Induced radioactivity in materials is important in a number of areas
of nuclear technology, including fission-reactor afterheat (heat
removal, meltdown accidents), fission-reactor operations (parasitic
absorption, decay contributions to heat production), fusion-reactor
waste discharge isotopics (waste disposal), accelerator operations
(personnel exposure, air activation, requirements for remote
handling, discharge isotopics of waste materials), and the production
of medical isotopes. Radioactivity calculations require four
components:
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Developing libraries of activation cross sections for use in radioactivity
calculations is very difficult because of the wide range of nuclides
involved in the different applications of interest. Fission reactor
calculations require good capture cross sections for about 200
important fission-product nuclides, and they need capture, fission,
and higher reactions for a large number of actinide isotopes. These
values have been commonly available for some time, but newer applications
proposed for high-current accelerators (such as tritium production,
waste transmutation, or even energy production) will require even more
cross sections. Basically, all reaction channels are needed, for all
targets with significant lifetimes, for energies up to perhaps 150 MeV,
for both neutrons and protons! These requirements total up to perhaps
10 or 20 thousand important reactions, and many of them have never been
measured. Our approach to this problem has three components: (1) gather all the best current evaluations for activation cross sections, (2) test against experimental data when available, (3) make incremental improvements to evaluations that disagree with experiment using nuclear model codes, and (4) do global calculations for unmeasured reactions using modern nuclear model codes. Some of the fruits of component 1 are available on this web site. The main libraries for ENDF/B-VII, JENDL-4, and JEFF-3.0 contain some of the activation cross sections of interest. However, many of the evaluation communities have also provided separate libraries of activation cross sections, and some of them are available through our nuclear data viewer; currently, the ADL-3T library of 20,000 activation cross sections from Russia and the EAF-2010 library from Europe with 66,000 reactions. |
When a nucleus fissions, the two fission fragments are not normally equal
in size. And every fission can result in slightly different products.
The result is a distribution of possible fission fragments known as
a fission-product yield distribution, which has two peaks. These
distributions are very important for the operation of fission power
reactors; they affect parasitic absorption, on-power decay heat,
decay heat after shutdown or emergency scram, and delayed neutrons
(which are important factors for reactor kinetics and control).
Because of their importance for both operations and safety,
fission-product yield distributions have been studied extensively
and made available in computer files for reactor calculations. In the
US, the standard set of data is part of ENDF/B-VII. Much of this work
was done in our Group, and the defining publication is available
on line in our publications
area under the title "Fission Product Yields." For the
convenience of the reader, we provide additional links here
to the paper T. R. England and B. F. Rider, "Evaluation and Compilation
of Fission Product Yields," Los Alamos National Laboratory report
LA-UR-94-3106 (Oct. 1994). Because this paper contains about 80,000
lines of text, we break into parts for this web site. The
General Discussion and Bibliography
is given as a Postscript file (1.75 MB). A
PDF version
is also available. The recommended yields, original data, and reference
sources are presented in a series of appendices in the form of
ordinary computer text files:
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The lifetimes and modes of decay of radioactive nuclei have been
studied intensively over the history of nuclear physics. The literature
for this active field is summarized in a bibliography called "Nuclear
Science References" (NSR) available at the
National Nuclear Data Center (NNDC).
These data are gathered together and evaluated by the members of the
US
Nuclear Data Network (USNDN). The evaluations make an attempt
to come up with a consistent set of "best" data for each nuclide.
The result is the Evaluated Nuclear Structure Data File,
ENSDF,
which is maintained at the NNDC. Printed information that has
resulted from this evaluation project is also available in the
Table of Isotopes
books, which have recently been upgraded and published by John
Wiley & Sons. However, this work has a definite physics outlook. The people most closely involved in it are most interested in things like spin assignments to levels, the effect of deformation on the level structure, and so on. For use in application calculations, different representations of the data are more convenient, and such a format is part of the Evaluated Nuclear Data File (ENDF). The decay-data sublibrary of ENDF/B-VII.1 contains sections that describe the decay modes, mode lifetimes, decay energies, and spectra of decay products (neutrons, alphas, beta-plus, beta-minus, gamma rays, and x rays). Because each section must be complete in order to be of use for application calculations, they have sometimes been extended past the experimental limits using model calculations. The final results are available from the NNDC. For the convenience of users around the world, we provide a simpler interface to the recent ENDF/B-VII.1 decay data. This link provides you with a list of all the nearly 4000 nuclides available. Clicking on one of the nuclides will bring up an HTML page with an interpreted version of the ENDF file. It is not necessary to know how to read the format to understand this listing. Just look for interesting parameters like "half life," "Egamma," or "spin & parity."
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Once the basic data as described above are available, you can proceed
to calculate the inventories of radionuclides as a function of time
for a system exposed to a flux of radiation (neutrons or protons,
usually). This calculation requires you to solve a set of coupled
differential equations stating the balance between producing (by
a reaction, or as a fission product) and destroying (by a reaction,
or by decay) each possible nuclide. Our code for computing nuclide inventories is called CINDER. It was originally written by Tal England for fission-reactor problems, and it went through a number of versions, culminating in ?? which is reported in ??. The classical CINDER code worked by dividing the process up into a set of "chains." This has the effect of reducing the size of the coupled system that has to be solved, thus giving more precise control over calculational errors. The final nuclide inventories were determined by summing up the contributions from the different chains. These chains were always chosen by hand to work well for fission reactor problems. More recently, CINDER has been extended to work for more general fusion and accelerator problems by importing libraries from the REAC code (reference) and teaching the code how to determine its chains automatically. This is necessary, because so many more reaction paths are available at higher energies, and many more nuclides are involved. The set of chains found for a given run depends on the conditions for that run. We are working on this section.... |