🔬 physics

Simultaneous Production of Molybdenum-99 and Actinium- 225 using Indonesia Research Reactor (RSG-GAS)

This study demonstrates that Indonesia's RSG-GAS research reactor, utilizing a modified low-enriched uranium electroplating capsule, can simultaneously produce Molybdenum-99 and enhance Actinium-225 yields to 4.3 GBq through optimized irradiation and multiple milking cycles.

Original authors: Amila Amatullah, Naoyuki Takaki

Published 2026-07-07

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Amila Amatullah, Naoyuki Takaki

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Big Picture: Two Medicines from One Machine

Imagine a nuclear research reactor as a giant, high-powered kitchen. Usually, this kitchen is used to bake one specific type of "radioactive bread" called Molybdenum-99 (99Mo). When this bread "stales" (decays), it turns into Technetium-99m, which doctors use to take X-ray-like pictures of the inside of the human body (like checking the heart or bones). This is the most common medicine used in nuclear imaging today.

However, there is a shortage of a different kind of "medicine" called Actinium-225 (225Ac). This one is used to treat cancer by shooting tiny, powerful alpha particles directly at tumor cells.

The Goal: The researchers wanted to see if they could bake both types of medicine at the same time in the same kitchen (Indonesia's RSG-GAS reactor) without breaking the oven or the ingredients.

The New Tool: A "Russian Doll" Capsule

To do this, the team designed a special container, which they call a modified LEU electroplating capsule. Think of this capsule as a set of Russian nesting dolls or a layered sandwich:

The Outer Shell: A stainless steel container.
The Filling (The "Batter"): A very thin layer of Low Enriched Uranium (LEU). When the reactor fires neutrons at this layer, it splits (fissions), creating the Molybdenum-99 needed for imaging.
The Center (The "Filling"): Right in the middle of the capsule, surrounded by helium gas, sits a tiny target of Radium-226.

How it works:
When the Uranium layer splits, it doesn't just make Molybdenum; it also shoots out "fast" neutrons (like high-speed bullets). These fast bullets hit the Radium-226 in the center. This collision transforms the Radium into Radium-225, which eventually decays into the cancer-fighting Actinium-225.

The Experiment: Finding the Perfect Thickness

The researchers had to figure out how thick the Uranium "batter" should be.

Too Thin: If the Uranium layer is too thin, it doesn't create enough fast neutrons to turn the Radium into Actinium efficiently.
Too Thick: If the Uranium layer is too thick, it creates too much heat. Imagine putting a giant steak in a small pan; the pan gets so hot it might melt the handle. In this case, the heat could melt the Radium target, ruining the experiment.

The Results:

They simulated different thicknesses. They found that making the Uranium layer thicker increased the production of both medicines.
However, they hit a "melting point" wall. If the layer was thicker than 0.04 cm, the heat would be too high for the Radium to survive.
The Sweet Spot: They determined that 0.04 cm is the perfect thickness. It's thick enough to make a lot of medicine but thin enough to keep the target from melting.

The Harvest: Milking the Medicine

Once the capsule is in the reactor for 26 days, the Radium-226 turns into Radium-225, which then turns into Actinium-225.

The researchers calculated a "milking" schedule (like milking a cow for milk). They found that if they wait 17.5 days and then extract (separate) the Actinium, they get the most yield.

By repeating this process 5 times, they could produce a total of 4.32 GBq (a measure of radioactivity) of Actinium-225.
This is about 19% more than they would get if they didn't use the Uranium layer at all.

The Safety Check: The "Ghost" Gases

The researchers also looked at the "trash" or impurities created during this process.

The Radon Problem: One of the byproducts is Radon gas. Specifically, a type called Radon-219. The paper notes that while this gas is radioactive, it decays so quickly (in seconds) that it likely won't travel far enough to be inhaled by workers if they are wearing standard masks.
The Radon-222 Issue: Another type of Radon (Radon-222) is produced from the Radium. This one is more dangerous because it stays around longer and its "children" (decay products) can cause lung damage if inhaled.
The Solution: The paper suggests that standard safety measures, like using negative pressure rooms (so air flows in rather than out) and wearing full protective gear, are sufficient to handle these risks.

The Bottom Line

The paper concludes that it is possible to use Indonesia's research reactor to produce both the imaging medicine (Molybdenum-99) and the cancer-treating medicine (Actinium-225) simultaneously. By using a specially designed capsule with a 0.04 cm thick Uranium layer, they can boost the production of the cancer-fighting Actinium without melting the target or breaking the reactor's safety limits.

What the paper does not say:

It does not claim that this medicine is ready to be injected into patients tomorrow.
It does not discuss how this will lower the cost of cancer treatment globally.
It focuses strictly on the physics, the heat calculations, and the production numbers within the reactor simulation.

Technical Summary: Simultaneous Production of Molybdenum-99 and Actinium-225 using Indonesia Research Reactor (RSG-GAS)

Problem Statement
The global demand for Technetium-99m ( $^{99m}$ Tc), the decay product of Molybdenum-99 ( $^{99}$ Mo), accounts for over 80% of nuclear medicine radioisotopes used for diagnostic imaging. Concurrently, Actinium-225 ( $^{225}$ Ac), a promising alpha-emitter for Targeted Radionuclide Therapy (TRT), faces significant production shortages. Current production methods for $^{225}$ Ac rely on limited stockpiles of Thorium-229 or accelerator-based methods (spallation, transmutation, photonuclear reactions). This study addresses the challenge of simultaneously producing $^{99}$ Mo and $^{225}$ Ac within a thermal research reactor environment, specifically the G.A. Siwabessy Research Reactor (RSG-GAS) in Indonesia, by modifying existing Low Enriched Uranium (LEU) electroplating capsules.

Methodology
The research utilized the OpenMC Monte Carlo particle transport code with the ENDF/B VIII.0 nuclear data library to model the RSG-GAS first working core. The core configuration consists of 12 fuel elements and 6 control elements, utilizing U $_3$ O $_8$ dispersed in an aluminum matrix with 19.75% $^{235}$ U enrichment.

A modified LEU electroplating capsule was designed for irradiation in the Central Irradiation Position (CIP). The modification involved implanting a 1-gram Radium-226 ( $^{226}$ Ra) target into the center of the capsule, surrounded by helium gas and a thin layer of LEU. The study hypothesized that fission reactions within the thin LEU layer would generate fast neutrons, thereby enhancing the $^{226}$ Ra(n,2n) $^{225}$ Ra reaction (threshold ~6.5 MeV), which subsequently decays to $^{225}$ Ac.

The simulation parameters included:

Irradiation: 26 days at a constant 15 MW thermal power.
Cooling: 7 days post-irradiation.
Variable: LEU layer thickness was varied between 0.01 cm and 0.25 cm to assess impacts on reactivity, power generation, neutron flux, and target viability.
Thermal Analysis: A 1D Fourier heat conduction equation in cylindrical coordinates was applied to calculate temperature distributions, ensuring the $^{226}$ Ra target (melting point 973 K) would not melt.
Milking Simulation: The production of $^{225}$ Ac was modeled over 120 days with five separation (milking) cycles occurring every 17.5 days.
Impurity Analysis: Radionuclide densities were analyzed to estimate radiation doses from impurities, specifically focusing on Radon isotopes ( $^{222}$ Rn, $^{219}$ Rn) and Polonium progeny.

Key Results

Neutronics and Reactivity: Increasing the LEU layer thickness correlated with higher reactivity insertion and total power generation. At the maximum tested thickness of 0.25 cm, the LEU layer generated approximately 1000 kW with a reactivity insertion of ~1.13$, approaching prompt criticality limits. Thicker layers also produced a "harder" neutron spectrum, increasing the flux of fast neutrons favorable for the (n,2n) reaction.
Production Yields:
- $^{225}$ Ra: Production increased by approximately 68.27% when the LEU thickness was increased from 0.02 cm to 0.25 cm.
- $^{99}$ Mo: Production increased by approximately 400% at 0.25 cm compared to 0.02 cm.
Target Viability: Thermal calculations indicated that LEU thicknesses of 0.04 cm or greater caused the $^{226}$ Ra target temperature to exceed its melting point (973 K). Consequently, 0.04 cm was identified as the maximum acceptable thickness to maintain target integrity.
Optimized Production: At the optimal thickness of 0.04 cm, the total production of $^{225}$ Ac after five milking cycles reached 4.32 GBq. This represents a 19% increase compared to a capsule without an LEU layer and a 12% increase compared to a 0.02 cm LEU layer.
Impurities: The analysis identified $^{219}$ Rn as having the highest inhalation dose among radon isotopes due to the generation of $^{227}$ Ac from $^{226}$ Ra irradiation, though its short half-life limits the risk. $^{222}$ Rn and its progeny ( $^{218}$ Po, $^{214}$ Po) present significant lung cancer risks if inhaled, necessitating strict ventilation and negative pressure controls in hot cells. Long-lived beta emitters $^{227}$ Ac and $^{210}$ Pb were identified as requiring careful waste containment.

Significance and Claims
The paper claims that the proposed modified LEU electroplating capsule offers a viable method for the simultaneous production of $^{99}$ Mo and $^{225}$ Ac within a thermal research reactor. The study demonstrates that utilizing the fission neutrons from a thin LEU layer can effectively enhance the production of $^{225}$ Ac precursors without compromising the primary production of $^{99}$ Mo.

The authors conclude that while thicker LEU layers yield higher production rates, thermal constraints limit the practical application. An LEU thickness of 0.04 cm provides the optimal balance between enhanced $^{225}$ Ac production (via increased fast neutron flux) and target safety. The research suggests that this approach could help alleviate the global shortage of $^{225}$ Ac while maintaining domestic $^{99}$ Mo supply capabilities for Indonesia. The authors note that future studies should address the logistical method for replacing the LEU layer every 6 days, as $^{99}$ Mo requires a shorter irradiation time (6 days) compared to $^{225}$ Ra (26 days).