Strong Neutron Absorbers Research Articles

D048 Analysis of Texture and Diffraction Data from Strong Neutron Absorbers

D048 Analysis of Texture and Diffraction Data from Strong Neutron Absorbers

Development and calibration of radio-isotope cone penetrometers

Radio-Isotope cone penetrometers (RI-cones) are developed to measure the basic soil properties like water content and soil density under in situ conditions in real time along with other parameters such as soil strength and hydrogeologic conditions of subsurface soils. In this paper, a brief description of these cone penetrometers is followed by their calibrations and the use of single calibration curve for different types of soils is discussed. Two case studies have been presented. It has been shown that under marine conditions a correction is required for the measured water content as the presence of chloride ions, a very strong neutron absorber, can underestimate the measured water content.

D-114 Analysis of Texture from Strong Neutron Absorbers

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Preparation of thin-film neutron converter foils for imaging detectors

With the pending delivery of the new Spallation Neutron Source at Oak Ridge National Laboratory, a new generation of highly efficient, position sensitive neutron detectors needs to be developed. One promising prospect for detector development has been the neutron imaging detector consisting of a conversion foil, which produce amongst other things /spl alpha/ and /spl gamma/ particles from incident neutrons, and a position sensitive charge particle detector. In order to understand the materials chemistry between layered converters as well as to maximize the production and transmission of the daughter products, we have prepared and characterized a series of thin-films from naturally occurring isotopes of the strong neutron absorbers lithium (Li), boron (B), and gadolinium (Gd). The films have been created using a combination of sputtering and evaporation techniques. The composition of the resulting films have been investigated using EDS and RBS, while the interfacial mixing between the layers has been studied using a combination of RBS and SEM experiments. Experiments studying the aging of the detector films in the detector gas as well as alloying of Li and the film substrate, have also been undertaken and the results are reported here.

Determination of the neutron flux in the reactor zones with the strong neutron absorption and leakage

The procedures for the numerical and experimental determination of the neutron flux in the zones with the strong neutron absorption and leakage are described in this paper. Numerical procedure is based on the application of the SCALE-4.4a code system where the Dancoff factors are determined by the VEGA2DAN code. Two main parts of the experimental methodology are measurement of the activity of irradiated foils and determination of the averaged neutron absorption cross-section in the foils by the SCALE-4.4a calculation procedure. The proposed procedures have been applied for the determination of the neutron flux in the internal neutron converter used with the RB reactor core configuration number 114.

The Critical Mass Laboratory at Rocky Flats

The Critical Mass Laboratory (CML) at Rocky Flats northwest of Denver, Colorado, was built in 1964 and commissioned to conduct nuclear experiments on January 28, 1965. It was built to attain more accurate and precise experimental data to ensure nuclear criticality safety at the plant than were previously possible. Prior to its construction, safety data were obtained from long extrapolations of subcritical data (called in situ experiments), calculated parameters from reactor engineering “models,” and a few other imprecise methods. About 1700 critical and critical-approach experiments involving several chemical forms of enriched uranium and plutonium were performed between then and 1988. These experiments included single units and arrays of fissile materials, reflected and “bare” systems, and configurations with various degrees of moderation, as well as some containing strong neutron absorbers. In 1989, a raid by the Federal Bureau of Investigation (FBI) caused the plant as a whole to focus on “resumption” instead of further criticality safety experiments. Though either not recognized or not admitted for a few years, that FBI raid did sound the death knell for the CML. The plant’s optimistic goal of resumption evolved to one of deactivation, decommissioning, and plantwide demolition during the 1990s. The once-proud CML facility was finally demolished in April of 2002.

Characterization of a Source Importance Function in an Accelerator-Driven System

An importance function of the external spallation neutrons in an accelerator-driven system (ADS) has been introduced and characterized to address the source multiplication in a subcritical blanket. For a model ADS problem with a central external source, the source importance function is evaluated with a neutron transport code system. For a homogeneous core, essential characteristics of the importance are identified from the viewpoint of spatial distributions and energy dependency, etc. The importance function is evaluated for two different beam tube diameters, and its dependency on the buffer thickness is also addressed. In order to assess the impact of the power distribution on the importance function, a heterogeneous core is considered, and its importance function is evaluated. The analyses show that the peak importance occurs in the inner fuel blanket zone, not in the central source region, and the neutron importance in a high-energy regime, above 7 to 20 MeV, is high and increases with the energy. Also, the effects of a neutron absorber on the source importance are studied, and it is found that the source importance could be drastically reduced by surrounding the source with a strong neutron absorber such as B4C. In addition, the source importance function is compared with the conventional λ-mode adjoint flux, which is used as an importance function of fission neutrons in critical reactors. The comparison reveals that the inhomogeneous source importance function could be quite similar to the homogeneous λ-mode adjoint flux in both spatial and spectral distributions for a wide range of subcriticality.

Matrix-type multiple reciprocity method applied to the modified Helmholtz neutron flux mode in nuclear criticality system

The matrix-type multiple reciprocity method has been reformulated to analyze the modified Helmholtz neutron flux mode in a domain which has a strong neutron absorption. The fundamental solutions for the modified Helmholtz mode can be given simply by replacing the imaginary quantity iB 1 in the conventional Helmholtz-type solutions by a real quantity k 1. Test calculations demonstrate the validity of the present reformulation and the usefulness when applied to the nuclear criticality analysis.

A Preliminary Consideration for Underground Autocatalytic Criticality by Vitrified High-Level Waste in Water-Saturated Geologic Repository

A preliminary assessment is conducted for the autocatalytic criticality phenomena for the Japanese repository concept, where the vitrified high-level waste (HLW) from reprocessed fuel of commercial light-water reactors is assumed to be disposed of in fractured granitic rock saturated with water. By a static neutronic analysis, the mass of 235U in 12%-enriched uranium required to make the transition from an under-moderated to an over-moderated critical system in water-saturated granite of a porosity of 0.3 is obtained 35 kg in a spherical core with a radius of 90 cm. The radionuclide transport analysis indicates that in any case the only fissile nuclide of concern for the autocatalytic criticality scenario is 235U. Other fissile nuclides are too short-lived, or are accompanied with a strong neutron absorber. This preliminary study indicates that the transport model can be simplified for uranium isotopes, with which a statistical uncertainty analysis can be performed to finally determine whether or not the autocatalytic criticality event can be screened out from the scenarios for the generic performance assessment of the conceptual repository.

Phonons in isotopically modified semiconductors and high Tc superconductors

Following the fall of the Iron Curtain there is an increasing availability of stable isotopes at affordable prices, a fact which has encouraged the production of single crystals with variable isotopic compositions. These samples have been copiously used to gain information on phonons. Here we briefly review the information obtained. It involves: (1) Dependence of the lattice parameters on isotopic masses. (2) Dependence of anharmonic self-energies on isotopic masses. (3) Thermal conductivity. (4) Dependence of frequencies and widths (self energies) on isotopic disorder. (5) Mass-disorder-induced first order Raman spectra. (6) Renormalization of electronic states through electron–phonon interaction. (7) Determination of phonon eigenvectors from the dependence of frequencies on isotopic mass. Not only elemental but also compound crystals have been studied; in the latter the mass dependence of a given physical property is different for the various constituent elements. Isotope substitution has also been used when one of the constituent atoms is a strong neutron absorber (e.g. 113Cd), to determine phonon dispersion relations by inelastic neutron scattering on crystals in which the absorbing isotope is absent. This work has been recently performed for CdS and CdSe.

Burnup waves

Burnup waves are regions of strong neutron absorption that move through absorbing media as a consequence of burnup of the absorbing nuclide(s). Such waves are analysed in both purely absorbing and diffusive media. The shape and propagation velocity of the reaction zone are studied, the latter depending on the type of boundary condition for the neutron source and the type of absorber as characterized by the ‘complete burnup’ and ‘partial burnup’ case. A preliminary discussion is presented of a possible application of burnup waves in diffusive media.

Odd-isotope enrichment studies of Gd by double resonance laser-ionization for the production of burnable nuclear reactor poison

Because of its large neutron capture cross-section, gadolinium (Gd) is widely used as burnable poison in nuclear reactors. Only two of its naturally occurring seven isotopes are strong neutron absorbers; hence, it would be desirable to enrich Gd in those isotopes. We demonstrate experimentally that significant enrichment can be achieved by two-step laser-ionization by relatively broad-band (bandwidth 1–2 GHz) lasers and by utilizing the isotope shifts only. Our results indicate that the content of the strong absorbers can be raised from the natural 30% to nearly 70%. We also compare the performance of this partially enriched Gd to natural Gd and to pure 157Gd, the most strongly absorbing isotope, as burnable poison.

Non-linear burn-up waves in opaque neutron absorbers / Nichtlineare Abbrandwellen in optisch dicken Neutronenabsorbern.

Unlike in thermonuclear fusion configurations, the phenomenon of burn-up waves is not known in strong thermal neutron absorbers. In the following analysis we investigate the burn-up of a strong and opaque neutron absorber exposed to a thermal neutron flux. It is shown that a bum-up wave propagates slowly through the absorber, obeying a non-linear partial differential equation. Since the shapes of the neutron flux, the particle density, and the reaction rate density do not change during the propagation, they are identified as soliton solutions stabilized by non-linearities alone.

Moisture Measurement by Neutron Moisture Cone Penetrometer: Design and Application

A new neutron-moisture cone penetrometer has been designed and developed at the Kyoto University. It uses Cf 252 as the source for fast neutrons and He 3 -filled proportional tube as the detector. In the present paper the principles of neutron action with the matter is briefly reviewed and some of the test results are presented. It has been shown that this newly developed Neutron-Moisture Cone Penetrometer (hereafter NM-cone penetrometer) can measure soil strength parameters such as penetration resistance ( q c ), friction ratio ( f s ), pore pressure ( u ) as well as natural water content ( w n %) for the entire depth of the penetration. This instrument enables us to have continuous data rather than discreet data which are obtained using other types of sampling technique. It has been observed that the presence of strong neutron absorber could give erroneous results, therefore, the knowledge of the composition of the soil is necessary. One of the biggest advantages of having such a system is that it eliminates the need to procure samples which may or may not represent the actual in-situ conditions.

Effect of alteration, formation absorption, and standoff on the response of the thermal neutron porosity log in gabbros and basalts: Examples from Deep Sea Drilling Project‐Ocean Drilling Program Sites

This study focuses on the effects of hydrous alteration minerals, formation absorption, and standoff on the response of the thermal neutron porosity log in the basaltic and gabbroic rocks logged at sites 395, 418, 504, 642, and 735 during the Ocean Drilling Program. The concentration of hydrogen present in the rocks in the form of free water (pore space) and bound water (hydrous minerals) is the primary factor controlling the neutron elastic scattering process, while the presence of other elements, such as chlorine, gadolinium, boron, lithium, and samarium in the fluids and in the rock matrix can largely affect the thermal diffusion phase. These neutron absorbers cause an increase of the capture cross section, and in turn of the apparent thermal porosity. Further perturbations occur when the recording conditions depart from those under which the tool has been calibrated; a large and irregular hole diameter and a lack of eccentralization both produce erroneous porosity readings. The effect of hydrous alteration minerals on the thermal neutron porosity log has been estimated from 922 core oxide measurements using an analysis program that calculates the slowing‐down length and converts it into apparent porosity. The results show that the computed apparent porosity ranges from less than 1% in fresh basalts and gabbros to about 30% in highly altered units. Depending on the alteration mineral assemblage, natural gamma ray, capture cross section, or hydrogen logs have been used to continuously predict the effect of bound hydrogen at each site. Corrected porosities generally show excellent agreement with core data for massive units, whereas they are higher for pillow basalts and fractured zones. The discrepancy is interpreted as the result of (1) difference in the volume of rock investigated (core specimens do not sample large vugs and fractures) and (2) frequent variations in the hole size and lack of tool contact with the borehole wall (standoff), not completely accounted for in the corrections. High capture cross section values measured on selected basalt samples seem to be associated with more altered basalts, suggesting a larger concentration of strong neutron absorbers in secondary minerals. The concentrations of elements with large capture cross section (Σ) measured at three of the five sites studied yield a maximum increase of 1% in the apparent neutron porosity. This is too low to explain the porosity values, some greater than 7%, observed in some of the fresh and unfractured massive units. A 1‐ to 2‐inch standoff due to the lack of eccentralization of the tool string used in the Ocean Drilling Program is considered the most probable cause of such discrepancy. This interpretation is confirmed by the neutron porosity values recorded at site 735, where the nuclear string was for the first time fully eccentralized, and only a correction for borehole size and the presence of hydrous minerals has been applied.

Control rod worth in high conversion PWR.

In a High Conversion PWR (HCPWR), the neutron spectrum is hardened in comparison with the conventional PWR and the nuclear characteristics of the control rod is much different. We have calculated the control rod worth in a HCPWR for several materials of strong neutron absorbers and selected B4C for the control rod material. For the B4C control rod, we have investigated the various nuclear characteristics such as the spatial and energy dependency of neutron absorption, the dependency of the control rod worth on moderator to fuel volume ratio and Pu enrichment etc., and obtained the useful data for the nulear design of control rods in a HCPWR.

Evaluation of the linear extrapolation distance for strong neutron absorbers by the Sn method

The neutron flux distribution has been calculated around a cylindrical, totally-absorbing rod immersed in moderators of various size and degree of absorption. Isotropic scattering was assumed as well as a uniform source distribution. From the results obtained with a standard ANISN code the linear extrapolation distance was derived. It was found necessary to introduce a correction for the fact that ANISN does not give an exact balance between production and loss of neutrons. After such a correction the linear extrapolation distance could be obtained to an accuracy of 1–2%. Through using two different definitions of the extrapolation distance it was possible to reproduce the values of Pellaud (1968) and Isakova (1968) and to explain the difference between their results.

Development of the moments method for neutron gauging

In the present investigation the new methodology of neutron moisture probe, based on measurements of the spatial moments of the slow neutron fluxes, is developed. Within the framework of the present work calibration curves for moments of low orders were calculated and recursive relations for high-order moments were obtained on the base of a P-1 approximation and diffusion theory. The neutron flux distributions obtained from a semiempirical method [5], three-group diffusion and age theories for the moments calculation, were investigated. It is shown that the spatial moments of neutron flux could serve as a basis for measurements of the volume weighted moisture and the content of strong neutron absorbers in the medium.

Elemental interferences in the analysis of uranium by delayed-neutron counting

Major physical and elemental effects influencing U analysis were investigated at the fully automated Risø delayed-neutron counting facility. Changes in the physical conditions at the analytical facility can be closely controlled and most physical interference parameters are of minor importance for the analysis. Experiments were performed with artificial granitic and basaltic rocks to estimate the actual influence of Be, B, Li, Al, Na, Mn, Fe, rare earth elements and Th on the U analysis. The neutron counting rate from U is increased by about 1% when 1000 ppm Be are added to a 5-g sample. For the strong neutron absorbers B and Cd the correction factors are about −1 and −2%, respectively, per 1000 ppm interfering element present. For the rare earth elements Sm and Gd these correction factors were −2.2% and −10.2% per 1000 ppm added element, respectively. High gamma-radiation fluxes from Al, Mn and Fe have no significant effect on the U analysis.

The Sidewall Epithermal Neutron Porosity Log

Abstract A sidewall epithermal neutron tool has been developed to substantially reduce environmental effects that have previously complicated neutron log interpretation. Designed for operation in uncased wells, the device provides increased accuracy in both liquid-filled and empty holes. A brief discussion of neutron moderation, diffusion and capture shows that logs using epithermal neutron detection depend on a smaller number of formation-characterizing parameters than those using thermal neutron or capture gamma ray detection. Thus, they come closer to providing an unambiguous determination of hydrogen content. In this new device a directionally sensitive epithermal neutron detection system has been incorporated in a sidewall source-detector skid to minimize borehole effects. The effects of variations in borehole size and shape, mud type, temperature and salinity are greatly reduced. Small residual borehole effects are then computationally accounted for in the surface control panel to provide a borehole-corrected neutron log. The log presents a direct recording of neutron-derived porosity on a linear scale. With corrections for bore hole effects already applied, this direct recording of porosity simplifies log interpretation. Furthermore, comparison with a linear porosity presentation of a formation density log permits sandstones, limestones and dolomites to be readily identified. Thus, in complex or variable lithology, porosity is determined with greater accuracy and reliability that heretofore. Laboratory data and field results demonstrate the improvements in neutron logging afforded by this new sidewall epithermal neutron logging device. Introduction The new Sidewall Neutron Porosity (SNP) logging system, designed for use in uncased wells, provides reliability and accuracy never before achieved with neutron logs. The effects of variations in borehole diameter and shape, fluid salinity, mud weight and temperature parameters that have long complicated neutron log interpretationare suppressed or corrected for by this sidewall epithermal neutron detection system. Furthermore, to simplify interpretation the SNP log presents a direct recording of Computed porosity on a linear scale. The performance improvements achieved by this new system arise primarily from the combination of two important design features. First, though not unique to this new tool, an epithermal neutron detection system is used, Epithermal neutron detection substantially reduces the perturbing influences of the thermal neutron absorption properties of rock matrices and water salinity. Second, the neutron detection system is mounted in a directionally sensitive sidewall skid to greatly minimize borehole effects. The purpose of this paper is to explain briefly the advantages offered by the detection of epithermal neutrons, to describe the SNP equipment and the log, to present calibration and correction data and to give examples of interpretation methods made especially convenient by this new system. ADVANTAGES OF EPITHERMAL NEUTRON DETECTION Epithermal neutron detection, of all the neutron methods in current commercial use, provides the simplest determination of formation hydrogen content. Advantages of epithermal neutron detectors, as compared with thermal neutron and gamma ray detectors, are probably best explained by a brief review of the life of a source-emitted neutron. A fast neutron from the source will eventually be captured by the nucleus of an atom. However, before capture is likely to occur, the fast neutron (energy of 100,000 electron volts (ev) or more) will be slowed down until it is in thermal equilibrium with its surroundings. The neutron is then considered to be a "thermal" neutron (average energy of 0.025 ev at 25C). The overwhelming majority of the slowing down thus occurs in reaching epithermal energies (energies just above thermal 0.5 to several ev). After reaching thermal energy, the neutron will "diffuse" through the formation with, on the average, no further energy change until capture. Upon the capture of the thermal neutron, relatively high energy gamma rays are usually emitted. Hydrogen plays a very important role in the process of slowing down fast neutrons. On the other hand, the absorption effects of water salinity and low concentrations of other strong thermal neutron absorbers relatively unimportant in the moderation process are very important in thermal diffusion, capture and the production of gamma rays. Thus, by detecting only epithermal neutrons with the SNP, spurious effects due to these thermal neutron absorbers are greatly minimized as compared to the effects on tools using thermal neutron or gamma ray detectors. The epithermal neutron detection system affords another important advantage. JPT P. 1351ˆ