Cross section versus recipes for fuel cycle transition analysis using ORION

Joshua Peterson-Droogh, Eva Davidson and Robbie Gregg

Oak Ridge National Laboratory

When modeling fuel cycle transitions, it is important to accurately capture the changes in radionuclide inventory of spent fuel at discharged as they can vary significantly from their associated steady-state conditions. Two methods for calculating the radionuclide inventory of discharged spent fuel for fuel cycle analysis includes using pre-calculated recipes and using cross sections.

Recipes are tabulated sets of feed and discharge compositions for a given fuel irradiation history. They provide the “transfer coefficients” that fuel cycle simulators need to convert mass flow radionuclide compositions for the feed fuel of a reactor into the mass flow radionuclide compositions of fuel discharged from the reactor. Recipes are calculated ahead of time using neutronics tools and then are input directly into the fuel cycle model. Although this approach works well for modeling fuel cycles with a fixed input and output composition (e.g., once-through LEU fuel cycles and simple single recycle scenarios) or fuel cycles already at equilibrium when compositions do not very significantly, it is difficult to accurately model more complex scenarios involving isotopic changes that occur during transition and on the approach to equilibrium. This includes changing from one fuel type to another or from one fuel cycle approach to another, such as transitioning from the current U.S. LWR fleet to a fleet of sodium fast cooled reactors.

Using problem-specific cross-section libraries generated by neutronics depletion tools is the second method for calculating the radionuclide inventory of discharged spent fuel for fuel cycle analysis. Unlike the recipes approach, the output stream in the reactor model is dynamic and changes based on input stream radionuclide composition. Another advantage of using cross-section libraries within fuel cycle evaluation codes is that some fuel cycle simulators can perform on the fly cross-section interpolation routines that generate reactor-, cycle-, and scenario-specific production and destruction routes during the fuel cycle calculation. This provides a means to capture the effects of changes in the neutron flux spectrum and magnitude of isotopic concentrations and cross sections during in-core irradiation.

Staff at ORNL has produced burnup-dependent cross-section libraries that can be used for the fuel cycles being evaluated in the DOE FCO campaign [1] including cross sections for thermal reactors (CANDU, BWR, PWR, MOX, MSR) and fast reactors (SFR). These cross sections have been used in ORION, a nuclear fuel cycle simulator developed at NNL [2]. It will be demonstrated with ORION that though recipes work well for static systems such as once-through fuel cycle models and steady-state scenarios, cross sections provide a higher fidelity results that can capture changes in more dynamic systems such as complex scenarios involving the multi-reuse and isotopic changes that occur during the transition and on the approach to equilibrium.

[1] Wigeland, R., et al. (October 8, 2014). Nuclear Fuel Cycle Evaluation and Screening - Final Report. FCRD-FCO-2014-000106. US DOE.
[2] R. Gregg, (October 2014) ORION User Guide –V4 Draft Issue 7. IMS_T_REP v.10