D. Ridikas

Modelling of HTRs with Monte Carlo: from a homogeneous to an exact heterogeneous core with microparticles

Abstract Recently a worldwide interest in HTR technology has been experienced. In this context the gas turbine modular helium-cooled reactor (GT-MHR) is a potential candidate for the maximum 239 Pu destruction in a once-through cycle. A particular feature of GT-MHR is that its coated fuel (TRISO particles) is supposed to provide an impermeable barrier to the release of fission products and, at the same time, to resist very deep burn-up rates (more than 90% for 239 Pu). In this paper a Monte Carlo approach is employed to characterise the neutron fluxes and the fuel evolution inside the tiny 200-μm diameter fuel kernels with an exact and finite geometry description. Our major goal is to obtain a quantitative comparison of different geometry sets, namely homogeneous versus single-heterogeneous and double-heterogeneous, in terms of k eff eigenvalues, the length of the fuel cycle, neutron characteristics and the evolution of fuel composition in particular. In all cases the same Monteburns (MCNP+ORIGEN) code system is used. We show that the performance of GT-MHR is considerably influenced by the way its geometry is modelled within the Monte Carlo approach. The spatial and energy shielding of the neutron flux even in such small particles cannot be neglected for important isotopes which have high resonance cross sections as 240 Pu, 241 Pu and 167 Er. Namely, the formation of 241 Pu and burn-up of 167 Er are responsible for the different length of the fuel cycle, being the shortest for a double-heterogeneous geometry. On the other hand, the evolution of 239 Pu at a constant reactor power and comparable neutron fluence is very similar for all three geometry configurations.

Detection of nuclear material by photon activation inside cargo containers

Photons with energies above 6 MeV can be used to detect small amounts of nuclear material inside large cargo containers. The method consists in using an intense beam of high-energy photons (bremsstrahlung radiation) in order to induce reactions of photofission on actinides. The measurement of delayed neutrons and delayed gammas emitted by fission products brings specific information on localization and quantification of the nuclear material. A simultaneous measurement of both of these delayed signals can overcome some important limitations due to matrix effects like heavy shielding and/or the presence of light elements as hydrogen. We have a long experience in the field of nuclear waste package characterization by photon interrogation and we have demonstrated that presently the detection limit can be less than one gram of actinide per ton of package. Recently we tried to extend our knowledge to assess the performance of this method for the detection of special nuclear materials in sea and air freights. This paper presents our first results based on experimental measurements carried out in the SAPHIR facility, which houses a linear electron accelerator with the energy range from 15 MeV to 30 MeV. Our experiments were also modeled using the full scale Monte Carlo techniques. In addition, and in a more general frame, due to the lack of consistent data on photonuclear reactions, we have been working on the development of a new photonuclear activation file (PAF), which includes cross sections for more than 600 isotopes including photofission fragment distributions and delayed neutron tables for actinides. Therefore, this work includes also some experimental results obtained at the ELSA electron accelerator, which is more adapted for precise basic nuclear data measurements.

Photonuclear Physics in Radiation Transport - III: Actinide Cross Sections and Spectra

Abstract This paper describes model calculations and nuclear data evaluations of photonuclear reactions on actinides such as 235U, 238U, 237Np, and 239Pu for incident photon energies from the reaction threshold up to 20 MeV. The calculations are done using the GNASH code, including the giant-dipole resonance for photoabsorption. The emission of secondary particles is computed using a preequilibrium theory, together with an open-ended sequence of the compound nucleus decay using the Hauser-Feschbach theory. The accuracy of the calculated and evaluated cross sections is assessed through extensive comparison with measured cross sections. This work also summarizes evaluation methods used to create actinide photonuclear files for the forthcoming ENDF/B-VII database, which will facilitate radiation transport studies related to photonuclear reactions in a number of technologies including production of photoneutrons and photofission fragments in electron accelerators, shielding studies, and nondestructive detection of nuclear material in particular.

Description of the European Helium-Cooled EFIT Plant: An Industrial-Scale Accelerator-Driven System for Minor Actinide Transmutation—II

In order to reduce the volume and the radiotoxicity of the nuclear waste coming from the operation of existing pressurized water reactors, accelerator-driven systems (ADSs) have been envisioned. The Helium-cooled (He) European Facility for Industrial-scale Transmutation (He-EFIT) concept is the EUROpean Research Programme for the TRANSmutation of High Level Nuclear Waste in Accelerator Driven System (EUROTRANS) Integrated Project (IP) (EUROTRANS IP) backup option, whereas Pb-cooled EFIT is the reference one. The plant has a power of [approximately]400 MW(thermal). Like all ADS plants, it consists of three main components: the accelerator, the spallation target module, and the subcritical core. This paper describes the He-EFIT design at the end of the EUROTRANS IP as well as the studies performed to support this design: spallation performances, trasmutation capabilities, and plant safety analyses. No specific technology deadlock has been identified, and it might be possible to build such a plant given necessary research and development in support.

Development of Fission Micro-Chambers for Nuclear Waste Incineration Studies

The INCA (INCineration by Accelerator) project of the Directorate for Science of Matter of the French Atomic Energy Authority (CEA/DSM) aims to outline the ideal physical conditions to transmute minor actinides in a high intensity neutron flux obtained either by hybrid systems or innovative critical reactors. To measure on-line the incineration rates of minor actinides, we are developing an innovative double deposit fission chamber (DDFC) working in current mode. Our method is based on a comparison between the isotope under study and a reference material whose nuclear parameters are well known, as 235 U and 239 Pu. This new fission chamber will be used in the High Flux Reactor in Grenoble/France in a neutron flux of 1.2 10 15 n . cm -2. s -1 for 50 days, the operating cycle of the reactor. These specific experimental conditions require substantial modifications of the existing chambers. The first experiment will be carried out in fall 2000.

Delayed neutron yields and spectra from photofission of actinides with bremsstrahlung photons below 20 MeV.

Experimental results for the photofission of 238U with an endpoint Bremsstrahlung emission of 15 MeV are presented. Absolute yield and time characteristics of the delayed neutrons are extracted. In parallel, calculations for fission fragment distributions and corresponding delayed neutron parameters are given and compared to data.

The Mini-Inca Project: Experimental Study of the Transmutation of Actinides in High Intensity Neutron Fluxes

A new experimental installation has been recently commissioned at the High Flux Reactor of Institut Laue-Langevin (ILL) in Grenoble (France). It gives access to high intensity neutron spectra from pure thermal (5.6 1014 n/s/cm2) to epithermal (2 1015 n/s/cm2). Several of low mass (10 µg) mono-isotopic targets of actinide elements are in the process of being irradiated and analyzed by a number of techniques, from nuclear spectroscopy to off-line mass spectrometry and innovative double-deposit fission micro-chambers. In the present paper we will present the first experiments carried out at the thermal neutron spectrum installation with 242Pu, 241-243Am samples.

Comparative Analysis of ENDF, JEF and JENDL Data Libraries by Modelling High Temperature Reactors and Plutonium Based Fuel Cycles

The gas turbine modular helium-cooled reactor (GT-MHR)1) is known probably as the best option for the maximum plutonium destruction in once-through cycle, even though the industrial fabrication of coated particle fuel still has to be proved. We perform detailed simulations along these lines by comparing different sets of data libraries in terms of keff eigenvalues, the length of the fuel cycle, neutronic characteristics and the evolution of fuel composition in particular. In all cases the same Monteburns code system2) is used making our results dependent only on the evaluated data tables. We show that in general the performance of GT-MHR is not considerably influenced by the choice of the data libraries employed. Nevertheless, a number of major differences among ENDF, 3) JENDL4) and JEF5) data files are identified and quantified in terms of the averaged one-group cross sections both for military (MPu) and civil (CPu) plutonium based fuel cycles.

Some Test Calculations with the IAEA Photonuclear Data Library

As a result of an international cooperation a recommended file of photonuclear reactions of 164 isotopes became available. The formal and practical quality of these data are examined by using them for calculation of photoneutron production in thick targets bombarded by photons and electrons. The Monte Carlo calculations are accomplished with a recent version of the MCNP code enhanced with a photonuclear capability. The energy range of our investigations is from the photonuclear threshold up to 150 MeV. In some cases, a comparison between our calculations and a new version of the MCNPX code, recently updated with the LANL photonuclear data files (12 isotopes) is established. Our results are also compared with the experimental data when available. These data include early measurements of the total neutron yields from thick targets bombarded by electrons as a function of energy, target thickness and atomic number, also, recent measurements of 238 U photofission yields. We show that in general the agreement between the experimental and calculated results is satisfactory. However, a number of particular examples when it is not the case seems to exist.

Status of the Photonuclear Data Library for CINDER’90

In this article we present the development of a new photonuclear activation library for the CINDER’90 evolution code (LANL). The IAEA evaluations for 164 isotopes are explicitly included and more than 600 isotopes are added using the HMS‐ALICE predictions. GNASH evaluations are used in the case of actinides.