Is There a Point to the Sodium Cooled Fast Reactor?

For two generations Argonne National Laboratory has obsessively pursued the development of the liquid sodium breeder reactor. Although Argonne claims they have solved all safety issues related to the liquid sodium breeder, in fact significant safety issues probably remain, and indeed lists of remaining research tasks related to the liquid sodium breeder reactors indicate a lack of assurance about safety claims being made about the LMFBR. In addition to safety concerns, there are cost questions. Sodium cooled reactor are viewed as having a significant cost penalty over conventional LWRs.

An 1998 Essay by Lawrence M. Lidsky and Marvin M. Miller of the Massachusetts Institute of Technology titled "Nuclear Power and Energy Security: A Revised Strategy for Japan", laid out significant questions about the usefulness the LMFBR.

While this essay raises serious questions about the LMFBR and indeed the LWR, it by no means is hostile to the LFTR. In their Introduction the authors of this essay stated.

'Nuclear power, in its present incarnation, has not lived up to its great promise. . . . There is good reason to suspect that other implementations of nuclear power technology might allow nuclear power to play a greater role in energy supply and energy security".

For example they note:

Although the LWR had been placed in a privileged position by the political situation, it had significant shortcomings, many of which were apparent from the beginning. It had low thermodynamic efficiency with little potential for improvement. Fuel burnup was limited. It was considerably less forgiving of mechanical or operational error than such competitive designs as the molten salt reactor and the gas-cooled reactor. The LWR’s necessary complexity (required to provide defense-in depth) implied “economies of scale” such that it could be economically competitive, if at all, only in very large sizes. These disadvantages were obvious enough to show that the LWR would be a poor choice to play the central role in nuclear generation strategies. Its shortcomings were tolerable only because the LWR was originally intended for a stop gap role, tobe substantially phased out by the breeder by 1990.

Oh wow! I hope Kirk Sorensen reads this.

The essay adds:

Thorium fuel cycles have also been promoted on the basis of lower long-term waste toxicity and greater proliferation resistance, . . . The initial rationale for introduction of the thorium cycle was the perception that it was more abundant than uranium, and that it could be used to breed U-233, an isotope with superior properties for use in thermal reactors. However, Its terrestrial abundance is not germane to Japan’s energy security concerns because Japan has no indigenous source of thorium and it is hard to imagine a scenario in which uranium is cut off but thorium is available. Conceivably, the use of U-233 in an advanced reactor could reduce the possibility of a common mode failure of a reactor fleet consisting of LEU-fueled LWRs and HTGRs. The Molten Salt Reactor would be a strong candidate for consideration for this role, with a solid research base and an international support group, . . .

Because the Lidsky-Miller essay offers an extensive account of the history of and the rationale for the LMFBR, it is worth quoting at length from its opening sections.

1. Introduction
The paradigmatic LWR/FBR nuclear power system was conceived in the United States over 50 years ago, and soon achieved “Official Technology” status, with resulting strong government support, preferential access to capital, and the capture of path dependent advantages. 5 The LWR/FBR approach rapidly became dominant, aided by its Official Technology status and by aggressive state-subsidized marketing. The LWR very quickly began to make significant contributions to power production in the U.S. and other industrialized countries and the FBR became the singular focus of development efforts.

However, after very rapid expansion in the 1970–90 time period, nuclear power’s rapid growth has slowed significantly; in some countries, installed nuclear capacity has actually started to shrink. Overall, the nuclear share of global electricity production, 17% in 1996, has begun to decline, and the current economic crisis in Asia does not bode well for growth in a region where rapid growth had been anticipated 6 .The other components of the LWR/FBR paradigm, the breeder and plutonium recycle, have not fared even as well. The U.S. and Germany have abandoned their breeder programs. The French government has recently announced that the 1200MWe Superphenix breeder reactor will be dismantled, while the Japanese demonstration breeder, Monju, remains shut down more than two years after a loss-of-sodium accident. The situation is almost as bleak for recycle of plutonium as Mixed-Oxide (MOX) fuel in LWRs; existing contracts are being honored but MOX fuel is not popular with reactor operators or the public. Nuclear power, in its present incarnation, has not lived up to its great promise. The fundamental question is whether such failure is inherent and unavoidable or if, perhaps, other technological embodiments of nuclear power systems can satisfy society’s economic and political requirements. There is good reason to suspect that other implementations of nuclear power technology might allow nuclear power to play a greater role in energy supply and energy security. The current LWR/LMFBR scheme is, after all, just one of many fundamentally different ways to exploit nuclear energy. It was chosen in response to the political and military conditions existing circa 1950, on the basis of contemporary assumptions regarding uranium and fossil fuel availability, the anticipated growth rate of nuclear power, and the predicted costs associated with both the FBR and the associated reprocessing technology. At the time, it was believed that uranium was in critically short supply and that fossil fuel prices would soon rise sharply, that nuclear power would become the dominant energy source, and that the costs of the FBR and its fuel cycle would actually be less than that of the LWR. All of these assumptions have proven to be false. Now, a better understanding of the actual situation along with improvements in technological capability make it possible to develop a clearer idea of nuclear power’s proper role in energy supply, and to develop technological embodiments that optimize the desired characteristics.

I ought to note at this point that the motive of Eugene Wigner and Alvin Weinberg in developing the fluid core thorium cycle reactor was to make sustainable nuclear power competitive on cost with coal. Fluid core reactors were never envisioned as providing weaponizable materials. This perhaps explains why the thorium fuel cycle MSR concept received such limited backing. It was not envisioned as being of value for weapons production.

2. History - Runup to Current Status

Status Nuclear reactors were developed in secrecy during the first decade of the nuclear era (1945–1955) at the National Laboratories of several countries, under the control of the military. The first reactors, fueled with natural uranium, were used to produce plutonium for weapons use. Shortly there after, the U.S. decided to use an enriched uranium fueled, light water cooled reactor, a LWR, for submarine propulsion. In the prevailing Cold War atmosphere, the development of nuclear powered submarines had very high priority. The pressurized LWR was chosen over several competitors for the submarine reactor because it employed “familiar” technology (liquid water and steam) and because it was capable of very high power density. When, in 1953, the race for dominance in the area of civilian nuclear power was set in motion bythe Atoms for Peace program, and the U.S. needed a rapid response to counter the British (commercial) and Soviet (propaganda) threats, the LWR was the obvious choice. Research into the use of reactors for civilian power production, although widespread, was still exploratory and unfocussed, and no other U.S. reactor design was ready for deployment as quickly. The LWR had a number of features in its favor. It had benefited from continuing research and development in the Navy’s ship propulsion program and there were manufacturers familiar with the required technology. It used enriched uranium, which was, for a time, a U. S. monopoly, and therefore gave American manufacturers an important competitive advantage with respect to potential competitors (France, England, and the Soviet Union). The U.S. hegemony in this area was further strengthened by a series of bilateral “Agreements for Cooperation” in which the U.S.provided loan funds which could be used only for the purchase of equipment, materials (including enriched uranium), and technical services from U. S. nuclear vendors. 7 This reinforced the“Official Technology” status of the LWR throughout most of the Western Block countries.

Although the LWR had been placed in a privileged position by the political situation, it had significant shortcomings, many of which were apparent from the beginning. It had low thermodynamic efficiency with little potential for improvement. Fuel burnup was limited. It wasconsiderably less forgiving of mechanical or operational error than such competitive designs as the molten salt reactor and the gas-cooled reactor. The LWR’s necessary complexity (required toprovide defense-in depth) implied “economies of scale” such that it could be economically competitive, if at all, only in very large sizes. These disadvantages were obvious enough to show that the LWR would be a poor choice to play the central role in nuclear generation strategies. Its shortcomings were tolerable only because the LWR was originally intended for a stop gap role, tobe substantially phased out by the breeder by 1990. Development of the breeder was the overarching goal of the scientists involved in both the military and civilian development of nuclear power. There was a pervasive belief that uranium wasa very limited resource, so limited that weapons production would be seriously impacted and significant civilian use would be impossible. Nuclear power proponents saw themselves in a race with fossil power generation schemes, and so needed a way to expand the number of nuclear power plants rapidly enough to gain market share and then to keep up with the anticipated very rapid rise in electricity demand. But not just any breeder would do. Because of the anticipated rapid growth of nuclear capacity, it would not be sufficient to breed at a rate capable of merely replenishing the fissile material burned. A “fuel factory” was needed which would produce enough excess plutonium not only to sustain itself but to simultaneously produce enough additional plutonium to serve as seed stock for a rapidly growing fleet of similar reactors. The measure of the ability to function as a fuel factory, not just as a self-sustaining reactor, is the "doubling time", and only the LMFBR had, at least in theory, the ability to achieve a short enough doubling time. The LMFBR performs best with an initial charge of plutonium to start the breeding process, which could be provided by extracting plutonium from spent LWR fuel, using methods and facilities similar to those developed for the weapons program. Thus, the LWR/LMFBR combination was thought to provide the most rapid path to a self-sustaining nuclear cycle. Even when it finally became obvious that uranium availability would not constrain the growth of nuclear power, the U.S. Atomic Energy Commission (AEC), and later the U.S. Department ofEnergy (DOE), remained firmly committed to the original plan, using both strategic and economic arguments to argue against any alternative to the LWR-LMFBR vision of the future. In 1969, Milton Shaw, director of the USAEC’s Division of Reactor Development and Technology, in a foreword to a study of alternative breeder reactors, wrote.

"The widespread acceptance of the light water reactor is an established fact. The large industrial commitments and improvements in technology should result in further improvements in performance. These factors will make difficult the introduction in the United States of any new system even though a potential economic gain is indicated. Because of the urgent need to introduce breeder reactors at the earliest date, the USAEC has committed itself to an extensive program involving LMFBR’s. For this reason, development funds for competing concepts are limited. The possible role of such reactors in the U. S. nuclear power economy is, therefore, not yet clear."

The degree of unwavering government support for its vision of the nuclear future is exemplified by the AEC’s 1973 (!) estimate that, by the year 2000, the U.S. would get half its electric power from 400 breeders and 600 LWRs. Only 41 reactors were ordered after 1973 and every one was subsequently canceled, as were nearly 70% of those ordered after 1970. In 1998 there are 103 licensed plants, all LWRs, and the number is expected to decrease substantially in the next decade. The U.S. utility industry was also advocating early introduction of the LWR. In 1970, a General Electric Company vice president, recalling the reasons for the decision to offer the “turnkey” loss-leader plants that started the nuclear stampede in the United States, said "If we couldn’t get orders out of the utility industry, with every tick of the clock, it became progressively more likely that some competing technology would be developed that would supersede the economic viability of our own. Our people understood that this was a game of massive stakes, and that if we didn’t force the utility industry to put those stations on line, we’d end up with nothing."

The strategy of a rapid buildup of LWR power generating capability, followed by an equally rapid conversion to reliance on LMFBR’s had a compelling technological logic. It had an equally attractive economic logic for the industrial participants who were eager to begin profiting from their enormous investments in nuclear technology. Unfortunately, for both the U.S. and those who followed the U.S. lead, both logical analyses were wrong because the underlying axioms and assumptions were untrue. The price now being paid for these errors is enormous in terms of both financial loss and lost opportunity. The financial loss is almost incalculable; it has been called the greatest managerial disaster in business history. 15 Moreover, even in countries where it has eventually failed, the LWR, by virtue of its Official Technology status, stifled the development and introduction of safer, cheaper nuclear power plants that might have taken advantage of modern technology and been better suited to contemporary constraints and the specific needs of various countries.

It is most gratifying to find that ideas which I have been advancing in Nuclear Green were understood and accepted by Lidsky and Miller ten years ago.

3.Current Status of the LWR/FBR Nuclear Power Paradigm

The cost and complexity of the systems needed to deal with the danger of severe accident makes the LWR a poor choice for large central station power plants. Ironically, it is the LWR’s high power density, the very reason it was chosen for submarine use, that is its Achilles heel. Even a 10-second interruption in the supply of cooling water at the surface of a fuel rod can lead to local overheating and irrevocable, cascading damage to the reactor core. As a result, the LWR must rely on defense-in-depth, a system of diverse and redundant backup devices, to guard against such an event. This is a widely used technique, but defense-in depth can not, by itself, guarantee absolute safety; it can only reduce the probability of a serious accident. All nuclear power plants,because of their cost and potential for off-site hazards, have a very low “acceptable” probability of failure. The larger the plant, the lower the acceptable probability of failure. Because the consequence of failure is so large in gigawatt-scale plants, LWR’s have been forced to employ engineered safety systems that promise unprecedentedly, and perhaps unattainably, low probability of failure.

The first LWRs employed defense-in-depth systems which were calculated to achieve failure probabilities of 10 -4 /year or less, i.e., an expected mean time before a major accident (such as core meltdown) of at least 10,000 years, for a single, given reactor. This is a commonly accepted level of risk for high capital cost industrial facilities from the standpoint of investment protection. However, it is clearly inadequate from the perspective of public safety for the case of nuclear reactors. 16 As a result, all reactors were required to have a confinement dome to protect the public, in addition to the engineered safety features which were of high-level industrial grade. Itwas clearly prudent to have such an extra level of protection for a new technology with possible unexpected failure modes, and largely ill understood consequences. The resulting risk of a given reactor undergoing a major accident with public health consequences was believed to be less than10 -6 /yr, with the confinement dome playing a major role in reducing the consequences of the accident. This arrangement made perfect sense for the first generation of 200-400 MWe LWR’s, but set a subtle trap for the next and successive generations of much larger reactors.

The complexity of defense-in depth safety systems leads to size-independent costs that are better borne if the costs are supported by the revenues of a larger power plant. This factor, in combination with the scale economy of steam generators and turbines, and the more difficult than anticipated competition with low cost fossil fuel, led to a very rapid scale up of LWR size. But above about 500-600 MWe, engineers could no longer guarantee the integrity of the confinement system 17 . It was not realized until it was too late to modify development plans that the inability to build a confinement vessel that could withstand a major accident in such a large reactor violated the initial safety concept. Because the confinement vessel could not be counted upon, defense-in-depth would have to be solely responsible for public safety. This meant that failure probability levels of 10 -6 /year, that is, a mean time before major accident of one million years, had to be achieved for the reactor itself, without reliance on any additional safety credit for the dome. This unprecedented level of safety for a defense-in-depth system, when applied to so complex a system as a nuclear reactor, meant that the safety system itself had to be enormously complex, which made it maintenance-intensive, and, as it happened, actually more problem prone than the device it was meant to protect. As a result, LWR power plants are expensive, complex, difficult to operate, and incapable of simultaneously competing with fossil fuels and achieving the desired level of safety . All of these problems are attributable, at least in part, to the reliance on defense-in-depth. However, despite all the attention given to the safety system, the public remains unconvinced of the safety for which so high a price is paid. This skepticism is well justified because insufficient data is available to calculate the true probability of a major accident and it is literally impossible to demonstrate, by definitive test, that the requisite level of safety has been achieved.

The sodium-cooled LMFBR was the device that was intended to replace the LWR when mined uranium supplies became prohibitively expensive. The LMFBR was chosen over other breeder reactor designs because it was, in theory, capable of very short fuel doubling times, shorter than that of any competing reactor design. The doubling time is the time required to produce an excess of fuel equal to the amount originally required to fuel the reactor itself. In other words, in one doubling time there would be enough fuel available to start up another reactor. In the absence of mined uranium, only a short doubling time would, it was believed, allow nuclear power to grow fast enough to compete with alternative sources of power. Unfortunately, the theoretical advantages of the LMFBR could not be achieved in practice. A successful commercial breeder reactor must have three attributes; it must breed, it must be economical, and it must be safe. Although any one or two of these attributes can be achieved in isolation by proper design, the laws of physics apparently make it impossible to achieve all three simultaneously, no matter how clever the design. The fundamental problem originates in the very properties of sodium that make the short doubling time possible. The physical characteristics of sodium and plutonium are such that a loss of sodium coolant in the center of the core of a breeding reactor (caused, for example, by overheating) would tend to increase the power of the reactor, thus driving more sodium from the core, further increasing the power in a continuous feedback loop. The resulting rapid, literally uncontrollable, rise in reactor power is clearly unacceptable from a safety standpoint. This effect, the so-called “positive void coefficient” can be mitigated by, for example, changing the shape of the core so that more neutrons leak out of the core, but this immediately compromises the reactor’s breeding potential. Safety and breeding are thus mutually antagonistic. This situation can be alleviated to some extent by making radical design changes, but these changes lead to greatly increased costs, and make the reactor prohibitively expensive. Even if the LMFBR could meet its original, highly optimistic, operating goals and the LWR/FBR power cycle were put into operation, it is unclear that the goal of energy security would be achieved. As discussed in the following sections, the measures that would have to be put in place to protect all parts of the fuel cycle against terrorism would have very high social costs. Equally important is the increased risk of accidental or maliciously-induced technological failure. Compared to light water reactors operating on the once-through fuel cycle, the breeder fuel cycle is much more complex and error-prone. This implies a higher probability that the entire nuclear system or a significant fraction thereof might need to be shutdown because of a generic problem, e.g., with sodium containment, in the reactors or an accident in one of the reprocessing or fuel fabrication plants that serve the system.

I will skip over much of the next section of the essay, although rest assured, gentle reader, it has interesting things to say, and may be consulted through the link which I supply at the beginning of this post. However, I could not skip over the following passage.

4.3.Technological Failure A breeder-based nuclear supply system is inherently more complex and error prone than one based on LWRs operating on a once-through fuel cycle. Not only does the breeder system have complex components which have no counterpart in an LWR system, e.g., reprocessing plants, but even when a counterpart exists, e.g., the reactors themselves, fuel fabrication plants, and a transportation network, they are more complex in a breeder-based system. Aside from the reactor, much of this added complexity is due to the radiological hazards, criticality risks, and security threats associated with the presence in the breeder fuel cycle of large quantities of unirradiated plutonium, i.e. plutonium without fission products. Actual failure of, or just loss of public confidence in, any component of the breeder fuel cycle could shut down a significant part of, or even the whole system, thus negating its potential energy security advantage. In the following, we comment briefly on the technological vulnerabilities of breeder reactors and the associated reprocessing and fuel fabrication plants. Proponents of the LMFBR have made many claims regarding the robust engineering base of sodium reactor technology, but experience around the world has demonstrated that sodium-cooled systems often suffer serious disruptions even in the event of relatively minor failures. 24 The potential for sodium-air and sodium-water reactions accounts for some of this sensitivity, and the problem is exacerbated by the opacity of sodium, which makes fault detection substantially more difficult for such systems than for those in which visual inspection is possible. These technological problems, in addition to the design difficulties associated with the tension between breeding and safety, strongly suggest that any large-scale LMFBR would be more problem-prone than the current generation of LWRs. Large plants for reprocessing LWR spent fuel in France and England have achieved high capacity factors in recent years. However, the radioactive effluents emitted by such plants during normal operation, and the accumulating stocks of separated plutonium as well as high-level and transuranic wastes are a source of growing concern among the public, the media, environmental groups, and bureaucracies in many countries. Moreover, because of the much higher fissile content and burnup of breeder compared with LWR spent fuel, reliable operation of breeder reprocessing plants will be more difficult and costly. That is, breeder plant equipment must be smaller to ensure criticality safety, the contact time between the extraction phases must be shorter to avoid radiative decomposition of process materials, and the need for higher fission product decontamination increases the volume of liquid waste streams. Similar remarks apply to plutonium fuel fabrication; as with reprocessing, fabrication of LMFBR fuel is more demanding than making MOX fuel for LWRs.

In sum, while the potential risks of both nuclear proliferation and terrorism as well as technological failure associated with a breeder-based nuclear supply system are difficult to quantify, they appear to be substantially greater than those associated with the current LWR-based system. Thus, in light of the strong adverse societal response to relatively minor mishaps with the present system, the chances and consequences of failure of any portion of a breeder-based system are too large to warrant reliance on it for a significant fraction of Japan’s electricity requirements. But is it possible to achieve energy security via nuclear power without the breeder?We believe that the answer is yes, and that the essential element is uranium stockpiling.

Finally we have the following statement from the conclusion:

This will provide a more realistic perspective on the need for nuclear power including the preferred technological embodiments and international institutional frameworks for dealing with safety and proliferation concerns. The role of nuclear power, decisions as to the optimal makeup of the power reactor fleet, and the degree of reliance upon seawater-derived uranium can be postponed until the technical, economic, and political issues are better resolved. However, it is of paramount importance to ensure that current actions do not unreasonably prejudice support for a nuclear component of energy supply. Strong support for plutonium recycle, with its associated technical risks and societal costs, in the face of increasing evidence that alternative strategies are superior, is clearly counterproductive.

Well there you have it. A very accurate though brief analysis of the history of nuclear power, and the flaws of the current thinking as well as the problems of the LMFBR concept, coupled with a firm statement that things can be cone much better.