L. L. Snead

Oak Ridge National Laboratory, Oak Ridge, TN, USA

M. Ferraris

Politecnico di Torino, Italy © 2012 Elsevier Ltd. All rights reserved.

Introduction

Background

Plasma-Facing Materials Particle-Matter Interactions The Advantages of Carbon as a PFC Plasma Impurities and the Need for Graphite Materials Thermomechanical Loading of PFMs Transient Loading of PFMs

Irradiation Effects on Thermophysical Properties of Graphite and CFCs

Graphite Irradiation Damage Surface Effects

Properties and Property Evolution of Graphite Fiber Composite

Irradiation-induced dimensional changes in CFCs Irradiation-induced changes in strength and modulus Thermal conductivity degradation Plasma-Particle Interactions Chemical Erosion

Doping of Graphite to Suppress Erosion Physical Sputtering Radiation-Enhanced Sublimation Erosion of Graphite in Simulated Disruption Events Tritium Retention in Graphitic Materials HHF Component Technology Joining of CFC to Heat Sink Evaluation of HHF Joint Summary and Conclusions

4.18.1 Introduction

4.18.1.1 Background

Graphite-moderated, gas-cooled reactors led the way into the nuclear age starting with the Chicago Pile-1 reactor, where the first controlled and sus­tained critical nuclear reaction was initiated in December 1942. The first commercial nuclear power plant, Calder Hall in the United Kingdom, went critical in 1956. As the graphite moderator was literally at the core of these early reactors, graphite became one of the first and most exten­sively studied nuclear materials. As discussed in Chapter 4.10, Radiation Effects in Graphite, the fission-born neutron results in significant thermo­physical property changes in graphite. Moreover, depending on the type of fission reactor, other environmental factors such as graphite oxidation become extremely important. In addition to being the moderator of gas-cooled reactors, graphite has found a number of new nuclear power applications. As examples, pyrolytic graphite is a key functional element in TRi ISOtropic (TRISO) fuels, which con­tinue to be developed and utilized for gas-cooled reactors; carbon fiber composites (CFCs) are now under development for core application in high — temperature gas-cooled reactors1 and have been widely used as plasma-facing components (PFCs) in fusion reactors.2 The latter application began in 1978, when the Princeton Large Torus made a transition from tungsten to graphite ‘limiters.’ This enabled the first thermonuclear temperatures, beginning the widespread application of graphite materials in fusion systems, the subject of this chapter. As will be discussed, the primary motivation for the use of graphite in fusion systems is not (as in fission reac­tors) for neutron moderation, but for reasons related to its exceptional high temperature performance and its relatively innocuous interaction with the plasma. However, the fusion reactor environmental effects on graphite, including irradiation-induced property evolution, are very similar to those of their fission reactor analogs.

In contrast to the fission of heavy elements such as uranium or plutonium, which releases a large amount of energy in their fission fragments and a moderate amount of energy in the form of neutron kinetic energy (mean about 1 MeV), fusion can occur for a number of light elements, some of which have reac­tions that release very high-kinetic-energy neutrons. Several possible routes to fusion are shown below in

eqn [1]:

1H1 +1 H1 !1 D2 + positron = 1.4MeV

1H1 +1 D2 !2 He3 = 5.5 MeV

1H1 +1 T3 !2 He4 = 19.9 MeV

1D2 +1 D2 !2 He3 + neutron = 3.3 MeV

1D2 +1 D2 !1 T3 +1 H1 = 4.0MeV

1D2 +1 T3 !2 He4 + neutron = 17.6MeV

1D2 +2He3 !2 He4 +1H1 = 18.2 MeV I1]

For any of these reactions to take place, the ionized atoms must be brought together with sufficient force to overcome the coulombic barrier. In thermonuclear fusion, this is accomplished by heating the ‘plasma’ of these atoms to the point where the kinetic energy is sufficient to overcome that barrier. Currently, it is thought that D+T fusion is the most accessible route to fusion, though the gaseous temperature required for D+T reaction is more than 50 million Kelvin.

Control and containment of high-temperature, high-density fusion plasmas is the primary challenge and obstacle to fusion power. Many reactor concepts have been studied in the past and attention is now focused on the ‘tokamak’ system. This toroidal con­finement machine system was developed in the mid-1960s in Russia. In this design, a high-strength twisted helix of magnetic lines forms a magnetic bottle. Ions, which are trapped within a certain gyro — radius, travel along these lines circulating around the helix in opposition to the plasma electrons. For non — collisional plasmas, the ions can be heated by mag­netic induction or through various external means to the extreme temperature necessary for the fusion reac­tion to take place. This concept is the basis for the four largest present-day fusion machines (Table 1), and is the premise for the ITER machine currently under construction. To give an idea of scale, in all of the present-day machines listed in Table 1, the helical cavity is big enough in size for an adult to walk within, and the radius from the center of the machine to the middle of the helix is typically several meters. A depiction of the inside of the JET torus, complete with beryllium-coated CFC wall, is given in Figure 1.