Despite rigorous safety systems in a nuclear reactor, there remains a finite probability that an accident can occur that can lead to the fuel in the core overheating or melting. If such an event were to occur, there is a chance that radioactive fission products may be released to the environment. The potential radiation exposure of the population will be influenced by many parameters such as: the amounts of the radionuclides released, the meteorological conditions affecting the dispersion and deposition of the released radioactive material, human and environmental factors and the effectiveness of any protective actions taken.
Isotopes of iodine particularly I-131, is likely to be important components of the release from a severe accident. Radioactive iodines can give rise to both external exposure and internal exposure (from inhalation and ingestion). Stable iodine prophylaxis is a protective action, for which preparedness arrangements can be made as part of the overall emergency response plan. This step can protect specifically against internal exposure from inhalation, and ingestion of radioiodines by consumption of I-131 contaminated milk/milk products. It should be noted that the term “iodine prophylaxis” refers to the blocking of the uptake of radioiodine by the thyroid gland after nuclear accidents.
The selective and rapid concentration and storage of radioactive iodine in the thyroid gland results in internal radiation exposure of the thyroid, Deterministic effects from thyroid exposure are hypothyroidism and acute thyroiditis. Stochastic effects from thyroid exposure are thyroid cancer and benign thyroid nodules.
As per the WHO document on iodine prophylaxis (update-1999), it recommended that in the management of nuclear reactor accidents, stable iodine prophylaxis for children up to the age of 18 years and for pregnant and lactating women be considered at 10 mGy dose, that is 1/10th of the generic intervention level dose of 100mGy for adults recommended in the IAEA International Basic Safety Standards for Protection against Ionizing Radiation.
The recommended single dosage of stable iodine for prompt administration for adults over 12 years is 130 mg of KI or 170 mg of KIO3, for children (3 to 12 years), the dose recommended is 65mg of KI and 85mg KIO3 and for infants (1 month to 3 years) the dose is 32 mg of KI and 42 mg of KIO3.
Tuesday, March 29, 2011
Monday, March 28, 2011
Loss-of-Coolant Accident (LOCA) – Nuclear reactors
A loss-of-coolant accident (LOCA) is a mode of failure for a nuclear reactor. If not managed properly and effectively, the results of a LOCA could result in reactor core damage. In every nuclear reactor, a separate Emergency Core Cooling System (ECCS) exists specifically to deal with the situation like LOCA.
Nuclear reactors generate heat internally in the fuel by the fission reaction, neutron with a fissile material uranium-235. This heat is removed by a coolant system to produce steam and is converted into useful electrical power. If this coolant flow is reduced, or lost altogether, the nuclear reactor's emergency shutdown system is designed to stop the fission chain reaction automatically. However, even after reactor shut down, due to radioactive decay of the fission products, the nuclear fuel will continue to generate a significant amount of heat. This decay heat needs to be taken out through secondary cooling system to maintain integrity of the fuel. If all of the independent cooling systems of the ECCS fail to operate as designed due to some reason such as failure of the pumps, this heat can increase the fuel temperature to the point of damaging the fuel and the reactor.
Damage to the reactor containment will result in the radioactive releases from the reactors. Iodine-131 and Xe-133 have half life of 8 days and 5.2 days and hence are detected in the environment in such situations. Depending upon the wind direction and speed, the radioactive isotopes will travel in dispersed form to large distances.
Nuclear reactors generate heat internally in the fuel by the fission reaction, neutron with a fissile material uranium-235. This heat is removed by a coolant system to produce steam and is converted into useful electrical power. If this coolant flow is reduced, or lost altogether, the nuclear reactor's emergency shutdown system is designed to stop the fission chain reaction automatically. However, even after reactor shut down, due to radioactive decay of the fission products, the nuclear fuel will continue to generate a significant amount of heat. This decay heat needs to be taken out through secondary cooling system to maintain integrity of the fuel. If all of the independent cooling systems of the ECCS fail to operate as designed due to some reason such as failure of the pumps, this heat can increase the fuel temperature to the point of damaging the fuel and the reactor.
Damage to the reactor containment will result in the radioactive releases from the reactors. Iodine-131 and Xe-133 have half life of 8 days and 5.2 days and hence are detected in the environment in such situations. Depending upon the wind direction and speed, the radioactive isotopes will travel in dispersed form to large distances.
Monday, March 7, 2011
ICRP Publication 111, 2009
In this document, the International Commission on Radiological Protection (ICRP) provides guidance on “Application of the Commission's Recommendations to the Protection of People Living in Long-term Contaminated Areas after a Nuclear Accident or a Radiation Emergency” for the protection of the people considering effects of: the pathways of human exposure, the types of exposed populations, and the characteristics of exposures. Although, the focus is on radiation protection considerations, the report also recognizes the complexity of post-accident situations, which cannot be managed without addressing all the affected domains of daily life, i.e. environmental, health, economic, social, psychological, cultural, ethical, political, etc. The report explains how the ICRP-103 (2007) Recommendations apply to this type of existing exposure situation, including consideration of the justification and optimization of protection strategies, and the introduction and application of a reference level to drive the optimization process.
The report also considers practical aspects of the implementation of protection strategies, both by authorities and the affected population. The role of radiation monitoring, health surveillance and the management of contaminated foodstuffs and other commodities is described. The Annex summarizes past experience of long term contaminated areas resulting from radiation emergencies and nuclear accidents, including radiological criteria followed in carrying out remediation measures (Extracted for the ICRP site).
The report also considers practical aspects of the implementation of protection strategies, both by authorities and the affected population. The role of radiation monitoring, health surveillance and the management of contaminated foodstuffs and other commodities is described. The Annex summarizes past experience of long term contaminated areas resulting from radiation emergencies and nuclear accidents, including radiological criteria followed in carrying out remediation measures (Extracted for the ICRP site).
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