A concern with HTGRs is the possible entry of air into the reactor and the resulting oxidation of the hot graphite. This possibility is discounted in the description of the PBMR provided in the Near-Term Deployment Roadmap:
Any concern of fire in the graphite core is avoided by showing that there is no method of introducing sufficient oxygen into a high-temperature core (> 1000 C) to achieve sustained oxidation. This
16 If heating of the graphite or loss of the helium has any appreciable effect on reactivity (and such an effect is not commonly mentioned), it is to provide negative feedback. The graphite expands slightly with heating, which makes it a less effective moderator, and loss of the helium coolant can only serve to decrease the moderation. Both effects would reduce the reactivity, although probably only by a small amount.
is achieved primarily by the structural design of the reactor structure and building. [14, p. G-1]
A more cautious view has been expressed by Kadak:
[I]t is very difficult to "burn" the graphite in the traditional sense, but it can be corroded and consumed. . .
The key issue for the pebble bed reactor is the amount of air available in the core from the reactor cavity and whether a chimney can form allowing for a flow of air to the graphite internal structure and fuel balls. Tests and analyses have shown that at these temperatures graphite is corroded and consumed but the natural circulation required for "burning" is not likely due to the resistance of the pebble bed to natural circulation flow. 
Continuing efforts are being made to determine if the air input can be sufficient to cause serious damage. Such studies are necessary to resolve the safety question and address criticisms such as those of Edwin Lyman. Considering the effects of a pipe break, he argues:
While the PBMR designers claim that the geometry of the primary circuit will inhibit air inflow and hence limit oxidation, this has not yet been conclusively shown.
The consequences of an extensive graphite fire could be severe, undermining the argument that a conventional containment is not needed. 
Here, Lyman touches on a major point, because proponents of HTGRs have sometimes viewed them as being so safe that a strong containment is not needed. However, omitting the containment is justified only if it can be unambiguously shown that the possibility of a significant radiation release from the reactor is negligible. Presumably, this will be a crucial point for the NRC to consider should it receive a license application for an HTGR. The containment issue is further complicated by concerns about terrorist attacks, which may provide an argument for a containment even for an otherwise accident-proof reactor.
The overall configuration of the GT-MHR is shown in Figure 16.3. The nominal module size is expected to be 600 MWt, corresponding to an electrical capacity of 286 MWe [14, p. H-3]. This sort of system, in which hot helium drives the turbine directly, with no steam generator, was suggested in the 1980s by Lawrence Lidsky and his collaborators at MIT . Lidsky et al. pointed out that this was not a new idea conceptually, but they argued that
technological developments and the push to modular design had by then made it practical.
In the GT-MHR, hot gas from the reactor drives the turbine. Gas exiting the turbine at reduced temperature and pressure passes through three heat exchangers and a two-stage compressor before returning to the reactor. The first heat exchanger (the recuperator) transfers heat from the helium leaving the turbine to the helium entering the reactor. The other two heat exchangers (the precooler and intercooler) further reduce the temperature of the gas in the compressor.17 Partly because the recuperator is very efficient in heating the gas returned to the reactor, the overall generating efficiency is expected to be about 48%. With the high enrichment, the burnup is in excess of 100 GWd/t [14, p. H-5]. Both performance levels are well above those of present reactors.
17 The nominal temperatures at various points in this cycle are as follows: turbine inlet and outlet, 848°C and 511°C; recuperator hot side inlet and outlet, 511°C and 125°C; recuperator cold side inlet and outlet, 105°C and 491°C [14, p. H-3].
The GT-MHR is now being developed in a program jointly sponsored by the U.S. DOE and the Russian Ministry for Atomic Energy (Minatom). The first construction target is for a prototype GT-MHR to be built in Russia and finished in 2009. U.S. plants would follow.
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