Thorcon
Thorcon is a proposed power generating nuclear reactor system. It includes not only the design, but also licensing, testing, manufacturing, installation and operation of a molten salt fuelled converter reactor. It is being developed as a modular replaceable power reactor by Thorcon Power, owned by Florida, US based Martingale Inc.
Introduction
Thorcon is a proposed nuclear reactor system, of the type molten salt reactor. Specifically, the Thorcon design is a single fluid molten salt fuelled converter reactor. It is being developed by Thorcon Power as a modular but large power station. The station will be rated at 1000 megawatts-electrical (MWe) and consists of four independent 250 MWe modules. These modules are lifted in and out to replace spent modules, like an ink cartridge.
A molten salt fuelled converter reactor uses molten salt nuclear fuel dissolved in a diluent (or carrier) salt. The diluent reduces the viscosity and melting point, and increases the heat capacity. Because of this, the fuel salt mixture is both primary coolant and fuel. See molten salt reactor and liquid fluoride thorium reactor. Most molten salt reactor proposals breed more fuel than they require. Hence the name breeder reactor. Thorcon is not a breeder, but a converter. A converter reactor does not breed enough fuel to sustain (breakeven) breeding. In stead it requires a supply of new makeup fuel, like today's solid fuelled reactors.
The Thorcon reactor design uses molten fluoride salt, NaF-BeF2-UF4(-ThF4). This salt is a mixture of sodium fluoride and beryllium fluoride as carrier, with uranium and/or thorium fluoride salt as the fuel charge dissolved in the carrier salt. This salt is the primary coolant and fuel, it is circulated through a critical, graphite moderated core followed by a closed heat exchanger. The heat exchanger transfer the thermal energy to a non radioactive, secondary salt loop made of bare carrier salt NaF-BeF2. For safety and containment, and to improve the interface with the power generating steam-side, a lower melting point, steam-compatible third salt loop is also added. The third loop consists of NaNO3-KNO3, which is a eutectic mixture of sodium nitrate and potassium nitrate.
The Thorcon reactor design, construction and operation draw heavily on shipyard-like assembly, in which large blocks are largely completed as modules in central production yards and then shipped to the construction site for final assembly in excavated (fully underground) silos.
Design
Thorcon is designed as a large yet modular reactor system. No online nuclear reprocessing is employed. The reactor is a converter reactor like today's solid fuelled reactors. The design focus is to use existing technologies to the extent possible, in order to build a prototype design quickly. Thorcon is a so single fluid reactor, meaning it has only one fuel salt loop. See molten salt reactor and liquid fluoride thorium reactor for more general explanation of this reactor type.
Thorcon uses replaceable, fully sealed, nuclear-grade stainless steel containment units called Cans. Each Can is housed in its own underground silo. The silos themselves sit inside a fully below grade service area. There are two silos for each operating can so that one unit can be operating while the previous, spent module can cool down for 4 years, allowing the radioactive decay of short lived radioactive elements. After this period the spent Can is removed and replaced with a shipped in fresh Can.
The Can itself forms the primary containment vessel, that together with the underground silo and shield tank, forms the equivalent of a containment building of today's operating reactors. The Can is fully replaceable as a unit, avoiding the need to open the primary reactor system itself or the containment. Replacing the Can as the graphite wears out allows the design to use graphite moderator in a reasonably high power density reactor, which would otherwise not last the full 40 to 60 years which modern nuclear plants are expected to last. Each Can contains one 557 MWt reactor, called the Pot. It also houses the primary loop heat exchanger, and one primary loop pump. This pump circulates fuel salt through the heat exchanger and back up the core again. When in the core, the fuel salt heats up from the critical chain reaction starting there. The core is critical because it is filled with graphite elements. Graphite slows down neutrons that are spontaneously emitted by the fuel in small quantities, causing the neutrons to be absorbed into fuel atoms they stream into. This process either causes the fuel atoms to break apart, which is termed fission, releasing 2 to 3 neutrons, or capture the neutron in its nucleus and become slightly heavier. The latter process often converts the atom to a fissile atom, which is called breeding, since it produces new fissile fuel indirectly. See nuclear reactor and neutron. The new neutrons that are created by the fissions are slowed down by the graphite and find more fuel atoms, thus creating a chain reaction that releases a large amount of heat while breeding more fuel. The fuel salt heats up, in the process it expands and absorbs more neutrons without fissioning, this causes the chain reaction to level itself out at a certain average temperature. For Thorcon, this design average temperature is around 634 °C (the operating inlet temperature is 564 °C and outlet 704 °C). The reactivity of the fuel (and with it the temperature and power output of the reactor) is maintained by adding new fuel periodically.
A molten salt reactor's operating pressure is low. The salt boiling point is over 1400 °C, so there is no need to pressurize the circuit to prevent boiling. The Pot pressure is around 3 atmospheres. The outlet temperature of 704 °C results in an overall plant thermal efficiency of 45%, and a net electrical output per Can of 250 MW. According to Thorcon modeling, each Can consumes 119 kg of fissile uranium per full-power year, about half as much as a light water reactor with the same electrical output. Thorcon has a flexible fuel usage; it can also operate on low enriched uranium only, without any thorium. This reduces the required enrichment level but increases the amount of uranium consumption (that is it requires more makeup fuel). The Can is 11.6 meters tall and 7.3 meters in diameter. Each can weighs around 400 tonnes. The pump impeller is the only moving part inside the Can.
Below the Can is a separate vessel. This vessel is a gravity drain tank for the Pot. In the event of extreme overheating of the primary fuel salt, a frozen plug of salt, called a freeze valve, will melt and this will drain the pot fuel salt to this drain tank by gravity. This tank is also used when the can must be replaced as this allows, after a few years cooling down time, the lifting of the can without the highly radioactive fuel charge. The old fuel charge can then be pumped out the drain tank and a new can and fuel charge can be added for the next cycle of operation.
Each Can is itself located in an underground Silo. The top of these Silos is about 29 meters underground. The secondary salt is a mixture of sodium fluoride and beryllium fluoride. It is essentially bare fuel carrier salt, without uranium or thorium. This fuel-salt-heated secondary salt 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 nitrate and potassium nitrate commonly called solar salt from its use as an energy storage medium in solar plants. The solar salt in turn transfers its heat to a steam loop, creating supercritical steam, and also reheating that steam to increase the plant’s efficiency.
This provides at least four barriers in the heat transfer part of the plant. Matching this number of barriers on the heat transfer side, the design incorporates four gas tight radiation barriers on the containment/building part of the plant:
* The fully sealed (all welded) Pot and primary heat exchanger boundary.
* The fully sealed Can (all welded, cannot be opened)
* The Silo and steam cells
* The silo hall above the Silo itself.
The first three of these barriers are more than 25 meters underground.
A major feature in the facility design is the membrane wall. This consists of a number of extra thick, corrosion resistant chromium steel vertical pipes that surround the Can like a curtain. The pipes are connected together with steel plates, forming a continuous "membrane" around the Can. The pipes extend downward to also surround the drain tank. Headers connect the pipes to risers and downcomers, these allow the water to rise up to the ground level where condenser coils, cooled by a pond full of separate cooling water, condense any steam and cool the water down. The cooled water sinks through the downcomer and rejoins the membrane wall.
The membrane wall serves critical cooling functions. In normal operation it keeps the Silo and Can at low temperature by radiative cooling. In an emergency it takes additional heat through constant radiative coupling to the Can, which in turn is radiatively heated from the inside by the Pot. Thermal radiation increases rapidly with increasing temperature and is independent of any coolant being present or not. This provides an effective and fully passive method of cooling both the silo and containment Can (directly) and the reactor itself (indirectly).
Safety
Thorcon is designed as a walk-away safe plant. This implies that no operator action, electricity supply, support systems (cooling water, safety instrumented systems, sensors), or coolant injection must be needed in the event of an accident. To achieve this extreme level of safety, Thorcon relies heavily on inherent safety and passive safety, as well as the fail-safe nature of the most critical systems.
As a molten salt reactor, the fuel and coolant are the same, inert, non volatile mixture. The boiling point is over 1400 °C, some 700 °C above the normal operating temperature. The salt is chemically stable up to and over the boiling point. There is very little stored energy in the system such as steam or gas pressure, and there is no potential for adverse chemical reactions such as hydrogen generation and detonation. In fact, the salt itself converts otherwise volatile fission products such as cesium, to much less volatile and chemically stable fluorides, providing an inherent chemical and physical sequestration of fission products. These physical and chemical features of the fluoride salt eliminate driving forces to push radionuclides into the environment.
The reactor itself is designed with inherent safety; a failure of the control system will simply produce a small temperature increase which will shut down the chain reaction on account of the reactor negative feedback coefficient. This is partly due to the salt expanding upon overheating, reducing fuel density in the core, and partly due to the physical fact that the fuel itself will absorb more neutrons as it overheats (even without expansion). The result is a fail-safe reactor core with inherent safety.
The primary cooling systems are designed to revert to natural circulation upon loss of the pump(s) or power to run the pumps. The heat exchangers are mounted above each other which results in a thermal convection driving force. The salt's large thermal expansion with increasing temperature enhances this natural circulation cooling effect. As a backup system, the membrane wall is provided. This wall consists of many water filled tubes surrounding the can like a curtain. Downcomers attached to the wall piping feed the wall with cold water, which rises due to radiative heating from the Can, causing it to be pushed out a riser and through a passive condenser/cooling coil. The coil itself is cooled by a large pond filled with water. The membrane wall has no isolation valves; it is always operating, removing a small amount of heat and keeping the silo at low temperature. In this mode it always operates as a passive containment cooling system, preventing overheating of both the can and silo. If the primary loop circulation is lost, the fuel salt temperature will increase and melt a frozen plug of salt in a pipe attached to the bottom of the Pot. This will passively drain (by melting and then gravity) the fuel salt to the drain tank. The drain tank is fitted with iron blocks as heat sink that provide additional short term cooling. Now, the water filled membrane wall does double duty as a passive backup core cooling system. Increased temperature in the pot and drain tank causes increased heat losses to the Can, which in turn radiates its heat to the surrounding membrane wall as normally, but with increased heat output. The membrane wall passively takes over the core cooling function. This works whether the fuel salt is in the Pot or the drain tank. Even a rupture of the membrane wall does not impede cooling as passive check valves open, flooding the pond water into the membrane wall piping, and then flooding the silo, providing deluge cooling. The result is a highly reliable and fail-safe cooling path even in the event of pipe ruptures.
The large number of containment barriers combined with an always operating passive containment cooling system, and the lack of pressure or other energetic driving forces, plus full undergrounding of the entire nuclear island, virtually assures containment integrity.
Thick biological shielding, consisting of thick steel tanks filled with water and lead blocks, combined with the full undergrounding of the nuclear island, provides protection for operating personnel. It also provides resistance to aircraft crash, earthquakes, storm-produced missiles and other external threats. In addition, above all this, the top of the silo hall building's ceiling, at grade level, is made up of a 3 meter thick steel beam reinforced steel tank that is backfilled with concrete.
Being a simplified converter reactor, there is no fuel reprocessing equipment onsite. This avoids leaks, corrosion and other potential hazards with reprocessing equipment.
Economics
ThorCon uses block type construction similar to the shipyard industry. Thorcon's principal engineer and architect, and former MIT professor, Jack Devanney, claims that applying the block type construction to nuclear construction will afford the high productivity currently seen in shipyards.
A panel line manufactures smaller blocks that are the combined to large block modules. The blocks can come with equipment such as airconditioning pre-installed. The blocks can weigh hundreds of tonnes. After assembly the blocks are dropped into place in a dock.
ThorCon is a simpler and smaller structure than today's large ships that are rapidly manufactured in large numbers by many shipyards in the world today.
Thorcon also plans to manufacture other equipment such as the Can, heat exchangers, steam cell equipment, in a similar block type way. The use of reinforced concrete, which is difficult to standardize and produce in panel lines, is minimized.
Block construction also allows tight tolerances to be held, assuring a high degree of quality control in a controlled, factory like setting. Any problems or equipment that is out of specification can be alleviated during the assembly process, avoiding surprises on the construction site itself.
The proposed Thorcon system extends beyond the design of the powerplant facility itself. Thorcon aims to guide the whole process from design through licensing-by-test, to construction and even operation and eventual decommissioning of the plant, as this is where cost overruns and quality issues can also arise.
Introduction
Thorcon is a proposed nuclear reactor system, of the type molten salt reactor. Specifically, the Thorcon design is a single fluid molten salt fuelled converter reactor. It is being developed by Thorcon Power as a modular but large power station. The station will be rated at 1000 megawatts-electrical (MWe) and consists of four independent 250 MWe modules. These modules are lifted in and out to replace spent modules, like an ink cartridge.
A molten salt fuelled converter reactor uses molten salt nuclear fuel dissolved in a diluent (or carrier) salt. The diluent reduces the viscosity and melting point, and increases the heat capacity. Because of this, the fuel salt mixture is both primary coolant and fuel. See molten salt reactor and liquid fluoride thorium reactor. Most molten salt reactor proposals breed more fuel than they require. Hence the name breeder reactor. Thorcon is not a breeder, but a converter. A converter reactor does not breed enough fuel to sustain (breakeven) breeding. In stead it requires a supply of new makeup fuel, like today's solid fuelled reactors.
The Thorcon reactor design uses molten fluoride salt, NaF-BeF2-UF4(-ThF4). This salt is a mixture of sodium fluoride and beryllium fluoride as carrier, with uranium and/or thorium fluoride salt as the fuel charge dissolved in the carrier salt. This salt is the primary coolant and fuel, it is circulated through a critical, graphite moderated core followed by a closed heat exchanger. The heat exchanger transfer the thermal energy to a non radioactive, secondary salt loop made of bare carrier salt NaF-BeF2. For safety and containment, and to improve the interface with the power generating steam-side, a lower melting point, steam-compatible third salt loop is also added. The third loop consists of NaNO3-KNO3, which is a eutectic mixture of sodium nitrate and potassium nitrate.
The Thorcon reactor design, construction and operation draw heavily on shipyard-like assembly, in which large blocks are largely completed as modules in central production yards and then shipped to the construction site for final assembly in excavated (fully underground) silos.
Design
Thorcon is designed as a large yet modular reactor system. No online nuclear reprocessing is employed. The reactor is a converter reactor like today's solid fuelled reactors. The design focus is to use existing technologies to the extent possible, in order to build a prototype design quickly. Thorcon is a so single fluid reactor, meaning it has only one fuel salt loop. See molten salt reactor and liquid fluoride thorium reactor for more general explanation of this reactor type.
Thorcon uses replaceable, fully sealed, nuclear-grade stainless steel containment units called Cans. Each Can is housed in its own underground silo. The silos themselves sit inside a fully below grade service area. There are two silos for each operating can so that one unit can be operating while the previous, spent module can cool down for 4 years, allowing the radioactive decay of short lived radioactive elements. After this period the spent Can is removed and replaced with a shipped in fresh Can.
The Can itself forms the primary containment vessel, that together with the underground silo and shield tank, forms the equivalent of a containment building of today's operating reactors. The Can is fully replaceable as a unit, avoiding the need to open the primary reactor system itself or the containment. Replacing the Can as the graphite wears out allows the design to use graphite moderator in a reasonably high power density reactor, which would otherwise not last the full 40 to 60 years which modern nuclear plants are expected to last. Each Can contains one 557 MWt reactor, called the Pot. It also houses the primary loop heat exchanger, and one primary loop pump. This pump circulates fuel salt through the heat exchanger and back up the core again. When in the core, the fuel salt heats up from the critical chain reaction starting there. The core is critical because it is filled with graphite elements. Graphite slows down neutrons that are spontaneously emitted by the fuel in small quantities, causing the neutrons to be absorbed into fuel atoms they stream into. This process either causes the fuel atoms to break apart, which is termed fission, releasing 2 to 3 neutrons, or capture the neutron in its nucleus and become slightly heavier. The latter process often converts the atom to a fissile atom, which is called breeding, since it produces new fissile fuel indirectly. See nuclear reactor and neutron. The new neutrons that are created by the fissions are slowed down by the graphite and find more fuel atoms, thus creating a chain reaction that releases a large amount of heat while breeding more fuel. The fuel salt heats up, in the process it expands and absorbs more neutrons without fissioning, this causes the chain reaction to level itself out at a certain average temperature. For Thorcon, this design average temperature is around 634 °C (the operating inlet temperature is 564 °C and outlet 704 °C). The reactivity of the fuel (and with it the temperature and power output of the reactor) is maintained by adding new fuel periodically.
A molten salt reactor's operating pressure is low. The salt boiling point is over 1400 °C, so there is no need to pressurize the circuit to prevent boiling. The Pot pressure is around 3 atmospheres. The outlet temperature of 704 °C results in an overall plant thermal efficiency of 45%, and a net electrical output per Can of 250 MW. According to Thorcon modeling, each Can consumes 119 kg of fissile uranium per full-power year, about half as much as a light water reactor with the same electrical output. Thorcon has a flexible fuel usage; it can also operate on low enriched uranium only, without any thorium. This reduces the required enrichment level but increases the amount of uranium consumption (that is it requires more makeup fuel). The Can is 11.6 meters tall and 7.3 meters in diameter. Each can weighs around 400 tonnes. The pump impeller is the only moving part inside the Can.
Below the Can is a separate vessel. This vessel is a gravity drain tank for the Pot. In the event of extreme overheating of the primary fuel salt, a frozen plug of salt, called a freeze valve, will melt and this will drain the pot fuel salt to this drain tank by gravity. This tank is also used when the can must be replaced as this allows, after a few years cooling down time, the lifting of the can without the highly radioactive fuel charge. The old fuel charge can then be pumped out the drain tank and a new can and fuel charge can be added for the next cycle of operation.
Each Can is itself located in an underground Silo. The top of these Silos is about 29 meters underground. The secondary salt is a mixture of sodium fluoride and beryllium fluoride. It is essentially bare fuel carrier salt, without uranium or thorium. This fuel-salt-heated secondary salt 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 nitrate and potassium nitrate commonly called solar salt from its use as an energy storage medium in solar plants. The solar salt in turn transfers its heat to a steam loop, creating supercritical steam, and also reheating that steam to increase the plant’s efficiency.
This provides at least four barriers in the heat transfer part of the plant. Matching this number of barriers on the heat transfer side, the design incorporates four gas tight radiation barriers on the containment/building part of the plant:
* The fully sealed (all welded) Pot and primary heat exchanger boundary.
* The fully sealed Can (all welded, cannot be opened)
* The Silo and steam cells
* The silo hall above the Silo itself.
The first three of these barriers are more than 25 meters underground.
A major feature in the facility design is the membrane wall. This consists of a number of extra thick, corrosion resistant chromium steel vertical pipes that surround the Can like a curtain. The pipes are connected together with steel plates, forming a continuous "membrane" around the Can. The pipes extend downward to also surround the drain tank. Headers connect the pipes to risers and downcomers, these allow the water to rise up to the ground level where condenser coils, cooled by a pond full of separate cooling water, condense any steam and cool the water down. The cooled water sinks through the downcomer and rejoins the membrane wall.
The membrane wall serves critical cooling functions. In normal operation it keeps the Silo and Can at low temperature by radiative cooling. In an emergency it takes additional heat through constant radiative coupling to the Can, which in turn is radiatively heated from the inside by the Pot. Thermal radiation increases rapidly with increasing temperature and is independent of any coolant being present or not. This provides an effective and fully passive method of cooling both the silo and containment Can (directly) and the reactor itself (indirectly).
Safety
Thorcon is designed as a walk-away safe plant. This implies that no operator action, electricity supply, support systems (cooling water, safety instrumented systems, sensors), or coolant injection must be needed in the event of an accident. To achieve this extreme level of safety, Thorcon relies heavily on inherent safety and passive safety, as well as the fail-safe nature of the most critical systems.
As a molten salt reactor, the fuel and coolant are the same, inert, non volatile mixture. The boiling point is over 1400 °C, some 700 °C above the normal operating temperature. The salt is chemically stable up to and over the boiling point. There is very little stored energy in the system such as steam or gas pressure, and there is no potential for adverse chemical reactions such as hydrogen generation and detonation. In fact, the salt itself converts otherwise volatile fission products such as cesium, to much less volatile and chemically stable fluorides, providing an inherent chemical and physical sequestration of fission products. These physical and chemical features of the fluoride salt eliminate driving forces to push radionuclides into the environment.
The reactor itself is designed with inherent safety; a failure of the control system will simply produce a small temperature increase which will shut down the chain reaction on account of the reactor negative feedback coefficient. This is partly due to the salt expanding upon overheating, reducing fuel density in the core, and partly due to the physical fact that the fuel itself will absorb more neutrons as it overheats (even without expansion). The result is a fail-safe reactor core with inherent safety.
The primary cooling systems are designed to revert to natural circulation upon loss of the pump(s) or power to run the pumps. The heat exchangers are mounted above each other which results in a thermal convection driving force. The salt's large thermal expansion with increasing temperature enhances this natural circulation cooling effect. As a backup system, the membrane wall is provided. This wall consists of many water filled tubes surrounding the can like a curtain. Downcomers attached to the wall piping feed the wall with cold water, which rises due to radiative heating from the Can, causing it to be pushed out a riser and through a passive condenser/cooling coil. The coil itself is cooled by a large pond filled with water. The membrane wall has no isolation valves; it is always operating, removing a small amount of heat and keeping the silo at low temperature. In this mode it always operates as a passive containment cooling system, preventing overheating of both the can and silo. If the primary loop circulation is lost, the fuel salt temperature will increase and melt a frozen plug of salt in a pipe attached to the bottom of the Pot. This will passively drain (by melting and then gravity) the fuel salt to the drain tank. The drain tank is fitted with iron blocks as heat sink that provide additional short term cooling. Now, the water filled membrane wall does double duty as a passive backup core cooling system. Increased temperature in the pot and drain tank causes increased heat losses to the Can, which in turn radiates its heat to the surrounding membrane wall as normally, but with increased heat output. The membrane wall passively takes over the core cooling function. This works whether the fuel salt is in the Pot or the drain tank. Even a rupture of the membrane wall does not impede cooling as passive check valves open, flooding the pond water into the membrane wall piping, and then flooding the silo, providing deluge cooling. The result is a highly reliable and fail-safe cooling path even in the event of pipe ruptures.
The large number of containment barriers combined with an always operating passive containment cooling system, and the lack of pressure or other energetic driving forces, plus full undergrounding of the entire nuclear island, virtually assures containment integrity.
Thick biological shielding, consisting of thick steel tanks filled with water and lead blocks, combined with the full undergrounding of the nuclear island, provides protection for operating personnel. It also provides resistance to aircraft crash, earthquakes, storm-produced missiles and other external threats. In addition, above all this, the top of the silo hall building's ceiling, at grade level, is made up of a 3 meter thick steel beam reinforced steel tank that is backfilled with concrete.
Being a simplified converter reactor, there is no fuel reprocessing equipment onsite. This avoids leaks, corrosion and other potential hazards with reprocessing equipment.
Economics
ThorCon uses block type construction similar to the shipyard industry. Thorcon's principal engineer and architect, and former MIT professor, Jack Devanney, claims that applying the block type construction to nuclear construction will afford the high productivity currently seen in shipyards.
A panel line manufactures smaller blocks that are the combined to large block modules. The blocks can come with equipment such as airconditioning pre-installed. The blocks can weigh hundreds of tonnes. After assembly the blocks are dropped into place in a dock.
ThorCon is a simpler and smaller structure than today's large ships that are rapidly manufactured in large numbers by many shipyards in the world today.
Thorcon also plans to manufacture other equipment such as the Can, heat exchangers, steam cell equipment, in a similar block type way. The use of reinforced concrete, which is difficult to standardize and produce in panel lines, is minimized.
Block construction also allows tight tolerances to be held, assuring a high degree of quality control in a controlled, factory like setting. Any problems or equipment that is out of specification can be alleviated during the assembly process, avoiding surprises on the construction site itself.
The proposed Thorcon system extends beyond the design of the powerplant facility itself. Thorcon aims to guide the whole process from design through licensing-by-test, to construction and even operation and eventual decommissioning of the plant, as this is where cost overruns and quality issues can also arise.
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