Page 200 - Managing Global Warming

162 Managing Global Warming

improved economics. A wide variety of options are currently considered: both
thermal-neutron and fast-neutron spectra are envisaged; both pressure-vessel and
pressure-tube configurations are considered, and, thus, use light water or heavy water
can be used as a moderator. The operation of a 30–150MW el technology demonstra-
tion reactor is targeted for 2022.
Unlike current water-cooled reactors, the coolant will experience a significantly
higher enthalpy rise in the core, which reduces the core mass flow for a given thermal
power and increases the core outlet enthalpy to supercritical conditions. For both
pressure-vessel and pressure-tube designs, a once-through steam cycle has been envis-
aged, omitting any coolant recirculation inside the reactor. As in a BWR, the super-
critical “steam” will be supplied directly to the high-pressure steam turbine and the
feed water from the steam cycle will be supplied back to the core. Thus, the SCWR
concepts combine the design and operation experiences gained from hundreds of
water-cooled reactors with those experiences from hundreds of fossil-fired power
plants operated with supercritical water (SCW). In contrast to some of the other Gen-
eration IV nuclear systems, the SCWR can be developed incrementally step by step
from current water-cooled reactors.
In general, SCWR designs have unique features that offer many advantages com-
pared to state-of-the-art water-cooled reactors. However, there are several technolog-
ical challenges associated with the development of the SCWR, and particularly the
need to validate transient heat-transfer models (for describing the depressurization
from supercritical to subcritical conditions), qualification of materials (namely,
advanced steels for cladding), and demonstration of the passive safety systems.
SCWR designs have unique features that offer many advantages compared to the
state-of-the-art water-cooled reactors:

l SCWRs offer increases in thermal efficiency relative to current-generation water-cooled
reactors. The efficiency of an SCWR can approach 44% or more, compared to 30%–36%
for current reactors.
l Reactor-coolant pumps are not required. The only pumps driving the coolant under normal
operating conditions are feed-water pumps and condensate-extraction pumps.
l The steam generators used in PWRs and steam separators and dryers used in BWRs can be
omitted since the coolant is supercritical in the core.
l Containment, designed with pressure-suppression pools and with emergency cooling and
residual heat-removal systems, can be significantly smaller than those of current water-
cooled reactors.
l The higher supercritical “steam” enthalpy allows to decrease the size of the turbine system
and thus to lower the capital costs of the conventional island.
These general features offer the potential of lower capital costs for a given electrical
power of the plant and of better fuel utilization, and thus a clear economic advantage
compared with current LWRs.
Preconceptual core design studies for a core outlet temperature of >500°C have
been performed in Japan, assuming either a thermal-neutron spectrum or a fast-
neutron spectrum (Oka et al. [14]). Both options are based on a coolant heat-up in
two steps with intermediate mixing underneath the core. Additional moderator for
a thermal-neutron spectrum is provided by feed water inside water rods. The fast-
spectrum option uses zirconium-hydride (ZrH 2 ) layers to minimize hardening of