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154 Managing Global Warming

Chinese HTR-10. For electricity generation, a helium-gas-turbine system can be
directly set in the primary-coolant loop, which is called a direct cycle, or at the lower
end of the outlet-temperature range, a steam generator can be used with a conventional
Rankine cycle. For nuclear heat applications such as process heat for refineries, pet-
rochemistry, metallurgy, and hydrogen production, the heat application process is
generally coupled with the reactor through an intermediate heat exchanger (IHX),
the so-called indirect cycle. The VHTR can produce hydrogen from only heat and
water by using thermochemical processes (such as the Sulfur-Iodine (S-I) process
or the hybrid-sulfur process), High-temperature steam electrolysis (HTSE), or from
heat, water, and natural gas by applying the steam-reformer technology.
While the original approach for VHTR at the start of the Generation IV program
focused on very high outlet temperatures and hydrogen production, current market
assessments have indicated that electricity production and industrial processes based
on high-temperature steam that require modest outlet temperatures (700–850°C) have
the greatest potential for application in the next decade, and, also, reduce technical risk
associated with higher outlet temperatures. As a result, over the past decade, the focus
has moved from higher outlet temperature designs such as gas-turbine modular helium
reactor (GT-MHR) and pebble-bed modular reactor (PBMR) to lower outlet temper-
ature designs such as high-temperature reactor pebble-bed modules (HTR-PM) in
China and the Next-Generation Nuclear Plant (NGNP) in the United States.
The VHTR has two typical reactor configurations, namely: (1) the pebble-bed type
and (2) the prismatic-block type. Although the shape of a fuel element for two con-
figurations is different, the technical basis for both configurations is same, such as the
TRISO-coated particle fuel in the graphite matrix, full ceramic (graphite) core struc-
ture, helium coolant, and low power density, in order to achieve high outlet temper-
ature and the retention of fission production inside the coated particle at normal
operation condition and accident condition. The VHTR can support alternative fuel
cycles such as U-Pu, Pu, MOX, and U-Th.

4.3.5.2 Gas-cooled fast reactor

The GFR (see Fig. 4.31) is a high-temperature helium-cooled fast-spectrum reactor
with a closed fuel cycle. The core outlet temperature will be of the order of 850°C.
It combines the advantages of fast-spectrum systems for long-term sustainability of
uranium resources and waste minimization (through fuel multiple reprocessing and
fission of long-lived actinides), with those of high-temperature systems (e.g., high
thermal-cycle efficiency and industrial use of the generated heat for hydrogen produc-
tion). It requires the development of robust refractory fuel elements and appropriate
safety architecture. The use of dense fuel such as carbide or nitride provides good per-
formance regarding plutonium breeding and minor-actinide burning. A technology
demonstration reactor needed for qualifying key technologies could be in operation
by 2020.
The GFR uses the same fuel-recycling processes as the SFR and the same reactor
technology as the VHTR. Therefore, its development approach is to rely, in so far as