The Molten Salt Reactor Family: Converters and Breeders

All reactors are capable of not only using nuclear fuel, but also of producing new nuclear fuel. This characteristic sets nuclear power apart from other energy sources. Nuclear fuel is fissionable, that is its atoms break apart after absorbing neutrons. Fissionable atoms are described as fissile. When a fissile atom fissions, it divides into two smaller atoms, and two or three neutrons. The neurons keep the nuclear process going. Atomic weapons are designed so that the neutrons will encounter more fissile atoms, which in turn split, releasing more neutrons. As long as there is only more fissile material for the neutrons to encounter, the cjain reaction will grow, until heat and energy produced by nuclear fission blows the device apart, ending the chain reaction.

In a reactor the chain reaction is much more control ed. The presence of atoms that can be converted into nuclear fuel will greatly alter the nuclear process found in atomic bombs. The atoms that can be converted into nuclear fuel are described firtile. When they absorb neutrons, firtile atoms undergo a process of nuclear transformation that turns them into fissionable atoms. These processes take time, and thus the conversion process is much slower than the chain reactions in atomic weapons.

Natural uranium contains two isotopes or types of atoms. U-235 and U-238. U-235 is fissile, but it is only 0.7% of all natural uranium. U-238, the other common uranium isotope makes up 99.3% of most uranium ores. Inder most circumstances the percentage of U-235 in uranium is too small to produce a chain reaction. There are, however a few exceptional circumstances that will allow natural uranium to produce a chain reaction. They involve the presence of one of two moderators. Moderators, at materials that slow neutrons down. Slow neutrons are more likely to be absorbed by U-235 than fast neureons, so much so in fact, that natural uranium, if moderated by pure graphite or heavy water, can be brought to a chain reaction.

U-235 has a love affair with slow neutrons, so the presence of a moderator heats things up, so to speak, as more and more neutrons get absorbed by U-235. But even with moderators some neutrons get absorbed by U-238, which then undergoes a conversion to Plutonium-239. Pu-239 is also fissile, so it becomes new nuclear fuel. But in Light Water Reactors plutonium is not the best nuclear fuel. Pu-239 fissions less often at thermal neutron speeds, and on average produces less than two neutrons per neutron absorption. In a thermal reactor Plutonium will only breed at a less than one to one ratio. U-235 produces just over two neutrons per neutron absorption, not enough to sustain breeding. in epithermal Light Water Reactors the neutron performance of both U-235 and Pu-239 are worse. Thus uranium fuel cycle, Light Water Reactors cannot be breeders.

There are two nuclear fuel cycle conversion processes involving thorium-232 and U-238:

U-238 (non-fissile) + n -> U-239 -> Np-239 -> Pu-239 (fissile)
Th-232 (non-fissile) + n -> Th-233 -> Pa-233 -> U-233 (fissile)

TU of Delft, the Netherlands, offers an explanation,

It is important to know how many new fissile nuclei can be formed by conversion for every fissile nucleus consumed. The ratio of (# of new fissile nuclei / # of consumed fissile nuclei) is known as the conversion ratio. This ratio can be estimated as follows:

For every fissile nucleus consumed, X new neutrons are released
For a stable chain reaction, one neutron is needed to sustain the reaction: X must be larger than 1
To have 1 new converted nucleus for every fissioned nucleus, one neutron is needed: X must be larger than 2
Neutrons will leak from the reactor, so X must be appreciably larger than 2 to make a practical reactor with a conversion ratio > 1.

The value of X is highly dependent on the energy of the incident neutrons, as shown in the figure, and X grows rapidly for high-energy interactions.

The following figure traces the effect of Neutron speed on neutron propagation for U-233, U-235 and Pu-239. Because some Neutrons are invariably lost in a reactor core, the breeding ratio will always be lower than eta.

Explanation: "In this figure the Greek letter eta is the symbol for X. Pu-239 has the highest value of X in the high energy region and thus Pu-239 is the best breeding fuel available. At thermal energies X=2.44 for U-235, which is enough to get a conversion ratio of around 0.5: for every U-235 nucleus consumed, 0.5 U-238 nucleus is converted to Pu-239. At higher energies X=3 or higher, providing the possibility of getting a conversion ratio of >1.

It should be noted however that when thorium is the fertile isotope thermal neutrons can produce a conversion ratio of > 1. There are very significant implications for this. As neutron speed declines the amount of fissionable material required to maintain a chain reaction declines as well. A thermal reactor can maintain a chain reaction with a little as 1/10 as much U-233 as a fast reactor. That means with a given amount of U-233, you can start 10 thermal breeders for every fast breeder you can start. The implications of this are enormous if you want to start a lot of breeder reactors quickly. Thorium Cycle Molten Salt Reactors (LFTRs) are capable of true breeding with a conversion ration of greater than one. Because it is possible to start so many more thermal reactors with a given amount of fissionable material, it is quite clear that fueling a very large number of LFTRs is possible. Over 40 years ago, Alvin Weinberg explained:

WHY DEVELOP MOLTEN-SALT BREEDERS?

Nuclear power, based on light-water-moderated converter reactors, seems to be an assured commercial success. This circumstance has placed upon the Atomic Energy Commission the burden of forestalling any serious rise in the cost of nuclear power once our country has been fully committed to this source of energy. It is for this reason that the development of an economical breeder, at one time viewed as a long-range goal, has emerged as the central task of the atomic energy enterprise. Moreover, as our country commits itself more and more heavily to nuclear power, the stake in developing the breeder rises—breeder development simply must not fail. All plausible paths to a successful breeder must therefore be examined carefully.

To be successful a breeder must meet three requirements. First, the breeder must be technically feasible. Second, the cost of power from the breeder must be low; and third, the breeder should utilize fuel so efficiently that a full-fledged-energy economy based on the breeder could be established without using high-cost ores. The molten-salt breeder appears to meet these criteria as well as, and in some respects better than, any other reactor system. Moreover, since the technology of molten-salt breeders hardly overlaps the technology of the solid-fueled fast reactor, its development provides the world with an alternate path to long-term cheap nuclear energy that is not affected by any obstacles that may crop up in the development of the fast breeder.

The molten-salt breeder, though seeming to be a by-way in reactor development, in fact represents the culmination of more than 17 years of research and development. The incentive to develop a reactor based on fluid fuels has been strong ever since the early days of the Metallurgical Laboratory. In 1958 the most prominent fluid-fuel projects were the liquid bismuth reactor, the aqueous homogeneous reactor, and the molten-salt reactor. In 1959 the AEC assembled a task force to evaluate the three concepts. The principal conclusion of their report was that the "molten-salt reactor has the highest probability of achieving technical feasibility."

This verdict of the 1959 task force appears to be confirmed by the operation of the Molten-Salt Reactor Experiment. To those who have followed the molten-salt project closely, this success is hardly surprising. The essential technical feasibility of the molten-salt system is based on certain thermodynamic realities first pointed out by the late R.C. Briant, who directed the ANP project at ORNL. Briant pointed out that molten fluorides are thermodynamically stable against reduction by nickel-based structural materials; that, being ionic, they should suffer no radiation damage in the liquid state; and that, having low vapor pressure and being relatively inert in contact with air, reactors based on them should be safe. The experience at ORNL with molten salts during the intervening years has confirmed Briant's chemical intuition. Though some technical uncertainties remain, particularly those connected with the graphite moderator, the path to a successful molten-salt breeder appears to be well defined.

We estimate that a 1000 MWe molten-salt breeder should cost $115 per kilowatt (electric) and that the fuel cycle cost ought to be in the range of 0.3 to 0.4 mill/kWh. The overall cost of power from a privately owned, 1000-MWe Molten-Salt Breeder Reactor should come to around 2.6 mills/kWh. In contrast to the fast-breeder, the extremely low cost of the MSBR fuel cycle hardly depends upon sale of byproduct fissile material. Rather, it depends upon certain advances in the chemical processing of molten fluoride salts that have been demonstrated either in pilot plants or laboratories: fluoride volatility to recover uranium, vacuum distillation to rid the salt of fission products, and for highest performance, but with somewhat less assurance, removal of protactinium by liquid-liquid extraction or absorption.

The molten-salt breeder, operating in the thermal Th-233U cycle, is characterized by a low breeding ratio: the maximum breeding ratio consistent with low fuel-cycle costs is estimated to be about 1.07. This low breeding ratio is compensated by the low specific inventory* of the MSBR. Whereas the specific inventory of the fast reactor ranges between 2.5 to 5 kg/MWe the specific inventory of the molten-salt breeder ranges between 0.4 to 1.0 kg/MWe. The estimated fuel doubling time for the MSBR therefore falls in the range of 8 to 50 years. This is comparable to estimates of doubling times of 7 to 30 years given in fast-breeder reactor design studies.

From the point of view of long-term conservation of resources, low specific inventory in itself confers an advantage upon the thermal breeder. If the amount of nuclear power grows linearly, the doubling time and the specific inventory enter symmetrically in determining the maximum amount of raw material that must be mined in order to inventory the whole nuclear system. Thus, low specific inventory is an essential criterion of merit for a breeder, and the detailed comparisons in the next section show that a good thermal breeder with low specific inventory could, in spite of its low breeding gain, make better use of our nuclear resources than a good fast breeder with high specific inventory and high breeding gain.

The molten salt approach to a breeder promises to satisfy the three criteria of technical feasibility, very low power cost, and good fuel utilization. Its development as a uniquely promising competitor to the fast breeder is, we believe, in the national interest.

Needless to say, Weinberg's cost estimates are not current, still there are plausible grounds for believing that both Molten Salt Converters and LFTRs will be less expensive than conventional reactors. In addition Molten Salt Reactors offer greater safety, a very significant reduction of the problems associated with disposal of used nuclear fuel, a reduced possibility of nuclear proliferation, and a probably lower energy costs. Molten Salt reactors offer a route to supplying peak demand electricity from post-carbon energy sources. They offer a potential for producing heat for industrial processes, and backing up renewable generated electricity.