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Radioactive scrap metal
Radioactive scrap metal is the situation when radioactive material enters the metal recycling process known as the scrap metal trade
A lost source accident is one where a radioactive object is lost or stolen. Such objects often end up in the scrap metal industry, as people mistake them for harmless bits of metal. The International Atomic Energy Agency has provided guides for scrap metal collectors on what a sealed source might look like. The best known example of this type of event is the Goiânia accident, in Brazil.
While some lost source accidents have not involved the scrap metal industry, they are still good examples of the likely scale and scope of a lost source accident. For example, the Soviet army left sources behind in Lilo. Another case occurred at Yanango where a radiography source was lost and at Gilan, Iran a radiography source harmed a welder .
Radioactive sources have a wide range of uses in medicine and industry, it is common for the design (and nature) of a source to be tailored to the application so it is impossible to state with confidence what the "typical" source looks like or contains. For instance antistatic devices include beta and alpha emitters. For instance polonium containing devices have been used to eliminate static electricity such devices include paint spraying equipment. An overview of the gamma sources used for radiography can be seen at Radiographic equipment, it is reasonable to consider this to be a good overview of small to moderate gamma sources.
In Tammiku (Estonia) a group of three men were responsible for a similar incident: They burgled a radioactive waste store to steal scrap metal. One of them picked up a metal pipe and placed it in his pocket. This metal pipe was a very strong 137Cs source which gave a high localised dose to the man’s leg (1800 Sv local, 4 Sv whole body). He was admitted a few days later to hospital where he claimed to have had an accident in the woods. He died shortly after as a result of whole body irradiation from the source. Before going to the hospital, he left the source in his house where it then irradiated other members of his family and his dog (which died as a result). His son suffered a localised radiation burn (Local dose of 25 Sv, whole body 3.6 Gy) which resulted in the amputation of fingers, when he inadvertently handled the source when looking for tools to repair his bicycle. When a medical doctor saw these burns it was understood that an ionising radiation accident was in progress. The man's wife got a 500 mSv dose while his mother got a 2.25 Sv dose.
It is interesting to note that the scrap metal industry was involved twice in this: the caesium source being originally found in a shipment of scrap metal which was brought into the country (at that point it was thought to be a 60Co source based on half thickness measurements). The source was placed in the radioactive waste store for safekeeping, which was subsequently entered by the men who were intent on stealing scrap metal.
At Samut Prakarn a 15.7 TBq (425 Ci) cobalt-60 teletherapy source was lost , attempts were made by some scrap metal workers to recycle the metal. During this time humans were subject to irradation by the source.
It was found that at the edge of the scrap yard the dose rate was about 1 to 10 mSv hr-1. The exact location of the source in the scrap yard was determined using a fluorescent screen which acted as a scintillator. This was held on the end of a long pole.
Isotopes and metals
137Cs vs 60Co (solubility in water)
The clean up operation for the Goiânia accident was hard because the source was opened, and the fact that the active material was water soluble. The event in Mexico wherein cobalt-60 was spilled in an almost identical event led to a very different pattern of contamination since, the cobalt in such a source is normally in the form of cobalt metal alloyed with some nickel to improve the mechanical properties of the active metal. If such a source is abused, then the cobalt metal fragments do not tend to dissolve in water or become very mobile. If a cobalt or iridium source is lost at a ferrous metal scrapyard then it is often the case that the source will enter a furnace, the radioactive metal will melt and contaminate the steel from this furnace. In the United States, some buildings have been demolished because of the level of cobalt-60 in the steel used to make them. Also, some of the steel which was rendered radioactive in the Mexican event was used to make table legs.
In the case of a caesium source being melted in such an electric arc furnace used for steel scrap, it is more likely that the caesium will contaminate the fly ash or dust from the furnace, while radium is likely to stay in the ash or slag. The EPA provide data about the fate of different contaminating elements in a scrap furnace . Four different fates for the element exist: the element can stay in the metal (as with cobalt and ruthenium); the element can enter the slag (as in lanthanides, actinides and radium); the element can enter the furnace dust or fly ash (as with caesium), which accounts for around 5%; or the element can leave the furnace and pass through the bag house to enter the air (as with iodine).
In the case of some high value scrap metals it is possible to decontaminate scrap metal, but this is best done long before the metal goes to a scrap yard.
It is normal to place silicon, aluminum scrap and flux in a furnace. This is heated to form molten aluminum. From the furnace three main streams are obtained, metal product, dross (metal oxides and halides which is skimmed off the molten metal product) and off gases which go to the baghouse. The cooled waste gasses are then allowed out into the environment.
It is normal that good quality copper scrap, such as that from a nuclear plant, will be refined in one furnace before being refined further in an electrochemical process. The furnace will generate metal, slag and dust. The dust will go to a bag house, the gases will leave the plant. The metal from the furnace will be refined in the electrochemical process.
If the copper refinary includes an electropurification step after the furnace then some of the elements will enter the anode slime. Here the partioning of the elements into the anode slime as well as the metal, slag, dust and exit gases is shown.
In the early part of the 20th century in the USA, gold which was contaminated with lead-210 entered the jewelry industry. This was from gold seeds which had held radon-222 which had been melted down (after the radon had decayed). The daughters of the radon are still radioactive.
Sealed vs. Unsealed source
In the Tammiku event, where a caesium source of similar strength was stolen, the accident site was easy to clean because the source remained sealed. All that needed to be done was to pick the source up, place it in a lead pot and transport this to the radioactive waste store. It is noteworthy that in that case the source recovery workers wore rubber gloves, but more importantly failed to use tongs. Gamma rays obey the inverse square law so by slightly increasing the distance between the recovery worker and the source the dose rate experienced by the worker can be reduced. In the recovery of lost sources the International Atomic Energy Agency consider that it is best to plan the recovery first, and to consider using a crane or other device to place shielding (such as pallet of bricks or a concrete block) near the source to allow the recovery worker to walk up to it while being shielded by the added dense object.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Radioactive_scrap_metal". A list of authors is available in Wikipedia.|