Rapid development of nuclear engineering, medicine and radiation technologies is accompanied by increase of the radioactive waste (RAW) including long-lived ones. A RAW handling assumes their element content and activity analysis (characterization), compacting and disposal. This problem is of ultimate importance after shutdown of Chornobyl nuclear power plant Unit 4. The activity of the RAW inside the unit is estimated as much as 20 MCi (mainly on account of the Cs-137). These circumstances ensure a necessity of elaboration of the especial methods for express-analysis of large RAW fluxes. An immobilization of the long-lived radionuclides is entailed in turn with the problem of their localization into stable matrix as well as placing in resistive to radiation containers and geological structures. The report contains an overview of methods elaborated in NSC KIPT for RAW characterization and investigation using the bremsstrahlung of the high-current electron accelerator of radiation stability of the artificial and natural barriers for radionuclide immobilization

Dikiy, N.P.

Dovbnya, N.A.

Sayenko, S.Yu.

Uvarov, V.L.

Наукова електронна бібліотека періодичних видань НАН України (Vernadsky National Library of Ukraine)

ELECTRON LINAC APPLICATION FOR CHARACTERIZATION AND DISPOSAL OF RADIOACTIVE WASTEN.P. Dikiy, N.A. Dovbnya, S.Yu. Sayenko, V.L. UvarovNational Science Center “Kharkov Institute of Physics and Technology”,61108, Kharkov, Ukraineuvarov@kipt.kharkov.ua Rapid development of nuclear engineering, medicine and radiation technologies is accompanied by increase of the radioactive waste (RAW) including long-lived ones. A RAW handling assumes their element content and activity analysis (characterization),  compacting and disposal.  This problem is of ultimate importance after  shutdown of Chornobyl nuclear power plant Unit 4. The activity of the RAW inside the unit is estimated as much as 20 MCi (mainly on account of the Cs-137). These circumstances ensure a necessity of elaboration of the especial methods for express-analysis of large RAW fluxes. An immobilization of the long-lived radionuclides is entailed in turn with the problem of their localization into stable matrix as well as placing in resistive to radiation containers and geolo-gical structures. The report contains an overview of methods elaborated in NSC KIPT for RAW characterization and investigation using the bremsstrahlung of the high-current electron accelerator of radiation stability of the artificial and natural barriers for radionuclide immobilization.PACS numbers: 29.17.+w, 28.41.Kw1  INTRODUCTIONRadioactive  waste  (RAW) management  includes  a number of procedures. First of all it is RAW characteri-zation  i.e.  determination  of  the  amount,  activity,  ra-dionuclide and element content etc. The available meth-ods of RAW characterization are based either on analy-sis of their inherent radiation (γ-, β-, α-spectrometry) or on the profound radiochemical treatment of the samples with an extraction of the corresponding analyzed frac-tion for its  further spectrometry.  The shortcomings of the first  group methods are  a  relatively low accuracy and a restriction to the analysis of only thin RAW layers (particularly  β-  and  α-active ones)  as well  as a  small number of identifiable elements. Second group methods are  devoid  of  these  shortcomings.  However  they  are rather labor-consuming, expensive and low operative (a duration of one radiochemical analysis is up to several days). It is known that the activation method based on secondary  radiation  of  electron  accelerators  is  widely used at  present  for  nondestructive  express-analysis  of samples of ore and different materials [1], fission mate-rials [2] and in other fields.Taking into account that  a RAW sample activated with high-energy braking photons emits a radiation that is caused both by its inherent activity and initiated one as a result of photonuclear reactions, then an analysis of such a radiation gives quantitative information about the radionuclide  and  element  composition  of  the  sample without its destruction.The next RAW management stage is their immobi-lization and disposal in the steady geological structures. This task calls elaboration of experimental prognostica-tion methods for lasting (up to thousand years) conduct of the disposal environment under complicated radiation and corrosive conditions.2  ACCELERATOR2.1. For analysis of large amounts of RAW samples by the γ-activation method and implementation of other concomitant programs it is needed an electron accelera-tor with a beam power up to 10 kW and a wide range of particle energy regulation.The complex LU-20 [3] designed at the “Accelera-tor” R&D Prod. Est.  of NSC KIPT satisfies these re-quirements (see Table 1).Table 1. Basic parameters of LU-20 LinacEnergy range, MeV 10...30Pulse duration, µs 4Maximum repetition rate, Hz 300Maximum peak current, mA 1000Maximum average current, µA 1000Beam scanning frequency, Hz 3Beam size at the accelerator exit, cm 2x30Absorbed dose rate (electrons), Gy/h up to 4⋅107Absorbed dose rate (braking photons), Gy/hup to 1⋅1052.2.  A necessary set of radiation forming and diag-nostics devices has been developed for ensuring γ–acti-vation analysis (Fig. 1).Fig. 1. Schematic of the radiation forming and diag-nostics devices.An  electron  beam  at  the  accelerator  A exit  is scanned  using  the  electromagnet  SM. A  continuous  ВОПРОСЫ АТОМНОЙ НАУКИ И ТЕХНИКИ. 2001. №3.                             Серия: Ядерно-физические исследования (38), с. 178-180.178beam current monitoring is carried out by the magne-toinductive  sensor  MIS and  linear  beam coordination using the beam position monitor BMP [4]. A converter assembly  C consists of a tantalum plate that is placed into an aluminium casing and is cooled by running wa-ter. The filter F (5 aluminium plates) absorbs the part of the electron beam that passed the converter assembly. A braking photon flux after filter  F measuring typical di-mensions 150×500 mm is controlled by a wide-aperture ionization chamber  IC-W [5]. The capsules with sam-ples being analyzed are placed just behind the IC-W.In case when the electron energy exceeds 10 MeV an isotropic stream of photoneutrons is emitted from the converter together with braking photons. These neutrons can be used also in the framework of the problem under consideration.3  RAW ANALYSISA RAW sample investigated is irradiated as a rule together with a sample of the standard isotope content. A concentration of this isotope in the sample is deter-mined by comparison of the induced  γ-activity of each sample along the lines corresponding to the given iso-tope (taking into account the mass of the specimen).As an example, Fig. 2, 3 show the induced  γ-spec-trum  for  two  samples  of  materials  of  the  destroyed ChPP Unit 4: fragments of the reactor concrete shield (Fig. 2)  and  lava-like  fuel-containing  mass  (LFCM), which was formed in under-reactor premises as a result of the accident (Fig. 3).400 800 1200 1600 2000channel number10100100010000countsjuvlSc47U237K43Ca47511K43Cs132K40Ni57Na24Ca47Na22Zr89Rb84Mn54Ca47Fig. 2. Induced γ-activity spectrum of concrete sample.400 800 1200 1600 2000channel number110100100010000countsjuvlU237Eu154U237511Cs137K40Eu154Eu154Sr89Eu154Eu154Cs134Fig. 3. Spectrum of activated LFCM sample.The spectrum in Fig. 2 includes the U-237 line. This isotope was generated in the sample under the activation process  in  the  238U(γ,n)237U  reaction.  This  example demonstrates the ability of the  γ-activation method to analyze the elements the identification of which is im-possible by means of a traditional spectrometry meth-ods. The quantitative data obtained concerning the ele-ment content in the samples allow to carry out a correla-tion analysis as well.Fig. 3 demonstrates also the ability of the  γ-activa-tion method in analysis of the samples having their own activity of a different nature. So, apart the lines of γ-ra-diating nuclides (Eu-154, Cs-137 and Cs-134) there are shown the lines of U-237 and Sr-89. The last result is especially important because the γ-radiating nuclide Sr-89 is created as a consequence of the β-radiating Sr-90 activation. It is known that an analysis of the γ-radiating nuclides  is  realized  technically  simpler  and  for  more thick RAW layers (up to 30 cm or so).The spectrum in Fig. 4 corresponds to the activated U-238  dioxide  water  solution  (with  concentration 30 mg U-238/l). These data show that  the  γ-activation method can be used also for the analysis of liquid RAW with identification limit no more then 2 µg U-238 /l.400 800 1200 1600 2000channel number110100100010000countsjuvlSc47U237511Ca47Na22Zr89237U KαFig. 4. γ-spectrum of the activated U-238 water so-lution.4  INVESTIGATION OF MATERIALS FOR RAW DISPOSALIt is known that during lasting disposal of the high-level RAW or nuclear spent fuel  a situation can arise when the RAW immobilization matrix (including geo-logical structure) will contact with ground water. Thus a structure of “RAW-water-geological barrier” type origi-nates. A radionuclide transport in such a structure deter-mines a reliability of the RAW disposal. Besides, such transport  depends  on  the  absorbed  dose  of  radiation from the RAW estimated as much as 108 Gy during dis-posal period.For research of radionuclide transport processes the granite samples (that is considered as a perspective en-vironment for disposal of long-lived RAW) were select-ed. A piece of granite was cut into the specimens in the form of blocks with the size of 10x10 mm in cross-sec-tion and 30 mm in thickness. Each block was covered with epoxy except for 10x10 surface.Isotope Yb-169 was used as a  γ-radiating nuclide-tracer which is  analogous to  actinides  in  its  chemical properties. For this nuclide production under reac-tion 168Yb(n, γ)169Yb the pellets of stable 168Yb2O3 were irradiated with photoneutrons. Then the pellet was dis-solved in concentrated HCl acid (0.2 ml) and finally the aqueous solution with pH=1.8 was prepared.The solution obtained (40 ml) together with the sam-ple irradiated up to the given dose value (3⋅106...3⋅107 Gy) were placed into the thermostable flask. The latter was heated with the water steam during 32 hours.Then each specimen was washed in distillate water during 24 hours and dried out at 60°C in the drying box. Further the layers (2...50  µm) from the free surface of the  specimen  were  removed  by  means  of  precision grinding. Material of the removed layers was used for γ-spectrometry with the Ge(Li)-detector. Typical spec-trum of the material removed of the sample, irradiated with  braking  photons  of  LU-20 accelerator  up  to  the dose value 3 107 Gy is shown in Fig. 5. Spatial distribu-tion of Yb-169 concentration within depth of the speci-men is demonstrated in Fig.6. These results allowed to determine a dose dependence of the radionuclide diffu-sion as well as to find out its mechanism [7].500 1000 1500channel number252525251101001000counts84 Ca47Rb511 keVYb169Fig. 5. γ-spectrum of irradiated granite (3⋅107 Gy).0 200 400depth,    m0.010.101.0010.00concentration,  *10   at/cm203µcbaFig. 6. Distribution of Yb-169 into granite:a – irradiated (3⋅107 Gy);b – pristine state (granite with pegmatite structure);c - pristine state (granite with uniform grain structure).5  COMPUTER SIMULATIONA method of the computer simulation based onthe standardized code GEANT (or other like it) can be used for  optimization  of  the  sample  irradiation  conditions, isotope generation modelling as well as for investigation of metrological characteristics of the measuring sensors under their interaction with radiation. Such code allows to calculate these parameters with appropriate accuracy (not less than 10%) considering real composition of the radiation forming systems as well  as of  the irradiated object [8].6  CONCLUSIONS1. High-current  electron  accelerator  with  the  energy range 10...30 MeV allows to solve effectively different problems of radioactive waste management, in particu-lar, operative analysis of the nuclide and element con-tent of the RAW using the γ-activation method without destruction of samples.This method provides also an ability of distant-read-ing analysis under automatic operation. Such facility is important for large amount of samples to be analyzed, for example, when extracting the RAW from Chernobyl power plant unit 4.2. A powerful (∼10 kW) electron accelerator is compa-rable, by its absorbed dose rate ability, with the Co-60 source having the activity up to 1 MCi. This circum-stance as well as a possibility to control the upper limit of the braking photons spectrum allow to use linacs for research of radiation and chemical stability of materials intended for immobilization and disposal of RAW. The radionuclides produced directly at linac can be used as tracers in these investigations.3. A linac provides the radiation of different intensity and  nature  (accelerated  electrons,  bremsstrahlung  and photoneutrons) that gives a possibility of radiation test of  materials  within  the  wide  range  of  their  operation conditions.Work is supported by STCU under contract N 1580.REFERENCES1 Toms M. Elaine. Photonuclear Activation Analysis with Ge(Li) Detector.  Nav. Res. Lab., USA, Rep. 7554, 1973.2 T.Gozani  et  al.  Measurement  of  Prompt  and  De-layed  Neutrons  from  Photofission  //  ANS  Trans. 1968, v. 11, p. 659.3 A.N.Dovbnya et al. Electron Linacs Based Radia-tion Facilities of Ukrainian National Science Center “KIPT” // Bul. of the Amer. Phys. Soc. 1997, v. 42, N 3, p. 1391.4 V.L.Uvarov,  V.N.Boriskin  et  al.  Calibration  of Electron Beam Measuring in Technological Linacs // Proc. of ICALEPS’99, Trieste, Italy, 1999.5 A.A.Butenko,  S.P.Karasyov  et  al.  Technological Measuring Channel for Bremsstrahlung Monitoring //  Problems of Atomic Science and Technology. Is-sue:  Nuclear-Physics  Research (35).  1999,  v.  4, p. 49.6 A.Borovoi.  Post-Accident  Management  of  De-stroyed Fuel  from Chernobyl:  Technologies  Used and Lessons Learned // IAEA. 1990, p. 15.7 N.P.Dikiy,  S.Yu.Sayenko,  V.L.Uvarov,  E.P.-Shevyakova. Application of Nuclear-Physics Meth-ods  for  Studying  the  Radionuclide  Transport  in Granite Rocks //  Problems of Atomic Science and Technology. Issue: Nuclear-Physics Research (36), 2000, v. 2, p. 54. (in Russian).8 S.P.Karasyov,  S.V.Maryokhin,  V.L.Uvarov  et  al. On Computer Modelling of Primary Transducers in Electron  Radiation.  Diagnostics  //  Proc.  of  EPAC’98, Stockholm, June 1998, p. 134.180

Electron linac application for characterization and disposal of radioactive waste

http://dspace.nbuv.gov.ua/bitstream/handle/123456789/79252/63-Dikiy.pdf?sequence=1

Electron linac application for characterization and disposal of radioactive waste

Abstract

Similar works

Full text

Available Versions

Наукова електронна бібліотека періодичних видань НАН України (Vernadsky National Library of Ukraine)