Assessment of Non-Traditional Isotopic Ratios by Mass Spectrometry for Analysis of Nuclear Activities

Technique can be used to distinguish between commercial nuclear reactor fuel cycles, fuel cycles for weapons grade plutonium, and products from nuclear weapons explosions.

The objective of this work is to identify isotopic ratios suitable for analysis via mass spectrometry that distinguish between commercial nuclear reactor fuel cycles, fuel cycles for weapons grade plutonium, and products from nuclear weapons explosions. Methods will also be determined to distinguish the above from medical and industrial radionuclide sources.

MCNPX fuel assembly models for the BWR (boiling water reactors), PWR (pressurized water reactors), and CANDU (Canadian natural deuterium) fuel assemblies.

There are many sources for radionu-clides in our environment. These include natural sources, the commercial nuclear industry, nuclear weapons, the medical industry, and other sources. Often times, the source of the radionuclide may be determined through just identification of the radionuclide. If radionu-clides are produced through different sources, the identification of the source is complex. In order to ascertain a specific source for attribution, radionu-clide ratios are often employed.

Production yields of radionuclides from fission are a function of many variables including: the fissile material, the energy spectrum of the neutron flux, the magnitude of the neutron flux, and the duration of the irradiation. As a result, the ratios of certain radionuclides are highly dependent on these variables and may be utilized to distinguish between radionuclides produced from nuclear weapons, medical waste, short nuclear fuel cycles (e.g. 239Pu production fuel cycles), and long nuclear fuel cycles (e.g., commercial nuclear fuel cycles). While the above is easily stated, the difficult part is to determine which radionuclide ratios should be utilized for best forensic value.

As an added complication, nuclear debris taken for forensic analysis often does not come directly from the source. There is often some type of chemical process or other process that may alter the sample composition. Chemical fractionation issues result and may significantly alter ratios of radionuclides of different elements. To mitigate this problem, it is best to examine isotopic ratios of individual elements since these ratios will be largely unaltered by chemical processes.

Traditional methods for radionu-clide detection depend upon measuring the energy released during radioactive decay. Decay counting is relatively simple, but sample prep and analysis takes time to complete. If short-lived radionuclides have already decayed, traditional counting can be quite slow. Mass spectrometry (MS) techniques often require comparable sample prep to decay counting, but analysis is faster since MS counts atoms rather than waiting for them to decay. Reducing time between sample collection in the field and reliable analytical results requires switching to MS.

There are numerous MS techniques capable of measuring isotopic ratios. The sample size, detection limit, dynamic range, sample prep requirements, and ease of analysis vary widely among the techniques. Some techniques have very simple sample prep, requiring only dissolution in acid or combustion prior to analysis. Others require extensive preprocessing that impedes quick turnaround. In practice, the selected MS technique will need to accurately measure isotope ratios in a range of interest as quickly as possible. To insure speedy analysis, the MS technique should probably be sufficiently robust to be field deployable inside a transportainer.

Extensive work was conducted during the first two years of this project. Nuclear reactor fuelcycles were modeled utilizing ORIGEN. Fuels cycles from pressurized water reactors (PWR),boiling water reactors (BWR), and Canadian natural deuterium (CANDU) reactors were all evaluated. Nuclear weapons were modeled by utilizing a bare sphere (keff =1.0) in MCNPX utilizing the BURN card. The production of every fission product, activation product, and transuranic was recorded and entered into a database.

An R value was then calculated for each possible isotopic ratio. This is a metric to evaluate the forensic value. R values greater than 100 or less than 0.01 are considered good. R values are-calculated as shown in the following equation.

Isotopic ratios were then prioritized by magnitude of the ratio, the absence of possible interferences in field monitoring, and the mass of isotope produced, and compiled into a table that shows the isotopic ratios identified as the best for distinguishing between an unknown reactor type and a known commercial reactor signature.

This work was done by Steven Biegalski of the University of Texas and Bruce Buchholz of Lawrence Livermore National Laboratory for the Defense Threat Reduction Agency. DTRA-0005

This Brief includes a Technical Support Package (TSP).
Assessment of Non-traditional Isotopic Ratios by Mass Spectrometry for Analysis of Nuclear Activities

(reference DTRA-0005) is currently available for download from the TSP library.

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