How PWRs and BWRs use different arrangements to manage released mass, energy and pressure
In Part 1, we established a basic rule: the reactor building and containment are not necessarily the same structure. The boundary is not defined by a dome, silhouette or building name, but by the function it is required to perform under defined accident conditions.
A comparison of pressurised water reactors and boiling water reactors shows why this rule is necessary.
Both reactor families must provide the fundamental safety function of confining radioactive material. In both cases, a credited boundary must maintain integrity, limit uncontrolled releases and permit the isolation of penetrations through that boundary. But the same fundamental function does not require the same geometry.
The arrangement of a containment system depends on where the main components of the reactor coolant system are located, where mass and energy would be released following a postulated break, how steam is condensed, where heat is transferred, and which systems prevent a continued rise in pressure and temperature over the long term.
This is where a typical PWR with a large dry containment differs fundamentally from a BWR with a pressure-suppression containment system.
Scope note: This article compares a typical PWR large dry containment with BWR Mark I and Mark II containment systems. It does not describe every configuration. PWR designs also include subatmospheric containments and ice-condenser containments, while BWR Mark III uses a different and substantially larger spatial arrangement. The exact boundary always depends on the specific plant design and its licensing basis.
Structure, system and supporting systems are not the same level
Before making the comparison, it is useful to distinguish three levels that are often merged in non-technical descriptions.
The containment boundary is the physical, leak-tight boundary that, together with its supporting structure, must withstand the design pressure, temperature and other loads. It includes walls or a steel shell, a leak-tight liner, penetration assemblies, hatches, airlocks, seals and isolation elements that collectively preserve the integrity and leak-tightness of the boundary.
The containment system is a broader functional concept. In addition to the structure, it includes the provisions needed to isolate containment, manage released mass and energy, limit radioactive releases and control combustible gases. Depending on the design, these may include isolation valves, containment spray systems, containment coolers, heat-removal systems, hydrogen recombiners and other systems.
The reactor building and other supporting structures provide space, access, maintenance capability, fuel and load handling, ventilation, fire protection and equipment protection. In some designs, they may also perform a secondary confinement or containment function, but this does not make them the primary containment pressure boundary.
The comparison that follows focuses mainly on the part of the containment system that receives the initial release from a damaged reactor coolant system and manages the associated increase in pressure and temperature.
PWR: a large volume around a distributed primary system
In a pressurised water reactor, or PWR, the water in the reactor coolant system remains liquid as it passes through the core because it is maintained at high pressure. It transfers heat to the steam generators, where steam for the turbine is produced on the secondary side.
The reactor coolant system is therefore not confined to the reactor vessel. It also includes the primary piping, reactor coolant pumps, the primary side of the steam generators and the pressurizer. These components are distributed around the reactor vessel and together form an extensive high-pressure system.
During a postulated loss-of-coolant accident, or LOCA, a large mass of coolant and its associated energy may be released from the damaged part of the system into containment. Some of the hot coolant flashes to steam as the pressure drops rapidly. Steam, droplets and hot surfaces then cause pressure and temperature to rise within the containment volume.
The containment structure, its penetrations and isolation elements must withstand the analysed loads. Energy-management systems must limit pressure and temperature and remove heat over the long term, including the decay heat that continues to be generated in the fuel after reactor shutdown.
In a large dry containment, the large free volume into which the reactor coolant system discharge expands provides an important part of the initial response. Structures and other internal surfaces also absorb heat temporarily. Depending on the design, containment spray systems, containment coolers or other heat-removal systems subsequently remove that heat.
The term dry does not mean that containment contains no water or water-based systems. It means that control of the initial pressure rise does not rely on routing released steam below the surface of a large water pool, as it does in BWR pressure-suppression containments.
This arrangement explains the characteristic appearance of many PWRs. Containment must enclose the reactor vessel, primary-system loops, steam generators, reactor coolant pumps and pressurizer, so the containment volume is large. The external protective structure may follow the inner pressure boundary so closely that, from the outside, the entire arrangement appears to be a single massive dome.
Yet external appearance does not tell the whole story, even for a PWR. The inner leak-tight shell, reinforced or prestressed concrete structure, any external shield building and other surrounding structures may perform different design functions. The Krško Nuclear Power Plant is a PWR and provides familiar Slovenian example of this spatial logic, but its configuration must not be generalised to all PWRs.
A specific PWR example: from KSNP+ to OPR-1000
A historical cutaway entitled KSNP+ Improved Korean Standard Nuclear Power Plant shows the design intended for Shin-Kori Units 1 and 2 and Shin-Wolsong Units 1 and 2. The 2003 poster uses the development designation KSNP+, while the IAEA PRIS database now identifies Shin-Kori Units 1 and 2 as OPR-1000 units.
The OPR-1000 is a two-loop pressurised water reactor. Its cutaway is useful because it shows several spatial and functional levels in a single image: the central containment building with the reactor system, the turbine building, auxiliary areas, fuel-handling areas and supporting systems.
The containment volume, heat transfer to internal structures, and the containment spray and cooling systems limit the increase in pressure and temperature. The prestressed-concrete shell carries the pressure loads, while the internal steel liner provides the leak-tight containment boundary.
The cutaway must nevertheless be read with discipline. The domed building indicates the general region of the containment system, but the illustration itself is not a licensing drawing of the containment pressure boundary. Determining the exact path of that boundary would require design documentation and information on the liner, penetrations, hatches, airlocks and isolation valves.
Naming note: Kori Unit 2 and Shin-Kori Unit 2 are not the same reactor unit or the same reactor design. Kori Unit 2 is an older two-loop Westinghouse PWR. Shin-Kori Units 1 and 2 are OPR-1000 units, while Shin-Kori Units 3 and 4 use the later APR-1400 design. A site name alone is therefore not sufficient when interpreting images and cutaways. The specific unit and reactor model must always be verified.
This example also demonstrates another important point: reactor standardisation does not erase the distinction between the containment function and the entire building. On the contrary, a carefully read cutaway makes that distinction visible.
BWR: the direct steam cycle changes the arrangement
In a boiling water reactor, or BWR, water boils inside the reactor vessel. Steam is separated from the water and dried in the upper part of the vessel before flowing through the main steam lines directly to the turbine. A conventional BWR therefore has no steam generators or pressurizer of the type used in a PWR.
This difference changes the spatial arrangement of the reactor coolant system. Much of the steam-generation and separation process occurs inside the reactor vessel. In Mark I and Mark II designs, the primary containment system can therefore be considerably more compact than a PWR large dry containment.
The direct steam path also means that the main steam lines and other piping cross the primary containment boundary. Their penetrations and their inboard and outboard isolation valves are therefore essential elements of the containment logic. If a path permits a release outside the credited boundary or bypasses the suppression pool, the issue is no longer merely one of shape. It becomes a potential containment bypass.
More compact does not mean less important. It means that released mass and energy are managed differently.
Mark I and Mark II: two volumes, one functional system
In a BWR Mark I, the primary containment system consists of the drywell and the wetwell, which contains the suppression pool.
The drywell houses the reactor vessel and associated parts of the recirculation system. The wetwell contains a large water pool, known as the suppression pool, and a gas space above the water surface. In Mark I, the wetwell has a characteristic annular shape and is commonly called the torus.
right: BWR Mark II. The drywell and the wetwell containing the suppression pool are arranged vertically in a common, more compact geometry. The pool is located below the drywell, and the released mixture is routed beneath its surface through vertical downcomers. The physical principle is the same as in Mark I, but the spatial arrangement, flow paths and hydrodynamic loads are different.
The drywell and wetwell are not two independent safety barriers. They are connected by a system of vents and downcomers and together form the primary containment system. Their relationship is functional: the first volume receives the initial release, while the second limits the pressure response by condensing steam.
Mark II uses the same basic physical principle but a different geometry. The suppression pool is generally located below the drywell and therefore does not form the distinctive separate torus associated with Mark I. The difference between Mark I and Mark II is not merely their silhouette, but also the arrangement of volumes, connecting paths and structural loads.
The pool is not an accessory but part of the pressure response
During a loss-of-coolant accident in a BWR Mark I or Mark II, a mixture of water and steam is released into the drywell. As drywell pressure rises, gases and steam are routed through the connecting system to outlets below the water surface in the wetwell.
When steam enters the cooler water, it condenses. Its specific volume decreases sharply and its energy is transferred to the pool water. Condensation therefore limits the pressure rise that would be substantially greater in the same small dry volume without a suppression pool.
Two paths that are often conflated in simplified diagrams must be distinguished.
During a LOCA in the drywell, released steam and gases are routed to the pool through the connecting system between the drywell and wetwell. When reactor safety-relief valves open, however, steam travels through separate discharge piping directly to outlets below the pool surface. Both paths use the condensation capacity of the suppression pool, but the flow path and initiating event are not the same.
The pool is not an infinite heat sink. Its water absorbs energy and heats up, while non-condensable gases accumulate in the wetwell gas space. To manage the event over the long term, residual heat-removal systems must cool the pool and maintain the required water inventory. Initial pressure suppression is therefore only the first part of the response.
The passage of gases and steam through the pool also produces hydrodynamic loads. Initial water clearing from the downcomers, bubble formation and collapse, condensation oscillations and pool swell can load structures, piping and equipment. The pool is therefore not simply a tank of water. It is an analysed part of the pressure, temperature and structural response of the containment system.
Nor is the pool itself the containment. The credited pressure boundary consists of the drywell, wetwell, their connections, structural shells, penetrations, hatches and isolation elements. The pool is a key functional component for managing energy, not the complete boundary.
Where, then, is the reactor building?
In BWR Mark I and Mark II plants, the primary containment system is generally located within a larger reactor building. This is one of the clearest examples of why the external building and the primary containment system are not the same thing.
The reactor building provides space for equipment, access, fuel handling, maintenance, ventilation and other operating functions. In many BWR designs, specified parts of the building are also credited as secondary containment.
Secondary containment is not another high-pressure version of primary containment. Its function is different. Following building isolation, an appropriate ventilation system can establish negative pressure so that leakage causes air to flow into the controlled area rather than allowing an uncontrolled outward release. The air is then routed through a prescribed path and, where required by the plant design, filtered before discharge.
The same reactor building can therefore perform several roles. During normal operation, it is part of the plant infrastructure. During specified events, it may support secondary containment and controlled air discharge. It is still not the primary containment pressure boundary that receives the initial release from a damaged reactor coolant system.
Mark III is not simply a larger Mark II
The designations Mark I, Mark II and Mark III do not describe three sizes of the same vessel.
Mark I is recognisable by its compact drywell and separate annular pressure-suppression chamber. In Mark II, the wetwell is generally located below the drywell. Mark III is a larger containment system with a different structural arrangement, although it also uses a water pool to condense steam and suppress pressure.
NRC data for U.S. designs indicate that the Mark III containment volume is approximately five times that of Mark I and approaches a substantial fraction of the volume of a PWR large dry containment. Mark III therefore does not fit the simplified assumption that every BWR has a small primary containment.
Unlike Mark I and Mark II, Mark III is generally not inerted with nitrogen and therefore uses hydrogen igniters for hydrogen control.
A technical explanation must always identify the generic containment design being discussed. The term BWR alone does not define containment geometry.
Where do VVERs fit into this comparison?
VVER is not a third thermohydraulic principle alongside PWR and BWR. VVERs are pressurised water reactors, but their design history includes substantially different approaches to containing the consequences of accidents.
Many VVER-1000 designs use a full-pressure containment that is broadly closer in principle to a large PWR containment system. The VVER-440/V213 uses a system of hermetic compartments and a bubble-condenser tower, in which water inventories contribute to steam condensation and pressure suppression. Earlier VVER-440/V230 designs used different accident-localisation and confinement systems that must not automatically be equated with a modern full-pressure containment.
After the check valves open, non-condensable gases and the remaining steam pass into the air traps. The valves then prevent reverse flow. In this way, the maximum pressure in the system of hermetic compartments and the bubble condenser is limited.
The designation PWR is therefore also insufficient for drawing conclusions from external appearance. VVER containment concepts require a separate discussion that distinguishes at least the V230, V213 and VVER-1000 variants and their subsequent safety upgrades. In this article, they serve primarily as a reminder that reactor family and containment-system type are not the same classification.
How to read a cutaway correctly
When reading a plant cutaway, first locate the reactor coolant system and ask where mass and energy would be released following a postulated break. Then follow the paths of steam, water and non-condensable gases.
Does the discharge remain within a large dry volume? Does a connecting system route it below the surface of a water pool? Which structure must contain the resulting pressure? Where are its penetrations, hatches and airlocks? Which valves isolate piping that crosses the boundary? Is there a path that could bypass containment? Which systems manage the initial pressure peak, and which remove heat over the long term?
Only after answering these questions can the primary containment boundary be identified and distinguished from the surrounding reactor building, secondary containment and supporting systems.
This method is more reliable than guessing from shape. In a PWR, a large dome may approximately follow the containment volume. In a BWR Mark I, the external reactor building may conceal a much smaller primary containment system composed of two functionally connected volumes. In a VVER-440/V213, the steam path may lead through a system of hermetic compartments to a bubble condenser.
In every case, the decisive factor is the path of mass and energy, not the external silhouette.
The same function does not require the same solution
PWRs and BWRs must preserve the containment function during an accident, but they do not manage the associated physical challenge in the same way.
In a typical PWR with a large dry containment, a substantial containment volume surrounds a spatially distributed reactor coolant system and receives its discharge. In BWR Mark I and Mark II designs, a more compact primary containment routes steam into a water pool, where condensation limits the pressure rise. The surrounding reactor building may provide infrastructure and, where credited by design, perform a secondary containment function.
None of these solutions is defined by building shape alone. Each results from the arrangement of components, analysed events, pressure and temperature loads, isolation requirements, allowable leakage rates, and the method selected for short-term and long-term energy removal.
A technical comparison of containment systems therefore does not begin with the dome. It begins with a question:
Where do mass and energy go when the reactor coolant system is no longer intact?
The answer reveals the location of the containment boundary, the systems that support it and the reason for its shape.
In the next part, we will examine the same event as a sequence in time: from the initial release during a loss-of-coolant accident to the pressure rise, heat transfer and long-term cooling of the containment system.
Terminology note
The terms drywell and wetwell describe specific functional parts of a BWR containment system. The drywell receives the initial release from the reactor coolant system. The wetwell contains the suppression pool and the gas space above it.
In this article, pressure suppression refers to limiting containment pressure through steam condensation in the suppression pool. It should not be confused with controlled containment venting.
The suppression pool is the water inventory used to condense routed steam and thereby contribute to pressure suppression.
Image and material notes
Hero image: Korea Kori NPP / IAEA Imagebank, via Wikimedia Commons, CC BY-SA 2.0. Crop and colour adjustment by By the Protocol. The adapted image is published under CC BY-SA 2.0.
KSNP+/OPR-1000: Detail from a cutaway of the KSNP+ design for Shin-Kori Units 1 and 2 and Shin-Wolsong Units 1 and 2. Source: Nuclear Engineering International, produced in cooperation with KOPEC. Design and illustration: Mahjong Partnership, 2003.
BWR Mark I, Mark II and Mark III: Historical schematic cutaways of General Electric containment designs. Used as comparative technical illustrations; original authorship and rights remain with General Electric.
VVER-440/V213: Adapted and translated diagram of a bubble-condenser containment system. Adapted from Blinkov, V. N., et al., Experimental Studies for the VVER-440/213 Bubble Condenser System for Kola NPP at the Integral Test Facility BC V-213, 2012, Figure 1, CC BY 3.0. Graphic and language adaptation by By the Protocol.
The text, editorial work and original graphic adaptations specifically identified as such are the work of Elite Studio 3D / By the Protocol. Copyright in external visual material remains with the respective authors and rights holders.
Sources for technical review
- IAEA SSG-53: Design of the Reactor Containment and Associated Systems for Nuclear Power Plants
- U.S. NRC: Issue 157, Containment Performance
- U.S. NRC: Issue 61, SRV Line Break Inside the BWR Wetwell Airspace of Mark I and II Containments
- U.S. NRC: Generic Letter 79-13, Suppression Pool Temperature Limits
- IAEA: Advanced PWR Simulator based on the OPR-1000
- IAEA PRIS: Shin-Kori 1, OPR-1000
- IAEA PRIS: Shin-Kori 2, OPR-1000
- IAEA: Development of New Nuclear Power Plants in the Republic of Korea, KSNP+
- IAEA INIS: Containment and Confinement Performance in NPPs with WWER-440/213 and WWER-440/230 Reactors
- Krško Nuclear Power Plant: Containment
bubbler condenser BWR Containment containment design drywell pressure suppression PWR reactor building VVER wetwell
Last modified: September 7, 2026
