SINTERING WITH SPS/FAST URANIUM FUEL WITH OR WITHOUT BURNABLE ABSORBERS

Abstract

The present invention relates to nuclear fuel compositions including uranium dioxide with integral fuel burnable absorber, and triuranium disilicide and a composite of uranium mononitride and triuranium disilicide with or without integral fuel burnable absorber, and methods of sintering these compositions. The sintering is conducted using SPS/FAST apparatus and techniques. The sintering time and temperature is reduced using SPS/FAST as compared to conventional sintering methods for nuclear fuel compositions. The nuclear fuel compositions of the present invention are particularly useful in light water reactors.

Claims

1. A method of sintering a fuel composition, comprising: forming a powder sample, comprising: a material selected from the group consisting of triuranium disilicide with or without an integral fuel burnable absorber, a composite of uranium mononitride and triuranium disilicide with or without an integral fuel burnable absorber and uranium dioxide with an integral fuel burnable absorber; employing a SPS/FAST system, comprising: a power supply; and a vacuum chamber structured to enclose components, comprising: an upper electrode and a lower electrode; an upper punch connected to the upper electrode and a lower punch connected to the lower electrode; and a die assembly constructed of a conductive material, positioned between the upper and lower punches, and structured to hold the powder sample; introducing the powder sample into the die assembly; passing pulsed direct current from the power supply through the die assembly; heating the powder sample; contacting and compressing the powder sample between the upper punch and the lower punch; and sintering the powder sample.

2. The method of claim 1, wherein the composite of uranium mononitride and triuranium disilicide comprises from greater than zero to about fifty percent by weight triuranium disilicide.

3. The method of claim 1, wherein the powder sample comprises a mixture of the triuranium disilicide and the integral fuel burnable absorber.

4. The method of claim 1, wherein the powder sample comprises a mixture of the composite of uranium mononitride and triuranium disilicide, and the integral fuel burnable absorber.

5. The method of claim 1, wherein the powder sample comprises a mixture of the uranium dioxide and the integral fuel burnable absorber.

6. The method of claim 1, wherein the integral fuel burnable absorber is selected from the group consisting of UB.sub.2, UB.sub.4, ZrB.sub.2, B, B.sub.4C, SiBn and mixtures thereof.

7. The method of claim 1, wherein the heating of the powder sample is to a temperature in a range from about 1000 C. to about 1700 C.

8. The method of claim 1, wherein the sintering of the powder sample is conducted in a time period of about 0.5 minute to about sixty minutes.

9. The method of claim 7, wherein the sintering of the powder sample is conducted in a time period of about five minutes to about ten minutes.

10. The method of claim 1, wherein the conductive material is selected from the group consisting of graphite, boron nitride, tungsten carbide, molybdenum, tantalum and mixtures thereof.

11. A method of forming a water corrosion resistant fuel microstructure, comprising: forming a powder sample, comprising: a composite of polycrystalline uranium mononitride grain bonded with triuranium disilicide with or without an integral fuel burnable absorber; employing a SPS/FAST system, comprising: a power supply; and a vacuum chamber structured to enclose components, comprising: an upper electrode and a lower electrode; an upper punch connected to the upper electrode and a lower punch connected to the lower electrode; and a die assembly constructed of a conductive material, positioned between the upper and lower punches, and structured to hold the powder sample; introducing the powder sample into the die assembly; passing pulsed direct current from the power supply through the die assembly; heating the powder sample to a temperature at or above the melting point of triuranium disilicide; contacting and compressing the powder sample between the upper punch and the lower punch; and sintering the powder sample.

12. The method of claim 10, wherein the powder sample comprises the composite of polycrystalline uranium mononitride grain bonded with triuranium disilicide and the integral fuel burnable absorber.

13. The method of claim 11, wherein the integral fuel burnable absorber is selected from the group consisting of UB.sub.2, UB.sub.4, ZrB.sub.2, BN and mixtures thereof.

14. The method of claim 12, wherein a USiB glass phase is formed.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The invention as set forth in the claims will become more apparent from the following detailed description of certain preferred practices thereof illustrated, by way of example only, and the accompanying drawings wherein;

[0024] FIG. 1 is a longitudinal view, partly in section and partly in elevation, of a prior art nuclear reactor to which the present invention may be applied;

[0025] FIG. 2 is a simplified enlarged plan view of the reactor taken along line 2-2 of FIG. 1, but with its core having a construction and arrangement of fuel in accordance with the present invention;

[0026] FIG. 3 is an elevational view, with parts sectioned and parts broken away for clarity, of one of the nuclear fuel assemblies in the reactor of FIG. 2, the fuel assembly being illustrated in a vertically foreshortened form;

[0027] FIG. 4 is an enlarged foreshortened longitudinal axial sectional view of a fuel rod of the fuel assembly of FIG. 3 containing fuel pellets;

[0028] FIG. 5 is a schematic of a known SPS/FAST system for use in certain embodiments of the invention; and

[0029] FIG. 6 is a schematic showing microstructures of a UN/U.sub.3Si.sub.2 composite as a result of sintering, in accordance with certain embodiments of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0030] The present invention relates to methods for sintering nuclear fuel compositions including triuranium disilicide (U.sub.3Si.sub.2) with or without integral fuel burnable absorber (IFBA), composites of uranium mononitride (UN) and triuranium disilicide (U.sub.3Si.sub.2) with or without integral fuel burnable absorber (IFBA), and materials of uranium dioxide (UO.sub.2) with integral fuel burnable absorber (IFBA) for use in light water reactors (LWRs). In the triuranium disilicide (U.sub.3Si.sub.2) and the composites of uranium mononitride (UN) and triuranium disilicide (U.sub.3Si.sub.2) nuclear fuel compositions, the presence of the IFBA is optional. The composite of UN and U.sub.3Si.sub.2 can include from greater than zero to about fifty percent by weight of the U.sub.3Si.sub.2. The composite can include polycrystalline UN grain bonded with U.sub.3Si.sub.2, with or without the IFBA. The sintering of the nuclear fuel compositions is conducted by employing Spark Plasma Sintering (SPS)/Field-Assisted Sintering Technique (FAST). The present invention is applicable to a variety of LWRs, including but not limited to, pressurized water reactors (PWRs) and boiling water reactors (BWRs). However, for simplicity in describing the details of the invention, the following description referring to the drawings will be in accordance with a PWR.

[0031] In the following description, like reference numerals designate like or corresponding parts throughout the several views. Also in the following description, it is to be understood that such terms as forward, rearward, left, right, upwardly, downwardly, and the like, are words of convenience and are not to be construed as limiting terms.

[0032] As previously mentioned, conventional nuclear fuel compositions for use in LWRs include UO.sub.2. The UO.sub.2 contains a significant amount of uranium-238 and a small amount of uranium-235. Further, as previously mentioned, there are economic benefits from increasing the content of uranium-235 in nuclear fuel compositions. Such benefits can include longer fuel cycles or the use of smaller batches. In addition, if a higher thermal conductivity can be obtained, then higher thermal duty can result therefrom. Thus, the use of U.sub.3Si.sub.2 in the fuel compositions of the invention provides an increased amount of uranium-235.

[0033] The invention relates to next generation fuels that include U.sub.3Si.sub.2 and UN/U.sub.3Si.sub.2 composite fuels. These fuels have accident resistant uranium compounds, which demonstrate one or more of the following properties: (i) resistance to water corrosion, (ii) higher thermal conductivity than uranium dioxide, (iii) a higher uranium loading than uranium dioxide, and (iv) a melting temperature that allows the fuel to stay solid under Light Water Reactor (LWR) normal operating and transient conditions.

[0034] U.sub.3Si.sub.2 and UN have higher thermal conductivity and higher uranium loading than UO.sub.2. Pure UN is not water corrosion resistant at a temperature of 300 C. and above, which prevents the use of UN alone in LWR fuel. However, U.sub.3Si.sub.2 has better water corrosion resistance than UN. Thus, a UN/U.sub.3Si.sub.2 composite can overcome the water corrosion issue related to the use of UN alone.

[0035] It is known in the art that the U.sub.3Si.sub.2 and UN/U.sub.3Si.sub.2 composite fuels are difficult to sinter using conventional techniques. For example, it is difficult to consolidate UN and U.sub.3Si.sub.2 using a conventional sintering method. The pellet density of U.sub.3Si.sub.2 is generally below ninety percent (90%) of theoretical density using conventional sintering techniques, unless extensive and expensive milling is applied to the powder beforehand. The UO.sub.2 pellets can reach above ninety-five percent (95%) of theoretical density. As for sintering with IFBA (e.g., different variations of boron, including but not limited to, UB.sub.2, UB.sub.4, ZrB.sub.2, B, B.sub.4C and SiB.sub.n), the absorbers easily decompose and evaporate at a high sintering temperature. Thus, a new sintering technique with more efficiency, lower sintering temperature and shorter sintering time is desired to produce U.sub.3Si.sub.2 and U.sub.3Si.sub.2/UN fuels with or without IFBA

[0036] It is also known in the art that conventional nuclear fuel compositions that include UO.sub.2 may also contain IFBA, which provides temporary reactivity control and compensate for excess reactivity early in the fuel cycle. However, as previously disclosed, IFBA, in particular, boron-based IFBA, cannot sinter with UO.sub.2 using conventional sintering technology. Thus, a new technique capable of sintering at lower temperatures, such as to reduce or preclude the IFBA from volatizing, and maintaining a consistent residual level of IFBA is desired for nuclear fuel including UO.sub.2 with IFBA. For example, in conventional sintering techniques, it has been found that the use of higher temperatures causes the boron of a boron-based IFBA to volatize and as a result, a consistent residual level of boron may not be maintainable.

[0037] The invention provides new sintering methods for UO.sub.2 with burnable absorbers, and U.sub.3Si.sub.2 and UN/U.sub.3Si.sub.2 composite fuels with or without burnable absorbers. It has been found that SPS/FAST provides effective apparatus and technique to sinter U.sub.3Si.sub.2 and UN/U.sub.3Si.sub.2 composite fuels, as well as fuels including UO.sub.2 with IFBA. This technique significantly decreases the sintering temperature and sintering time needed, as compared to conventional sintering techniques used for lower U-235 density fuel material. The SPS/FAST provides for heating a powder fuel sample to a temperature in a range from about 1000 C. to about 1700 C., and sintering the powder sample in a time period from about 0.5 minute to about 60 minutes. Moreover, it has been found that SPS/FAST minimizes porosity, which can result in enhanced resistance against corrosion in water/steam. The sintering time for the SPS/FAST process can be minutes, as compared to hours for conventional sintering processes. Generally, SPS/FAST is a low voltage, direct current (DC) pulsed current activated, pressure-assisted sintering, and synthesis technique. SPS/FAST is similar to a conventional hot pressing (HP) technique, but is distinguishable because the mechanism for producing and transmitting heat to the sintering material is different in SPS/FAST as compared to HP. A primary characteristic of the SPS/FAST sintering technique is that DC pulsed current directly passes through a conductive (e.g. graphite) die, as well as a powder compact, for conductive samples. Joule heating has been found to play a dominant role in the densification of powder compacts, which results in achieving near theoretical density at a lower sintering temperature compared to conventional sintering techniques. The heat generation is internal, in contrast to conventional hot pressing, where the heat is provided by external heating elements. Internally generating the heat facilitates a very high heating or cooling rate (up to 1000 K/min), hence the sintering process generally is very fast, e.g., within a few minutes as compared to several hours or more with conventional sintering techniques. The general speed of the process ensures it has the potential of densifying powders with nanosize or nanostructure while avoiding coarsening, which accompanies standard densification methods.

[0038] FIG. 5 is a schematic which shows a known FAST/SPS apparatus 100 for use in the invention, which consists of a mechanical loading system that serves as a high-power electrical circuit, placed in a controlled atmosphere. FIG. 5 includes a power mechanism 110 to supply DC pulsed current, and a water-cooled vacuum chamber 112. Positioned within the chamber 112 is an upper electrode 114 and a lower electrode 116, an upper punch 118 and a lower punch 120. Positioned between the upper and lower punches 118, 120 is a die assembly 122. A powder sample 124 is placed in the die assembly 122. Heat is quickly and efficiently transferred to the sample. The process can take place under vacuum or protective gas at atmospheric pressure. The heated parts are located in the water-cooled vacuum chamber 112.

[0039] Without intending to be bound by any particular theory, it is believed that the quasi-static compressive stress applied in the SPS/FAST system, e.g., the pressure exerted by the upper and lower punches, provides better contact between particles, changes the amount and morphology of those contacts, enhances existing densification mechanisms present in free sintering (grain boundary diffusion, lattice diffusion, and viscous flow) or activates new mechanisms.

[0040] In accordance with the invention, the SPS/FAST process generally includes obtaining the U.sub.3Si.sub.2 or UN/U.sub.3Si.sub.2 with or without burnable absorbers, or UO.sub.2 with burnable absorbers, in a dry, powder form; placing the powder in a die assembly between an upper punch and a lower punch; providing pulsed current flow through the die assembly to cause rapid heating; contacting and compressing the powder between the upper and lower punches; and rapidly and efficiently transferring heat from the die assembly to the powder for sintering. The powder may be heated to the melting temperature of U.sub.3Si.sub.2 (i.e., 1665 C.) or higher.

[0041] In conventional sintering techniques, for example, for UO.sub.2, a sintering temperature above about 1750 C. is used in combination with a holding time of approximately five hours. In contrast, for the SPS/FAST process in accordance with the invention, the sintering temperature may be about 1050 C. with a holding time of approximately 0.5 minute. For UN/U.sub.3Si.sub.2, the conventional sintering temperature is greater than about 1800 C. with approximately forty hours of milling prior to sintering. In contrast, for UN/U.sub.3Si.sub.2, SPS/FAST sintering may be accomplished at a temperature of about 1500 C. for a period of approximately ten minutes, without pre-milling to achieve ninety percent (90%) theoretical density. In other embodiments, for UN/U.sub.3Si.sub.2, a temperature of about 1650 C. for approximately three minutes results in above ninety-nine percent (99%) theoretical density.

[0042] As a result of rapid sintering (in minutes), the boron-based burnable absorbers (ZrB.sub.2, BN, etc.) have limited time to volatilize and therefore, remain (e.g., are present) in the fuels during the sintering process.

[0043] In certain embodiments, the sintering time for the powder sample (fuel composition) is from about 0.5 minute to about 60 minutes. In other embodiments, the sintering time for the powder sample (fuel composition) is from about 5 minutes to about 10 minutes.

[0044] Further, as a result of rapid heating of the powder in the SPS/FAST sintering process, high local temperature gradients and non-uniform temperature distribution may exist and cause thermal stress. UN and U.sub.3Si.sub.2 have high temperature conductivity and therefore, the thermal stress is mitigated.

[0045] The most commonly used conductive material for a SPS/FAST die (e.g., die assembly 122 in FIG. 5) is graphite. However, because graphite is a moderator, it may not be suitable for mass production in nuclear fuels. Therefore, in accordance with the invention, a material, such as boron nitride, tungsten carbide, or a metal other than graphite, such as, but not limited to molybdenum, tungsten, tantalum, and the like, may be used for the die.

[0046] In order to achieve water corrosion resistance, the microstructure of the UN/U.sub.3Si.sub.2 composite may be optimized. FIG. 6 shows UN/U.sub.3Si.sub.2 composite microstructures in accordance with certain embodiments of the invention. View A in FIG. 6 illustrates a UN/U.sub.3Si.sub.2 composite having a desired microstructure, which includes polycrystalline UN grains (140) and grain boundaries (144) there between. A portion of the grain boundaries (144) include a thin layer of U.sub.3Si.sub.2 (142), to bond the polycrystalline UN grains (140) with U.sub.3Si.sub.2 to prevent grain boundary segregation. View B in FIG. 6 shows a UN/U.sub.3Si.sub.2 composite having an ideal or optimum microstructure, wherein all of the grain boundaries include a thin layer of U.sub.3Si.sub.2 (142) in the microstructure, to bond all of the polycrystalline UN grains (140) with U.sub.3Si.sub.2 to prevent grain boundary segregation. The use of the SPS/FAST process provides increased or improved control over the microstructure as compared to conventional sintering processes. For example, it has been found that with UN/U.sub.3Si.sub.2 sintered near the melting temperature of U.sub.3Si.sub.2 (i.e., 1665 C.), the liquid phase or near-liquid phase of U.sub.3Si.sub.2 can be readily distributed along grain boundaries of the UN. Since SPS/FAST can be performed in a short time period (a few minutes), the risk of evaporation of the liquid-phase U.sub.3Si.sub.2 is mitigated. Furthermore, the moderate pressure applied to the powder sample in the die through the upper and lower punches allows for a more homogeneous distribution of U.sub.3Si.sub.2 and UN, which provides for improvement in polycrystalline UN grain bonded with U.sub.3Si.sub.2 at the UN grain boundaries (e.g., as shown in View B of FIG. 6).

[0047] In certain embodiments, a USiB glass as a water proofing phase is formed for the composite of polycrystalline UN grain bonded with U.sub.3Si.sub.2 with IFBA, as well as for U.sub.3Si.sub.2 with IFBA.

[0048] Whereas particular embodiments of the invention have been described herein for purposes of illustration, it will be evident to those skilled in the art that numerous variations of the details may be made without departing from the invention as set forth in the appended claims.