective velocity inter-val. Accidents with speeds above 80 km/h are treated as accidents with a velocity of 110 km/h. Re-leases from fire impact are modelled assuming a fire fully engulfing waste packages with a thermal energy input equivalent to a fire either of 800°C and 30 mm duration or of 800°C and 60 mm duration.
4.3 Probabilistic source term determination
A computer code was developed to simulate a wide spectrum of waste transport and accident configu-rations using Monte Carlo sampling techniques In a first step a large number (e.g. 10000) of source terms are generated to represent possible releases of radionuchdes from transport accidents. Accident events in which the integrity of waste packages is retained and consequently no releases occur are also recorded. Source terms are determined separately for road and rail transports.
A source term generated by the accident simulation program represents the released activities of indi-vidual radionuclides for the simulated accident configuration. The radionuciide-specific activities are determined by the activity content of die waste packages involved in the accident and the fraction as-sumed to be released into the atmosphere.
For the purpose of subsequent analysis of possible radiological consequences and their expected fre-quencies of occurrence the following information is assigned to each source term'
• The seventy category (k = 1.2,3.. 9)
• The conditional probability of the accident configuration (given an accident occurs)
• A radiological hazard index calculated from the radionuclide-specific activity which permits an approximate relative classification of different source terms with respect to potential radiological consequences
To facilitate the analysis of environmental consequences, the large number of source terms must first be appropriately grouped into a limited number of source term groups. In a next step for each source term group a representative source term is determined designated as release category
The source terms are first arranged in ascending order according to the radiological hazard index. This is done separately for purely mechanical and combined mechanical/thermal seventy categories. The reason for this is that in the calculation of radiological consequences an effective release height of 2 m is assumed for accidents with only mechanical impact and of 50 m in the case of mechanical impact followed by a fire
Source term groups are then formed by combining source terms with approximately equal hazard in-dices in a way that the range of radiological hazard inin-dices of source terms having high hazard inin-dices does not differ substantially. This procedure is intended to assure representativeness particularly for the source terms resulting in higher radiological consequences.
In a next step for each source term group a representative source term, called release category, is derived. Without going into detail here it can be demonstrated that the limited number of release cate-gories determined in this way very well represent the spectrum of potential releases from transport ac-cidents including their probabilities of occurrence. In summary, ten such release categories each have been generated by the simulation program for accidents during transportation by goods train, by truck, and in the Braunschweig marshalling yard. In each case 5 release categories arc representative for ac-cidents with purely mechanical impact on shipping units, and 5 release categories for acac-cidents with mechanical impact and subsequent fire. The expected frequency of occurrence has been determined for each of these release categories.
5 RESULTS
Potential radiological consequences such as radiation exposure of persons and ground contamination have been calculated by using the accident consequence code UFOMOD. In calculating radiauon ex-posure, the following exposure pathways are considered.
• cloudshine (radiation from the passing cloud)
• inhalation (intake of activity with respiratory air)
• groundshine (external radiation from radionuchdes deposited on surfaces. 70 a)
• ingestion (intake of activity with food, integration time 70 a)
• rcsuspension (reentry of radionuclidcs deposited on surfaces into the air with subsequent inhala-tion, 70 a integration time)
The calculations take into account the relative frequency of different atmosphenc dispersion condi-tions in the region of the final repository on the basis of long-term measurements of a meteorological
station near Braunschweig The calculation with the accident consequence code UFOMOD are made for each of (lie 10 release categories representative for the following shipping scenanos
• 100% transportation by rail
• 80% rail / 20% road
• marshalling yard of Braumclnvcig
For each scenario the results for the 10 release categories are then superimposed taking into account the relative frequency of occurrence of each release category. The final presentation of risks from transport accidents is m the form of cumulative complementary frequency distribution (ccfd) relating radiological consequences and (he associated expected frequencies of their occurrence The expected frequencies refer to the region (25 Km zone) of the final waste repository Fig 3 shows as an example for the 80% rail / 20% road scenano the expected frequencies of effective doses which could result anywhere within the 25 km radius in downwind direction from the location of an accident
By displaying frequency distnbutions for different downwind distances of 250 m. 1250 m and 6250 m the additional information is given how radiological consequences decrease on average with distance from the location of an accident From Fig 3 the following information can be derived.
• The frequencies shown on the vertical axis refer to the entire region of the waste repository, that is to say to ihe /one within a radius of 25 km around the installation
• The effective dose given on the honzonW axis indicates the potential dose to a person residing permanently in close proximity to the accident site in the direction of atmospheric transport of the contaminant
• Accidents of trucks or rail cars carrying radioactive waste arc expected to happen on average ev-ery 75 years
• As a result of the accident analysis every second accident involving a truck or rail cars loaded with radioactive waste containers would lead to a release of radioactivity But it has to be stressed that this rather high fraction is Ihe result of a cumulation of conservative assumptions within the risk analysis. Nevertheless, in many accidents with airborne release of radioactive material poten-tial radiological consequences would be quite small
• The chances that for this shipping sccnano an accident in the region of the repository would lead without countermeasurcs to an effective dose in 250 m downwind distance from the location of the accident equivalent to or exceeding the n?tural radiation exposure of one year are about 1 in 75 for an operating period of 40 years
• Effective lifetime doses of 50 mSv in 250 m down-wind direction from the location of an acci-dent would be expected ivith a chance of I in 10000 during an operating period of the repository of 40 years. As can be seen from Fig "> potential radiation exposures decrease on average rapidly with distance from the location of the accident, starting from 250 m up to about 1200 m by a fac-tor of 10 and a further facfac-tor of 10 at a distance of about 6200 in.
• No protective counlermeasures are assumed in calculating the potential radiation doses. That is to say. that the removal of radioactive substances deposited on vegetation and other surfaces after an accident or other measures to reduce potential radiation exposure arc not assumed
It would also have been possible to calculate cumulative complementary frequency distributions of collective effective doses resulting from transport accidents anywhere within the 25 km zone around
Predicted frequency of road vehicle transport accidents
1 in 80 years 1 in 170 years Accidents with release
Annual natural 250 mV i radialen expos«
1 in 3500 years Design guideline
exposure (§28)
1150m"
10-6 1CT5 10-* Iff3 Iff2 10'1 10°
Effective dose D in plume direction ——
Sv
Fig. 3: Frequency distribution of the effective lifetime dose from waste transport accidents:
80% rail / 20% road transport.
the repository by assuming a uniform population density. But collective doses arc much more difficult
the repository by assuming a uniform population density. But collective doses arc much more difficult