Integrated Fission Fragment Yields

The Integrated Fission Fragment Yields (IFFY) database is a tool for nuclear waste analysis.  It provides the history of a generic reactor in terms of its accumulated waste.  In effect, IFFY peeks inside the core of a reactor as it is operating. The main output of the IFFY simulation is a database table covering:

  • 31 Snapshots of the waste composition from 1, 2, 4, 8…230 seconds (34 years),
  • Fourteen fuel isotopes, fissionable by fast neutrons, slow neutrons or both
    • 23 Fuel and Speed combinations
  • 1322 Nuclides (isotopes and their isomer cousins) Current Objective

Get the simulation validated by a competent authority, and no longer be iffy.

IFFY Database Components

IFFY resides in a MS Access database.   The focus of the database is nuclear physics.  For readers who need a bit of background, we wove the relevant physics into our Definitions.  You’ll have to read them eventually anyway.

The description below is expanded in a post that covers the excruciating details of the database.

Integrated Nuclear data

The raw nuclear data came from several Internet sources.  Integrating these was a considerable effort.  If nothing else from IFFY survives, these should be valuable for educational or general analytic purposes.


IFFY uses two models to simulate the evolution of fission fragments.  The Decay model is the main driver.  The Capture model is based on the (iffy!) concept of Half-life Equivalency.  These processes operate inside our really tiny reactor, which splits one atom per second.

  • Caveat: The IFFY developer is an engineer, but not a nuclear engineer. 

Our Visual Basic simulation code implements the models.  This is available for verification and validation purposes HERE.  The methodology has other components, but they don’t need much blessing.

Output Tables

The Snapshots table provides the complete picture of fission fragment waste composition.  The full Snapshots table flirts with the size limit for MS Access (31 x 23 x 1322 records), so we provide some short versions HERE.

Production components

Outside the code, several SQL queries are relevant to the processing, as are some special tables and new isotope characteristics.

IFFY Features

The goal of the IFFY project is to track the contents of a nuclear reactor while it is operating.  You can find data for the initial proportions of fission products (yields) at the exact moment a fuel atom is split.  Also, you can find data for nuclear waste as it decays.  However, it is harder to find data for fission fragments that are being created and decaying concurrently.

Generic reactor

The project’s original focus was molten salt reactors (MSR).  Brand-new or slightly aged fission products are accessible in this type of reactor.  For this we need time-dependent cumulative yields.  However, the data are generic, so the results should apply to any type of reactor.

The IFFY reactor is tiny, operating at one fission per second.  Accordingly, we assume the results can be scaled up to any size reactor.

Data cloning

The IFFY simulation is deterministic, so one event is like any other.  For each fuel, we cloned the data from splitting one fuel atom a billion times.

Exponential time

The exponential treatment of time in this model is unusual.  We number Snapshots from 0-30.  Two, raised to the snapshot number, equals the number of fission events that have happened so far.  (Multiply this by two to get the number of fission fragment atoms.)

The data fall into neat slices of a thousand, a million, and a billion seconds.

We will use the KiloSec data in an analysis of a molten salt reactor’s off-gas system (coming soon).

Nuclear engineers might use these data to address other issues.  They should find snapshots 6-18 useful in an analysis of 135-Xe and other neutron poisons over time.  Waste composition of the waste at various ages is useful for corrosion analysis

We assign names to the last eleven snapshots :  MegaSec, 3wk, 6wk, 3mo, 6mo, 1/2/4/8/17yr, and GigaSec.

 Error budget

There is a rule-of-thumb in nuclear engineering that, after ten half-lives, the radioactivity is gone.  This rule is only 99.9% accurate.  We use this value, or more precisely 1023/1024, as the error budget for the project.

Our corollary is that after ten half-lives in an operating reactor, the population of an unstable isotope comes to equilibrium.  This is one of the IFFY concepts that a competent authority needs to verify.


We use a variety of analogies to explain nuclear processes and our representation of those processes.

  • Brookhaven National Lab manages the NuDat site, which has a flexible graphic of the Stable Valley.  It shows stable isotopes at the bottom of a valley, with hillsides full of unstable isotopes.
  • We adapt that by calling a stable isotope a Sink, toward which the unstable isotopes drain.
  • We also use a Decay Ladder analogy to emphasize the step-by-step nature of radioactive decay.

Future Efforts – a full fleet of reactors

The IFFY project focuses on a single reactor.  Originally, we were only interested in molten salt reactors.  Now, the notional reactor is more generic.

The next project will focus on a fleet of reactors.  Specifically, we can use the output of regular (slow/thermal) power reactors as input for fast reactors.  This creates the potential for an elegant waste disposal plan.

Fleet Sim Concepts

As with IFFY, we need a competent nuclear expert to vet the the Fleet Sim concepts.  IFFY focused on the evolution of fission fragments.  In contrast, the Fleet Sim focuses on the evolution of fuel.

Failed Fuel

Fission fragments comprise most of the nuclear waste stream: ~80% today, but ~98% for a Thorium-powered fleet.  The other major component of nuclear waste is failed fuel.  The 18% of 235-U atoms that fail to fission when a neutron hits them become 236-U.  The fate of 236-U depends on its cross section for capturing neutrons.  We evolve the heavy atoms using the concept of Half-Life Equivalency.

Reactor speed

In testing a new reactor design, the engineer will be able to manipulate one variable, the reactor’s Speed.  The designer can chose any combination of fast and slow neutrons.

Fuel schedules

The current IFFY output data assumes a single fuel.  This doesn’t happen in real reactors.  Today’s reactors start with pure Uranium fuel; 4-5% 235-U and the rest 238-U.  Some of the 238-U becomes 239-Pu.  Some of that fails to fission and become 241-Pu.  Plutonium might contribute one-third of the power by the time the fuel batch needs replacement.  Future reactor operators might vary the fuel mix on purpose.  We will need a special table and some new code to capture this possibility.

Fleet data repository

The next generation IFFY model will use a repository to store the results of a variety of reactor designs and fuel schedules.  This will allow us to analyze nuclear waste for an entire fleet of reactors.  One interesting option is to use the waste from slow reactors as fuel for fast reactors.  Then we can design an entire fleet of reactors optimized for minimal waste.