Once-through nuclear reactor fuel compounds

The improved oxide, once-through plutonium fuel compound that can be used for nuclear fission in currently operating light-water reactors and fast reactors has a composition in the range defined by the lines that connect the three compositional points of a three-component system consisting of plutonium dioxide (PuO.sub.2), a plutonium host phase and alumina (Al.sub.2 O.sub.3). The compound also has such a phase structure that two phases, the plutonium host phase having plutonium dioxide dissolved therein and the alumina phase, are in equilibrium.

BACKGROUND OF THE INVENTION 
This invention relates to compounds that can be used as nuclear reactor 
fuels. More particularly, it relates to oxide, once-through pIutonium Fuel 
compounds that can be used for nuclear Fission in currently operating 
light-water reactors and fast reactors. 
The nuclear fuel compounds that are used in currently operating light-water 
reactors and Fast reactors are uranium dioxide (UO.sub.2), gadolinia 
(Gd.sub.2 O.sub.3) doped uranium dioxide, and mixed uranium and plutonium 
dioxide ((U,Pu)O.sub.2). These nuclear Fuel compounds are pressed into 
pellets and enclosed in metal cladding, which is made of either a 
zirconium alloy in light-water reactors or stainless steel in fast 
reactors. The thus manufactured fuel assembly is used in a nuclear 
reactor, where it is irradiated with neutrons and part of the uranium and 
plutonium in the fuel compounds undergoes fission to produce a group of 
elements that are called "fission products" (FP). The resulting thermal 
energy is used for electric power generation. 
The proportion by which uranium and plutonium in a fuel compound undergoes 
nuclear fission is called "burnup" and expressed in percent (%). Burnup, 
which is determined primarily by the operating conditions of a nuclear 
reactor and the stability of fuel cladding, is in the range from 3 to 5%. 
In other words, 3-5% of the uranium and plutonium in the fuel compound 
undergoes nuclear fission. The spent nuclear fuel is dissolved in acid, 
and the uranium and plutonium which are useful as fuels are separated from 
fission products. The procedure involving these steps is called 
"reprocessing". The separated uranium and plutonium are re-converted to a 
nuclear fuel compound for another use. The fission products are melted in 
glass and subsequently solidified (vitrified). The solid glass, which is 
referred to as a high-level radioactive waste, is buried in a deep 
geological formation. Since the flows of uranium and plutonium form a 
cycle, the materials flow described above is conventionally referred to as 
"a nuclear fuel cycle". 
With the constant pressure for nuclear disarmament added to the production 
of plutonium in the nuclear fuel cycle, the excess amount of plutonium has 
been a global problem and efforts are being made in various countries of 
the world to develop effective methods that permit plutonium to be used or 
processed at sites other light-water reactors and fast reactors. The 
proposals made to date are classified to fall within one of the following 
two categories (W. J. Broad, Inter. Herald Tribune, Apr. 7, 1993): 
(1) Use as a fuel in a new type of reactor 
A helium-cooled reactor is newly developed so that it can be operated with 
plutonium used as a fuel. The problem with this idea is that huge amounts 
of expenditure and time are necessary to develop the new type of reactor. 
(2) Disposal after vitrification 
Excess plutonium is simply converted to waste without further use. The 
processing cost is smaller than in the case of developing the first 
method; however, from the viewpoint of nuclear fuel cycle which aims at 
effective utilization of plutonium, the loss of resources is extremely 
great and, furthermore, the long-term stability of the solid glass is also 
an important consideration since it has solidified after incorporating a 
large amount of molten plutonium. 
SUMMARY OF THE INVENTION 
An object, therefore, of the present invention is to provide a new 
plutonium fuel compound that is free from the problems with the 
utilization and processing methods that have heretofore been developed 
with a view to reducing the amount of excess plutonium, i.e., (1) that can 
be developed at low cost and (2) that obviates the need for special waste 
management, thereby causing only a small impact on the existing nuclear 
fuel cycle. 
A further object of the present invention is to provide a new plutonium 
fuel compound of a once-through type that has the following 
characteristics: 
(1) with the currently operating nuclear facilities being used as such, the 
new fuel compound undergoes substantially complete nuclear fission in a 
light-water reactor or fast reactor for electric power generation; and 
(2) the spent fuel compound need not be reprocessed or solidified but can 
be directly handled as a high-level waste.

DETAILED DESCRIPTION OF THE INVENTION 
The plutonium fuel compound of the present invention has the following 
advantages: 
(1) it can be manufactured by means of the technology currently employed to 
fabricate oxide fuels: 
(2) since it can be used in currently operating light-water reactors or 
fast reactors for electric power generation, the huge amounts of 
expenditure and time that would otherwise be required to develop a new 
type of reactor can be reduced; and 
(3) its composition and phase structure are adjusted in such a way as to 
eliminate the need for reprocessing or solidification of spent fuels and, 
hence, the compound can be managed as a high-level radioactive waste as it 
is enclosed in metal cladding. Since the high-level radioactive waste is 
expected to be so chemically stable and resisting to weathering that it 
can solve all the problems associated with the methods so far proposed for 
volume reduction of excess plutonium. 
A further advantage of the fuel compound of the present invention is that 
It does not interfere with the efforts currently made in various parts of 
the world to recycle nuclear fuels and that therefore it can be used in 
harmony with the currently employed nuclear fuels. 
Having these advantages, the plutonium fuel compound of the present 
invention must satisfy the following three fuel conditions in terms of 
composition and phase structure. 
Fuel condition 1: The fuel compound under consideration should be an oxide 
system that is either a single-phase compound capable of becoming a 
plutonium host phase with 35 mol % of plutonium being dissolved or a 
multi-phase equilibrium compound in which a plutonium host phase can exist 
in a thermodynamically stable manner; 
Fuel condition 2: The behavior of the plutonium host phase in the process 
of nuclear fission and the behavior to irradiation should be capable of 
being estimated and evaluated from the heretofore accumulated technical 
database; and 
Fuel condition 3: The fission products in the spent fuel compound should 
react with other components to produce stable high-level radioactive 
wastes without any special processing. 
The present inventor conducted extensive studies on the changes in the 
physical and chemical properties of oxide nuclear fuels that would take 
place in light-water reactors and fast reactors during nuclear fission, as 
well as on the stable product of solidification of high-level radioactive 
wastes, and the made comparative review on the thermodynamic properties 
and the like of various compounds. As a result, the inventor has found 
that both (1) a plutonium dioxide-thoria-alumina system and (2) a 
plutonium dioxide-stabilized zirconia-alumina system satisfy the three 
fuel conditions set forth in the preceding paragraph. The two systems are 
individually described below in detail. 
(1) Plutonium dioxide-thoria-alumina system 
The compound of this system contains three compounds, plutonium dioxide 
(PuO.sub.2), thoria (ThO.sub.2) and alumina (Al.sub.2 O.sub.3), and is 
characterized by two phase at equilibrium, i.e., a phase with a fluorite 
type structure which is a solid solution of plutonium dioxide and thoria 
and an alumina phase. In this compound system, thoria having a fluorite 
type structure phase provides a plutonium host phase, whereby fuel 
condition i is satisfied. 
The plutonium host phase has a crystalline structure of the same fluorite 
type as conventional fuels and, hence, the physicochemical changes that 
occur upon irradiation can be estimated and evaluated from the technical 
database on conventional fuels, whereby fuel condition 2 is satisfied. 
The plutonium host phase does not react with alumina (Al.sub.2 O.sub.3). On 
the other hand, alumina reacts with fission products such as alkali metal 
elements and alkaline earth metal elements that can dissolve in limited 
amounts in the plutonium host phase, thereby forming stable compound. This 
satisfies fuel condition 3. 
(2) Plutonium dioxide-stabilized zirconia-alumina system 
The compound of this system contains three compounds, plutonium dioxide 
(PuO.sub.2), stabilized zirconia and alumina (Al.sub.2 O.sub.3), and is 
characterized by two phases at equilibrium, i.e., a phase with a fluorite 
type structure which is a solid solution of plutonium dioxide and 
stabilized zirconia and an alumina phase. In this compound system, 
stabilized zirconia having a fluorite type structural phase provides a 
plutonium host phase. This satisfies fuel condition 1. 
The plutonium host phase has a crystalline structure of the same fluorite 
type as conventional fuels and, hence, the physicochemical changes that 
occur upon irradiation can be estimated and evaluated from the technical 
database on conventional fuels. This satisfies Fuel condition 2. 
The plutonium host phase does not react with alumina (Al.sub.2 O.sub.3). On 
the other hand, alumina reacts with fission products such as alkali metal 
elements and alkaline earth metal elements that can dissolve in limited 
amounts in the plutonium host phase, thereby forming stable compounds. 
This satisfies fuel condition 3. 
The plutonium fuel compound of the present invention has either thoria 
(ThO.sub.2) or stabilized zirconia, both with a fluorite type structural 
phase, present as a plutonium host phase and two phases, the plutonium 
host phase and an alumina phase (Al.sub.2 O.sub.3), are in equilibrium. 
The characteristic utility of the compound is two-fold: (A) it is stable, 
both physically and chemically, as a fuel; (B) when spent, the compound is 
already stable as a high-level radioactive waste. 
(A) Physical and chemical stability as fuel 
The stability of the plutonium fuel compound of the present invention in a 
nuclear reactor may be considered in the following two aspects: the 
radiation stability of the plutonium host phase having plutonium dissolved 
therein; and the solubility of fission products in the plutonium host 
phase. 
The plutonium host phase has a fluorite type structure and is well known to 
have high stability to radiations. It is also known that the plutonium 
host phase is highly capable of dissolving various elements, in 
particular, zirconium, rare earth elements, alkaline earth metal elements, 
etc. which account for at least 404 of the fission products. 
Hence, one may safely assume that the plutonium fuel compound has not only 
high radiation resistance but also great ability to dissolve fission 
products. 
(B) Stability as high-level radioactive waste 
The plutonium fuel compound of the present invention, once it has been 
spent, has the following four phases at equilibrium. The properties and 
stability features of the respective phases are described below. 
(1) Plutonium host phase 
This phase contains thoria (ThO.sub.2) or stabilized zirconia as a main 
component, with plutonium having being extinguished during fission. 
Fission products that can be dissolved in this host phase include 
zirconium, rare earth element (cerium, neodymium. etc.) and some alkaline 
earth metal elements (strontium and barium). 
Both thoria (ThO.sub.2) and stabilized zirconia are well known to be two of 
the most chemically stable ceramics. Hence, they are assumed to retain the 
high resistance to weathering, water, etc. even if they have the 
above-mentioned fission products dissolved in small amounts. 
(2) Magnetoplumbite type phase 
Fission products, in particular, alkali metal elements (cesium and 
rubidium) and alkaline earth metal elements (strontium and barium) will 
react with alumina in the plutonium fuel compound of the present invention 
to form magnetoplumbite type phases (e.g., SrO.6Al.sub.2 O.sub.3). 
The magnetoplumbite type phases have the same crystalline structure as 
natural stable hibonite (CaO.6Al.sub.2 O.sub.3) and one may well assume 
that they have high resistance to weathering, water, etc. 
(3) Alloy phase 
The interior of the fuel cladding for enclosing the plutonium fuel compound 
of the present invention is filled with a low-oxygen potential atmosphere 
as in the case of currently used fuels. Hence, certain fission products 
(e.g., molybdenum, ruthenium, palladium and rhodium) are reduced to the 
metallic form, thereby forming alloy phases. 
The alloy phases are so-called "noble metal alloys" and their chemical 
stability is well known. Alloy phases of the same type are also generated 
in currently employed fuels and are known to be slightly soluble in acids. 
(4) Alumina phase 
The plutonium fuel compound of the present invention has alumina added 
desirably in an excess amount so as to promote the progress of the 
formation of magnetoplumbite type phases and its composition is adjusted 
accordingly. Hence, in the spent fuel at least 80% of alumina remains 
unreacted. 
It is widely known that like thoria and stabilized zirconia, alumina is one 
of the most chemically stable ceramics. 
Therefore, the high-level radioactive waste in which the four phases 
(1)-(4) are in equilibrium is composed not only of compounds similar to 
the natural rocks and ores that can remain stable over several million 
years but also of chemically stable ceramics. In addition, compounds of a 
similar phase structure have already been shown to exhibit high water 
resistance. Hence, the high-level radioactive waste under consideration is 
also anticipated to display satisfactory resistance to weathering, water, 
etc. (see T. Muromura, "The Geological Disposal of High Level Radioactive 
Wastes", (Ed) D.C. Brookins, pp. 263-289, Theophrestus Publications, S. A. 
Athens (1987)). 
Shown below are several examples of the present invention that verify the 
following two important facts about the plutonium fuel compound of the 
present invention: it contains two phases, a plutonium host phase (with a 
fluorite type structure) and an alumina phase, in equilibrium; when spent, 
it contains four phases in equilibrium, which are primarily a plutonium 
host phase (with a fluorite type structure), a magnetoplumbite type phase, 
an alloy phase and an alumina phase. Further, the compositional range of 
the plutonium fuel compound of the present invention was determined on the 
basis of the results obtained in the examples. 
Example 1: Experiment to Verify the Availability of Plutonium Fuel Compound 
System Consisting of Plutonium Dioxide, Thoria and Alumina 
The following three samples were prepared and the availability of the fuel 
compound system mentioned above was reviewed. 
Sample 1--1: Plutonium-free sample 
Solutions of thorium and aluminum, as rendered acidic with nitric acid, 
were mixed at mol % ratios of 47.4:52.6 (thoria/alumina) and evaporated to 
dryness. The dried product was heated at 800.degree. C. in air atmosphere 
to form a mixture of oxides. In accordance with the current practice of 
nuclear fuel fabrication, the mixture of oxides was shaped into pellets 
each having an outside diameter of 7 mm and weighing about 500 mg; the 
pellets were sintered by heating at 1,500.degree. C. for 4 h in a hydrogen 
(H.sub.2) stream. The sinter was then ground into a powder, which was 
subjected to X-ray diffraction for identifying the produced phases. 
The X-ray diffraction scan showed that the reaction product contained two 
phases in equilibrium, which were a thoria phase, or a plutonium host 
phase (with a fluorite type structure) having a lattice constant of 
5.596.ANG. and an alumina phase. 
Sample 1-2: Plutonium-containing sample 
Solutions of plutonium, thorium and aluminum, as rendered acidic with 
nitric acid, were mixed at mol % ratios of 5:45:50 (plutonium 
dioxide/thoria/alumina) and evaporated to dryness. The dried product was 
heated at 800.degree. C. in air atmosphere to form an oxide. In accordance 
with the current practice of nuclear fuel fabrication, the oxide was 
shaped into pellets each having an outside diameter of 7 mm and weighing 
about 500 mg; the pellets were sintered by heating at 1,500.degree. C. for 
4 h in a hydrogen (H.sub.2) stream. The sinter was ground into particles, 
which were subjected to X-ray diffraction for identifying the produced 
phases. 
The X-ray diffraction scan showed that the reaction product contained two 
phases at equilibrium, which were a plutonium host phase (with a fluorite 
type structure) having a lattice constant of 5.575.ANG. and an alumina 
phase. 
Sample 1-3: Sample containing simulated fission products 
Solutions of simulated fission products (see Table 1), thorium and 
aluminum, as rendered acidic with nitric acid, were mixed at mol % ratios 
of 5:45:50 (simulated fission products/thoria/alumina) and evaporated to 
dryness. The quantities of the simulated fission products are equivalent 
to the fission of all plutonium in sample 1-2. In other words, one gram of 
fission products is equivalent to the fission of one gram of plutonium. 
The respective simulated fission products and their relative quantities 
are shown in Table 1. With a view to promoting the reaction of alumina 
with certain fission products (i.e., alkali metal elements and alkaline 
earth metal elements), alumina was added in an amount about 5 times as 
many as the valve required to form magnetoplumbite. The dried product was 
calcined by heating at 500.degree. C. in a stream consisting of a mixture 
of 4% hydrogen (H.sub.2) and 96% helium (He). The calcine was shaped into 
pellets each having an outside diameter of 7 mm and weighing about 500 mg; 
the pellets were sintered by heating at 1,500.degree. C. for 4 h in a 
stream consisting of 504 carbon dioxide (CO.sub.2) and 50% carbon monoxide 
(CO). The mixed stream was employed to simulate the oxygen potential in 
nuclear fuels (ca. -300 kJ/mol O.sub.2). The sinter was ground into 
particles and subjected to X-ray diffraction for identifying the 
constitutional phases. 
The X-ray diffraction scan showed that the reaction product contained four 
phases, a plutonium host phase (with a fluorite type structure), a 
magnetoplumbite phase, an alloy phase and an alumina phase, at 
equilibrium. 
TABLE 1 
______________________________________ 
Simulated Plutonium Fission Products (from Fast Reactor) 
Element g.multidot.atom % 
wt. % substituting for 
______________________________________ 
1. Zr (zirconium) 
11.31 9.02 none 
2. Ce (cerium) 
13.74 16.83 part of La and Pr 
3. Nd (neodymium) 
13.86 17.50 part of Pm, Sm, 
Eu, Gd and Y 
4. Cs (cesium) 
13.07 15.21 part of Rb 
5. Sr (strontium) 
6.26 4.80 part of Ba 
6. Mo (molybdenum) 
16.38 13.76 part of Tc 
7. Ru (rubidium) 
13.20 11.68 none 
8. Rh (ruthenium) 
3.65 3.28 none 
9. Pd (palladium) 
8.49 7.90 none 
______________________________________ 
Summary of Example 1 
The lattice constant of the plutonium host phase (with a fluorite type 
structure) decreased from 5.596.ANG. in sample 1--1 to 5.575.ANG. in 
sample 1-2 and this is evidence for the dissolution of plutonium in the 
plutonium host phase. The experiment on sample 1-3 verified that the spent 
fuel had four phases at equilibrium as designed for high-level radioactive 
wastes. 
Thus, the availability of a plutonium fuel compound was verified that 
consisted of plutonium dioxide, thoria and alumina in mol % ratios of 
5:45:50 and which contained two phases, plutonium host phase (with a 
fluorite type structure) and alumina phase, at equilibrium. 
Example 2: Experiment to Verify the Availability of Plutonium Fuel Compound 
System consisting of Plutonium Dioxide, Stabilized Zirconia and Alumina 
The following three samples were prepared and availability of the fuel 
compound system mentioned above was reviewed as in Example 1. 
Sample 2-1: Plutonium-free sample 
A solution of 11 mol % gadolinia (Gd.sub.2 O.sub.3) and 89 mol % zirconia 
(ZrO.sub.2) was rendered acidic with nitric acid for use as an acidic 
solution of stabilized zirconia. This solution and an aluminum solution as 
rendered acidic with nitric acid were mixed in mol % ratios of 47.4:52.6 
(stabilized zirconia/alumina) and evaporated to dryness. The dried product 
was heated at 800.degree. C. in air atmosphere to form an oxide. 
Simulating the current practice of nuclear fuel fabrication, the oxide was 
shaped into pellets each having an outside diameter of 7 mm and weighing 
about 500 mg; the pellets were sintered by heating at 1,500.degree. C. for 
4 h in a hydrogen stream. The sinter was ground into particles and 
subjected to X-ray diffraction for identifying the produced phases. 
The X-ray diffraction scan showed that the reaction product contained two 
phases at equilibrium, which were a plutonium host phase (with a fluorite 
type structure) laving a lattice constant of 5.162.ANG. and an alumina 
phase. 
Sample 2--2: Plutonium-containing sample 
Solutions of plutonium, stabilized zirconia and aluminum, as rendered 
acidic with nitric acid, were mixed at mol % ratios of 5:45:50 (plutonium 
dioxide/stabilized zirconia/alumina) and evaporated to dryness. The dried 
product was heated at 800.degree. C. in air atmosphere to form an oxide. 
Simulating the current practice of nuclear fuel fabrication, the oxide was 
shaped into pellets each having an outside diameter of 7 mm and weighing 
about 500 mg; the pellets were sintered by heating at 1,500.degree. C. for 
4 h in a hydrogen stream. The sinter was ground into a powder, which was 
subjected to X-ray diffraction. 
The X-ray diffraction scan showed that the reaction product contained two 
phases in equilibrium, which were a plutonium host phase (with a fluorite 
type structure) having a lattice constant of 5.186.ANG. and an alumina 
phase. 
Sample 2-3: Sample containing simulated fission products 
As in the case of the preparation of sample 1-3, solutions of simulated 
fission products, stabilized zirconia and aluminum, as rendered acidic 
with nitric acid, were mixed at mol % ratios of 5:45:50 (simulated fission 
products/stabilized zirconia/alumina) and evaporated to dryness. The dried 
product was calcined by heating at 500.degree. C. in a stream consisting 
of a mixture of 4% hydrogen and 96% helium. The calcine was shaped into 
pellets each having an outside diameter of 7 mm and weighing 500 mg; the 
pellets were sintered by heating at 1,500.degree. C. for 4 h in a stream 
consisting of a mixture of 50% carbon dioxide and 50% carbon monoxide. The 
sinter was ground into a powder and subjected to X-ray diffraction for 
identifying the constitutional phases. 
The X-ray diffraction scan showed that the reaction product contained four 
phases in equilibrium, which were a plutonium host phase (with a fluorite 
structure), a magnetoplumbite type phase an alloy phase and an alumina 
phase. 
Summary of Example 2 
The lattice constant of the plutonium host phase (with a fluorite type 
structure) increased from 5.162.ANG. in sample 2-1 to 5.186.ANG. in sample 
2--2 and this is evidence for the dissolution of plutonium in the 
plutonium host phase. The experiment on sample 2-3 verified that the spent 
fuel had four phases at equilibrium as designed for high-level radioactive 
wastes. 
Thus, the availability of a plutonium fuel compound was verified that 
consisted of plutonium dioxide, stabilized zirconia and alumina at mol % 
ratios of 5:45:50 and which contained two phases, plutonium host phase 
(with a fluorite type structure) and alumina phase, in equilibrium. 
The compositional Range of the Plutonium Fuel Compound 
The compositional range in which the plutonium fuel compound of the present 
invention was available was determined from the following parameters: (1) 
the equilibria of a three-component system consisting of plutonium 
dioxide, plutonium host phase and alumina; (2) the compositional range as 
determined from the alumina content; (3) the compositional range as 
determined from the amount of plutonium dissolved in the plutonium host 
phase; and (4) the compositional range as determined from the density of 
plutonium in the fuel. 
(1) Equilibria of a three-component system consisting of plutonium dioxide, 
plutonium host phase and alumina 
FIG. 1 is a phase diagram showing the equilibria of a three-component 
system, consisting of plutonium dioxide, plutonium host phase and alumina, 
at 1,900.degree. C. and below as constructed on the basis of the results 
obtained in Examples 1 and 2. Component A is the plutonium host phase 
which is composed of either thoria or stabilized zirconia; component B is 
plutonium dioxide; and component C is alumina. The thick line AB 
designates the solid solution of plutonium dioxide in the plutonium host 
phase. 
Point P on line AC refers to the composition of samples 1--1 and 2-1, each 
consisting of 47.4 mol % plutonium dioxide and 52.6 mol % alumina, and 
point Q within the diagram refers to the composition of samples 1-2 and 
2--2, each consisting of 5 mol % plutonium dioxide, 45 mol % plutonium 
host phase and 50 mol % alumina. Point Q also refers to the composition of 
spent fuel samples 1-3 and 2-3. The phase equilibria of those samples 
showed that the compound within the triangle ABC contained two phases in 
equilibrium, which were the plutonium host phase having plutonium dioxide 
dissolved therein and alumina. 
(2) Compositional range as determined from the alumina content 
(2)--a: the chemical equivalent amounts of alumina that were necessary to 
form magnetoplumbite type phases from alkali metal elements and alkaline 
earth metal elements as fission products were calculated from chemical 
formulae and indicated by line AI in the diagram. Point I refers to 67 mol 
% alumina and 33 mol % plutonium dioxide. Hence, the area within the 
triangle AIC designates a maximum compositional range over which the 
plutonium fuel compound of the present invention is available. 
(2)--b: It was verified from samples 1-(2) and 1-(3), as well as from 
samples 2-(2) and 2-(3)that when alumina was added in amount five times as 
many as the value necessary to generate magnetoplumbite type phases from 
alkali metal elements and alkaline earth metal elements as fission 
products, phase equilibria could be attained rapidly. Line AV plots the 
values of alumina content that were five times as many as the value 
necessary to produce the magnetoplumbite type phases. Point V refers to 91 
mol % alumina and 9 mol % plutonium dioxide. 
Thus, the examples verified the availability of the intended plutonium Fuel 
compound at a composition within the triangle AVC. Note that point P and Q 
lie on sides of the triangle AVC. 
(3) Compositional range as determined from the amount of plutonium 
dissolved in the plutonium lost phase 
The two-phase equilibrium verified by samples 1--1 and 2-1 at point P shows 
that line AC is a conjugation line for the plutonium host phase and 
alumina. The two-phase equilibrium verified by samples 1-2 and 2--2 at 
point Q shows that line XC is a conjugation line for the plutonium host 
phase (having 10 mol % plutonium dioxide dissolved) and alumina point X 
refers to 10 mol % plutonium dioxide and 90 mol % plutonium host phase. 
Thus, the availability of the intended plutonium fuel compound was 
verified. Further, samples 1-3 and 2-3 showed that when spent, samples 1-2 
and 2--2 at point Q were already the as-designed high-level waste. 
It thus became clear that the desired plutonium fuel compound was available 
at a composition within the triangle AXC. Note that points P and Q lie on 
sides of the triangle AXC. 
(4) Compositional range as determined from the density of plutonium in the 
fuel 
As already mentioned in connection with the description of the prior art, 
the burnup of light-water reactor fuels is 3-5%. Line DE in the FIG. 1 
diagram designates 5 mol % plutonium dioxide and line FG refers to 3 mol % 
plutonium dioxide. 
Thus, it became clear that the plutonium fuel compound of the present 
invention was available with the compositional range within the trapezoid 
DEFG. Note that point Q also lies on a side of the trapezoid DEFG. 
Combining the four compositional ranges discussed above, one can determine 
the range in which the plutonium fuel compound of the present invention 
can actually be obtained and that is the area bounded by triangle aQb, in 
which point a is at the intersection between lines AV and FG and point b 
is at the intersection between lines XC and FG. Point a refers to the 
composition consisting of 3 mol % plutonium dioxide, 67 mol % plutonium 
host phase, and 30 mol % alumina; point Q refers to the composition 
consisting of 5 mol % plutonium dioxide, 45 mol % plutonium host phase, 
and 50 mol % alumina; and point b refers to the composition consisting of 
3 mol % plutonium dioxide, 27 mol % plutonium host phase, and 70 mol % 
alumina. 
Composition of Stabilized Zirconia 
The stabilized zirconia used in Example 2 consisted of 11 mol % Gd.sub.2 
O.sub.3 and 89 mol % ZrO.sub.2 but it should be apparent to one skilled in 
the art that the stable zirconia phase can exist over the compositional 
range of 8-53 mol % Gd.sub.2 O.sub.3 and 92-47 mol % ZrO.sub.2. 
The present invention provides a once-through plutonium fuel compound 
having its composition and phase structure adjusted in the manner recited 
in the appended claims. The fuel compound offers the following benefits. 
(1) It can be manufactured by the conventional fuel fabrication technology 
and can be used together with currently used nuclear fuels in currently 
operating light-water reactors or fast reactors. 
(2) Nuclear fuels comprising the compound under consideration can be burned 
almost completely by designing an appropriate layout within reactor and, 
hence, there is no need to reprocess spent fuels. 
(3) The fuel compound is so adjusted in composition and phase structure 
that, when spent, it requires no special chemical treatment or 
solidification treatment to be converted to a stable high-level 
radioactive waste. 
(4) As will be apparent to one skilled in the art, the fuel compound can 
also be used in fuels that substitutes enriched uranium or transuranic 
elements (neptunium, americium, etc.) for plutonium.