The IFFY simulation uses several types of nuclear data to evolve fission fragments and (eventually) failed fuel. The IFFY database has five key input data tables.
Nuclear Data Tables
|Nuclides||Half-life||Lawrence Livermore (LLBL)|
|Absorbers||Barns||Nat Inst Standards|
Nuclides table – home of the Primary key
The Nuclides table is the central table of the database. Here we define the Nuclide, which is the primary key that links the various tables together. We use AtWt-Symbol to define an isotope. We add another dash and the energy Level (1 or 2) to define an isomer.
|Field Name||Data Type||Description|
|ID||Integer||Order of processing|
|Nuclide||Short text||Isotope/Isomer name|
|Level||Integer||Metastable energy level|
|Rung||Double||Number of excess neutrons|
|Time_Bin||Short text||Half-life in time units|
|EZA||Integer||Adaptation of TORI key|
|Source||Short text||Source of half-life data|
The TORI_2 Parents table from the Lawrence Livermore/Berkeley Lab (LLBL) web site provided the basic structure for the Nuclides table. Unfortunately, LLBL no longer offers the TORI_2 download. Originally, we renamed the table from Parents to Isotopes. The existence of isomers forced us to use Nuclides, a term far less familiar.
ID: First, we sorted the nuclides in the order that decay is processed, i.e., heavy-to-light and top-rung-down. Then we added the ID field, using Access’s auto-number feature, and so saved the processing order.
Nuclide: This is the name of the isotope or isomer, in the form ##[#]-(A-Z)[(a-z)][-#].
Half-Life: Half-life is the primary characteristic needed for decay calculation. We store half-life in seconds. To avoid divide-by-zero errors we gave stable isotopes a half-life of 1.0e+35 seconds.
AtWt: Atomic weight is the total number of protons and neutrons in the nucleus.
Protons: The number of protons is also called atomic number. The Protons field provides the link to the Elements table.
Level: This integer represents the number of energy levels above the ground state for an isomer. It is zero for isotopes, but we don’t put “-0” at the end of an isotope’s name.
Rung: In the Decay Ladder analogy, unstable isotopes are located above the stable isotope they will someday become. They are unstable because they have too many neutrons. The Rung value counts the excess neutrons. We assign a Rung value of zero to the stable terminator at the bottom of the ladder. Isomers have Rung values of 0.1 or 0.2 higher than the rung value of their ground state isotope. The Rung is critical for processing decay properly, that is, in one pass.
Time_Bin: This is a rough estimate of the half-life of each nuclide. If the half-life is recorded in seconds, we assign it to the 1_Sec bin. We had to translate some numbers in the data integration phase. For instance, an isotope with a half-life listed as 36 hours was put in the 4_Day bin. Thus we aggregate the 1322 nuclides into nine bins. This allows interesting charts, sorted properly because of the prepended number.
EZA: This is an adaptation of the awful TORI_2 primary key. For further info, see TORI_2 Issues.
Source: This is the source of the half-life data, usually TORI_2. However, we added some nuclides, and the reason is captured here.
If we had mass data for the isomers, we’d add an amu field, for atomic mass units, to the Nuclides table. This would be useful in heat calculations and in describing E=mc2 issues.
There are 1322 records in the Nuclides table, but 1704 in the Branches table. Some nuclei decay in more than one way. The Pct field tracks how often an nucleus will decay by that branch. We use the common familial analogy that a parent decays to a daughter. Technically, putting the daughter in the table is redundant. We could derive the daughter from the decay mode. Did we mention we are not database purists?
|Nuclide||Short text||The decaying parent nuclide|
|Mode||Short text||Type of decay|
|Source||Short text||Source for mode & %|
|Daughter||Short text||Resulting nuclide|
The Branches table is patterned after the Parents2 table from the TORI_2 database. The International Atomic Energy Agency’s JEFF-20 report provided most of the decay mode data that produce isomers. These decay mode names end in “-m”. The “m” is short for meta-stable, another way of referring to isomers.
Nuclide: This is the parent, which is about to die and leave an inheritence (some percent of its starting population) to its daughter(s).
Mode: The mode is the type of radioactive decay. Most popular is B- (Beta Minus) decay, which is named after the Beta particle ejected in this form of decay. Actually, the Beta particle is an electron.
Pct: This value reflects the probability that the parent will decay by the named mode.
Source: This is the source of the decay mode and percent data, usually TORI_2 or JEFF-20. If not, it reflects the reason for an assumed value.
Daughter: The daughter is the isotope that emerges from the decay process, or the isomer that emerges when the decay mode ends with “-m”.
The initial yields provide the probabilities that a given nuclide (fission fragment) will emerge when a fuel atom splits. The sum of the probabilities should be 2.000 because two nuclei emerge from each fission. However, probabilities should always add up to 1.000. Since we don’t want to be harassed by the statistics police we call them yields.
The initial yields from Brookhaven’s Sigma database include data for a few higher energy neutrons, possibly for fusion research. We do not include these in the Yield_Matrix table. The matrix is rectangular for ease of processing. We added some zero-yield nuclides to pad the periphery of the matrix. We also added other nuclides to complete a decay chain. (Gaps appeared when we were integrating the decay mode data, especially the “-m” modes.)
|Fuel||Short Text||Isotope being split|
|Speed||Short Text||Neutron energy|
|Nuclide||Short Text||Fission fragment|
|Yield||Double||Number of atoms|
Fuel This is the nuclide that splits into two fission fragments. Not all the given fuels will split when hit with a slow neutron. Hence, this table needs three keys (fuel, speed, and nuclide) to identify each data record uniquely.
Speed: The neutron that initiates fission can be moving fast or slow. The original data specified energy levels of 0.0253eV or 0.5Mev. Floating point numbers don’t make good keys, so we called these the Fast and Slow data sets.
Nuclide: This is one of the fragments that emerge from the fission event.
Yield: This is the probability that the given nuclide is produced. Yield_Matrix values are less than one atom. While this is physically impossible, the math works.
This table maps the number of Protons to an element Name (not used) and Symbol. The symbol is found within the nuclide name. It is used separately in some charts.
This table covers fission fragments that have a significant cross-section for capturing thermal (slow) neutrons. These can rob the reactor of some of the neutrons it needs to continue the chain reaction. For this reason, another name for these isotopes is neutron poisons. In addition, this table includes the stable isotopes of some lighter elements.
|Field Name||Data Type||Description|
|Nuclide||Short text||Neutron absorber|
|Abs_xs||Double||Cross section barns|
|Source||Short text||Barns source|
Note that the name of the field is Abs_xs, where abs is short for absorption. Some references use absorb and capture ambiguously. Fuel nuclei absorb neutrons before they fission. Here, we limit capture to absorption where fission doesn’t occur. Capture can lead to instability, and then to the next higher element via transmutation. In some cases the nucleus simply gets “plumper”. The element stays the same, for instance 154-Gd becomes 155-Gd. Both of these are stable. Some references still call this “transmutation”, but we don’t.
New Capture Table: LocalHLE
The Absorbers table once had a field called HLEsec, the half-life equivalent for an isotope when the fuel was 235-U. Since then, we made the HLE values dependent on (local to) the fuel. The absorbers table is no longer used in the IFFY code. It did, along with the Fuel_Barns (below), provide the local HLEsec values. It still provides the Class value in the View235QC download, for charts and other analysis.
|Nuclide||Short text (10)||Fission fragment: Neutron poison (absorb barns >0)|
|Fuel||Short text (10)||Actinide|
|HLEsec||Double||Half-life Equivalent in seconds|
Fuel_Barns data (in development)
This table includes fission cross-sections in addition to capture cross-sections. Cross-section data are available for some minor nuclear reactions, and we may include some of these at a later date. For example, the n,2n reaction creates 232-U in a Thorium fueled reactor. 232-U is basically a contaminant, but is not entirely bad. It has a minor role in non-proliferation.
|Field Name||Data Type||Description|
|Nuclide||Short text||Fuel isotope|
|Source||Short text||Data source|
The new field here is the Region, which describes the conditions under which the barns values were developed. Here, the primary distinction is between slow (thermal) and fast neutrons. Some data exist for the region between, called epi-thermal. These would be useful in a more detailed simulation. The other regions define different ways of measuring neutron energy.