Shielding Calculation And Methods Evaluation For Normal γ Sources

In industry,agriculture,medicine and other civilian applications of nuclear technology,theγradiation sources which may lead to great radioactive accidents include: 60Co (cobalt 60),137Cs (Cs 137),192Ir (iridium 192); 60Co are widely used in food irradiation processing, radiation therapy and non-destructive testing; 137Cs is mainly used for food irradiation processing, radiation therapy, and industrial measurement; 192Ir is mainly used for industrial flaw detection and install treatment. These sources have different applications and activities. The activity of the sources used for radiation processing is large,while the activity of the sources used for install treatment, short-distance radiotherapy is relatively small. However, the pollution of the sources on environment and the action to the body are very strong. The effective radiation shielding is needed to ensure that the radiation dosages accepted by the staff and closed public must be under the dose limit. It is increasingly important to find out how to evaluate the radiation protective effect of commonγsources fast and accurately, and how to determine the way of radiation protection fast and accurately in response to nuclear terrorism, nuclear accidents and other unexpected events.The paper has compiled the parameters for shielding calculation from publications of ICRP and other references. It makes the fast calculation of security shield thickness for commonγsources by using the empirical formula achieved. Aiming at the three common sources, including high activity 60Co, medium activity 192Ir and low activity 137Cs,to establish the storage,usage and transportation scenarios,we calculated the security thickness of radiation shielding materials used (lead,concrete,iron,water) by empirical formula. This paper applied the MCNP simulation to test and evaluate the protective effect contrasting with the empirical formula , and put forward optimization recommendations for protection programs. It provides references for ensuring the annual dose of the staff and the public below the does limit with the lowest cost, and coping with events which may occur or emergency response after events.For the calculation of three scenarios: well storage,lead shielding and protective concrete walls of high activity 60Co sources,relative to the MCNP simulated values,if we use the semi-reduced thickness method to calculate the thickness,the dose equivalent rate of the observation point is above the national limit, and in this case the protection is not valid. If we use multiple attenuation method to calculate the thickness,the dose equivalent rate of the observation point is below the national limit,and in this case the protection is valid. Compared with MCNP simulating values, the minimum safe thickness of water and lead shields has minor deviation and we can directly use this method in fast calculation. For the minimum safe thickness of concrete shielding, compared with MCNP simulated values, the error of calculated value is larger,so it is necessary to correct the corresponding coefficient.For the calculation of two scenarios: lead shielding and protective concrete walls of medium activity 192Ir sources,we consider the shielding of the work area, relative to the MCNP simulated values. If we use the semi-reduced thickness method to calculate the thickness,the dose equivalent rate of the observation point is above the national limit, and in this case the protection is not valid. If we use multiple attenuation method to calculate the thickness,the dose equivalent rate of the observation point is below the national limit,and in this case the protection is valid. For the shielding of the public area, the thickness calculated by multiple attenuation method and the semi-reduced thickness method is both larger than the minimum safety value,so the protection is valid. Comparing the two methods,in accuracy, the error of the thickness calculated by the semi-reduced thickness method is less than that of multiple attenuation method. The thickness of lead shielding protection calculated by the two methods both meets the safety conditions, but larger error. Therefore it is necessary to correct the corresponding coefficient.For the calculation of two scenarios: lead shielding and iron shielding of low activity 137Cs sources, relative to the MCNP simulated values,for iron shielding,If we use the semi-reduced thickness method to calculate the thickness,the dose equivalent rate of the observation point is above the national limit, and in this case the protection is not valid. If we use multiple attenuation method to calculate the thickness, the dose equivalent rate of the observation point is below the national limit, and in this case the protection is valid. For lead shielding,the thickness calculated by methods is both larger than the minimum safety value. The dose equivalent rates of observation points are both below the national limit,so the protection is valid. Comparing the two methods, the error of multiple attenuation method is less,so this method can be used in fast calculation directly.