Page 202 - Managing Global Warming

164 Managing Global Warming

include SCW, including depressurization transients to subcritical conditions. Flow
stability in the core has been studied numerically. As in BWRs, flow stability can
be ensured using suitable inlet orifices in fuel assemblies.
A number of candidate cladding materials have been tested in capsules, autoclaves,
and recirculating loops up to 700°C at a pressure of 25MPa. Stainless steels with
>20% chromium (Cr) are expected to have the required corrosion resistance up to
a peak cladding temperature of 650°C. More work is needed to develop alloys suitable
for use at the design peak cladding temperatures of 850°C for the Canadian SCWR
concept. Further work is also needed to better identify the coolant conditions that lead
to stress-corrosion cracking. It has been shown that the creep resistance of existing
alloys can be improved by adding small amounts of elements, such as zirconium
(Zr). In the longer term, the steel experimental Oxide Dispersion Strengthened
(ODS) alloys offer an even higher potential, whereas nickel-base alloys are being con-
sidered for use in ultra-supercritical fossil-fired plants are less favorable for use in
SCWRs due to their high neutron absorption and associated swelling and
embrittlement.
Key water chemistry issues have been identified by Guzonas et al. [18]: Predicting
and controlling water radiolysis and corrosion product transport (including fission
products) remain the major R&D areas. In this regard, the operating experience using
nuclear steam reheat at the Beloyarsk NPP in Russia is extremely valuable [1,19].

4.3.6 Additional reactor classifications

In several countries, the term “SMR” has been introduced representing the initials of
“Small Modular Reactor” or “Small-/Medium-sized Reactor,” which addresses some
of the Generation IV goals. Not presently deployed, these ideas or preliminary designs
cover a size range from 20 to 600MW el ), and feature claims for lower costs due to
more factory-based construction techniques and the use of multiple “modules” to
reduce investment and financial risk. SMRs include many design concepts that span
from the entire range of the six Generation IV concepts and coolants (PBMRs,
LMFBRS, etc.) plus others, which are essentially scaled-down LWR systems, e.g., as
in the NuScale concept currently undergoing licensing review. The idea is to have a
more flexible system that may also be suitable for remote regions, as being presently
undertaken by Russia using “ice-breaker” reactor technology, or, of course, for mil-
itary applications beyond nuclear-powered submarines. Various industry and investor
groups have promoted these concepts, while seeking fiscal support, first-of-a-kind
(FOAK) or prototype funding and market acceptance. Some of these SMR concepts
were recently examined in detail by an extensive Australian Royal Commission study,
which found them to be not economically competitive on a relative basis [20].It is
therefore premature to speculate here on the degree or extent to which these smaller
units may be deployed. (More details on SMRs can be found in [1].)

4.3.7 Summary
In summary, Table 4.11 lists estimated ranges of thermal efficiencies (gross)
of Generation IV NPP concepts for reference purposes (Pioro and Duffey [1,2]).