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AGR Reactor

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The Advanced Gas-cooled Reactor (AGR) has a core with nuclear fuel, circulating fans and steam generators located in a common pressure vessel made of prestressed reinforced concrete. Explore the 3D online model of the AGR reactor.

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AGR Reactor

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AGR Reactor

Gas baffle

A steel envelope known as a gas baffle surrounds the entire graphite core and shielding. The function of the gas baffle is to provide a route for the cooling gas through the core. The baffle consists of three main parts: the dome at the top, the cylinder and the skirt. There are a number of penetrations in the dome that allow the passage of fuel assemblies and control rods into the core. A diagrid forms the lower end of the gas baffle cylinder.

Reactor core

The core consists of a graphite structure that acts as a moderator and provides individual channels for the fuel assemblies and control rods and ensures reliable flow of cooling gas for fuel cooling. Two types of graphite blocks are stacked in the core: one type is basically circular and contains the fuel channels and in between are square profile blocks containing either control rods or coolant holes.

Fuel assembly

The fuel in AGR reactors is in the form of cylindrical pellets made from slightly enriched uranium (up to 3 %). The pellets are contained in sealed stainless steel finned tubes and form fuel pins approximately 980 mm long. The fuel assembly then consists of 36 fuel pins and is arranged in graphite sleeves. Eight fuel assemblies are linked together with a tie bar to form the fuel string.

Control rods

A total of 89 control rods, divided into two categories, pass through the top cap of the reactor vessel. Approximately half are regulating rods primarily used to control the power output of the reactor core. They contain a weaker neutron absorber and are constantly moving in and out of the core during operation. The second category are rods with a strong neutron absorber serving as the primary shutdown device. The control rod assembly in each standpipe consists of a control rod, a control plug unit, a control rod actuator and the standpipe closure unit.

Gas circulator

Carbon dioxide gas under high pressure is used in AGR reactors to transfer heat from the fuel in the core to the steam generators. It is circulated through the core by 8 gas circulators - two circulators for each quadrant. Circulators are located in large penetrations at the bottom of the pressure vessel and each delivers approximately 500 kg of CO₂ every second.

Steam generator

In the reactor, 12 boiler units are installed between the gas baffle and the pressure vessel liner, partitioned into four quadrants. Each quadrant contains three boiler units and two gas circulators. The boiler unit is of the once-through type and consists of an economizer, evaporator and superheater sections. The carbon dioxide is cooled by approximately 300 °C by passing through the steam generator. The heat transferred generates about half a ton of steam every second.

Diagrid

The reactor core is supported by a system of steel plates known as the diagrid. The diagrid also forms the lower end of the gas baffle cylinder and is supported from pressure vessel floor by diagrid supports.

Fuel Assembly

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Fuel Assembly

Bottom grid

The fuel pins are spaced at the bottom and supported by the bottom grid, which is fixed to the outer graphite sleeve.

Support braces

Along the height of the fuel element are two (upper and middle) support braces that ensure the correct positioning of the pins and limit their possible bowing. The entire assembly of the grid, braces and inner graphite sleeves is secured at the top by a screwed graphite retaining ring.

Guide tube

In the center of each fuel element is a stainless steel guide tube through which the tie bar passes. This bar holds together the 8 fuel elements stacked on top of each other in order to form a stringer. The stringers are loaded vertically by the loading machine into channels in the graphite blocks of the core.

Fuel pins

The AGR fuel typically consists of cylindrical pellets of uranium dioxide, slightly enriched to 2.5-3.5% ²³⁵U. The pellets have a central bore acting as a plenum and are stacked in a ribbed austenitic stainless steel tube, which also acts as a partial neutron shielding. The inner space of the hermetic fuel pins is filled with helium.

Graphite sleeve

The fuel element is encased in a pair of graphite sleeves. The inner, thinner one is divided by a central brace into upper and lower sleeves. The outer sleeve is thicker and has a handling slot at the top. Graphite sleeves act as both structural support and moderator.

Fuel bundle

The fuel bundle (element) consists of a circular array of 36 fuel pins, is approximately 1 meter long and weighs about 43 kg. The bundle is encased in a graphite sleeve that serves as a moderator. The resulting fuel assembly (stringer) can be assembled from eight such bundles stacked on top of each other.

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