Observables of interest for the characterisation of Spent Nuclear Fuel

The characterisation of Spent Nuclear Fuel (SNF) in view of intermediate storage and final disposal is discussed. The main observables of interest that need to be determined are the decay heat, neutron and -ray emission spectra. In addition, the inventory of specific nuclides that are important for criticality safety analysis and to verify the fuel history has to be determined. Some of the observables such as the decay heat and neutron and -ray emission rate can be determined by Non-Destructive Analysis (NDA) methods. Unfortunately, this is not always possible especially during routine operation. Hence, a characterisation of SNF will rely on theoretical calculations combined with results of NDA methods. In this work the observables of interest, also referred to as source terms, are discussed based on theoretical calculations starting from fresh UO2 and MOX fuel. The irradiation conditions are representative for PWR. The Serpent code is used to define the nuclides which have an important contribution to the observables. The emphasis is on cooling times between 1 a and 1000 a.JRC.G.2-Standards for Nuclear Safety, Security and Safeguard

Effect of hydrogen gas and leaching solution on the fast release of fission products from two PWR fuels

To study the dissolution of UOX spent nuclear fuel in a deep geological environment and the fast release of a selection of relevant radionuclides for long-term safety of this high level waste, leaching experiments were performed with spent nuclear fuel samples originating from the pressurized water reactors (PWRs) Tihange 1 and Gösgen with a similar burnup (50 – 55 MWd.kgHM−1) but different irradiation histories. Six experiments were conducted to investigate the effect of two critical parameters: (1) the highly alkaline environment caused by the presence of cementitious materials in the “Supercontainer design”, which is currently the reference design for the long-term management of the high-level nuclear waste forms in Belgium, and (2) the reducing conditions imposed by the presence of hydrogen from the corrosion of iron-based materials present in the engineered barriers. The experiments were performed using autoclaves under pressure from 1 to 40 bar with a pure Ar atmosphere or a mixture of H2/Ar. Divided into two consecutive phases, the total experimental duration was about 1400 days. The Phase I provided mainly information about the fast release of the fission products while the perspective of the Phase II was to study the long-term evolution of the spent fuel matrix. During the leaching experiment, concentrations of a selection of radionuclides (238U, 129I, 137Cs, 90Sr and 99Tc) were monitored in solution and the amounts of Kr and Xe were measured in the gas phase. Based on results of the experiments conducted for up to 40 months (i.e. during Phase I of the experimental program), we observe that there is a continuous release of 137Cs, 90Sr and of the fission gases and a clear impact of the irradiation history on the release of certain fission products

Topological (Sliced) Doping of a 3D Peierls System: Predicted Structure of Doped BaBiO3

At hole concentrations below x=0.4, Ba_(1-x)K_xBiO_3 is non-metallic. At x=0, pure BaBiO3 is a Peierls insulator. Very dilute holes create bipolaronic point defects in the Peierls order parameter. Here we find that the Rice-Sneddon version of Peierls theory predicts that more concentrated holes should form stacking faults (two-dimensional topological defects, called slices) in the Peierls order parameter. However, the long-range Coulomb interaction, left out of the Rice-Sneddon model, destabilizes slices in favor of point bipolarons at low concentrations, leaving a window near 30% doping where the sliced state is marginally stable.Comment: 6 pages with 5 embedded postscript figure

Solvent Optimization Studies for a New EURO-GANEX Process with 2,2’-Oxybis( N,N -di- n -decylpropanamide) (mTDDGA) and Its Radiolysis Products

The diglycolamide 2,2’-oxybis(N,N-di-n-decylpropanamide) (mTDDGA) is being studied as an extractant for actinides and lanthanides in the European Grouped Actinide Extraction (EURO-GANEX) process. The aim is the development of a more simplified process using a single extractant instead of a mixture of extractants used in the current EURO-GANEX process. This work presents solvent optimization studies of mTDDGA, with regards to the extraction characteristics of the different diastereomers of mTDGA and of mixed diastereomer solutions. Also radiolysis behavior has been studied by irradiation of solvent extraction systems in a gamma irradiation facility using $^{60}$Co. The availability of irradiated organic solutions made it possible to gain valuable insights into the plutonium loading capacity after gamma-irradiation of the solvent up to 445 kGy and to quantify degradation compounds. Solvent extraction characteristic of the major degradation compounds themselves were determined. Like other methylated diglycolamides, we found a remarkable difference in extraction of up to two orders of magnitude between the two diastereomers. High plutonium loading (36 g L$^{−1}$) is feasible using this single extractant, even after absorbing a dose of 445 kGy. This remarkable observation is possibly promoted by the presence of the main degradation compound which extracts plutonium verywell

A non-destructive method to determine the neutron production rate of a sample of spent nuclear fuel under standard controlled area conditions

A method to determine the neutron production rate of a sample of spent nuclear fuel by means of non-destructive analysis conducted under controlled-area conditions is described, validated and demonstrated. A standard neutron well-counter designed for routine nuclear safeguards applications is applied. The method relies on a transfer procedure that is adapted to the hot-cell facilities at the laboratory for high and medium level activity of the SCK CEN. The sample transfer and measurement procedures are described together with results of Monte Carlo simulations. Experiments with radionuclide sources were carried out at the Joint Research Centre to test the procedures and to determine the performance characteristics of the detection device. Finally, measurements of a segment of a spent nuclear fuel rod were carried out at the SCK CEN to validate and demonstrate the method.JRC.G.2-Standards for Nuclear Safety, Security and Safeguard

An absolute measurement of the neutron production rate of a spent nuclear fuel sample used for depletion code validation

A method to determine the neutron production rate of a spent nuclear fuel segment sample by means of non-destructive assay conducted under standard controlled-area conditions is described and demonstrated. A neutron well counter designed for routine nuclear safeguards applications is applied. The method relies on a transfer procedure that is adapted to the hot cell facilities at the Laboratory for High and Medium level Activity of SCK CEN in Belgium. Experiments with 252Cf(sf) sources, certified for their neutron emission rate, were carried out at the Joint Research Centre to determine the characteristics of the detection device. Measurements of a segment of a spent nuclear fuel rod were carried out at SCK CEN resulting in an absolute and non-destructive measurement of the neutron production rate avoiding any reference to a representative spent nuclear fuel sample to calibrate the device. Results of these measurements were used to study the performance of depletion codes, i.e., ALEPH2, SCALE, and Serpent2. The study includes a code-to-code and code-to-experiment comparison using different nuclear data libraries

Depth-Sensing Indentation on REBa2Cu3O(7-\delta) Single Crystals obtained from Xenotime Mineral

A natural mixture of heavy rare earths oxides extracted from xenotime mineral have been used to prepare large single crystals of high-temperature REBa2Cu3O(7-\delta) superconductor grown using the CuO-BaO self-flux method. Its mechanical properties along the ab-plane were characterized using instrumented indentation. Hardness and elastic modulus were obtained by the Oliver and Pharr method and corresponds to 7.4 \pm 0.2 GPa and in range 135-175 GPa at small depths, respectively. Increasing the load promotes the nucleation of lateral cracks that causes a decrease in hardness and the measured elastic modulus by instrumented indentation at higher loads. The indentation fracture toughness was estimated by measuring the radial crack length from cube-corner indentations at various loads and was 0.8 \pm 0.2 MPa.m1/2. The observed slip systems of REBa2Cu3O(7-\delta) single crystals were [100](001) and [010](001), the same as for YBa2Cu3O(7-\delta) single crystals. The initial stages of deformation and fracture in the indentation process were investigated. The hardness and elastic modulus were not strongly modified by the crystallographic orientation in the ab-plane. This was interpreted in terms of the resolved shear stresses in the active slip systems. Evidence of cracking along the {100} and {110} planes on the ab-plane was observed. As a conclusion, the mechanical properties of REBa2Cu3O(7-\delta) single crystals prepared from xenotime are equivalent to those of YBa2Cu3O(7-\delta) single crystals produced by conventional rare earths oxides.Comment: The paper will appear in Volume 42 (2012) of the Brazilian Journal of Physic