WO2005088649A1

WO2005088649A1 - Method of distinguishing different types of nuclear fuel elements and nuclear fuel element provided with identification means

Info

Publication number: WO2005088649A1
Application number: PCT/IB2005/050734
Authority: WO
Inventors: Leszek Andrzej Kuczynski
Original assignee: Pebble Bed Modular Reactor Pty Ltd
Current assignee: Pebble Bed Modular Reactor Pty Ltd
Priority date: 2004-03-01
Filing date: 2005-03-01
Publication date: 2005-09-22
Anticipated expiration: 2006-09-01
Also published as: AR047915A1

Abstract

This invention relates to a method of distinguishing at least two different types of fuel elements (10), for use in a nuclear reactor, each fuel element (10) including a core (12), comprising a plurality of fuel particles (14), each having a kernel of fissile material, dispersed in a matrix (16), and at least one coating (18, 20) around the core (12), which method includes providing in the at least one coating (18, 20) of each fuel element (10) of the predetermined first type identification means whereby the fuel element (10) of said first type are distinguishable from fuel elements (10) of another type. The invention extends to a nuclear fuel element (10).

Description

METHOD OF DISTINGUISHING DIFFERENT TYPES OF NUCLEAR FUEL ELEMENTS AND NUCLEAR FUEL ELEMENT PROVIDED WITH IDENTIFICATION MEANS

THIS INVENTION relates to nuclear fuel. More particularly, the invention relates to a method of distinguishing fuel elements for use in a nuclear reactor and to a nuclear fuel element.

In a nuclear reactor of the high temperature gas-cooled type, use is made of fuel comprising a plurality of spherical fuel elements. Each fuel element includes a core comprising particles, each having a kernel of a fissile material, dispersed in a matrix. The fuel spheres are known as pebbles and the reactor of this type is generally known as a pebble bed reactor.

10 In a pebble bed reactor it is known to operate a multi-pass fuelling scheme in which fuel spheres are passed through a core of the reactor more than once in order to optimise burn-up of fuel. The nuclear reactor thus typically includes a fuel handling system intermediate one or

15 more outlets and one or more inlets of the reactor core, for cycling fuel elements (and/or moderator elements) from and to the core. The fuel handling system defines a flow path intermediate the outlet(s) and the inlet(s) and may include a storage system for storing fuel elements and for feeding fuel elements at predetermined intervals into the reactor core via the inlet(s).

20 Equilibrium, or "standard", fuel, used during normal operating conditions of the nuclear reactor and highly enriched with fissile material, is distinguished from "start-up" fuel, which has a lower enrichment and is intended for use during reactor start-up. "Start-up" fuel is typically maintained

25 at cooler conditions and is not as significantly contaminated with fission products as standard/equilibrium fuel. It is important that the "start-up" fuel elements and the "standard" fuel elements are readily distinguishable from one another in order that they can be separated within the fuel handling system during a transition phase when switching from one type of fuel element to the other, eg. when the operating conditions of the reactor change.

According to one aspect of the invention, there is provided a method for enabling at least two different types of fuel elements, for use in a nuclear reactor, to be distinguished from one another, each fuel element including a core, comprising a plurality of fuel particles, each having a kernel of fissile material, dispersed in a matrix, and at least one coating around the core, which method includes providing in the at least one coating of each fuel element of a predetermined first type identification means whereby the fuel elements of said first type are distinguishable from fuel elements of another type.

For example, each fuel element which is intended to provide start-up fuel may have provided in a coating thereof identification means by which it can be distinguished from each fuel element of the standard fuel type, which may be provided with distinct identification means in its coating.

Typically, the fuel elements are generally spherical. The fuel particles may be generally spherical and may have one or more fission product-retentive coatings deposited thereon. According to another aspect of the invention, there is provided a method of distinguishing nuclear fuel elements, each fuel element including a core, comprising a plurality of fuel particles, each having a kernel of fissile material, dispersed in a matrix, which method includes the step of applying a coating including an identification element, around a core of a predetermined first type of fuel element to be distinguished to facilitate identification of said first type of fuel element.

The method may include the step of applying at least one coating including a different identification element around the core of at least one predetermined other type of fuel element to be distinguished to facilitate identification of said at least one other type of fuel element and distinguishing thereof from fuel elements of the first type. The or each identification element may be provided by a chemical element or compound.

In one embodiment of the invention, the method includes depositing a coating including diamond (as the identification element) around the core of each fuel element of said first type, eg. intended for use during start-up of a nuclear reactor (so-called "start-up" fuel). The method may include depositing a coating including silicon-nitride-bonded-silicon-carbide around the core of each fuel element of said at least one other type, eg. providing equilibrium or so-called "standard" fuel for use during normal reactor operating conditions. ^■ Naturally, however, any other suitable chemical element/compound, not being diamond, may be included in the coating applied to the surface of said other type of fuel to be distinguished.

In another embodiment of the invention, the method includes applying a coating of graphite incorporating an identification element around the core of each fuel element of said first type or said at least one other type. The identification element may be in powder form. The identification element may be provided by at least one chemical compound selected from the group consisting of zirconium carbide, titanium carbide and chromium carbide. The identification element may be incorporated in the graphite coating in powder form in a mass percentage of between about 0.05 % and about 0.65 % by mass, preferably about 0.5 % by mass.

It will be appreciated that spectroscopic methods may be used to distinguish nuclear fuel elements providing different fuel types, eg. start-up and equilibrium fuel, where at least one of the fuel types includes fuel elements incorporating identification means in a coating applied to the fuel element, in accordance with the invention. The identification means/element serves to render the fuel elements of a predetermined particular fuel type distinguishable from fuel elements of at least one other predetermined type.

According to still another aspect of the invention, there is provided a nuclear fuel element, which includes a core comprising a plurality of fuel particles, each incorporating a kernel of fissile material, dispersed in a matrix; and a coating including at least one chemical compound selected from the group consisting of diamond, silicon-nitride-bonded-silicon-carbide, zirconium carbide, titanium carbide and chromium carbide on the core, the chemical compound providing identification means by which the fuel element is identifiable and distinguishable, eg. by spectroscopic methods.

In the case of the at least one chemical compound including zirconium carbide, titanium carbide or chromium carbide, the chemical compound may be incorporated in powder form in a coating of graphite. More particularly, the at least one chemical compound (at least one of zirconium carbide, titanium carbide and chromium carbide) may be incorporated in a graphite coating in powder form in a mass percentage of between about 0.05 % and about 0.65 % by mass, preferably about 0.5 % by mass.

The invention will now be described, by way of example, with reference to the accompanying diagrammatic drawing and the following Example:

Example 1

A plurality of nuclear fuel particles was formed by atomisation of uranyl nitrate to form microspheres. The microspheres were then gelled and baked at a high temperature, ie. calcinated, to yield uranium dioxide particles, each to provide a kernel of fissile material for a nuclear fuel particle.

A batch of uranium dioxide particles was suspended in a deposition chamber of a chemical vapor deposition reactor, the deposition chamber having an argon environment. The deposition chamber was heated to a temperature of about 1000 degrees Celsius.

Carbon material and carbon compounds, selected from the group consisting of porous carbon, pyrolytic carbon, silicon carbide, silicon- nitride-bonded-silicon-carbide and diamond, were deposited on the surface of the uranium dioxide particles to yield generally spherical fuel particles of diameter about 1 millimeter. Some 15 000 of these coated fuel particles were mixed with approximately 200 g of a matrix material comprising 80 % by mass graphite powder and 20 % by mass phenolic resin. The resulting mixture was pressed into spheres which were machined to a diameter of about 50 millimeters to yield cores of nuclear fuel elements, each core including a plurality of fuel particles, incorporating a fissile material, dispersed in a matrix and having a mass of about 210 grams.

A first batch of such nuclear fuel element cores was suspended in a deposition chamber of a chemical vapor deposition reactor, the deposition chamber having an argon environment. To ensure free stable flotation of the fuel element cores within the coater volume, chemical precursors for deposition were transported through the deposition chamber at a rate of flow in accordance with their temperature and pressure so as to maintain the fuel element cores in a suspended condition.

Graphite was deposited on the cores to a thickness of 5 millimetres. Thereafter, silicon carbide was deposited on the surfaces of the fuel element cores to a thickness of about 240 micrometres. The deposition of silicon carbide was carried out a pressure of 1.6kPa.

The source gases for deposition were then switched to pure argon at a pressure of 1.65 kPa in a manner that ensured minimal disturbance of the floating bed dynamics of the coater volume. The temperature of the circulating argon was decreased to 950 degrees Celsius and argon circulation was maintained for fifteen minutes.

A chlorine (Cl₂) source was then connected to an inlet end of the deposition chamber whilst maintaining the temperature of about 950 degrees Celsius. The silicon carbide was reduced to diamond through the full depth of silicon carbide.

A second batch of nuclear fuel element cores was suspended in a deposition chamber of a chemical vapor deposition reactor, a methylchlorosilane source was connected to an inlet end of the deposition chamber and the temperature was raised to 1375 degrees Celsius. Graphite was deposited on the cores to a thickness of 5 millimetres. Thereafter, a mixture comprising silicon and silicon carbide was deposited to a thickness of about 240 micrometres, by the decomposition of the methylchlorosilane, on the surfaces of the fuel element cores. The deposition was carried out a pressure of 1.6 kPa.

The fuel elements were then nitrided, by furnacing in a nitrogen atmosphere at a temperature of 1820 degrees Celsius and a pressure of approximately 1.7 kPa, to yield a coating including a mixture of silicon carbide and silicon nitride crystals. The fuel elements were treated in the nitrogen atmosphere for 4.5 hours to permit nitrogen to permeate the full depth of the silicon and silicon carbide to yield silicon-nitride-bonded-silicon-carbide.

Stoichiometric silicon carbide was thereafter deposited on the silicon-nitride-bonded-silicon-carbide at a temperature of 1840 degrees Celsius and a pressure of 1.75 kPa. The batch of fuel elements was finally treated in an argon environment at a pressure of 300 MPa, in which they were heated rapidly by use of a microwave heater to a surface temperature of 1 800 degrees Celsius. This was followed by stable cooling to room temperature for at least four hours. Reference is made to Figure 1 of the drawings, which shows a cross-sectional perspective view of a nuclear fuel element in accordance with the invention. In Figure 1 , reference numeral 10 refers generally to a coated nuclear fuel element of the invention, prepared in accordance with Example 1 above. The coated fuel element 10 includes a core 12 of spherical form and having a diameter of about 50 millimetres. The core 12 includes of the order of 15 000 coated fuel particles 14, each having a kernel of fissile material (not shown), dispersed in a matrix 16. The matrix 16 comprises graphite powder and phenolic resin. About 5 millimetres of graphite coating 18 is deposited on the core 12 followed by diamond or silicon-nitride-bonded-silicon-carbide 20, as the case may be, to a thickness of about 240 micrometres.

Discussion

Instead of adding chlorine gas to the argon stream in the case of the first batch of fuel elements, hydrogen chloride vapor may be used as a reducing agent. The reduction of silicon carbide occurs in accordance with the following reaction: SiC + CI₂ - C + SiCI₂, or, where hydrogen chloride is used as reducing agent: SiC + 2HCI - C + SiCI₂ + H₂

At the temperature of about 950 degrees Celsius, the carbon crystallises in an sp2 hybridised face-centred cubic structure, ie. to yield its allotrope of diamond. It will be appreciated that, in the case of silicon-nitride-bonded- silicon-carbide coated fuel elements, a strong nitrogen signal will be present in an absorption spectrum derived by spectroscopic methods of analysis for such fuel elements. The type of fuel including such fuel element (eg. equilibrium or standard fuel) will therefore be distinguishable from a fuel type not having such a coating, eg. start-up fuel having a diamond coating on a core thereof, on this basis.

Similarly, in the case of zirconium carbide, titanium carbide or chromium carbide powder incorporated in a graphite outer layer of a particular type of fuel element, eg. start-up fuel, a gamma spectrometer, incorporated into the fuel handling system of a nuclear plant, will be capable of distinguishing this fuel type from another fuel type not incorporating the zirconium carbide, titanium carbide or chromium carbide fuel marker/identification element.

The Applicant believes that the low radiation creep of diamond and its long term stability at lower temperatures suits diamond to use as a coating on start-up fuel to allow start-up fuel to be distinguished. "Start-up" fuel is typically maintained at cooler conditions and is not contaminated with fission products and is susceptible to re-use when it is below predetrmined burn-up design limits. Further, it is believed it will be possible to use the diamond-coated start-up fuel, distinguishable as it is with its diamond coating from standard fuel, several times for start-up of different reactors or for restarting of a reactor after maintenance shut down.

Claims

Claims:

1. A method of distinguishing at least two different types of fuel elements, for use in a nuclear reactor, each fuel element including a core, comprising a plurality of fuel particles, each having a kernel of fissile material, dispersed in a matrix, and at least one coating around the core, which method includes providing in the at least one coating of each fuel element of a predetermined first type identification means whereby the fuel elements of said first type are distinguishable from fuel elements of another type.

2. A method as claimed in Claim 1, which includes applying a coating including an identification element around the core of each fuel element of said first type, to facilitate identification of said first type of fuel element.

3. A method as claimed in Claim 2, which includes applying a coating including a different identification element around the core of at least one predetermined other type of fuel element, to facilitate identification of said at least one other type of fuel element and distinguishing thereof from fuel elements of the first type.

4. A method as claimed in Claim 3, in which the or each identification element is provided by a chemical element or compound.

5. A method as claimed in Claim 4, which includes depositing a coating including diamond around the core of each fuel element of said first type.

6. A method as claimed in Claim 4 or Claim 5, which includes depositing a coating including silicon-nitride-bonded-silicon-carbide around the core of each fuel element of said at least one other type.

7. A method as claimed in Claim 4, which includes applying a coating of graphite incorporating at least one identification element selected from the group consisting of zirconium carbide, titanium carbide and chromium carbide around the core of the core of each fuel element of said first type.

8. A method as claimed in Claim 7, in which the at least one identification element is incorporated in the graphite coating in powder form in a mass percentage of between 0.05 % and 0.65 % by mass.

9. A method as claimed in Claim 8, in which the at least one identification element is incorporated in the graphite coating in powder form in a mass percentage of 0.5 % by mass.

10. A nuclear fuel element, which includes a core comprising a plurality of fuel particles, each incorporating a kernel of fissile material, dispersed in a matrix; and a coating including at least one chemical compound selected from the group consisting of diamond, silicon-nitride-bonded-silicon-carbide, zirconium carbide, titanium carbide and chromium carbide around the core, the chemical compound providing identification means by which the fuel element is identifiable and distinguishable.

11. A nuclear fuel element as claimed in Claim 10, in which, where the at least one chemical compound includes zirconium carbide, titanium carbide or chromium carbide, the at least one chemical compound is incorporated in powder form in a coating of graphite.

12. A nuclear fuel element as claimed in Claim 11 , in which the chemical compound is incorporated in the graphite coating in a mass percentage of between 0.05 % and 0.65 % by mass.

13. A nuclear fuel element as claimed in Claim 12, in which the identification element is incorporated in the graphite coating in a mass percentage of 0.5 % by mass.