Frazier Bronson

Nuclear measurement technologies & solutions implemented during nuclear accident at Fukushima

Fukushima accident imposed a stretch to nuclear measurement operational approach requiring in such emergency situation: fast concept development, fast system integration, deployment and start-up in a very short time frame. This paper is describing the Nuclear Measurement that AREVA-BUNM (CANBERRA) has realized and foresight at Fukushima accident site describing the technical solution conceived developed and deployed at Fukushima NPP for the process control of the treatment system of contaminated water. A detailed description of all levels design choices, from detection technologies to system architecture is offer in the paper as well as the read-out and global data management system. This paper describes also the technical choices executed and put in place to overcome the challenges related to the high radiological contamination on site.

Mathematical efficiency calibration methods for high quality laboratory based gamma spectrometry systems

The efficiency calibration of laboratory based gamma spectrometry systems typically involves the purchase or construction of calibration samples that are supposed to represent the geometries of the unknown samples to be measured. For complete and correct calibrations, these sample containers must span the operational range of the system, which at times can include difficult configurations of size, density, matrix, and source distribution. The efficiency calibration of a system is dependent not only on the detector, but on the radiation attenuation factors in the detector–source configuration, and therefore is invalid unless all parameters of the sample assay condition are identical to the calibration condition. An alternative to source-based calibrations is to mathematically model the efficiency response of a given detector–sample configuration. In this approach, the measurement system is calibrated using physically accurate models whose parameters can generally be easily measured. Using modeled efficiencies, systems can be quickly adapted to changing sample containers and detector configurations. This paper explores the advantages of using mathematically computed efficiencies in place of traditional source-based measured efficiencies for laboratory samples, focusing specifically on the possibility of sample optimization for a given detector, uncertainty estimation, and cascade summing corrections.

The application of mathematical modeling for commercial nuclear instrument design, development, and calibration

Mathematical modeling of nuclear instruments has evolved over the last 40 years. First came simple curve fitting operations; then Monte Carlo processes were made available to large research facilities; and then, due to the power of the computer, became widely available to normal radiation instrumentation users. Canberra has been extensively using MCNP for design and calibration of large and complicated instrumentation to create and optimize designs that would be prohibitively expensive with radioactive sources. For gamma spectroscopy applications we have developed a software application called ISOCS that combines MCNP pre-calculations and ray-tracing operations for speed. Examples of the use of these tools are presented.

Mathematical efficiency calibration with uncertain source geometries using smart optimization

The In Situ Object Counting Software (ISOCS), a mathematical method developed by CANBERRA, is a well established technique for computing High Purity Germanium (HPGe) detector efficiencies for a wide variety of source shapes and sizes. In the ISOCS method, the user needs to input the geometry related parameters such as: the source dimensions, matrix composition and density, along with the source-to-detector distance. In many applications, the source dimensions, the matrix material and density may not be well known. Under such circumstances, the efficiencies may not be very accurate since the modeled source geometry may not be very representative of the measured geometry. CANBERRA developed an efficiency optimization software known as “Advanced ISOCS” that varies the not well known parameters within user specified intervals and determines the optimal efficiency shape and magnitude based on available benchmarks in the measured spectra. The benchmarks could be results from isotopic codes such as MGAU, MGA, IGA, or FRAM, activities from multi-line nuclides, and multiple counts of the same item taken in different geometries (from the side, bottom, top etc). The efficiency optimization is carried out using either a random search based on standard probability distributions, or using numerical techniques that carry out a more directed (referred to as “smart” in this paper) search. Measurements were carried out using representative source geometries and radionuclide distributions. The radionuclide activities were determined using the optimum efficiency and compared against the true activities. The “Advanced ISOCS” method has many applications among which are: Safeguards, Decommissioning and Decontamination, Non-Destructive Assay systems and Nuclear reactor outages maintenance.

Optimization of the Canberra UltraRadiac GM Tube Wrapping

The UltraRadiactrade personal radiation monitor is a ruggedized handheld dose rate and total dose monitoring device intended for use by first responders in the harsh environments that firefighters, Hazmat teams, paramedics and military personnel may encounter. The device is based on Geiger-Mueller (GM) tubes with lead and tin wrappings. Different options exist for different dose ranges. The photon radiation interacts with the GM tube wrapping through Compton scattering and the scattered electrons are counted through ionization in the fill gas. The amount of the ionization created in the Geiger discharge saturates and becomes independent of the initial energy deposit in the gas. The thickness of the wrapping material must be carefully optimized however in order to obtain a fairly uniform energy response to the incident photons that give rise to the secondary ionizing electrons which are actually counted. MCNP simulations and benchmark measurements were performed to establish a suitable energy compensation filter design. The results of the study will be presented and the performance of the selected design summarized in this paper.

Probabilistic ISOCS Uncertainty Estimator: Application to the Segmented Gamma Scanner

Traditionally, high resolution gamma-ray spectroscopy (HRGS) has been used as a very powerful tool to determine the radioactivity of various items, such as samples in the laboratory, waste assay containers, or large items in-situ. However, in order to properly interpret the quality of the result, an uncertainty estimate must be made. This uncertainty estimate should include the uncertainty in the efficiency calibration of the instrument, as well as many other operational and geometrical parameters. Efficiency calibrations have traditionally been made using traceable radioactive sources. More recently, mathematical calibration techniques have become increasingly accurate and more convenient in terms of time and effort, especially for complex or unusual configurations. Whether mathematical or source-based calibrations are used, any deviations between the as-calibrated geometry and the as-measured geometry contribute to the total measurement uncertainty (TMU). Monte Carlo approaches require source, detector, and surrounding geometry inputs. For non-trivial setups, the Monte Carlo approach is time consuming both in terms of geometry input and CPU processing. Canberra Industries has developed a tool known as In-Situ Object Calibration Software (ISOCS) that utilizes templates for most common real life setups. With over 1000 detectors in use with this product, the ISOCS software has been well validated and proven to be much faster and acceptably accurate for many applications. A segmented gamma scanner (SGS) template is available within ISOCS and we use it here to model this assay instrument for the drummed radioactive waste. Recently, a technique has been developed which uses automated ISOCS mathematical calibrations to evaluate variations between reasonably expected calibration conditions and those that might exist during the actual measurement and to propagate them into an overall uncertainty on the final efficiency. This includes variations in container wall thickness and diameter, sample height and density, sample non-uniformity, sample-detector geometry, and many other variables, which can be specified according to certain probability distributions. The software has a sensitivity analysis mode which varies one parameter at a time and allows the user to identify those variables that have the largest contribution to the uncertainty. There is an uncertainty mode which uses probabilistic techniques to combine all the variables and compute the average efficiency and the uncertainty in that efficiency, and then to propagate those values with the gamma spectroscopic analysis into the final result. In the areas of waste handling and environmental protection, nondestructive assay by gamma ray scanning can provide a fast, convenient, and reliable way of measuring many radionuclides in closed items. The SGS is designed to perform accurate quantitative assays on gamma emitting nuclides such as fission products, activation products, and transuranic nuclides. For the SGS, this technique has been applied to understand impacts of the geometry variations during calibration on the efficiency and to estimate the TMU.

Gamma spectroscopy with automated efficiency optimization for nuclear safeguards applications

In all applications of gamma-ray spectroscopy, one of the important parts of the data analysis is to measure the detection efficiency which depends on the geometrical conditions of the source-detector arrangement. As the samples commonly measured can vary greatly in volume and shape, it is impractical to manufacture standard sources for large and complex measurement items and packagings for the purpose of efficiency calibration. Canberra Industries developed the mathematical efficiency software called In Situ Object Calibration Software (ISOCS) to overcome the difficulties presented above. Recently, Canberra has extended the capability of ISOCS and has developed a software package for the International Atomic Energy Agency (IAEA) that simplifies, automates, and optimizes mathematical efficiency analysis for a given set of experimental parameters. The new optimization software called the “Advanced-ISOCS” varies the “not well known” parameters of the source geometry within user specified intervals and determines the optimal efficiency. The efficiency optimization capability is carried out using either a random search based on standard probability distributions or using numerical technique that carry out more directed search. The radionuclide mass is determined using the optimum efficiency and compared against the known mass. Results of optimizations carried out using the numerical technique are presented in this paper.