Study of the selective determination of 90 Sr in environmental. and nuclear power plant samples

Transcription

1 Study of the selective determination of 90 Sr in environmental and nuclear power plant samples SUMARY OF THE Ph.D. THESIS by RÓBERT CHOBOLA Supervisor: Dr. József Solymosi D.Sc. Department of Physical Chemistry Budapest University of Technology and Economics 2006

2 Introduction The radionuclide 90 Sr, a fission product in spent fuel of nuclear power plants, has become an ingredient of the natural background radiation of an artificial origin, due to emissions from reprocessing plants and atmospheric nuclear weapons tests ended not so long ago, as well as because of the regular and accidental emissions from nuclear power plants. 90 Sr, which can also be found in foodstuff and in natural waters and can be incorporated into the human body, is one of the most dangerous isotopes considering its physiological effects. On one hand it is especially dangerous because of its chemical nature and its metabolism within the body as it can be integrated into the skeleton in calcium atom locations, and, on the other hand, because of its radiochemical properties as it is a pure beta emitter. The permanent environmental monitoring of 90 Sr is therefore a necessity, both for radiological protection and nuclear environmental protection. Considering these aspects, the studies on selective separation and determination of radio-strontium, which can facilitate faster and simpler monitoring of 89 Sr and/or 90 Sr, will remain important and relevant questions as long as there is nuclear energy industry. Subject of the Ph.D. thesis Developing a complex analytical procedure for determining the 89/90 Sr content of low- and intermediate-level liquid radioactive waste of the Paks Nuclear Power Plant (evaporation concentrates, used ion-exchange resins, active dross and other contaminated technological solutions), as well as enhancing the so-called fast water method previously developed at the Department of Physical Chemistry of the Budapest University of Technology and Economics for 2

3 analyzing the 90 Sr content of natural waters, in order to eliminate the effect of 234 Th which disturbs the measurement. Timeliness of the research During the operation of the WWER-440 type reactor of the Paks NPP β emitting isotopes may get into the primary coolant owing to the surface contamination of fuel elements, leakage or damage of the enclosure, or as activation products and these radio-nuclides may then get into regeneration wastes formed during the purification of the primary coolant water, and into other wastes via intended or unintended leakage. Radioactive wastes produced by various reactor types have reactor-specific sample matrices, so for each reactor type a different analytical procedure is needed to determine the amounts of β-emitting radio isotopes. Description of the problem At the location of the Paks NPP low- and intermediate-level liquid waste (LILW) is collected in containers. The sources of this waste are the technological waste waters of the power plant. Considering the type and activity of their radionuclide content, the release of these waste waters into the environment of the power plant is strictly controlled by regulations issued by the relevant authorities. This is also one of the reasons why a routine, periodic verification of the 90 Sr content of the containers is necessary. In nuclear measurements the problem of separation of complex beta-spectra and thus the determination of the amounts of the individual isotopes is still not completely solved. Therefore, in order to perform a nuclide-specific measurement the application of some kind of selective 3

4 separation technique is needed. Most of these are time consuming and/or they involve complicated chemical operations, so they cannot, or can hardly be automatically performed. Chromatographic procedures (e.g. procedures based on solid state extraction and ion exchange), however, facilitate fast sample preparation, and thus make it possible to apply these techniques in an automatically controlled way. Methods for the efficient extraction of strontium as a precipitate, as well as chromatographic methods for its selective chemical separation are well known from the international literature. Procedures using both these types of methods are also known. However, in order to increase the reliability of the corresponding analysis, it seemed appropriate to determine the conditions for the applicability of a complex separation technique which combines both of the abovementioned separation principles. An important feature of the complex procedure which I applied and further developed is that from each investigated sample two extracts are prepared using different methods. This greatly contributes to the success of the analysis: in addition to the radioactive isotopes of strontium ( 90 Sr or 90 Sr/ 89 Sr together), 90 Y, a daughter of 90 Sr, which is in secular equilibrium with its parent, is also extracted from the investigated sample. One type of the methods for the determination of 90 Sr in natural-water samples is based on the selective separation of strontium which is then followed by the measurement of its activity. Another group of methods is based on the assumption that 90 Sr and its daughter 90 Y are already in secular radiochemical equilibrium. Then yttrium is selectively extracted and the radioactive concentration of 90 Sr is calculated from the measured activity of yttrium. The latter procedure is justified by the fact that the activity of 90 Y can be measured easier than that of 90 Sr. By analysing a large number of water samples at the Department of Physical Chemistry of the Budapest University of Technology and Economics, the Laboratory for Environmental Control of the Paks NPP has tested the so-called fast water method previously developed for the 4

5 measurement of the radioactive 90 Sr isotope found in natural-water samples. This method is simple, fast and can be performed automatically. By a detailed analysis of the decay kinetics of the 90 Y samples prepared by this procedure it turned out that the samples are contaminated with the naturally occurring 234 Th isotope, which has a short half-life. The presence of 234 Th makes the results of the method unreliable, so its disturbing effect should be eliminated. New scientific results of the thesis The results of the complex analytical procedure for determining the 89/90 Sr content of the liquid radioactive waste of the Paks Nuclear Power Plant (NPP), as well as the results of the enhancement of the so called fast water method previously developed at the Department of Physical Chemistry of the Budapest University of Technology and Economics for the assay of the 90 Sr content of natural waters are summarized as follows. 1. By adapting the known radioanalytical separation techniques for the assay of strontium I have developed a complex chemical procedure for the simultaneous determination of 89 Sr and 90 Sr activity concentration in the liquid waste of the Paks NPP. In particular, I have determined the efficiency and optimal conditions of precipitate forming, and of the applicability of the element selective techniques, which are the basis of the developed procedure. Using this optimized procedure I have analyzed a great number of samples taken from the Paks NPP and proved its successful practical applicability. 2. I have verified the applicability of the procedure on evaporation concentrates and dredge like slurry samples. I have found that the higher concentrations of the organic complex forming agent decreases the chemical yield of the procedure, this is why the analysis cannot be directly applied for these samples. This is because the higher 5

6 complex forming agent content binds a considerable amount of the inactive carrier material. 3. Based on the experimental results I extended the applicability of this procedure to the determination of the 90 Sr content of natural soil samples. The effectiveness of the studies was confirmed by the 90 Sr radioactive concentration measurements of the samples collected from the environment of the Paks NPP, which were of ~1 2 Bq/kg. These results are of the same order of magnitude as the ones obtained by the OSSKI (NRIRR) Hungarian Institute and other international laboratories. 4. I have enhanced the so called fast water method previously developed at the Department of Physical Chemistry of the Budapest University of Technology and Economics for analyzing the 90 Sr content of natural waters, and eliminated the highly disturbing effect of the natural 234 Th which biased the beta measurement. In particular, in order to determine the effect of 234 Th, I have established a mathematical model for the chemical method, which I used for calculating the optimal parameters for the separation of the Sr 2+,Y 3+ and Fe 3+, Th 4+ ions respectively on the cation exchange resin. 5. Next I have verified experimentally the precision of the model using artificially composed solutions. I have showed that with a properly selected and selective complex forming agent (citric acid) the Y 3+ and Th 4+ ions can be completely separated from each other. 6. Starting from the above experiences, I have further developed the so called fast water method for natural water samples, complementing the previously developed one with a cation exchange based ion separation. 7. I have tested the optimized procedure using self made standard solutions and proved its good reproducibility. Relying on the experimental results I have determined the 90 Sr 6

7 activity concentration in the water wells around the Paks NPP, which were of < mbq/dm 3. Based on these achievements I have proposed an amendment for the automatic sample preparation phase of the previously used method and I also determined the conditions for the applicability of the procedure. Practical applicability of the results The methods used in Hungary by the expert authorities for the determination of the 90 Sr content of natural waters are complicated procedures consisting of more steps, and therefore they are time consuming. The enhanced fast water method which I developed eliminates these drawbacks and enables the Laboratory for Environmental Control of the Paks NPP to routinely use this method for the determination of the 90 Sr content of natural waters around the power plant. 7

8 Publications and lectures Publications related to the subject of the Ph.D. thesis 1. I. GRESITS, S. TÖLGYESI, J. SOLYMOSI, R. CHOBOLA, L. GY. NAGY, L. SZABÓ and P. ORMAI: Determination of Transuranium elements in Nuclear Power Plant Wastes. J. Radioanal. Nucl. Chem., 203, , CHOBOLA R., VINCZE Á., ERDŐS E., SOLYMOSI J., GRESITS I., ORMAI P., FRITZ A., PINTÉR T. és SÜVEGES M.: 90 Sr meghatározása atomerőművi folyékony hulladékoból. Magyar Kémiai Folyóirat, 103.évf., 9, , Á. VINCZE, E. ERDŐS, J. SOLYMOSI, I. GRESITS, P. MELL, L. TÖRÖK, R. CHOBOLA, P. ORMAI, A. FRITZ, T. PINTÉR. Determination of Radiostroncium in Samples of NPP Origin. Proceedings of the IRPA Regional Symposium on Radiation Protection. Prague, 8-12 Sep pp , R. CHOBOLA, E. ERDŐS, P. MELL, J. SOLYMOSI, Á. VINCZE, L. TÖRÖK, T. RANGA and G. VOLENT: Separation of 234 Th from 90 Y. Czech. J. Phys., 49, , R. CHOBOLA, P. MELL, L. DARÓCZI and Á. VINCZE: Rapid Determination of Radiostrontium Isotopes in Samples of NPP Origin. J. Radioanal. Nucl. Chem., 267(2), , L. SZENTMIKLÓSI, ZS. RÉVAY, R. CHOBOLA, P. MELL, S. SZAKÁCS and I. KÁSA: Characterization of CaSO 4 -Based Dosimeter Materials with PGAA and Thermoluminescent Methods. J. Radioanal. Nucl. Chem., 267(2), ,

9 7. I. KÁSA, R. CHOBOLA, P. MELL, S.SZAKÁCS and A. KEREKES: Preparation and Investigation of Thermoluminescence Properties of CaSO 4 :Tm,Cu. Accepted for publication in Radiat. Prot. Dosim., Lectures and posters related to the subject of the Ph.D. thesis 1. E. ERDŐS, J. SOÓS, J. SIMON, T. PINTÉR, P. ZAGYVAI, Á. GUJGICZER, R. CHOBOLA, J. SOLYMOSI and L. GY. NAGY: Continuous monitoring of radioiodines in NPP primary coolant. 2 nd International Seminar on Primary and Secondary Side Water Chemistry of Nuclear Power Plants. Hungary, Balatonfüred 19-23, September CHOBOLA R.: 90 Stroncium meghatározása atomerőművi hulladékokból. Őszi Radiokémiai Napok MKKE Radioanalitikai Szakcsoportja. Balatonkenese, VINCZE Á., CHOBOLA R., ERDŐS E. és SOLYMOSI J.: Radiostroncium gyors mérése atomerőművi mintákból. MKKE Vegyészkonferencia. Eger, Á. VINCZE, E. ERDŐS, J. SOLYMOSI, I. GRESITS, R. CHOBOLA, P. ORMAI, A. FRITZ, T. PINTÉR and M. SÜVEGES: Analysis of Radiostrontium isotopes in evaporation concentrates from the water purification system of NPP Paks. 3 nd International Seminar on Primary and Secondary Side Water Chemistry of Nuclear Power Plants. Hungary, Balatonfüred 16-20, September E. ERDŐS, I. GRESITS, P. MELL, R. CHOBOLA, Á. VINCZE, J. SOLYMOSI and P. ORMAI: Completion of the IVO Technology for the Separation of 90 Sr Isotope in the Decontamination of Waste Waters. EPRI 1998 International LLW Conference and Exhibit Show. Orlando, Fl, July 15-17, CHOBOLA R., VINCZE Á., SOLYMOSI J., MELL P. és RANGA T.: Th-234 elválasztása Y- 90-től. Őszi Radiokémiai Napok MKKE Radiokémiai Szakcsoportja.Tata,

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