Patent Number: 
Section: claims

1. A nuclear plant, comprising:a nuclear reactor;a containment structure having one or more inner surfaces at least partially defining a containment environment in which the nuclear reactor is located;a passive containment cooling system, includinga coolant reservoir configured to hold a coolant fluid,a coolant channel coupled to the containment structure such that the coolant channel extends vertically from a coolant channel inlet at a bottom of the coolant channel to a coolant channel outlet at a top of the coolant channel,a coolant supply conduit extending downwards from an inlet of the coolant supply conduit that is open to a lower region of the coolant reservoir, an outlet of the coolant supply conduit is coupled to the coolant channel inlet, such that the coolant supply conduit is configured to direct a flow of the coolant fluid downwards out of the lower region of the coolant reservoir and into the bottom of the coolant channel via the coolant channel inlet according to gravity, such that the coolant fluid rises through the coolant channel from the bottom of the coolant channel to the top of the coolant channel according to a change in buoyancy of the coolant fluid based on the coolant fluid absorbing heat rejected from the nuclear reactor in the containment environment via at least the containment structure, anda coolant return conduit having an inlet coupled to the coolant channel outlet at the top of the coolant channel, the coolant return conduit extending upwards from the inlet of the coolant return conduit to an outlet of the coolant return conduit that is open to an upper region of the coolant reservoir that is above the lower region of the coolant reservoir, such that the coolant return conduit is configured to direct a flow of the coolant fluid to rise out of the top of the coolant channel via the coolant channel outlet and into the upper region of the coolant reservoir according to increased buoyancy of the coolant fluid at the top of the coolant channel over buoyancy of the coolant fluid at the bottom of the coolant channel; anda first check valve assembly at a first vertical depth below a top surface of the coolant fluid in the coolant reservoir, the first check valve assembly in fluid communication with the coolant reservoir through the coolant channel and in fluid communication with the containment environment, whereinthe first check valve assembly includes one or more check valves coupled between a first check valve assembly inlet and a first check valve assembly outlet, the first check valve assembly inlet being open to the containment environment, the first check valve assembly outlet being in fluid communication with the coolant reservoir through the coolant channel,the one or more check valves are configured to open in response to a pressure at an inlet of the one or more check valves being equal to or greater than a first threshold magnitude, the first threshold magnitude at least partially corresponding to a hydrostatic pressure of the coolant fluid at the first check valve assembly outlet at the first vertical depth, andthe first check valve assembly is configured to selectively enable one-way flow of a containment fluid, from the containment environment via the first check valve assembly inlet to the coolant reservoir through the coolant channel via the first check valve assembly outlet and the coolant channel, based on the one or more check valves opening in response to a pressure of the containment environment at the first check valve assembly inlet at the first vertical depth being equal to or greater than the first threshold magnitude. 2. The nuclear plant of claim 1, wherein the first threshold magnitude is greater than a reference hydrostatic pressure of the coolant fluid at the first vertical depth below the top surface of the coolant fluid in the coolant reservoir that results from the coolant reservoir being filled to a reference reservoir depth. 3. The nuclear plant of claim 1, whereinthe first check valve assembly is configured to, subsequently to selectively enabling the one-way flow, inhibit the one-way flow of the containment fluid based on the one or more check valves closing in response to the pressure of the containment environment at the first check valve assembly inlet being less than the first threshold magnitude. 4. The nuclear plant of claim 1, whereinthe one or more check valves include a series connection of a plurality of check valves between the first check valve assembly inlet and the first check valve assembly outlet,each check valve of the plurality of check valves is configured to open in response to a pressure at an inlet of the check valve being equal to or greater than the first threshold magnitude, andthe first check valve assembly is configured to selectively enable the one-way flow based on all check valves of the series connection of the plurality of check valves opening. 5. The nuclear plant of claim 1, whereinthe one or more check valves include a parallel connection of a plurality of sets of one or more check valves between the first check valve assembly inlet and one or more check valve assembly outlets,each check valve of the plurality of sets of one or more check valves is configured to open in response to a pressure at an inlet of the check valve being equal to or greater than the first threshold magnitude, andthe first check valve assembly is configured to selectively enable the one-way flow based on any set of one or more check valves of the parallel connection of the plurality of sets of one or more check valves. 6. The nuclear plant of claim 1, whereinthe first check valve assembly includes a burst disc coupled in series with the inlet of the one or more check valves and the first check valve assembly inlet, the burst disc configured to rupture in response to the pressure of the containment environment at the first check valve assembly inlet being equal to or greater than a particular set point pressure magnitude. 7. The nuclear plant of claim 1, further comprising:a second check valve assembly at a second vertical depth below the top surface of the coolant fluid in the coolant reservoir, the second check valve assembly in fluid communication with the coolant reservoir through the coolant channel and in fluid communication with the containment environment, the second vertical depth being less than the first vertical depth,wherein the second check valve assembly is configured to selectively enable one-way flow of the containment fluid, from the containment environment to the coolant reservoir through the coolant channel, based on one or more check valves of the second check valve assembly opening in response to a pressure of the containment environment at an inlet of the second check valve assembly being equal to or greater than a second threshold magnitude, the second threshold magnitude at least partially corresponding to a hydrostatic pressure of the coolant fluid at an outlet of the second check valve assembly at the second vertical depth. 8. The nuclear plant of claim 1, whereinthe first check valve assembly extends through the containment structure and into the coolant channel at the first vertical depth, and the first check valve assembly is open to the coolant channel, andthe first check valve assembly is configured to selectively enable the one-way flow of the containment fluid, from the containment environment via the first check valve assembly inlet, to the coolant channel via the first check valve assembly outlet. 9. The nuclear plant of claim 1, further comprising:a fusible plug in fluid communication with the coolant reservoir through the coolant channel and in fluid communication with the containment environment at a bottom vertical depth below the top surface of the coolant fluid in the coolant reservoir, the bottom vertical depth being greater than the first vertical depth, such that a hydrostatic pressure of the coolant fluid at the bottom vertical depth is greater than the hydrostatic pressure of the coolant fluid at the first check valve assembly outlet at the first vertical depth,wherein the fusible plug is configured to at least partially melt in response to a temperature in the containment environment at an end of the fusible plug that is open to the containment environment being equal to or greater than a threshold temperature, such that the fusible plug exposes a flow conduit extending between the coolant reservoir into the containment environment through the coolant channel to at least partially flood the containment environment with at least some of the coolant fluid. 10. The nuclear plant of claim 9, wherein the first check valve assembly is configured to, based on selectively enabling the one-way flow of the containment fluid in response to the pressure in the containment environment at the first check valve assembly inlet being equal to or greater than the first threshold magnitude, maintain a pressure in the containment environment at the bottom vertical depth at a magnitude that is less than the hydrostatic pressure of the coolant fluid at the bottom vertical depth, to enable flow of the coolant fluid through the exposed flow conduit and into the containment environment through the coolant channel in response to the fusible plug at least partially melting. 11. The nuclear plant of claim 5, whereineach separate set of one or more check valves between the first check valve assembly inlet and the one or more check valve assembly outlets includes a series connection of check valves between the first check valve assembly inlet and the first check valve assembly outlet,each check valve of each series connection of check valves is configured to open in response to a pressure at an inlet of the check valve being equal to or greater than the first threshold magnitude, andthe first check valve assembly is configured to selectively enable the one-way flow based on all check valves of at least one series connection of check valves opening.