Patent ID: 12211627

SUMMARY OF THE INVENTION

A system is herein disclosed as including a containment vessel configured to prohibit a release of a coolant, and a reactor vessel mounted inside the containment vessel. An outer surface of the reactor vessel is exposed to below atmospheric pressure, wherein substantially all gases are evacuated from within the containment vessel.

A reactor module is herein disclosed as including a containment vessel, a reactor vessel mounted inside the containment vessel, and a containment region located between the reactor vessel and the containment vessel. The containment region substantially surrounds the reactor vessel in a partial vacuum, wherein the containment region is substantially evacuated of gases during operation of the reactor module to substantially eliminate all convective heat transfer between the reactor core and the reactor vessel

A power module assembly is herein disclosed as including a reactor core immersed in a coolant and a reactor vessel housing the coolant and the reactor core. An internal dry containment vessel is submerged in liquid and substantially surrounds the reactor vessel in a gaseous environment. During an over-pressurization event the reactor vessel is configured to release the coolant into the containment vessel and remove a decay heat of the reactor core through condensation of the coolant on an inner surface of the containment vessel.

A nuclear reactor module is herein disclosed as including a containment vessel designed to prohibit a release of a liquid and a reactor vessel mounted inside the containment vessel, wherein an outer surface of the reactor vessel is exposed to a below atmospheric pressure condition. The nuclear reactor module further includes a reactor core submerged in the liquid and a steam vent connected to the reactor vessel, wherein when the reactor core becomes over-heated the steam vent is configured to vent steam into the containment vessel.

A method is herein disclosed, wherein the method includes maintaining a containment region at a below atmospheric pressure; identifying a high pressure event for a reactor vessel; and releasing coolant into the containment region located between a containment vessel and the reactor vessel to remove decay heat from the reactor vessel. The reactor vessel is substantially surrounded by the containment region, wherein substantially all gases are evacuated from the containment region.

A method of cooling a nuclear reactor is herein disclosed, wherein the method includes scramming the nuclear reactor in the event of a high pressure event indicated in a reactor vessel and releasing coolant into a containment region located between a containment vessel and the reactor vessel. The containment region surrounds the reactor vessel and is substantially dry prior to the high pressure event. The method further includes condensing the coolant on an inner wall of the containment vessel, transferring a decay heat to a liquid medium surrounding the containment vessel, and maintaining the pressure in the containment region within design limits through the condensation of the coolant on the inner wall.

The invention will become more readily apparent from the following detailed description of a preferred embodiment of the invention which proceeds with reference to the accompanying drawings.

DESCRIPTION OF EXAMPLE EMBODIMENTS

Conventional nuclear facilities are expensive to license and build, with significant upfront investment costs and delayed return of profits. In addition to energy cost considerations, efficiency requirements, and reliability concerns, today's nuclear reactor designs must also take into account issues of nuclear proliferation, terrorist activities, and a heightened awareness of environmental stewardship.

Developing countries that could otherwise greatly benefit from nuclear power are frequently left to resort to other energy sources such as coal, gas or hydroelectric power generators that produce significant amounts of pollution or have other detrimental environmental impact. These developing countries may not have the technological or natural resources that enable them to build a nuclear power plant. Countries that have already developed nuclear power may be hesitant to introduce these technologies into the developing countries out of concern of the loss of control of the nuclear materials or technology.

Passively safe nuclear power systems help address some of these concerns. Further system improvements and innovative designs are expected to usher in a new era of nuclear power as a globally viable primary energy source.

FIG.2illustrates a novel power module assembly50including an internally dry containment vessel54. The containment vessel54is cylindrical in shape, and has spherical upper and lower ends. The entire power module assembly50may be submerged in a pool of water16which serves as an effective heat sink. The containment vessel54may be welded or otherwise sealed to the environment, such that liquids and gas do not escape from, or enter, the power module assembly50. The containment vessel54may be bottom supported, top supported or supported about its center. Supporting the containment vessel54at the top may facilitate maintenance and removal of the power module assembly50from the pool of water16.

In one embodiment, the containment vessel54is suspended in the pool of water16by one or more mounting connections80. The mounting connections80may be attached to the upper portion of the containment vessel54. The mounting connections80may be rigid or flexible members that help locate the containment vessel54approximately at the center of the pool of water16. During seismic activities, such as earthquakes, the pool of water16acts as a protective cushion about the containment vessel54to avoid damage that may otherwise result if the containment vessel54came into contact with the reactor bay26. Flexible mounting connections such as chains or cables attached to the wall of the reactor bay26, may reduce an amount of vibration or stress that might otherwise be transferred to the containment vessel54. In one embodiment, a flexible tie down connector is attached to the bottom of the containment vessel54to reduce sway or lateral movement. The power module assembly50may be arranged to float in the pool of water16to minimize support requirements and provide seismic resistance. A support base may be provided on the bottom of the containment vessel54to support the power module assembly50in a standing position.

A reactor vessel52is located or mounted inside the containment vessel54. An inner surface of the reactor vessel52may be exposed to a wet environment including a coolant100or liquid, such as water, and an outer surface may be exposed to a dry environment such as air. The reactor vessel52may be made of stainless steel or carbon steel, may include cladding, and may be supported within the containment vessel54.

The power module assembly50may be sized so that it can be transported on a rail car. For example, the containment vessel54may be constructed to be approximately 4.3 meters in diameter and 17.7 meters in height (length). By completely sealing the containment vessel54, access to the reactor core6may be restricted. Any unauthorized access or tampering may be monitored. Furthermore, the subterranean profile of a nuclear power system makes it less visible and easier to conceal. The pool of water16may be covered with a protective shield (not shown) to further isolate the power module assembly50from outside threats or airborne objects such as planes or missiles.

Refueling of the reactor core6may be performed by transporting the entire power module assembly50by rail car or overseas, for example, and replacing it with a new or refurbished power module assembly which has a fresh supply of fuel rods. Refueling and maintenance activities may be performed by unbolting flanges or cutting the vessels in the cylindrical portion at an elevation above the reactor core6. Refueling may be accomplished once every 2 to 10 years or even longer, depending on fuel type and system specifications.

The containment vessel54encapsulates and, in some conditions, cools the reactor core6. It is relatively small, has a high strength and may be capable of withstanding six or seven times the pressure of conventional containment designs in part due to its smaller overall dimensions. Given a break in the primary cooling system of the power module assembly50no fission products are released into the environment. Decay heat may be removed from the power module assembly50under emergency conditions.

The reactor core6is illustrated as being submerged or immersed in a primary coolant100, such as water. The reactor vessel52houses the coolant100and the reactor core6. A shroud22surrounds the reactor core6about its sides and serves to direct the coolant100up through an annulus23and out a riser24located in the upper half of the reactor vessel52as a result of natural circulation of the coolant100. In one embodiment, the reactor vessel52is approximately 2.7 meters in diameter and includes an overall height (length) of 13.7 meters. The reactor vessel52may include a predominately cylindrical shape with spherical upper and lower ends. The reactor vessel52is normally at operating pressure and temperature. The containment vessel54is internally dry and may operate at atmospheric pressure with wall temperatures at or near the temperature of the pool of water16.

The containment vessel54substantially surrounds the reactor vessel52in a dry or gaseous environment identified as containment region44. Containment region44may be filled with air. The containment vessel54includes an inner surface55or inner wall which is adjacent to the containment region44. The containment region44may include a gas or gases instead of or in addition to air. In one embodiment, the containment region44is maintained at a below atmospheric pressure condition, for example as a partial vacuum. The partial vacuum may be provided or maintained at a level which substantially inhibits all convective heat transfer.

Removal of convective heat transfer in air occurs generally at about 50 torr (50 mmHG) of absolute pressure, however a reduction in convective heat transfer may be observed at approximately 300 torr (300 mmHG) of absolute pressure. In one embodiment, the partial vacuum is provided or maintained below 300 torr (300 mmHG). In another embodiment, the partial vacuum is provided or maintained below 50 torr (50 mmHG). Gas or gasses in the containment vessel may be removed such that the reactor vessel52is located in a complete or partial vacuum in the containment region44. The complete or partial vacuum may be provided or maintained by operating a vacuum pump43or steam-air jet ejector.

By maintaining the containment region54in a vacuum or partial vacuum, moisture within the containment region54may be eliminated, thereby protecting electrical and mechanical components from corrosion or failure. Additionally, the vacuum or partial vacuum may operate to draw or pull coolant into the containment region54during an emergency operation (e.g. over-pressurization or over-heating event) without the use of a separate pump or elevated holding tank. The vacuum or partial vacuum may also operate to provide a way to flood or fill the containment region54with coolant during a refueling process.

During normal operation, thermal energy from the fission events in the reactor core6causes the coolant100to heat. As the coolant100heats up, it becomes less dense and tends to rise up through the annulus23and out of the riser24. As the coolant100cools down, it becomes relatively denser than the heated coolant and is circulated around the outside of the annulus23, down to the bottom of the reactor vessel52and up through the shroud22to once again be heated by the reactor core6. This natural circulation causes the coolant100to cycle through the reactor core6, transferring heat to a secondary coolant system, such as the secondary cooling system30ofFIG.1to generate electricity.

The natural circulation may be enhanced by providing a two phase condition of the coolant100in the riser24. In one embodiment, gas is injected into or near the reactor core6to create or augment the two phase condition and increase a flow rate of the coolant100through the riser24. Whereas voiding the reactor core6produces a negative insertion of reactivity, a steady-state condition followed by a non-voided condition may result in a positive insertion of reactivity. In one embodiment, the reactivity is further controlled through a combination of managing control rod insertion rates and temperature sensitive control rod trips.

FIG.3illustrates the power module assembly50ofFIG.2during an emergency operation. The emergency operation may include a response to an overheating of the reactor core6, or an over-pressurization event of the reactor vessel52, for example. During the emergency operation, the reactor vessel6may be configured to release the coolant100into the containment region44of the otherwise dry containment vessel54. A decay heat of the reactor core6may be removed through condensation of the coolant100on the inner surface55of the containment vessel54. Whereas the containment vessel54may be immersed in a pool of water16, the inner surface55of the containment vessel54may be completely dry prior to the emergency operation or over-pressurization event. For example, the suppression pool12ofFIG.1does not exist in the containment vessel54during normal operation.

A flow limiter58or steam vent may be mounted on the reactor vessel52for venting the coolant100into the containment vessel54during the emergency operation. The coolant100may be released into the containment vessel54as a gas or vapor41, such as steam. The flow limiter58may be connected or mounted directly to an outer wall of the reactor vessel52, without any intervening structures such as piping or connections. In one embodiment, the flow limiter58is welded directly to the reactor vessel52to minimize the likelihood of any leaking or structural failures. The flow limiter58may be a Venturi flow valve sized to release coolant100into the containment vessel54at a controlled rate. In one embodiment, the coolant100is released only in the form of steam or vapor from the reactor vessel52. The condensation of the vapor41may reduce pressure in the containment vessel54at approximately the same rate that the vented vapor41adds pressure to the containment vessel54. In one embodiment, the flow limiter58is configured to release approximately five megawatts of heat contained in the vapor41.

Coolant100that is released as vapor41into the containment vessel54condenses on the inner surface55of the containment vessel54as a liquid, such as water. The condensation of the vapor41causes the pressure in the containment vessel54to decrease, as the vapor41is transformed into the liquid coolant100. A sufficient amount of heat may be removed from the power module assembly50through the condensation of the vapor41on the inner surface55of the containment vessel to manage the removal of decay heat from the reactor core6. In one embodiment, there is no release of the liquid coolant100from the reactor vessel52even during an emergency operation. The condensed coolant100descends to the bottom of the containment vessel54and collects as a pool of liquid. As more vapor41condenses on the inner surface55, the level of the coolant100in the bottom of the containment vessel54gradually rises. Heat stored in the vapor41is transferred through the walls of the containment vessel54into the pool of water16that acts as an ultimate heat sink. Heat stored in the coolant100located at the bottom of the containment vessel54is transferred through liquid convection and conduction heat transfer on the inner surface55.

Heat removed from the steam or vapor41may be transferred to the relatively cold inner surface55through condensation on the inside walls of the cold containment vessel54and by natural convection from the hot coolant to the inner surface55. Heat may be transferred to the pool of water16by conduction through the containment vessel walls and through natural convection on an outside surface of the containment vessel54. The coolant100remains confined within the power module assembly50after the reactor core6becomes over-heated and during the emergency operation. The heat transferred to the pool of water16may provide adequate passive decay heat removal for three or more days without any operator intervention.

The containment vessel54may be designed to withstand the maximum pressure that would result given an instantaneous release of the high-pressure fluid from the reactor vessel52into the containment vessel54. The pressure inside the containment vessel54may be designed to equilibrate with the pressure inside the reactor vessel52, stopping break flow caused by the pressure difference. Over time, the amount of pressure in the containment vessel54may be made to equalize with the amount of pressure in the reactor vessel52, resulting in coolant level100A in the reactor vessel52and coolant level100B in the containment vessel54as shown inFIG.3. The coolant level100B is shown elevated with respect to the coolant level100A due to higher coolant temperatures in the reactor vessel52as compared with temperatures in the containment vessel54.FIG.3shows that the coolant levels100A and100B may equilibrate such that the coolant level100A in the reactor vessel52remains above the top of the reactor core6, keeping the reactor core6covered with coolant100at all times.

A flow valve57may be provided to allow the coolant100to flow from the containment vessel54back into the reactor vessel52once a steady state condition of the coolant levels100A,100B is achieved. Coolant100that is allowed to reenter the reactor vessel52through the flow valve57replenishes the coolant100that was vented as vapor41through the flow limiter58. The flow of coolant100through the flow valve57may be achieved through the natural circulation of the passive system due to the different water densities that result from temperature differences in the vessels52,54. No mechanical or electrical pumps or motors are required. In one embodiment, the flow valve57restricts the flow of coolant100in a single direction, from the containment vessel54to the reactor vessel52.

When the reactor core6becomes over-heated the flow limiter58or steam vent is configured to vent the coolant100, for example as steam or vapor41, into the containment vessel54at a rate that maintains an approximate constant pressure in the containment vessel54during a steady state condition. In one embodiment, the containment vessel54undergoes an initial pressure spike prior to reaching the steady state condition. By controlling the rate of pressure increase in the containment vessel54, the thickness of the containment vessel wall can be designed with less material strength on account of the lower, controlled pressures therein. Decreasing the wall thickness can lessen the transportation weight of the power module assembly50and decrease manufacturing and delivery costs.

Whereas a complete or perfect vacuum may be commercially or technically impractical to achieve or maintain, a partial vacuum may be created in the containment vessel54. Any reference to a vacuum herein is therefore understood to be either a partial or complete vacuum. In one embodiment, the containment region44is maintained at a vacuum pressure that significantly reduces convective and conductive heat transfer through the containment gases. The containment region44may be provided or maintained at a vacuum pressure that significantly reduces or eliminates convective heat transfer between the reactor vessel52and the containment vessel54. By substantially removing gases from the containment region44, for example by maintaining a vacuum within the containment vessel54, the initial rate of condensation of vapor41on the inner surface55is increased. Increasing the rate of condensation increases the rate of heat transfer through the containment vessel54.

A vacuum within the containment region44acts as a type of thermal insulation during normal operation, thereby retaining heat and energy in the reactor vessel52where it can continue to be utilized. As a result, less material insulation may be used in the design of the reactor vessel52. In one embodiment, a reflective insulation is used instead of or in addition to conventional thermal insulations. Reflective insulation may be included on one or both of the reactor vessel52or the containment vessel54. The reflective insulation may be more resistant to water damage compared to conventional thermal insulation. In addition, reflective insulation does not impede a transfer of heat from the reactor vessel52as much as the conventional thermal insulation during an emergency condition. The combination of a vacuum and reflective insulation therefore provides thermal insulation during normal operation and promotes the transfer of heat away from the reactor core6during the emergency condition. In certain embodiments, no fibrous or other conventional material thermal insulations are used for the reactor vessel52, or with the containment vessel54.

In the event of a loss of the vacuum in the containment region44, the introduced gases or liquids provide a further passive safety cooling mechanism to transfer heat between the reactor vessel52and the containment vessel54through natural convection. For example, by reducing or eliminating conventional thermal insulation, a more effective heat transfer from the reactor vessel52can be made during the emergency operation due to the condensed liquid coolant100which pools at the bottom of the containment vessel54. Heat is able to be transferred from the reactor vessel52through the liquid coolant100to the containment vessel54.

Additionally, the removal of air, oxygen and other gases from the containment region44may reduce or completely eliminate the need for any hydrogen recombiners that are typically used to reduce combustible mixtures of gases that might otherwise develop. During an emergency operation, steam may chemically react with the fuel rods to produce a high level of hydrogen. When hydrogen mixes with air or oxygen, this may create a combustible mixture. By removing a substantial portion of the air or oxygen from the containment vessel54, the amount of hydrogen and oxygen that is allowed to mix is minimized or eliminated. In one embodiment, any air or other gases that reside in the containment region44are removed or voided when an emergency condition is detected.

Certain gases are considered non-condensable under operating pressures that are experienced within a nuclear reactor system. These non-condensable gases include hydrogen and oxygen, for example. The gases that are voided or evacuated from the containment region44may comprise both non-condensable gases and condensable gases. Condensable gases may include any steam that is vented into the containment region.

During an emergency operation, whereas steam may be vented into the containment region44, only a negligible amount of non-condensable gas (such as hydrogen) may be vented or released into the containment region. In such cases, one can assume from a practical standpoint, that substantially no non-condensable gases are released into the containment region44together with the steam. Accordingly, substantially no hydrogen gas is vented into the containment region44together with the steam, such that the levels or amounts of hydrogen together with any oxygen that may exist within the containment region44are maintained at a non-combustible level. This non-combustible level of oxygen-hydrogen mixture may be maintained without the use of hydrogen recombiners.

FIG.4illustrates an example rate of condensation of released coolant100into the containment vessel54. As previously described, the coolant100may be vented as steam or vapor41that condenses on the inner surface55of the containment vessel54. A rate of condensation of the coolant100on the inner surface55may be higher by virtue of the evacuated gases. Gases that would normally accumulate at the inner surface55to inhibit the condensation of coolant100are either at such low levels or are swept from the inner surface55due to the natural convection of the coolant11, that the rate of condensation is maximized.

The flow limiter58controls the rate of release of coolant100as vapor41into the containment vessel54, such that the rate of increase of coolant level100B in the containment vessel54may be determined or managed. According to the graph ofFIG.4, approximately 110 inches of coolant100may collect at the bottom of the containment vessel54after a period of 9500 seconds or about 2 hours and 38 minutes. Of course, this rate of increase in coolant level100B will depend on the size of the reactor vessel52and containment vessel54as well as the design of the flow limiter58.

In one embodiment, the rate of increase of the coolant level1006flattens out at a near constant value once the pressures in the reactor vessel52and containment vessel54equalize or reach steady state. A flow of coolant100through the flow valve57ofFIG.3into the reactor vessel52may remove approximately the same amount of coolant100that condenses as liquid on the inner surface55of the containment vessel54.

The flow limiter58connected to the reactor vessel52may vent the vapor41at a rate that maintains an approximate constant pressure in the containment vessel54during a steady state condition.FIG.5illustrates an example pressure fluctuation in the containment vessel54during an over-pressurization event. In one embodiment, the pressure inside the containment vessel54may be at or near atmospheric pressure prior to the over-pressurization event. In another embodiment, the pressure inside the containment vessel is maintained as a vacuum. The containment vessel54may then undergo a pressure spike that increases the pressure up to some predetermined upper threshold value.

In one embodiment, the upper threshold pressure value is approximately 300 psia. Once the pressure reaches the upper threshold value, the flow limiter58may close or otherwise prohibit the further release of coolant100as vapor41into the containment vessel54. The pressure within the containment vessel54then decreases due to the condensation of the vapor41into a liquid. The pressure may be allowed to decrease down to some predetermined lower threshold value. In one embodiment, the lower threshold pressure value is less than 150 psia. Once the pressure reaches the lower threshold value, the flow limiter58may open or otherwise allow the release of additional coolant100into the containment vessel54. The pressure within the containment vessel54then increases until it once again reaches the upper threshold value, continuing the cycle of pressurization and depressurization while the decay heat is being removed from the reactor core6. The pressure within the containment54may therefore be maintained between the upper and lower threshold values.

A steam nozzle flow area of the flow limiter58may be calculated according to measurements or estimated values of steam condensation rate in the containment vessel54, an energy removal rate from the containment vessel54, and a heat up rate of the pool of water16ofFIG.3. A rate of change of liquid level in the containment vessel54may be approximately 0.0074 inches per second in one embodiment. According to the principle of conservation of mass, the mass flow rate of steam condensed to liquid may be determined according to the following equation:
dML/dt=ρLAC(dL/dt)C=m(1)

The heat transfer rate to the inner surface55of the containment vessel54may be given by the following equation:
q=mhfg(2)

The heat up rate for the pool of water16may be determined using the following equation:
MCPdT/dt=q(3)

Assuming that the cooling pool mass, the cooling pool water specific heat at constant pressure, and the heat input are constants, allows one to integrate equation (2) to obtain the time required to heat the pool of water16according to the following equation:
Δt=MCPΔT/q(4)

In one embodiment, the upper temperature of the pool of water16is set at below boiling, such as 200 degrees Fahrenheit. Finally, the equation for the choke flow of steam may be given by the following equation:
m=CdA[KgcμgP](1/2)(5)
where Cdis a discharge coefficient of approximately 0.95 and where
K=γ[2/(γ+1)](γ+1)/(γ−1).

An initial 6% decay heat may be experienced by the primary cooling system during the first 100 seconds of a steam blow-down scenario, however this flattens out to 2% or 3% in the steady state condition. Releasing the pressure into the containment vessel54can result in approximately 3% of the decay heat being transferred from the reactor vessel52, which accommodates the amount of decay heat being released at steady state. This is accomplished through the passive emergency feedwater and decay heat removal system described herein, without the need for a pre-existing source of water or suppression pool being located inside the containment vessel54.

FIG.6illustrates an alternate embodiment of a power module assembly60including a containment vessel64having fins65to increase a cooling surface area. The cooling fins65may be attached to an outside wall of the containment vessel64to remove decay heat of a reactor core during an emergency operation. During normal operation of the power module assembly60, the inside of the containment vessel64remains dry, whereas a reactor vessel62contains a coolant as well as a reactor core. In one embodiment, the containment vessel64is in a depressurized state or vacuum during normal operating conditions. The coolant may be a liquid or gas. In the emergency operation, such as over-pressurization of the reactor vessel62, coolant is released out of flow limiter68into the containment vessel64. The coolant circulates within and releases heat into the wall of the containment vessel64. The heat is then removed from the containment vessel through convection or conduction into a surrounding heat sink66.

Heat sink66may be a fluid such as water or gas. In one embodiment, the heat sink is composed of earth (e.g. rock, soil or other solid material) that completely surrounds the containment vessel66. Fins65may be attached to the containment vessel64and provide additional surface area with which to transfer the decay heat to the heat sink66. The fins65may encircle the containment vessel64. In one embodiment, the fins65are orientated in horizontal planes. The heat sink66may be contained in a containment structure61such as concrete. A cover63, which may also be made of concrete, may completely enclose the power module assembly60and heat sink66. The containment structure61and cover63may serve to protect against an impact from foreign projectiles and also operate as a biological shield.

FIG.7illustrates an embodiment of a power module assembly70including multiple containment regions71,72. The containment region may be compartmentalized into a first containment region71and a second containment region72. The first containment region71may be located in an upper portion of the containment vessel74and the second containment region72may be located in a lower portion of the containment vessel74. The first containment region71may be maintained at atmospheric pressure, whereas the second containment region72may be maintained at a below atmospheric pressure.

One or more valves75may be provided between the first and second containment regions71,72. Valve75may operate in the event of an emergency condition to release pressure. In one embodiment, valve75operates to transfer liquid coolant that condenses in the first containment region71such that it collects in the second containment region72. In one embodiment, conventional thermal insulation76is included in the first containment region71and reflective insulation78is included in the second containment region72. Any number of containment regions may be provided for, some or all of which may be maintained as a vacuum.

FIG.8illustrates a novel method of cooling a power system, such as the power module assembly50ofFIG.3. The method may be understood to operate with, but not limited by, means illustrated or described with respect to the various embodiments illustrated herein asFIGS.1-7. At operation810, the power module assembly50is scrammed in the event of a high pressure event indicated in a reactor vessel, such as the reactor vessel52ofFIG.3.

At operation820, coolant is released into a containment region, such as containment region44ofFIG.3, located between a containment vessel, such as containment vessel54ofFIG.3and the reactor vessel52. The containment region54surrounds the reactor vessel52and may be substantially dry prior to the high pressure event. The coolant, such as coolant100, may be released as a vapor41or steam into the containment vessel54. In one embodiment, steam that is released from the secondary coolant system30ofFIG.1as a result of failure or loss of pressure integrity may also be vented into the containment vessel54.

At operation830the vapor41condenses on an inner wall, such as inner wall55of the containment vessel54. The vapor41may be condensed into a liquid, such as water.

At operation840, a decay heat is transferred to a liquid medium surrounding the containment vessel54. The decay heat may be transferred via condensation of the vapor41, as well as convection and conduction of the condensed liquid.

At operation850, the pressure in the containment region44is limited or maintained within design limits through the condensation of the coolant on the inner wall. A steam flow limiter such as flow limiter58ofFIG.3may be sized to limit a rate of pressure increase in the containment vessel54. The rate of pressure increase may be substantially offset by the condensation of the vapor41into liquid. The steam flow limiter58may be selectively or intermittently opened such that a pressure in the containment vessel54is limited to a maximum value and allowed to depressurize when the flow limiter58is closed.

The condensation of the vapor41may reduce pressure within the containment region44by approximately a same amount as the released coolant increases pressure in the containment region44. The coolant100may be released into the containment region44as vapor41or steam, and a decay heat of the reactor core6may be removed from the power module assembly50by condensing the vapor41on the inner wall55of the containment vessel54.

FIG.9illustrates a novel method of cooling a reactor module. The method may be understood to operate with, but not limited by, means illustrated or described with respect to the various embodiments illustrated herein asFIGS.1-7.

At operation910, a containment region is maintained at a below atmospheric pressure. In one embodiment, the below atmospheric pressure is less than 300 mmHG absolute. In one embodiment, the below atmospheric pressure prohibits substantially all convective heat transfer between the reactor vessel and the containment vessel.

Substantially all gases may be evacuated from the containment region. In one embodiment, the evacuated gases comprise non-condensable gases. The evacuated gases may comprise air. In one embodiment, the levels of oxygen and hydrogen within the containment region are maintained below a combustible mixture without employing a hydrogen recombiner.

At operation920, a high pressure event is identified for a reactor vessel. In one embodiment, the containment region is substantially dry prior to the high pressure event.

At operation930, coolant is released into the containment region located between a containment vessel and the reactor vessel to remove decay heat from the reactor vessel. The reactor vessel may be substantially surrounded by the containment region. In one embodiment, the coolant condenses on an inner wall of the containment vessel during an over-pressurization or over-heating event.

In one embodiment, the containment region remains substantially evacuated of all non-condensable gases after the coolant is released into the containment region. The levels of oxygen and hydrogen may be maintained below a combustible mixture after the coolant is released into the containment region without employing a hydrogen recombiner.

At operation940, a pressure within the containment region increases when the coolant is released. The pressure may continue to increase until the pressure reaches an upper pressure limit.

At operation950, the pressure is decreased through condensation. The pressure may decrease until the pressure reaches a lower pressure limit. In one embodiment, the further release of coolant into the containment region is prohibited while the pressure within the containment region is being decreased. The pressure within the containment region may be cyclically increased and decreased by alternatively releasing and prohibiting the release of coolant into the containment region. In one embodiment, operations940and950are continuously cycled through until an over-pressurization or over-heating condition of the reactor vessel falls below some predetermined value.

Although the embodiments provided herein have primarily described a pressurized water reactor, it should be apparent to one skilled in the art that the embodiments may be applied to other types of nuclear power systems as described or with some obvious modification. For example, the embodiments or variations thereof may also be made operable with a boiling water reactor. A boiling water reactor may require larger vessels to produce the same energy output.

The rate of release of the coolant into the containment vessel, the rate of condensation of the coolant into a liquid, and the rate of increase of pressure in the containment vessel, as well as other rates and values described herein are provided by way of example only. Other rates and values may be determined through experimentation such as construction of full scale or scaled models of the nuclear reactor.

Having described and illustrated the principles of the invention in a preferred embodiment thereof, it should be apparent that the invention may be modified in arrangement and detail without departing from such principles. We claim all modifications and variation coming within the spirit and scope of the following claims.