Graphite radiation effects

The pebble zone is a volume 7 ft 4 in. square by 9 ft high surrounding the reactor tank and filled by some 7T)0,000 l-in.-diameter AGOT graphite pebbles. The pebbles rest on a base plate.mounted on the lower thermal shield plate and are free to expand upward. In this way any expansion due to radiation effects or temperature increase will merely raise the level of the balls without exerting undue stress on the tank wall or the permanent graphite. 

Other factors, such as radiation damage, chemical stability, and structural requirements, can be important in the choice of the moderator material for a reactor concept. The radiation effects on graphite and BeO have received a great deal of attention. Detailed discussions of this subject can be found in the literature. Only a very brief review will be included here. 

T. D. Burchell, Radiation effects in graphite and carbon-based msltenaAs, " MRS Bulletin, 22(2), 29-35 (1997). 

As mentioned in Sect. 3.2, a thin layer of rhenium/iridium alloy will be ideal to protect the core and reflector from oxidation as well as external and internal contaminants at high temperature. This would require a development program to investigate the thickness needed for the degree of sealing desired, the bond strength with the graphitized foam, radiation effects, and the mechanical resistance to internal pressure buildup. 

Experimental data for the mechanical and physical properties of the selected graphite candidates (IG-110, IG-430, NBG-18, and NBG-25) were produced. In addition, the fracture and oxidation behaviors were estimated. To understand the radiation effects in nuclear-grade graphite, an atomistic stmctural change in IG-110 irradiated with 3 MeV H" " and gamma—irradiation effects were characterized (Kim et al., 2009b Hong et al., 2012 Corwin et al., 2008). 

The limiting temperature for graphite use in fusion systems is defined by tliermal sublimation (--1500-2000°C). However, a process which is very similar to thermal sublimation (in cause and in effect) appears to define the current temperature limit. This phenomenon, which is known as radiation enhanced sublimation (RES), is not clearly understood but dominates above a temperature of about 1000°C and increases exponentially with increasing temperatme. 

As area detectors (other than multiwire systems) are not energy discriminating devices, apotential source of error lies in the contamination of the data with harmonics of the assumed wavelength of the primary beam. The importance of this effect has been estimated for molybdenum Ka radiation using a graphite monochromator [1],... 

Figure 20. Effects of radiation on the thermal expansion of T300/5208 graphite/epoxy composite. (Reproduced from reference 18.)...

Tests of a graphite-reinforced polyimide composite (C6000/PMR15) did not show any effect of radiation exposure (1 MEV electrons 6x109 rad total dose) on the thermal expansion behavior (14). DMA curves for unirradiated and irradiated composites were essentially identical over the temperature range of the thermal expansion measurements. 

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