Low-cost, clean energy can help solve many world problems, from global warming to overpopulation to GDP stagnation. Nuclear power can be such an energy source, but the costs of solid fuel reactors continue to rise. Needed energy cost innovation may arise from a new technology -- liquid fuel nuclear reactors.

Global warming arises principally from CO2 emissions from coal combustion, rising as developing nations seek to increase prosperity. Proposed new coal-fired plants will double the world’s 1400 GW of coal-generated electricity. Repeated fruitless climate-treaty negotiations from Kyoto to Durban illustrate that nations’ economic needs trump global politics. Replacing coal combustion requires clean energy technologies cheaper than coal. Wind and solar sources are far too expensive and intermittent. Availability of advanced nuclear power cheaper than coal will dissuade nations burning coal for energy, through the immutable law of economic self-interest.

Rising populations in the poor nations are unsustainable, leading to more contention for natural resources. Affordable electric power can end energy poverty and develop lifestyles that include diminishing birthrates.

Petroleum is the second largest source of CO2 emissions. As petroleum-sourced fuels become more expensive, low-cost, high-temperature energy sources may be used to fabricate economic, carbon-neutral, synthetic vehicle fuels.

World GDP growth continues to drop as the availability of low-cost energy wanes; the fiscal crisis worsens with the loss of the ability to use economic growth to escape from mounting national debts. Cheaper energy sources may drive GDP growth sufficiently high to mitigate these consequences and avoid fiscal crises.

Nuclear power can be such an energy source. It is inexhaustible, emits no CO2, disturbs a tiny fraction of the land of coal mining, and has the smallest deathprint of any energy source. But despite improvements in solid fuel nuclear reactors, their costs continue to rise. Energy cost innovation may arise from a new technology -- liquid fuel nuclear reactors.

This article is presented in three parts

Part 1: Liquid Fuel Nuclear Reactors

Part 2: Energy cost innovation

Part 3: Global Impacts of Low-Cost Clean Energy

LIQUID FUEL NUCLEAR REACTORS

Since the 1970s conventional nuclear power reactors have been cooled by ~300°C water pressurized up to approximately 160 atmospheres to remain liquid to transfer heat. The term light water reactors (LWR) differentiates them from Canadian heavy water reactors. LWRs use water-moderated, slow neutrons because the U-235 fission probability is orders of magnitude higher for slow neutrons than for the fast neutrons produced directly by the fission process.

Several new nuclear technologies are competing for the attention of utilities, governments, and industry. The new commercial reactor designs are Gen III+ LWR power plants, which will enter service in 2014. In China AREVA is building two European Pressurized Reactors and Westinghouse is completing two of four AP1000 reactors. Four AP1000 reactors are now under construction in the US.

Small modular reactors (SMRs) are LWRs that are being developed and marketed to utilities unable to risk billions of dollars of up-front capital. Babcock & Wilcox and NuScale lead in SMR development.

The US DOE supports design work for the Next Generation Nuclear Plant (NGNP), a high-temperature, gas-cooled reactor with ceramic-coated-particle uranium-oxide fuel. Shown in the film Pandora’s Promise, Argonne National Laboratories designed the Integral Fast Reactor (IFR), gaining experience with liquid-metal fast breeder reactors (LMFBRs) such as EBR-II. GE used this research to design the Prism fast reactor, now on offer by GE to the UK. New entrant TerraPower’s traveling wave reactor (TWR) design has morphed to a design similar to IFR.

Features

New power plants are marketed to utilities, and to regulators such as Nuclear Regulatory Commission (NRC), and also to the broad public. The marketing messages from nuclear reactor companies focus more on features than cost. NRC requires core melt-down risk be less than 1 in 10,000 reactor-years; competitors use probability risk assessment to claim less than 1 in a million reactor-years. The Fukushima accident educated the public and utilities about the importance of cooling of fission products, especially without electric power for pumps, sensors, and controls. Although no one was killed by radiation, the financial loss was huge. New reactor designs provide more reliable cooling of fission product decay heat. The amount of spent fuel waste concerns politicians; new LWRs obtain more energy from fuel rods before they are disposed of, reducing waste.

Costs

High costs are concerning. With the advent of shale gas, utility executives find that natural gas generators can provide electric power cheaper than nuclear power plants. Four US nuclear plants have shut down. Some new reactors are under construction as the nuclear industry tries to address costs through evolutionary design improvements. The Westinghouse AP1000 has 35% fewer safety-grade pumps, 50% fewer valves, 80% less pipe, and 85% less cable. But technology evolution may not be sufficient to economically displace coal and natural gas power plants.

Solid fuel

LWR fuel rods contain ceramic pellets of UO2 with fissile uranium U-235 expensively enriched to 4% or more, the remainder being U-238. After about 5 years of use the fuel is spent and must be removed, because fissile material is depleted, noble gases such as krypton and xenon build up, and other fission products such as cesium, strontium, and samarium accumulate and absorb neutrons, keeping them from sustaining the chain reaction. The fuel rods are stressed by uneven temperatures up to 2000°C at the center, by radiation damage that breaks covalent UO2 bonds, and by fission products that disturb the solid lattice structure. As the ceramic fuel swells and distorts, the irradiated zirconium cladding tubes must continue to contain it and all fission products while in the reactor and for centuries thereafter in a waste storage repository.

The spent nuclear fuel contains transuranic elements such as plutonium Pu-239, created after U-238 nuclei absorb neutrons. Some Pu-239 is fissioned during operation, contributing as much as a third of reactor power. All such transuranics could eventually be destroyed in the neutron flux, either by fission or transmutation to a fissile element that later fissions, except that the solid fuel must be removed long before. Nuclear waste storage concerns stem primarily from the decay heat of these long-lived transuranics, which could largely be consumed by leaving them in the neutron flux.

Figure 1. New elements form when actinides fission or absorb neutrons.

Figure 1. New elements form when actinides fission or absorb neutrons.

A liquid fuel irradiated and fissioned in a neutron flux is not subjected to the structural stresses of a solid fuel. Fission products like xenon gas bubble out, so they do not absorb neutrons from the chain reaction. Noble fission product solids like silver precipitate out. The transuranic elements can remain in the fluid fuel to absorb neutrons or fission, eventually releasing energy.

Early liquid fuel reactors

Enrico Fermi, who created the first nuclear reactor in a pile of graphite and uranium blocks at the University of Chicago, also started up the world’s first liquid fuel reactor, using uranium sulfate fuel dissolved in water. Nobel laureate Eugene Wigner conceived this technology in 1945; Alvin Weinberg built it. The water carries the dissolved fuel, moderates neutron speeds, transfers heat, and expands as the reaction heat increases, lowering moderation and stabilizing fission rate.

The hydrogen in H2O sometimes absorbs a neutron, robbing it from the chain reaction, so the aqueous reactor doesn’t reach criticality unless fueled with uranium enriched beyond the natural 0.7% isotopic abundance of U-235. Deuterium absorbs few neutrons, so by using expensive heavy water, D2O, aqueous reactors can use inexpensive, unenriched uranium.

This aqueous reactor at Oak Ridge National Laboratory fed 140 kWe into the electric grid for 1000 hours. In operation it successfully removed xenon fission products. The intrinsic reactivity control was so effective that shutdown was accomplished simply by turning off the steam turbine generator. Babcock & Wilcox is again proposing an aqueous reactor to produce the fission product molybdenum-99 for medical diagnosis and therapy.

Figure 2. At Oak Ridge in 1953, Richard Engel adds 300 g of uranium in 500 ml of heavy water

Figure 2. At Oak Ridge in 1953, Richard Engel adds 300 g of uranium in 500 ml of heavy water to generate electric power for 2 months, doing the work of 1,000 tons of coal.

US national laboratories also experimented with liquid metal fuels. Brookhaven designed a liquid-metal fuel reactor with circulating molten bismuth and uranium in the 1950s. This fluid fuel had the advantages of easy fuel handling and intrinsic criticality control, but the limited solubility of uranium in bismuth required enriched uranium to achieve criticality, and no such reactors were constructed.

Planning for a time when U-235 might become exhausted, Los Alamos National Laboratories developed the Los Alamos Molten Plutonium Reactor Experiment (LAMPRE) reactor. LAMPRE had a 600°C core with molten plutonium and iron contained in tantalum thimbles cooled by liquid sodium. This 1 MWt test reactor ran from 1961 until 1963.

Molten salt reactor

Oak Ridge scientists proposed the idea of a fluid fuel reactor with UF4 dissolved in molten fluoride salts. A mixture of LiF and BeF2 salts is fluid at temperatures as low as 360°C.  Light Li-7 and Be-9 nuclei, along with C-12 graphite, slow neutrons to lower energies for efficient uranium fission. Reactivity is stable as the expanding hot core salt dilutes the dissolved fuel and moderator and expresses some out the critical core volume. As temperatures rise, more neutrons are absorbed reducing the fission rate. The hot salt circulates to transfer thermal energy out of the reactor. The strong ionic bonds of the fluid fluoride salts are stable under irradiation at high temperature.

Oak Ridge built the first molten salt reactor (MSR), which ran for 100 hours in 1954 at temperatures up to 860°C – red hot! This Aircraft Reactor Experiment demonstrated intrinsic reactivity stability and automatically adjusted power, without control rods, as the 2.5 MWt heat exchanger airflow varied.

Figure 3. Jet engines would use heat from the liquid fuel nuclear reactor.

Figure 3. Jet engines would use heat from the liquid fuel nuclear reactor.

This success led to the design of a compact, 1.4 m diameter reactor containing a fluid core of UF4 dissolved in molten salt in a beryllium metal sphere. It heated liquid NaK (sodium and potassium metal) to transfer 200 MWt of power to aircraft turbine jet engines. This Fireball Reactor was never built because practical in-flight refueling allowed sustaining a fleet of airborne bombers, backed up by ICBMs in submarines and on land.

Thorium

Within a nuclear reactor the neutrons not only cause fissions but some are absorbed to create new elements. In today’s LWRs some U-238 becomes U-239, which beta decays to Np-239 and then to Pu-239, some of which is fissioned, providing up to one third of the power. Similarly if thorium is placed in a nuclear reactor some Th-232 becomes Th-233, which beta decays to protactinium-233  and then to U-233, which is fissile. Relatively little plutonium is produced starting from Th-232 because six more neutron absorptions are required than from  U-238.

Figure 4. Neutrons transmute fertile thorium-232 to fissile uranium-233.

Figure 4. Neutrons transmute fertile thorium-232 to fissile uranium-233.

Thorium fuel was successfully tested at the Shippingport power reactor, the first US power reactor. Thorium was used in the German THTR-300 pebble bed reactor. Alvin Radkowski founded Thorium Power to develop fuel rods to use thorium and plutonium in existing reactors. Carlo Rubbia at CERN conceived an accelerator-driven thorium reactor. India’s Kalpakkam sodium-cooled fast reactor under construction will also convert Th-232 to U-233. India plans to produce 30% of its electricity from thorium by 2050. China and Canada are testing thorium in heavy-water-moderated CANDU reactors. But all these use solid fuel forms and are constrained by the common limitations of all solid fuel reactors. Thorium is especially well suited to a liquid fuel reactor because it can be readily converted to uranium with slow neutrons, keeping the critical mass and the reactor small.

Thorium molten salt reactor – an ambitious breeder reactor

Eugene Wigner and Alvin Weinberg had already conceived the liquid-fuel thorium-to-uranium breeder reactor by 1943. The aqueous reactor was but the first step. In the 1960s Oak Ridge director Alvin Weinberg led development of the molten salt reactor for electric power generation, convinced that “humankind’s whole future depended on” this inexhaustible energy. Weinberg had rather accurately predicted the 21st century climate crisis

“….atmospheric concentration of 375-390 ppm may well be a threshold range at which climate change from CO2 effects will be separable from natural climate fluctuations … The consequences of an increase of this magnitude in atmospheric CO2 make it prudent to proceed cautiously in the large-scale use of fossil fuels."

Fig 5. In this single-fluid MSR U-235 is fissioned, Th-232 is bred to U-233

Fig 5. In this single-fluid MSR U-235 is fissioned, Th-232 is bred to U-233 that is fissioned, and U-238 is bred to Pu-239 that is fissioned. The noble gases, noble and semi-noble metals, and many fission product fluorides are continually removed.

The Molten Salt Reactor Experiment (MSRE) drew on the 1950s experience with the Aircraft Reactor Experiment. MSRE operated successfully over 4 years through 1969. To facilitate engineering tests Th-232 was not bred in the MSRE; the U-233 came from other reactors breeding Th-232. No turbine generator was attached; the fission energy heat was dissipated with a salt-to-air radiator.

MSRE was a success. It operated with U-235 and subsequently with U-233. Fission product xenon gas was continually removed to prevent unwanted neutron absorptions and avoid a hazardous buildup as in solid fuels. Online refueling was demonstrated. Minor intergrain boundary corrosion of the Hastelloy vessel was addressed. Oak Ridge also developed chemistry for separation of thorium, uranium, and fission products in the fluid fluoride salts. For example, UF4 (in solution) + F2 (gas) → UF6 (gas), so bubbling fluorine gas through the salt could remove the bred fissile uranium, leaving the thorium fluoride behind. Fluorination and distillation processes could separate fission products from the salt.

The Oak Ridge work was stopped when President Nixon decided instead to fund work on the solid fuel LMFBR in his home state, California, whose congressman Chet Holifield chaired the Joint Committee on Atomic Energy. The LMFBR bred plutonium-239 faster than the MSR bred uranium-233. Weinberg argued against this; he also expressed concerns about the safety of LWRs. In 1973 Holifield told him, “if you are concerned about the safety of reactors, then I think it may be time for you to leave nuclear energy.” Then he was fired and MSR funding ceased. Later Weinberg said “It was a successful technology that was dropped because it was too different from the main lines of reactor development.” MSR development requires chemical engineering expertise, and the liquid fuel technology is still unfamiliar to most nuclear engineers today, who are schooled in solid fuel reactor technologies.

Denatured molten salt reactor – a simple burner reactor

The DMSR (denatured molten salt reactor) depicted in Figure 5 contains fissile U-235 and bred U-233 fuel that is always diluted (denatured) with over 80% U-238, so the uranium is never suitable for weapons. The DMSR converts thorium to 75-80% of the fissile U-233 it would need to sustain criticality, so additional fissile U-235 is added. A DMSR’s bred Pu-239 fuel is mixed with Pu-240, making the plutonium unsuitable for weapons.. The simplicity of the single-fluid DMSR indicates it will be a low-cost, first-to-market version of MSR. It avoids the complexities of breeding, which today is not considered an necessary objective for nuclear innovation.

Part 2: Energy Cost Innovation, coming tomorrow, August 27, will introduce the opportunities for substantial cost reductions and present in detail the attributes of the molten salt reactor that lead to lower costs for energy.