In Part 2, we compared different spatial configurations. Now we look at the same event as it unfolds over time.
During a loss-of-coolant accident, or LOCA, the containment system is not merely an empty volume passively waiting to receive a release. It becomes a thermodynamic system. Within seconds and minutes, water, steam, air and energy are redistributed; structures and equipment heat up; steam condenses; water collects at the lowest points; and safety systems must turn the initial response into a condition that can be managed over the long term.
This is not a single pressure pulse represented by a single curve. It is a sequence of physically distinct phases whose significance depends on the location and size of the break, the plant’s initial state, the volume and configuration of the containment system, and the heat-removal systems available.
A LOCA Is Not a Single Accident
The term LOCA describes a loss of coolant from the pressure boundary of the reactor coolant system. By itself, however, it does not define the size or location of the break, or the rate of release. A small leak, a break in an auxiliary line and a postulated large break in a main coolant line produce very different system blowdown transients.
In a larger break, hot, high-pressure coolant flows into a lower-pressure space. Part of the water flashes to steam as a result of the sudden pressure drop. The break therefore releases not only liquid, but a two-phase jet of water and steam with high momentum and enthalpy. During the first moments, the transient is governed by reactor coolant system blowdown, flashing and steam expansion. Later, condensation, heat transfer to structures, decay heat, and the operation of core and containment cooling systems become increasingly important.
An important distinction. Mass and energy release analysis describes the thermal-hydraulic discharge from the damaged system. The radiological source term describes the quantity and isotopic composition of radionuclides, the timing of their release, and their physical and chemical forms. The two are related, but they are not the same analysis.
The Break Releases More Than Mass
If containment received only a given mass of cold water, the problem would be considerably simpler. During a LOCA, however, the coolant carries energy as well as mass, and steam occupies a much larger volume than liquid water. The relationship between the released mass, its energy and the available free volume determines the initial pressure response.
Part of the energy is consumed in vaporising water and heating the atmosphere. Some is absorbed by steel and concrete structures, piping, equipment, and water on the floor or in a pool. Steam condenses on cooler surfaces and releases latent heat. At the same time, containment compartments do not necessarily behave as a single volume that is instantaneously and perfectly mixed: the jet may create locally high temperatures, pressure differentials and loads before the atmosphere becomes sufficiently mixed.
The maximum pressure and maximum temperature are therefore not necessarily reached at the same time or in the same location. In addition to the overall response of the containment volume, design analyses address local compartments and flow paths, and assess pressure differentials and the environmental conditions that safety-significant equipment must withstand.
The First Seconds: Release and Pressure Rise
In a typical large dry PWR containment, the two-phase release expands into a large containment volume. The larger free volume limits the pressure rise, while internal surfaces temporarily absorb part of the heat. Their heat capacity is important during the initial response, but it does not constitute a long-term heat sink. Concrete, steel and water heat up during the event, so their capacity to continue absorbing heat changes over time.
In BWR Mark I and Mark II containments, the path is different. A release into the drywell increases the pressure, driving the mixture of steam and gases through connecting flow paths and below the water level in the wetwell suppression pool. The steam condenses, its volume decreases sharply, and its energy is transferred to the water. The pool therefore limits the initial pressure rise, but it heats up in the process, while non-condensable gases remain in the gas space above the water level.
The VVER-440/V213 uses a third spatial arrangement. From the hermetic compartments, the mixture is directed towards the bubbler condenser tower, where it passes through water trays on successive levels. Steam condensation limits the pressure, while non-condensable gases collect in designated air traps. The physical principle resembles pressure suppression by water in a BWR, but the structures and flow paths are not the same.
Three architectures, one question. The large dry PWR volume receives the release; the BWR directs steam into a suppression pool for condensation; and the VVER-440/V213 routes it through the levels of a bubbler condenser. None of these solutions makes the energy disappear. Each simply determines where it is stored temporarily and how it is subsequently removed.
The First Pressure Peak Is Not the End of the Event
Once the initial blowdown subsides, some of the energy has already been transferred from the reactor coolant system to the containment system, but the event is not over. The core continues to generate decay heat after shutdown, while heated metal masses and coolant release the energy already stored within them. Water in the containment sump, suppression pool or other inventories grows warmer, while structures first absorb energy and may later release it back into the atmosphere.
The long-term challenge is therefore not limited to preventing the initial design pressure and temperature from being exceeded. A stable heat-transfer path must be established from the core and containment system to the ultimate heat sink. Depending on the design, this path may include emergency core cooling, the containment sump, pumps, heat exchangers, service-water systems, containment coolers or suppression-pool cooling.
If water is merely circulated within the containment system without effective heat transfer to an external sink, energy is redistributed rather than removed. After an initial decrease, the pressure may stabilise at an excessively high level or begin to rise again. This is why regulations and design criteria distinguish between rapidly reducing pressure and temperature and maintaining them at acceptably low levels over the long term.
What the Containment Spray System Actually Does
In many PWRs, the containment spray system is one of the systems used to mitigate the consequences of a LOCA. Nozzles in the upper part of the containment produce a large number of fine droplets. These cooler droplets provide a large interfacial area for heat and mass transfer with the atmosphere. Steam therefore condenses and the atmosphere cools. As the steam content and temperature decrease, the pressure also falls.
The water then falls into the sump at the bottom of the containment. During the initial phase, the system may draw from a dedicated water supply; in the long-term phase, where provided by the design, it may be realigned to recirculate water from the sump. For this to constitute actual heat removal rather than the mere circulation of hot water, the flow path must provide cooling through a heat exchanger or another engineered heat sink.
The containment spray system may also perform a radiological function. Droplets remove aerosols from the atmosphere, while the removal efficiency for certain forms of iodine also depends on the chemical composition of the solution and the conditions inside containment. This function must be distinguished from the thermodynamic effect of steam condensation: the same system may contribute to both, but each function is demonstrated against its own analyses and criteria.
The containment spray system does not directly cool the core. Nor is it a universal feature of every containment system, and it is not always the principal mechanism for limiting the initial pressure rise. In BWR Mark I and Mark II containments, this function is performed primarily by the suppression pool; in other designs, containment coolers, passive heat transfer or other systems may play an important role.
The sprayed water also affects the rest of the system. It changes the temperature and humidity of the atmosphere, washes material from surfaces, increases the amount of water in the sump, and affects conditions at the pump suction. Water inventories, chemistry, filters and strainers, available net positive suction head, heat exchangers, electrical power, and the transition to recirculation are therefore all part of the same functional chain.
The Analysis Does Not Describe an Average — It Defines the Limiting Case
The design pressure–temperature response is not a prediction of the most likely transient. Its purpose is to demonstrate that the containment boundary and safety-significant equipment can withstand the analysed conditions with appropriate margins. Initial conditions, the break, and combinations of available equipment are therefore selected to establish a conservative or limiting case for each acceptance criterion.
The assessment addresses, among other factors, mass and energy release, flashing and condensation, heat transfer to structures, the operation of heat-removal systems, local pressures and temperatures, the availability of required flow paths, and environmental conditions for equipment. For water-based systems, important considerations also include sump or pool heat-up, pressure losses, pump suction conditions, and the long-term capability to transfer heat to the ultimate heat sink.
A single smooth pressure curve can therefore create a misleading impression of simplicity. Behind it lie a containment model, geometric and material data, time-dependent releases, heat-transfer models, assumptions concerning mixing and condensation, and the specific configuration of safety systems. The result applies to the analysed design and its licensing basis, not to every plant within the same reactor family.
Reducing Steam Pressure Is Not the Same as Managing Hydrogen
During an accident, the containment atmosphere may contain steam, air or an inert gas, as well as other gases. Their partial pressures add up to the total pressure, but this does not mean that they all have the same safety significance or are managed by the same system.
Steam condensation can rapidly reduce the steam contribution to pressure. Hydrogen does not condense under the same conditions. Its generation, mixing, local accumulation and potential combustion therefore require different measures and criteria. The containment spray system may affect atmospheric temperature and mixing, but that alone does not make it a hydrogen-management system.
Part 4 of this series will address this subject: Hydrogen Is Not Just a Question of Pressure
Containment Is a Process, Not Just a Structure
During a LOCA, the pressure boundary of the reactor coolant system is breached, and the containment system assumes the next level of protection. It must do more than preserve its integrity: it must receive the release, limit the initial rise in pressure and temperature, manage local loads, collect water, support emergency core cooling, and remove heat over the long term.
The large dry PWR dome, the BWR drywell and wetwell, and the VVER-440/V213 hermetic compartments with a bubbler condenser are different responses to the same balance. Mass does not disappear. Energy does not disappear. Both must be given a controlled path.
The most important question after a break is therefore not simply how much pressure the wall can withstand. It is whether the entire chain, from the release compartment to the ultimate heat sink, remains intact for long enough to bring the plant to a stable and safe state.
Terminology and Editorial Note
In the Slovenian version, containment spray system is translated as sistem za prhanje zadrževalnega hrama, in accordance with the Slovenian Nuclear Society’s ePojmovnik terminology database. Brizgalni sistem is understandable, but it is not the preferred term used in this article.
This article describes the physical and functional logic of containment systems. It does not replace the safety analysis report, design analyses or licensing documentation for any specific plant.
Image Credits
Header image: iStock, used under the appropriate commercial licence.
The diagrams and system illustrations are original works by Elite Studio 3D / By the Protocol and were created specifically for this article as simplified conceptual representations of physical processes and functional relationships. They do not constitute technical documentation, design drawings or the actual configuration of any specific nuclear power plant.
Sources and Technical Documentation
- IAEA, SSG-53: Design of the Reactor Containment and Associated Systems for Nuclear Power Plants
- IAEA, SSR-2/1 (Rev. 1): Safety of Nuclear Power Plants: Design
- OECD/NEA, Bubbler Condenser Related Research Work, NEA/CSNI/R(2001)3
- U.S. NRC, NUREG-0800, Chapter 6: Engineered Safety Features
- U.S. NRC, Appendix A to 10 CFR Part 50, General Design Criterion 38: Containment Heat Removal
- U.S. NRC, Generic Letter 96-06: Assurance of Equipment Operability and Containment Integrity During Design-Basis Accident Conditions
- U.S. NRC, Item C-10: Effective Operation of Containment Sprays in a LOCA
- U.S. NRC, Generic Letter 97-04: Assurance of Sufficient Net Positive Suction Head for Emergency Core Cooling and Containment Heat Removal Pumps
- Krško Nuclear Power Plant: Containment
- Krško Nuclear Power Plant: Nuclear Safety
accident management Containment containment spray system heat removal LOCA pressure and temperature PWR recirculation Safety Systems ultimate heat sink
Last modified: September 7, 2026
