Nuclear Reactor Core

Introduction

This is a 3D model of a Nuclear Reactor Core.

3D Model Annotations

Reactor

This 3D model represents a pressurised water reactor (PWR) used in the majority of the world’s nuclear power plants. PWRs are a type of light water reactor (LWR) as they use normal water as coolant and for neutron moderation. A nuclear power plant reactor is the initial heat source of the power plant. Heat is generated in the reactor by a nuclear fission chain reaction, this heat is then passed to a primary coolant circuit, then to a secondary coolant circuit. The secondary coolant circuit is heated until it boils and produces steam, this steam is delivered to a steam turbine, which causes the turbine rotor to rotate. Finally, a generator rotor directly coupled to the steam turbine rotor also rotates, which induces electrical current in the generator stator, and electrical power can then be dispatch to consumers via an electrical transformer.

Hot Leg

Primary coolant is heated in the reactor. Coolant that has been heated by the reactor is referred to as ‘hot’. The hot primary coolant circuit is referred to as the ‘hot leg’. The hot leg is the part of the coolant circuit that extends from the reactor primary coolant outlet, to the once through steam generator (OTSG) inlet. This reactor has two hot legs and thus two primary coolant discharge nozzles.

Cold Leg

Primary coolant that has transferred its heat energy to the secondary coolant circuit, is referred to as ‘cold’. The cold primary coolant circuit is referred to as the ‘cold leg’. The cold leg is the part of the coolant circuit that extends from the once through steam generator (OTSG) discharge to the reactor inlet. This reactor has four cold legs and thus four primary coolant inlet nozzles.

Body/Shell

The body of the reactor vessel forms its largest pressure boundary; it contains the fuel rods and control rods. A typical body reactor is cylindrical in shape with a removable head that allows for fuel loading. Reactor bodies may operate at pressures of approx. 170 bar (2,465 psi) at 350⁰C (662⁰F).

Control Rod Drive (CRD)

Control rod drives are used to extend and retract control rods into, or out of, a reactor. Extending or retracting the control rods will respectively decrease or increase the fission reaction rate. Upon loss of power, the control rod drive disengages, and the control rods are lowered due to gravity; the fail-safe position of the control drive is thus that the control rods are fully engaged and the rate of fission restricted.

Core Barrel

The core barrel has a long cylindrical shape and is manufactured from corrosive resistant materials. Fuel assembles are fixed to the lower core plate, which is welded to the base of the core barrel. The weight of the barrel is transferred to the reactor body; it forms part of the lower core support structure. As primary coolant enters the reactor, it flows downwards between the body and the barrel, this space is referred to as the ‘downcomer’.

Fuel Assembly

Reactors use Uranium as nuclear fuel. Processed uranium is encapsulated in small ceramic pellets, then the pellets are poured into long thin metal tubes; these tubes are called ‘fuel rods’. Fuel rods are bundled together to form a fuel assembly. A typical reactor may have several hundred assemblies depending upon the power plant’s megawatt (MW) capacity. Fuel rods are submerged in water to cool and moderate (reduce the number of neutrons produced by the fission reaction) the nuclear reaction.

Control Rod

Control rods are an integral safety and control item for all nuclear reactors. A control rod consists of long, cylindrical shaped, rods, manufactured from neutron absorbing material (boron, cadmium etc.). Control rods are bundled together to form clusters, which are further grouped to form a bank/group. Control rods absorb neutrons, which reduces the number of neutrons available for fission, and thus reduces the amount of heat generated by the reactor. Control rods can be extended or retracted into the reactor using control rod drives. Extending or retracting the control rods will respectively increase or decrease the reactivity of the reactor.

Neutron Reflector / Absorber

Neutron reflectors may be of the core baffle or heavy reflector design. The core baffle design uses vertical, stainless-steel baffles, and horizontal formers, that have been installed around the reactor core. A heavy reflector consists of multiple stainless-steel slabs that have been stacked vertically around the reactor core. Both reflector designs reduce the effects of irradiation embrittlement on the reactor body by reducing neutron leakage (neutrons are reflected to the fuel assemblies, which increases the reactor’s overall efficiency).

Vessel Head

The reactor vessel head is attached to the top of the reactor vessel body. Penetrations in the head are required to allow the control rod drives to attach to the control rods. Additional penetrations are required for the reactor level measurement probes.

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