Radiochemical studies in the development of deep geological repository in the Czech Republic

In this contribution the main achievements of radiochemical studies performed within the framework of the Czech DGR development programme are summarized and further plans outlined. The results of selection of the most dangerous radionuclides in spent fuel assemblies from VVER 440 reactors, based on spent fuel inventory calculations and analyses of migration rate of radionuclides to the environment, are presented in the first part of the contribution. It is shown that ¹⁴C, ¹²⁹I, ¹²⁶Sn, ¹³⁵Cs, ³⁶Cl, ⁷⁹Se, ²²⁶Ra, ²³⁷Np, ²²⁹Th, and ²⁴²Pu belong among the most dangerous radionuclides in the Czech disposal concept. Problems with the determination of migration parameters of radionuclides are described in the second part of this contribution.     

A new code for modelling the near field diffusion releases from the final disposal of nuclear waste

The canisters with spent nuclear fuel produced during the operation of WWER reactors at the Czech power plants are planned, like in other countries, to be disposed of in an underground repository. Canisters will be surrounded by compacted bentonite that will retard the migration of safety-relevant radionuclides into the host rock. A new code that enables the modelling of the critical radionuclides transport from the canister through the bentonite layer in the cylindrical geometry was developed. The code enables to solve the diffusion equation for various types of initial and boundary conditions by means of the finite difference method and to take into account the non-linear shape of the sorption isotherm. A comparison of the code reported here with code PAGODA, which is based on analytical solution of the transport equation, was made for the actinide chain 4N+3 that includes ²³⁹Pu. A simple parametric study of the releases of ²³⁹Pu, ¹²⁹I, and ¹⁴C into geosphere is discussed.     

New trlfs laboratory at the CTU in Prague

Early in 2006, construction and equipment of a new TRLFS (Time-Resolved Laser-induced Fluorescence Spectroscopy) laboratory was accomplished at the Czech Technical University in Prague, Czech Republic. In this contribution, the system is described in detail, the results of the first measurements obtained in the new laboratory for a standard UO ₂ ²⁺ - SO ₄ ²⁻ system are presented, and the future research plans are outlined.     

Variants of Regenerated Fissile Materials Usage in Thermal Reactors as the First Stage of Fuel Cycle Closing

At present, 240 000 t of spent nuclear fuel (SF) has been accumulated in the world. Its long-term storage should meet safety conditions and requires noticeable finances, which grow every year. Obviously, this situation cannot exist for a long time; in the end, it will require a final decision. At present, several variants of solution of the problem of SF management are considered. Since most of the operating reactors and those under construction are thermal reactors, it is reasonable to assume that the structure of the nuclear power industry in the near and medium-term future will be unchanged, and it will be necessary to utilize plutonium in thermal reactors. In this study, different strategies of SF management are compared: open fuel cycle with long-term SF storage, closed fuel cycle with MOX fuel usage in thermal reactors and subsequent long-term storage of SF from MOX fuel, and closed fuel cycle in thermal reactors with heterogeneous fuel arrangement. The concept of heterogeneous fuel arrangement is considered in detail. While in the case of traditional fuel it is necessary to reprocess the whole amount of spent fuel, in the case of heterogeneous arrangement, it is possible to separate plutonium and ²³⁸U in different fuel rods. In this case, it is possible to achieve nearly complete burning of fissile isotopes of plutonium in fuel rods loaded with plutonium. These fuel rods with burned plutonium can be buried after cooling without reprocessing. They would contain just several percent of initially loaded plutonium, mainly even isotopes. Fuel rods with ²³⁸U alone should be reprocessed in the usual way.     

球床高温气冷堆燃料颗粒中放射性核素的累积释放份额研究

本文利用Fick扩散定律,给出经典方法和改进的两种累积释放份额的严格数学表达式,并分析与之相关的穿透时间的物理内涵,并分别用以上两种方法计算137Cs,90Sr,110mAg三种重要放射性核素通过SiC层扩散的累积释放份额.研究表明,传统的"穿透时间"概念并非放射性核素释放到SiC层外的时间,本文提出的改进方法可以更准确的描述放射性核素在燃料元件中的输运和扩散行为.     

A setup for active neutron analysis of the fissile material content in fuel assemblies of nuclear reactors

An active neutron method for measuring the residual mass of ²³⁵U in spent fuel assemblies (FAs) of the IRT MEPhI research reactor is presented. The special measuring stand design and uniform irradiation of the fuel with neutrons along the entire length of the active part of the FA provide high accuracy of determination of the residual ²³⁵U content. AmLi neutron sources yield a higher effect/background ratio than other types of sources and do not induce the fission of ²³⁸U. The proposed method of transfer of the isotope source in accordance with a given algorithm may be used in experiments where the studied object needs to be irradiated with a uniform fluence.     

An optical absorption method of detecting iodine-containing substances in liquid acid media formed during the reprocessing of spent nuclear fuel

We propose an absorption method for determining the concentrations of iodine-containing substances (I₂ and IO ₃ ^? ) in real time in liquid acid media formed when spent nuclear fuel is reprocessed at nuclear fuel cycle plants. The essence of the method is to measure simultaneously at two wavelengths the intensity of the radiation that passes through the solution being analyzed. The sensitivity of this method allows reliable and real-time control to be exercised over both the technological process of fuel reprocessing and the operations of purification systems.     

Characteristics of radionuclide contamination of different zones of Semipalatinsk Nuclear Test Site “Opytnoe pole”

Data on the spatial distribution of radionuclides (²⁴¹Am, ²³⁹Pu, ¹³⁷Cs and ¹⁵²Eu) formed during nuclear explosions of different types near P2 SNTS test site are presented. Radionuclide contamination induced by the explosions varies in the concentrations of individual radionuclides, their proportions and species. Examination of the variations is a crucial task to plan remediation activities as well as those aimed at decrease of radiation risk for population and prevention of repeated contamination. Concentrations of ²⁴¹Am and ^239+240Pu that are the most toxic radionuclides in the area lie in hundred thousands of Bqkg^?1. The most contaminated areas are classified by the radionuclide concentration, ratio and form present in soil.     

Confinement and migration of radionuclides in a nuclear waste deep repository

The safety of the geological disposal of nuclear waste is evaluated, among other factors, through the ability of radionuclides to be confined, first by waste packages then by engineered barriers and host rocks. Deep underground conditions favour the immobilisation of most radionuclides by sorption or precipitation, which means that mobile radionuclides, such as ¹²⁹I or ³⁶Cl, may contribute to most of the ultimate dose release during up to a million years. However, degraded evolution scenarios must be taken into account (oxygen intrusion, faults, …) to assess earlier dose releases. To cite this article: P. Toulhoat, C. R. Physique 3 (2002) 975–986.     

Diffusion of 99-technetium in compacted bentonite under aerobic and anaerobic conditions

The main aim of this study was to investigate diffusion of technetium ⁹⁹Tc under different conditions. Because technetium represents one of the most dangerous fission products due to its very long halftime and high mobility in aerobic conditions diffusion experiments of technetium (as ⁹⁹TcO ₄ ⁻ anion) in Czech bentonite from Rokle locality have been carried out. For performance and evaluation of experiments the through-diffusion method was chosen and apparent (D_a) and effective (D_e) diffusion coefficients were evaluated. The effects of particle mesh-size, dry bulk density and aerobic or anaerobic conditions on diffusion were studied. In the presence of oxygen, technetium occurs in oxidation state VII, as an anion, soluble and mobile in the environment. However, under reducing conditions it occurs in a lower oxidation states, mainly as insoluble oxides or hydroxides. Aerobic experiments were carried out under laboratory conditions and anaerobic experiments were performed in a nitrogen atmosphere in a glove box, to simulate the real underground conditions.     

The procedure and results of calculations of the equilibrium isotopic composition of a demonstration subcritical molten salt reactor

A subcritical molten salt reactor with an external neutron source is studied computationally as a facility for incineration and transmutation of minor actinides from spent nuclear fuel of reactors of VVER-1000 type and for producing ²³³U from ²³²Th. The reactor configuration is chosen, the requirements to be imposed on the external neutron source are formulated, and the equilibrium isotopic composition of heavy nuclides and the key parameters of the fuel cycle are calculated.     

Studies of the radionuclide content of fuel particles sampled from the chernobyl nuclear power plant’s No. 4 reactor unit

Spectroscopic studies are performed of the L _x , K _x , and ?? emanations from fuel particles sampled in 2011 inside the Chernobyl Nuclear Power Plant??s (ChNPP??s) No. 4 reactor unit. The isotope ratios for ^134,135Cs, ^154,155Eu, Pu isotopes, ^241,243Am, and ²⁴³Cm are measured. The data on ?? emitters for all radionuclides above ²⁴¹Am exhibit considerable inconsistency with the theoretically calculated values. A systematic deviation of the ⁹⁰Sr and ¹³⁷Cs ratios for the fuel component from the 1986 data is observed. Zirconium is shown to be the main radionuclide in the fuel particles.     

CR-39 alpha particle spectrometry for the separation of the radon decay product 214Po from the thoron decay product 212Po

In this work, it is presented an experimental assembly based on electric capture of radionuclides in order to discriminate the alpha activity of ²¹⁴Po (radon decay product) from ²¹²Po (thoron decay product) one. The alpha activity was registered with CR-39 analyzing etch pit diameters and optical density. This discrimination may allow for the estimation of the relative contribution of the alpha activity due to thoron decay products compared to the total activity due to radon decay products.     

Radionuclide contamination of the LIRA faciliticies territory

Radiation situation at the LIRA site has been investigated. It served as a test site for 6 underground nuclear explosions. In comparison with background in the region the explosions caused a slight increase of the level of radionuclide contamination in the territory of the site. It was found that the release of ¹³⁷Xe and ⁹⁰Kr onto the surface is the main reason for formation of additional contamination with ¹³⁷Cs and ⁹⁰Sr. The peculiarities of the distribution and migration of the radionuclides in dependence on the relief and soil conditions are taken into consideration.     

Separation of long-lived radionuclides from high active nuclear waste

The management of vitrified high activity waste issued from the reprocessing of spent nuclear fuel is often made questionable by the presence within this waste of long-lived radionuclides (LLRNs). New management scenarios of this waste are under study at the CEA since the beginning of the 1990s, involving the development of separation processes for some LLRNs. Six LLRNs are the targets of these studies; they are: (i) the minor actinides: neptunium, americium and curium; (ii) the fission products: technetium, iodine and caesium. The present paper highlights the main achievements of the research performed at the CEA in this field. To cite this article: C. Madic et al., C. R. Physique 3 (2002) 797–811.     

Present status and trends of radioecological research

Radioecology can be defined as the science concerned with the interactions between radionuclides and the environment and with the effects of radionuclides on man and biotic communities. Ecology advanced from radioecological studies because tracer radionuclides were instrumental in understanding many ecological processes. During the period of fallout from nuclear weapons testing the behaviour of the most important radionuclides (typically:⁹⁰Sr,¹³¹I,¹³⁷Cs,^239,240Pu) in the most critical pathways was studied. After the Chernobyl accident studies were extended to natural and semi-natural ecosystems and to other less common radionuclides. In principle the implementation of radiation protection criteria for man, which are based on the protection of a single individual, assures the protection of organisms other than man. In case of serious contamination scenarios there is a need for realistic evaluations of the effectiveness of the countermeasures that could be undertaken. In particular, studies on biogeochemical transfer mechanisms which control the cycling of radionuclides should be developed. With reference to future developments of the nuclear industry, some attention should be paid to the operation and the releases from, e.g., intrinsically safe reactors and fusion plants. Besides such potential new-type sources, the decontamination and possible site restoration of military installations has to be considered.     

<Superscript>210</Superscript>Pb and <Superscript>210</Superscript>Po in fossil fuel combustion at the Šoštanj thermal power plant (Slovenia)

The main aim of the present study was to evaluate ²¹⁰Pb and ²¹⁰Po emissions from the ?o?tanj TPP and to evaluate their mass balance in unit 4. Samples of coal, fly ash, slag and flue gases were analysed for ²¹⁰Pb and ²¹⁰Po content. The results showed that these radionuclides are mostly concentrated in ash (71% and 81% for ²¹⁰Pb and ²¹⁰Po, respectively). Only a small part of the input activity was detected in flue gases. The activities of ²¹⁰Pb and ²¹⁰Po in unit 4 were from 1.1 to 2.7 Bq m^?3 and from 0.37 to 0.56 Bq m^?3, respectively. The mass balance of the two radionuclides in unit 4 show only 6 and 10% differences between the annual activities of the input and output samples.     

Analysis of the electroproduction structure functions in the lowQ 2 region,combining the vector meson dominance and the parton model with possible scaling violation

The nucleon structure functionF ₂ is constructed and analysed for low values ofQ ² using the generalised vector meson dominance representation with the largeQ ² spectral function calculated from the analytic continuation of the parton model structure function. Various parametrisations of the parton distributions are considered. Possible effects of the largeQ ² scaling violation on the lowQ ² part of the structure function are investigated. The magnitude of the total contribution given by this asymptotic part can be as high as 50% of the vector meson contribution in the low-Q ², low-χ region. The contribution of the valence quarks alone to the structure function at lowQ ² turns out to be at least as important as the corresponding non-Pomeron Regge-pole-like terms coming from the vector meson part. Increase of the structure function with ν coming from the increase of the quark sea in the limit of small χ implied by QCD turns out to be relatively weak at lowQ ². Predictions of the model are compared with available experimental data. The photoproduction cross section and the nuclear effects in the structure function are also briefly discussed.     

Nuclear flow in heavy ion collisions

Short overview of the flow phenomenon from the low upto relativistic heavy ion energies is given. Strict difference between spectator and participant flow is emphased. The latter is described by the modification of the nuclear blast model. Thermal source expanding in a plane perpendicular to the line connecting the centers of projectile and target is assumed. Observed dependences of squeeze-out, side-splash and of mean transverse-energy in relativistic heavy-ion collisions on the particle's mass are reproduced. Partioning of initial projectile energy into thermal and compressional parts in²⁰⁹Bi(1 GeV/u)+²⁰⁸Pb and¹⁹⁷Au(150–800 MeV/u)+¹⁹⁷Au collisions is discussed.Invited lecture given at the International School-Workshop Relativistic Heavy-Ion Physics, Prague (Czech Republic), 19–23 September 1994.Experimental data obtained at Bi+Pb collisions [6] are the result of the group effort. The contribution of V. Wagner, M. Pachr, Yu. G. Sobolev, R. Simon, S. Hlava, M. umbera as well as of my student R. Pleska, who carried out most of the fits, is gratefully acknowledged. This work was supported by the Granting Agency of the Czech Republic under contract No. 202/93/1144. A.K. gratefully acknowledges support from EC during his stay at KVI Groningen, where part of this work was done.     

Treatment of spent NTA-based decontamination solutions

This study deals with potential treatment of spent NTA-containing decontamination solutions for final disposal. The method proposed is based on the degradation of organic substances followed by the separation of radionuclides. The influence of various parameters (pH value, irradiation time, temperature, catalyst amount, type and various combinations of catalysts) on photocatalytic degradation of NTA has been studied. Photo-Fenton reagent (Fe³⁺/H₂O₂) as a homogenous catalyst was found to be much more efficient than the TiO₂-based heterogeneous catalyst Degussa P25. Under optimum conditions NTA in a simulant of a spent decontamination solution without or with hydrazine could be degraded within 5 or 9 hours, respectively. The study of sorption properties of a series of absorbers revealed that radiostrontium and radiosilver can be effectively removed from the simulant of a spent decontamination solution even in the presence of NTA, while total NTA degradation is necessary for effective radiocobalt separation.