A Graphite Isotope Ratio Method: A Primer on Estimating Plutonium Production in Graphite Moderated Reactors

Gesh, Christopher J.

doi:10.2172/15010619

Cited by 12 publications

(7 citation statements)

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“…GIRM exploits isotopic ratio changes that occur in the impurity elements present in the graphite to infer cumulative exposure and hence the reactor's lifetime cumulative plutonium production. Refer to Gesh, et. al.…”

Section: Introductionmentioning

confidence: 99%

Uranium Oxide Aerosol Transport in Porous Graphite

Blanchard¹,

Gerlach²,

Scheele³

et al. 2012

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Conditions exist in carbon dioxide (CO 2) gas-cooled, graphite moderated reactors that might allow for gaseous transport of uranium oxide (UO 2) particles into the graphite moderator. The transport of UO 2 in the reactor coolant system, and subsequent deposition of this material in the graphite, is of interest due to the potential to influence the application of the Graphite Isotope Ratio Method (GIRM). GIRM was developed to validate the declared operation of graphite moderated reactors. Uranium impurities in nuclear grade graphite are one of several possible indicator elements that can be used for a GIRM assessment. During fuel failures, uranium metal in the fuel is exposed to the CO 2 coolant and oxidizes as either UO 2 or U 3 O 8. Measurements in adjacent fuel channels indicate that CO 2 coolant readily flows through the porous graphite moderator blocks. Therefore, the potential exists for the coolant gas to transport UO 2 particles, as an aerosol, into the porous graphite, thereby invalidating uranium as an indicator element. Scoping calculations indicated that a mass of UO 2 particles sufficient to impact typical background levels in the graphite could be produced during the life of a graphite reactor. Air flow velocities in these reactors were more than sufficient to transport UO 2 particles to channels adjacent to where the fuel failure occurred. The objective of the work, summarized in this report, was to address the feasibility and extent of UO 2 particle transport in porous graphite. Using a theoretical model, based on classical aerosol filtration, a conservative, worst-case, particle size distribution (PSD) was determined. Experiments were then designed to evaluate the aerosol flow, using the worst case PSD, through well-characterized porous graphite samples. The graphite samples were analyzed to determine the UO 2 concentration profile and quantify the depth of penetration. The experimental results were also compared to model data to further understand the physics of UO 2 aerosol flow in porous graphite. The graphite penetration tests indicate that it is possible for uranium from fuel failures to be transported by the CO 2 gas coolant system and be deposited at depths as great as 35 mm in the graphite blocks of a graphite moderated reactor. Since a nominal GIRM graphite sample is taken within the first 19 mm, uranium from fuel failures has the potential to adversely impact a GIRM assessment. The penetration of uranium seen in these worst case tests may not be realized when UO 2 is generated from fuel failures in an actual reactor environment. Nonetheless, these tests show that steps must be taken to identify and ensure that uranium contamination from fuel failures does not adversely impact a GIRM assessment. A standard protocol to evaluate whether a GIRM graphite sample has been compromised by uranium from fuel failures should be implemented and appropriate steps should be taken to ensure that any compromised sample uses other indicator elements for GIRM assessment.

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Section: Introductionmentioning

confidence: 99%

Uranium Oxide Aerosol Transport in Porous Graphite

Blanchard¹,

Gerlach²,

Scheele³

et al. 2012

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“…Only recently, however, have mass spectrometric methods capable of measuring isotope ratios of extremely low concentration impurity elements become available. In the mid-1990s, an effort was undertaken at Pacific Northwest National Laboratory to develop a technology for verifying plutonium production in graphite-moderated reactors (Gerlach et al 1998, Gesh 2004, Reid et al 1999.…”

Section: Introductionmentioning

confidence: 99%

Final Report on Isotope Ratio Techniques for Light Water Reactors

Gerlach¹,

Gesh²,

Hurley³

et al. 2009

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The Isotope Ratio Method (IRM) is a technique for estimating the energy or plutonium production in a fission reactor by measuring isotope ratios in non-fuel reactor components. The isotope ratios in these components can then be directly related to the cumulative energy production with standard reactor modeling methods.All reactor materials contain trace elemental impurities at parts per million levels, and the isotopes of these elements are transmuted by neutron irradiation in a predictable manner. While measuring the change in a particular isotope's concentration is possible, it is difficult to correlate to energy production because the initial concentration of that element may not be known. However, if the ratio of two isotopes of the same element can be measured, the energy production can then be determined without knowing the absolute concentration of that impurity since the initial natural ratio is known. This is the fundamental principle underlying the IRM. Extremely sensitive mass-spectrometric methods are currently available that allow accurate measurements of the impurity isotope ratios in a sample. Additionally, indicator elements with stable activation products have been identified so that their post-irradiation isotope ratios remain constant.This method has been successfully demonstrated on graphite-moderated reactors. Graphite reactors are particularly well-suited to such analyses since the graphite moderator is resident in the fuel region of the core for the entire period of operation. Applying this method to other reactor types is more difficult since the resident portions of the reactor structure available for sampling are either outside the active core or non-cladding structural parts of individual fuel assemblies. The goal of this research is to evaluate whether the IRM can produce meaningful results for light water-moderated reactors and to propose the design of a fuel assembly monitor that would simplify sampling and analysis.In this paper, we use the IRM to estimate the energy production in one specific light water moderated reactor -a boiling water reactor (BWR) based upon measurements taken from a fuel assembly channel. These channels, like the cladding and structural materials of the fuel assembly, is made of Zircaloy, either Zircaloy -2 or Zircaloy -4. Both of these alloys have been used in both pressurized water and boiling water reactors throughout the history of the domestic US nuclear industry.Our preliminary results are in good agreement with the actual operating history of the reactor during the time the fuel assembly channel was resident in the core. We will also present chemical analysis protocols, identify potential materials for purpose-built fluence monitors, and present a fluence monitor design that may be suitable for a commercial power reactor.v

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“…The isotope ratios in these components then can be related directly to the cumulative energy production with standard reactor calculations. Isotope ratio methods have been used to estimate total fluence at research reactors by measuring impurities in aluminum core supports (Cliff et al 2005) and by measuring impurities in graphite from graphite-moderated reactors (Reid et al 2001;Gesh 2004). One technique for measuring the relative abundance of two isotopes is the secondary ionization mass spectrometer (SIMS).…”

Section: Introductionmentioning

confidence: 99%