First, the role of radiochemical analysis in the field of radiation protection
In 1896, the French scientist Becquerel discovered that uranium compounds spontaneously emit radiation that can penetrate the black paper to make the film sensitive, thus discovering the natural radioactivity for the first time and bringing the world into the atomic energy era. Only two years later, the Curies discovered strontium and radium, which are more active than uranium, from uranium ore. They found that radium can emit three rays of alpha, beta and gamma. It can be said that radiochemistry was born with the discovery of radioactivity, and it also grew with the development of the atomic energy business. Radiochemistry is a branch of chemistry that is derived from the characteristics of radionuclides (very small amounts, spontaneous decay, radiation, and radiation protection). Because radionuclide is completely the same as its own elements except for its radiological characteristics, radiochemical analysis is essentially a branch of elemental analysis chemistry, that is, separation and purification by analytical chemistry to determine the radionuclide activity concentration or specific activity, just use Radiation is measured and attention should be paid to low concentrations and protection against radiation. Radiation protection is the protection of the harmful effects of radiation on the human body in the application of radioactive sources and radionuclides. Radiochemical analysis plays an important role in the development of radiation protection, and even in most areas of the atomic energy business.
The development of the atomic energy industry in the world (usually for nuclear fuel refining and nuclear testing or nuclear power facility research) has promoted the formation and development of radiochemical analysis for the separation and purification of radionuclides, the testing of content and the monitoring of workers and public safety. The research of the Health and Nuclear Safety Sector is primarily aimed at the establishment and research needs of radionuclide monitoring methods in environmental and human-related biological samples. The nuclear weapons research program in the early days of developed countries (such as the Manhattan Project of the United States) included the necessary research on corresponding radiochemical analysis methods. Radionuclides have been recognized as a class of harmful substances. Developing countries (including China) in the 1950s and 1960s were trying to detect the impact of the nuclear test of two superpowers on the country, and later to understand and respond to the possible impact of the radioactive falloff of the Chernobyl nuclear accident in the former Soviet Union on the country. The national atomic energy application monitors the radioactive contamination (mainly food, water and air) caused by workers and the public and the environment, in accordance with the International Radiological Protection Committee (ICRP) Basic Recommendations [1-3] . 1 The sanitary standards for environmental, food, drinking water and atmospheric radionuclide limits and the corresponding supporting test method standards have been formulated. For example, in the early 1960s, I was assigned to set up a national radioactive background monitoring center headed by Zhu Changshou (later transferred to the Beijing Industrial and Health Center). Under the leadership of the corresponding department of the Ministry of Health, the radioactive background of each province, city and autonomous region was uniformly arranged. The monitoring work of the monitoring station and the statistical analysis of the national results were carried out. The initial aim was to monitor the US and Soviet nuclear tests, and later transferred to the national data on the national trends and pollution levels of radioactive aerosols and sediments in China's previous nuclear tests [4] . In the early 1970s, in view of the need for “no radioactive pollution†visas for international trade in food, the Ministry of Health commissioned the task of establishing national standards for the concentration and testing methods of food radioactivity in China. In 1994, we were responsible for the development of the "Limited Concentration Standards for Radioactive Materials in Foods" (GBl4882-1994) [5] and "Tests for Radioactive Substances in Foods" (GB/T 14883.1-1994-GB/T 14883.10-1994) [6] . Later, it was supplemented with the “Limited 241 Limits in Foods†and “Determination of Radioactive Substances in Foods 241†(WS/T234-2002), which was released in 2003 [7] . At present, these standards are absorbing the development of basic standards for radiation protection and the progress of relevant inspection methods since implementation, and comprehensively revising the problems found, and strive to provide the regulatory authorities with advanced and as far as possible international standards. Author: Chinese Academy of Medical Sciences, Chinese Academy of Medical Sciences Institute of Radiation Medicine, Tianjin 300192, China Author: Zhu Hongda (1940), male, Suzhou, Jiangsu, researcher, main research direction: radiological health and radiochemical analysis. With fast, reasonably applicable activity concentration limits and corresponding test method standards.
Second, the main method of radiochemical analysis
Radionuclides usually coexist in the environment with their radioactive parent, daughter, other radioactive or stable nuclides. Radiochemical analysis is mainly used in nuclear fuel production, extraction and recovery, radionuclide and radioactive source preparation, radiolabeled compounds and nuclear drug synthesis. With the determination of radionuclides in production, environmental and biological samples, it is first necessary to separate, concentrate, and purify the radionuclide to prepare sample preparation. Radiochemical analysis mainly uses the following main methods.
(1) Coprecipitation method
The coprecipitation method is a method for separating, enriching and purifying nuclide or trace elements by using a trace substance to form a precipitate with a constant substance. It has been used in the development of atomic energy science and has played an important role, but due to poor separation efficiency, The advantages of low chemical recovery rate, large amount of waste liquid, cumbersome operation, and difficulty in continuous automation of the production process are gradually replaced by solvent extraction and chromatography in industrial scale production. It is still widely used in environmental and biological samples for radionuclide analysis and wastewater treatment. Measuring environmental and biological samples, for example, in "60 Co stable cobalt used as a carrier, as precipitant potassium nitrite, potassium cobalt nitrite generated carrier tape precipitation, the concentration of trace 60 Co, the precipitate was further purified measurable" 60 Coβ - a radioactivity. Aluminum, iron hydroxide or phosphate coprecipitation adsorbing radioactive waste and effective method for purifying contaminated water. In addition, some drug-releasing drugs are also based on the principle of co-precipitation. For example, ferrocyanide and co-precipitation of radioactive cesium in vivo are often used to prevent the absorption of 137 Cs in the body.
(2) Solvent extraction method
Solvent extraction is the separation of the various components dissolved in one liquid phase (mostly the aqueous phase) from the difference in the partition coefficient of another immiscible solution phase (such as the organic phase). The method for separating these components is simple and rapid, and is particularly suitable for short-lived radionuclide separation; high selectivity and recovery, good separation effect, can be used for preparing carrier-free radioactive materials and effectively separating trace radionuclides from a large amount of impurities; The device is simple and easy to operate, and it is easy to realize continuous operation and remote automatic control in industrial production; there are many extracting agents available. Moreover, it is also possible to synthesize an extractant having excellent performance according to requirements. The disadvantage is that most organic solvents are volatile, flammable and toxic, and should be used safely. The extractant is usually expensive and difficult to recycle. This method has one of the most commonly used separation methods for nucleation fuel production and radionuclide separation and extraction. For example, analysis of trace amounts of uranium in environmental water is usually carried out by TBP as an extractant, back extraction with uranium reagent III aqueous solution, and spectrophotometric determination.
(three) chromatography
Chromatography used to be called chromatography, chromatography or ion exchange. The components are separated by the difference in affinity between the stationary phase and the mobile phase. When the mobile phase continuously flows through the stationary phase, the components are distributed multiple times in two phases, allowing the differential affinity to accumulate so that the components can be sufficiently separated. The method has high selectivity, good separation effect, and can achieve satisfactory separation and high recovery rate for similar elements, which is particularly useful for concentration and extraction of trace elements and separation of carrier-free radionuclides; simple and convenient for long-distance operation And protection. Disadvantages are slower flow rate, longer separation time; smaller ion exchange capacity; some ion exchangers have poor thermal stability and radiation stability, which limits the application. Currently, ion exchange chromatography is the most widely used in radiation protection. Ion exchangers can be broadly classified into inorganic and organic ion exchangers, each of which is natural and synthetic. Synthetic inorganic cation exchangers, ammonium phosphomolybdate (AMP) commonly used in the separation of cesium. The most widely used is the synthetic organic ion exchanger (ie ion exchange resin).
(4) Electrochemical separation method
It is a general term for various separation methods that utilize differences in the electrochemical properties of elements. Separation is only possible if the two electrodes are separated by a large difference in potential. therefore. As a separation method, its application is not extensive, but radioactive sources are often prepared by electrochemical separation. Commonly used are electrochemical replacement methods, electrolytic deposition methods and paper electrophoresis methods. Electrochemical displacement method is the use of substances to be separated to separate ions spontaneous redox reaction at the electrodes, when the electrode potential of the metal below the element to be separated is higher than the reduction potential of Li solution reduction potential impurity element, an element can be separated Electrochemical displacement reaction spontaneously with the metal electrode precipitates on the surface of the electrode. The reactor was prepared from 239 Po bismuth irradiation target may use copper as an electrode sheet so as to separate the target material cast 209 Bi: Cu just below the standard electrode potential than bismuth and polonium. The electrolytic deposition method is a separation method in which an ion to be separated is subjected to a redox reaction (ie, electrolysis) on an electrode under the action of an external voltage. The basic principle is that the appropriate applied voltage is selected so that the potential of the cathode electrode is lower than the critical deposition potential of the metal ion to be separated and higher than the critical deposition potential of the impurity ion, so that the metal ions to be separated are selectively precipitated on the electrode to be separated. At present, electrolytic deposition is mainly used for the preparation of radioactive film sources. Paper electrophoresis is a method of electrophoresis using paper as a support. Different ions or charged particles are used to separate the migration direction and velocity on the paper impregnated with electrolyte under the action of J, bjJn electric field. It has fast, simple and separation effect. Good advantages, suitable for the separation and identification of trace radionuclides.
Third, the application of radiochemical analysis in China's nuclear energy development and radiation protection monitoring
(1) Leaching, concentration, separation and purification of nuclear fuel
Uranium and thorium are natural nuclear fuels, and initial nuclear energy development was first used for nuclear weapons testing military and nuclear energy production purposes. As fossil fuels continue to be consumed. Nuclear energy has been recognized by the world as the main energy source in the future and will surely grow substantially. Of the natural elements, only uranium and thorium can be used in reactors to produce nuclear and fissile materials, see equation (1)-(2). The reactor active zone and regeneration zone are mainly composed of uranium and thorium, depending on their slow neutron effective nuclear fission cross section. Although the 238 U and 232 Th themselves are slow neutrons, the effective nuclear fission cross section is small. However, the cross-section of 239 Pu and 233 U formed by the reaction with neutrons is very large, and it becomes a fissile material and participates in the fission chain reaction [8] , as shown in Table 1.
Take the preparation of uranium as an example, from ore to mechanical and high temperature treatment, leaching, uranium oxide production, refining (precipitation, extraction and ion exchange adsorption), fluoride production. The gas diffusion or centrifugal separation of uranium isotopes and the preparation of the final metal uranium, the production process together with the grade test are based on its physical and chemical properties and radiochemical analysis principles.
(2) Production of radionuclides, labeled compounds and nuclear medicines and disposal of nuclear waste
In addition to the production of nuclear fuel elements, most of the radionuclides produced by fission and neutron activation are prepared by radiochemical separation and purification of the target of the fuel rod or accelerator after the reactor irradiation. The synthesis of labeled compounds and nuclear medicines is generally based on the usual medicinal chemical methods combined with radiochemical methods to produce radionuclides at specific locations as radioactive tracer atoms, and nuclear waste treatment based on conventional radiation treatment methods combined with nuclear radiation. Features to handle and verify the effect.
(3) Radiological monitoring of environmental and biological samples
1. Monitoring and evaluation of aerosol and sedimentation levels in nuclear reactor accidents and nuclear tests Nuclear tests or explosions are uncontrolled fission chain reactions, while nuclear reactors are fission-chain reactions under controlled regulation, both of which use nuclear fuel. 235 U, 233 U or 239 Pu, all produce fission or activation product nuclide with different half-lives. These nuclides can contaminate the environment through aerosols and sediments, and then enter the body through food, the atmosphere or the skin. Nuclear tests or accidents in aerosols and sediments that diffuse through atmospheric clouds are short-lived fission product nuclide (eg 131 I and 140 Ba, etc.) often monitored as signals, while long-lived nuclide (eg 90 Sr, 137 Cs and 106) Ru- 106 Rh, etc., mainly requires monitoring and hygienic evaluation due to long time and toxicity in the body.
2. Emergency monitoring of nuclear radiation terrorist incidents After the September 11 incident, nuclear radiation was taken seriously by countries as a means of incident. Such terrorist incidents have an unpredictable form and nuclides compared to nuclear reactor accidents and nuclear tests. Its form is broadly divided into nuclear device explosions or dirty bombs, (radioactive material) release, attacks on nuclear power plants or other nuclear facilities. Therefore, the scope of monitoring for nuclear radiation terrorist incidents is broader, with emphasis on rapid detection methods for radioactive contamination in externally irradiated, surface-contaminated air, drinking water, food and other samples and related technical conditions [9] . Although the FAO/WHO Codex Committee is recommended for food international trade [10] and China's basic standards only for the ten most important artificial radionuclides in food (including 137 Cs, 134 Cs, 131 I, 90 Sr, 89) Sr, 103 Ru, 106 Ru, 238 Pu and 239 Pu) stipulate a general level of action [3] , but there are many sources of radionuclides in food, and nuclear terrorism involves more unpredictable nuclides, such as nuclear fuel elements (such as uranium,é’ and depleted uranium), natural radionuclides (such as 210 Po, etc., which caused deaths from internal pollution in the UK) cannot be ruled out. On the other hand, referring to the WHO Guidelines for Drinking Water Quality, which is based on China's current standard “Sanitary Standards for Drinking Water†[11] , it is considered that the natural and artificial radionuclides that should be included include alpha radiators ( 234 U). , 238 U, 232 Th, 224 Ra, 226 Ra, 210 Po, 239 Pu) and beta radiator ( 60 Co, 137 Cs, 134 Cs, 129 I, 131 I, 90 Sr, 89 Sr, 210 Pb, 228 Ra ) [12] . There are 12 kinds of radionuclides with concentration limits according to the original Standard for Confined Concentrations of Radioactive Substances in Foods (GBl4882-94), including: natural radionuclides (or elements) U, Th, 226 Ra, 210 Po, 228 Ra And artificial radionuclides (transuranic, fission products and activation products) 239 Pu, 147 Prn, 137 Ca, 131 L, 90 Sr, 89 Sr, 3 H [5-7] . Depleted uranium (DU) is a by-product of the extraction of 235 U from natural uranium. There are millions of tons of DU reserves in the world. The United States used the depleted uranium bombs for the first time in the 1991 Gulf War. The current depleted uranium is a means of terrorist radiation. These radionuclides should be the focus of recent monitoring.
3. Monitoring of radioactive contamination in human body With the development of the application of atomic energy industry and radionuclide in various sectors of the national economy, the number of occupational population involved in radionuclide internal irradiation has increased dramatically, and specialized radiology departments have been established in occupational disease hospitals. One of the important objectives of a radioactive occupational health check that may have radioactive contamination in the body is to estimate the internal dose caused by the amount of contamination in the body and conduct a hygienic evaluation. Among them, the detection of uranium and strontium content has long been used to estimate uranium and carcass load, and the uranium value can also be used to estimate the uranium concentration in the working environment air, the most commonly used indicators for medical observation and health protection of occupational personnel. However, there are many factors affecting the urine content. The literature believes that the current uranium value can only be used for reference when understanding the body load, and further research is needed [13] . Recently, we observed the quantitative relationship between the daily urine output of 40 healthy adult male volunteers and 68 elements (including uranium and plutonium) in whole blood. The daily output of urine was normalized by creatinine content, which was initially obtained. The difference between different elements [14] .
4, α spectrometer and low energy beta radioactive liquid flash measurement sample source preparation of alpha radioactivity (such as 235 U and 239 Pu) and low energy 13 radioactivity (such as 3 H and 106 Ru) penetration ability is weak, should not be measured by thick layer sample radioactivity. These nuclides are usually required to be electrochemically concentrated, separated, purified, electrochemically prepared for electroplating using an Ot spectrometer or mixed with a suitable scintillation fluid using a liquid scintillation spectrometer. In addition, radiochemical analysis methods are used in a wide range of fields such as geology and archaeology for prospecting and dating.
Fourth, the future outlook
(1) The irreplaceability of functions
On the one hand, the irreplaceability of radiochemical analysis depends on different types of radiation characteristics. The alpha particles have the least penetration ability and can only pass through a very short distance in a dense medium. It is still difficult for β particles to penetrate in the medium despite the strong penetrating ability. The measurement is directly measured by the sample. Only gamma radiation has a large penetrating ability in the medium. The gamma particles of various energies emitted by different gamma radionuclides can respond to the corresponding energy region of the gamma spectrometer, so it is possible to directly determine the sample content by using the gamma spectrometer through the corresponding characteristic energy peak. . on the other hand. The radionuclide in the sample usually coexists with its radioactive parent, daughter and other elements of radioactive or stable nuclide. Therefore, in most radionuclides (especially for α and β radionuclides), the radionuclide must be tested first. Separation, enrichment and purification of the pigments until preparation, which must be done by radiochemical means.
(2) The application fields and requirements are still expanding, and there are still some weak links in China.
At present, the development of nuclear energy in China's nuclear energy and the application itself and its need for radiation protection monitoring are growing. The application and the need to measure the radionuclide focus have shifted from fission and neutron activation products to transuranic elements. It has been greatly expanded, especially the nuclear terror emergency is more unpredictable. Although the atmospheric nuclear test has basically stopped, nuclear power plants have become recognized as the future promising clean energy, and China has established a positive development policy. Nuclear tests, nuclear power plants, and reactor operations all emit waste into the environment, and their radionuclide components are similar, so their monitoring projects are roughly the same. However, the current radioactive background monitoring stations of the health systems of various provinces, municipalities and autonomous regions are engaged in the retiring or diversion of most of the radiochemical analysis personnel. The problem of lack of new talents is a worrying and necessary remedy.
The discipline of radiochemical analysis is accompanied by the development of nuclear applications and nuclear science and technology. The types and tasks for the determination of nuclides are also expanding. At present, the following weak links have emerged in the field of radiochemistry in China, and attention should be paid to strengthening:
1. Research and application of advanced radioactivity measurement and ultra-trace element nuclear analysis technology and instruments The rapid development of China's atomic energy industry (60s and 70s of the last century) led to the research and development of radioactive measuring instruments. At present, advanced ultra-micro-element nuclear analysis techniques, quality control and statistical methods have progressed, and will continue to promote the reduction of the lower limit and accuracy of radionuclide and trace element determination methods. Relatively speaking, the development and development of domestic radioactive measuring instruments. The momentum is not as good as before and should be strengthened. We have recently reported the use of ICP-MS, ICP-AES and GFAAS technology to determine the content of 56 elements in important organs and tissues of human body in China [15] . Among them, the minimum measurable concentration of uranium and thorium has reached the order of 10 -13 g / g, and should be used for the detection of uranium and thorium general action levels (limited) in food, and these important radionuclides ( 234 U, 235 U, The 238 U, 228 Th, 230 Th and 232 Th) determination methods are the weak links in the radiochemical analysis of environmental and human biological samples in China so far and lead to the corresponding national data vacancies. It is valuable to find out the content and proportion of uranium and terpene in these samples in the normal background. It is worthwhile for China's national background dose estimation, radioactivity monitoring, especially the nuclear emergency to judge the depleted uranium pollution and the improvement of China's necessary national conditions. In addition, both 103 Ru and 106 Ru are important nuclear fission products, and the standard for radioactive inspection of pronuclear foods has not been included, considering the newly issued International Safety Standards for Ionizing Radiation Protection and Radiation Source Safety (IAEA Series No. 115). [16] and China's "Basic Standards for Protection of Ionizing Radiation and Safety of Radiation Sources" (GBl8871-2002) [17] have developed the general action level of these two nuclear foods, and the "Handbook of Medical Response to Nuclear Radiation Incidents" also As a nucleus to be monitored [9] , in order to meet the current domestic food and international food trade radioactive pollution inspection and nuclear terror emergency needs, it is feasible to supplement the development of more mature 103 Ru, 106 Ru and 238 Pu determination methods. of.
2, food testing methods urgently need to supplement the total reference level of total alpha and total beta activity concentration and supporting test standards. China's "Standards for Drinking Water Hygiene" stipulates that the total alpha and total beta radioactivity guide values ​​are used as the primary screening level, only in the total The radionuclide test is required only when the concentration of the radioactivity exceeds the guide value. This is not only logically reasonable, but also economical for the use of China's inspection resources [11] . The food radioactive limit and inspection standard methods in China should be supplemented and improved.
3. Strengthening the rapid and multi-nuclear combined determination method The main disadvantage of most radionuclide radiochemical analysis methods in the past is that the process is long and time-consuming, and it is difficult to meet the requirements of rapid results and evaluation. Internationally as early as the 60s and 70s of the last century
Conduct a variety of radionuclide simultaneous determination and rapid test methods. In the 1970s and 1980s, we organized the relevant units to establish a joint determination method including: 55 Fe, 59 Fe, 60 Co and 65 Zn; 2 90 Sr, 137 Cs and 144 Ce; 3 octanucleotide; uranium, thorium, joint determination of 226 Ra and 228 Ra method total strontium, radium total radioactivity measurement and so on, most of the country has been applied to investigate the seafood u scraping and national food radiological survey [19]. Any nuclides that can be measured by gamma radiation emitted by themselves or their daughters should be determined by gamma spectroscopy as much as possible. This method has the advantages of no need for time-consuming sample pretreatment and simultaneous determination of multiple gamma radionuclides. . Of course, while promoting and vigorously promoting gamma spectroscopy, it should be eliminated as a misunderstanding of omnipotent and replaceable radiochemical analysis.
In addition to the above technical aspects, it is recommended that relevant state departments should pay attention to the necessary adjustments and rationalization of the current system that is not suitable for sustainable development in the future. For example, the radiochemistry majors in higher education institutions should be retained and the corresponding personnel training should be moderately strengthened. The relevant radiation protection safety and radionuclide regulation and health standard management system should be rationalized and adjusted.
references:
[1]ICRP. Recommendations of ICRP[R]. ICRP Pub26, Annalsof the ICRP 1 (3) 1997. R eprinted(with additions)in 1987.
[2] ICRP. 1990 Recommendations of ICRP[R]. ICRP Pub. 60. AnBals of the ICRP 21 (1-3). Pergamon Pr∞s, Oxford, 1991.
[3] IAEA, FAO, WHO, etc. International safety standards for ionizing radiation protection and radiation source safety [s]. Security series, NO. 115, 1997.
[4] Ministry of Health of the People's Republic of China China's environmental radioactivity level and health evaluation [Z]. 1985.
[5] GB 14882-94, the limit concentration standard for radioactive substances in food [s].
[6] GBl4883 1—10—94, method for testing radioactive materials in food [s].
[7] WS/T-234, 2002, Determination of radioactive materials in foods é•… 241 Measure [s].
[8] Editor of the Editorial Committee of the Nuclear Science Committee of the Chinese Academy of Sciences. Natural uranium, bismuth technology [z]. 1961, (internal information).
[9] Su Xu, editor-in-chief Liu Ying. Nuclear Radiation Incident Medical Handbook [M]. Beijing: People's Medical Publishing House, 2005.
[10]FAO. , WHO. Standard Joint Program, Nutrition Codex Committee, Nutrition Code [M]. (1991) Section 6.1, “Radioactive Nuclide Levelsâ€.
[11] GB 5749-2006, Sanitary Standard for Drinking Water [s].
[12] WHO. Guidelines for drinking water quality. Second editio[P]. Vol 1 Recommendations, Geneva, 1993.
[13] Zhu Shoupeng, editor-in-chief. Radiotoxicology [M]. Beijing: People's Medical Publishing House 1982: 112-115.
[14] Zhu Hongda, Wang Jingyu, Wu Quan, et al. Chinese adult male elemental urine discharge and the content of homologous whole blood and urine samples and their relationship [J]. Chinese Journal of Radiation Medicine and Protection, 2007, 27(4): 63-67.
[15] Zhu Hongda, Wang Jingyu, Wu Quan, et al. Study on the content of 56 elements in 18 kinds of organs and tissues of Chinese adult males [J]. Radiation Protection, 2007, 27(3): 129-140.
[16] GB 18871-2002, Ionizing radiation protection and radiation source safety iron standard [S].
[17] Edited by the editorial group of Radioactive Investigation of Seafood. Marine food radioactivity survey [M]. Beijing, Atomic Energy Press. 1983.
[18] Zhang Jingyuan, Zhu Hongda editor. Chinese food radioactivity and induced internal dose [M]. Beijing: China Environmental Science Press, 1989.
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