ANSTO :: Browsing by Author "Ritchie, AIM"

Browsing by Author "Ritchie, AIM"

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Application of the pulsed neutron technique to fast metal systems
(Australian Atomic Energy Commission, 1973-01) Moo, SP; Rainbow, MT; Ritchie, AIM
The experimental apparatus and techniques which have been developed up to the end of 1971 at the AAEC Research Establishment at Lucas Heights to allow pulsed experiments to be performed in fast metal systems are described. The details of a metallic thorium assembly, the pulsed neutron source, timing systems and shielding and room return problems are discussed.
The calculation of angular neutron spectra from the thick target Li(p,n) source
(Australian Atomic Energy Commission, 1977-10) Rainbow, MT; Ritchie, AIM; Sullivan, L
Neutron energy spectrum of the thick target Li(p,n) source is well suited to neutronics experiments on fast reactors. In such applications the angular distribution of the source energy spectrum is required in a form suitable for reactor calculations. A method is described of using various experimental and theoretical estimates of the angular cross section of the Li(p,n) reaction to estimate the angular yield from a thick target in a form suitable for reactor calculations. The problem is discussed of determining the integration limits for angle integrated quantities appropriate to particular values of incident proton energy and target thickness. The resulting calculational techniques have been incorporated in a computer code LIPNA written for an IBM360/65. An ancillary, interactive program, written for a NOVA computer, prepares the input data for LIPNA, obviating the need for the occasional user to remember details of the code input requirements.
Flow and diffusion of a two-species gas mixture in a porous slab.
(Australian Atomic Energy Commission, 1977-10) Ritchie, AIM
Problem of mass transport of oxygen is considered in a soil whose upper surface is open to air at atmospheric pressure and in which the oxygen is preferentially removed at some reaction surface within the body of the soil. The soil is assumed to be a permeable slab filled with a two-species gas mixture, the first of which is removed at one face of the slab, while the other face is maintained at constant pressure (~ atmospheric) . It is shown that the maximum removal rate of the first gas is largely determined by the diffusion coefficient of the second gas, but enhanced to some extent by gas flow due to the pressure difference set up by gas removal. This pressure difference is very small (~ 10-4 atmospheres) even at the maximum removal rate. It is also shown that under some conditions, which are satisfied in the case of oxygen removal from air, the concentration of the first gas has the same functional dependence on distance, as expected in the case of diffusion with no gas flow, but with an enhanced diffusion coefficient. For the problem of interest, this enhancement is about 20 per cent.
From Rum Jungle to Wismut - Reducing the environmental impact of uranium mining and milling
(The Institution of Engineers Australia, 1994-05-01) Zuk, WM; Jeffree, RA; Levins, DM; Lowson, RT; Ritchie, AIM
Australia has a long history of uranium mining. In the early days, little attention was given to environmental matters and considerable pollution occurred. ANSTO has been involved in rehabilitation of a number of the early uranium mining sites, from Rum Jungle in Australia's Northern Territory to Wismut in Germany, and is working with current producers to minimise the environmental impact of their operations. ANSTO's expertise in amelioration of acid mine drainage, radon measurements and control, treatment of mill wastes, management of tailings, monitoring of seepage plumes, mathematical modelling of pollutant transport and biological impacts in a tropical environment are summarized.
Heap leaching: a gas diffusion rate-limited model.
(Australian Atomic Energy Commission, 1977-11) Ritchie, AIM
The consequences are examined of assuming that the oxidation rate of pyrites in an overburden dump is determined by the rate at which the oxygen required can be supplied from the top surface of the heap by diffusion through the pore space of the heap. It is shown that the oxidation reaction proceeds at a reaction front that moves away from the top surface of the heap at a rate determined entirely by the concentration of pyrites in the heap, the diffusion coefficient of oxygen in the pore space, the concentration of oxygen in the air and the ratio of the mass of oxygen to the mass of pyrites consumed in the oxidation. Applied to White's overburden dump at Rum Jungle, the model indicates oxidation rates of the same order as those inferred from field measurement. It also predicts that at the present stage of that heap's development the reaction front lies between 3 and 5 m from the surface, depending on the porosity, which is assumed to be between 20 and 40 per cent, and that the front advances 7.5 to 13 cm per year. If the reaction is catalysed by iron-oxidising bacteria, then on the basis of the model the bacterial population will be greatest in this comparatively small region, zero in the anaerobic region below the front, and small in the upper levels of the heap. Estimates are also made of the temperature rise at the reaction front and the temperature distribution, assuming that the only heat loss is by conduction through the soil. However, heat loss to water passing through the heap could be significant and a more sophisticated treatment may be required.
In situ determination of thermal conductivity of waste rock dump material
(Springer, 1997-09) Tan, Y; Ritchie, AIM
In situ determinations of thermal conductivity of waste rock dumps at three mine sites: Aitik, Sweden; Kelian, Indonesia; and Heath Steele, Canada are described. Measurements were carried out at various locations and depths at each site. The estimated thermal conductivities range from 0.62 to 1.63 W m-1K-1 with an average of 1.21±0.27 W m-1K-1 for the waste rock pile at the Aitik site, 1.04 to 1.25 W m-1K-1 with an average value of 1.17(0.11 W m-1K-1 for the Heath Steele site, and 1.57 to 2.08 W m-1K-1 with an average value of 1.89±0.21 W m-1K-1 for the Kelian site. The higher average thermal conductivity at the Kelian site could be attributed to higher moisture content in the waste rock pile because of high rainfall at this site. Thermal diffusivity was also evaluated at a region of the Aitik waste rock dump where the thermal conductivity was also determined using measured temperature profiles. The thermal diffusivity value determined this way was (5.75 ± 0.75) × 10-7 m2s-1, consistent with that calculated from the thermal conductivity values determined for the same location. © 1997, Springer.
Investigation of the response of a neutron moisture meter using a multigroup, two-dimensional diffusion theory code.
(Australian Atomic Energy Commission, 1984-12) Ritchie, AIM; Wilson, DJ
A multigroup diffusion code has been used to predict the count rate from a neutron moisture meter for a range of values of soil water content ω thermal neutron absorption cross section Sα (defined as SIGMA Σα/ρ) of the soil matrix and soil matrix density ρ two dimensions adequately approximated the geometry of the source detector and soil surrounding the detector. Seven energy groups the data for which were condensed from 128 group data set over the neutron energy spectrum appropriate to the soil-water mixture under study proved adequate to describe neutron slowing-down and diffusion. The soil-water mixture was an SiO2-water mixture with the absorption cross section of SiO2 increased to cover the range of SIGMA Σα required. The response to changes in matrix density is in general linear but the response to changes in water content is not linear over the range of parameter values investigated. Tabular results are presented which allow interpolation of the response for a particular ω, Sα and ρ. It is shown that R(ω.Sα. ρ)= ρ M(Sα) + C (ω)is a crude representation of the response over a very limited range of variation of ω and Sα. As the response is a slowly varying function of ρ, Sα, and ω a polynomial fit will provide a better estimate of the response for values of ρ, Sα and ω not tabulated.
Measurement of the slowing down time to 0.3 eV in beryllium oxide.
(Australian Atomic Energy Commission, 1970-07) Rainbow, MT; Ritchie, AIM
The reaction rates of plutonium-239 and boron (1/v) were measured as a function of space and time in a large block of beryllium oxide following the injection of a pulse of fast neutrons. The time-dependent reaction rates associated with the various Fourier spatial modes were derived from the experimental results. The experimental value of the peaking time of the plutonium-239 reaction rate associated with the fundamental spatial mode was found to be 19.85 + 1.1 μ sec. A theoretical value of 16.5 μ sec was obtained using a time-dependent zero-dimensional diffusion code and a crystal scattering kernel for beryllium oxide. It is in marked disagreement with the experimental value. Measurements made of the time to peak in the plutonium-239 reaction rate as a function of distance from the source showed a total variation of a 4 μ sec in peaking time over a distance of 53 cm. Theoretical calculations of the peaking time as a function of distance showed a similar variation and the general trends were in good agreement with the experimental result. There was, however, a constant discrepancy between the theoretical and experimental results of the order of that noted for the fundamental mode.
Measurement of the thermal neutron wave dispersion relations in BeO.
(Australian Atomic Energy Commission, 1971-05) Ritchie, AIM; Whittlestone, S
The amplitudes and phase shifts of thermal neutron waves were measured in a block of BeO 60.96 x 60.96 x 58.42 cm3 of average density 2.87 g cm-3 and having nominal transverse buckling of 0.0048 cm-2. The attenuation constants (α) and phase shift constants (ζ) were derived from such measurements at 18 different frequencies covering the range 52.4 to 506.1 Hz. The parameters of α2 - ζ2 and 2αζ were evaluated, normalised to a reference density of 2.96 g cm-3, and corrected for the density normalised transverse buckling of 0.00513 cm-2. The normalised and corrected parameters α2 - ζ2 and 2αζ were fitted to polynomials in the angular frequency ω, and the resulting thermal neutron diffusion parameters were compared with those derived from a λ(B2) experiment carried out on the same block of BeO. The parameters λα and L determined by the two methods are in good agreement, but the diffusion coefficient and diffusion cooling constant derived from the sine wave experiment are significantly higher. The high diffusion coefficient (by ~ 9 per cent) could be due to the effect of the sub-Bragg continuum although the values of the parameters α and ζ appeared to be independent of the distance from the source. Agreement of the parameters α2 - ζ 2 and 2αζ with the theoretical values is not good and reasons for the discrepancy are advanced.
Measurement of thermal neutron waves at high frequencies in BeO
(Australian Atomic Energy Commission, 1972-03) Ritchie, AIM; Whittlestone, S
The amplitudes and phases of neutron waves have been measured in a BeO assembly in the frequency range 515.7 Hz. The results confirm earlier measurements that below ~ 520 Hz the neutron wave has the properties of a discrete mode of propagation. At frequencies > 720 Hz the attenuation α(z) and phase shift parameters ζ(z) change with distance from the source, the change being more marked the higher the frequency. The observed spatial variations of α(z) and ζ(z) do not agree with present theoretical predictions. Interference effects predicted by theory have not been observed.
Measurements in pulsed BeO assemblies with decay constants in the region of Corngold's limit.
(Australian Atomic Energy Commission, 1968-05) Rainbow, MT; Ritchie, AIM
Decay curves have been measured in BeO assemblies with bucklings in the range 1.93 x 10 -2 cm-2 to 5.34 x 10 -2 cm-2 at times from immediately after the initial pulse to at least 4.5 ms after the pulse. The decay curves for assemblies with B2 < 2.91 x 10 -2 cm-2 can be described by an exponential, from a minimum of 1.5 ms after the pulse to greater than 3.0 ms after the pulse. For bucklings greater than B2 = 3.3 x 10 -2 cm-2 the decay curves cannot be described by an exponential in this time region, and the departure from exponential increases with increased buckling. The results indicate a transition region at B2 ~ 3.0 x 10 -2 cm-2, (λ ~ 3.65 x 10 3 sec-1) which is consistent with the expression proposed by Kothari for the upper limit on discrete decay constants and well above Corngold's limit of λ* = (υΣinel) min ~ 2.5 x 10 3 sec-1. The transition point does not have any marked shape dependence. There is some shape dependence in the behaviour of decay curves for assemblies with B2 > 3.3 x 10 -2 cm-2 but little dependence of pulse length, or counter shape and position.
Model for the description of oxidation in sulfidic waste rock dumps.
(Australian Nuclear Science and Technology Organisation, 2000-03) Pantelis, G; Ritchie, AIM; Stepanyants, YA; Bennett, JW
Basic mathematical equations which describe the processes of sulfide oxidation and gas and water transport in waste rock dumps are presented and discussed. The governing equations account for gas and water flow vaporisation and condensation with latent heat effects heat transport and mass balance. Gas water and solid phases are assumed to be in local thermal equilibrium at all times. Air is approximated as an ideal three-component gas. Different semi-empirical relationships between physical values are used: Darcy's law for fluid flow ideal gas law the Van Genuchten formula for the relationship between degree of water saturation and pressure head Mualem's formula for the relative hydraulic conductivity as a function of pressure head etc. Some important global quantities such as the fraction of sulfide sulfur oxidised and the global oxidation rate are defined and considered as functions of time. The full set of equations is collected and presented in explicit form convenient for further numerical modelling. The glossary of some technical terms and the table of definitions of the main parameters as well as their units and characteristic values are displayed.
Neutron yields and energy spectra from the thick target Li(p,n) source.
(Australian Atomic Energy Commission, 1975-06) Ritchie, AIM
Angle-dependent spectra of neutrons, emitted by a thick lithium target when bombarded with protons in the energy range 1.881 to 3 MeV, have been evaluated from experimental and theoretical values of the angular distribution of neutrons emitted in the 7Li(p,n)7Be reaction. The variation of d2N/dEndΩ, with proton energy at different angles of emission, and with angle of emission for different neutron energies, is presented for the ground and excited state reactions. The angle-integrated neutron spectrum is given for different incident proton energies.
A pulsed neutron measurement in BeO and derivation of the thermal neutron diffusion parameters from the λ (B2) curve
(Australian Nuclear Science and Technology Organisation, 1967-11) Ritchie, AIM
The decay constant of the neutron population was measured in BeO assemblies of various sizes. Particular attention was paid to the problem of 'room return', establishment of the asymptotic mode, and the appearance of the continuous mode of decay. The best fit to the λ(B2) curve gave λa = (1.954 > 0.076) x 10 2 sec -1 Do = (1.343 + 0.011) x 10 5 sec -1 cm 2 C = (4.88 + 0.31) x 10 5 sec - 1 cm 4, for BeO density 2.96 g cm -3 at a temperature of 22oC. The parameters can be used to derive L = 26.22 + 0.51 cm λtr = 1.619 + 0.013 cm. The parameterisation of the λ(B2) curve is discussed in conjunction with the problem of deciding when agreement exists between two separate measurements.
Square and sine wave modulation of the beam from a 3 MeV Van de Graaff accelerator
(Australian Atomic Energy Commission, 1967-09) Fraser, HJ; Ritchie, AIM; Whittlestone, S
Methods of producing square and sine wave modulation of a 3 MeV Van de Graaff accelerator are described. Square pulses are produced at frequencies from 100kc/s to d.c. with up to a 100 per cent duty cycle and a minimum pulse length of 8 μs. The ratio of the off-current level to the on-current level is less than 5 x 10 -4 and can be further reduced by a post-acceleration deflection system to less than 10 -5. The unit, which is triggered from a command pulse external to the machine, is in the top terminal of the Van de Graaff and has shown itself to be highly reliable over some 1,000 hours of machine operation. The sine wave modulation unit described here operates in the range 80-1000 c/s with a modulation depth of up to 90 per cent and higher harmonic contamination of less than 1 per cent.
Time dependent 237Np, 235U and 239Pu fission rates in a thorium assembly during the interval 0 to 200 ns using a pulsed 9Be(d,n) source part I - experiment
(Australian Atomic Energy Commission, 1973-03) Moo, SP; Rainbow, MT; Ritchie, AIM
This paper describes a series of integral pulsed neutron experiments performed in a 0.4 x 0.4 x 0.4 m3 metallic thorium assembly in such a way as to allow direct comparison of space independent reaction rates with calculated reaction rates derived from a code which uses the asymptotic reactor theory approximation to describe leakage. The technique relies on the Fourier decomposition of measured space-time dependent reaction rates and the extraction from these of the reaction rate corresponding to the fundamental three-dimensional Fourier spatial mode. The reaction rates measured were the fission rates of 235U, 239Pu and 237Np following a short (~ 10 ns) burst of neutrons with a mean energy of ~ 2.7 MeV.
Time dependent 237Np, 235U and 239Pu fission rates in a thorium assembly during the interval 0 to 200 ns using a pulsed 9Be(d,n) source part II - theory
(Australian Atomic Energy Commission, 1974-02) Moo, SP; Rainbow, MT; Ritchie, AIM
Calculations are presented of the 237Np, 235U and 239Pu fundamental mode fission rates in a thorium assembly based on a diffusion theory code with a DB2 leakage term. Three different thorium cross section sets derived from the ABBN set, the UKNDL68 file and the ENDF/B-II file were used in the calculations and the results compared with each other and with experimental results reported previously. The sensitivity of the instantaneous decay constant of the fundamental mode to changes in partial cross sections is investigated.
The use of flat-vee weirs for measuring runoff from mine waste dumps
(Australian Atomic Energy Commission, 1981-11) Ritchie, AIM
A description is given of the installation of prefabricated, timber, flat-vee weirs to measure the discharge rates in the principal runoff streams from waste rock dumps at the abandoned mine site at Rum Jungle, Northern Territory. Details are presented of the tapping point, stilling well, stilling well/tapping point connection and the technique used to measure gauge height. The relationship between discharge rate and gauge head for the weir structure installed is discussed. The high gradient and high silt load of the run-off streams have led to silt deposition behind the weirs. Information is given on the silt levels and their effect of the discharge/gauge head relationship. The total error on the discharge from uncertainties in stream bed depth, uncertainties in the discharge coefficient and uncertainties in the measured gauge height, is estimated to be less that 10% in the range 5 x 10-2 m3 s-1 to 0.9 m3 s-1, the upper rate being just greater than the maximum discharge rate observed. The error on the total discharge in an event, a parameter of interest in estimating runoff coefficients and total pollution loads, was in general less than this. The weirs have proved robust under field conditions; the weir on White's runoff channel is still in use five years after installation in October 1075, and that on the Intermediate runoff channel, in use three years after installation in November 1977.

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