Spencer Michael Luker

Simultaneous Evaluation of Neutron Spectra and 1-MeV-Equivalent (Si) Fluences at SPR-III and ACRR

Simultaneous least-squares adjustment of calculated neutron spectra in the central cavity of SPR-III and in the Pb-B4C bucket at ACRR is described, and the resulting 1-MeV-equivalent fluences are compared with damage measurements in 2N2222A transistors.

Reactor Pulse-Repeatability Studies at the Annular Core Research Reactor

The Annular Core Research Reactor (ACRR) at Sandia National Laboratories is a water-moderated pool-type reactor designed for testing many types of objects in the pulse and steady-state mode of operations. Personnel at Sandia began working to improve the repeatability of pulse operations for experimenters in the facility. The ACRR has a unique UO2–BeO fuel that makes the task of producing repeatable pulses difficult with the current operating procedure. The ACRR produces a significant quantity of photoneutrons through the 9Be(γ, n)8Be reaction in the fuel elements. The photoneutrons are the result of the gammas produced during fission and in fission product decay, so their production is very much dependent on the reactor power history and changes throughout the day/week of experiments in the facility. Because the photoneutrons interfere with the delayed-critical measurements required for accurate pulse reactivity prediction, a new operating procedure was created. The photoneutron effects at delayed critical are minimized when using the modified procedure. In addition, the pulse element removal time is standardized for all pulse operations with the modified procedure, and this produces less variation in reactivity removal times.

Diamond PCD for Reactor active dosimetry applications

This work reports on the development, calibration, and application of a radiation-hardened diamond photoconducting detector (PCD) for active ionizing dose measurements in research reactors. Results are reported for gamma irradiations between 10 and 10/sup 12/ rad(C)/s.

Radiation characterization summary

This document presents the facility-recommended characterization of the neutron, prompt gamma-ray, and delayed gamma-ray radiation fields in the Annular Core Research Reactor (ACRR) for the 44-inch-long lead-boron bucket in the central cavity on the 32-inch pedestal at the core centerline. The designation for this environment is ACRR-LB44-CC-32-cl. The neutron, prompt gamma-ray, and delayed gamma-ray energy spectra are presented as well as radial and axial neutron and gamma-ray flux profiles within the experiment area of the bucket. Recommended constants are given to facilitate the conversion of various dosimetry readings into radiation metrics desired by experimenters. Representative pulse and steady-state operations are presented with conversion examples.

Development of an Active Detector for the Characterization of the Late-Time Radiation Environment from a Reactor Pulse

This paper discusses the use of a commercially available 235U fission chamber, with a matching compensating ion chamber, originally sold as a single-ended detector with the signal conducted over the shield of a coaxial cable. The authors designed an aluminum housing that isolates the two detectors and converts the signals to full differential mode as a noise-reduction technique. The signals are processed using the switched resistor technique to extend the signal range to longer times from the peak of the pulse [Luker, S. M., Griffin, P. J., King, D. B., and Suo-Anttila, A. J., “Improved Diagnostics for Analysis of a Reactor Pulse Radiation Environment,” 13th International Symposium on Reactor Dosimetry, Akersloot, Netherlands, May 25, 2008, pp. 4–6.]. The newly configured fission chamber assembly has been used at the annular core research reactor at Sandia National Laboratories to provide a high-fidelity characterization of the neutron time profile from a pulsed operation.

Characterizing the Time- and Energy-Dependent Reactor n /γ Environment

In a reactor pulse, the early radiation has a neutron/gamma component resulting from the prompt fission neutron and gamma radiation and from the neutron-induced secondary gammas. However, after the primary reactor pulse, the radiation environment also includes a time-dependent delayed neutron and gamma component. At even later times, the material activation dominates the source term. When active tests are conducted in the reactor, the radiation environment at a particular time may not be well characterized by the time-integrated spectrum provided by the typical radiation transport calculations. This paper defines the steps that are required to adequately model the time- and energy-dependent radiation environment in the reactor environment. This paper also compares the time-dependent response from a range of active dosimeters in a reactor pulsed environment and shows how the n/γ mixed-field response for the dosimeters and the changing radiation field can influence the interpretation of the dosimetry.

Development of a Silicon Calorimeter for Dosimetry Applications in a Water-Moderated Reactor

High fidelity active dosimetry in the mixed neutron/gamma field of a research reactor is a very complex issue. For passive dosimetry applications, the use of activation foils addresses the neutron environment while the use of low neutron response CaF2:Mn thermoluminescent dosimeters (TLDs) addresses the gamma environment. While radiation-hardened diamond photoconducting detectors (PCD) have been developed that provide a very precise fast response (picosecond) dosimeter and can provide a time-dependent profile for the radiation environment, the mixed field response of the PCD is still uncertain and this interferes with the calibration of the PCD response. In order to address the research reactor experimenters need for a dosimeter that reports silicon dose and dose rate at a test location during a pulsed reactor operation, a silicon calorimeter has been developed. This dosimeter can be used by itself to provide a dose in rad(Si) up to a point in a reactor pulsed operation, or, in conjunction with the diamond PCD, to provide a dose rate. This paper reports on the development, testing, and validation of this silicon calorimeter for applications in water-moderated research reactors.

Radiation Characterization Summary: ACRR Polyethylene-Lead-Graphite (PLG) Bucket Located in the Central Cavity on the 32-Inch Pedestal at the Core Centerline (ACRR-PLG-CC-32-cl).

This document presents the facility-recommended characterization of the neutron, prompt gamma-ray, and delayed gamma-ray radiation fields in the Annular Core Research Reactor (ACRR) for the polyethylene-lead-graphite (PLG) bucket in the central cavity on the 32-inch pedestal at the core centerline. The designation for this environment is ACRR-PLG-CC-32-cl. The neutron, prompt gamma-ray, and delayed gamma-ray energy spectra, uncertainties, and covariance matrices are presented as well as radial and axial neutron and gamma-ray fluence profiles within the experiment area of the bucket. Recommended constants are given to facilitate the conversion of various dosimetry readings into radiation metrics desired by experimenters. Representative pulse operations are presented with conversion examples. Acknowledgements The authors wish to thank the Annular Core Research Reactor staff and the Radiation Metrology Laboratory staff for their support of this work. Also thanks to David Ames for his assistance in running MCNP on the Sandia parallel machines.

Multiplexer/amplifier test results for SP‐100

Multiplexer and amplifier systems must be designed with transistors that can perform satisfactorily over ten years to a total gamma dose of 120E6 rads and a total neutron fluence of 1.6E15 nvt for the SP‐100 reactor system. Series of gamma and neutron tests have been completed to measure transistor degradation as a function of total dose, fluence, and temperature. Test results indicate that modest increases in temperature result in substantial improvement of transistor performance at a neutron flux of 8E8 n/cm2/s.