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\title{Improved Stability of Thermoelectric Generators Using Bi-Isotopic Fuel: Pu-241 and Am-241}
\author{Dmitrii Kouznetsov}
\date{}

\begin{document}

\maketitle

\begin{abstract}
We analyze the potential advantages of using a mixed-isotope fuel composed of \textsuperscript{241}Pu and \textsuperscript{241}Am in long-duration radioisotope thermoelectric generators (RTGs). This bi-component mixture may improve thermal stability over long timescales by compensating the decay of \textsuperscript{241}Pu with the growing contribution of its decay product, \textsuperscript{241}Am. Challenges related to fuel processing, neutron emission, and shielding are also discussed.
\end{abstract}

\section{Introduction}

The traditional choice of isotopes for RTG power sources, such as \textsuperscript{238}Pu, prioritizes high specific power and low radiation shielding requirements. However, isotopes such as \textsuperscript{241}Am and \textsuperscript{241}Pu are produced in nuclear reactors and can be found in nuclear waste. We propose that a bi-component mixture of \textsuperscript{241}Pu and \textsuperscript{241}Am could provide a more stable heat output over century-long missions.

\section{Isotopic Properties and Heat Output}

Table~\ref{tab:data} shows key data for selected isotopes relevant to RTG fuel, including decay type, half-life, heat generation, estimated cost, required shielding, and references.

\begin{table}[h!]
\centering
\begin{tabular}{|c|c|c|c|c|c|c|}
\hline
Isotope & Half-life & Power (W/g) & Decay Type & Est. Cost (\$/g) & Shielding & Ref. \\
\hline
\textsuperscript{241}Am & 432 y & 0.115 & $\alpha$ & 1500--3000 & 2–3 cm borated poly & [1][2] \\
\textsuperscript{241}Pu & 14.4 y & 0.0042\footnote{Net heat from beta decay; antineutrino energy not included.} & $\beta^-$ & High / classified & minimal & [1][3] \\
\textsuperscript{238}Pu & 87.7 y & 0.57 & $\alpha$ & classified & 1–2 cm Pb & [1] \\
\hline
\end{tabular}
\caption{Properties of selected isotopes for RTG fuel.}
\label{tab:data}
\end{table}

\section{Heat Output Stability: Single vs. Bi-Isotope Fuel}

Figure~\ref{fig:stability} compares the heat output of two fuel configurations over a 100-year interval:
\begin{itemize}
    \item \textbf{Single-isotope:} 1 gram of \textsuperscript{241}Am
    \item \textbf{Bi-component:} 0.85g \textsuperscript{241}Am + 0.15g \textsuperscript{241}Pu
\end{itemize}

The single-isotope curve shows a slow, monotonic decay due to the 432-year half-life of \textsuperscript{241}Am. The bi-component configuration shows a slightly more stable output: it begins with a small increase in power due to the decay of \textsuperscript{241}Pu into \textsuperscript{241}Am, followed by a mild decline.

\begin{figure}[h!]
\centering
\includegraphics[width=0.9\textwidth]{bicomponent_vs_single.png}
\caption{Heat output over 100 years for single-isotope and bi-component RTG fuels. The bi-component configuration shows a flatter power profile.}
\label{fig:stability}
\end{figure}

\section{Discussion}

The decay chain:
\[
\mathrm{^{241}Pu} \rightarrow \mathrm{^{241}Am} + \beta^- + \bar{\nu}_e
\]
has the potential to enhance long-term power stability. Although \textsuperscript{241}Pu itself produces minimal heat, its decay product contributes significantly.

Neutron emission in the bi-component fuel arises mainly from $(\alpha,n)$ reactions of \textsuperscript{241}Am. A shielding layer of 2–3 cm of borated polyethylene is typically sufficient.

Although \textsuperscript{241}Pu is fissile, it is not practical for weapon use due to its high spontaneous fission rate and short half-life, which increase neutron background and complicate triggering.

We also considered whether the bi-component approach allows a reduction in total fuel mass. In our example, using 0.85g of \textsuperscript{241}Am and 0.15g of \textsuperscript{241}Pu produces the same minimum heat output over 100 years as 1g of pure \textsuperscript{241}Am. However, the total power gain is small, and the total mass saving is marginal. While the gain in thermal stability is clear, the gain in mass efficiency appears limited.

\section{Conclusion}

A bi-component RTG fuel combining \textsuperscript{241}Pu and \textsuperscript{241}Am can offer improved thermal output stability over multi-decade missions. The reduced variation in power may simplify thermal management systems and extend mission reliability. However, the mass savings from this approach appear minimal. Further modeling and experimental studies may better quantify the trade-offs for space and terrestrial applications.

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