Contributors..............................................................................................................................................ix Introduction..............................................................................................................................................xi 1. Potential Sources of Contamination in Inhabited Areas Raimo Mustonen..................................................................................1 2. The Dispersion, Deposition and Resuspension of Atmospheric Contamination in the Outdoor Urban Environment Ken W. Nicholson............................21 3. Airborne Contamination Inside Dwellings Miriam A. Byrne...............................................................................................55 4. Contamination of Humans: In the Respiratory Tract and on Body Surfaces Miriam A. Byrne................................................................77 5. Migration of Radionuclides on Outdoor Surfaces Kasper G. Andersson....................................................................................107 6. Estimation of Doses in Inhabited Areas Kasper G. Andersson, J. Arthur Jones and Thomas W. Charnock....................................................147 7. Measurement and Screening of Contaminated Inhabited Areas Peter Zombori...............................................................................187 8. Countermeasures for Reduction of Dose in Contaminated Inhabited Areas Kasper G. Andersson.............................................................217 9. Non-Radiological Perspectives: Holistic Value Assessment of Countermeasure Strategies Deborah H. Oughton and Ellen-Marie Forsberg.....................259 10. Strategies for Restoration of Contaminated Inhabited Areas Kasper G. Andersson.......................................................................297 Concluding Remarks Kasper G. Andersson...................................................................................................................327 Author Index..............................................................................................................................................331 Subject Index.............................................................................................................................................339
Raimo Mustonen
Contents
1. Introduction 1 2. Background 2 3. Accidents at Nuclear Installations 3 4. Accidents with Highly Radioactive Sources 7 5. Transport Accidents 10 6. Nuclear-Powered Satellites Entering the Atmosphere 11 7. Malicious Use of Radiation and Radiological Terrorism 13 8. Special Problems Related with Radioactive Contamination of Residential Areas 17 References 19
1. Introduction
There are several types of events that could result in dispersion of radioactive substances in an inhabited environment. These include both intentional and unintentional events. Releases of radioactive substances could range from major events involving a nuclear facility or a nuclear weapon to small events such as a transportation accident. The extent of the contamination and impact on the environment and people depend greatly on the specific event and the radionuclides involved. However, many aspects of assessing and remediating the consequent situation and of protecting people will be the same or similar regardless of the spatial scale and specific radionuclides involved. Of course, the time needed to prepare these actions depends on the event. This chapter deals with potential sources of radioactive contamination of urban or inhabited areas. It is aimed at providing an overview of different types of conceivable scenarios that could lead to contamination on different scales and the radionuclides that would be of primary concern. Examples are given of experience from the past.
2. Background
Worldwide there are hundreds of medical, industrial and academic applications using radioactive sources of significant strength. These applications encompass inter alia sterilisation of foodstuffs and pharmaceutical products, industrial and medical radiography, teletherapy, exploring for oil, research, etc. Seven reactor-produced radionuclides are of particular concern due to their radiotoxicity, their widespread use and their sufficiently long half-lives: 241Am (432 years), 252Cf (2.6 years), 137Cs (30 years), 60Co (5.3 years), 192Ir (74 days), 238Pu (88 years) and 90Sr (29 years) (Ferguson et al., 2003). These radionuclides are used in many applications with different activities as shown in Figure 1 (IAEA, 2001).
There are many factors decreasing the overall security of a radioactive source during its life cycle. There can be weaknesses in organisations, in regulations and procedures, in the proper working of regulatory bodies and in regulatory enforcement. Often a lack of knowledge and awareness is the reason for getting the source lost. The follow-up of registries can be a problem, especially in the case of bankruptcy or change of ownership. In addition, security, for example during transport, can be deficient. Furthermore, there can be inhibitions to legal disposal because of the high costs involved; or worse, legal disposal can be non-existent. It is estimated that in the USA there are between 500,000 and 2,000,000 sources which are no longer needed and up to 375 sources are yearly reported to be orphaned. The figures for the European Union are 30,000 disused sources and up to 70 sources per year reported to be orphaned. In the former Soviet Union it is estimated that there are thousands of orphan sources of high-risk category (Ferguson et al., 2003).
The past six decades have shown that various accidents related to the use of radioactive and nuclear materials must be taken into consideration although today the likelihood of major accidents is small and releases of radioactive substances into the environment are minimised with effective safety and security systems. As a consequence of the terrorist attacks on the US World Trade Center and the Pentagon Headquarters, the subway systems in Spain, Japan and the UK, embassies, nightclubs and hotels, political leaders and authorities have become more aware of the need to reassess existing threats and our preparedness for them. There are several lessons learned from the recent attacks and other events where radioactive or nuclear materials have been involved. The first of these is the intent of terrorists to stage multiple events simultaneously, and this must be taken into account in emergency planning today. The second factor, which is very difficult to predict, is the concept of suicide scenarios. A third lesson is that terrorists deliberately choose improbable or unexpected events and we can no longer rely on historical factors such as the probability of failure rates of various components to predict the likelihood of an event. The fourth lesson is the realisation of a terrorist event combining multiple hazardous agents. Thus, planning for a radiological incident alone is an outmoded concept and authorities need to be able to recognise and respond to a situation where there is a combined chemical, biological and radiological hazard.
3. Accidents at Nuclear Installations
In 2006, there were 442 commercial nuclear power reactors operating in 31 countries, 11 reprocessing plants in 9 countries, 284 nuclear research reactors in 56 countries and 220 nuclear-powered ships and submarines (World Nuclear Association, 2008). Most of them are situated quite close to inhabited areas and accidents occurring with them might have severe consequences for the local population. There are different types of nuclear reactors. Most are used for power generation, but some can also produce plutonium for weapons and fuel. Two components are common to all reactors: control rods and a coolant. Control rods determine the rate of fission by regulating the number of neutrons. These rods consist of neutron-absorbing elements such as boron. The coolant removes the heat generated by fission reactions. Water is the most common coolant, but pressurised water, helium gas and liquid sodium have also been used. Slow-neutron reactors operate on the principle that 235U undergoes fission more readily with thermal or slow neutrons. Therefore, these reactors require a moderator to slow down neutrons from high speeds upon emerging from fission reactions. The most common moderators are graphite (carbon), light water (H2O) and heavy water (D2O). Since slow reactors are highly efficient in producing fission of 235U, slow-neutron reactors operate with slightly enriched uranium. Light-water reactors are classified as either pressurised-water reactors (PWR) or boiling-water reactors (BWR), depending on whether the coolant water is kept under pressure or not.
Fuel amounts in reactors vary with their size. Small research reactors may contain only a few kilograms of uranium, plutonium or thorium, whereas reactors used in submarines and surface vessels can contain a few hundreds of kilograms, and power plant reactors up to more than 100 tons of uranium. The composition of nuclear fuel varies depending on the reactor type and application. Natural uranium, containing about 99.3% 238U and about 0.7% 235U, is used in some power reactors to produce plutonium. Low-enriched uranium (enriched to increase the concentration of 235U up to some 20%) is used in most commercial power reactors to produce electricity and in some research reactors. Highly enriched uranium (235U from 20% up to more than 90%) is used in many research reactors, naval propulsion reactors, nuclear weapons and reactors producing tritium and plutonium. Mixed plutonium–uranium oxide (MOX) fuel is used in some research and experimental reactors and also in some power reactors.
The consequences of severe accidents at nuclear installations relate to the amount of fuel in addition to the type of accident. Hundreds of accidents and incidents have occurred with small research reactors and nuclear-powered ships and submarines. Some of them have resulted in loss of lives and human exposure to radiation at different levels. Accidents in nuclear submarines and vessels may lead to serious consequences in inhabited areas only if they happen in harbours. A damaged reactor may result in dispersion of radioactive materials within an area of a few tens of square kilometres calling for protective actions and later on also some cleanup actions. Small research reactors are normally close to or inside inhabited areas and their severe accidents may also contaminate areas of a few tens of square kilometres; hence, protective and clean-up actions might be needed.
A few severe accidents have happened in nuclear power reactors, the best known being at the Chernobyl plant in Ukraine in 1986, at the Three Mile Island plant in Pennsylvania in the USA in 1979 and at Windscale in Cumbria in Northern England in 1957. The reactor of the Windscale accident was using natural uranium as the fuel, graphite as the moderator and air as the cooling medium. The accident occurred during annealing of the graphite. Annealing is an operation which is periodically necessary to liberate the energy built up in the graphite as a result of exposure to neutrons. The operation leads to a significant liberation of energy. The event initiating the accident was an erroneous temperature indication. As a result of an underestimation of the temperatures, the nuclear power was raised until two fuel elements, which already had damaged cans, ignited. The fire spread over 20% of the reactor core. Fission product release was not monitored, because of an instrumentation failure. The fire was finally detected visually. After 12 h of great effort, the fire was brought under control and finally extinguished. A significant fraction of the affected core inventory was released over a period of 21 h. The releases of the most important radionuclides are summarised in Table 1. The maximum individual dose received was estimated at 9 mSv.
The Three Mile Island accident began early in the morning on 28 March 1979 when the plant experienced a failure in the secondary, non-nuclear section of the plant (U.S. Nuclear Regulatory Commission, 2008). The main feeding water pumps stopped running, caused by either a mechanical or electrical failure, which prevented heat removal from the steam generators. First the turbine and then the reactor automatically shut down. Immediately after that, the pressure in the primary system, inside the nuclear portion of the plant, began to increase. In order to prevent that pressure from becoming excessive, the pilot-operated relief valve opened. The valve should have closed when the pressure decreased by a certain amount, but it did not. As a result, cooling water poured out of the open valve and caused the core of the reactor to overheat. Because adequate cooling was not available, the nuclear fuel overheated to the point at which the zirconium metal tubes, holding the nuclear fuel pellets, ruptured and the fuel pellets began to melt. A considerable part of the reactor core melted during the early stages of the accident. A dose rate of 60 Gy h-1 was measured in the building. The major risk was linked to presence of hydrogen in the pressure vessel and its possible explosion if contacted with oxygen. This did not happened. A leak of contaminated liquids at the beginning of the accident caused a release of rare gases (see Table 1). The most important exposure route was by direct exposure to the passing radioactive cloud. The low speed and variable direction of the wind caused very complex atmospheric dispersion. Evacuation of pregnant women and young children was imposed. This decision gave rise to the spontaneous evacuation of more than 100,000 persons. The maximum dose received per individual was estimated at 0.4 mSv. Melting of nuclear fuel did not however lead to a breach of the walls of the containment building, which would have led to a release of great quantities of radioactive materials to the environment. The accident led to no deaths or injuries to plant workers or members of the nearby community.
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