ThorConIsle is an offshore 500-MWe thorium molten salt reactor constructed inside a ship’s hull, ready to provide power from navigable waterways. The ThorCon “pot” operates at a pressure of 3 bar gauge, similar to garden hose pressure, has one moving part—the pump impeller—and uses a four-loop steam cycle, attaining 45% efficiency. The design could be a game-changer for the nuclear industry.
Numerous factors are considered when screening candidate sites to construct and operate a land-based nuclear plant. Important site characteristics (such as access to cooling water, distances to large population centers, and seismic activity in the area) are verified through field reconnaissance, and weighting factors are combined with decision-analysis methods to differentiate and select acceptable sites. The process appears straightforward, but the subjective nature of balancing environmental amenities against economics, social benefits, and other agendas can create indefinite delays. A reasonable alternative to accommodate these desired site characteristics is to look offshore.
POWER has previously reported on Indonesia’s interest in ThorCon’s thorium molten salt reactors (see “Indonesia Considers Thorium Molten Salt Reactors” in the May 2017 issue and online at powermag.com). ThorCon co-founder, Robert Hargraves, recently spoke to POWER and provided an update on ThorConIsle, an offshore 500-MWe nuclear plant (Figure 1).
1. This plan view of ThorConIsle (with dimensions in meters) shows the fission island on the left, which takes up about one-third of the ship’s total length, turbine hall in the center, and gas-insulated switchgear (GIS) hall on the right. Pre-fabricated, modular power blocks are encapsulated inside the hull using a shipyard-style assembly process. Courtesy: ThorCon
Surface and submerged offshore nuclear plants have operated for decades, specifically by the U.S. Navy’s fleet of aircraft carriers and submarines, among others. A civilian example is the Academik Lomonosov floating nuclear plant, which is being deployed to provide power in Pevek, Russia (see “Novel Floating Power Plants on the Horizon” in the December 2018 issue).
There is also an extensive history with sodium-cooled reactors. The Los Alamos Molten Plutonium Reactor Experiment (LAMPRE) and the USS Seawolf (SSN-575) offer a couple of examples. Thus, the ThorCon design needs no new technology and reflects a scaled-up version of the molten salt reactor experiment conducted at the Oak Ridge National Laboratory in Tennessee in the 1960s (see “Molten Salt Reactors: Military Applications Behind the Energy Promises,” in the December 2018 issue).
ThorConIsle can be located on navigable waterways, with its electrical power transmitted to a land-based switching station. This is significant because half the world’s population lives within 100 kilometers of the sea, so coastal power is in high demand. A breakwater and water depth protect against maritime traffic collisions and tsunamis, respectively.
Feasibility Study: Indonesia
Indonesia is an 18,000-island archipelago in Southeast Asia. The majority of the country’s power comes from coal-fired generation (56% in 2015), with natural gas (25%), oil (9%), hydro (6%), and geothermal (4%) also in the mix. A commitment to reduce emissions by 29% is expected by 2030.
Indonesia is projected to be the world’s fifth-largest economy by 2030. Despite canceling 22 GW from independent power producers through 2026—the bulk being combined cycle gas turbine and renewable projects—it is projected to need significant power generation investments. A public opinion survey conducted in 2016 showed 77.5% of the population favored nuclear power.
The recommended ThorConIsle site is expected to be safe from external hazards, such as tropical cyclones and tsunami waves from historical tectonic earthquakes and Krakatoa volcanic activity. Regulatory guidance is obtained from the International Atomic Energy Agency and various Indonesian institutions.
The ThorCon Can: A Pot, a Pump, and a Still
ThorConIsle is divided into two 250-MWe power modules for a combined 500 MWe. Each module contains two replaceable reactors inside sealed “cans” sitting inside silos. Each can is 11.6 meters tall, 7.3 meters in diameter, and weighs 400 tons. The units have only one moving part: the pump impeller (Figure 2).
2. This image shows the ThorCon nuclear steam supply system and safety features. The “can” includes everything within the red colored volume. Courtesy: ThorCon
The can encapsulates the reactor, called the “pot,” which contains molten fuel salt—a homogeneous mixture of sodium, beryllium, and thorium fluorides with low-enriched (19.7%) uranium-235 (LEU). The pot operates at 3 bar gauge pressure, similar to a typical garden hose. Hidden behind the header tank (colored blue in Figure 2), the primary loop pump circulates the fuel salt (3,000 kilograms/sec at 565C) into the pot, containing graphite moderator slabs, which slow neutrons to fission uranium. The fuel salt is heated as it flows upward through the pot.
ThorCon employs four loops—the same steam cycle used in many modern coal plants—converting fission heat to electricity. The heated fuel salt (704C) travels through the piping down through the primary-loop heat exchanger (PHX) to secondary salt piping (colored green in Figure 2), leading to a secondary heat exchanger.
Heat is transferred to a “solar salt” loop, which contains a sodium-potassium nitrate mixture. This mixture is called solar salt because it is used as an energy storage medium in some solar plants. The solar salt loop then transfers its heat to a single-reheat, supercritical steam-turbine loop, yielding 45% plant efficiency, and a 60% reduction in cooling water usage compared to light-water reactors (LWRs).
Molten salt designs typically use a low-chromium, high-nickel specialty steel called Alloy N for surfaces contacting fluoride salts. However, in the ThorCon design all four loops, heat exchangers, and pumps are constructed from a standard stainless steel—SUS316Ti. Advantages of selecting SUS316Ti include its commercial availability, abundant supply, durability against high-radiation fields, and no specialized fabrication requirements. Adequate component thickness, including a thickness allowance for corrosion, allows suitable functional performance over its design lifetime.
Directly below the can is a helium-cooled freeze valve plug (colored gray in Figure 2). If the primary-loop fuel salt temperature increases above operating limits, the plug thaws, draining the fuel salt into the 32-segment, vertical, unmoderated fuel-salt drain tank, which is passively cooled by radiating heat to the silo cold wall.
The passive cooling process is sabotage-resistant, requiring neither operator intervention nor outside power. The can is cooled by thermal radiation to the silo cold wall, consisting of two concentric steel cylinders. Water in the void space is naturally circulated to a cooling tower, condensed, and returned. While the reactor has black start capability, that is, it restores power without an external transmission network, the can is designed to remain safe if left unattended for one year.
ThorConIsle’s robust hull design (25-millimeter dual-steel walls and 3-meter void space filled with sand), and deck (25-millimeter steel) can withstand a vertical 777 aircraft strike. The silo, can, and pot barrier system creates multiple, independent and redundant defense-in-depth layers. Furthermore, when the fuel salt heats up, fissioning slows down due to the negative temperature coefficient, referred to as the Doppler effect. This negative feedback stabilizes the reactor and will passively stop fissioning should the reactor ever overheat. Furthermore, the wide margin between the fuel salt operating temperature (704C) and boiling temperature (1,430C) prevents fuel salt temperature excursions causing phase changes and catastrophic energy releases.
The fuel salt has several advantages over LWR fuel. They include:
- ■ It chemically bounds key radionuclides, strontium-90, iodine-131 and cesium-137.
- ■ Sodium fluoride eliminates elemental sodium-oxygen interactions encountered with sodium-cooled fast reactors, which precludes potential sodium fires.
- ■ A six-stage offgas system removes noble gas fission products, xenon and krypton. Tritium is captured by accumulators in the silo hall, secondary heat exchanger module, and solar salt.
- ■ High-radiation areas and buildup of proliferation-resistant radionuclides (such as LEU and plutonium diluted with thorium) make it unattractive weapons material.
ThorConIsle is designed to have all key parts regularly replaced while each 500-MWe unit is down for turbine-generator maintenance. For example, a used can sits idle and is replaced every four years, thereby replacing the entire primary loop.
The fuel salt is transferred to a fuel salt cask, and a dedicated CanShip transfers the fuel salt cask and used can to a centralized recycling facility (CRF) supporting up to 50 ThorCon plants. Therefore, disassembly, decontamination, and waste handling are shifted from the plant to the CRF, achieving up to an 80-year life span.
ThorCon is estimated to generate 9 meter 3 /GWe-year of high-level radioactive waste that is stored in dry casks. The recycling plan is to re-inject recovered salt with LEU as new fuel. When decommissioning occurs, components can be reused, deconstructed, or buried, according to local radioactive waste regulatory requirements.
Path to Commercial Operation
U.S. laws limit transferring nuclear materials and technology, but fewer restrictions exist with countries like Indonesia that have signed and ratified the non-proliferation treaty. Because ThorCon intellectual property is guarded as trade secret information, this transfer requires non-disclosure agreements.
The U.S. Department of Energy’s National Nuclear Security Administration affirms that exporting ThorCon information to Indonesia is correctly registered under Part 810 of Title 10, Code of Federal Regulations, Assistance to Foreign Atomic Energy Activities. This assures exported U.S. nuclear technologies will be used for peaceful purposes.
A recommended ThorConIsle prototype site is expected to be announced by the National Nuclear Energy Agency of Indonesia (BATAN) in 2019. A step-by-step commissioning process according to agreed upon milestones will then be used to achieve commercial operation.
Hargraves expects ThorCon to have a commercial scale prototype up and running within four years. This includes two years dedicated to building and testing a pre-fission, full-scale test platform. Once a shipyard has built the 500-MWe demonstration plant, it will be towed, ballasted to the seabed floor, and connected to the grid. After that, two years of extensive plant testing will lead to a licensed design for mass production. With a projected cost of electricity of less than 5¢/kWh, the ThorCon plant could play an important role in energizing emerging economies. ■
—James M. Hylko ([email protected]) specializes in safety, quality, and emergency management issues and is a frequent contributor to POWER.