Introduction to Graphite in Nuclear Industry
Contents
1. Introduction
2. Overview
3. Detailed reports
1. Introduction
When the group of scientist led by Enrico Fermi decided in 1942 to attempt to produce a self-sustaining nuclear chain reaction, they chose graphite as the moderator because it was the only suitable material available at that time. The first pile, CP-1 was constructued on a squash court under the West Stands of Staag Field at the University of Chicago. Earlier Fermi and his collaborators had assembled the first exponential graphite-uranium structures at Columbia University for the purpose of determining the multiplication factor ( k ), the ratio of the number of neutrons in any one generation to the number of corresponding neutrons in the previous generation. If k could be made greater than one, then a nuclear chain reaction could be produced. The first exponential pile, an 8-ft cube, was assembled from graphite blocks and contained about 7 tons of uranium oxide in iron containers; it was completed in July 1941.
2. Overview
a) Current Applications
i. Reactor Moderator
Although the properties of graphite make it sustainable for many nuclear applications (moredator, reflector, fuel-channel sleeve, thermal column, fuel matrix, and control-rod material), by far the greatest use has been as a moderator and reflector. Nuclear graphite with an absorption cross section for thermal neutrons as low as 3.5 to 3.8 mb can now be manufactured in quantity at relatively low cost. Furthermore, graphite is strong enough to serve as structural component, eliminating the necessity of employing metals with their higher cross sections. Although graphite is compatible with most reactor materials up to high temperatures, it must be protected from hot oxidizing gases.
Most serious engineering problems in the use of graphite as a moderator have resulted from radiation-damage effects: expansion at low temperature energy in the Windscale, BEPO, and X-10 reactors; and hazards from possible uncontrolled burning in air-cooled reactors. These problems have arisen in the reflector graphite to much lesser degree because of the greatly reduced fast-neutron flux in the reflector. Problems in the use of graphite in the new high-temperature reactors will include oxidation and radiation-induced contraction of the moderator stack.
ii. Matrix Material
The development of fuel materials capable of producing high specific powers has been necessary to increase the efficiency and lower the capital costs of power reactors. Higher specific powers result in increased fuel temperatures. Higher temperatures, in turn, have led to the development of high-temperature ceramic fuel materials such as the uranium oxides and carbides, which can be used alone or dispersed in a high-temperatures ceramic matrix. Although the irradiation behavious of fueled graphite materials has not yet been thoroughly tested, the high-temperature strength, stability, and thermal conductivity of graphite make it a good matrix-material candidate.
ii.Other Applications
Graphite is used in a number of other special applications in the nuclear energy industry. In the reactor core boronated graphite is sometimes used as a high-temperature control-rod material. Recent designs for gas-cooled reactors employ graphite sleeves that support the fuel elements and channel the coolant. Graphite is useful as a container for reactor irradiation experiments. Since it does not become highly radioactive, it can be handled with a minimum of shielding following irradiation. furthermore, graphite possesses the high-temperature strength, stability, and chemical compatibility necessary in many experimental applications. Intricate parts can be machined to close tolerances.
b) Future Applications
New methods of converting nuclear energy into useful power, such as very high temperature reactor cores, controlled fusion, direct heat-to-electrical energy conversion, and nuclear rocket propulsion, will involve temperatures in the range of 1500 to 3000 °C and higher. Unless improved high-temperature materials are developed, the capability of these systems will be seriously limited. It is safe to assume that graphite and graphite-based materials will find increasingly important uses in these fields.
3. Detailed Reports
Taken with permission from
R E Nightingale (Ed.) Nuclear Graphite, Academic Press, 1962, New York and London
