Impact of local burnup on prediction of power density in the NIST research reactor

Nicholas R. Brown, Albert L. Hanson, David J. Diamond

    Research output: Contribution to journalArticle

    1 Citation (Scopus)

    Abstract

    This study addresses the overprediction of local power when the burnup distribution in each half-element of the National Institute of Standards and Technology (NIST) research reactor, known as the NBSR, is assumed to be uniforma constraint in the full-core model used for neutronie analysis. A single-element model was utilized to quantify the impact of axial and platewise burnup on the power distribution within the NBSR fuel elements for both high-enriched uranium (HEU) and (proposed) low-enriched uranium (LEU) fuel. To validate this approach, key parameters in the single-element model were compared to parameters from an equilibrium core model, specifically, neutron energy spectrum, power distribution, and integral 235U vector. The power distribution changes significantly when incorporating local bumup effects and has lower power peaking relative to the uniform bumup case. In the uniform burnup case, the axial relative power peaking is overpredicted by as much as 59% in the HEU single element and 46% in the LEU single element. In the uniform burnup case, the platewise power peaking is overpredicted by as much as 23% in the HEU single element and 18% in the LEU single element. The degree of overprediction increases as a function of burnup cycle, with the greatest overprediction at the end of fuel element life. However, the overprediction in local power is always conservative in terms of the minimum critical heat flux ratio, a key safety parameter that depends on the local heat flux condition. The thermal flux peak is always in the midplane gap; this causes the local cumulative bumup near the midplane gap to be significantly higher than the fuel element average. Uniform bumup distribution throughout a half-element also causes a bias in fuel element reactivity worth particularly near end of life, primarily due to the importance of the fissile inventory in the midplane gap region. Despite this bias, comparisons of cycle length exhibit very good agreement between the core model with uniform bumup and the NBSR, which has many decades of operational experience with HEU fuel.

    Original languageEnglish (US)
    Pages (from-to)103-116
    Number of pages14
    JournalNuclear Technology
    Volume187
    Issue number1
    DOIs
    StatePublished - Jul 2014

    Fingerprint

    Research reactors
    Uranium
    uranium
    radiant flux density
    nuclear fuel elements
    reactors
    predictions
    Heat flux
    heat flux
    cycles
    causes
    neutron spectra
    Power spectrum
    safety
    Neutrons
    energy spectra
    reactivity
    Fluxes

    All Science Journal Classification (ASJC) codes

    • Nuclear and High Energy Physics
    • Nuclear Energy and Engineering
    • Condensed Matter Physics

    Cite this

    Brown, Nicholas R. ; Hanson, Albert L. ; Diamond, David J. / Impact of local burnup on prediction of power density in the NIST research reactor. In: Nuclear Technology. 2014 ; Vol. 187, No. 1. pp. 103-116.
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    abstract = "This study addresses the overprediction of local power when the burnup distribution in each half-element of the National Institute of Standards and Technology (NIST) research reactor, known as the NBSR, is assumed to be uniforma constraint in the full-core model used for neutronie analysis. A single-element model was utilized to quantify the impact of axial and platewise burnup on the power distribution within the NBSR fuel elements for both high-enriched uranium (HEU) and (proposed) low-enriched uranium (LEU) fuel. To validate this approach, key parameters in the single-element model were compared to parameters from an equilibrium core model, specifically, neutron energy spectrum, power distribution, and integral 235U vector. The power distribution changes significantly when incorporating local bumup effects and has lower power peaking relative to the uniform bumup case. In the uniform burnup case, the axial relative power peaking is overpredicted by as much as 59{\%} in the HEU single element and 46{\%} in the LEU single element. In the uniform burnup case, the platewise power peaking is overpredicted by as much as 23{\%} in the HEU single element and 18{\%} in the LEU single element. The degree of overprediction increases as a function of burnup cycle, with the greatest overprediction at the end of fuel element life. However, the overprediction in local power is always conservative in terms of the minimum critical heat flux ratio, a key safety parameter that depends on the local heat flux condition. The thermal flux peak is always in the midplane gap; this causes the local cumulative bumup near the midplane gap to be significantly higher than the fuel element average. Uniform bumup distribution throughout a half-element also causes a bias in fuel element reactivity worth particularly near end of life, primarily due to the importance of the fissile inventory in the midplane gap region. Despite this bias, comparisons of cycle length exhibit very good agreement between the core model with uniform bumup and the NBSR, which has many decades of operational experience with HEU fuel.",
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    Impact of local burnup on prediction of power density in the NIST research reactor. / Brown, Nicholas R.; Hanson, Albert L.; Diamond, David J.

    In: Nuclear Technology, Vol. 187, No. 1, 07.2014, p. 103-116.

    Research output: Contribution to journalArticle

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