Patent Number: 051805278
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the nuclear fuel pellets according to the present invention, a deposition phase of substances having high thermal conductivity is continuously present in the grain boundaries in the pellets. Thus, the thermal conduction in the pellets is efficiently performed through the continuous deposition phase. As a result, the average thermal conductivity of the pellets is increased, and then the distribution of temperature in the pellets becomes more uniform than that of the conventional nuclear fuel pellets. Further, the nuclear fuel pellets according to the present invention are manufactured in the following manner. Specifically, high-thermal conductivity substances, at least a part of which liquefies at a temperature near or below the sintering temperature thereof, are added to nuclear fission substances, and sintered. Thus, the high-thermal conductivity substances are melted into liquid when they are sintered. As a result, the thus liquefied substances are deposited in the grain boundaries of uranium oxide or mixed oxides, and become continuous grain layers after cooling. Moreover, the following substances are preferable as the above-described high-thermal conductivity substances. Specifically, they include beryllium oxide alone, or a mixture of beryllium oxide and at least one of or one oxide of titanium, gadolinium, calcium, barium, magnesium, strontium, lanthanum, yttrium, ytterium, silicon, aluminum, samarium, tungsten, zirconium, lithium, molybdenum, uranium, and thorium or an eutectic matter being obtained by heating the above mixture, whereby the melting point thereof being decreased. Hereinafter, nuclear fuel pellets of embodiments according to the present invention will be described with reference to the accompanying drawings. FIG. 1 is an enlarged schematic diagram illustrating one of nuclear fuel pellets, which is observed commonly in respective embodiments according to the present invention. In FIG. 1, a deposition phase 8 of substances having high thermal conductivity is continuously deposited in the grain boundaries of nuclear fission substances 7. First Embodiment Beryllium oxide (BeO) powder was added to uranium oxide (UO.sub.2) powder, and mixed therewith. The amount of beryllium oxide was 1.5 wt. % at a maximum (5.0 vol. % at a maximum) with respect to the total amount of the uranium oxide powder and the beryllium oxide powder. The thus mixed powder was molded by pressing with a pressure of about 2.5 through about 3.0 t/cm.sup.2, and a mold of about 50 through about 55% TD was obtained. The mold was sintered at about 2100.degree. C., the temperature being higher than the eutectic point thereof. As a result, pellets having an average grain diameter of about 110 through 160 .mu.m were obtained. In the process of sintering, at least a part of the pellets liquefied and covered at least half the grain boundaries. As the grain boundary-covering factor of the thus liquefied pellets increases, the thermal conductivity of the pellets increases monotonously. Next, the relative thermal conductivities of such pellets were measured varying the amount of beryllium oxide to be added. The measurements thereof are shown in FIG. 2. Second Embodiment Beryllium oxide powder and titanium oxide powder were mixed, and the mixture thereof was melted at a temperature higher than the eutectic point thereof, and then ground. The thus ground powder was mixed with uranium oxide powder, and molded by pressing with a pressure of about 2.5 through 3.0 t/cm.sup.2. Thereafter, the thus obtained mold was sintered in a reduction atmosphere at about 1700.degree. C., the temperature being higher than the eutectic point (about 1670.degree. C.). As a result, pellets having an average grain diameter of about 50 through about 110 .mu.m were obtained. In this embodiments, the beryllium oxide powder and the titanium oxide powder were added to the uranium oxide powder in the following proportions expressed by wt. % with respect to the total amount of the pellets: ______________________________________ Beryllium oxide powder Titanium oxide powder ______________________________________ 1.5 wt % 1.0 wt % 3.0 wt % 2.0 wt % 1.0 wt % 1.0 wt % 1.0 wt % 4.0 wt % 1.0 wt % 8.0 wt % ______________________________________ The thermal conductivities (K) of the pellets obtained by use of the above-described proportions were compared with the thermal conductivity (Ko) of conventional pellets. The comparison results are shown in FIG. 3. Third Embodiment Beryllium oxide powder and gadolinium oxide powder were mixed, and the mixture thereof was melted at a temperature high than the eutectic point thereof, and then ground. The thus ground powder was mixed with uranium oxide powder, and molded by pressing with a pressure of about 2.5 through about 3.0 t/cm.sup.2. Thereafter, the thus obtained mold was sintered in a weak oxidation atmosphere (moist hydrogen of an oxidation potential of about -300 kJ/mol) at about 1700.degree. C., the temperature being higher than the eutectic point (about 1500.degree. C.). As a result, pellets having an average grain diameter of about 15 through about 20 .mu.m were obtained. The thermal conductivity of the thus obtained pellets was about 1.11 through about 1.13 times that of the conventional pellets such that gadolinium oxide powder of about 10 wt. % was added to the uranium oxide powder (the comparison was made at a temperature of about 1000K). In this embodiment, the adding proportions of the beryllium oxide powder and the gadolinium oxide powder to the uranium oxide powder were about 1.5 wt. % and about 1.0 wt. %, respectively to the total amount of the pellets. Fourth Embodiment Beryllium oxide powder and silicon oxide powder were mixed, and the mixture thereof was mixed with uranium oxide powder. The thus mixed powder was molded by pressing with a pressure of about 2.5 through about 3.0 t/cm.sup.2. Thereafter, the thus obtained mold was sintered in a reduction atmosphere at about 1700.degree. C., the temperature being higher than the eutectic point (about 1670.degree. C.). Further, besides the above, beryllium oxide powder and silicon oxide powder were mixed, and the mixture thereof was melted at a temperature higher than the eutectic pint, and then ground. The thus ground powder was mixed with uranium oxide powder, and the mixture thereof was sintered in the same manner as above. As a result, pellets having an average grain diameter of about 40 through 50 .mu.m were obtained. The thermal conductivity of the thus obtained pellets was about 1.08 times that of the conventional pellets of uranium oxide (the comparison as made at a temperature of 1000K). In this embodiment, the beryllium oxide powder and the silicon oxide powder were added to the uranium oxide powder in the following proportions expressed by wt. % with respect to the total amount of the pellets: ______________________________________ Beryllium oxide powder Silicon oxide powder ______________________________________ 0.9 wt % 0.1 wt % 0.9 wt % 0.3 wt % ______________________________________ Fifth Embodiment Beryllium oxide powder and aluminum oxide powder were mixed, and the mixture thereof was mixed with uranium oxide powder. The thus mixed powder was molded by pressing with a pressure of about 2.5 through 3.0 t/cm.sup.2. Thereafter, the thus obtained mold was sintered in a reduction atmosphere at about 1900.degree. C. or about 2000.degree. C., the temperatures being higher than the eutectic point (about 1840.degree. C.). Further, besides the above, beryllium oxide powder and aluminum oxide powder was mixed, and the mixture thereof was melted at a temperature higher than the eutectic point, and then ground. The thus ground powder was mixed with uranium oxide powder, and the mixture thereof was sintered in the same manner as above. As a result, pellets of two different kinds were obtained. Specifically, the thermal conductivity of the pellets obtained by sintering at about 1900.degree. C. was about 1.08 times that of uranium oxide. Further, the thermal conductivity of the pellets obtained by sintering at about 2000.degree. C. was about 1.12 times that of uranium oxide (the comparison was made at a temperature of 1000K). In this embodiment, the beryllium oxide powder and aluminum oxide powder were added to the uranium oxide in the following proportions expressed by wt. % with respect to the total amount of the pellets: ______________________________________ Beryllium oxide powder Aluminum oxide powder ______________________________________ 0.9 wt % 0.1 wt % 0.9 wt % 0.3 wt % ______________________________________ Further, the average grain diameters in the case of 0.9 wt. % - beryllium oxide powder and 0.1 wt. % - aluminum oxide powder were as follows depending on the sintering temperatures; about 60 .mu.m when sintered at about 1900.degree. C., and PA0 about 110 .mu.m when sintered at about 2000.degree. C. PA0 about 90 .mu.m when sintered at about 1900.degree. C., and PA0 about 140 .mu.m when sintered at about 2000.degree. C. Moreover, the average grain diameters in the case of 0.9 wt. % - beryllium oxide powder and 0.3 wt. % - aluminum oxide powder were as follows depending on the sintering temperatures; Sixth Embodiment Beryllium oxide powder, titanium oxide powder and gadolinium oxide powder were mixed, and the mixture thereof was melted at a temperature high than the eutectic point, and then ground. (Besides this, the mixture thereof was not melted depending on conditions). The thus obtained powder was mixed with uranium oxide powder, and this mixed powder was molded by pressing. Thereafter, the thus obtained mold was sintered in a weak oxidation atmosphere. The average grain diameter was about 30 .mu.m, and the thermal conductivity of the thus obtained pellets was about 1.11 through 1.13 times that of the conventional pellets consisting of uranium oxide and gadolinium oxide. In this embodiment, the beryllium oxide powder, titanium oxide powder and gadolinium oxide powder were added to the uranium oxide powder in the following proportions expressed by wt. % with respect to the total amount of the pellets: ______________________________________ Beryllium oxide Titanium oxide Sadolinium oxide powder powder powder ______________________________________ 1.5 wt % 0.5 wt % 10 wt % 1.5 wt % 1.0 wt % 10 wt % ______________________________________ In all of the above-described embodiments, a part of the additive substance having high thermal conductivity is liquefied during the sintering. Further, at least a half of the grain boundaries is continuously covered with the thus liquefied high-thermal conductivity substance. As the grain boundary-covering factor of the liquefied substance increases, the thermal conductivity of the pellets increases monotonously. In all cases, when the pellets have the same density, the thermal conductivity thereof is increased in proportion to the increase of the amount of the additives. Further, even when a very small amount of additive (e.g., beryllium oxide of 0.3 wt. %) is added, high-density pellets can be obtained, and the thermal conductivity thereof can also be increased. Moreover, the relative densities of the molds with respect to the theoretical densities were about 50% TD. The relative densities of the thus obtained sintered pellets were about 95 through 99.7%. Furthermore, nuclear fuel pellets having the same advantages as above can also be obtained by use of the following high-thermal conductivity substances, a part of which or the entire of which is melted at a temperature near or below their sintering temperatures. Specifically, such substances include beryllium oxide alone, or a mixture of beryllium oxide and at least one of or one oxide of barium, calcium, magnesium, strontium, aluminum, lanthanum, yttrium, ytterbium, silicon, titanium, uranium, zirconium, tungsten, lithium, molybdenum, samarium, thorium, and gadolinium. As described above, according to the present invention, the thermal conductivity of nuclear fuel pellets can be significantly increased. Thus, the temperature in the center of the nuclear fuel rod can be reduced, whereby the discharge amount of gases generated on the nuclear fission can be efficiently reduced. Obviously, numerous additional modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.