triso_fuel.tex

\section{TRISO Fuel}
\label{sec:triso_fuel}

This thesis adapts the approach of Bachmann et al.
\cite{bachmann_enrichment_2021} to focus on \gls{triso} fueled reactor designs
alongside current commercial fuel forms at various enrichments. The \gls{triso} fuel form can accommodate any classification of enrichment; several reactor designs use different fuel enrichments that are all \gls{triso}. This section will distinguish the production of \gls{triso} from the traditional metallic fuels used in \glspl{lwr} outlined in Section \ref{sec:nfc}.

Coating the fuel particles is a critical step in the fabrication of \gls{triso}
fuel, and the idea has existed in nuclear fuel design spaces since the 1950s
\cite{price_dragon_2012} with the Dragon project. In 1957, the Harwell facility
began coating spherical fuel particles, and in 1961, researchers modified the
particle coating to include a silicon carbide layer to trap cesium, strontium,
and barium--which diffused through the single pyrolytic carbon layer.
Concurrently, in 1958, a report to the \gls{aec} introduced the concept of a
pebble bed pile (first proposed by Daniels
\cite{f_b_daniels_suggestions_1944}) to the broader nuclear community.
Researchers in Germany, China, and the UK have proposed, built, and operated
similar designs since then, with companies in the \gls{usa} looking to deploy
modern versions of the technology.

In a 2019 paper, authors Demkowicz, Liu, and Hunn \cite{particle_review_2019}
authors describe the fuel as a particle encapsulated in layers of pyrolytic
carbon and silicon carbide; a \gls{fbcvd} applies each of these layers. The
fuel kernel, shown by Figure \ref{fig:triso_layers}, is typically composed of
uranium dioxide, and it is surrounded by a porous carbon buffer using acetylene
in the \gls{fbcvd} as it has a relatively low density. The silicon carbide
layer encapsulates the pyrolytic carbon layers and provides a barrier to
fission products. A mix of methyltrichlorosilane and hydrogen is sufficient for
SiC deposition without argon. The inner and outer pyrolytic carbon layers
isolate the silicon carbide layer and provide a barrier to the coolant; the
\gls{fbcvd} applies these layers using a mix of propylene, acetylene, and argon.

\begin{figure}[H]
    \centering
    \includegraphics[scale=0.98]{images/triso_review/triso_layers.pdf}
    \caption{TRISO fuel particle layers \cite{particle_review_2019}.}
    \label{fig:triso_layers}
\end{figure}

Through a heat and pressure setting process, fabricators create a graphite
matrix that accepts the coated particles.
% These matrices hold the \gls{triso} particles in graphite and carbonized resin and are set under heat and pressure, often overcoated to prevent particle-particle interactions.
The resulting fuel type is incredibly robust and can reduce proliferation
concerns due to the difficulty of separating the particles from the matrix and the fuel from the particles. The fuel can withstand high temperatures and burnups, which can be advantageous in reactor designs that require high temperatures for efficient operation.