Nuclear's Model T

The future of nuclear energy could lie in plants that can be factory built,
shipped to a site, and operated 30 years without refueling.

By Craig F. Smith

http://memagazine.asme.org/Articles/2009/July/Nuclears_Model_T.cfm

[Images at the link]

It has become commonplace to say that we are at the beginning of a global
revitalization of the nuclear energy enterprise. The scope and timing of this
"nuclear renaissance," however, remain somewhat uncertain. What is known is that
in countries around the globe, including the United States, significant numbers
of new nuclear energy projects are under way or in various stages of planning,
and this activity represents a departure from that of recent decades.

As in the past, there appears to be a continuing trend toward larger plant
capacities as the industry leverages economy of scale to optimize the economic
performance of the new plants. According to the World Nuclear Association, the
average power rating of today's 436 operating power reactors is 854 MW. In
contrast, for the 112 new reactors on order or planned globally, the average
size is 1,171 MW. And for the 31 plants under consideration or proposed in the
U.S., the average is 1,284 MW.

The conclusion to be drawn from this seems clear: Although existing power plants
include reactors with outputs as low as 12 MW, the major trend is toward larger
plant size. This is easy to understand in light of the high capital costs
associated with nuclear energy plants. Given the large capital outlay, and the
fact that costs do not scale linearly, there seems to be a relentless pressure
toward increasing plant size.

But is bigger necessarily better? While new conventional nuclear plants trend
toward larger size, there has also been continuing and growing interest in small
and medium size plants. Nuclear generating stations that can operate at that
smaller scale could enable broader use of this source of clean, abundant energy
in a rapidly growing world economy. Such plants could be installed in locations
that would not be able to accept the large quantity of electricity generated by
a gigawatt-scale reactor, and there is some indication that, properly designed,
a small plant could be cost competitive with the larger ones currently planned.

Along with my colleagues, I have worked on developing the class of reactors
known as the Secure Transportable Autonomous Reactor (STAR), and the Small
Secure Transportable Autonomous Reactor (SSTAR) in particular, and I believe
this concept could play an important role in a global renaissance in nuclear
energy. Reactors of this kind could be built in a central location and shipped
to locations around the world, even developing nations that do not have the
capability to build nuclear plants themselves. Indeed, small nuclear power
plants could be a key technology for curbing greenhouse gas emissions in
emerging economies.



It is true that most modern power plants exceed 1,000 MW in output and are
trending toward even larger sizes. But smaller plants—those rating below 100
MW—have historically played an important part in nuclear energy development. The
first nuclear power plant to generate electricity was the experimental reactor
EBR-I that, in 1951, operated in Idaho with an output of merely 100 kW. Later,
in 1954, the first plant to provide power to an electricity grid was the 5 MW
Obninsk Atomic Power Station (APS-1) in the U.S.S.R. And the first commercial
nuclear power station, Calder Hall in England, was operated in 1956 at 50 MW,
though later upgraded to 200 MW. The first commercial nuclear power plant in the
United States, the Shippingport Plant, began operating on December 2, 1957 and
continued for 25 years of operation with an output level of 60 MW.

In addition to those pioneering efforts, small-size reactors are employed in
training, isotope production, research, naval propulsion, and in some space
applications.

But what role could small-scale nuclear reactors have in generating central
station power? The International Atomic Energy Agency indicates that more than
50 new concepts and designs for advanced small or moderate-size reactors are
under development in more than 15 IAEA member states. Proponents of such designs
believe they have the potential to meet such needs as providing energy for
islands that are not served by a national grid or for regions lacking the
infrastructures and grid capacity needed for large plants. Small reactors could
also power such energy-intensive industrial activities as water desalinization
or the extraction of oil from tar sands.

Nuclear's Model T - Current SSTAR reactor design

Nuclear's Model T - Current SSTAR reactor designThe current SSTAR reactor design
features a small, open-lattice cassette core (depicted in red) immersed in
molten lead. A heat-exchange system (green) sends 550 °C CO2 to a gas turbine.




The broad interest in developing new small reactor system concepts seems to be
in conflict with the trend toward ever-larger central station power plants,
which is driven by the principle of economy of scale. But there is the
possibility to offset the advantages of economy of scale through a combination
of factors that can serve to improve the economic performance of small systems.

Among these factors are the interrelated impacts of factory fabrication and mass
production. Modern nuclear power plants are field-constructed using factory
fabricated parts and components, and though individual plant designs offer some
consistency from site to site, the experience in the United States has been that
each individual plant is unique in many of its details. For some of the small
reactor concepts being considered, however, the entire power plant would be
built in a factory and shipped to the site; this would be expected to yield
substantial economic benefits. Henry Ford long ago established the industrial
benefits of mass production; and the practice of factory fabrication of complex
mechanical systems, such as large passenger aircraft, is well-developed modern
practice.

Another factor is the simplicity of design that can follow from the development
of small nuclear power systems. Small reactors should not be considered
miniature versions of large plants—to the contrary, there is great potential for
plant simplification and alternate design features that can be obtained as a
result of the lower power rating. An example of this is the possibility of
eliminating main coolant pumps in reactors that can rely on natural convection
circulation. That approach is difficult to achieve in a larger reactor, which
must rely on forced coolant circulation.

Small plants can be operated differently from conventionally sized nuclear
plants, which are typically used to provide base load power to a large regional
or national grid. Especially if they are located on smaller, more isolated power
distribution systems, small-scale plants would be expected to operate at varying
power levels in addition to providing base load power. The ability to operate
with a high level of autonomous load following not only facilitates meeting this
goal, but it also reduces the operator burden. With small modular reactors, the
option exists for incremental development of a site which in turn enables
incremental financing that lowers financial risk and overall cost.

Finally, a small reactor size offers a substantially reduced footprint for both
security and operations, and these factors act to reduce the operational
complexity and cost of the plant.

Taken in combination, these factors would offset the loss of economy of scale of
small systems; in fact, it is possible that they could eliminate the
disadvantage in terms of cost per unit energy generated. Whether that would
happen depends on such variables as the level of design simplification, the
number of units produced, the degree of duplication in mass production, and the
regulatory hurdles small reactors would face.



One frequently cited drawback to widespread use of nuclear power is the risk of
fissionable material being diverted to produce weapons. In the 1990s,
researchers at Lawrence Livermore National Laboratory began looking at reactor
system designs intended to minimize the potential for nuclear weapons
proliferation. The initial research effort concluded that this goal could be met
by a sealed reactor that was transportable and autonomous in operation and that
would have a very long reactor core lifetime. Such a reactor would eliminate the
need for handling or processing fresh or spent nuclear fuel and otherwise
minimize the potential for any possible misuse of the reactor.

In addition, several desired features were identified to address, in particular,
the anticipated lack of existing industrial and human resource infrastructures
in developing nations. Those requirements included, among others, the need to
provide relatively small increments of electric power on distributed grids; the
desire for simple controls, passive safety, and low maintenance characteristics;
and the requirement for high reliability in power availability over long periods
of time. The need for stability in energy prices and low investment risk was
also stressed.

Nuclear's Model T - A complete SSTAR module

A complete SSTAR module would be capable of transport by ship or heavy-haul
ground transporter. Modules would be mass produced at a central factory and
shipped to remote sites or to countries that lack the technology infrastructure
to build their own nuclear reactors.


Continued research determined that certain design objectives for a reactor
system would meet those requirements. For instance, to reduce the risk that
nuclear material would be diverted, the core could operate for 15 to 30 years
without refueling and reside in a sealed, tamper-proof vessel. The reactor would
be small enough to enable factory fabrication and shipment. And the system
should exhibit autonomous control to allow load following and reduction of
operational burdens. This combination of desired features came to be known as
the Secure Transportable Autonomous Reactor, or STAR.

To achieve these objectives, several different reactor types were envisioned,
and research was launched in multiple parallel directions. For example, the
Encapsulated Nuclear Heat Source effort led by the University of California
sought to develop a modular encapsulated reactor system that is a self-contained
fission power source cooled by heavy liquid metal coolant. A team led by the
Westinghouse Corp. pursued the STAR-LW concept, a light-water cooled reactor
variant that was the predecessor to the current system known as IRIS. In
addition, many other small reactor concepts, such as the Japanese 4S (Super
Safe, Small, and Simple) reactor, were influenced by the STAR effort. More
recently, the IAEA has organized an effort focused on small reactors without
on-site refueling (to which they assign the acronym SRWOR), and they identify no
fewer than 30 such reactor concepts currently under consideration.



Researchers have continued to pursue the STAR concept in an effort to bring it
closer to realization. A team that included Lawrence Livermore National
Laboratory, Argonne National Laboratory, Los Alamos National Laboratory and the
University of California developed a new reactor design designated the Small
Secure Transportable Autonomous Reactor. SSTAR arose from the conclusion that
the best approach to achieve the overall STAR objectives, and in particular the
long core life characteristic, was through development of a fast spectrum
reactor cooled by heavy liquid metal. Indeed, SSTAR is the only such advanced
system designed from the start as a small system intended to meet the stringent
STAR objectives. (The effort was supported by the Department of Energy under its
Generation IV advanced reactor initiative.)

The resulting pre-conceptual design, developed primarily by the reactor design
team at Argonne, is a 20 MW electric (45 MW thermal) transportable reactor
system that features molten elemental lead as a coolant, natural circulation
heat transfer, and power generation based on a 44 percent efficient
supercritical CO2 Brayton cycle energy conversion system. The compact active
core is not accessible by the user but can be removed by the supplier as a
single cassette and replaced by a fresh core.

The lead coolant is contained inside a reactor vessel surrounded by a guard
vessel. An alloy of lead and bismuth had also been considered as a coolant
because it has a lower melting point than pure lead. But lead was selected as
the coolant in order to dramatically reduce the amount of an alpha
particle-emitting polonium isotope (210Po) formed in the coolant—a problem with
the alloy—and to eliminate dependency upon bismuth that might be a limited or
expensive resource.

The design has some interesting properties. The fast neutron energy spectrum and
the strong reactivity feedbacks within the reactor core enable the reactor to
adjust its power to match the heat removal, a property known as autonomous load
following. And the system incorporates carbon dioxide heat exchangers inside the
reactor vessel to provide a compact system design, low primary system pressure,
and separate coolant and working fluids.

It's an exciting concept, though one that is certainly not ready for immediate
deployment. For one thing, the SSTAR design assumes that a number of advanced
technologies will be successfully developed. One of those technologies is a
yet-to-be-developed and qualified advanced cladding and structural materials
that will enable service in lead for the 15 to 30 years core lifetime at peak
temperatures of up to about 650 °C. Other technologies that need to be developed
are qualified transuranic nitride fuel meeting performance requirements, a
whole-core cassette refueling system, and a means for in-service inspection of
components immersed in lead coolant. The hope is, though, that the promise of
the SSTAR concept can provide a driver for the development of the advanced
technologies incorporated in the design.

In addition to these technology advances, the deployment of reactors broadly
into the developing world, whether of the SSTAR or any other type, will require
a framework for regulation and governance that does not currently exist.

As the SSTAR concept has been developed, a parallel effort in Europe is being
pursued to design a larger (600 MW) lead-cooled reactor system for central
station electricity generation. The European Lead-cooled System, or ELSY,
incorporates a number of innovative features and emphasizes the use of existing
technologies to minimize the need for additional research and development prior
to building an initial demonstrator reactor system. The initial ELSY design
efforts demonstrate the great potential for system simplification and
compactness that is a characteristic of lead-cooled reactor systems. The SSTAR
and ELSY concepts are distinct and different in important ways, but they
nevertheless share a number of common features and have the potential to benefit
greatly from parallel and coordinated development efforts.



One of the great opportunities for small reactors is to bring zero
carbon-emission nuclear power to many new regions of the world. To fulfill this
need, small size is not enough by itself. Reactors suitable for use in remote
areas and in nations lacking the elaborate technology infrastructures that we
take for granted in the developed world need to include design features to
address operations, safety, and proliferation risk management considerations.
The STAR concept and the SSTAR reactor in particular provide good examples of
additional design features that could make the introduction of such reactors
more readily accepted while offering the potential for economic performance that
makes sense in comparison to other alternative sources of energy.

In this way, the SSTAR reactors could finally fulfill the first promise of
nuclear energy. During the Cold War, President Dwight D. Eisenhower laid out a
new vision for peaceful uses of nuclear energy culminating in a program known as
Atoms for Peace. In this vision, technology and assistance for peaceful civilian
uses of nuclear energy would be provided to states that agreed to forgo the
development of nuclear weapons.

Implicit in his vision was the idea that atomic energy could be an important
force to improve the socioeconomic condition of all of mankind. The results were
a remarkably rapid international deployment of nuclear reactors. Today, some 16
percent of the world's electricity is generated by this technology.

Has the Atoms for Peace vision been fully achieved? Many would say no. Although
the initial expansion of nuclear energy was significant, the 436 power reactors
that exist globally provide nuclear energy in only 30 of the nearly 200
countries of the world. These 30 nations include many highly developed countries
and a few of the largest developing nations, such as India and China, but a
large part of the world has not been included in the initial global deployment
of nuclear energy.

It is likely that small, autonomous reactors such as SSTAR could be the key to
bringing the benefits of nuclear energy to the developing world while assuring
safety, security, and proliferation resistance. I am not alone in believing that
this is a promising new direction that should be vigorously pursued.


To learn more:

For more detailed information on the prospects for small nuclear power plants,
please see:

+ International Atomic Energy Agency (IAEA), "Innovative small and medium sized
reactors: Design features, safety approaches and R&D trends," Report
IAEA-TECDOC-1451, June 2004.

+ Robert N. Schock, Neil W. Brown, and Craig F. Smith, "Nuclear Power, Small
Nuclear Technology, and the Role of Technical Innovation: An Assessment,"
UCRL-JC-142964, May 2001.

+ International Atomic Energy Agency (IAEA), "Status of Small Reactor Designs
Without On-Site Refuelling," Report IAEA TECDOC Series No. 1536, 2007.

+ Craig F. Smith, William G. Halsey, Neil W. Brown, James J. Sienicki, Anton
Moisseytsev, and David C. Wade, "SSTAR: The US Lead-Cooled Fast Reactor (LFR),"
Journal of Nuclear Materials, Volume 376,
Issue 3, pp 255-259, 15 June 2008.

+ L. Cinotti, C. F. Smith, J. J. Sienicki, H. Aďt Abderrahim, G. Benamati, G.
Locatelli, S. Monti, H. Wider, D. Struwe, A. Orden, and I. S. Hwang, "The
Potential of the LFR and the ELSY Project," Paper 7585, 2007 International
Congress on Advances in Nuclear Power Plants, Nice, France, May 13-18, 2007.

+ "Atoms for Peace after 50 Years: The New Challenges and Opportunities," Report
UCRL-TR-200927,
December 2003.

+ S. Hattori and A. Minato, "The Super Safe Small and Simple Reactor (4S-50),"
International Conference On Design and Safety of Advanced Nuclear Power Plants
(ANP `92), Tokyo, Japan October 1992.


Craig F. Smith is the Lawrence Livermore National Laboratory Chair Professor at
the Naval Postgraduate School, in Monterey, Calif. He is a Fellow of the
American Nuclear Society and the American Association for the Advancement of
Science.

http://memagazine.asme.org/Articles/2009/July/Nuclears_Model_T.cfm