In my previous article I had proposed a submerged underwater nuclear reactor design. In this article I will be proposing revolutionary Uranium free thorium fuel rods. These rods were made possible thanks to the revolutionary closed loop low pressure direct steam cycle design. The simplicity of the design allowed a new rector fuel bundle concept.
The objective is to use Thorium 232 as the fuel without additional fissile material. Thorium by itself is not fissile like U 235. It needs to capture one neutron to become Uranium 233. Uranium 233 would then need to capture a slow neutron to fission. This requires neutron multipliers. I decided to use Beryllium for that. The fuel would be ignited by Americium-Beryllium (AmBe) in a fused manner.
The major difference of my design compared to classical nuclear cores is that each fuel rod is a standalone reactor. The rods do not interact with the neighboring rods or external moderators. There would be a hexagonal tubular grid of 110 cm in height. Each hexagonal tubular section would have 3 void ducts around it. Each fuel rod would be sharing a wall with three neighboring rods. This structure would be strong and have very large thermal mass for efficient thermal coupling with the cooling water. The hexagonal structure would be arranged to form a circle to generate circular steam which fits with the steam duct above the core. The entire hexagonal core block would be suspended on a floating expansion joint inside the main reactor hull.
The closed sections of the fuel structure would be filled with fine Thorium metal powder up to 100 cm. In the center of each fuel rod, a 2mm wide hexagonal Be rod would be placed as a neutron multiplier and moderator. The fast neutron emitted would turn Thorium 232 into Uranium 233 and Be moderated neutrons would fission the Uranium 233. The thick (2mm) copper making the fuel structure would be keeping most of the neutrons inside to increase neutron efficiency. Copper structure would be coated with Diamond Like Carbon (DLC). DLC would protect Copper from corrosions and form a low friction surface for the bubbling gasses. Once the fuel is loaded into the structure it would be filled with distilled water and frozen. The water would be filled slightly above the fuel level. This distance would determine the delay for the ignition mechanism AmBe. After addition of AmBe powder on top of the frozen water, the rest of the fuel container section would be filled with water and frozen solid. Once the fuel structure is ready, it would be placed inside the reactor core. The core would be filled with cold water at a temperature close to the freezing point of water and frozen quickly afterwards. So, the closed loop water of the reactor and the water inside the fuel rods would be frozen together.
In frozen state, the nuclear plant would be lowered to the depth it would be suspended at, roughly 20 meters. After deployment and safety control checks are verified, the reactor would be heated from its condenser section to speed up the melting of the ice inside. The insulated core section would keep the big ice mass frozen for several hours to allow time for installment and safety checks. Once the ice melts and the ice suspended AmBe falls on to the Th fuel and the Be rod on the center, the reactor would start. Neutrons emitted from AmBe would be captured by the Thorium atoms and turn them into Uranium 233 after quick radioactive decays. As a result, non fissile Thorium would be breeding its own fuel Uranium 233. The neutrons scattered from AmBe would be multiplied by the Be rod in the center and more Thorium atoms would be turned into Uranium. The scattered neutrons would be fast. This is good for Thorium but would not fission Uranium. Beryllium rod would act as a moderator in that case and slow down the neutrons to allow Uranium atoms to capture them and fission.
The fission reaction would then self-sustain itself and progress towards the bottom of the rod. The fine powder structure of the fuel and its open top would allow the rod to exhaust fission byproducts directly into the steam. Additionally, downward movement of the fission would leave the solid fission byproducts on the upper section of the rod where the fuel has already consumed. As a result, the proceeding fission reaction would have no fission killers on their path. The fuel and the neutrons would be efficiently consumed. The pressure of the exhausted He, Kr and Xe gasses would keep the fuel free of liquid water, but in a saturated steam environment.
The reactors condenser section would be designed to sink much more heat than the core’s nominal thermal capacity. Coupled with the infinite heat capacity of deep-sea water, the reactor would not run away. Thorium’s two step fission characteristic also help to that.
Here is a technical detail about how the Thorium oxide is reduced and processed into the optimal porous fuel powder:
To achieve the necessary fuel porosity and remove oxygen, a confined plasma reduction setup is used. Thorium oxide powder is injected into an Argon plasma torch alongside vaporized Calcium metal. Because Calcium has a much higher chemical affinity for Oxygen than Thorium does, it strips the Oxygen atoms away, forming Calcium oxide. This high temperature reaction takes place directly above a cryogenic trap cooled by liquid nitrogen. As the heavy Thorium metal particles fall out of the plasma phase, they are flash frozen in the trap, preventing the newly formed metal particles from clumping. The lighter Calcium oxide dust and Argon gas are continuously extracted by the vacuum system. This process yields a highly pure, oxygen free fuel powder with the exact fractured geometry needed to allow gaseous fission byproducts to escape the fuel structure. After enough Thorium is collected inside the container, the ice would be let to melt and fine Thorium particles would be transferred to the fuel structure and frozen there again. Turning Thorium into fine particles and removing Oxygen atoms from the fuel increases the neutron capture capacity of Thorium and allow gaseous byproducts to escape from the fission region faster.
Fission Byproduct Harvesting
A unique advantage of this open-top fuel bundle is the continuous extraction of gaseous fission byproducts. As the 152 degrees Celsius steam exits the turbine and enters the uninsulated condenser, it rapidly changes phase back into liquid water. The non-condensable fission gases, primarily Helium, Xenon, and Krypton, naturally separate from the water loop at this vacuum junction.
To capture these gases, the condenser is equipped with a tandem redundant harvest pod system. Two separate capture tanks operate in parallel at the turbine exit. While one tank actively collects the exhausted gases, the second remains on standby. Once the primary tank reaches its capacity, the system automatically diverts the gas flow to the standby tank. This redundancy allows a remotely operated vehicle to periodically detach and swap the full tank without interrupting the continuous operation of the reactor.
The harvested tanks are then transported to a land based separation facility. The captured gases represent a significant economic asset. While some isotopes require a temporary hold for radioactive decay, the stable isotopes of Xenon and Krypton are highly sought after for industrial applications, including aerospace ion thrusters, advanced medical imaging, and specialized electronics. Furthermore, the Helium generated by alpha decay is a critical commodity for cryogenic and scientific industries. By separating and selling these stable, non-radioactive gases, the reactor transforms what is traditionally considered nuclear waste into a lucrative revenue stream that significantly offsets operational costs.
In conclusion, this uranium-free thorium architecture offers a fundamental departure from traditional nuclear fuel rods by replacing complex active safety systems with passive geometric physics. Conventional rods rely on solid ceramic uranium pellets sealed inside thin zirconium cladding, a design that traps expanding fission gases, induces mechanical swelling, and risks hydrogen generation if external cooling pumps fail. In stark contrast, this hexagonal copper honeycomb design utilizes a porous thorium metal powder that naturally exhausts helium, krypton, and xenon directly into the steam flow, eliminating internal pressure buildup. By replacing external moderators and mechanical control rods with a dedicated 2 mm beryllium spine and utilizing the natural thermal expansion of the fuel bed, the reactor achieves inherent reactivity control (This is technically known as the Doppler Feedback and Fuel Expansion Coefficient. It means the brakes for the fission reaction are built into the atoms themselves, not on a computer or a motor.). Furthermore, the thick diamond-like carbon coated copper walls provide vastly superior thermal conductivity compared to zirconium, transferring heat immediately to the internal water loop. Ultimately, this open-top, direct-boiling fuel bundle transforms the reactor core from a pressurized hazard requiring constant mechanical intervention into a self-regulating thermal engine.
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