Spallation Source SINQ Research Articles

Experimental investigation of residual stress distribution on girth welds fabricated at proximity using neutron diffraction technique

Maintaining minimum distance between repair and existing welds often becomes impractical due to the presence of compact layouts, original welds of branches, nozzles, etc. on offshore structural elements, pipelines, and piping. Although some international codes and standards provide criteria for maintaining a minimum distance between proximity welds, most of them lack technical justification in relation to their effect on the structural integrity of welded components. The development of residual stresses has a significant effect (i.e., a negative effect on tensile stresses and a positive effect on compressive stresses) on the integrity of the welds fabricated at proximity. Hence, it is important to investigate the residual stress distribution on welds fabricated at proximity, especially at a distance away from the weld toe. This study presents findings on the characterization of residual stresses by neutron diffraction at the proximity region between two girth welds. The two welds were fabricated at proximity, using two different welding procedure specifications and at three different distances, on a structural steel pipe, grade S355. The three different welding distances between weld toes were maintained at 5, 10, and 15 mm respectively. The neutron diffraction-based residual stress distributions were investigated at the POLDI neutron instrument at the Swiss spallation source SINQ in Switzerland. The axial, hoop, and radial components of the residual stresses were experimentally investigated between proximity welds. The findings revealed that residual axial stresses at a 5-mm proximity distance were increased beyond the yield strength of the structural steel, grade S355. The findings of this study enable practitioners to take remedial actions to minimize the residual stresses developed in girth welds fabricated at proximity. Also, the findings enable us to derive technical justification for maintaining a minimum distance and developing welding procedure for welds fabricated at proximity.

Evolution of the tensile properties of the tempered martensitic steel Eurofer97 after spallation irradiation at SINQ

Small disk tensile specimens of the reduced activation tempered martensitic steel Eurofer97 were tested from −100 °C up to 370 °C before and after irradiation in the spallation source SINQ at Paul Scherrer Institute. The calculated irradiation dose and helium content are respectively 11 dpa and 540 appm. Even for such a large dose, the tensile curves showed a significant total elongation at all testing temperatures. Fractographic observations revealed a predominance of cleavage facets at −100 °C for unirradiated and irradiated specimens while at higher temperature the fracture surfaces were dominated by ductile dimples. True stress/strain curves were derived using a finite element based inverse method for simulating small disk tensile tests. The finite element simulations indicated that the local fracture stress in the neck is consistent with that calibrated based on fracture tests and that the micro-mechanisms of fracture were not affected by helium up to 540 appm.

In situ Polarized Neutron Reflectometry: Epitaxial Thin-Film Growth of Fe on Cu(001) by dc Magnetron Sputtering

The step-wise growth of epitaxial Fe on Cu(001)/Si(001), investigated by in-situ polarized neutron reflectometry is presented. A sputter deposition system was integrated into the neutron reflectometer AMOR at the Swiss neutron spallation source SINQ, which enables the analysis of the microstructure and magnetic moments during all deposition steps of the Fe layer. We report on the progressive evolution of the accessible parameters describing the microstructure and the magnetic properties of the Fe film, which reproduce known features and extend our knowledge on the behavior of ultrathin iron films.

Radiochemical separation of 7Be from the cooling water of the neutron spallation source SINQ at PSI

Summary 7Be is a key radionuclide for investigation of several astrophysical processes and phenomena. In addition, it is used as a tracer in wear measurements. It is produced in considerable amounts in the cooling water (D2O) of the Spallation Induced Neutron Source (SINQ) facility at PSI by spallation reactions on 16O with the generated fast neutrons. A shielded ion-exchange filter containing 100 mL of the mixed-bed ion exchanger LEWATIT was installed as a bypass for the cooling water into the cooling loop of SINQ for three months. The collected activity of 7Be was in the range of several hundred GBq. Further, the 7Be was separated and purified in a hot-cell remotely-controlled using a separation system installed. With the exception of 10Be, radioactive byproducts can be neglected, so that this cooling water could serve as an ideal source for highly active 7Be-samples. The facility is capable of producing 7Be with activities up to 1 TBq per year. The 7Be sample preparation is described in detail and the possible uses are discussed. In particular some preliminary results of 7Be ion beam production are presented.

Cold-neutron tomography of annular flow and functional spacer performance in a model of a boiling water reactor fuel rod bundle

Dryout of the coolant liquid film at the upper part of the fuel pins of a boiling water reactor (BWR) core constitutes the type of heat transfer crisis relevant for the conditions of high void fractions. It is both a safety concern and a limiting factor in the thermal power and thus for the economy of BWRs. We have investigated adiabatic, air–water annular flows in a scaled-up model of two neighboring subchannels as found in BWR fuel assemblies using cold-neutron tomography. The imaging of the double suchannel has been performed at the ICON beamline at the neutron spallation source SINQ at the Paul Scherrer Institute, Switzerland. Cold-neutron tomography is shown here to be an excellent tool for investigating air–water annular flows and the influence of functional spacers of different geometries on such flows. The high-resolution, high-contrast measurements provide the spatial distributions of the coolant liquid film thickness on the fuel pin surfaces as well as on the surfaces of the spacer vanes. The axial variations of the cross-section averaged liquid hold-up and its fraction in the gas core shows the effect of the spacers on the redistribution of the two phases.

Cold neutron tomography of annular coolant flow in a double subchannel model of a boiling water reactor

Dryout of the liquid coolant film on fuel pins at the top of boiling water reactor (BWR) cores constitutes the type of heat transfer crisis relevant for the conditions of high void fractions. It is a limiting factor in the thermal power, and therefore the economy, of BWRs. Ongoing research on multiphase annular flow, specifically the liquid film thickness, is fundamental not only to nuclear reactor safety and operation but also to that of evaporators, condensers, and pipelines in a general industrial context. We have performed cold neutron tomography of adiabatic air water annular flow in a scaled up model of the subchannel geometry found in BWR fuel assemblies today. All imaging has been performed at the ICON beamline at the neutron spallation source SINQ at the Paul Scherrer Institut in Switzerland. Neutron tomography is shown to excel in investigating the interactions of air water two phase flows with spacer vanes of different geometry. The high resolution, high contrast measurements provide spatial distributions of the coolant on top of the surfaces of the spacer, including the vanes, and in the subchannel downstream of the spacers.

POLDI: Materials Science and Engineering Instrument at SINQ

The time-of-flight (TOF) diffractometer POLDI (Pulse Overlap DIffractometer) operational at the continuous spallation source SINQ was specifically designed and optimised for the study of residual stresses and mechanical behaviour in engineering materials. The novel feature of POLDI compared to typical TOF instruments is the use of pulse overlap, whereby the faster neutrons of a pulse emitted from the slits of a chopper can catch up the slower neutrons from the previous slit. In this case, the flight time cannot be simply calculated by the arrival time of the neutron, but by recording the angular dependence of the neutron arrival time and the flight time of the neutron, the lattice spacing d can be determined [1]. In a plot of intensity vs. arrival time and scattering angle, each Bragg reflection is represented by a Bragg line (see Fig. 1). The slopes of these Bragg lines are given by Eq. (1):

Advanced on-target beam monitoring for spallation sources

The correct distribution of the incident proton beam density is crucial for the safe operation of high-power spallation targets. In all installations the proton beam footprint has to be monitored and any deviation from tight reference values must invariably lead to the prompt interruption of the irradiation. Based on several years’ experience with the dedicated optical system VIMOS at the Paul Scherrer Institute (PSI) neutron spallation source SINQ, issues, which turned out important and difficult, are highlighted.

Validation of Monte-Carlo simulations with measurements at the ICON beam-line at SINQ

ICON is the new cold neutron imaging facility at the neutron spallation source SINQ. The ICON facility is placed at beam-line S52 with direct view to the cold liquid D 2 moderator. The beam-line includes a 4.4 m long collimation section followed by a 11 m long flight path to the imaging system. The essential part of the collimation section is composed of six revolving drums and a variable aperture wheel. Depending on the investigated object, different apertures are used. Measurements have shown that each setup has a different spatial neutron flux distribution and specific beam profiles. Measured beam profiles have been used to validate results of simulations coupling the Monte-Carlo program MCNPX with the neutron ray-tracing program McStas. In a first step, MCNPX was used to calculate neutron spectra closed to the SINQ target, at the entrance of the collimation section. These results served as an input for McStas where the beam-line itself was simulated. In the present paper, experimental and theoretical results will be compared and discussed.

Diffusion of Cu in AlCu alloys of different composition by quasielastic neutron scattering

Quasielastic cold neutron scattering experiments on molten Al1−xCux alloys (x=0.10, 0.17 and 0.25) have been performed at three different temperatures on the FOCUS time-of-flight spectrometer at the Swiss neutron spallation source SINQ. As the main part of the scattering at small Q is incoherent and originating from Cu, self-diffusion coefficients D for the Cu ions could be determined from the widths of quasielastic peaks. The derived values are considerably larger than those found in capillary tube measurements. Even if the accuracy in the determination of D is better than about 4% it could not be concluded whether the temperature variation of D follows an Arrhenius law or a power law.

The 2D small angle detector project at the FOCUS time-of-flight spectrometer at SINQ

A small-angle detector will complete the performance of the cold time-of-flight spectrometer FOCUS at the Swiss Spallation Source SINQ of the Paul Scherrer Institute, Villigen, Switzerland. It is constructed from position-sensitive 3 He tubes located

Monte Carlo simulations for instrumentation at SINQ

The Paul Scherrer Institut (PSI) operates a spallation source SINQ equipped with 11 different neutron scattering instruments. Beside the optimization of the existing instruments, the extension with new instruments and devices are continuously done at PSI. For design and performance studies different Monte Carlo packages are used. Presently two major projects are in an advanced stage of planning. These are the new thermal neutron triple-axis spectrometer Enhanced Intensity and Greater Energy Range (EIGER) and the ultra-cold neutron source (UCN-PSI). The EIGER instrument design is focused on an optimal signal-to-background ratio. A very important design part was to realize a monochromator shielding which covers best shielding characteristic, low background production and high instrument functionality. The Monte Carlo package MCNPX was used to find the best choice. Due to the sharp energy distribution of ultra-cold neutrons (UCN) which can be Doppler-shifted towards cold neutron energies, a UCN phase space transformation (PST) device could produce highly monochromatic cold and very cold neutrons (VCN). The UCN-PST instrumentation project running at PSI is very timely since a new-generation superthermal spallation source of UCN is under construction at PSI with a UCN density of 3000–4000 n cm −3. Detailed numerical simulations have been carried out to optimize the UCN density and flux. Recent results on numerical simulations of an UCN-PST-based source of highly monochromatic cold neutrons and VCN are presented.

The TOF-strain scanner POLDI with multiple frame overlap—concept and performance

The new neutron diffractometer POLDI for residual stress measurements at the continuous spallation source SINQ is in full operation since 2003. The instrument is a novel type of time-of-flight diffractometer with multiple frame overlap, a method, which allows tuning to high intensity and high resolution simultaneously. In this paper the concept and the most relevant components of the instrument are presented and an example for the performance is given.

The Digital User Office (DUO)

The Digital User Office (DUO) has been developed at the Swiss Light Source (SLS) of the Paul Scherrer Institut (PSI) in order to deal efficiently with the large number of users typical of a synchrotron radiation facility. DUO is based on an Oracle database. Thus, in addition to the canonical functions expected from such a system, such as support of the proposal processing and preparation of conferences, it offers additional novel functions. Active linking of the DUO database to other databases permits the creation of computer accounts, automatic setup of experiment parameters at the start of a measurement or preparing information on publications for reporting semi-automatically. Developed initially for the SLS at the Paul Scherrer Institut, DUO is currently being extended to include in an integrated central PSI user office the user communities of the other large facilities at PSI: the neutron spallation source SINQ, the μSR facility SμS, and the proton accelerator. Currently, the system is dealing effectively with far over one thousand users per year and hundreds of proposals so that in terms of scalability the multiplication of these numbers expected in coming years should present no problem.

Small punch tests on austenitic and martensitic steels irradiated in a spallation environment with 530 MeV protons

Mechanical properties of the austenitic steel 316L (both solution annealed and cold worked) and the low activation martensitic steel Optifer have been investigated by small punch tests. The samples, in the form of TEM disks, were irradiated in the target of the spallation source SINQ to displacement doses of up to 11 dpa. Values for yield strength, ductility and fracture energy as evaluated from punch tests are given.

Non-destructive analysis of nuclear fuel by means of thermal and cold neutrons

The article describes the experimental procedures, detection methods and resulting information when thermal or cold neutrons are used for non-destructive inspections and quantitative analysis of fuel elements from nuclear power plants. Due to the high penetration capability of slow neutrons through natural uranium and different attenuation properties for the individual uranium isotopes, a high sensitivity in respect to the enrichment in U-235 exists. Both the structural integrity and the material distribution inside the pellets can be determined by neutron imaging methods.This holds for fresh fuel before insertion into the reactor core but also for spent fuel after reloading and storage. In the latter case, more demanding effort with respect to shielding is needed and adapted detection methods have to be applied because of the high radiation background from the fuel itself. The facility NEUTRA at the spallation source SINQ is able to manage the high quality neutron imaging procedures with fuel, the special set-up NEURAP allows it to do such measurements with pellets of high burn-up too.

New features in cold neutron radiography and tomography Part II: applied energy-selective neutron radiography and tomography

Abstract The neutron attenuation coefficient drops for many solid materials quite drastically at a defined cold neutron energy known as a Bragg-cut-off in the cross-section diagrams. In many cases, the drop in attenuation for the corresponding elements is significant and this behavior can be exploited to change the material contrast in radiography and tomography images by modifying the spectrum of the applied neutron beam. The energy-dependent experiments were performed at the Prompt Gamma-ray Activation beam line where the irradiation position is at the end of a curved neutron guide, which delivers cold neutrons from the spallation source SINQ (PSI, Switzerland). This beam position gave the opportunity to perform radiography and tomography at low neutron energies. An effective monochromatization of the primary neutron beam was obtained by using a neutron velocity selector. The intensity of the modified beam was still reasonable for radiography images at different neutron energies and the experiments were performed in relatively short measuring times. A variety of samples were studied to illustrate possible applications of energy-selective radiography and tomography. This new neutron imaging technique provided encouraging results and projects of developing permanent facilities for such investigations at PSI and FRM II are under study.

The k0-method in cold-neutron prompt gamma-ray activation analysis

The k0-standardization method has been applied and evaluated at the cold-neutron prompt gamma-ray activation (PGA) facility of the Swiss spallation source SINQ (Paul Scherrer Institute). The k0-factors for 26 elements of interest were measured using chlorine as a comparator. The results showed good agreement with the values determined at other cold and thermal neutron guided beams, except for a few elements. Then, standard reference materials were analyzed to assess the accuracy of the method using the obtained k0-factors. Finally, the technique was used for multielement determination in various samples coming from nuclear waste storage, geochemistry and geology. In addition, the non-destructive nature of PGAA offered an interesting application in archeology.

A high-temperature cell for in situ small-angle neutron scattering studies of phase separation in alloys

Abstract A furnace for small-angle neutron scattering on samples heated from room temperature to 1500°C was constructed and is now available on the SANS instrument at the spallation source SINQ of the Paul Scherrer Institut in Villigen, Switzerland. Solid solution treatments and quick temperature steps are feasible combined with a motorized positioning system to precisely adjust the sample. The furnace works under high vacuum (better than 10 −4 mbar ) or under controlled atmosphere. Special measures were taken to reduce parasitic scattering from the sample environment. Remote control when installed on the instrument as well as stand-alone operation for calibration and test purposes are feasible. Results of small-angle neutron scattering experiments on a polycrystal and a single crystal of Ni– 11.3 at % Ti aged in situ at 600°C for up to 100 h are presented.

New features in cold neutron radiography and tomography: Part I: thinner scintillators and a neutron velocity selector to improve the spatial resolution

The tomography setup developed at the PGA facility of the Swiss spallation source SINQ has provided encouraging results in the field of cold neutron imaging. Performances of the detection system based on a CCD camera and a converter screen have been recently improved using 6LiF/ZnS:Ag scintillators with different thickness. Indeed, reducing the layer of the scintillator improved considerably the spatial resolution while keeping a reasonable efficiency. Furthermore, a neutron velocity selector was temporarily added to the setup to perform radiography and tomography experiments with monochromatic neutron beams. Basic properties of transmitted beams were studied to assess the applicability of this device in neutron imaging. Of interest was the enhancement of the L/ D-ratio by selecting neutron beams of short wavelengths. Cold neutron tomography demonstrated to be a useful technique in various spheres of activity, such as aerospatial industry or radioactive waste storage. Various applications of the neutron velocity selector are described in the second part, namely, New features in cold neutron radiography and tomography—Part II: Applied energy-selective neutron radiography and tomography.