How Microreactors Work: Interactive 3D Tour | Microreactors.org

Building the plant

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Guide

How a sodium-cooled microreactor works, and how it powers a data center

A microreactor is a small nuclear reactor, usually under 20 megawatts of electricity, that a factory builds and a truck delivers. This model shows a representative sodium-cooled, pool-type design of about 10 MWe, the building around it, the switchyard and transmission line, and the data center it powers.

Liquid sodium carries heat from a compact core to heat exchangers at near atmospheric pressure. Control drums in the reflector set the power. Shutdown rods and passive air cooling keep the core safe with no pumps and no operator action. The whole module ships on one skid and sets into a concrete shield silo.

Use the 3D model to cut the reactor open, explode it, search every part, run the start-up sequence, and change the physics live. The guide below lists every labeled component with its role and typical specifications.

Reactor core

Reactor core

The core is the compact region where fission takes place. Fuel assemblies sit in a tight hexagonal lattice, and the reflector surrounds them.

Thermal power: ≈ 30 MWt
Active height: ≈ 1.0 m
Diameter: ≈ 1.2 m
Refueling: Whole-core swap every 5–10 years

Fuel assemblies

Each assembly is a hexagonal steel duct that holds a bundle of fuel pins. Sodium flows upward through the duct and carries the heat away.

Count: 61
Duct material: HT-9 ferritic steel
Lattice pitch: ≈ 135 mm
Pins per assembly: 91, wire-wrapped

Fuel pins

Fuel pins are sealed steel tubes filled with uranium fuel pellets. A wire wrapped around each pin keeps the pins apart and lets sodium flow between them.

Fuel form: U-Zr metal or UO₂, HALEU (< 20% U-235)
Cladding: HT-9 steel
Pin diameter: ≈ 9 mm
Peak cladding temp.: ≈ 650 °C

Neutron reflector

The reflector ring surrounds the core and bounces escaping neutrons back in. It lets a small core sustain a chain reaction with less fuel.

Material: Beryllium oxide or graphite in steel cans
Radial thickness: ≈ 280 mm
Height: ≈ 1.1 m
Also houses: 12 control drums

Control drums

Control drums are rotating cylinders inside the reflector. One face is a neutron absorber: turned toward the core it lowers power, turned away it raises power.

Count: 12
Absorber: Boron carbide (B₄C) sector, 120°
Diameter: ≈ 220 mm
Drive: Stepper motor, 0–180° rotation

Shutdown rods

Shutdown rods drop into the core to stop the chain reaction. Gravity inserts them when the holding magnets lose power, so the system needs no operator action.

Count: 3
Absorber: Boron carbide (B₄C)
Insertion time: < 2 s
Hold: Electromagnetic latch, fail-safe

Primary system

Reactor vessel

The reactor vessel is a stainless-steel tank that holds the core and the sodium pool. It operates near atmospheric pressure because sodium boils at 883 °C, far above operating temperature.

Material: 316H stainless steel
Height: ≈ 5.5 m
Diameter: ≈ 2.4 m
Design pressure: Near atmospheric

Guard vessel

A second steel vessel surrounds the reactor vessel with a small gap. If the inner vessel leaks, the guard vessel catches the sodium and keeps the core covered.

Gap: ≈ 150 mm, argon filled
Material: Stainless steel
Leak detection: Gap sensors and level probes
Function: Second barrier for sodium

Sodium coolant pool

Liquid sodium fills the vessel and carries heat from the core to the heat exchangers. It conducts heat very well and stays liquid from 98 °C to 883 °C.

Core inlet: ≈ 400 °C
Core outlet: ≈ 550 °C
Inventory: ≈ 20 tonnes
Cover gas: Argon at ~1 atm

Electromagnetic pumps

EM pumps drive sodium with magnetic fields instead of impellers. They have no moving parts, no seals, and nothing to wear out.

Count: 2
Type: Annular linear induction
Flow: ≈ 150 kg/s each
Backup: Natural circulation

Intermediate heat exchangers

The IHX moves heat from the radioactive primary sodium to a clean intermediate loop. Thousands of thin tubes give the two fluids a large contact area without mixing.

Count: 2
Type: Shell-and-tube, counter-flow
Duty: ≈ 15 MWt each
Secondary fluid: Sodium or molten salt

Reactor head & drives

The head closes the vessel and carries every penetration. Rod drives, instrument lines, and pipe nozzles pass through it, and a rotating plug gives access for refueling.

Deck thickness: ≈ 300 mm steel
Rotating plug: ≈ 1.8 m diameter
Penetrations: Rod drives, IHX nozzles, instruments
Shield plug: Borated steel and concrete below deck

Safety & shielding

Decay heat removal

After shutdown the fuel still produces a few percent of full power. Air ducts around the guard vessel let natural convection carry that heat to a chimney with no pumps and no power.

Type: Reactor vessel auxiliary cooling (RVACS)
Capacity: ≈ 1 MWt
Driving force: Natural draft
Availability: Always on, passive

Radiation shielding

Thick concrete and steel around the vessel absorb neutrons and gamma rays. Workers can approach the building while the reactor operates.

Material: Reinforced concrete and borated steel
Radial thickness: ≈ 1.4 m
Top shield: Rotating shield plug
Design goal: Public dose limits met at site boundary

Instrumentation

Neutron detectors, thermocouples, and sodium level sensors report on the core in real time. The control system uses these readings to adjust the drums automatically.

Neutron detectors: 4 ex-core fission chambers
Temperature: ≈ 200 thermocouples
Leak detection: Guard-vessel gap sensors
Operation: Autonomous with remote monitoring

Power conversion

Intermediate loop piping

Insulated pipes carry the clean intermediate fluid from the IHX to the power conversion unit and back. The loop keeps radioactive sodium inside the reactor vault.

Pipes: 2 hot legs, 2 cold legs
Hot leg: ≈ 520 °C
Cold leg: ≈ 370 °C
Insulation: Mineral wool with steel jacket

Power conversion unit

The PCU turns heat into electricity. Hot intermediate fluid heats a working gas that spins a turbine and a generator.

Cycle: Supercritical CO₂ Brayton
Net output: ≈ 10 MWe
Efficiency: ≈ 35–40%
Footprint: One truck-transportable skid

Air-cooled heat rejection

Fans blow air across finned tubes to reject the waste heat of the power cycle. Dry cooling means the plant needs no cooling water.

Type: Forced-draft dry cooler
Heat rejected: ≈ 20 MWt
Fans: 6 × 3 m diameter
Water use: None

Site & structure

Reactor building

A steel building encloses the reactor module and the power unit. Inside it, a concrete shield silo holds the module, gives seismic protection, and forms a final barrier for sodium.

Footprint: ≈ 18 × 12 m
Shield silo: ≈ 1.4 m reinforced concrete
Structure: Steel frame, insulated panels
Seismic: Module on isolated base

Bridge crane

An overhead bridge crane runs the length of the building. It lifts the reactor head, the shield plug, and the spent core cask during refueling.

Capacity: ≈ 40 t
Span: ≈ 12 m
Hoist: Electric wire-rope, two speeds
Use: Refueling and major maintenance

Module delivery truck

A heavy-haul trailer brings a complete reactor module from the factory. The crane lifts the skid off the trailer and sets it in the silo.

Load: One module on its skid, ≈ 80 t
Trailer: Lowboy, multi-axle
Permits: Standard oversize-load route
Factory to site: Days, not months

Control & operations building

Operators monitor the plant from this building. The reactor runs on its own, and the control system needs only oversight, so the room is small.

Staff on shift: 2 operators, 1 security
Monitoring: Local control room plus remote center
Also inside: Offices, instrument shop, break room
Footprint: ≈ 14 × 6 m, two floors

Used fuel cask pad

After a core swap, the used core cools in a steel and concrete cask on this pad. The casks are passive, need no power, and ship off site later.

Casks: 4 positions
Shielding: Steel canister in a concrete overpack
Cooling: Passive air, no power
Dose at fence: Below public limits

Transport skid

The reactor module ships from the factory on a steel skid. The skid sets the module in the shield silo and acts as its seismic base.

Fits: Standard heavy-haul trailer
Mass: ≈ 80 t with module
Transport: Truck or rail
Install time: Weeks, not years

Grid connection

Transmission line

A short line carries the plant output to the campus substation. A second circuit on lattice towers ties the plant to the regional grid.

Voltage: 69 kV, three phase
Campus line: ≈ 30 m on steel monopoles
Grid tie: Lattice towers to the regional network
Capacity: ≈ 12 MW, far above plant output

Campus substation

The campus substation steps the voltage down for the data center. Two transformers share the load so one can rest during maintenance.

Transformers: 2 × 8 MVA, 69 kV to 13.8 kV
Redundancy: N+1, either unit carries the hall
Protection: Breakers and relays in the switch house
Feed to hall: Underground duct bank

Switchyard & transformer

The generator output steps up through a transformer to the local grid voltage. Switchgear cabinets protect the plant and let operators isolate it from the grid.

Transformer: ≈ 12 MVA, 13.8 kV to 69 kV
Switchgear: Metal-clad, 3 cabinets
Connection: Single feeder to local grid or microgrid
Islanding: Can run isolated with a load bank

Data center

Data hall

The data hall holds the servers that use the reactor output. One microreactor carries roughly 8 MW of IT load plus cooling and losses.

Footprint: ≈ 48 × 22 m
IT load: ≈ 8 MW
Racks: 320 at ≈ 25 kW each
Uptime target: Tier III, concurrently maintainable

Server racks

Servers sit in rows of racks with hot-aisle containment. Cold air enters the front of each rack and hot air leaves into a sealed aisle.

Rows: 8 rows × 40 racks
Rack power: ≈ 25 kW, liquid-ready
Containment: Hot aisle, doors at each end
Distribution: Overhead busway, A and B feeds

UPS & electrical room

The electrical room receives the feed from the substation and conditions it. UPS units ride through short dips while the generators start.

UPS: 2 × 5 MW, N+1
Ride-through: ≈ 5 minutes at full load
Switchgear: 13.8 kV to 480 V
Distribution: PDUs feed the overhead busway

Battery storage

Battery containers smooth the load the reactor sees. The reactor runs flat out, and the batteries absorb or supply the difference.

Capacity: ≈ 10 MWh in 3 containers
Power: ≈ 5 MW
Chemistry: Lithium iron phosphate
Role: Load following and black start

Backup generators

Standby generators cover the hall if the reactor or the line is out. With the reactor as prime power, they run only for tests.

Units: 2 × 3 MW
Fuel: Diesel, 24 hours on site
Start time: < 10 s to full load
Duty: Standby, monthly test

Cooling plant

Rooftop coolers reject the heat the servers make. Fans pull air across coils, and the chilled loop returns to the hall.

Heat rejected: ≈ 9 MW
Units: 6 rooftop dry coolers
Loop: Warm water, 30 °C supply
Water use: None, dry cooling

Heat-pipe reactor (park)

Heat-pipe reactor module

This sister plant has no pumps and no liquid coolant loop. The whole reactor ships inside one steel canister on a truck, and the lid is off here so you can see inside.

Electric output: ≈ 5 MWe
Thermal power: ≈ 15 MWt
Canister: ≈ 3 m dia × 10 m
Core life: ≈ 8 years, then swap the module

Monolith core

The core is one solid block with fuel channels and heat-pipe channels drilled through it. Heat moves from the fuel into the block, then into the pipes. Nothing flows inside the core.

Fuel: TRISO particles in compacts
Block: Graphite or steel monolith
Diameter: ≈ 1.6 m
Length: ≈ 2.4 m

Sodium heat pipes

Each pipe is a sealed tube with a little sodium inside. Sodium boils at the hot end, the vapor flows to the cold end, condenses, and a wick pulls the liquid back. No pump, no valve, no power.

Count: ≈ 100 to 200
Working fluid: Sodium, a few hundred grams per pipe
Operating temperature: ≈ 650 °C
Heat per pipe: ≈ 100 kW

Control drums

Drums in the reflector turn to bring an absorber face toward or away from the core. They are the same idea as the drums in the sodium plant.

Count: 12
Absorber: Boron carbide
Drive: Electric motor, fail-safe spring

Heat-pipe heat exchanger

The cold ends of the heat pipes pass through this block. Air or carbon dioxide flows around them, picks up the heat, and goes to the turbine.

Working gas: Air or supercritical CO₂
Gas outlet temperature: ≈ 600 °C
Size: ≈ 2 × 2 × 1.5 m

Open-air Brayton skid

A gas turbine and generator sit on one skid outside the canister. Air comes in through the filter house, goes through the heat exchanger, spins the turbine, and leaves through the stack.

Cycle: Open-air Brayton
Electric output: ≈ 5 MWe
Cooling water: None
Footprint: ≈ 12 × 4 m

Gas-cooled reactor (park)

Helium pressure vessel

This plant cools its core with helium gas under pressure. Helium does not react with anything and cannot boil. The vessel is a tall steel cylinder with the core in the middle.

Coolant: Helium at ≈ 7 MPa
Core outlet: ≈ 630 °C
Thermal power: ≈ 15 MWt
Core life: ≈ 20 years

TRISO fuel

Each fuel kernel is smaller than a poppy seed and wears four ceramic coatings. The coatings hold in fission products to well above 1,600 °C. The display case shows one particle cut open at large scale.

Kernel: Uranium oxycarbide, ≈ 0.5 mm
Coatings: Buffer, PyC, SiC, PyC
Particle size: ≈ 1 mm
Particles per reactor: Billions

Helium circulator

A motor-driven blower on top of the vessel pushes helium down the outside of the core and up through the fuel. If it stops, the core cools itself by conduction and radiation.

Type: Electric, magnetic bearings
Flow: ≈ 10 kg/s
Power draw: ≈ 300 kW

Helium-to-salt heat exchanger

Hot helium hands its heat to a molten salt here. The salt then leaves the reactor building, so no nuclear system runs through the power plant next door.

Hot side: Helium, 630 °C in
Cold side: Nitrate salt, 560 °C out
Type: Printed-circuit or shell-and-tube

Molten-salt storage tanks

Two tanks store hot and cold salt. The reactor runs flat out all day. The salt releases heat to the turbine when the data center needs it, like a thermal battery.

Salt: Sodium and potassium nitrate
Hot tank: ≈ 560 °C
Storage: ≈ 8 hours at full output
Tank size: ≈ 4.5 m dia × 8 m

Steam turbine-generator

Hot salt boils water in a steam generator, and the steam drives this turbine. It is ordinary power-plant equipment, and it can run faster or slower than the reactor because the tanks sit in between.

Cycle: Steam Rankine
Electric output: ≈ 5 MWe
Steam: ≈ 540 °C
Cooling: Dry air-cooled condenser

Molten-salt reactor (park)

Molten-salt reactor vessel

In this plant the fuel is dissolved in the coolant. A hot salt with uranium in it flows through channels in a graphite block, where it goes critical, and then out to a heat exchanger.

Fuel salt: Fluoride salt with uranium
Temperature: ≈ 650 °C at near atmospheric pressure
Thermal power: ≈ 30 MWt
Moderator: Graphite

Fuel-salt pump

A pump on the vessel head moves the fuel salt around the loop. The motor sits above the salt, out of the heat, with a long shaft down to the impeller.

Type: Vertical shaft, sump pump
Flow: ≈ 0.5 m³/s
Shaft seal: Inert gas

Primary heat exchanger

Fuel salt on one side, a clean coolant salt on the other. Fission products stay in the primary loop. The coolant salt carries the heat to the steam plant.

Type: Shell-and-tube
Material: Nickel alloy
Secondary salt: FLiBe or FLiNaK

Freeze plug and drain tank

A small blower keeps a plug of salt frozen in the drain line. If power fails, the blower stops, the plug melts, and the fuel drains by gravity into this tank, where it cannot stay critical and cools passively.

Plug: Frozen salt, ≈ 20 mm pipe
Drain time: Minutes
Tank: Flat, subcritical shape with cooling fins

Off-gas system

Gaseous fission products bubble out of the liquid fuel. The off-gas system pulls them into charcoal beds, holds them while they decay, and keeps the loop clean.

Gases: Xenon, krypton, tritium
Hold-up: Charcoal beds, days to weeks
Carrier: Helium sweep gas

Steam plant

Coolant salt boils water in the steam generator, and the steam turns the turbine-generator under the canopy. The reactor cell wall sits between here and the fuel.

Electric output: ≈ 10 MWe
Steam: ≈ 550 °C
Cooling: Dry air-cooled condenser

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