FISPIN - A Code for Nuclide Inventory Calculations

FISPIN is a fuel depletion code. It calculates the changes in the numbers of atoms of the nuclides of various species – heavy isotopes or actinides, fission products, and structural or activation materials – as a sample of nuclear fuel element is subjected to periods of irradiation and cooling.

FISPIN is a 'point' code — the whole sample of nuclear fuel element is subject to the same irradiation conditions. Usually, the sample of reactor fuel initially contains atoms of a few uranium and plutonium isotopes and, optionally, atoms of some structural material such as pin clad, graphite moderator or impurities in the fuel pin. The atomic number densities of the various actinides, fission products and structural material nuclides are of interest in themselves.

Other useful information may be obtained by multiplying each atomic number density by the corresponding radioactive decay constant, to obtain the radioactivity in becquerels (disintegrations/second) or curies (3.7 x 1010 disintegrations/second). These include the α, β and γ energy (MeV), the number of neutrons emitted by spontaneous fission or from (α,n) reactions in oxygen (in oxide fuel), the γ energy spectrum (MeV) and multiplicity (number of photons in a given γ energy band or group).

FISPIN can 'edit' the atomic number densities of the nuclides, using the decay constant and these associated quantities, to obtain useful information such as the γ source spectrum and α, β and γ contributions to decay heat. Please note that FISPIN can only calculate the heat from decay, not from neutron-induced reactions.

It is useful to present the totals of these quantities for the actinides, fission products and structural material nuclides. Users who are involved in fuel reprocessing often want these quantities broken down by chemical element.

To calculate the changes in the numbers of atoms of the various nuclides, FISPIN requires the radioactive decay constant and the modes of decay and branching ratios between them, for the various nuclides. It also requires the independent fission yields of the fission products created by the fission of the 19 actinides which undergo significant amounts of fission. These data, and the associated quantities per decay mentioned above, do not change between one type of thermal reactor fuel and another.

The other class of nuclear data required by FISPIN is the cross sections for various neutron induced reactions in the various nuclides. For example many of the actinides undergo fission, (n,2n) reactions and neutron capture (n,γ), sometimes to several distinct states of the daughter product nuclide. These cross section data change markedly between different types of reactor fuel, and can also change with burnup, due to changes in neutron spectrum and to changes in resonance shielding.

The ANSWERS TRAIL code is often used in conjunction with the WIMS reactor physics code to prepare burnup dependent cross section libraries for use with FISPIN. The integrated WIMS/TRAIL route provides reactor specific and burnup dependent cross sections that are then augmented with burnup independent cross sections from a TRAIL database. These cross sections are then output in a form suitable for use with FISPIN.

FISPIN is often used to model the irradiation of fuel at a given power rather than at a given neutron flux, so data are required which give the energy released per fission of an actinide, to permit the relationship between flux and power to be calculated. It is also possible to calculate fission product composition, emitted radiation and decay heat following specified periods of fisson rates and cooling.

FISPIN is issued with JEF2.2 and JEFF3.1 reactor independent nuclear cross section libraries including 38 different reactions. The JEF2.2 file contains data for 326 actinides, 1075 fission products and 1917 structural materials; respectively 324, 1380 and 1913 for the JEFF3.1 file.

A Graphical User Interface, FISGUI is available to simplify the production of standard FISPIN calculations using a set of generic PWR, BWR, AGR and Magnox reactor cross sections without the need to run WIMS/TRAIL calculations. The FISGUI also has the capability to generate neutron production for (α,n) reactions for arbitrary compounds. It is possible within FISGUI to calculate the neutron emissions from fuel of any composition and from liquors or waste derived from fuels. Using the LET and REPROC option, it is also possible to trace waste streams, where components have been fully or partially removed eg liquors where 90% of uranium and 98% of plutonium have been removed.

FISPIN is available on most major workstations and PC's

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