Study On Optimization Control Of Radioactive Effluent Discharge To The Environment From Nuclear Facilities

According to the Basic Safety Standards for Protection against Ionizing Radiation and for the Safety of Radiation Sources, radiation and radioactive substances are natural and permanent features of the environment, and thus the risks associated with radiation exposure can only be restricted not eliminated entirely. The basic principles of radiation protection and safety can be summarized as that the practice must be justified, protection and safety should be optimized, and individual doses due to the combination of exposures from all relevant practices should not exceed specified dose limits. In 1995, the IAEA published the safety fundamentals entitled "Principles of Radioactive Waste Management". The application of principles on radioactive waste management requires the implementation of measures that will afford protection of human health and the environment, since improper management of radioactive waste could result in adverse effects on human health or the environment, now and in the future.Radioactive waste management means the whole sequence of operations starting with the generation of waste and ending with disposal. The waste disposal covers the discharge of effluents and solid waste disposal. Waste disposal strategies can be divided into two conceptual approaches: "Dilute and Disperse" or "Concentrate and Retain". Early or deferred releases of radionuclides to the environment would inevitably result from either of these strategies and therefore an objective of no release is not feasible. Both strategies are in common use and are not mutually exclusive.The control of radioactive effluent discharge to the environment is an important content of waste management. Radioactive effluent discharge to the environment is early release of radioactive waste in the way of "Dilute and Disperse". Two aspects should be paid attention in the study on the discharge of effluents to the environment, e.g. optimization control of radioactive effluent discharge and monitoring of important nuclides in radioactive effluents. From sustainable development point of view, it is not feasible for "zero discharge" of radioactive effluent from nuclear installations to the environment, because in such way the treatment process on liquid/gaseous waste will greatly increase so that result in sharply increase of quantity of radioactive solid waste. The disposal of thesolid waste will become more difficult with the increase of solid waste quantity and it will increase possibility of potential exposure hazard to future generation. Meanwhile, it is also not feasible for excessive discharge of radioactive effluents to the environment, because this will result in additional radiation exposure to public. Therefore, it is necessary to conduct a study on optimization control of radioactive effluent discharge to the environment based on the optimization principle of radiation protection. And monitoring on important radionuclides is very important for control of radioactive effluent discharge. Up to now, measurement for 131I during its production is not sufficient, such as no monitoring is conducted for iodine in different forms that include element iodine, organic iodine and aerosol iodine.This study is divided into two parts. The first part emphases the analysis and calculation, which includes a study methodology of optimization control on liquid effluent discharge to the environment in nuclear facilities. The initial step in optimizing is to ensure that the individual doses to the critical group due to the discharges anticipated, with the control options considered, comply with the dose constraint. Any control option that does not satisfy this condition would normally be excluded from the optimization process. The optimized dose target value on liquid effluent discharge, which is the dose to individuals of the critical group from optimization analysis, is analyzed and determined. The author selected Qinshan Nuclear Power Plant as an example to calculate its annual optimized discharge quantity in Bq/a and concentration of liquid effluents in Bq/M3. The basis of optimization of radioactive waste disposal is that generation of radioactive waste shall be kept to the minimum practicable. The second part of the study focuses on experiment and measurement on several issues of gaseous effluent discharge, in which the production and application of 131I are selected as example to discuss and analyze the instrumentation and measurement method on gaseous discharge.Concerning the optimization control of radioactive liquid effluent discharge, the author establishes the methodology to determine optimized target dose value on liquid effluent discharge, and analyzes various factors to affect optimization control of the liquid effluent discharge and gives all related parameters used for individual dose calculation to critical group. The engineering judgment and formal decision aiding techniques e.g. multi-criteria methods, are used in the optimization process. Eight options supposed ondifferent release of radioactivity are considered and the optimized individual doses to the critical group are calculated in different impact weighting factors applying multi-criteria methods. Protection is then optimized by choosing, from among eight options which satisfy the dose constraint condition, the one for which radiation doses are as low as reasonably achievable, economic and social factors being taken into account. In order to analyze the influence of weighting factors, the curves of optimized value of individual dose to total functional value for option i, Ui, are linear regressed by minimum square method. And the average optimized dose target value is calculated using geometry average calculation method. The author analyzes and recommends the value of optimized target dose value for liquid effluent discharge to the environment in Chinese nuclear facilities. Qinshan NPP is taken as an example, its annul quantity of liquid effluent to be discharged and release concentration are calculated based on above optimized target dose value. During the optimization analysis, the engineering judgment is also used.Concerning radioactive gaseous effluent discharge control, some issues related monitoring release of 131I during production and use of iodine are discussed. The production and application of 131I in China is investigated. Two experimental plans for iodine production and use are designed. One experiment is to investigate present situation of 131I production lines, its technical design and waste gas treatment, and to sample the gaseous radioactive iodine effluents for measurement under normal operation in the 131I production lines in Institute of Atomic Energy of China. The investigation includes production process, daily production capacity, production cycle, gaseous waste treatment facility and operation situation in the line. The environmental aerial sample monitoring results arising from 131I products production, loading, packaging process is also studied. Another experiment is to investigate radioactive iodine application situation and 131I waste treatment in Beijing Union Medical College Hospital. Meanwhile, the continuing sampling and measurement are carried out in discharge points of relevant ventilation cabinet or glove box under the hospital normal operation.The main conclusions and recommendations are as follows:(1) The radioactive waste disposal covers both liquid/gaseous effluent discharge to the environment and solid waste disposal. Early or deferred releases of radionuclides to the environment would inevitably result from either of these two ways and therefore anobjective of no release is not feasible. These two strategies, e.g. effluent discharge to the environment and solid waste disposal, are in common use and are not mutually exclusive. A comprehensive consideration based on optimization principle should be conducted in order to ensure the safety of human health and the environment, now and in the future. The whole process management, including adopting necessary engineering and technical measurements, should be implemented to ensure the minimum of the generation of radioactive liquid waste. In this condition, the optimized control of radioactive liquid effluent discharge from nuclear facilities to the environment is more effective and efficient.(2) Optimization of radiation protection is process that is at the heart of a successful radiological protection program. It is forward-looking, aimed at preventing unnecessary exposures before they occur. It is ongoing and iterative, taking into account both technical and social-economic developments, and requires both qualitative judgment and quantitative analysis. Thus, while quantitative methods provide an input to optimization, they should never be the sole input given the many qualitative factors. The dose constrain is the upper limit of optimization process of radiation protection, stakeholder involvement is the important input of decision making process. And nuclear safety culture is also a very important content of optimization of radiation protection.(3) The study propose that, based on optimization principle of radiation protection, the optimized target dose value of radioactive liquid effluent discharge to the environment in Chinese nuclear facilities be Hop= 30 μ Sv/a.(4) Based on analysis of sea water dilution in Qinshan area and inventory of discharge in Qinshan NPP, it is calculated that total annual optimized discharge quantity of radioactive liquid effluent for Qinshan area is 9.62 × 1011 Bq/a. And total annual optimized discharge quantity of radioactive liquid effluent for Qinshan NPP is Qop=107 GBq/a, corresponding discharge concentration is 4 MBq/m3, and discharge concentration constrain is 20 MBq/m3. Subsequently, individual dose to critical group arising from radioactive liquid effluent discharge to the environment in Qinshan NPP, with annual maxium liquid discharge 1 × 104m /a at concentration 4 MBq/m3, is 1.2 μ Sv/a. And with annual average liquid discharge 5500 m3/a, individual dose is about 0.6 μ Sv/a.(5) To strengthen the monitoring on important nuclides of radioactive effluent is very important. Based on the investigation and monitoring during 131I production in CIAE, the annual production of 131I product is 4.5×1013 Bq/a, the daily iodine discharge is 6.56 × 107 Bq, in which, the aerosol iodine is 1.38× 105Bq, the element iodine is 5.01 × 105Bq, the organic iodine is 6.51 × 107Bq, the organic iodine contribute 99%. The monitoring results show that the discharge rate of gaseous iodine effluent during production is 0.016%. The annual iodine discharge from gaseous effluent in CIAE is 7.2×109Bq/a.According to the statistics of iodine isotope production from present factories, the production capability is 1.11 × 10 14Bq in the Chinese Nuclear Gaotong Isotope Company Co.Ltd., and 4.5 × 1013 Bq in CIAE. The annual iodine production in China is 1.56 × 10 14 Bq. It is estimated that total 131I industrial gaseous discharge is 2.5×1010Bq/a if the discharge rate of gaseous iodine effluent to environment is 0.016%.(6) The investigation and monitoring from the Beijing Union Medical College Hospital shows that most of hospitals purchase 131I capsule from special isotope companies and store them in the special pharmacy. Generally, there is no environmental monitoring program for 131I discharge in hospital. During use of 131I capsule by the patients in the hospital, the gaseous 131I will be discharged into environment. However, the impact from gaseous 31I during use of the capsule are small that can be neglected.In order to strengthen the optimization control for radioactive effluents discharge to the environment, and strengthen the monitoring for important nuclides in radioactive effluents, the following issues need to be paid more attention in future. Firstly, the research and development work on radioactive waste minimum should also be conducted. Secondly, it is important to develop research work on optimization control of gaseous effluent discharge from nuclear facilities and liquid effluent discharge from other nuclear reactors at river. The methodology and project design may refer to this study. Thirdly, the discharge of radioactive liquid effluents must comply with the principle of controlling both total discharge quantity and discharge concentration. Fourthly, the effluent monitoring (or source monitoring) and environment monitoring should be implemented and improved by both operating organizations and regulatory body. Lastly, the study on optimization of radiation protection in medical service should be developed based on the problems inhospitals on 131I application. Some issues should be paid more attention, such as exposure to the family members and persons who provide medical service. And patient excretes should be collected and keep for decay.