Design

Thorcon requires no new technology. Thorcon requires sound engineering, but nothing we don’t already know how to do. ThorCon uses only commercially available components and materials, such as SUS316H steel, but avoiding unobtainable lithium-7 for molten salt. The Thoron 500 feeds its heat to the same standard super-critical steam cycle used by coal plants around the world for decades. The result is nil power loop development risk and sourcing from competitive suppliers.

The high-temperature, low-pressure liquid fuel leads to low cost and intrinsic safety. The reactor itself is in a Can that is replaced on a four-year service schedule. Molten fluoride salt with dissolved thorium and uranium fuel is similarly removed and replaced, on an eight-year cycle. Shipyards can rapidly fabricate 500 MW power plants on hulls to be towed to near-shore sites. A Thorcon 500 prototype could begin testing by 2029.

ThorCon Overview

Thorcon is a molten salt fission reactor. Unlike all current nuclear reactors, the fuel is in liquid form. It can be moved around with a pump, and passively drained in the event of a casualty.The Thorcon 500 reactor operates at about the same pressure as your garden hose. Standard nuclear reactors operate at up to 160 bar (2300 psi). They require 9 inch thick pressure vessels and massive piping. The key forgings can only be done by a few specialized foundries. Worse, if there is a big piping failure, the pressurized water explodes into steam, which might spray radioactivity all over the place. This means the reactor, heat exchangers and pumps must be entombed in a massive, reinforced concrete mausoleum, where they are extremely difficult to repair or replace. Therefore, we pretend they will need little or no maintenance for the life of the plant. Reinforced concrete construction is horribly slow, nearly impossible to automate, difficult to inspect, and even more difficult to repair. In contrast, ThorCon uses normal piping thicknesses and easily automated, ship-style steel plate construction.

The Thorcon 500 plant is manufactured and installed in two hulls. The nuclear module hull (fission island) contains the nuclear reactors in Cans in two independent Power Modules (PMODs). Each PMOD also contains the secondary (clean) salt loop and secondary heat exchanger that transfers heat energy to solar salt. Solar salt is a mixture of NaNO3 and KNO3 commonly used for heat storage and heat transfer in thermal solar power plants.

The steam module hull accepts the heated solar salt from both PMODs of the nuclear module. The solar salt heats a steam generator to produce 550°C 257 bar supercritical steam, which turns the 500 MWe steam-turbine generator.

The turbine hall contains the turbogenerator, exciter, condensers, feedheaters, pumps, and condensate treatment. The auxiliary boiler and sentry turbine are also located in the turbine hall. These components are used during start up — ThorCon has the capability of a black start — and the sentry turbine plays a role in certain upsets and grid failures. The Gas Insulated Switchgear, which steps up the 25 kV generator voltage to grid voltage, is located in the gishall at the aft end of the hull.

The generator bus bars run aft to the low voltage Generator Circuit Breaker positioned on the hull centerline at the aft end of the turbine hall. The three single phase, water cooled transformers are located at the forward end of the gishall. The high voltage buses run aft to the High Voltage Circuit Breaker bays, and then up to the deck. If ThorCon is sited close to shore, the power will go ashore on conventional, air insulated transmission lines. Further offshore, subsea cables will be used. If the site is more than 50 km offshore, then we will need to rectify the AC to DC. Space has been reserved in the gishall for the necessary Voltage Source Converters.

Nuclear module

ThorCon is divided into 250 MWe power modules or PMODs. Each module contains two replaceable reactors in sealed Cans. The Cans sit in silos. At any one time, just one of the Cans of each module is producing power. The other Can is in cooldown mode. Every four years the Can that has been cooling is removed and replaced with a new Can. The fuelsalt is transferred to the new Can, and the Can that has been operating goes into cool down mode.

The Can contains the reactor, called the Pot (in orange color), a primary loop heat exchanger (PHX, in blue), and a primary loop pump (PLP, in purple), which is a centrifugal pump. Fuelsalt flowing in the primary loop is a mixture of sodium, beryllium, and uranium fluorides. The PLP pulls fuelsalt from the Pot at 704°C and pushes the fuelsalt to the primary heat exchanger (PHX) at 3305 kg/s.

Flowing down through the PHX, the fuelsalt transfers heat to a secondary salt, and is cooled to 560°C in the process. The cooled fuelsalt then flows downward inside the Pot along the steel walls to its bottom, then rises through channels in the reactor core, which is mostly filled with a moderator made of graphite blocks. This graphite slows the neutrons, allowing thermal fission of uranium fuel, heating the fuelsalt to 704C as it rises through the Pot.

The Pot pressure is 3 bar gage, about the same as a garden hose. The outlet temperature of 704C results in an overall plant efficiency of 46%, and a net electrical output per Can of 250 MW. The uranium in the fuelsalt loop is enriched to 2.2% U-235. As it is fissioned the enrichment drops, so some fuelsalt is replaced from a makeup tank with uranium enriched to 4.95% U-235. This replacement forces some fuelsalt to be exuded from the primary loop and stored in an overflow tank in the silo cell of the PMOD. After 8 full-power years of operation the nuclear module is towed to a maintenance facility, with the used fuel salt inside. The used fuelsalt is suitable for starting up another Thorcon 500, with no reprocessing.

Primary loop metal in contact with fuelsalt is stainless steel 316H. Corrosion is suppressed by managing redox potential via the ratio of UF4 to UF3 in the fuelsalt. Steel surface corrosion will be well under 100 microns per year.

The Can contains the primary loop plus the drain tanks in the passively water-cooled calandria. The interior volume of the Can is sealed and contains an inert gas. The Can is cooled by a water-filled annulus between the silo wall and the Can.

Water and steam rise in the annulus by convection. The top of this annulus connects via water piping to passively air-cooled radiators in chimneys. Water-steam circulation is via natural convection.

Drain tank

Water and steam rise by convection. The top of this annulus connects via water piping to passively air-cooled radiators in chimneys. Water/steam circulation is via natural convection.

The airflow in the chimneys is hastened by radiator fans. These fans are not important to safety; they do improve heat transfer to the atmosphere, keeping the basement water at a lower temperature than if the radiators are cooled only by natural air convection.

The Thorcon 500 has two PMODs. Each PMOD has two two chimneys with water-to-air convection cooling systems. Each Can and Calandria system is cooled by bifurcated convective water/steam circulation to the PMOD chimney pair.

The air-cooled cooling water is returned to the basement where the Can silos are located. Pipes to the bottom of the silo wall allow flow of basement water into the bottom of the cold-wall annulus between the silo and the Can.

Fuelsalt is normally circulated through the primary loop by the primary loop pump (PLP). A side-stream of fuelsalt is continually drained by gravity from the Pot to the fuelsalt sump. Electrically powered refill pumps continually pump the drained fuelsalt back into the header tank of the primary loop.

In the event of a power failure or control system command the refill pumps stop and the fuel salt passively drains from the primary loop to the Fuelsalt Drain Tank (FDT) by gravity. A full drain takes 20 minutes, but fission stops in about 5 minutes as the fissile mass drops below criticality within the moderating graphite. Also, fission will stop due to nuclear physics fundamentals if fuelsalt temperatures approach or exceed 800°C.

The FDT is composed of dozens of high-temperature tolerant metal cylinders cooled by heat radiation to individual metal sleeves immersed in water in the calandria tank. The water circulates in the same convection cooling water system that cools the Can.

The cold-wall water keeps the Can interior below 350°C during normal operation and keeps the Can and FDT from over-heating after a drain. The cooling water is always flowing by convection, confirming readiness to remove decay heat in any shutdown without changing any valve settings or electric switches. If a problem develops in the cooling water loop, it is expected to be detected before a serious event occurs.

A breach of the primary loop would spill fuelsalt onto a spill guide that diverts escaping fuelsalt to the salt catcher at the bottom of the Can.

Power conversion

ThorCon employs four loops for converting fission heat to electric power:

The primary loop inside the Can
The secondary salt loop
A solar salt loop
A supercritical steam loop

Salt piped through heat exchangers converts Pot heat to steam. The secondary salt is a mixture of sodium and beryllium fluoride containing no uranium or thorium. Hot secondary salt, depicted in green, is pumped out of the top of the Primary Heat Exchanger to a Secondary Heat Exchanger where it transfers its heat to a mixture 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 pink, in turn transfers its heat to a supercritical steam loop, shown in red and orange. The solar salt loop captures any tritium that has made it to the secondary loop. The solar salt loop standpipe limits the solar salt pressure in the event of a rupture in the high-pressure steam generator. This prevents excessive steam pressure from subsequently rupturing both the primary and secondary heat exchangers.

Turbine-generator system

The Thorcon 500 steam loop is a standard, first generation, single reheat, super-critical steam cycle, essentially the same as that currently used by coal power plants with the boiler replaced by a pollution free steam generator. The turbine generator and auxiliaries required to implement this power conversion loop are not only existing technology but nearly off the shelf. Thanks to the solar salt loop, no special high pressure feedwater preheater is required.

The turbine is fitted with 100% cascade by-pass. A loss of load need not trip the reactor. Thorcon 500 is a load-following power plant, meeting EU requirements. Changes in power generation are automatic, not the responsibility of skilled operators.

The Thorcon 500 does not depend on an outside source of electric power. It can be the anchor providing stable, reliable electric power to a developing grid. ThorCon 500 has black start capability. It is equipped with a 50 MWt oil fired auxiliary boiler and a 15 MWe Sentry turbine in the turbine hall. The auxiliary boiler provides steam to heat up and roll the main turbine during a cold start. This boiler also provides steam for the Sentry turbogenerator whose power in turn is used to heat up the first power module to be brought on line. The boiler allows us to use steam turbine driven main feed pumps improving cycle efficiency and reducing the requirement for diesel generator power during start up.

Sentry turbine-generator

The Sentry turbine-generator helps keep the plant ready to run in a power down. It normally operates in parallel with the main turbine-generator, taking steam from the cold reheat line. In a power down, decay heat from one 250 MWe module makes steam to feed the Sentry TG until the auxiliary boiler fires up to make steam indefinitely. This maintains circulation in the four loops, allowing a warm restart, avoiding a drain and longer restart time.