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Hindley M.P.,Pebble Ltd | Groenwold A.A.,Stellenbosch University | Blaine D.C.,Stellenbosch University | Becker T.H.,Stellenbosch University
Nuclear Engineering and Design | Year: 2014

This paper describes the process for approximating the optimal size of a link volume required for weakest link failure calculation in nuclear graphite, with NBG-18 used as an example. As part of the failure methodology, the link volume is defined in terms of two grouping criteria. The first criterion is a factor of the maximum grain size and the second criterion is a function of an equivalent stress limit. A methodology for approximating these grouping criteria is presented. The failure methodology employs finite element analysis (FEA) in order to predict the failure load, at 50% probability of failure. The average experimental failure load, as determined for 26 test geometries, is used to evaluate the accuracy of the weakest link failure calculations. The influence of the two grouping criteria on the failure load prediction is evaluated by defining an error in prediction across all test cases. Mathematical optimisation is used to find the minimum error across a range of test case failure predictions. This minimum error is shown to deliver the most accurate failure prediction across a whole range of components, although some test cases in the range predict conservative failure load. The mathematical optimisation objective function is penalised to account for non-conservative prediction of the failure load for any test case. The optimisation is repeated and a link volume found for conservative failure prediction. The failure prediction for each test case is evaluated, in detail, for the proposed link volumes. Based on the analysis, link design volumes for NBG-18 are recommended for either accurate or conservative failure prediction. © 2014 Elsevier B.V.

In the multi-pass fuel management scheme employed for the pebble bed modular reactor the fuel pebbles are re-circulated until they reach the target burn-up. The rate at which fresh fuel is loaded and burned fuel is discharged is a result of the core neutronics cycle analysis but in practice (on the plant) this has to be controlled and managed by the fuel handling and storage system and use of the burnup measurement system. The excess reactivity is the additional reactivity available in the core during operating conditions that is the result of loading a fuel mixture in the core that is more reactive (less burned) than what is required to keep the reactor critical at full power operational conditions. The excess reactivity is balanced by the insertion of the control rods to keep the reactor critical. The excess reactivity allows flexibility in operations, for example to overcome the xenon build up when power is decreased as part of load follow. In order to limit reactivity excursions and to ensure safe shutdown the excess reactivity and thus the insertion depth of the control rods at normal operating conditions has to be managed. One way to do this is by operational procedures. The reactivity effect of long-term operation with the control rods inserted deeper than the design point is investigated and a control rod insertion limit is proposed that will not limit normal operations. The effects of other phenomena that can increase the power defect, such as higher-than-expected fuel temperatures, are also introduced. All of these cases are then evaluated by ensuring cold shutdown is still achievable and where appropriate by reactivity insertion accident analysis. These aspects are investigated on the PBMR 400 MW design. © 2011 Elsevier B.V.

De Villiers G.J.,Pebble Ltd | Treurnicht J.,Stellenbosch University | Dobson R.T.,Stellenbosch University
Applied Thermal Engineering | Year: 2012

The Pebble Bed Modular Reactor Pty. Ltd. (PBMR) has called for research into the possibility of distributed in-core temperature measurement. In this paper, several methods for distributed temperature measurement in high-pressure, -radiation and -temperature environments have been investigated by means of a literature study. The literature study revealed fiber-Bragg grating (FBG) temperature sensors to be the most feasible solution to the temperature measurement challenge. Various parameters affecting the propagation of light in optical fibers and consequently the FBG reflection profile was investigated. The differential equations describing FBG structures were solved and implemented in Matlab in order to simulate wavelength division multiplexing (WDM) of a distributed FBG sensing system. Distributed sensing with apodized FBGs written into the sapphire optical fibers is considered. Temperature measurement using wavelength division multiplexing of apodized FBGs written into silica optical fibers were also demonstrated in a test platform. The measured results corresponded with the theory. It was found that when there is a strong temperature gradient across the FBG, spectral widening of the reflection profile occurs. This fact should be taken into account when allocating bandwidth to a certain FBG and choosing a demodulation algorithm. Sapphire FBGs were also acquired and the optical properties investigated. Furthermore, high temperature stable FBGs written with femtosecond laser radiation in silica Sumitomo Z-Fiber have been evaluated and shown to be a good option for temperature measurement below 1000 °C. Finally, the implementation of FBGs in a high temperature nuclear reactor is discussed and recommendations are made for future work. © 2012 Elsevier Ltd. All rights reserved.

Mathur R.,Juniata College | Munk L.,University of Alaska Anchorage | Nguyen M.,Juniata College | Gregory M.,Pebble Ltd | And 3 more authors.
Economic Geology | Year: 2013

Copper isotope ratios measured in minerals and shallow groundwater and surface waters provide insight into high-temperature mineralization and active weathering processes at the Pebble porphyry Cu-Au-Mo deposit, Alaska. The West zone of the deposit contains hypogene mineralization with a supergene overprint and a thin oxide leached capping, whereas the contiguous East zone contains only hypogene mineralization. Sulfide- rich rock powders and mineral separates have δ65Cu values that range from 0.78 to 2.28% (hypogene West), 0.02 to 1.55% (hypogene East), -3.49 to 1.88% (oxide West), and -5.04 to 1.27% (supergene West). The results from hypogene samples show that there is a systematic increase in δ65Cu values from deeper to shallower portions of the deposit. Furthermore, the δ65Cu values correlate with silicate alteration assemblages; mostly positive values correspond to quartz-illite-pyrite, sericite and quartz-pyrophyllite alteration zones which formed at relatively lower temperatures, whereas negative values characterize the higher temperature potassic and sodic-potassic domains. This empirical evidence could indicate that fractionation of Cu isotopes during hypogene alteration is controlled by pH and/or temperature variations. Shallow surface waters proximal to the deposit, and which likely interacted with underlying concealed mineralization, have heavy δ65Cu values which contrast with lighter values in waters distal from the deposit. Patterns measured in the copper isotope ratios of both solids and surface waters demonstrate the potential use of copper isotope distribution as a vectoring tool in mineral exploration and aid in understanding the sources of copper in the surface and near-surface environments. © 2013 Society of Economic Geologists, Inc.

Harraden C.L.,Pebble Ltd | Harraden C.L.,Hecla Mining Company | McNulty B.A.,Pebble Ltd | McNulty B.A.,University of British Columbia | And 3 more authors.
Economic Geology | Year: 2013

The Pebble Cu-Au-Mo porphyry deposit is located approximately 320 km southwest of Anchorage, Alaska. Shortwave infrared (SWIR) spectroscopy on drill core from the deposit has been used to document the distribution of alteration assemblages characterized by subtle variations in phyllosilicate minerals that cannot be confidently distinguished by visual criteria alone. At Pebble, these phyllosilicate alteration types have different histories of metal introduction and/or redistribution. Delineation of the distribution of these assemblages is critical to the geologic and genetic interpretations of the deposit. Spectral absorption features between 1,300 and 2,500 nm (in particular, small shifts in the position of the absorption wavelength related to AlOH bonds around 2,200 nm) allows distinction among illite-, sericite-, kaolinite-, and pyrophyllite-bearing alteration assemblages. Electron microprobe and X-ray diffraction analyses were used to validate the chemical composition and crystallinity of the phyllosilicate minerals identified using spectral data. The results confirm the use of SWIR spectroscopy to confidently identify and spatially delineate phyllosilicate alteration assemblages at Pebble. These alteration types include potassic, illite ± kaolinite, quartz-illite-pyrite, sericite, pyrophyllite, quartzsericite- pyrite, and sodic-potassic assemblages. The highest gold and copper concentrations within the deposit are in the eastern pluton and are coincident with low AlOH values associated with pyrophyllite and sericite alteration. An additional zone of low AlOH values, not associated with high metal grades, occurs in the northeast along the margins of the deposit, coincident with quartz-sericite-pyrite alteration. The approach described here has significantly improved three-dimensional alteration mapping and shows that short wave infrared spectroscopy may successfully distinguish variations in phyllosiclicate species. This has implications for exploration because clay speciation is genetically related to the distribution of metals in the Pebble deposit. The recognition and utilization of these relationships has produced a robust three-dimensional alteration model, which can be applied to optimizing mine planning, comminution, and mineral process design. © 2013 Society of Economic Geologists, Inc.

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