Thermal Neutron Multiplicity Measurements Using the Pyrochemical Multiplicity Counter at Lawrence Livermore National Laboratory

Thermal Neutron Multiplicity Measurements Using the Pyrochemical Multiplicity Counter at Lawrence Livermore National Laboratory
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Total Pages: 9
Release: 1993
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The pyrochemical multiplicity counter designed and built at Los Alamos has been undergoing tests and evaluation at Lawrence Livermore National Laboratory (LLNL). Measurements have been performed using a variety of plutonium oxide and metal materials. The pyrochemical multiplicity counter uses the information contained in the higher moments of the neutron multiplicity distribution to deduce the three unknowns in the assay problem: 24°Pu-effective mass, ([alpha], n) neutron rate, and self-multiplication. This is an improvement over conventional neutron coincidence counting, which must rely on some estimate of the ([alpha], n) neutron rate or self-multiplication to deduce an assay result. Such conventional techniques are generally unsatisfactory for impure materials for which these quantities are unknown. We present the assay results obtained with the pyro-chemical multiplicity counter and discuss the procedures necessary to produce good assay results. Using these procedures, we have obtained assay accuracies of 1%--2% for oxide materials in 1/2 hour measurement times. We also compare these results to those that would have been obtained using conventional neutron assay techniques and discuss the correlations we have observed between assay results and the ratio of total neutron counts in the different rings of the pyrochemical counter.

The Application of Neutron Multiplicity Counting to the Assay of Bulk Plutonium Bearing Materials at RFETS and LLNL.

The Application of Neutron Multiplicity Counting to the Assay of Bulk Plutonium Bearing Materials at RFETS and LLNL.
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Total Pages: 8
Release: 1995
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ISBN:

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In the past several years, several facilities have identified a need for a large multiplicity counter to support safeguards of excess weapons materials and the measurement control and accountability of large, unusual samples. The authors have designed and fabricated two large thermal neutron multiplicity counters to meet this need at two DOE facilities. The first of these counters was built for Rocky Flats Environmental Test Site for use in the initial inventory inspection of excess weapons plutonium offered to International Atomic Energy Agency safeguards. The second counter was built for the Lawrence Livermore National Laboratory (LLNL) to support their material control and accountability program. For the LLNL version of the counter, a removable, fast-neutron interrogation assembly was added for the measurement of large uranium samples. In the passive mode these counters can accommodate samples in containers as large as a 30-gal. drum. This paper reports on the measured performance of these two counters and the data obtained with them.

Energy Research Abstracts

Energy Research Abstracts
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Total Pages: 922
Release: 1993
Genre: Power resources
ISBN:

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An Analysis Technique for Active Neutron Multiplicity Measurements Based on First Principles

An Analysis Technique for Active Neutron Multiplicity Measurements Based on First Principles
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Total Pages:
Release: 2012
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ISBN:

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Passive neutron multiplicity counting is commonly used to quantify the total mass of plutonium in a sample, without prior knowledge of the sample geometry. However, passive neutron counting is less applicable to uranium measurements due to the low spontaneous fission rates of uranium. Active neutron multiplicity measurements are therefore used to determine the 235U mass in a sample. Unfortunately, there are still additional challenges to overcome for uranium measurements, such as the coupling of the active source and the uranium sample. Techniques, such as the coupling method, have been developed to help reduce the dependence of calibration curves for active measurements on uranium samples; although, they still require similar geometry known standards. An advanced active neutron multiplicity measurement method is being developed by Texas A & M University, in collaboration with Los Alamos National Laboratory (LANL) in an attempt to overcome the calibration curve requirements. This method can be used to quantify the 235U mass in a sample containing uranium without using calibration curves. Furthermore, this method is based on existing detectors and nondestructive assay (NDA) systems, such as the LANL Epithermal Neutron Multiplicity Counter (ENMC). This method uses an inexpensive boron carbide liner to shield the uranium sample from thermal and epithermal neutrons while allowing fast neutrons to reach the sample. Due to the relatively low and constant fission and absorption energy dependent cross-sections at high neutron energies for uranium isotopes, fast neutrons can penetrate the sample without significant attenuation. Fast neutron interrogation therefore creates a homogeneous fission rate in the sample, allowing for first principle methods to be used to determine the 235U mass in the sample. This paper discusses the measurement method concept and development, including measurements and simulations performed to date, as well as the potential limitations.

Annual Book of ASTM Standards

Annual Book of ASTM Standards
Author: ASTM International
Publisher:
Total Pages: 1196
Release: 2003
Genre: Materials
ISBN:

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The State-of-the-art of Thermal Neutron Multiplicity Counting

The State-of-the-art of Thermal Neutron Multiplicity Counting
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Total Pages: 12
Release: 1997
Genre:
ISBN:

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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.

Government Reports Annual Index

Government Reports Annual Index
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Total Pages: 1372
Release: 1994
Genre: Government reports announcements & index
ISBN:

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Thermal Neutron Multiplicity Counting of Samples with Very Low Fission Rates

Thermal Neutron Multiplicity Counting of Samples with Very Low Fission Rates
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Total Pages: 14
Release: 1997
Genre:
ISBN:

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When samples with very low fission rates are assayed using thermal neutron multiplicity counting, shift register electronics, and the moments formalism for the singles, doubles, and triples count rates, large counting statistics errors can occur in the triples rates because of cosmic ray events. These counting statistics errors can be greatly reduced using gate multiplicity analysis when cosmic ray events produce high multiplicity neutron bursts. The gate multiplicity analysis method, measurement data, and results are presented.