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.
Foorsberg 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.
