Guest post by Robert Hargraves, co-founder of ThorCon. ThorCon are a leader in developing thorium fuelled molten salt reactor technology with full passive safety. I invited them to submit a guest post several weeks ago and they have duly obliged with their submission to the IAEA small modular reactor booklet.

Introduction

ThorCon is a molten salt fission reactor. Unlike all current operating reactors, the fuel is in liquid form. The molten salt can be circulated with a pump and passively drained in the event of an accident. The ThorCon reactor operates at garden hose pressures using normal pipe thicknesses and easily automated, ship-style steel plate construction methods.

Market Target

The first planned application of ThorCon reactors is to generate electric power in developing nations with fragile grids, so ThorCon is capable of demand discontinuities and black start without grid power. Capital cost and generated electricity costs are critical in these markets. ThorCon is cheaper than coal and deployable as rapidly. Indonesia completed a ThorCon pre-feasibility study in 2017.

Figure 1 This picture shows two hull-mounted 500 MWe ThorConIsle power plants. A CanShip is changing the Can containing the reactor vessel and radioactive primary loop. Decay heat cooling towers are in the foreground. The yellow rectangles are hatches for access by gantry cranes.

Figure 2 From right to left, this cutaway shows the cooling towers, fission island, heat exchangers, steam turbine-generator, and switchgear.

Figure 3 Basement water is used as a third, backup decay heat sink.

Design Philosophy

ThorCon is Walkaway Safe. If the reactor overheats for any reason, it will automatically shut itself down, drain the fuel from the primary loop, and passively remove the decay heat. There is no need for any operator intervention; the operators cannot prevent the draining and cooling. ThorCon has three gas tight barriers between the fuel salt and the environment. In a primary loop rupture, there is no coolant phase change and no dispersal energy. Spilled fuel merely flows to the drain tank where it is passively cooled. The most troublesome fission products, including I-131, Sr-90, and Cs-137 are chemically bound to the salt. They will end up in the drain tank as well.

ThorCon is Ready to Go. The ThorCon design needs no new technology. ThorCon is a scale-up of the successful Molten Salt Reactor Experiment (MSRE). A full-scale dual 250 MWe ThorConIsle prototype can be operating under test within four years. This prototype will be subject to the failures and problems that the designers claim the plant can handle. As soon as the prototype passes these tests, commercial production can begin.

ThorCon is Rapidly Deployable. The entire ThorConIsle plant is designed to be manufactured in blocks on a shipyard-like assembly line. These 150 to 500 ton, fully outfitted, pre-tested blocks are then assembled into a hull containing the complete power plant, to be towed to a customer site and firmly settled in 5-10 m of water. A 500 MWe power station will require fewer than 100 blocks. Compared to traditional on-site nuclear power plant construction, this improves productivity, quality control, and build time. A single large reactor yard can turn out 30 500 MWe ThorConIsles per year. Alternatively the blocks can be barged to the site and assembled in an excavation, creating an underground fission island linked to a standard above-grade turbine hall. ThorCon is much more than a power plant; it is a system for building power plants.

ThorCon is Fixable. No complex repairs will be attempted on site. Hatches and cranes permit everything in the fission island to be replaced with little interruption in power output. The primary loop is totally contained within a Can. Every four years the Can is changed out, returned to a centralized recycling facility, decontaminated, disassembled, inspected, and refurbished. The instrumentation design and monitoring system is designed to identify incipient problems before they can lead to failures. A fission power plant following such a change-out strategy can in principle operate indefinitely. Decommissioning should be little more than removing the Cans without replacing them, then towing the hull away.

ThorCon is Cheaper than Coal. ThorCon requires far fewer resources than a coal plant. Assuming efficient, evidence-based regulation, ThorCon will produce clean, reliable, carbon-free electricity at less than the cost of coal.

Fission Steam Supply System

Figure 4 ThorCon is divided into 250 MWe power modules. Each module contains two replaceable reactors in sealed Cans. The Cans, depicted in red, sit in silos. Just one of the Cans of each module produces power at a time, while the other is in cooldown mode. After four years the cooled Can is replaced with a fresh Can, the fuel salt transferred to it, and used Can starts its 4-year cool down. The fuel salt is a mixture of sodium, beryllium, uranium and thorium fluorides at 704˚C.

ThorCon Can

Figure 5 The (red) Can contains the (orange) reactor called the Pot. Hidden behind the (blue) header tank, the primary loop pump pushes the fuel salt at 3000 kg/s through the (red) piping down through the (blue) primary loop heat exchanger (PHX).

The PHX transfers heat to secondary salt in (green) piping. The fuel salt at 565C is then piped into the Pot. There the graphite moderator slows neutrons, which fission uranium in the fuel salt as it rises through the Pot, heating the salt. Neutron absorption also converts some fertile thorium and U238 to fissile fuel.

The Pot pressure is 3 bar gage at the maximum stress point. The outlet temperature of 704C results in an overall plant efficiency of 46% with net electric power output of 250 MW per Can. Consumption of fissile uranium is 112 kg per year. The Can is 11.6 m high and 7.3 m in diameter. It weighs about 400 tons. The Can has only one major moving part, the pump impeller.

Directly below the Can is the (green) 32-segment Fuel salt Drain Tank (FDT). In the bottom of the Can is a freeze valve. At normal operating temperatures, the fuel salt in the freeze valve is kept frozen by cold flowing helium creating a plug. If the Can overheats for any reason, the helium flow stops, the plug will thaw, and the fuel salt will drain to the FDT. This drain is totally passive. There is nothing an operator can do to prevent it. Fission in the Pot stops as the drain begins. The 32 segmented vertical drain tanks have no moderator, and re-criticality is impossible in all events, including flooding.

Silo Cold Wall

An important feature of ThorCon is the silo cold wall (blue). The silo wall is made up of two concentric steel cylinders, shown in blue. The annulus between these two cylinders is filled with water. The top is connected to a condenser in a decay heat pond. The outlet of this condenser is connected to the basement in which the Can silos are located. This basement is flooded. Openings in the bottom of the outer silo wall allow the basement water to flow into the bottom of the annulus. The Can is cooled by thermal radiation to the silo cold wall. This heat converts a portion of the water in the wall annulus to steam. This steam/water mixture rises by natural circulation to the cooling pond, where the steam is condensed, and returned to the bottom of the cooling wall via the basement.

The silo cold wall also cools the Fuel salt Drain Tank (FDT). The drain tank is a circle of (green) vertical tanks to contain hot fuel salt drained from the Pot. This provides sufficient radiating area to keep the peak tank temperature after a drain within the limits of the tank material. This cooling process is totally passive, requiring neither operator intervention nor any outside power.

Thereactor core is inside the Pot inside the Can. The core is 90% filled with hexagonal graphite logs which moderate neutron energies. The core is 5 m diameter, and 5.7 m high.

The primary reactivity control is temperature and fuel salt flow rate. Makeup fissile uranium fuel salt additions increase reactivity slowly. Adding fertile thorium fuel salt decreases reactivity.

The fuel salt is NaF-BeF2-ThF4-UF4 76/12/9.5/2.5 where the uranium is 19.7% enriched. As fissile is consumed more fissile U-233 and Pu-239 is generated, but not enough to replace the fuel burned. The reactor has no excess reactivity, no burnable poisons, no poison control rods. Makeup fuel must be added daily.

The Pot reactor pressure vessel is never under high pressure. Since no high pressure is present that can act as a driving force to disperse radioactive content into the environment, ThorCon’s reactor pressure vessel does not have the central safety importance that it does in a LWR.

The secondary salt is a mixture of sodium and beryllium fluoride containing no uranium or thorium. It is pumped to a secondary heat exchanger where it transfers its heat to a tertiary molten salt loop of sodium and potassium nitrate, commonly called solar salt from its use as an energy storage medium in solar plants. The solar salt, shown in purple pipes, in turn transfers its heat to a steam generator to power the supercritical turbine-generator.

Safety Features

Approach to engineered safety systems. Passive reconfiguration into positive shutdown and passive, infinite grace time decay heat removal with no requirement for electricity or operator actions to initiate or continue any safety systems.

Passive temperature control. The ThorCon negative temperature coefficient provides passive temperature stability. The large margin between the operating temperature of 700C and the fuel salt boiling temperature of 1430C exceeds any possible temperature excursions, so radioactive material can never be vaporized. If the temperature of the fuel salt rises much above the operating level, physical principles decrease reactivity and shut fission down. Additionally, if high temperature somehow persists, the freeze valve will thaw and drain the fuel salt from the primary loop to the drain tank, which radiates heat to the cold wall to passively remove the decay heat. No operator intervention is needed at any time. No valves need be realigned by operators nor control systems. In fact, operators can do nothing to prevent the shutdown, drain, and cooling. The decay heat is transferred to the external pond which has sufficient water for 145 days cooling. After evaporation exposes the condenser at the pond bottom, natural air flow suffices for cooling indefinitely. If the pond cooling line is lost, there is enough water in the basements to handle 1.5 years of decay heat.

Release Resistance. ThorCon has three gas tight barriers between the fuel salt and the environment. ThorCon reactor operates at near-ambient pressure. In the event of a primary loop rupture, there is little dispersal energy and no phase change and no vigorous chemical reactions (like zirconium and steam). The spilled fuel merely flows to the drain tank where it is passively cooled. Moreover, the most troublesome fission products, including iodine-131, strontium-90 and cesium-137, are chemically bound to the salt. They will end up in the drain tank as well. Even if all three radioactive material barriers are somehow breached, few of these salt-soluble fission products could disperse.

Spent fuel salt. ThorCon uses an eight-year fuel salt processing cycle, after which the used salt is drained to the fuel drain tank cooled by the cold wall. Within 4 weeks the liquid fuel salt is pumped to a shipping cask in its own silo alongside the Can, within the power module, within the hull. The silo is passively cooled by basement water. The fuel salt cooling in its shipping cask is as well protected as the fuel salt being burned. After 4 years the cooled fuel cask is transferred to the visiting CanShip and shipped to a fuel salt handling facility for future uranium re-enrichment and fuel salt recycling.

Four loop separation of steam and fuel salt. ThorCon employs four loops to transfer heat from the reactor to the steam turbine — the fuel salt loop, the secondary salt loop, the solar salt loop, and the steam loop. Oxygen in the solar salt captures any created tritium that may have penetrated hot heat exchangers. A pressure limiting standpipe in the solar salt loop ensures that a rupture in the high pressure steam generator just vents harmlessly into the Steam Generating Cell.

Plant Safety and Operational Performances

Load following is accomplished by changing primary loop pump speed while keeping the temperatures relatively constant. ThorCon meets EU Utility Requirements for Light Water Reactors: 5% full power per minute operating at 50% to 100% power. Since the off-gases are continuously removed xenon poisoning and power oscillations are not an issue. No neutron poisons are used in the control of the reactor, reducing fuel consumption.

Instrumentation and Control Systems

Instrumentation and control systems are not safety-critical for ThorCon. Argonne National Lab is adapting its isotopic concentration sensors to monitor ThorCon fuel salt components. Commercial instrumentation and sensors will record and report the condition of power generation. Statistical process control will track trend lines and detect incipient failures such as bearing wear. A common control center for all modules in a power plant minimizes staffing requirements. A central engineering facility will monitor conditions at all plants, allowing fleet wide analysis, detect unusual activity, and provide expert advice for any plant experiencing unusual conditions.

Plant Arrangement

The control room is shared by all 250 MWe power modules in a power plant. Commonly two power modules will drive a single 500 MWe turbine/generator. This allows using competitively-priced, efficient supercritical steam turbine-generators. while also remaining suitable for smaller 250 MWe power plants.

Design and Licensing status

Basic design is complete. Some detailed designs are being discussed with specialty component suppliers. License discussions have started with the Indonesian regulator, Bapeten.

Development Milestones