College of Engineering Unit:
Criticality safety is a major aspect of the nuclear industry, with in-depth analysis applications in reactor design and fuel processing, storage, and transportation. This project investigates the application of the alpha-static eigenvalue in criticality safety analysis of nuclear systems to develop a more robust and comprehensive safety metric for modern designs. Current criticality safety standards are based on the k-static which is derived by replacing the time dependence of a system with a multiplication factor, k. Alpha-static is derived by assuming the time dependence of the neutron population in the system as exponential with time – similar to the decay constant in radioactive decay – to maintain the time characteristic of the system, being defined by time-independent conditions.
Current safety analysis using k-static is not universally applicable to all different types of systems. It has worked well historically because it is easy to do computationally, and most current reactors operate with thermal neutrons. However, some newer generation reactors start utilizing neutrons in the fast regime, and the current safety analysis method may not be robust enough. So, this project was done to look at the viability of a new safety analysis method and to see if a universal criticality safety margin could be achieved from it.
With the intention to utilize the developmental version of the OpenMC alpha-static code package, the first step was to verify the code package to examine its performance and accuracy in k-static and alpha-static calculations. Various fuel configurations and their corresponding computational benchmarks from several papers were compared to k-static, alpha-static, and removal time calculated using the OpenMC code package. With the difference in nuclear data libraries used, the calculated results were not equal to the exact benchmark values, but both values were close enough in the same order of magnitude. However, the energy spectra generated by the OpenMC matched those in the benchmark paper, proving the capability of OpenMC in simulating neutron behavior in various system designs. After the alpha-static code package was verified, it was then used to simulate and calculate the alpha-static values of various systems. Each system was represented by a spherical fuel with varying fuel material, radius, level of enrichment, type of moderator, and fuel-to-moderator ratio. One special case – the Uranium-Thorium fuel with FLiBe moderator and Zirconium cladding configured in an infinite hexagonal lattice - was later examined in addition to the general systems.
The results of these experiments showed a correlation between alpha-static values, k-static values, and the system energy regime. Prompt and delayed results were similar for supercritical systems, yet delayed alpha-static was limited by the delayed neutron precursor minimum decay constant, making prompt alpha-static more desirable for energy dependent. From the data, it was determined that multiplying the prompt alpha-static by the associated mean generation time yielded a generally linear relationship with k-static, where the slope was based on the system. This calculated value was determined to be equivalent to the k-static derived reactivity, thus meaning these values are the alpha-static reactivity.
