Graphite has been used as a nuclear moderator in nuclear fission reactors since the very beginning of the nuclear age.1 Indeed, the Chicago Pile No. 1 (CP-1), constructed by Enrico Fermi under the stands at Stagg Field, University of Chicago, used The National Carbon Company’s AGOT grade graphite. On 2 December 1942, Enrico Fermi and his research team achieved the world’s first nuclear chain reaction in CP-1. Subsequently, the early weapons materials production reactors constructed in the United States, United Kingdom, France, and the former Union of Soviet Socialist Republics (USSR) were all graphite­moderated reactors, as were the first commercial power generating fission reactors.

The core of a graphite-moderated reactor is com­prised of stacks of graphite blocks that are usually keyed to one another to facilitate transmission of mechanical loads throughout the core. Vertical chan­nels penetrate the core into which fuel stringers are placed via the reactor charge face using a refueling machine. The nuclear fuel, which may be natural ura­nium (a mixture of 238U, 235U, and 234U) or enriched uranium, is usually sheathed (clad) in a metallic clad­ding. Typically, the cladding is a light alloy (aluminum or magnesium), but it can also be stainless steel (requir­ing enriched fuel) if a higher fuel temperature is desired (>600 °C). The fuel stringer and cladding material may be one and the same, as in the United Kingdom designed Magnox reactor,1 or the fuel may be in the form of stainless steel clad ‘pins’ arranged in graphite fuel sleeves, which are joined to one another and form the fuel stringer as in the UK Advanced Gas-Cooled Reactor1 (AGR). The metallic fuel clad retains the gaseous fission products that migrate from the fuel during the fission reaction and prevents con­tact between the gaseous coolant and the fuel.

An alternative core layout uses integral fuel/mod — erator elements in which the uranium fuel is placed directly into cavities in the graphite moderator block,

Table 1 Currently available nuclear grade graphites

Grade Manufacturer Coke type Comments

Isostatically molded, candidate for high-dose regions of NGNP concepts

Isostatically molded, candidate for high-dose regions of NGNP concepts

Extruded, candidate for high-dose regions of NGNP pebble bed concepts; PBMR core graphite Vibrationally molded, candidate for high-dose regions of NGNP prismatic core concepts Vibrationally molded, candidate for high-dose regions of NGNP pebble bed concepts; PBMR core graphite Extruded, candidate for high-dose regions of NGNP prismatic core concepts

Large blocks for permanent structure in a prismatic core Isostatically molded, candidate for permanent structures in a prismatic core

Isostatically molded, candidate for permanent structures in a prismatic core

and the entire block is discharged from the reactor when the fuel is spent. Fuel elements of this design typically utilize ceramic (UO2 or UC2) rather that metallic fuel so as to be capable of reaching higher fuel temperatures. The ceramic fuel kernel is over coated with layers of SiC and pyrolytic carbon to pro­vide a fission product barrier and to negate the use of a metallic fuel clad (see Chapter 3.07, TRISO — Coated Particle Fuel Performance), allowing the reactor core to operate at very high temperatures (> 1000 “C).1 The coated particle fuel is usually formed into fuel pucks or compacts but may be consoli­dated into fuel balls, or pebbles.1 The US designed modular high-temperature gas-cooled reactor (MHTGR) and Next Generation Nuclear Plant (NGNP), and the Japanese high-temperature test reactor (HTTR) are examples ofgas-cooled reactors with high-temperature ceramic fuel.

Additional vertical channels in the graphite reac­tor core house the control rods, which regulate the fission reaction by introducing neutron-adsorbing materials to the core, and thus reduce the number of neutrons available to sustain the fission process. When the control rods are withdrawn from the core, the self-sustaining fission reaction commences. Heat is generated by the moderation of the fission frag­ments in the fuel and moderation of fast neutrons in the graphite. The heat is removed from the core by a coolant, typically a gas, that flows freely through the core and over the graphite moderator. The coolant is forced through the core by a gas circulator and passes into a heat exchanger/boiler (frequently referred to as a steam generator).

The primary coolant loop (the reactor coolant) is maintained at elevated pressure to improve the cool­ant’s heat transfer characteristics and thus, the core is surrounded by a pressure vessel. A secondary coolant (water) loop runs through the heat exchanger and cools the primary coolant so that it may be returned to the reactor core at reduced temperature. The secondary coolant temperature is raised to produce steam which is passed through a turbine where it gives up its energy to drive an electric generator. Some reactor designs, such as the MHTGR, are direct cycle systems in which the helium coolant passes directly to a turbine.

The reactor core and primary coolant loop are enclosed in a concrete biological-shield, which pro­tects the reactor staff and public from g radiation and fission neutrons and also prevents the escape of radioactive contamination and fission product gasses that originate in the fuel pins/blocks. The charge face, refueling machine, control rod drives, discharge area, and cooling ponds are housed in a containment structure which similarly prevents the spread of any contamination. Additional necessary features of a fission reactor are (1) the refueling bay, where new fuel stringers or fuel elements are assembled prior to being loaded into the reactor core; (2), a discharge area and cooling ponds where spent fuel is placed while the short-lived isotopes are allowed to decay before the fuel can be reprocessed.

The NGNP, a graphite-moderated, helium cooled reactor, is designed specifically to generate elec­tricity and produce process heat, which could be used for the production of hydrogen, or steam gener­ation for the recovery of tar sands or oil shale.

Two NGNP concepts are currently being consid­ered, a prismatic core design and a pebble bed core design. In the prismatic core concept, the TRISO fuel is compacted into sticks and supported within a graphite fuel block which has helium coolant holes running through its length.1 The graphite fuel blocks are discharged from the reactor at the end of the fuel’s lifetime. In the pebble bed core concept, the TRISO fuel is mixed with other graphite materi­als and a resin binder and formed into 6 cm diameter spheres or pebbles.1 The pebbles are loaded into the core to form a ‘pebble bed’ through which helium coolant flows. The pebble bed is constrained by a graphite moderator and reflector blocks which define the reactor core shape. The fuel pebbles migrate slowly down through the reactor core and are discharged at the bottom of the core where they are either sent to spent fuel storage or returned to the top of the pebble bed.

Not all graphite-moderated reactors are gas — cooled. Several designs have utilized water cooling, with the water carried through the core in zirconium alloy tubes at elevated pressure, before being fed to a steam generator. Moreover, graphite-moderated reactors can also utilize a molten salt coolant, for example, the Molten Salt Reactor Experiment (MSRE)1 at Oak Ridge National Laboratory (ORNL). The fluid fuel in the MSRE consisted of UF4 dissolved in fluorides of beryllium and lithium, which was circu­lated through a reactor core moderated by graphite. The average temperature of the fuel salt was 650 °C (1200 °F) at the normal operating condition of 8 MW, which was the maximum heat removal capacity of the air-cooled secondary heat exchanger. The graphite core was small, being only 137.2 cm (54 in.) in diameter and 162.6 cm (64 in.) in height. The fuel salt entered the reactor vessel at 632 °C (1170 °F) and flowed down around the outside of the graphite core in the annular space between the core and the vessel. The graphite core was made up of graphite bars 5.08 cm (2 in.) square, exposed directly to the fuel which flowed upward in passages machined into the faces of the bars. The fuel flowed out of the top of the vessel at a temperature of 654 °C (1210 °F), through the circulating pump to the primary heat exchanger, where it gave up heat to a coolant salt stream. The core graphite, grade CGB, was specially produced for the MSRE, and had to have a small pore size to prevent penetration of the fuel salt, a long irradiation lifetime, and good dimen­sional stability. Moreover, for molten salt reactor mod­erators, a low permeability (preferably <10~8 cm2 s-1) is desirable in order to prevent the build up of unacceptable inventories of the nuclear poison 135Xe in the graphite. At ORNL, this was achieved by sealing the graphite surface using a gas phase carbon deposition process.1