The invention relates generally to the nuclear energy field, and more particularly to pressurized water reactor containment internal passive heat removal systems (C PHRS), and is designed for reactor containment cooling by natural circulation of the cooling liquid (water) in the system circuit.

According to the background of the invention, there are numerous designs of reactor containment heat removal systems based on natural heat circulation.

Russian patent RU2125744, G21C15/18 dated 1/27/1999 discloses a system for passive heat removal from the nuclear reactor containment structure internal volume including the first heat exchanger located outside of the containment structure, the second heat exchanger inside the reactor containment structure. The first heat exchanger and second heat exchanger are hydraulically connected in a closed circuit by means of pipes with the coolant passing through the containment structure and the exhaust pipe above open to the atmosphere. The system also includes a tank filled with water to the set level that is connected to the containment structure and located near its upper wall. The first heat exchanger is immersed in the water in the tank and extended vertically from the base plate adjoining the bottom of the tank to the upper section dividing the tank into two hydraulically connected volumes. The tank is equipped with an cover shaping the first channel and the second channel, each covering its corresponding area formed by the vertical heat exchanger, and connected to the corresponding area only. One of the channels is connected to the outer air intake, the other is connected to the exhaust pipe, and the water in the tank blocks the connection between the channels when the tank is filled to the set level.

Russian patent RU2302674, G21C9/00 dated 7/10/2007 discloses a containment heat removal system comprising a heat exchanger installed under the containment, its inlet and outlet passing through the containment and connected to the closed circulation circuit of low-boiling coolant, including a turbine with an electric generator, a power unit with a steam generator located under the containment, and power unit safety systems, one of which having a hydraulic unit and a steam water turbine. The heat exchanger is installed under the containment dome and is designed as two-tiered circular tubes connected by means of C-shaped finned tubes, the ends of the tubes facing the containment wall and enclosing the hydraulic unit to ensure power unit safety.

The closest analog of the claimed invention is the PHRS system disclosed in Russian utility model patent RU85029, G21C15/18 dated 7/20/2009 and comprising a coolant circulation circuit, including at least one heat exchanger located inside the containment and a tank with coolant supply installed above the heat exchanger outside the containment interconnected by the inlet and outlet pipelines. The system is also equipped with a steam receiver installed in the coolant supply tank and hydraulically connected to the latter and connected to the outlet pipeline.

The disadvantage of the said devices is potential water hammer in the system.

The purpose of the invention is to provide a system for efficient heat removal from the reactor containment.

The technical result of the invention is increase of heat removal efficiency, flow stability in the circuit (no water hammer) and, consequently, system operation reliability.

The said technical result is achieved owing to the fact that the pressurized water reactor containment internal passive heat removal system with at least one cooling water circulation circuit comprises a heat exchanger located inside the containment and including an upper header and a lower header interconnected by heat-exchange tubes, a riser pipeline and a downtake pipeline connected to the heat exchanger, a cooling water supply tank located above the heat exchanger outside the containment and connected to the downtake pipeline, a steam relief valve connected to the riser pipeline and located in the water supply tank and hydraulically connected to the latter. The upper header and the lower header of the heat exchanger are divided into heat-exchange tube sections on the assumption that: $$\mathrm{L}/\mathrm{D}\le \mathrm{20},$$

• where L is the header section length,
• D is the header bore,

the riser pipeline design provides the minimum riser section height h_rs to meet the following criterion: $${{\mathrm{\Delta P}}^{\mathrm{c}}}_{\mathrm{res}}={\mathrm{\Delta \rho }}_{\mathrm{rs}}{\mathrm{gh}}_{\mathrm{rs}}+{\mathrm{\Delta \rho }}_{\mathrm{he}}{\mathrm{gh}}_{\mathrm{he}},$$ $${\mathrm{h}}_{\mathrm{rs}}=\left({{\mathrm{\Delta P}}^{\mathrm{c}}}_{\mathrm{res}}-{\mathrm{\Delta \rho }}_{\mathrm{he}}{\mathrm{gh}}_{\mathrm{he}}\right)/{\mathrm{\Delta \rho }}_{\mathrm{rs}}\mathrm{g},$$

• where ΔP^c_res is the circuit total hydraulic resistance,
• h_he is the heat exchanger height,
• g is the gravity factor, $${\mathrm{\Delta \rho }}_{\mathrm{rs}}={\mathrm{\rho }}_{\mathrm{cw}}-\left(\mathrm{\rho }\prime \left(\mathrm{1}-\mathrm{x}\right)+\mathrm{\rho }"\mathrm{x}\right),$$ $${\mathrm{\Delta \rho }}_{\mathrm{he}}={\mathrm{\rho }}_{\mathrm{cw}}-{\mathrm{\rho }}_{\mathrm{hw}},$$

  • Where ρ_cw is the downtake pipeline water density,
  • ρ_hw is the riser pipeline water density in the within the heat exchanger height range,
  • ρ', ρ" are the water and steam saturation density,
  • x is the mean mass steam quality of the two-phase mixture in the riser section.

The above technical result is also achieved in specific options of the invention owing to the fact that:

• the system includes four channels, each comprising four cooling water circulation circuits,
• at least a part of the riser pipeline from the upper headers of the heat exchanger sections to the steam relief valve has an upward inclination to an angle of at least 10° in relation to the horizontal line,
• the riser pipeline includes sections with an inclination angle of less than 10° in relation to the horizontal line, the length of such sections is L_sec1 and the bore is D_see1 meeting the following criterion: L_sec1/D_sec1 ≤ 10,
• at least a part of the downtake pipeline has a downward inclination to an angle of at least 10° in relation to the horizontal line,
• the downtake pipeline includes sections with an inclination angle of less than 10° in relation to the horizontal line, the length of such sections is L_sec2 and the bore is D_sec2 meeting the following criterion: L_sec2/D_sec2 ≤ 10,
• the heat-exchange tube height ensures that the criterion of the turbulent convection on the heat exchanger outer surface is met, namely: $$R_a>{4.10}^{12},$$ where $R_a=\frac{g{l}^3{S}_c}{v^2}\cdot \frac{{\rho }_w-{\rho }_{\mathit{30}}}{{\rho }_{\mathit{30}}}:$

  • R_a is the Rayleigh criterion,
  • g is the gravity factor,
  • l is the typical structure size - heat exchanger tube height,
  • ν is the steam-air kinematic viscosity coefficient,
  • ρ_w is the steam-air medium density on the outer wall of the heat exchanger tubing,
  • ρ_c is the steal-water medium density in the containment,

  • is the Schmidt number,

  • D_dif is the steam diffusion factor.

    • the heat exchanger is located under the containment dome,
    • the heat exchanger section has a single-row vertical bundle,
    • the spacing between any adjacent tubes in the heat exchanger section meets the equivalent plane wall criterion.

For the purposes of this application, the riser section means the portion of the riser pipeline where the coolant is a steam-water (two-phase) mixture with mean mass steam quality x. The section is referred to as "riser" as it makes a major contribution to development of natural circulation in the circuit and determines its intensity.

The experiments conducted show that the above system parameter correlations provide the most efficient heat removal without water hammering or coolant mass-flow rate perturbation due to selection of the best system geometry: the correlation between the length and bore of the heat exchanger header sections, length of the circulation circuit riser section, height of the heat-exchange tubes and optimized arrangement of the system heat exchangers in the containment.

The correlation of the section length and bore of the heat exchanger headers is selected so as to minimize the non-uniformity of coolant flow distribution among the heat exchanger tubes, i.e. to reduce the so-called "header effect". The uniform distribution of flow in the tubing is one of the main conditions for improved energy efficiency and performance of heat exchangers. One of the methods used to improve coolant distribution among the header heat exchanger channels is pressure loss reduction of the medium flow in the header. This is achieved by reducing the header length and increasing its bore within the device manufacturing process capabilities and other design features. For headers meting the L/D ≤ 20 criterion, pressure loss along the header length is minimal, and distribution of coolant flows among the heat exchanger tubes is the most uniform. When the said criterion is exceeded, the uniformity of medium distribution among the heat exchanger channels degrades, which results in the coolant mass flow instability and perturbation and, subsequently, reduced heat output of the heat exchanger.

The design of the invention is illustrated by drawings, where:

• Fig. 1 shows the cooling water circulation circuit design,
• Fig. 2 shows the experimental dependence of the C PHRS cooling circuit output on the pressure of the steam-gas fluid in the tank,
• Fig. 3 shows the calculated dependency of pressure and temperature on time in the course of an accident.

The claimed system is a combination of cooling water circulation circuits. In the preferable embodiment of the invention, the claimed system consists of four completely independent channels, each comprising four such circulation circuits.

The circulation circuit (Fig. 1) comprises a heat exchanger (1) located inside the containment (under the dome) and including an upper header (2) and a lower header (3) interconnected by heat-exchange tubes (4) forming a single-row vertical heat-exchange bundle. A riser pipeline (5) and a downtake pipeline (6) are connected to the heat exchanger (1). A cooling water supply tank (emergency heat removal tank (EHRT)) (7) connected to the downtake pipeline (6) is located above the heat exchanger outside the containment. A steam relief valve (8) connected to the riser pipeline (5) is located in the cooling water supply tank (7) and connected to the same hydraulically. The steam relief valve (8) is designed for elimination of condensate-induced water hammer and increased vibration level in the system riser pipeline (5). The riser pipe of the steam relief valve (8) has a connection hole enabling it to fulfill these functions.

The upper header (2) and the lower header (3) of the heat exchanger are divided into heat-exchange tube sections on the assumption that: $$\mathrm{L}/\mathrm{D}\le \mathrm{20},$$

• where L is the header section length,
• D is the header bore,

the riser pipeline design provides the minimum riser section height h_rs to meet the following criterion: $${{\mathrm{\Delta P}}^{\mathrm{c}}}_{\mathrm{res}}={\mathrm{\Delta \rho }}_{\mathrm{rs}}{\mathrm{gh}}_{\mathrm{rs}}+{\mathrm{\Delta \rho }}_{\mathrm{he}}{\mathrm{gh}}_{\mathrm{he}},$$ $${\mathrm{h}}_{\mathrm{rs}}=\left({{\mathrm{\Delta P}}^{\mathrm{c}}}_{\mathrm{res}}-{\mathrm{\Delta \rho }}_{\mathrm{he}}{\mathrm{gh}}_{\mathrm{he}}\right)/{\mathrm{\Delta \rho }}_{\mathrm{rs}}\mathrm{g},$$

• where ΔP^c_res is the circuit total hydraulic resistance,
• h_he is the heat exchanger height,
• g is the gravity factor, $${\mathrm{\Delta \rho }}_{\mathrm{rs}}={\mathrm{\rho }}_{\mathrm{cw}}-\left(\mathrm{\rho }\prime \left(\mathrm{1}-\mathrm{x}\right)+\mathrm{\rho }"\mathrm{x}\right),$$ $${\mathrm{\Delta \rho }}_{\mathrm{he}}={\mathrm{\rho }}_{\mathrm{cw}}-{\mathrm{\rho }}_{\mathrm{hw}},$$

  • ρ_cw is the downtake pipeline water density,
  • ρ_hw is the riser pipeline water density in the within the heat exchanger height range,
  • ρ', p" are the water and steam saturation density,
  • x is the mean mass steam quality of the two-phase mixture in the riser section.

The heat exchanger section has a single-row vertical bundle. It is preferable that the spacing between any adjacent section tubes meets the equivalent plane wall criterion.

In the preferable embodiment of the invention, the heat-exchange tube height ensures that the criterion of the turbulent convection on the heat exchanger outer surface is met, namely: $$R_a>{4.10}^{12},$$ where $R_a=\frac{g{l}^3{S}_c}{v^2}\cdot \frac{{\rho }_w-{\rho }_{\mathit{30}}}{{\rho }_{\mathit{30}}}:$

• R_a is the Rayleigh criterion,
• g is the gravity factor,
• l is the typical structure size - heat exchanger tube height,
• ν is the steam-air kinematic viscosity coefficient,
• ρ_w is the steam-air medium density on the outer wall of the heat exchanger tubing,
• ρ_c is the steal-water medium density in the containment,

• is the Schmidt number,

• D_dif is the steam diffusion factor.

The riser pipeline from the upper heat exchanger section headers to the steam relief valve has an upward inclination to the angle of a least 10° in relation to the horizontal line, except for certain sections with an inclination less than 10°, having length L_sec1 and bore D_sec1, meeting the following criterion: L_sec1/D_sec1 ≤ 10.

The downtake pipeline has a downward inclination to the angle of a least 10° in relation to the horizontal line, with the exception of certain sections with an inclination less than 10°, length L_sec2 and bore D_sec2, meeting the following criterion: L_sec2/D_sec2 ≤ 10.

In the specific embodiment of the invention for the Leningrad-2 NPP reactor plant, the heat exchangers (1) of the circuits are located along the perimeter on the containment inner wall above elevation 49.3 m. Each heat exchanger has a heat-exchange area of 75 m². The heat-exchange bundle height is 5 m and is built up by 38×3 mm vertical tubes. The total heat-exchange area of each channel amounts to 300 m². The length (L) of the upper and lower sections of the heat exchanger headers equals 2,755 mm. The outer/inner diameter (D) of the upper header is 219/195 mm, the one of the lower header is 194/174 mm.

The system heat output is selected so as to reduce and maintain pressure in the containment inside pressure within the design limits during beyond design basis accidents of reactors, including those involving severe core damage.

Isolating valves (9) and (10) designed for isolation of the heat exchanger (1) in the event of its leakage are mounted on the riser pipeline (5) and downtake pipeline (6). To prevent overpressurization of the C PHRS circuits in case of emergency closing of the isolating valves, safety valves (not shown) are installed to discharge fluid below the tank (7) level.

The isolating and safety valves are located in the reactor building envelope annulus compartments at elevation +54.45 m.

The claimed system operation is based on coolant natural circulation and requires no startup actions. Heat energy is removed from the containment by steam condensation from the steam-air mixture on the outer surface of the heat exchanger (1) from where it is transferred to the water supply tank (7) by means of natural circulation. Heat is ultimately removed from the water supply tank to the ultimate heat sink by evaporation of the water in the tank. The coolant is supplied from the steam relief valve (8) to the cooling water supply tank (7), followed by the cooled coolant (water) return to the heat exchanger (1) through the downtake pipeline (6). Thus, heat energy is transferred from the containment internal volume to the ultimate heat sink, the environment, by means of evaporation of the water in the tank (7) using the circulation circuit.

For experimental justification of the proposed system design efficiency, a significant amount of experimental work has been performed on several experimental setups.

Research has been performed on a full-scale model of the C PHRS cooling circuit installed on the JSC "Afrikantov OKBM" test stand. The C PHRS circuit model included a heat-exchanger-condenser model, operational pipelines located in the containment model tank, and a operational steam relief valve located in the water supply tank.

The heat removal capacity of the tested cooling circuit and parameters of the steam-gas medium in the tank are approximated to the actual reactor accident conditions of the operational system to the maximum extent. Therefore, with the geometry and parameters of the C PHRS cooling circuit practically comparable to the full-scale cooling circuit design, the research results obtained for the C PHRS cooling circuit model are representative and may be applied to the operational C PHRS cooling circuit.

The tests performed on the full-scale C PHRS cooling circuit loop shows that at the maximum cooling water temperature of 100 °C in the cooling water tank, and the specified design capacity per cooling circuit loop, the pressure in the tank will not exceed the design limit pressure of 500 kPa.

Fig. 2 shows the experimental dependence of the C PHRS cooling circuit output on the pressure of the steam-gas fluid in the tank.

Fig. 3 shows how the functioning of the C PHRS influences parameters inside the containment in case of a beyond design basis accident involving depressurization of the reactor plant primary circuit (large leak) and safety system failure (line I shows parameters without PHRS operation, and line II shows parameters with PHRS operation).

The full-scale C PHRS cooling circuit model tests performed show that the circuit design parameters are met both in terms of heat removal efficiency and circuit flow stability. Within the whole range of cooling circuit operation (power operation from the initial state to water boiling), no water hammering in the tank or vibration of the elements and structures of the tested circuit were observed that could affect its operability.

Therefore, the claimed system allows to maintain the pressure under the containment below the design level without operator's intervention for a long period of time and within the whole range of beyond design basis accidents involving release of mass and energy under the containment.