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 SUS316 steel, but avoiding unobtainable lithium-7 for molten salt. ThorCon 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 prototype could begin testing in four years.
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.ThorCon’s 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.
Each ThorCon plant is based on one or more hulls, each containing two 250 MWe power modules, a 500 MW super-critical turbogenerator, gas insulated switchgear (GIS), a decay heat pond, and auxiliaries. The fission island is at the forward end of the hull. Aft of the fission island is the Steam Generating Cell (SGC). Aft of the SGC is the turbine hall, which contains the turbogenerator, exciter, condensers, feedheaters, pumps, and condensate treatment.
Aft of the SGC is the turbine hall, which 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.
Fission Island
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 is ThorCon’s heart. The Can contains the reactor, which we call the Pot, a primary loop heat exchanger (PHX), and a primary loop pump (PLP). The pump takes liquid fuelsalt — a mixture of sodium, beryllium, uranium and thorium fluorides — from the Pot at 704C, and pushes the fuelsalt over to the PHX at a rate of just under 3000 kg/s. Flowing downward through the PHX, the fuelsalt transfers heat to a secondary salt, and is cooled to 565C in the process. The fuelsalt then flows over to the bottom of the Pot, and rises through the reactor core, which is mostly filled with graphite blocks, called the moderator. This graphite slows the neutrons produced by the fissile uranium, allowing a portion of the uranium in the fuelsalt to fission as it rises through the Pot, heating the salt to 704C, and also converting some thorium to protactinium which becomes fissile uranium, all within the circulating molten fuelsalt. It’s just that simple; and just that magical.
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. To produce this power, the Can requires only 39 liters of uranium per year enriched to 19.7%. The Can is a cylinder 11.6 m high and 7.3 m in diameter. It weighs about 400 tons. The Can has only one major moving part, the primary loop pump.
Drain tank
Directly below the Can is the Fuelsalt Drain Tank (FDT), shown in green. At the bottom of the Pot is a freeze valve shown in gray. The freeze valve is an insulated low point in the drain line. This valve is cooled by a flow of helium which freezes the fuelsalt in the valve creating a plug. The helium flow is controlled by a thermal switch which opens passively if the temperature at the top of the loop exceeds 750C. The plug then thaws and the fuelsalt drains to the FDT. Fission cannot take place in the drain tank since it has no moderator. This drain is totally passive. There is nothing an operator can do to prevent it.
A critically important feature of ThorCon is the silo cooling wall or cold-wall. The cold-wall is made up of two concentric steel cylinders, shown in blue. The annulus between these two cylinders is filled with water. The top of this annulus is connected to a condenser in a decay heat pond located at the forward end of the hull. 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 cold-wall allow the basement water into the bottom of the annulus.
The Can is cooled by thermal radiation to the 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 cold-wall via the basement. In this process, some of the water in the pond is evaporated. The decay heat cooling towers keep the pond close to wet bulb temperature. If the pond cooling line is lost, there is enough water in the basement to handle the first 354 days of decay heat.
The cold-wall also cools the Fuelsalt Drain Tank (FDT). The drain tank is divided into a circle of cylinders. This arrangement 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 no operator intervention nor any outside power.
The cold-wall is what makes the ThorCon work.
1. The cold-wall allows us to keep the Can interior below 350C during normal operation and keeps the Can from over-heating after a drain. The fact that the cold-wall is always operating is an important safety feature. If a problem develops in the cooling wall loop, we will find out before a casualty occurs rather than during.
2. The wall allows us to capture any tritium permeating through the Can or drain tank in the inert gas in the annulus between the Can/FDT and the walls.
3. The wall cools more rapidly as the Can/FDT tank heats up, but more slowly as the Can/drain tank cool down, which is exactly what we want to handle both emergencies and avoid salt freeze ups.
4. The wall maintains a double barrier between the fuelsalt and the cold-wall water, even if the primary loop is breached.
5. And the cold-wall does all this without any penetrations into the Can or the fuelsalt drain tank.
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, and more importantly ensures that a rupture in the steam generator creates no nasty chemicals and harmlessly vents to the Steam Generating Cell via an open standpipe.
Unlike almost all current nuclear reactors, ThorCon is a high temperature reactor. This translates to thermal efficiency of 46% compared to about 33% for a standard light water reactor. This reduces capital costs and cuts cooling water requirements by 60%. It also allows us to use the same steam cycle as a modern coal plant.
Turbine-generator system
The ThorCon 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 does not depend on an outside source of electric power. ThorCon can be the anchor providing stable, reliable electric power to a developing grid. ThorCon 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.
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.
ThorCon is a load-following power plant, meeting EU requirements. Changes in power generation are automatic, not the responsibility of skilled operators.