As per US-EPA (US-DOE, 2007) the maximum concentration level (MCL) in drinking water is 0.03mg/L or 27pCi/L for natural uranium. Level of 67pCi/L of natural uranium in drinking water is also suggested by some authors. It is assumed that 02 to 5% of the intake is absorbed in to the blood stream of which 22% is deposited on bone, 12% in kidneys and rest excreted. The uranium deposited in kidneys is excreted within few days. The mean U concentration in drinking water detected in some SU cities is 2.55 micro-g/L
WHO threshold value to prevent sub-clinical renal effects is 0.002mg/L.
The chemical toxicity value for soluble salts of uranium is 0.003mg /kg-day. This is the highest dose that can be taken every day, over a life time without causing adverse health effects. This is generated based on animal studies and normalized to humans using an uncertainty factor of 1000. Lowest observed adverse health effect is at an intake level of soluble uranium of 3mg/kg-day. Daily intake of uranium by adults (60kg) through air, water and food is 2.2 micro-g and total body burden of uranium in humans is 40 micro-g. At equilibrium, the excretion rate through urine is 4.4 micro-g per day.
In alkaline conditions in the kidney, uranium hydrogen carbonate complex is most stable and hence uranium is excreted out from the kidney. In acidic conditions the uranium (as uranyl ion) gets deposited in the tubular wall in the kidney. The most common renal injury caused by uranium in experimental animals is damage to the proximal convoluted tubules in kidney. Nephritis is the primary induced effect of uranium in humans.
The threshold uranium concentration for chemical toxicity is 1 micro-g/g of kidney tissue. For protection of the public, further safety factor of 10 is applied and values in the range of 0.1 to 0.3 micro-g/g of the tissue are reported as threshold limits for public by some authors.
Thursday, March 31, 2011
Monday, March 28, 2011
Physical Barriers to Environmental Release of radioactive releases in case of reactor accidents: Defense-in-Depth concept
The following five barriers are built into the reactor design to prevent radioactivity escaping from the reactor to the public:
1. Ceramic Fuel - The ceramic uranium dioxide fuel in pellet form entraps most of the fission products generated due to the fission reaction in the fuel. These fission products would be released if the fuel were to melt. The fuel has a high melting point, but continuous cooling is required so that the fuel is not over-heated resulting in melting of the fuel. Another safety feature of the ceramic fuel is that it is relatively chemically inert with the heavy water coolant.
2. Fuel Sheath (cladding) - The fuel pellets are enclosed in a high integrity, welded zircalloy sheath. This sheath contains the gaseous and volatile fission products which escape from the pellets. The sheath is designed to withstand the stresses resulting from pellet thermal expansion, gaseous fission product build-up and external hydraulic pressure.
3. Heat Transport System Boundary - The high integrity pressure tubes, piping, and vessels contain most fission products escaping via sheath defects until they are removed via the coolant purification system.
4. Containment Boundary - This is designed to withstand the pressure surge of a worst case LOCA, with a small ‘puff release’ during the overpressure transient. Post LOCA containment venting via a filtered, monitored pathway minimizes the environmental radioactive release.
5. Exclusion Zone - No permanent residence is allowed within a 1.6 km radius from nuclear reactor. This ensures significant dilution of an airborne radioactive release before it reaches any public habitation, thus reducing the resulting public dose.
Inadequate fuel cooling due to cooling system failure, the situation which is prevailing now at Fukushima nuclear power reactor in Japanese, results in overheating of the fuel, with potential for large scale fuel failures. In the event of large scale fuel failures, at least two of the five physical barriers would be breached, i.e., the fuel and the fuel sheath. In the case of a LOCA, the third barrier, the heat transport system is also breached, leaving only the containment and exclusion zone barriers.
In the case of a LOCA coincident with containment failure (dual failure), only the exclusion zone would remain as a physical barrier. Thus, the Containment boundary is a very strategic defense-in-depth barrier to fission product release in to the public domain. .
1. Ceramic Fuel - The ceramic uranium dioxide fuel in pellet form entraps most of the fission products generated due to the fission reaction in the fuel. These fission products would be released if the fuel were to melt. The fuel has a high melting point, but continuous cooling is required so that the fuel is not over-heated resulting in melting of the fuel. Another safety feature of the ceramic fuel is that it is relatively chemically inert with the heavy water coolant.
2. Fuel Sheath (cladding) - The fuel pellets are enclosed in a high integrity, welded zircalloy sheath. This sheath contains the gaseous and volatile fission products which escape from the pellets. The sheath is designed to withstand the stresses resulting from pellet thermal expansion, gaseous fission product build-up and external hydraulic pressure.
3. Heat Transport System Boundary - The high integrity pressure tubes, piping, and vessels contain most fission products escaping via sheath defects until they are removed via the coolant purification system.
4. Containment Boundary - This is designed to withstand the pressure surge of a worst case LOCA, with a small ‘puff release’ during the overpressure transient. Post LOCA containment venting via a filtered, monitored pathway minimizes the environmental radioactive release.
5. Exclusion Zone - No permanent residence is allowed within a 1.6 km radius from nuclear reactor. This ensures significant dilution of an airborne radioactive release before it reaches any public habitation, thus reducing the resulting public dose.
Inadequate fuel cooling due to cooling system failure, the situation which is prevailing now at Fukushima nuclear power reactor in Japanese, results in overheating of the fuel, with potential for large scale fuel failures. In the event of large scale fuel failures, at least two of the five physical barriers would be breached, i.e., the fuel and the fuel sheath. In the case of a LOCA, the third barrier, the heat transport system is also breached, leaving only the containment and exclusion zone barriers.
In the case of a LOCA coincident with containment failure (dual failure), only the exclusion zone would remain as a physical barrier. Thus, the Containment boundary is a very strategic defense-in-depth barrier to fission product release in to the public domain. .
Thursday, March 24, 2011
Contamination of foods and feedstuffs by the nuclear disaster
The releases from the damaged reactors are contaminating food and feedstuffs in the nearby areas in Japan. Radioactive iodine will be seen quickly in the milk. As most of the radioactive particles may end up in the ocean it is imperious to avoid sea foods of any kind. Water sources are likely to be contaminated. The Japan government declared that the tap water in Tokyo is contaminated with radioactive iodine and is unsafe for infants.
Continuous monitoring for radioactivity of the food products, milk and drinking water is being done to ensure that the radiation exposure of the public does not exceed the acceptable limits prescribed by the WHO. Since some of the radioactive isotopes released are having half lives in years, the food chain safety will be compromised in the next decades by the situation in Fukushima.
The WHO guideline levels in drinking water, for the most predominant radio-nuclides, Iodine-131 and Cesium-137, in the releases from the reactors are: 0.01 Bq/mL. These guideline levels of radionuclides in drinking water were calculated on the basis of an annual dose criterion of 0.1 mSv (10mR) from drinking 2 liters of water per day. The average natural background radiation to which we are all exposed is 2 mSv in a year.
The International Atomic Energy Agency (IAEA) has also been monitoring the environment for radioactivity and radiation levels at the nearby areas from the Fukushima plant in Japan. As per IAEA update, the dose-rate results ranged from 0.8 to 9.1 micro-Sievert per hour. The beta-gamma activity contamination measurements ranged from 0.08 to 0.9 MBq per square metre.
The Agency continues to receive data confirming high levels of radioactivity (above permissible limits) in food, notably spinach, in samples taken from 37 locations in the vicinity of five cities south of the Fukishima site. Higher levels of both Iodine-131 and Caesium-137 have been measured by the Japanese authorities in milk, water, in spinach and some other fresh vegetables. In view of this, it is reported that distribution of food from the areas affected has been restricted. The Japanese authorities are monitoring the situation in the rest of the country.
Further radioactivity releases from the stricken reactors in to the environment should be controlled somehow to keep the public exposures as low as possible.
Continuous monitoring for radioactivity of the food products, milk and drinking water is being done to ensure that the radiation exposure of the public does not exceed the acceptable limits prescribed by the WHO. Since some of the radioactive isotopes released are having half lives in years, the food chain safety will be compromised in the next decades by the situation in Fukushima.
The WHO guideline levels in drinking water, for the most predominant radio-nuclides, Iodine-131 and Cesium-137, in the releases from the reactors are: 0.01 Bq/mL. These guideline levels of radionuclides in drinking water were calculated on the basis of an annual dose criterion of 0.1 mSv (10mR) from drinking 2 liters of water per day. The average natural background radiation to which we are all exposed is 2 mSv in a year.
The International Atomic Energy Agency (IAEA) has also been monitoring the environment for radioactivity and radiation levels at the nearby areas from the Fukushima plant in Japan. As per IAEA update, the dose-rate results ranged from 0.8 to 9.1 micro-Sievert per hour. The beta-gamma activity contamination measurements ranged from 0.08 to 0.9 MBq per square metre.
The Agency continues to receive data confirming high levels of radioactivity (above permissible limits) in food, notably spinach, in samples taken from 37 locations in the vicinity of five cities south of the Fukishima site. Higher levels of both Iodine-131 and Caesium-137 have been measured by the Japanese authorities in milk, water, in spinach and some other fresh vegetables. In view of this, it is reported that distribution of food from the areas affected has been restricted. The Japanese authorities are monitoring the situation in the rest of the country.
Further radioactivity releases from the stricken reactors in to the environment should be controlled somehow to keep the public exposures as low as possible.
Tuesday, March 22, 2011
Radiation dose control from releases from “damaged” nuclear reactors
The recent nuclear crisis in Japan where four nuclear power reactors were damaged due to earthquake has raised questions with respect to the public exposures. In such accidental situations, the atmospheric releases from the damaged reactor core are mainly radio-nuclides in vapour form, such as of Iodine. Let us take an example of radioactive Iodine isotope, I-131.
The half-life - the time required for the radioactivity to reduce by one-half - for I-131 isotope is 8 days. Since the half life is considerably long, the isotope can travel long distances along the direction of the wind. On the way, the iodine isotope gets deposited on the soil, on water surfaces and on grass and “contaminates” them. Human consumption of such contaminated items results in radiation dose to the exposed population. Biologically, the thyroid hormone contains iodine in stable (not radioactive) form. If one is exposed to radioactive iodine, it will get uploaded into the thyroid and gets deposited there, giving radiation dose. Prophylaxis is a process where the thyroid is deliberately saturated by “stable” iodine and hence the uptake of “radioactive” iodine from the releases is minimized. A tablet of Potassium iodide/iodate (around 130 mg) is consumed, just before or just after the releases, for “blocking” the thyroid from radioactive iodine uptake.
In general, staying indoors, use of proper respirators and consumption of stable iodine tablets (if advised), reduces exposure of people from the atmospheric releases of radioactive isotopes/materials.
Public needs to be sensitized on this aspect for their protection.
The half-life - the time required for the radioactivity to reduce by one-half - for I-131 isotope is 8 days. Since the half life is considerably long, the isotope can travel long distances along the direction of the wind. On the way, the iodine isotope gets deposited on the soil, on water surfaces and on grass and “contaminates” them. Human consumption of such contaminated items results in radiation dose to the exposed population. Biologically, the thyroid hormone contains iodine in stable (not radioactive) form. If one is exposed to radioactive iodine, it will get uploaded into the thyroid and gets deposited there, giving radiation dose. Prophylaxis is a process where the thyroid is deliberately saturated by “stable” iodine and hence the uptake of “radioactive” iodine from the releases is minimized. A tablet of Potassium iodide/iodate (around 130 mg) is consumed, just before or just after the releases, for “blocking” the thyroid from radioactive iodine uptake.
In general, staying indoors, use of proper respirators and consumption of stable iodine tablets (if advised), reduces exposure of people from the atmospheric releases of radioactive isotopes/materials.
Public needs to be sensitized on this aspect for their protection.
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