Nuclear Energy: Fuel of the Future?

The Plutonium Problem

There are considerable stockpiles of plutonium in both civilian and military facilities. The plutonium has accumulated in nuclear waste storage facilities from the nuclear power programs of different countries. There is general public and political concern about the misuse of plutonium for nuclear weapons and the accidental release of radiotoxic elements into the environment, so plutonium inventories should be reduced (1). Due to these concerns, plutonium must be kept under strong security. The cost of tight security increases the cost of plutonium.

The Thorium Solution

In order to reduce the amount of plutonium in circulation, it can be used in nuclear reactors as fuel. Firstly, plutonium can be mixed with uranium fuel to form mixed oxide fuel (MOX). Second generation plutonium is then produced, which can then be combined with thorium to be burned again. The thorium cycle is preferable from a non-proliferation point of view (see Figure below). Thorium-based fuel can be used in Canada Deuterium Uranium (CANDU) reactors (1). Sahin et al. tested the efficiency of two different fuel compositions in a CANDU reactor. The fuels they used were 96% thoria (ThO₂) + 4% PuO₂ and 91% ThO₂ + 5% UO ₂ + 4% PuO₂. Reactor grade plutonium can be used as a booster fissile fuel. Russia is also working on developing a thorium-plutonium fuel in order to use up the weapons-grade plutonium.
Figure 1: The face-centered cubic structure of plutonium dioxide structure, Pu atoms in green, O atoms in red

Thorium is much more plentiful in the environment than uranium. The thorium fuel cycle also produces much less plutonium, which means there is less risk of nuclear weapon proliferation from spent nuclear waste. How can thorium be used as a nuclear fuel? Although it is not fissile itself, it will absorb slow neutrons and produce uranium-233 (2). One advantage of U-233 is that it is better than uranium-235 and plutonium-239 because of its higher neutron yield per neutron absorbed. For conventional pressurized-water reactors, fuel assemblies can be arranged such that the thorium fuel rods are surrounded by a more-enriched seed element which contains U-235. The uranium-235 provides neutrons to the subcritical blanket. The U-233 produced there is burned for fuel.
Figure 2: The Thorium fissile process
Thorium fuel can be developed such that it is nuclear proliferation resistant. Each fuel assembly can have 20% U-235 fuel assembly. The blanket should consist of thorium with some U-238. Any uranium that is chemically separable from it is unsuitable for weapons. Spent blanket fuel decays rapidly because it creates handling problems, which confers proliferation resistance. The plutonium produced by the spent fuel will also have a large proportion of Pu-238, which generates a lot of heat and makes it unuseable for weapons.

Thorium-Plutonium Fuel versus MOX

Thorium-plutonium fuel has several advantages over mixed-oxide fuel (MOX). Thorium-plutonium fuel cannot be converted easily into nuclear weapons. Existing reactors only need slight modifications to burn the thorium-plutonium fuel. A lot more fuel can be used in a single fuel assembly with thorium-plutonium fuel than MOX fuel, so more plutonium is disposed of. There is less spent fuel leftover from the thorium-plutonium fuel than from MOX, and the spent fuel is less likely to allow recovery of weapons-usable material because less fissile plutonium remains in it (2).

Other Fuel Options

Other fuels are currently being researched in addition to thorium fuel. Increasing the thermal conductivity is desired because uranium oxide has a low thermal conductivity. The thermal conductivity also decreases as the temperature goes up. Metal fuels have the advantage of a much higher heat conductivity than oxide fuels but they cannot survive equally high temperatures.
TRIGA fuel
TRIGA fuel is used in TRIGA (Training, Research, Isotopes, General Atomics) reactors. The fuel used consists of a uranium zirconium hydride matrix (3). TRIGA fuel is safer than other nuclear fuels because in even that it reaches a high temperature, the hydrogen’s cross section in the fuel is shifted to higher energies, allowing more neutrons to be lost, and less to be thermalized. Most cores that use this fuel are “high leakage” cores where the excess leaked neutrons can be utilized for research.
Actinide Fuel
In a fast neutron reactor, the minor actinides produced by neutron capture of uranium and plutonium can be used as fuel. Metal actinide fuel is typically an alloy of zirconium, uranium, plutonium and the minor actinides. It can be made safe by changing the thermal expansion of the metal alloy in order to increase neutron leakage.
Ceramic Fuels
Ceramic fuels have high heat conductivities and melting points, but they are less understood than oxide fuels. They are also more prone to swelling. Uranium nitride is the ceramic fuel used by NASA. Uranium carbide is an attractive fuel because of its high heat conductivity and melting point and its lack of oxygen, which cannot form O₂ during the course of radiation. In Kalpakkam, India, a Fast Breeder Test Reactor has been operating on mixed carbide fuel since 1985. It is used in advanced liquid metal cooled fast breeder reactors (4). The thermal conductivity of mixed carbide fuels increases with temperature, which has an advantage over uranium dioxide fuel.
Figure 3: Thermal Conductivity of MKI [(Pu_0.70U_0.30)C]and MKII [(Pu_0.55U_0.45)C] as a function of temperature (2)

TRISO Fuels
Tristructural isotropic fuel consists of UO_x in the center coated with four layers of three different isotropic materials. Starting from the inside, the first layer is porous carbon buffer layer. The next layer is a layer of pyrolytic carbon (PyC), followed by a ceramic layer of silicone carbide (SiC) in order to hold in the nuclear fission products and give the TRISO fuel pellet more structure. Finally, the outer layer is also PyC. TRISO fuels are designed so that they will not crack, even due to fission gas pressure or thermal expansion up to 1600 degrees Celsius. This design makes the fuel especially resistant to accidents in a properly designed reactor. The pebble-bed modular reactor (PBMR) and a prismatic-block gas cooled reactor can both handle TRISO fuels properly. These two designs are both Generation IV reactors.
Figure 4: TRISO fuel (5)

Liquid fuels
There are two important types of liquid fuel. Molten anhydrous salts include fuels where the fuel is dissolved in the coolant. They were used in the molten salt reactor experiment and numerous other liquid core reactor experiments. The liquid fuel for the molten salt reactor was LiF-BeF2-ThF4-UF4 (72-16-12-0.4 mol%), it had a peak operating temperature of 705 °C in the experiment but could have gone to much higher temperatures since the boiling point of the molten salt was in excess of 1400 °C. The second type of fuel, the aqueous solution of uranyl sulfate, is used in the Aqueous Homogeneous Reactors. This homogenous reactor type has not been used for any large power reactors. One of its disadvantages is that the fuel is in a form which is easy to disperse in the event of an accident.