Speciation Of Fission Products Research Articles

Fission product speciation across a UO2 spent nuclear fuel

This article presents the preparation, the realization and the results of the first X-ray absorption spectroscopy analysis of a bulky irradiated UO2 sample. The studied sample was taken from a high burnup fuel rod irradiated for 5 years in a nuclear reactor. HERFD-XANES spectra at the K edge of molybdenum and at the L3 edge of cæsium were measured at different radial positions of the original pellet. Important information on the chemical forms of Mo in the spent fuel could be obtained: Mo is mainly in the form of two metallic phases, or in solution in UO2, the nature of these “dissolved” species is discussed in details. At the Cs L3 edge, several experimental difficulties were encountered for this first experiment, and the data obtained, although promising, are more difficult to interpret. They show, nonetheless, that the cæsium is in the same chemical form(s) in the center and on the periphery of the pellet, probably predominantly dissolved in UO2.

Coupled Thermal-Hydraulic Analysis and Species Mass Transport in a Versatile Experimental Salt Irradiation Loop (VESIL) Using CTF

With the resurgence of interest in molten salt reactors, there is a need for new experiments and modeling capabilities to characterize the unique phenomena present in this fluid fuel system. A Versatile Experimental Salt Irradiation Loop (VESIL) is currently under investigation at Idaho National Laboratory to be placed in the Advanced Test Reactor (ATR). One of the key phenomena this proposed experiment plans to elucidate is fission product speciation in the fuel-salt and the subsequent effects this has on the fuel-salt properties, source term generation, and corrosion control. Specifically, noble gases (Xe & Kr) will bubble out to a plenum or off-gas system, and noble metals (Mo, Tc, Te, etc.) will precipitate and deposit in specific zones in the loop. This work extends the mass transfer and species interaction models in CTF (Coolant-Boiling in Rod Arrays—Two Fluids) and applies these models to give a preliminary estimation of fission product behavior in the proposed VESIL design. A noble metal–helium bubble mass transfer model is coupled with the thermal-hydraulic results from CTF to determine the effectiveness of this insoluble fission product (IFP) extraction method for VESIL. Amounts of IFP species extracted to the off-gas system and species distributions in VESIL after a 60-day ATR cycle are reported.

Modelling of fission products release in VERDON-1 experiment with cGEMS: Coupling of severe accident code MELCOR with GEMS thermodynamic modelling package

The treatment of chemistry and thermodynamics in the integral severe accident codes is typically limited. A more accurate treatment of the chemistry during the severe accident modelling is, therefore, of great interest. For this purpose, the work is focused on the development of coupling of the MELCOR code with chemical thermodynamic calculations using GEMS codes and HERACLES database. Developed coupling between the two codes, called cGEMS, allowed for the improved thermodynamic description of the fission product release from the nuclear fuel under severe accident conditions.VERDON-1 test was selected as an experimental reference for the simulations. Experimental release behaviour of Mo, Cs and Ba observed in VERDON-1 experiment was reproduced by the developed coupled code. Performed simulations provided detailed information about the fission product speciation at different redox conditions.The obtained information and the developed code provides a more accurate description of the fission product behaviour and release during severe accidents.

Fission product speciation in the VERDON-3 and VERDON-4 MOX fuels samples

Within the framework of the International Source Term Programme, the VERDON-3 and VERDON-4 complementary tests were devoted to studying MOX fuel and fission product (FP) behaviour at very high temperature (>2300 °C) under oxidising and reducing atmospheres. Post-test qualitative and quantitative characterisations of the VERDON-3 and VERDON-4 samples were carried out in the LECA-STAR facility at the CEA Cadarache centre. This article focuses on the impact of the atmosphere on fission product speciation and more precisely on the behaviour of Cs, Mo and Ba. The well-known white inclusions composed of metallic Mo, Ru, Rh, Pd and Tc underwent transformations during the two tests implying melting and/or composition changes. The semi-volatile behaviour of Mo was once again confirmed. Almost no Cs was seen to remain in the samples after the VERDON-3 and VERDON-4 tests, which is consistent with its volatile behaviour. The behaviour of Ba, which is known to be released in high quantities in reducing atmospheres, provided complex to assess as significant releases seems to have taken place in both VERDON-3 and VERDON-4 tests.

Modeling of Aerosol Fission Product Scrubbing in Experiments and in Integral Severe Accident Scenarios

Aerosol scrubbing experiments are recalculated for selected POSEIDON-II series tests and TEPCO-TOSHIBA-HITACHI tests with the MELCOR/SPARC code and with the BUSCA code. The major uncertainties in the calculations are identified and the results of the analyses are used in sensitivity simulations for long-term station blackout accident sequence in a boiling water reactor (BWR) with MELCOR/SPARC. The accident sequence is similar in nature to what happened in Fukushima Unit-3.In the analyses of experiments, the basic characteristics of the thermal-hydraulic behavior were captured very well by MELCOR/SPARC. The trends in the calculated values of decontamination factors (DFs) for aerosols were in good agreement with the data, predicting the dependence of decontamination on the effect of the submergence depth, of steam content in the gas, and of aerosol particle size. However, the absolute values of DFs as calculated by the codes were rather sensitive to changes in the default input options and the agreement with experiments was not convincing.In the integral BWR sequence simulations, the predicted DFs for aerosols in the wetwell (WW) was sensitive in the same way as for the experiments. For this type of a scenario, the fission product (FP) releases to containment—represented by Cs and I compounds—were mostly discrete events of short duration. The release path was from the reactor vessel to WW during the operation of the safety relief valves and subsequent venting of the containment from the WW gas space. Even though the passage through the WW water in these simulations was the only way for fission products to reach the environment, the sole scrubbing potential of the wetwell was not the determining contributor to the FP retention. At least of the same importance were the details of the sequence progression, timing of events, FP speciation, and other factors.

Local atomic structure of Pd and Ag in the SiC containment layer of TRISO fuel particles fissioned to 20% burn-up

The structure and speciation of fission products within the SiC barrier layer of tristructural-isotropic (TRISO) fuel particles irradiated to 19.6% fissions per initial metal atom (FIMA) burnup in the Advanced Test Reactor (ATR) at Idaho National Laboratory (INL) was investigated. As-irradiated fuel particles, as well as those subjected to simulated accident scenarios, were examined. The TRISO particles were characterized using synchrotron X-ray absorption fine-structure spectroscopy (XAFS) at the Materials Research Collaborative Access Team (MRCAT) beamline at the Advanced Photon Source. The TRISO particles were produced at Oak Ridge National Laboratory under the Advanced Gas Reactor Fuel Development and Qualification Program and sent to the ATR for irradiation. XAFS measurements on the palladium and silver K-edges were collected using the MRCAT undulator beamline. Analysis of the Pd edge indicated the formation of palladium silicides of the form PdxSi (2 ≤ x ≤ 3). In contrast, Ag was found to be metallic within the SiC shell safety tested to 1700 °C. To the best of our knowledge, this is the first result demonstrating metallic bonding of silver from fissioned samples. Knowledge of these reaction pathways will allow for better simulations of radionuclide transport in the various coating layers of TRISO fuels for next generation nuclear reactors. They may also suggest different ways to modify TRISO particles to improve their fuel performance and to mitigate potential fission product release under both normal operation and accident conditions.

Fission products and nuclear fuel behaviour under severe accident conditions part 3: Speciation of fission products in the VERDON-1 sample

Qualitative and quantitative analyses on the VERDON-1 sample made it possible to obtain valuable information on fission product behaviour in the fuel during the test. A promising methodology based on the quantitative results of post-test characterisations has been implemented to assess the release fraction of non γ-emitter fission products. The order of magnitude of the estimated release fractions for each fission product was consistent with their class of volatility.

When the dust settles: stable xenon isotope constraints on the formation of nuclear fallout

Nuclear weapons represent one of the most immediate threats of mass destruction. In the event that a procured or developed nuclear weapon is detonated in a populated metropolitan area, timely and accurate nuclear forensic analysis and fallout modeling would be needed to support attribution efforts and hazard assessments. Here we demonstrate that fissiogenic xenon isotopes retained in radioactive fallout generated by a nuclear explosion provide unique constraints on (1) the timescale of fallout formation, (2) chemical fractionation that occurs when fission products and nuclear fuel are incorporated into fallout, and (3) the speciation of fission products in the fireball. Our data suggest that, in near surface nuclear tests, the presence of a significant quantity of metal in a device assembly, combined with a short time allowed for mixing with the ambient atmosphere (seconds), may prevent complete oxidation of fission products prior to their incorporation into fallout. Xenon isotopes thus provide a window into the chemical composition of the fireball in the seconds that follow a nuclear explosion, thereby improving our understanding of the physical and thermo-chemical conditions under which fallout forms.

Modelling of fission-product transport in the reactor coolant system

The Phébus fission product (FP) programme studies the phenomenology of severe accidents in water-cooled nuclear reactors. Five tests were performed in the frame of the programme covering fuel-rod degradation and FP behaviour released via the coolant system into the containment. To model FP transport and behaviour in the coolant system, numerous physical and chemical phenomena have to be taken into account. In the vapour phase, for example, FP speciation, vapour condensation and vapour/surface or vapour/aerosol reactions have to be considered. The aerosol phase has to be modelled with nucleation, growth and deposition processes. Finally, remobilisation phenomena like resuspension and revaporisation have to be taken into account for delayed release into the containment. Four Phébus FP tests (FTP0, FPT1, FPT2, FPT3) have been modelled with the ASTEC/SOPHAEROS code. Modelling shows an overall good estimation of retention for the main FPs (e.g., I, Cs, Mo). Furthermore, a strong connection is revealed in the gaseous phase chemistry between I, Cs, Cd and Mo which has a great impact on gaseous iodine release into the containment. The Phébus FP test modelling also exposes disagreement on FP retention when laminar gaseous flow is not well developed. Finally, probably the most significant shortcoming in modelling that Phébus-FP tests highlighted concerns vapour-phase iodine-chemistry modelling at low temperature. The study of this latter point is on going with the experimental programme ISTP/CHIP.

LWR Severe Accident Simulation Fission Product Behavior in FPT2 Experiment

The Phebus Fission Product (FP) program studies key phenomena and phenomenology of severe accidents in water-cooled nuclear reactors. In the framework of the Phebus program, five in-pile experiments were performed that cover fuel rod degradation and behavior of FPs released via the coolant circuit into the containment vessel. Analyses of FP behavior were performed using standard stand-alone versions of codes with input data mainly taken from measured boundary conditions.The FPT2 test used 33 GWd/t uranium dioxide fuel enriched to 4.5%, reirradiated in situ for 7 days to a burnup of 130 MWd/t. This test was designed to study low-pressure FP release and transport through a primary cooling system that included a noncondensing steam generator, with release into the containment vessel in steam-poor conditions. This test also investigated how diluted boric acid in the injected steam influenced FP speciation. In the containment vessel, the objective was to study iodine chemistry in an alkaline sump under evaporating conditions.The analytical approach consisted of progressive studies to explore and explain the main disagreements between base-case calculations and experimental results. Regarding releases in slightly degraded fuel zones, the fission gas behavior and characteristics are shown to be satisfactorily reproduced by the calculations. Electron microprobe analyses also validate the mechanisms for Mo and Ba releases, while the Cs mechanism requires further investigation.Concerning the transport of FPs, a strong connection is shown to exist between Cs, I, Mo, and Cd that substantially impacts vapor-phase chemistry in equilibrium and iodine volatility. Most of the cesium released from fuel is shown to rapidly convert into cesium borates and then into cesium molybdates when the molybdenum release becomes significant at the end of the hydrogen production phase. The main predicted iodine vapor species is cesium iodide. A low fraction of gaseous hydrogen iodide is also calculated at low temperature, but this fraction was found to be strongly dependent on the Cd release kinetics. Hydrogen iodide is the main candidate predicted by equilibrium chemistry calculations to explain the persistence at low temperatures of volatile iodine. Nevertheless, potential limitations on chemical kinetics in the primary circuit zones, characterized by a sharp decrease in temperature, could also be an explanation and are currently under investigation.In the containment, gas-phase reactions were found to predominate in governing iodine chemistry. As for the previous Phebus tests, the gaseous iodine fraction measured in the containment early in the test is thought to come from the primary circuit. However, this low gaseous iodine was hardly tractable by the few dedicated samplings mounted in the primary circuit cold leg upstream from the containment entrance. By favoring hydrolysis reactions of volatile iodine species, the alkaline sump is shown to act as an iodine trap despite the evaporating conditions that prevail during the long-term chemistry phase. However, during this latter phase, a persistent, low-level concentration of gaseous iodine was reached in the long term, as during the previous FPT0/1 tests, indicative of a competition in the containment between iodine traps and sources. Aside from the aerosol particles injected by the primary circuit, in situ iodine oxide particles were found to be continuously forming from the decomposition of I2 and ICH3 by air radiolysis products. These particles are suspected to be fine, implying that they predominantly deposit by diffusion on all the containment surfaces. Therefore, in the long term, both the persistence of gaseous iodine and the survival of iodine oxide particles are shown to exist in the containment.

Separation and speciation of fission products by zirconium phosphate prepared by solid-solid reaction

Zirconium phosphate hase been prepared by solid-solid reaction. The characterization of this inorganic ion exchanger was done with respect to capacity, pH titration, dissolution, etc. The preconcentration and separation of fission products, Sr(II), Co(II), Eu(III), Gd(III), Hg(II) and Am(III) has been achieved from different acidic media, HCl, HNO3, H2SO4, HClO4. The speciation of these cations in different media has been investigated. From the results obtained, it is obvious that these cations can be preconcentrated in low acidic medium, whilst at high acidity in the range 1-2 M, they be can separated from each other.

Speciation of fission products in raw cow milk by high-performance size exclusion radiochromatography

Speciation of fission products (90Sr,99Tc,137Cs and152Eu) in raw cow milk was studied using the high-performance size exclusion radiochromatography (HP-SERC) by Biosep-SEC-S® column (Phenomenex, USA). The separation fraction were identified by a UV detector (280 nm) and a scintillation radiodetector. The mobile phase was Jenness-Koops buffer that simulated milk serum. Radiocesium was present only in non-protein fractions. The non-protein fractions complexed 17% of technetium and 57% of strontium. About 32% of strontium and 24% of europium were bound on the high molecular weight proteins (micellar casein). Low molecular weight proteins bound about 11% of strontium and 76% of europium. Non-size effects play a dominant role at HPSERC speciation of technetium, 83% of which follow the lightest fractions of milk.