Drawing No. EH–NR–003 // Nuclear Engineering & Radiation

Basic BWR Simulator

Reviewed August 2026

BWR means Boiling Water Reactor. In this type of nuclear power plant, water boils inside the reactor vessel. The steam goes directly to a turbine, the turbine turns a generator, and the steam is then cooled back into water and returned to the reactor. Use this simulator to see that cycle in action and learn how the plant reacts when something changes.

Beginner friendly no reactor experience needed One main control choose reactor power Live physics not scripted animations 9 experiments normal and failure conditions

Start here: three ideas to know

About 60 seconds

The reactor makes heat

Atoms of uranium split in the fuel. This process is called nuclear fission. The released energy heats the water flowing through the reactor core.

Steam carries the energy

Water boils inside the reactor vessel. The steam flows to a turbine, which turns an electrical generator. A condenser then turns the steam back into water.

Cooling is needed after shutdown

Stopping fission quickly reduces reactor power, but radioactive material in the fuel continues to produce decay heat. Water must keep covering and cooling the fuel.

How to use the page: first change reactor power and watch the diagram. Then try one failure at a time. You do not need to operate individual pumps or valves—the simulator handles the normal automatic controls for you.

1 Choose reactor power

Your main control

The experiment starts with the plant at full power. Move the slider to ask for less or more power. Automatic systems gradually move the control rods, change water flow through the core, adjust the turbine and keep reactor water level and pressure near normal.

Requested reactor power100%

100%

Try 50%. Watch reactor heat output fall first, then notice how steam flow, electricity production and core water flow follow.

SCRAM is the common nuclear-industry word for a rapid reactor shutdown. The control rods insert quickly into the core to stop the fission chain reaction.

2 Watch these six indicators

What is changing?

Reactor heat output100.0%

How much thermal power the reactor core is producing compared with full power.

Electricity output1350MW

Electrical power from the generator. MW means megawatt: one million watts.

Pressure inside reactor7.17MPa

This is absolute pressure. High pressure lets reactor water boil at a much higher temperature than water in an open pot.

Water level+0.00m

A teaching indication of reactor-vessel water level. Keeping the fuel covered with water is essential for cooling.

Water flow through core100%

Relative coolant flow through the fuel. In a BWR, flow also changes how many steam bubbles form in the core.

Pressure around reactor vessel0.0kPa

Pressure rise in the surrounding containment space. A coolant leak can make this value increase above normal atmospheric pressure.

3 See what automation is doing

Normally automatic

Control rods84.8% withdrawn

Neutron-absorbing blades. Moving them farther into the core reduces fission power.

Recirculation pumps10 of 10 running

Internal pumps move water through the core. More flow usually means fewer steam bubbles and more reactor power.

Emergency coolingStandby

Backup water systems that automatically help maintain core cooling when normal water supply is not enough.

4 Follow the energy through the plant

Orange = steam · blue = water · green = backup cooling

Interactive process steps: select buttons 1–5 on the diagram (or the cards below) to trace how heat becomes electricity and how backup cooling supports the reactor.

1Water boils

Heat from nuclear fission produces steam inside the reactor vessel.

2Steam turns the turbine

The turbine spins the generator, producing electricity.

3Steam is cooled

The condenser removes heat and turns exhaust steam back into liquid water.

4Water returns

The feedwater pump sends the water back to the reactor so the cycle can repeat.

5Backup cooling can inject water

During some failures, automatic safety systems supply additional water to protect core cooling.

Plant trend

What changed over time?

This graph overlays three different indicators on one 0–120 teaching scale so you can compare their direction and timing. It is for trends, not for reading exact engineering values.

Reactor powerReactor pressureWater-level index

5 Introduce a failure

One at a time

Choose a failure and watch what happens next. The failure buttons do not play a fixed animation: they change the plant condition and the same reactor model calculates the response.

What should I watch?

For every experiment, look first at reactor power, then pressure and water level. Finally check whether the reactor has shut down and whether backup cooling has started.

Plant condition

Automatic warnings

No active warnings. The plant is operating normally.

What just happened?

Newest event first

Four good first experiments

Suggested order

Set power to 50%. See normal automatic control.
Stop 3 recirculation pumps. Notice that power falls without an immediate shutdown.
Stop feedwater. Watch water level and backup cooling.
Try station blackout. Compare normal electrically powered equipment with steam-driven backup cooling.

Reactor controls

FMCRD / neutron control

Reactor mode switchSHUTDOWN

Aggregate FMCRD withdrawal0.0%

Automatic power control

Emergency reactivity actions

Rod withdrawal blockCLEAR

Reactor recirculation

10 reactor internal pumps

RIP speed command0%

Automatic flow/power control

RIP speed changes core flow and void fraction. Increasing flow generally reduces voids and inserts positive reactivity; decreasing flow does the opposite.

Pressure & steam

pressure regulator / bypass

Pressure setpoint0.10 MPa(a)

Automatic pressure control

Turbine control valve0%

Turbine bypass valve0%

Main steam isolation valvesOPEN

Feedwater & level

three-element educational model

Level setpoint0.0 m

Automatic level control

Feedwater command0%

Turbine-generator

HP / MSR / LP train

Grid frequency50 Hz

Electrical load demand0 MWe

ECCS & containment

3 safety divisions

Automatic ECCS logic

RCIC0 kg/s

HPCF B0 kg/s

HPCF C0 kg/s

LPFL A0 kg/s

LPFL B0 kg/s

LPFL C0 kg/s

Total ECCS water injection0 kg/s

Off-site ACAVAILABLE

Safety ACAVAILABLE

DC battery model24.0 h

Wetwell30.0°C

Failure scenarios

Inject / clear

Plant process mimic

ABWR-style mass, energy & safety paths

Plant trends

Last 15 simulated minutes

PowerPressureLevel indexCore flowElectrical

Normalized plant overview0–120% display scale

Live instrumentation

Reduced control-room view

Active alarms

0 active

Operator event console

Newest first

Startup procedure trainer

Educational workflow · left-to-right sequence

Guided startup automates this simplified teaching sequence. It is not an operating procedure.

Diagnostics & engineering views

See how forced-flow BWR physics changes the plant

These views link operator actions to simplified core behaviour. They are conceptual educational displays, not replicas of proprietary ABWR core-monitoring or protection systems.

Conceptual 872-assembly core power map

Stable radial power shape with local control-rod suppression and global void/flow feedback. Geometry is deliberately schematic.

Power / core-flow operating map

Public ABWR documentation defines the normal operating area using minimum-pump-speed, constant-speed, steam-separator, natural-circulation and rod-pattern/flow-control boundaries. The redrawn chart separates the operating envelope from startup/low-flow and avoid regions, with continuous boundary lines identified in a clean legend below the plot. Coordinates remain educational approximations, not plant limits.

Reactivity balance

Approximate contributions from FMCRDs, voids, fuel temperature, moderator, xenon and standby liquid control.

ECCS and heat-removal margin

Compares decay heat and inventory support with condenser/RHR status and active RCIC, HPCF and LPFL functions.

Learn the plant

How a boiling water reactor works

The simulator is easier to understand if you know what each part is doing. These explanations use the same ideas shown in the live diagram above.

Fission heats the fuel

Inside the reactor core, uranium atoms split. Each split releases energy and neutrons. Some of those neutrons cause more atoms to split, creating a controlled fission chain reaction.

Water boils in the reactor

Water flows upward through the hot fuel. Part of it becomes steam. Unlike a pressurized water reactor, a boiling water reactor sends this steam directly to the turbine.

The turbine makes mechanical power

High-pressure steam expands through turbine blades and turns the turbine shaft. The shaft drives the generator, which converts rotation into electricity.

The condenser recycles the steam

After leaving the turbine, steam enters the condenser. Cooling water on the other side of metal tubes removes heat, turning the steam back into liquid water.

Feedwater returns to the reactor

Pumps send the condensed water back to the reactor. This returning water is called feedwater. During normal operation, feedwater flow is automatically adjusted to control reactor water level.

How reactor power is controlled

Control rods absorb neutrons. Inserting them farther into the core reduces the chain reaction; withdrawing them allows power to increase.

A BWR can also adjust recirculation flow. More water flow through the core tends to reduce steam bubbles and increase nuclear power. Less flow tends to increase bubbles and reduce power.

Why steam bubbles affect power

A steam bubble contains much less liquid water than the same volume of water. Liquid water helps slow neutrons to energies that efficiently sustain fission in this reactor design. More bubbles therefore usually reduce reactor power.

The percentage of coolant volume occupied by steam is called the void fraction.

What happens during a SCRAM?

A SCRAM is a rapid automatic or manual reactor shutdown. Control rods insert quickly and the fission chain reaction falls to a very low level.

Heat does not instantly become zero. Radioactive fission products continue to release decay heat, so the plant must keep removing heat after shutdown.

Why reactor water level matters

Water removes heat from the fuel. During normal operation, automatic feedwater control keeps the indicated level near its target. A loss of normal feedwater makes level fall and can trigger reactor protection and backup water injection.

The simulator's displayed level is a simplified teaching coordinate, not a plant operating procedure setpoint.

What is the suppression pool?

The suppression pool is a large body of water inside the containment. If safety or relief valves discharge steam, that steam can be routed below the water surface. Condensing the steam helps limit pressure.

This water can also serve as an important source for emergency cooling systems.

What is emergency core cooling?

Emergency Core Cooling System (ECCS) is a general name for several backup systems that help provide water to the reactor during abnormal conditions.

Some systems can add water while reactor pressure is still high; others work after the reactor has depressurized. The Basic simulator operates these systems automatically.

Plain-language glossary

Terms and abbreviations used on this page

You do not need to memorize these. This section is here whenever a nuclear term is unfamiliar.

BWR — Boiling Water Reactor

A reactor in which water boils inside the reactor vessel and the steam goes directly to the turbine.

ABWR — Advanced Boiling Water Reactor

A modern large BWR design used as the public reference plant for this educational model.

Reactor core

The region containing nuclear fuel where fission produces heat.

Control rod

A neutron-absorbing blade used to control or rapidly stop the fission chain reaction.

Reactor vessel

The strong steel vessel containing the core, high-pressure water and steam.

Recirculation pump

A pump that moves reactor water through the core. The reference design uses internal recirculation pumps.

Feedwater

Water returned from the condenser to the reactor vessel.

Condenser

A heat exchanger that cools turbine exhaust steam back into liquid water.

Condenser vacuum

Very low pressure maintained at the turbine exhaust. A good vacuum helps the turbine produce power efficiently.

SCRAM

A rapid reactor shutdown in which control rods are inserted to stop the fission chain reaction.

Decay heat

Heat that continues after shutdown because radioactive fission products keep decaying.

ECCS — Emergency Core Cooling System

The group of backup systems that can supply reactor cooling water during abnormal conditions.

RCIC — Reactor Core Isolation Cooling

A steam-driven system that can supply water to the reactor when normal feedwater is unavailable.

HPCF — High Pressure Core Flooder

An electrically powered system that can inject cooling water while reactor pressure is relatively high.

LPFL — Low Pressure Flooder

A high-flow cooling function used after reactor pressure has been reduced sufficiently.

ADS — Automatic Depressurization System

A system that can open valves to reduce reactor pressure so low-pressure cooling can inject water.

SRV — Safety/Relief Valve

A valve that can release reactor steam to protect against excessive pressure.

LOCA — Loss-of-Coolant Accident

The engineering term for a leak or pipe break that causes reactor coolant to escape.

Station blackout

Loss of normal alternating-current electrical power and emergency alternating-current power at the plant.

Containment / drywell

The pressure-resistant structure and space surrounding the reactor vessel and primary piping.

Suppression pool

A large water pool in containment used to condense discharged steam and support safety functions.

Void fraction

The fraction of coolant volume occupied by steam bubbles rather than liquid water.

Negative void feedback

The natural BWR tendency for more steam bubbles in the core to reduce nuclear power.

MW, MWth and MWe

MW means megawatt. MWth means megawatts of heat; MWe means megawatts of electricity.

MPa, kPa and psi

Pressure units: megapascal, kilopascal and pounds per square inch. “Absolute” pressure is measured from a perfect vacuum; “above atmosphere” is measured relative to surrounding air pressure.

For curious readers: what is happening underneath the simple interface?

The visible controls are simple, but the reactor calculation is not a fixed animation. The model solves a reduced-order dynamic representation of the neutron population, reactor heat, decay heat, steam formation, water inventory, pressure, recirculation flow, turbine response, containment pressure and automatic safety actions. It includes delayed neutrons—a small fraction of neutrons emitted after a short delay by fission products—which are essential to controllable reactor dynamics.

Point kinetics is the method used to calculate how the overall neutron population changes with time. Reactivity describes whether the chain reaction tends to grow or shrink. The model also includes simplified fuel-temperature feedback (often called Doppler feedback, where hotter fuel tends to reduce reactivity), coolant/steam-bubble feedback and a slow xenon-135 effect from a neutron-absorbing fission product. These are useful for education but do not replace the detailed three-dimensional analysis used for real nuclear-plant design and safety work.

Common questions

BWR questions for first-time learners

Is the water inside the reactor really supposed to boil?

Yes. That is the defining feature of a Boiling Water Reactor. Water boils as it passes upward through the reactor core, and the resulting steam is separated and dried before going to the turbine.

Does the turbine control the nuclear chain reaction?

Not directly. The reactor's nuclear power is controlled mainly by control rods and core coolant flow. The turbine controls how much steam energy is converted to mechanical and electrical power. Because the reactor and turbine are connected by the steam system, a sudden turbine problem can change reactor pressure and trigger automatic protection.

Why can stopping recirculation pumps reduce reactor power?

Lower coolant flow creates more steam bubbles in the core. Steam is much less effective than liquid water at slowing neutrons. In this BWR design, more bubbles therefore reduce the fission rate. This useful self-stabilizing effect is called negative void feedback.

If SCRAM stops fission, why does the reactor still need cooling?

After a rapid shutdown, the main chain reaction collapses quickly, but radioactive fission products continue to decay and release heat. This decay heat falls with time but is initially large enough that continued water inventory and heat removal are essential.

What is the difference between feedwater and emergency cooling water?

Feedwater is the normal water returned from the condenser during everyday operation. Emergency core cooling is backup water supplied by safety systems when normal water supply is insufficient or unavailable.

Why can indicated water level move strangely during pressure changes?

Boiling water contains many steam bubbles. When pressure falls, bubbles can expand and make indicated water level temporarily rise; this is often called swell. When pressure rises, bubbles collapse and indicated level can temporarily fall, called shrink. The simulator includes a simplified version of this behaviour.

Is this a real nuclear operator-training simulator?

No. It is an educational browser model designed to teach cause and effect. Real operator-training simulators reproduce a specific plant, its control systems, procedures, setpoints and detailed equipment behaviour under a controlled training programme.

Next learning step

Ready to operate individual plant systems?

This Basic simulator is designed to answer “what happens, and why?” The Advanced BWR Simulator adds individual reactor-control, recirculation, steam, feedwater, turbine-generator and emergency-cooling controls after you are comfortable with the terms used here.

Open Advanced BWR Simulator →

Optional technical reading

Where the simulator numbers come from

You can use the simulator without reading this section. It is included so students and technical readers can see which parts are tied to public reactor information and which parts are simplified for browser-based education.

Public reference-plant values used by the model

The model uses the Advanced Boiling Water Reactor (ABWR) as its public reference design. Important reference points include approximately 3,926 megawatts of thermal heat (MWth), about 1,350 megawatts of electricity (MWe), operating pressure near 7.17 megapascals absolute, 10 internal recirculation pumps, 872 fuel assemblies, and 205 control rods. Public documents also give core coolant flow of roughly 52,200 tonnes per hour and steam flow of roughly 7,640 tonnes per hour.

These values anchor the normal full-power state. They do not imply that this browser model reproduces proprietary plant setpoints or vendor safety-analysis software.

How the browser model is simplified

The reactor core is represented as one overall dynamic region rather than thousands of separate fuel locations. Overall neutron behaviour is calculated with a six-group point-kinetics model, which is a standard educational way to represent prompt and delayed neutron response without solving a full three-dimensional neutron field.

Water mass, steam production, reactor pressure, water level, turbine response and containment pressure use reduced-order balances and calibrated correlations. The displayed water-level coordinate is for teaching trends and is not a real plant operating setpoint.

How the failure behaviour was checked

The most useful public transient benchmark is loss of recirculation pumps. Public United Kingdom regulatory assessment material describes a three-pump trip reaching a new equilibrium near 80% reactor power without a reactor shutdown. The simulator settles close to that behaviour. The same public material describes loss of all internal recirculation pumps producing a rapid-flow shutdown signal at about 2 seconds, which the simulator also reproduces.

Other beginner scenarios are checked mainly for correct physical sequence: loss of feedwater lowers reactor water inventory and calls for protection and makeup water; condenser degradation raises turbine exhaust pressure and eventually trips the turbine/reactor; loss of electrical power removes normal pumps; coolant leaks raise containment pressure, reduce inventory and call for emergency cooling. Exact timings for these simplified cases should not be interpreted as plant predictions.

Important limitations

This tool is for education. It does not calculate detailed three-dimensional power distribution, local fuel-cooling margins, the highest fuel-cladding temperature, radiological releases, severe-accident progression or licensing safety margins. It must not be used for nuclear-plant operation, operator training, safety analysis or engineering decisions.

Primary public sources

United States Nuclear Regulatory Commission — Advanced Boiling Water Reactor design certification

Hitachi-GE — introduction to the United Kingdom Advanced Boiling Water Reactor

United Kingdom Advanced Boiling Water Reactor Pre-Construction Safety Report — reactor coolant and reactivity control systems

United Kingdom Office for Nuclear Regulation — fault-studies assessment for the Advanced Boiling Water Reactor