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Abstract In 1992, a large aircraft containing pieces of depleted uranium crashed into an apartment building complex in the Netherlands. A risk analysis of the depleted uranium was carried out in 1998. The results are presented here. 1. Introduction On 4 October 1992, a Boeing 747 cargo plane crashed into an apartment
building complex near Schiphol Airport in the Netherlands, leading to |
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2. Risk Assessment Literature studies and model calculations were used to estimate the exposure of bystanders to uranium and to evaluate the risk. The risk assessment aimed at giving conservative, but realistic results by using the most likely value for each parameter on the basis of the information available; conservative values were employed where no information was available. In addition to this, a worst-case approach was used to investigate the potential upper limit of the exposure. Figure 1: Overview of the crash site 2.1 Local Situation The local situation is shown in Figure 1. The aircraft collided almost perpendicularly into the intersecting point of an eleven-floor apartment building complex. Cargo was dispersed over a large area in front of the building and caught fire there. Two burning sites could be distinguished: namely, the site where the plane hit the apartments and the (rest of the) cargo plane ended up, and the site where cargo had been dispersed in the public garden in front of the apartment building. The surface area of the fire at the site of the crash was estimated at about 270 m2, based on the area of the apartments which had collapsed. The (effective) area of the burning cargo in the public garden was estimated at a factor of 2 larger, i.e. 540 m2. The duration of the fire was estimated to be one hour. At the time of the accident, a strong (12 m s-1) northeast breeze favoured a strong dispersion of the contaminants. The source term of smoke gases was derived from the burning rate in the fire and the burning area. The burning rate is estimated to be 0.05 kg m-2 s-1, based on typical values for kerosene and chemical waste [3,4]. Since the fire was in the open air, it was not oxygen limited. A stoichiometric air to fuel mass ratio is assumed, so that combustion of 1 kg kerosene results in 16 kg combustion products. The mass flows of combustion products generated in the fires were calculated at 216 kg s-1 and 432 kg s-1 for the crash and cargo sites, respectively. The heat available for plume rise per kg material burned is estimated to be 20 MJ [3,4,5]. |
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2.2 Source term of depleted uranium The cargo plane contained about 24 pieces of depleted uranium, the mass of a single piece ranging from 6 to 30 kg, with specific areas of 0.05 – 0.15 cm2 g-1. Following the clean-up of the crash area, counter balance weights having a total mass of 152 kg uranium were missing. To date, it is still not known what happened to the missing uranium. The possibility exists that pieces of uranium were treated as ordinary waste material and dumped with the contaminated soil. Pieces of uranium may also have been (partly) oxidised in the fire and dispersed into the environment. In the analysis of this scenario, it is assumed that the pieces of uranium were evenly distributed over the two fire sites. To estimate the source term and its consequence, the oxidation rate, chemical appearance and particle distribution of the uranium oxide were determined. Based on a literature study, it is estimated that under the conditions likely to be present in these fires, a maximum of 30% of the missing uranium (46 kg) is oxidised within one hour [6]. The respirable fraction, i.e. the fraction having a particle size diameter less than 20 m m, is less than 1 per cent by weight. The uranium oxides formed under these conditions are UO2 and U3O8, which are poorly soluble [7]. The worst case approach assumes the total mass of the missing uranium to be located at the crash site and completely dispersed in respirable form. |
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2.3 Dispersion of smoke gases and uranium Various models were used to calculate the dispersion of uranium and smoke gases in the environment, namely a very simple, rule-of-thumb calculation [8], a pool fire model in combination with a free-field dispersion model [9] and a 3D Computational Fluid Dynamics (CFD) model [10]. The results of the CFD calculations are shown in Figure 2. The calculation shows the smoke gases from both the fires at the crash and cargo burning sites transported upwards through the newly created gap in the apartment building, leading to reduced concentrations at ground level. The results of the CFD calculations are confirmed by the available observations on video. The highest concentrations, found for bystanders situated in the area downwind of the crash site ranges from 1 – 10 g m-3, where a minimum distance of 20 metres to the fire is assumed. The magnitudes of the smoke gas concentrations were confirmed by the other model calculations. Since the source term of uranium is very small relative to the amount of smoke gases, the presence of uranium will not affect the dispersion properties of the plume of smoke gases. The concentration of the aerosol-bound uranium in the environment can be derived from the calculated concentration of smoke gases by considering the mass fraction of uranium versus the mass fraction of the smoke gases. As a result, bystanders are calculated to have possibly been exposed to a concentration of uranium of 3 m g m-3 (best case) for one hour; the upper limit is calculated as 2 mg m-3 (worst case). Intake of uranium by inhalation in one hour is calculated at 4 m g (best estimate) to 6 mg (worst case).
(a) Release from the crash site
(b) Release form the site of burning cargo Figure 2: Concentrations of smoke gases
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2.4 Exposure and consequences Since the uranium oxides formed in the fire are poorly soluble, the radiological toxicity is more important than the chemical toxicity. The intake of uranium by inhalation results in an effective radiation dose of 0.5 m Sv (best estimate) to 0.7 mSv (worst case). The radiation dose is therefore comparable to eight hours of exposure to natural radiation in the Netherlands (best estimate). The radiation dose calculated in the worst-case scenario is comparable to the natural radiation dose in one year and is less than the yearly limit of 1 mSv for exposure to radiation by human action [11]. Inhalation of depleted uranium is therefore concluded as not resulting in detectable adverse effects to the bystanders. 3. Conclusion On the basis of literature studies and model calculations, we estimated the exposure of bystanders to depleted uranium after a Boeing 747 aircraft crash. Various dispersion models used to estimate the concentrations of uranium in air yielded consistent results. The model calculations show that the concentrations and radiation doses were considerably below the levels at which acute health complaints can occur. It is therefore highly improbable that exposure of bystanders to uranium would result in the health complaints reported. |
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References
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the immediate death of 43 people [1]. The aircraft,
cargo, fuel and damaged apartment building caught fire immediately after the crash and
burned for more than one hour. In the years following the accident, an increasing number
of people began reporting various physical and mental health complaints, which they
attributed to the exposure to dangerous substances during the fire after the crash.
Especially depleted uranium was pointed out as a possible cause of their health problems,
since the aircraft contained 
depleted uranium to function
as counterbalance weight and because about 150 kg out of the total 282 kg uranium was
missing following clearance of the crash area. This paper presents the risk analysis
carried out to determine whether the dispersion of uranium following the air crash could
have led to long-term health complaints of bystanders present near the crash area. The
reader is further referred to reference [2] for details not included in this paper.
Reference [2] also describes the risk analysis, using a similar approach, of other
hazardous substances issuing from the cargo or other burned material, such as HCl, PACs
and heavy metals.
