leup.tex

\section{Fuel Enrichment}
\label{sec:leup}

In 2020, a \gls{haleu} workshop report led by Monica Regalbuto \cite{regalbuto_high_assay_2020} highlighted the unique regulatory challenges of establishing a \gls{haleu} fuel cycle in the \gls{usa}. It noted that part of enriching \gls{haleu} is first to produce \gls{leup}, defined as between 5\% and 10\% $^{235}U$ enrichment. The report notes that \gls{leup} facilities would fall under a similar category of regulations as the existing \gls{leu} fuel cycle, allowing existing enrichment servicers to leverage their experience and infrastructure before taking on the increased regulatory burden of producing \gls{haleu}. If a reactor could be redesigned to accommodate it, using \gls{leup} could delay the demand for \gls{haleu}. Table \ref{tab:enrichment_levels} shows the various levels of enrichment for uranium fuel in the \gls{usa}

\begin{table}[H]
   \centering
   \caption{Enrichment levels and their ranges.}
   \label{tab:enrichment_levels}
   \begin{tabular}{l l}
      \hline
      \textbf{Enrichment Level} & \textbf{Range [\%  $^{235}$U]} \\
      \hline
      Natural & < 0.711 \\
      \gls{leu} & 0.711-5 \\
      \gls{leup} & 5-10 \\
      \gls{haleu} & 10-20 \\
      \gls{heu} & $\geq$ 20  \\
      \hline
   \end{tabular}
\end{table}

One of the primary advantages of a fuel cycle containing \gls{leup} is that the facility to produce it would fall under the same licensing category as \gls{leu} fuel. The \gls{nrc} defines a \textit{special nuclear material of low strategic significance} as meeting one of three criteria, the most notable of which for the purposes of this thesis is "(3) 10,000 grams or more of uranium-235 (contained in uranium enriched above natural but less than 10 percent in the U–235 isotope)," \cite{nrc_catiii}. This facility definition is where the upper limit of the \gls{leup} range arises.

Facilities like Centrus in Piketon, Ohio enrich \gls{haleu} fuels compliant with regulations for \textit{special nuclear material of moderate strategic significance} (Category II), but the commercial facilities in the \gls{usa} do not have contemporary experience enriching \gls{haleu} fuels and this demonstration will need to be expanded to support a fleet of higher enriched reactors. Additionally, Category II facilities come with increased construction and operation costs that would increase the barriers to establishing these facilities before a consistent demand has been established. Thus, \gls{leup} is an attractive intermediary step for servicers wishing to minimize the size of a Category II facility (thereby reducing costs) as it is the same category as historically licensed enrichment facilities for \gls{leu}.

Traditional \glspl{lwr} could receive benefits from using \gls{leup} fuel; as outlined by L\'{o}pez-Luna et al. \cite{24_month_cycle_bwr}, incorporating such fuel rods would allow for a 24-month cycle in the \gls{bwr} design they studied and would reduce the levelized cost of the nuclear fuel cycle they simulated. In October 2024, Framatome announced that their 6 wt$\%$ $^{235}$U GAIA fuel assemblies completed their third 18-month fuel cycle at the Vogtle plant in Georgia \cite{framatome_press_2024}, with the eventual goal of this process being commercialization of new accident-tolerant fuels that can potentially support \gls{leup}.

Increased prevalence of higher enrichment fuels will require modifications to the existing supply chain, particularly to ensure the continued safety of workers and the public. A 2022 report from Shaw and Clarity out of \gls{ornl} highlighted that existing nuclear fuel vault configurations at \glspl{bwr} and \glspl{pwr} did not have sufficient margins to satisfy regulatory requirements when fully flooded \cite{leup_atf_storage_impacts}. Their report only studied the impacts of 6.5 wt$\%$ and 8 wt$\%$ fuel, but they concluded that \gls{haleu} fuel would similarly require significant changes to existing fuel storage infrastructure.