Charles Forsberg's views on Generation IV nuclear costs

I am reposting this 2020 essay, because I believe that it makes important points about the future of nuclear power.  Charles Forsberg has retired from ORNL, and currently directs nuclear fuel cycle research at MIT.

ORNL MSR development work focused almost exclusively on MSBRs, although Ed Bettis's reactor design shop did design some deep burn MSR converters. The AEC was interested in breeder reactors, so the ORNL focus was on the development of a MSBR, rather than on possibly simpler converters. During the 1960's the cost of Light Water Reactors (LWRs) was believed to be low. Indeed by the time the dramatic reactor cost inflation of the 1970's had taken place, the MSR was no longer in the picture, and thus its potential for competing with the LWR on costs never became a topic for discussion. ORNL designers during the early 1970's had concluded that the cost of the MSBR was competitive with the cost of LWRs, but no attempt had been made to compare the cost of a MSR converter, to LWR costs.

In retrospective the failure to view the MSR as a potential replacement to the LWR, was an unfortunate product of faulty assumptions based on incomplete information. The incomplete information pertained to Light Water Reactor costs, and the faulty assumptions had to do with the desirability of the LWR as a competitor of coal fired power plants. As it turned out the LWR was by the early 1980's at a definite cost disadvantage compared to coal fired power plants, and was widely seen by the public as suffering from disadvantages with respect to the environment, and human health and safety. In retrospective the health and safety issues appear to have been largely solved by 1980. The Three Mile Island accident showed that even a major reactor accident would produce no casualties or environmental costs. Thus Three Mile Island demonstrated that the health, safety and environmental protection approaches philosophy adopted by American reactor manufacturers was sound. However, the technology protecting health, safety and the environmental came at a considerable monetary cost, a cost which was to cripple prospects for further growth of the nuclear industry for over a generation.

In the meantime Molten Salt Reactor technology languished, although a small group of ORNL staff members and a similarly small group of MSR international fans sought to revive interest in Molten Salt Nuclear technology.

It was only after the beginning of the 21st century that the use of Molten Salt coolants began to be seen as a low cost alternative Generation IV approach to nuclear power. This view emerged from Charles Forsberg one of the ORNL MSR old hands. Forsberg's view appears to have been that breeder technology was an encumbrance on Molten Salt development, and that a marriage of technology used for gas cooled reactors and molten salt based coolants had many attractive features. While the development of Molten Salt Reactor technology had largely stood still for a generation, the development of gas cooled reactors had advanced, and that technology was ready for implementation. Yet Gas cooled reactors suffered from a technical flaw, that would lead to high costs. Gasses are relatively unsatisfactory reactor coolants, especially when compared to liquid coolants like water, sodium, of molten salts. As a consequence, a lot of gas is required to cool a reactor core, and consequently the core must be large. This means a lot of material will go into gas cooled reactor construction compared to reactor power output. Liquid salts used in the Molten Salt Reactor are excellent coolants. What Forsberg noticed that aside from the size differences, there were a lot of structural similarities between Molten Salt Reactors and Gas Cooled Reactors. Both reactor types featured a coolant flowing through a graphite nuclear core. The graphite provided both core structure and neutron moderation.

The largest difference between the Gas Cooled Reactor and the Molten Salt Reactor was

the placement of the nuclear fuel. In the gas cooled reactor the fuel was embedded in the graphite, while in the MSR, the fuel (U-233, U-235. or Pu-239) was mixed with the molten salt coolant. The classic MSR was useful for a nuclear economy that assumed a limited or expensive uranium supply. Uranium and possibly thorium mixed with the MSR carrier salts, could be easily processed along with their nuclear byproducts. Processing uranium or thorium embedded in core graphite, while not impossible, was potentially more complicated. Forsberg's view only made since if nuclear breeding would be unnecessary for the next century or so. As it turned out this is Forsberg's view. Thus Forsberg concluded that it was not only possible to build a hybrid reactor using already mature Molten Salt and graphite embedded fuel technologies. Not only was it possible, but the resulting reactor, the Advanced High Temperature Reactor (AHTR) was very attractive. Forsberg did not directly compare the AHTR to the LWR but he did offer comparisons between the AHTR and other Generation IV reactor types. Forsberg compared variants of the AHTR with two other Generation IV reactor designs, an IFR, the General Electric sodium-cooled S-PRISM, and the gas cooled General Atomic Modular High-Temperature Reactor (GT-MHR). Forsberg argued that the AHTR would cost between 55% and 49% of the cost of the S-PRISM, and 61% and 53% of the GT-MHR.

Forsberg noted that several factors would would contribute to the lower AHtR cost:

• Higher efficiency. The higher temperature implies higher efficiency (~50% vs 42%). This results in lower costs per kilowatt (electric) because of the smaller power conversion equipment, cooling systems to reject heat from the power cycle, and smaller decay-heat-removal systems.

• Passive decay heat removal. The higher AHTR temperatures, combined with the high-temperature fuel, enable the development of passive safety systems for large reactors. Passive safety systems have the potential for lower costs.

• Reduced containment requirements. The molten salt coolant avoids the potential for steam−sodium interactions, absorbs radionuclides that escape the fuel, and eliminates highly energetic accidents, all of which lower containment requirements.

• Reduced equipment sizes. Volumetric heat capacities for molten salts are several times larger than those for sodium. This reduces the size of pipes, valves, and heat exchangers per unit of energy transferred.

• Transparent coolant. Unlike liquid metals, molten salts are transparent. This simplifies maintenance and inspection of the primary system with significant cost advantages.

it should be noted that Forsberg's thinking did not extend to the potential cost savings advantages of small modular reactors. But Per Peterson was shortly to refine Forsberg's analysis in a number of respects, and his findings. in my next post I intend to review Peterson's analysis.

It should be noted, however, that Forsberg's cost estimates are far too low. Thus it is not the cost estimate but the relationship between reactor costs for different nuclear technologies. It should be noted that TVA rebuilt its Browns Ferry unit 1 reactor between 2002 and 2007 at a cost of $1.9 billion, $1720 per kW, that is higher than Forsberg's estimate of new Generation IV reactor costs. Despite these difficulties, it would appear that Forsberg's hybrid reactor offered a promising rout to lower nuclear power costs.

MSR/LFTR Development: Forsberg

Two American scientists, Drs. Ralph Moir and Charles Forsberg have attempted to assess the state of MSR/LFTR development. (Access to papers discussed can be found here.)

In this post, I will discuss Forsberg's views on MSR/LFTR development as stated in a 2006 paper, Molten-Salt-Reactor Technology Gaps (Proceedings of ICAPP ‘06, Reno, NV USA, June 4–8, 2006, Paper 6295), in which he lays out both the case for development and discusses the developmental research required to achieve a commercial LFTR product.

Forsberg pointed to two major advantages of MSR/LFTR technology:

"As a liquid-fuel reactor, the MSR has two sets of unique characteristics relative to solid-fuel reactors.

• Safety. Under emergency conditions, the liquid fuel is drained to passively cooled critically safe
dump tanks. By the use of freeze valves (cooled sections of piping) and other techniques, this
safety system can be passively initiated upon overheating of the coolant salt. MSRs operate at
steady-state conditions, with no change in the nuclear reactivity of the fuel as a function of
time. Last, the option exists to remove fission products online and then solidify those
radionuclides into a stable waste form. This minimizes the radioactive inventory (accident
source term) in the reactor core and potential accident consequences.

• Fuel cycles. The liquid fuel allows online refueling and a wide choice of fuel cycle options:
burning of actinides from other reactors, a once through fuel cycle, a thorium-233U breeder cycle,
and a denatured thorium-233U breeder cycle. Some of the options, such as a thermal-neutron-
spectrum thorium-233U breeder cycle require online refueling and thus can not be practically
achieved using solid fuels. The use of a liquid fuel also avoids the need to develop fuel or
fabricate fuel".

Forsberg believes that recent advances in Brayton cycles gas turbines, actually solve many MSR developmental issues, and point theway to a significant improvement in MSR efficiency:

• "Efficiency. MSRs are naturally high-temperature reactors. Depending upon the choice of salt, the
freezing points are between 320 and 500°C. The heat transfer properties (viscosity, thermal
conductivity, etc.) improve rapidly with increasing temperature. Consequently, the
detailed 1000-MW(e) conceptual design of the MSR had a reactor-core fuel-coolant exit
temperature of 705°C. However, because of corrosion and other constraints in steam cycles,
peak steam cycle temperatures are between 500 and 550°C. In the 1960s designs, high-
temperature heat was inefficiently dumped to lower temperatures to match what the steam
cycle could tolerate. This process reduces heat exchanger sizes but has a large penalty in terms
of efficiency. In contrast, many Brayton cycles operate above 1000°C. The adoption of closed
helium or nitrogen Brayton power cycles enables the power cycle to efficiently use the high-
temperature heat generated by the MSR. This capability allows a 15% improvement in
electrical power output without changing the temperatures of the fuel salt exiting the reactor
core.

According to Forsberg, the choice to use Brayton cycle power generating technology would be a considerable developmental resolve these developmental issues:

• Freeze protection
• Tritium control. Adoption of a Brayton cycle provides an alternative tritium trapping option where the tritium is removed from the helium in the Brayton power cycle. This is potentially a high
performance low-cost option based on demonstrated inexpensive methods to remove tritium gas or tritiated water from helium. Helium-cooled high-temperature reactors produce tritium from nuclear reactions with 3He and from leaking fuel; consequently, these reactors are equipped with systems to remove the tritium from the helium.
• Chemical reactions. "Changing from a steam cycle to a gas Brayton cycle eliminates this class of challenges."

Forsberg also reported that Pebble Bed Reactors are "being constructed in South Africa with a helium Brayton power cycle. Additional technology development would be required for an MSR; however, the closed Brayton cycle technology istransitioning to a commercial technology".)

Forsberg notes second developmental shortcut:

"In the last decade, compact plate-fin and printed circuit high-temperature heat exchangers have been developed for the aircraft, chemical, and offshore-oil industries. The adoption of compact heat exchangers drastically reduces the molten fuel salt inventory in the heat exchangers and may reduce the inventory of fuel salt in the reactor by up to 50%. There are major benefits in using such heat exchangers products used in industry. These heat exchangers are being considered for use in high-temperature helium-cooled reactors and in the transport of heat from high-temperature gas-cooled reactors to hydrogen production plants using liquid-fluoride-salt heat- transport systems. Additional work is required to fully evaluate their use in MSRs".

Forsberg reports the advantages of new technology heat exchanges to include:

* Fuel salt inventory. Reducing the fuel inventory reduces both fuel salt costs and nonproliferation
risks, because the total fissile inventory in the nuclear system is decreased.

• Fuel salt processing. "In an MSR, volatile fission products (including xenon) are removed continuously, which creates a large parasitic neutron sink in solid-fuel reactors. For nonvolatile fission products, the fuel salt is processed online or off-line, depending upon design goals. Reducing the salt inventory reduces the quantities of salt to be processed".

• Heat exchanger size "The size of the heat exchangers is reduced by a factor of 3 or more.

• Tritium control. "The aircraft and other industries have developed compact heat exchangers with buffer gas zones to separate different fluids that may react explosively—such as hot gases vaporizing fuels in aircraft. The same technologies enable trapping of tritium from the primary system in the heat exchanger".

Forsberg also addresses four further MSR/LFTR developmental issues:
* Fuel Storage
* Noble Metal Plate-Out
* High-Level Waste (HLW) Form
* Peak Reactor Temperature

The curious can read what Forsberg says about the first three by accessing Ralph Moir's MSR paper collection. I will briefly report of Forsberg's suggestions on peak reactor temperature.

According to Forsberg with current reactor construction materials, materials, the peak operating temperature is limited to around 750 degrees C. Forsberg suggests that there are a number of good reasons making higher temperature operations desirable. He states, "Carbon-carbon composites are potentially an enabling technology for very high temperature MSRs. However, there are major technical uncertainties including joining technologies". Indeed there are. The same radiation problem that effect core graphite will effect carbon-carbon composites. One possibility which Forsberg did not consider is the use of carbon nanotubes in MSR core construction. "Pound for pound, carbon nanotubes are stronger and lighter than steel . . ."

Boris Yakobson, a Rice University professor of mechanical engineering and materials science and of chemistry, noted a unique feature of some carbon nanotubes, there ability to self repair. When damaged, tiny blemishes crawl over the skin of the damaged tubes, sewing up larger holes as they go. Yakobson stated, "The shape and direction of this imperfection does not change, and it never gets any larger," "We were amazed by it, but upon further study we found a good explanation. The atomic irregularity acts as a kind of safety valve, allowing the nanotube to release excess energy, in much the way that a valve allows steam to escape from a kettle."

Yakobson and a research associate Feng Ding found that this mechanism had the power to heal damage to carbon nanotubes caused by radiation. Whether the nanotubes can self repair in a reactor core has yet to be tested, but carbon nanotubs have an interesting potential both as a MSR/LFTR core moderator, and as a heat and radiation resistant structural material.