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Moisture corrections in neutron coincidence counting of PuO/sub 2/

Description: Passive neutron coincidence counting is capable of 1% assay accuracy for pure, well-characterized PuO/sub 2/ samples that contain plutonium masses from a few tens of grams to several kilograms. Moisture in the sample can significantly bias the assay high by changing the (..cap alpha..,n) neutron production, the sample multiplication, and the detection efficiency. Monte Carlo calculations and an analytical model of coincidence counting have been used to quantify the individual and cumulative effects of moisture biases for two PuO/sub 2/ sample sizes and a range of moisture levels from 0 to 9 wt %. Results of the calculations suggest a simple correction procedure for moisture bias that is effective from 0 to 3 wt % H/sub 2/O. The procedure requires that the moisture level in the sample be known before the coincidence measurement.
Date: January 1, 1987
Creator: Stewart, J.E. & Menlove, H.O.
Partner: UNT Libraries Government Documents Department

Direct fissile assay of highly enriched UF/sub 6/ using random self-interrogation and neutron coincidence response

Description: A new nondestructive method for direct assay of /sup 235/U mass contained in Model 5A uranium hexafluoride (UF/sub 6/) product storage cylinders has been successfully tested in the laboratory and under field conditions. The technique employs passive neutron self-interrogation and uses the ratio of coincidences-to-totals counts as a measure of bulk fissile mass. The accuracy of the method is 6.8% (1 sigma) based on field measurements of 44 Model 5A cylinders, 11 of which were either only partially filled or contained reactor return material. The cylinders contained UF/sub 6/ with enrichments from 5.96% to 97.6%. Count times were 3 to 6 min depending on /sup 235/U mass. Samples ranged from below 1 kg to over 16 kg of /sup 235/U. Because the method relies primarily on fast neutron self-interrogation, complete sampling of the UF/sub 6/ takes place. This feature alleviates inhomogeneity problems and offers increased assurance of the presence of stated amounts of bulk fissile material as compared with current verification methods.
Date: January 1, 1983
Creator: Stewart, J.E. & Menlove, H.O.
Partner: UNT Libraries Government Documents Department

Data analysis for neutron monitoring in an enrichment facility. [To detect high-enriched uranium production]

Description: Area monitoring of neutron radiation to detect high-enriched uranium production is a potential strategy for inspector verification of operations in the cascade area of a centrifuge enrichment facility. This paper discusses the application of statistical filtering and hypothesis testing procedures to experimental data taken in an enrichment facility. The results demonstrate that these data analysis methods can enhance detection of facility misoperation by neutron monitoring.
Date: January 1, 1982
Creator: Markin, J.T.; Stewart, J.E. & Goldman, A.S.
Partner: UNT Libraries Government Documents Department

Neutron multiplicity assay of impure materials using four different neutron counters

Description: During an advanced IAEA inspector training course given at Los Alamos in November, 1997, the opportunity existed for an intercomparison study of various neutron detectors to quantify measurement performance using pure and impure plutonium oxide and mixed uranium-plutonium oxide (MOX) items. Because of the cost of counters designed specifically for multiplicity analysis, it was desired to explore the limits of other, less costly and less efficient detectors. This paper presents and intercompares neutron coincidence and multiplicity assay performance for five detectors, which vary widely in detection efficiency. Eight pure plutonium oxide and twelve impure plutonium oxide and MOX working standards were used in the study.
Date: December 1, 1998
Creator: Stewart, J.E. Krick, M.S.; Langner, D.G. & Wenz, T.R.
Partner: UNT Libraries Government Documents Department

The state-of-the-art of thermal neutron multiplicity counting

Description: Neutron multiplicity counting is a maturing technology. It has been implemented at many facilities to address the increasing need to rapidly measure impure plutonium bearing materials. At Hanford Site and Rocky Flats Environmental Technology Site, multiplicity counting has also been used with excellent results by the International Atomic Energy Agency to verify excess plutonium inventories now under their safeguards. Neutron multiplicity counting as currently implemented, however, will not address all forms of impure plutonium. Materials containing large concentrations of matrix elements like fluorine and beryllium cannot be assayed successfully without extremely long count times. Assays of compact plutonium metals and oxides having a large uranium concentration relative to their plutonium content tend to bias low because of a breakdown in the theoretical model now used to translate the measured multiplicity distributions to plutonium mass. In this paper, the authors will discuss the most recent efforts to extend the range of materials that can be measured successfully with thermal neutron multiplicity counting and a use of multiplicity counting to detect sample changes during long-term storage.
Date: November 1, 1997
Creator: Langner, D.G.; Krick, M.S.; Stewart, J.E. & Ensslin, N.
Partner: UNT Libraries Government Documents Department

Methods of Verification, Accountability and Control of Special Nuclear Material

Description: This session demonstrates nondestructive assay (NDA) measurement, surveillance and analysis technology required to protect, control and account (MPC and A) for special nuclear materials (SNM) in sealed containers. These measurements, observations and analyses comprise state-of-the art, strengthened, SNM safeguards systems. Staff member specialists, actively involved in research, development, training and implementation worldwide, will present six NDA verification systems and two software tools for integration and analysis of facility MPC and A data.
Date: May 3, 1999
Creator: Stewart, J. E.
Partner: UNT Libraries Government Documents Department

Preparation of Pure Plutonium Metal Standards for Nondestructive Assay

Description: To calibrate neutron coincidence and neutron multiplicity counters for passive assay of plutonium, certain detector parameters must be determined. When one is using small plutonium metal samples, biases can be introduced from non-zero multiplication and impurities. This paper describes preparing small, pure plutonium metal standards with well-known geometries to enable accurate multiplication corrections and with acceptably low levels of impurities. To minimize multiplication, these standards are designed as 2-cm-diameter foils with varying thicknesses and masses of 1.4, 3.6, and 7.2 g plutonium. These standards will significantly improve characterization and calibration of neutron coincidence and multiplicity counters. They can also be equally useful for gamma-ray spectrometry and calorimetry. Five sets will be made: four for other US Department of Energy plutonium facilities, and one set to remain at Los Alamos. We will also describe other nondestructive assay standards that are planned for the next few years.
Date: November 1, 2000
Creator: Hsue, S. -T.; Stewart, J. E. & Krick, M. S.
Partner: UNT Libraries Government Documents Department

Guide to nondestructive assay standards: Preparation criteria, availability, and practical considerations

Description: For certification and measurement control, nondestructive assay (NDA) instruments and methods used for verification measurement of special nuclear materials (SNMs) require calibrations based on certified reference materials (CRMs), or working reference materials (WRMs), traceable to the national system of measurements, and adequately characteristic of the unknowns. The Department of Energy Office of Safeguards and Security is sponsoring production of a comprehensive guide to preparation of NDA standards. The scope of the report includes preparation criteria, current availability of CRMs and WRMs, practical considerations for preparation and characterization, and an extensive bibliography. In preparing the report, based primarily on experience at Los Alamos, we have found that standards preparation is highly dependent on the particular NDA method being applied. We therefore include sections that contain information specific to commonly used neutron and gamma-ray NDA techniques. 16 refs., 4 figs., 2 tabs.
Date: October 1, 1997
Creator: Stewart, J.E.; Hsue, S.T. & Sampson, T.E.
Partner: UNT Libraries Government Documents Department

Energy dependent bias in plutonium verification measurements using thermal neutron multiplicity counters

Description: Multiplicity analysis algorithms are extended to include the effect of ({alpha}, n) neutron energies on the detector efficiencies, induced fission probabilities, and induced fission factorial moments. The analysis is restricted to plutonium oxide. Bias is calculated as a function of ({alpha}, n) neutron energy for six thermal neutron coincidence counters: HLNC, AWCC, 3RMC, PSMC, PYRO, and 5RMC. Ring ratio data for the 3RMC are used to reduce energy dependent bias in the verification of impure plutonium oxide. The utility of the AWCC as a multiplicity counter is considered.
Date: October 1, 1997
Creator: Krick, M.S.; Langner, D.G. & Stewart, J.E.
Partner: UNT Libraries Government Documents Department

Application of detailed fission-product decay gamma spectra in the calculation of photoneutron spectra from D(. gamma. ,n) reactions in PWR fuel

Description: The magnitude, average neutron energy, and spectrum of the photoneutron source in a PWR fuel is calculated for shutdown times to 100 y using CINDER radionuclide inventory calculations, ENDF/B-V ..gamma.. line decay spectra, MCNP ..gamma.. transport calculations, and PHONEX D(..gamma..,n) reaction physics calculations.
Date: January 1, 1985
Creator: Wilson, W.B.; Stewart, J.E.; England, T.R. & Perry, R.T.
Partner: UNT Libraries Government Documents Department

Confirmatory measurements of UF/sub 6/ using the neutron self-interrogation method

Description: A passive neutron counting method has been developed for measurement of the /sup 235/U mass in Model 5A cylinders of UF/sub 6/. The unique neutronic properties of UF/sub 6/ containing highly enriched uranium (HEU) permit /sup 235/U assay using only passive neutron counting. The sample effectively assays itself by self-interrogation. Shipped from enrichment plants and received at fuel fabrication and conversion facilities, 5A UF/sub 6/ cylinders hold up to approx.17 kg of /sup 235/U each. Field measurements at the Portsmouth Gaseous Diffusion Plant (GDP) showed an average assay accuracy of 6.8% (1sigma) for 44 cylinders with enrichments from 6 to 98% and with a range of fill heights. Further measurements on 38 cylinders containing 97%-enriched material yielded an accuracy of 2.8% (1sigma). Typical counting times for these measurements were less than 5 min. An in-plant instrument for receipts confirmation measurements of 5A UF/sub 6/ cylinders has been developed for the Savannah River Plant. The Receipts Assay Monitor (RAM) is currently being tested and calibrated. It is designed to confirm declared fissile mass in all incoming 5A cylinders containing HEU in the form of UF/sub 6/. One of the computer-controlled features is a removable cadmium liner for the sample cavity. The liner allows a sample fill-height correction, which significantly improves assay accuracy.
Date: January 1, 1985
Creator: Stewart, J.E.; Ensslin, N.; Menlove, H.O.; Cowder, L.R. & Polk, P.J.
Partner: UNT Libraries Government Documents Department

Calibration parameters from Monte Carlo simulations for neutron coincidence assay of MOX (mixed oxide) fuel elements: A substitute for standards

Description: Results from application of a calculational model for the two- parameter (singles and doubles) passive neutron coincidence assay of finished Fast Breeder Reactor (FBR) subassemblies are compared with calibration measurements. Two assay instruments are considered; the Universal Fast Breeder Reactor Subassembly Counter (UFBC) and the Capsule Counter installed at the Japanese Plutonium Fuel Production Facility (PFPF). In the case of US Fast Flux Test Facility (FFTF) fuel, the absolute ratio of calculations to measurements for the multiplication-corrected coincidence calibration constant is +1.1 /+-/ 1.0% (average of four subassemblies) for the UFBC and /minus/1.3 /+-/ 0.6% (average of five subassemblies) for the Capsule Counter. For initial measurements of Japanese fuel in the Capsule Counter, the absolute ratio is /minus/1.0 /+-/ 0.7% for three JOYO subassemblies and +0.8 /+-/ 0.7% for one MONJU subassembly. Calculations of relative effects such as the change in coincidence response from, for example, subassembly can thickness of U enrichment are more accurate (better than 0.5%) than absolute calibration parameters. This very good accuracy offers more effective and less costly inspector verification of finished FBR fuel elements by reducing reliance on physical standards to expand the cross-calibration databases. 11 refs., 8 figs., 5 tabs.
Date: January 1, 1989
Creator: Stewart, J.E.; Ferran, R.R.; Simmonds, S.M. & Menlove, H.O.
Partner: UNT Libraries Government Documents Department

NDA (nondestructive assay) training for new IAEA inspectors at Los Alamos

Description: The history of the evolution of nondestructive assay (NDA) training for international inspectors at Los Alamos is described. The current NDA training course for International Atomic Energy Agency inspectors is presented in terms of structure, content, and rationale. Results of inspector measurement exercises are given along with projections for future developments in NDA inspector training. 3 refs.
Date: January 1, 1987
Creator: Stewart, J.E.; Reilly, T.D.; Belew, W.; Woelfl, E. & Fager, J.
Partner: UNT Libraries Government Documents Department

A two-fold reduction in measurement time for neutron assay: Initial tests of a prototype dual-gated shift register

Description: Neutron coincidence counting (NCC) is used routinely around the world for nondestructive mass assay of uranium and plutonium in many forms, including waste. Compared with other methods, NCC is generally the most flexible, economic, and rapid. Many applications of NCC would benefit from a reduction in counting time required for a fixed random error. We have developed and tested the first prototype of a dual- gated, shift-register-based electronics unit that offers the potential of decreased measurement time for all passive and active NCC applications.
Date: September 1, 1996
Creator: Stewart, J.E.; Bourret, S.C.; Krick, M.S.; Hansen, W.J. & Harker, W.C.
Partner: UNT Libraries Government Documents Department

Design of standards for nondestructive assay of special nuclear material

Description: Nondestructive assay (NDA) of special nuclear material (SNM) involves a variety of measurement techniques, instruments, and nuclear materials. High-quality measurements require well-characterized SNM standards that represent the expected range of mass, chemical composition, and physical properties of the SNM to be measured. Due to the very limited commercial availability of NDA standards, facilities must usually produce their own standards, both to meet their specific measurement needs and to comply with existing regulations. This paper will describe the current extent to which NDA standards are commercially available. The authors will further describe the types of NDA standards used to calibrate and verify the measurement techniques commonly used in the safeguards of SNM. Several types of NDA standards will be discussed in detail to illustrate the considerations that go into specifying and designing traceable, representative standards for materials accounting measurements.
Date: May 1997
Creator: Smith, H. A., Jr.; Stewart, J. E. & Ruhter, W.
Partner: UNT Libraries Government Documents Department

Passive neutron assay of irradiated nuclear fuels

Description: Passive neutron assay of irradiated nuclear fuel has been investigated by calculations and experiments as a simple, complementary technique to the gamma assay. From the calculations it was found that the neutron emission arises mainly from the curium isotopes, the neutrons exhibit very good penetrability of the assemblies, and the neutron multiplication is not affected by the burnup. From the experiments on BWR and PWR assemblies, the neutron emission rate is proportional to burnup raised to 3.4 power. The investigations indicate that the passive neutron assay is a simple and useful technique to determine the consistency of burnups between assemblies.
Date: February 1, 1979
Creator: Hsue, S.T.; Stewart, J.E.; Kaieda, K.; Halbig, J.K.; Phillips, J.R.; Lee, D.M. et al.
Partner: UNT Libraries Government Documents Department

Advances in passive neutron instruments for safeguards use

Description: Passive neutron and other nondestructive assay techniques have been used extensively by the International Atomic Energy Agency to verify plutonium metal, powder, mixed oxide, pellets, rods, assemblies, scrap, and liquids. Normally, the coincidence counting rate is used to measure the {sup 240}Pu-effective mass and gamma-ray spectrometry or mass spectrometry is used to verify the plutonium isotopic ratios. During the past few years, the passive neutron detectors have been installed in plants and operated in the unattended/continuous mode. These radiation data with time continuity have made it possible to use the totals counting rate to monitor the movement of nuclear material. Monte Carlo computer codes have been used to optimize the detector designs for specific applications. The inventory sample counter (INVS-III) has been designed to have a higher efficiency (43%) and a larger uniform counting volume than the original INVS. Data analyses techniques have been developed, including the ``known alpha`` and ``known multiplication`` methods that depend on the sample. For scrap and other impure or poorly characterized samples, we have developed multiplicity counting, initially implemented in the plutonium scrap multiplicity counter. For large waste containers such as 200-L drums, we have developed the add-a-source technique to give accurate corrections for the waste-matrix materials. This paper summarizes recent developments in the design and application of passive neutron assay systems.
Date: February 1, 1994
Creator: Menlove, H. O.; Krick, M. S.; Langner, D. G.; Miller, M. C. & Stewart, J. E.
Partner: UNT Libraries Government Documents Department

Neutron-based measurements for nondestructive assay of minor actinides produced in nuclear power reactors

Description: Because of their impacts on long-term storage of high-level radioactive waste and their value as nuclear fuels, measurement and accounting of the minor actinides produced in nuclear power reactors are becoming significant issues. This paper briefly reviews the commercial nuclear fuel cycle with emphasis on reprocessing plants and key measurement points therein. Neutron signatures and characteristics are compared and contrasted for special nuclear materials (SNMs) and minor actinides (MAs). The paper focuses on application of neutron-based nondestructive analysis (NDA) methods that can be extended for verification of MAs. We describe current IAEA methods for NDA of SNMs and extension of these methods to satisfy accounting requirements for MAs in reprocessing plant dissolver solutions, separated products, and high-level waste. Recommendations for further systems studies and development of measurement methods are also included.
Date: October 1, 1996
Creator: Stewart, J.E.; Eccleston, G.W.; Ensslin, N.; Cremers, T.L.; Foster, L.A.; Menlove, H.O. et al.
Partner: UNT Libraries Government Documents Department

Assay of scrap plutonium oxide by thermal neutron multiplicity counting for IAEA verification of excess materials from nuclear weapons production

Description: The US Nonproliferation and Export Control Policy commits the US to placing under International Atomic Energy Agency (IAEA) safeguards excess nuclear materials no longer needed for the US nuclear deterrent. As of January 1,1996, the IAEA has completed Initial Physical Inventory Verification (IPIV) at the Oak Ridge Y-12 plant, the Hanford Plutonium Finishing Plant, and a plutonium storage vault at Rock Flats. Two IPIVs were performed at Hanford . This paper reports the results of thermal neutron multiplicity assay of plutonium residues during the second IPIV at Hanford. Using the Three Ring Multiplicity Counter (3RMC), measurements were performed on 69 individual cans of plutonium residues, each containing approximately 1 kg of material. Of the 69 items, 67 passed the IAEA acceptance criteria and two were selected for destructive analysis.
Date: September 1, 1996
Creator: Stewart, J.E.; Krick, M.S.; Xiao, J.; LeMaire, R.J.; Fotin, V.; McRae, L. et al.
Partner: UNT Libraries Government Documents Department

Guide to nondestructive assay standards: Preparation criteria, availability, and practical considerations

Description: For certification and measurement control, nondestructive assay (NDA) instruments and methods used for verification measurements of special nuclear materials (SNMs) require calibrations based on certified reference materials (CRMs), or working reference materials (WRMs), traceable to the national system of measurements, and adequately characteristic of the unknowns. The Department of Energy Office of Safeguards and Security is sponsoring production of a comprehensive guide to preparation of NDA standards. The scope of the report includes preparation criteria, current availability of CRMs and WRMs, practical considerations for preparation and characterization, and an extensive bibliography. In preparing the report, based primarily on experience at Los Alamos, they have found that standards preparation is highly dependent on the particular NDA method being applied. They therefore include sections that contain information specific to commonly used neutron and gamma-ray NDA techniques. They also present approaches that are alternatives to, or minimize requirements for physical standards.
Date: October 1, 1997
Creator: Hsue, S.T.; Stewart, J.E.; Sampson, T.E.; Butler, G.W.; Rudy, C.R. & Rinard, P.M.
Partner: UNT Libraries Government Documents Department

The Ephithermal Neutron Multiplicity Counter Design and Performance Manual: More Rapid Plutonium and Uranium Inventory Verifications by Factors of 5-20

Description: Thermal neutron multiplicity counters (TNMCs) assay {sup 240}Pu-effective mass, isolating spontaneous-fission (SF), induced-fission, and ({alpha},n) neutrons emitted from plutonium metal, oxide, scrap, and residue items. Three independent parameters are measured: single, double, and triple neutron-pulse-coincidence count rates. TNMC assays can become precision limited by high ({alpha},n) neutron rates arising from low-Z impurities and {sup 241}Am. TNMCs capture thermal neutrons in 4-atm {sup 3}He tubes after fast-source-neutron moderation by polyethylene. TNMCs are {approximately}50% efficient with {approximately}-{micro}s die-away times. Simultaneously increasing efficiency and reducing die-away time dramatically improve assay precision. Using 10-atm {sup 3}He tubes, they've developed and performance-tested the first of a new generation of neutron assay counters for a wide range of plutonium items. The Epithermal Neutron Multiplicity Counter (ENMC) has an efficiency of 65% and a 22-{micro}s die-away time. The ENMC detects neutrons before thermalization using higher {sup 3}He pressure counters and less moderator than TNMCs. A special insert raises efficiency to 80% for small samples. For five bulk samples containing 50 to 875 g of {sup 240}Pu-effective, ENMC assay times are reduced by factors of 5 to 21, compared with prior state-of-the-art TNMCs. The largest relative gains are for the most impure items, where gains are needed most. In active mode, the ENMC assay times are reduced by factors of 5 to 11, compared with the Active Well Coincidence Counter (AWCC). The ENMC, with high precision and low multiplicity dead time (37 ns), can be used in standards verification mode to precisely and accurately characterize plutonium standards and isotopic sources. The ENMC's performance is very competitive with calorimetry. This report describes the ENMC; presents results of characterization, calibration, and verification measurements; and shows the clear performance and economic advantages of implementing the ENMC for nuclear materials control and accountability.
Date: August 1, 2000
Creator: Stewart, J. E.; Menlove, H. O.; Mayo, D. R.; Geist, W. H.; Carrillo, L. A. & Herrera, G. D.
Partner: UNT Libraries Government Documents Department