An analysis of fission product gamma spectra : the use of nuclear systematics to estimate gamma spectra Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/0r967615b

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  • The gamma contribution to reactor afterheat depends on gamma energy since high-energy commas are more likely to escape from tilt. fuel. The objective of this work is to investigate the gamma-energy spectrum of a pure U-235 fission reactor. Fission-product nuclides are first listed in order' of importance as contributors to total gamma energy at various times after shutdown. In particular, lists are drawn up using LNDF/B-IV data for 1, 10, 100, 1,000, and 10,000 seconds after a fission burst and after an operating time of 24 hours. Luclides from these lists are then compared with nuclides from a magnetic tape entitled "radioactive Nuclide Decay Data -File-CWR 17-74" by Charles W. Reich of the Aerojet Nuclear Corporation. The Reich tape provides little data about important gamma contributors in the early stages after short irradiation times. A simrlified approach is chosen to estimate the gamma spectra of nuclides without Reich data. That is 1 nuclides in the Reich. tape are sorted into "boxes" (collections of nuclides) on the basis of simple nuclear systematics and the "box" averages are assumed to represent the nuclides without Reich data. Isomeric states, nuclides with less than ten gamma spectral lines, and nuclides with atomic mass numbers above 200 are generally excluded from sorting. Each sorting into boxes is done in two ways: (1) by the "absolute-energy" method, where gamma spectra are split into "energy groups" of fixed intervals, such as 250 keV, and (2) by the "adjusted-energy" method, where the gamma spectrum of each nuclide is split into "energy groups" found by dividing that nuclide's Q value by a fixed number, such as ten. The second method is preferred. In a "zeroeth" sorting, all nuclides are put into just one box. In the first sorting, nuclides are put into four boxes (even-even, even-odd, odd-even, and odd-odd) based on the oddness or evenness of proton number and of neutron number. In the second sorting, nuclides are put into four boxes (1 -Z magic, N-0 magic, 0-Z magic, and 0-0 magic) on the basis of distance from the magic numbers 28, 50, 82, and 126. A nuclide is deemed "N-magic" (for its neutron number) or "Z-magic" (for its proton number) according to its "closeness" to a magic number, where "closeness" is defined by a bandwidth around each magic number. In the third sorting, nuclides are put into one of four boxes according to distance from beta stability, which is equal to the number of consecutive beta-minus decays required to reach a stable nuclide (or a very longlived nuclide). In the fourth sorting, nuclides are put into two boxes (positive and negative) on the basis of the parity of the ground states. For each box, an average spectrum (normalized to 100%) is computed to indicate the percentage of gammas contributed by each energy group. Also calculated for each energy group is its standard deviation from the average spectrum. Since the standard deviations are usually too large to be of value, "composite" spectra are created by weighting the average spectra from various individual sortings. In the "tri-composite" case, each of three factors (odd-evenness, distance from magic number, and distance from beta stability) are used in several combinations of weighting. In the "optimum" combination (where the sum of the squares of the standard deviations is minimized), odd-evenness has about twice the weight of distance from magic number, while distance from beta stability is insignificant. Finally, the optimized composite spectra are used to provide estimates of actual gamma spectra at various instants after reactor shutdown. several operating histories prior to shutdo,cm are considered for purposes of comparison.
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