GB2184235A

GB2184235A - Determining oxygen metal ratio in nuclear fuel oxides

Info

Publication number: GB2184235A
Application number: GB08629018A
Authority: GB
Inventors: Toshiaki Hiyama
Original assignee: Doryokuro Kakunenryo Kaihatsu Jigyodan
Current assignee: Doryokuro Kakunenryo Kaihatsu Jigyodan
Priority date: 1985-12-05
Filing date: 1986-12-04
Publication date: 1987-06-17
Anticipated expiration: 2006-12-04
Also published as: FR2591343B1; DE3641668A1; FR2591343A1; JPH068813B2; GB8629018D0; GB2184235B; JPS62133352A

Abstract

A method for determination of oxygen-to-metal ratio in a nuclear fuel oxide. The method comprises placing a mixture of a nuclear fuel oxide and a metal flux in a graphite crucible, heating said mixture in said crucible in a reducing atmosphere of an inert gas to melt said mixture thereby generating carbon monoxide, accurately determining an amount of the generated carbon monoxide to calculate an amount of oxygen in said oxide, and calculating oxygen-to-metal ratio in said oxide based on the calculated amount of oxygen.

Description

SPECIFICATION Method for determining oxygen-to metal ratio in nuclear fuel oxide Background of the invention The present invention relates to a method for rapid determination of oxygen-to-metal ratio (hereinafter referred to as "O/M ratio") in a nuclearfuel oxide with excellent accuracy.

It is no exaggeration to say that the O/M ratio in a nuclearfuel oxide such as uranium oxide, plutonium oxide, thorium oxide, orthe like is an important physical property which influencesthethermal conductivity, creep rate, diffusion of nuclear fission products, electrical conductivity, self-diffusion, etc. and determines the value ofthe nuclearfuel oxide.

Forexample,when M is U (uranium), the creep rate varies depending on the O/U ratio. Namely, the larger the O/U ratio, the largerthe creep rate. Further, it is known thatthe largerthe O/U ratio, the largerthe electrical conductivity and the smallerthe activation energy. It is also known thatthe largerthe O/U ratio,the smallerthevalue of ITs k(T)dTwhich indicates the value necessary for causing melting or recrystallization.

Tmc k(T)dT is a so-called thermal conductivity integral wherein k(T) is the thermal conductivity at atemperature of T, Tsthetemperature of a fuel element on its surface and Tcthetemperature of a fuel element at its central part.

Examples of the methods for determining the O/M ratio in a nuclearfuel which have conventionally been adopted include an X-ray diffractometry in which the amount of oxygen present in the oxide is determined from the lattice constant and a gravimetry in which the increase and decrease in the amount of oxygen dueto a redox reaction is determined with a balance.

However, in the above-mentioned conventional methods, the amount of oxygen is indirectly determined from the physical properties of a nuclear fuel oxide. Therefore, the conventional methods are disadvantageous in that errors of determination are large and that much time is needed for analysis and determination.

Summary ofthe invention Accordingly, an object of the present invention is to provide a method capable of directly determining the O/M ratio in a nuclearfuel oxide with high accuracy and efficiency in a shortened period oftime.

In accordance with the present invention, a nuclearfuel oxide is placed in a graphite crucible togetherwith a metal flux and the mixture ofthe oxide and the metal flux in the crucible is heated in a reducing atmosphere of an inert gas such as argon or helium to thereby melt the mixture. Oxygen generated from the oxide by the reductive reaction immediately reacts with carbon ofthe graphite crucibletoform carbon monoxide. Thus, the amount of oxygen can be calculated by accurately determining the amount ofthe generated carbon monoxide, and the O/M ratio can be calculated from the amount of oxygen.

A powder of a metal such as iron ortin may be used as the metal flux to be mixed with the nuclearfuel oxide. The use of such metal flux serves to lowerthe melting temperature ofthe nuclear fuel oxide and improve the uniformity ofthe melting.

Preferred apparatusfordetermining the amount of carbon monoxide include a non-dispersive infrared gas analyzerwhich takes advantage of infrared absorption by carbon monoxide and a thermal conductivitydet ectorwhich takes advantage ofthe difference in the thermal conductivity between a carriergascomprising an inert gas and carbon monoxide.

Briefdescription ofthe drawing Byway of example and to make the description more clear, reference is made to the accompanyng drawing which is an illustration of an apparatus for practicing the method ofthe present invention.

Preferred embodiments of the invention Referring now to the accompanying drawings, the reference numeral 1 designates a gas extraction furnace within which a graphite crucible 2 is placed. The gas extraction furnace is constructed so that the contents of the graphite crucible 2 are melted with a heating apparatus (not shown). Examples of the heating apparatus incl ude a high-frequency heating apparatus and a resistance heating constant-voltage power source using the graphite crucible as a resistor.

A mixture of a nuclearfuel oxide, e.g. uranium dioxide, with a metal flux, e.g. an iron powder, is placed in the graphite crucible 2. Then, an inert gas, e.g. argon is introduced into the gas extraction furnace, andthe mixture in the crucible 2 is heat-melted in an argon atmosphere. In this connection, it is noted that, as mentioned above, the melting temperature ofthe nuclearfuel oxide can be lowered by adding a metal flux. For example, when uranium dioxide is used alone, it begins to melt at atemperature as high as 2850"C. On the other hand, by adding a metal flux, it beginsto melt to meltatabout2500 C.

Oxygen is generated from the oxide by a reductive reaction accompanying the melting ofthe oxide and metal flux. Due to the presence of the reducing atmosphere comprising argon, the generated oxygen im mediately reacts with carbon of the graphite crucible 2toform carbon monoxide. The reaction can be ex- pressed bythefollowing reaction formula: MO2+flux+ 2C (carbon of graphite crucible) reductive reaction

M-flux + ZCU wherein MO2 standsfora nuclearfuel oxide,fluxa powdered metal flux such as iron ortin, Ocarbon and CO carbon monoxide.

The carbon monoxidethusformed is passedtogetherwith argon from the gas extraction furnace 1 through a dustfilter7 and then introduced into a carbon monoxide determining apparatus 3 such as a non-dispersive infrared gas analyzer, where the amount of carbon monoxide contained in argon is accurately determined. If necessary, the determined value is recorded with a recorder 4. A mixed gas comprising carbon monoxide and argon discharged from the carbon monoxide determining apparatus 3 is released through a gas flowme- ter 5 to the outside.

The gas flowmeter 5 is installed as an aid in monitoring whether the flow rate of the carrier gas (inert gas) is kept constant. With regard to the above-mentioned discharge of the mixed gas to the outside, it is noted that in the case of the analysis of a sample before irradiation (i.e., a sample of U02 before irradiation), itwill suffice if a dustfilter7 is installed since oxygen is not in an activated state. However, in the case of the analysis ofan irradiated sample, there are possibilitiesthatthe sample contains an FP gas (nuclearfission product gas) and that oxygen contained therein is in an activated state. Therefore, in such a case, an infa 11 ible filter means, e.g.

a HEPA filter or an activated carbon filter, should be employed.

In the determination of the O/M ratio in a sample of a mixed oxide of PuO2-UO2, the gas extraction furnace 1 and carbon monoxide determining apparatus 3 should be provided within a glove box 10 and an open port box 20 which are indicated with a dotted line in the drawing.

The reference numeral 6 designates a standard gas generatorwhich is used in calibrating the carbon monoxide determining apparatus 3. Namely, a mixed gas comprising a carbon monoxide gas and an inert gas, the carbon monoxide concentration of which is known, is preliminary run as a standard gas from the generator 6 to the carbon monoxide determining apparatus 3 to obtain response data indicated bythedeter- mining apparatus 3.The obtained data are graphed, and the linearity ofthe graph is examined to usethe results in calibration.

The amount of oxygen Co (% by weight) can be calculated based on the amount of carbon monoxide CcO (% by weight) determined with the carbon monoxide determining apparatus 3 by the following equation (1): C Wo Wo fw00 x Cco wherein WO standsforthe atomic weight of oxygen and Withe atomic weight ofcarbon.

The O/M ratio R can be determined from the amount of oxygen Co obtained by the equation (1), bythe following equation (2): Co = WMW+%oR.R RR x100 Co.WM 1 #R = 100 - Co. Wo (2) wherein WM stands for the average atomic weight of the metal atom M in an oxide MO2.

It is also possible to automatically calculate the O/M ratio by programming the above-mentioned equations (1 ) and (2) into a data processor (not shown) and inputting the data, which have been determined with the carbon monoxide determining apparatus 3, into the data processor.

Further, it is possible to continuously and automatically carry outthe determination according tothe pre- sent invention by preliminarily placing various mixtures of a nuclearfuel oxide to be determined and a metal flux in a plurality of graphite crucibles respectively, disposing the crucibles on a turn table and supplying the crucibles one by one in each test succesively to the gas extraction furnace by the rotation of the the turn table.

As explained above, according to the present invention, the O/M ratio can be directly determined by extra cting the whole quantity of oxygen in a nuclearfuel oxide, which leads to an improvement in accuracy of determination overthe conventional O/M ratio determination methods. Further, since the process compris ing the melting of a sample of a nuclearfuel oxide, fermentation of carbon monoxide and determination can be completed in a rate of about 10 minutes per sample, the present invention can remarkably shorten the determining time of the conventional methods which require about7 hours to 1 day per sample. Further, the amountofthe nuclearfuel oxideto be used as a sample may be about0.1 to 0.5 g, i.e., can be reduced to 1/10 to 1/2 of those required in the conventional methods. The present invention also has an advantage that the dose of exposure resulting from the handling of a nuclearfuel oxide can be reduced by virtue oftheshort ened determining time and decrease in the amount of sample to be used.

Although the method of the present invention is particularly effective in determining the amount of oxygen and the O/M ratio in a nuclearfuel oxide, it is also applicabletothe determination ofthe amount of oxygen and the O/M ratio in various oxides.

The foregoing description illustrates specific embodiments within the scope of this invention and are not to be construed as limitating said scope. It isto be understood that variations and modification thereof may be made by those skilled in the art without departing from the scope of the invention.

Claims

1. A method for determination of oxygen-to-metal ratio in a nuclearfuel oxide, said method comprising placing a mixture of a nuclearfuel oxide and a metal flux in a graphite crucible, heating said mixture in said crucible in a reducing atmosphere of an inertgasto melt said mixturethereby generating carbon monoxide, accurately determining an amount of the generated carbon monoxide to calculate an amount of oxygen in said oxide, and calculating oxygen-to-metal ratio in said oxide based on the calculated amount of oxygen.

2. The method according to claim 1, wherein said nuclearfuel oxide is selected from a group consisting of uranium oxide, plutonium oxide, thorium oxide and a uranium-plutonium mixed oxide.

3. The method according to claim 1,wherein said metal flux is selected from a group consisting of iron powderandtin powder.

4. The method according to claim 1, wherein said inert gas is selected from a group consisting of argon and helium.

5. The method according to claim 1, wherein said mixture ofthe nuclearfuel oxide and the metal flux is heated by high-frequency heating or resistance heating.

6. The method according to claim 1,wherein the amount of carbon monoxide is determined by using a non-dispersive infrared gas analyzer or a thermal conductivity detector.

7. A method according to claim 1, substantially as hereinbefore described with reference to and as shown in the accompanying drawing.