The reactor core is the heart of any nuclear reactor and consists of fuel elements made of a suitable fissile material. There are presently four radioactive materials that are suitable for fission by thermal neutrons. They are uranium-233 (²³³ U), uranium-235 (²³⁵U), plutonium-239 (²³⁹Pu), and plutonium-241 (²⁴¹Pu). The isotopes uranium-238 (²³⁸U) and thorium-232 (²²²Th) are fissionable by fast neutrons. The following text discusses plutonium, uranium, and thorium as used for nuclear fuel.

Plutonium

Plutonium is an artificial element produced by the transmutation of ²³⁸U. It does exist in small amounts (5 parts per trillion) in uranium ore, but this concentration is not high enough to be mined commercially.

Plutonium is produced by the conversion of ²¹¹U into ²¹⁹pu according to the following reaction.

This reaction occurs in reactors designed specifically to produce fissionable fuel. These reactors are frequently called breeder reactors because they produce more fissionable fuel than is used in the reaction. Plutonium is also produced in thermal ²³⁵U reactors that contain ²¹⁸U. Plutonium can be obtained through the processing of spent fuel elements. To be useful as a fuel, plutonium must be alloyed to be in a stable phase as a metal or a ceramic.

Plutonium dioxide (PuO₂) is the most common form used as a reactor fuel. Pu0₂ is not used alone as a reactor fuel; it is mixed with uranium dioxide. This mixture ranges from 20% plutonium dioxide for fast reactor fuel to 3 % to 5 % for thermal reactors.

Plutonium-239 can serve as the fissile material in both thermal and fast reactors. In thermal reactors, the plutonium-239 produced from uranium-238 can provide a partial replacement for uranium-235. The use of plutonium-239 in fast reactors is much more economical, because breeding takes place, which results in the production of more plutonium-239 than is consumed by fission.

Uranium

The basic nuclear reactor fuel materials used today are the elements uranium and thorium. Uranium has played the major role for reasons of both availability and usability. It can be used in the form of pure metal, as a constituent of an alloy, or as an oxide, carbide, or other suitable compound. Although metallic uranium was used as a fuel in early reactors, its poor mechanical properties and great susceptibility to radiation damage excludes its use for commercial power reactors today. The source material for uranium is uranium ore, which after mining is concentrated in a "mill" and shipped as an impure form of the oxide U₃O₈ (yellow cake). The material is then shipped to a materials plant where it is converted to uranium dioxide (UO₂), a ceramic, which is the most common fuel material used in commercial power reactors. The UO₂ is formed into pellets and clad with zircaloy (water-cooled reactors) or stainless steel (fast sodium-cooled reactors) to form fuel elements. The cladding protects the fuel from attack by the coolant, prevents the escape of fission products, and provides geometrical integrity.

Oxide fuels have demonstrated very satisfactory high-temperature, dimensional, and radiation stability and chemical compatibility with cladding metals and coolant in light-water reactor service. Under the much more severe conditions in a fast reactor, however, even inert U0₂ begins to respond to its environment in a manner that is often detrimental to fuel performance. Uranium dioxide is almost exclusively used in light-water-moderated reactors (LWR). Mixed oxides of uranium and plutonium are used in liquid-metal fast breeder reactors (LMFBR).

The major disadvantages of oxide fuels that have prompted the investigation of other fuel materials are their low uranium density and low thermal conductivity that decreases with increasing temperatures. The low density of uranium atoms in U0₂ requires a larger core for a given amount of fissile species than if a fuel of higher uranium density were used. The increase in reactor size with no increase in power raises the capital cost of the reactor. Poor thermal conductivity means that the centerline temperature of the fuel and the temperature difference between the center and the surface of the fuel rod must be very large for sufficient fission heat be extracted from a unit of fuel to make electric power production economical. On the other hand, central fuel temperatures close to the melting point have a beneficial fission product scouring effect on the fuel.

Thorium

Natural thorium consists of one isotope, ²³²Th, with only trace quantities of other much more radioactive thorium isotopes. The only ore mineral of thorium, that is found in useful amounts is monazite. Monazite-bearing sands provide most commercial supplies. The extraction and purification of thorium is carried out in much the same manner as for uranium. Thorium dioxide (ThO₂) is used as the fuel of some reactors. Thorium dioxide can be prepared by heating thorium metal or a wide variety of other thorium compounds in air. It occurs typically as a fine white powder and is extremely refractory (hard to melt or work) and resistant to chemical attack.

The sole reason for using thorium in nuclear reactors is the fact that thorium (²³²Th) is not fissile, but can be converted to uranium-233 (fissile) via neutron capture. Uranium-233 is an isotope of uranium that does not occur in nature. When a thermal neutron is absorbed by this isotope, the number of neutrons produced is sufficiently larger than two, which permits breeding in a thermal nuclear reactor. No other fuel can be used for thermal breeding applications. It has the superior nuclear properties of the thorium fuel cycle when applied in thermal reactors that motivated the development of thorium-based fuels. The development of the uranium fuel cycle preceded that of thorium because of the natural occurrence of a fissile isotope in natural uranium, uranium-235, which was capable of sustaining a nuclear chain reaction. Once the utilization of uranium dioxide nuclear fuels had been established, development of the compound thorium dioxide logically followed.

As stated above, thorium dioxide is known to be one of the most refractory and chemically nonreactive solid substances available. This material has many advantages over uranium dioxide. Its melting point is higher; it is among the highest measured. It is not subject to oxidation beyond stoichiometric (elements entering into and resulting from combination) Th0₂. At comparable temperatures over most of the expected operating range its thermal conductivity is higher than that of U0₂. One disadvantage is that the thorium cycle produces more fission gas per fission, although experience has shown that thorium dioxide is superior to uranium dioxide in retaining these gases. Another disadvantage is the cost of recycling thoria-base fuels, or the "spiking" of initial-load fuels with ²³³U. It is more difficult because ²³³U always contains ²³²U as a contaminant. ²³²U alpha decays to ²²⁸Thwith a 1.9 year half-life. The decay chain of ²²⁸Thproduces strong gamma and alpha emitters. All handling of such material must be done under remote conditions with containment.

Investigation and utilization of thorium dioxide and thorium dioxide-uranium dioxide (thoria-urania) solid solutions as nuclear fuel materials have been conducted at the Shipping port Light Water Breeder Reactor (LWBR). After a history of successful operation, the reactor was shut down on October 1, 1982. Other reactor experience with ThO₂ and ThO₂-UO₂ fuels have been conducted at the Elk River (Minnesota) Reactor, the Indian Point (N.Y.) No. 1 Reactor, and the HTGR (High-temperature Gas-cooled Reactor) at Peach Bottom, Pennsylvania, and at Fort St. Vrain, a commercial HTGR in Colorado.

As noted above, interest in thorium as a contributor to the world's useful energy supply is based on its transmutability into the fissile isotope ²³³U. The ease with which this property can be utilized depends on the impact of the nuclear characteristics of thorium on the various reactor systems in which it might be placed and also on the ability to fabricate thorium into suitable fuel elements and, after irradiation, to separate chemically the resultant uranium. The nuclear characteristics of thorium are briefly discussed below by comparing them with ²³⁸U as a point of reference.

First, a higher fissile material loading requirement exists for initial criticality for a given reactor

system and fissile fuel when thorium is used than is the case for an otherwise comparable system using ²³⁸U.

Second, on the basis of nuclear performance, the interval between refueling for comparable thermal reactor systems can be longer when thorium is the fertile fuel. However, for a given reactor system, fuel element integrity may be the limiting factor in the depletion levels that can be achieved.

Third, ²³³Pa (protactinium), which occurs in the transmutation chain for the conversion of thorium to ²³³U, acts as a power history dependent neutron poison in a thorium-fueled nuclear reactor. There is no isotope with comparable properties present in a ²³⁸U fuel system.

Fourth, for comparable reactor systems, the one using a thorium-base fuel will have a larger negative feedback on neutron multiplication with increased fuel temperature (Doppler coefficient) than will a ²³⁸U-fueled reactor.

Fifth, for comparable reactor configurations, a ²³²Th/²³³U fuel system will have a greater stability relative to xenon-induced power oscillations than will a ²³⁸U/²³⁵U fuel system. The stability is also enhanced by the larger Doppler coefficient for the ²³²Th/²³³U fuel system.

And sixth, the effective value of ( for ²³²Th/²³³U systems is about half that of ²³⁵U^-fueled reactors and about the same as for plutonium-fueled reactors. A small value of means that the reactor is more responsive to reactivity changes.

In conclusion, the nuclear properties of thorium can be a source of vast energy production. As demonstrated by the Light Water Breeder Reactor Program, this production can be achieved in nuclear reactors utilizing proven light water reactor technology.

Nuclear Fuel Selection

The nuclear properties of a material must be the first consideration in the selection of a suitable nuclear fuel. Principle properties are those bearing on neutron economy: absorption and fission cross sections, the reactions and products that result, neutron production, and the energy released. These are properties of a specific nuclide, such as ²³²Th, and its product during breeding, ²³³U. To assess these properties in the performance of the bulk fuel, the density value, or frequency of occurrence per unit volume, of the specific nuclide must be used.

Once it has been established that the desired nuclear reaction is feasible in a candidate fuel material, the effect of other material properties on reactor performance must be considered. For the reactor to perform its function of producing usable energy, the energy must be removed. It is desirable for thermal conductivity to be as high as possible throughout the temperature range of operations and working life of the reactor. High thermal conductivity allows high power density and high specific power without excessive fuel temperature gradients. The selection of a ceramic fuel represents a compromise. Though it is known that thermal conductivities comparable to those of metals cannot be expected, chemical and dimensional stability at high temperature are obtained.

Because the thermal conductivity of a ceramic fuel is not high, it is necessary to generate relatively high temperatures at the centers of ceramic fuel elements. A high melting point enables more energy to be extracted, all other things being equal. In all cases, the fuel must remain well below the melting point in normal operation, but a higher melting point results in a higher permissible operating temperature.

The dimensional stability of the fuel under conditions of high temperature and high burnup is of primary importance in determining the usable lifetime. The dimensional stability is compromised by swelling, which constricts the coolant channels and may lead to rupture of the metal cladding and escape of highly radioactive fission products into the coolant. The various other factors leading to the degradation of fuel performance as reactor life proceeds (the exhaustion of fissionable material, the accumulation of nonfissionable products, the accumulation of radiation effects on associated nonfuel materials) are all of secondary importance in comparison to dimensional stability of the fuel elements.

The main cause of fuel element swelling is the accumulation of two fission product atoms for each atom fissioned. This is aggravated by the fact that some of the fission products are gases. The ability of a ceramic fuel to retain and accommodate fission gases is therefore of primary importance in determining core lifetime.

The chemical properties of a fuel are also important considerations. A fuel should be able to resist the wholesale change in its properties, or the destruction of its mechanical integrity, that might take place if it is exposed to superheated coolant water through a cladding failure. On the other hand, certain chemical reactions are desirable.

Other materials such as zirconium and niobium in solid solution may be deliberately incorporated in the fuel to alter the properties to those needed for the reactor design. Also, it is generally advantageous for some of the products of the nuclear reaction to remain in solid solution in the fuel, rather than accumulating as separate phases.

The physical properties of the fuel material are primarily of interest in ensuring its integrity during the manufacturing process. Nevertheless they must be considered in assessments of the integrity of the core under operating conditions, or the conditions of hypothetical accidents. The physical and mechanical properties should also permit economical manufacturing. The fuel material should have a low coefficient of expansion.

It is not possible to fabricate typical refractory ceramics to 100% of their theoretical density. Therefore, methods of controlling the porosity of the final product must be considered. The role of this initial porosity as sites for fission gas, as well as its effects on thermal conductivity and mechanical strength, is a significant factor in the design.