Nuclear reactor having a layer protecting the surface of zirconium alloys

Abstract

A layer protecting the surface of zirconium alloys used as materials for nuclear reactors is formed by a homogenous polycrystalline diamond layer prepared by chemical vapor deposition method. This diamond layer is 100 nm to 50 m thick and the size of the crystalline cores in the layer ranges from 10 nm to 500 nm. Maximum content of non-diamond carbon is 25 mol %, total content of non-carbon impurities is maximum up to 0.5 mol %, RMS surface roughness of the polycrystalline diamond layer has a value less than 40 nm and thermal conductivity of the layer ranges from 1000 to 1900 Wm.sup.1K.sup.1. Coating of the zirconium alloys surface with the described polycrystalline diamond layer serves as a zirconium alloys surface protection against undesirable changes and processes in the nuclear reactor environment.

Claims

1. A nuclear reactor, comprising: a nuclear fuel rod comprising a zirconium alloy tube; cladding on the nuclear fuel rod, the cladding comprising a layer coated on an outer surface of the zirconium alloy tube, wherein, under standard operating conditions that include a temperature of about 300 C., the layer comprises a homogenous polycrystalline diamond layer formed by chemical vapor deposition, is from 100 nm to 50 m thick, includes crystalline cores, sizes thereof being in the range from 10 nm to 500 nm, has a maximum content of non-diamond carbon of 25 mol %, has a maximum total content of non-carbon impurities of 0.5 mol %, has a RMS surface roughness value less than 40 nm, and has a thermal conductivity of the layer ranges from 1000 to 1900 W.Math.m.sup.1.Math.K.sup.1, and wherein, when the nuclear reactor is operated at a temperature above 850 C., the polycrystalline diamond layer undergoes a phase change and is transformed into a mixture of crystalline graphite, amorphous carbon, and graphene, wherein the transformation is configured to consume environment energy to lower environment temperature.

2. The nuclear reactor as set forth in claim 1, wherein the mixture forms a carbide.

3. The nuclear reactor as set forth in claim 1, wherein, after operation at a temperature above 1100 C., a thermally transformed carbon layer comprises a mixture of carbon, oxygen, and atoms from the zirconium alloy tube.

4. A method of protecting a nuclear reactor against corrosion, comprising: providing in the nuclear reactor a nuclear fuel rod comprising a zirconium alloy tube; providing cladding on the nuclear fuel rod, the cladding comprising a layer coated on an outer surface of the zirconium alloy tube, wherein, under standard operating conditions that include a temperature of about 300 C., the layer comprises a homogenous polycrystalline diamond layer formed by chemical vapor deposition, is from 100 nm to 50 m thick, includes crystalline cores, sizes thereof being in the range from 10 nm to 500 nm, has a maximum content of non-diamond carbon of 25 mol %, has a maximum total content of non-carbon impurities of 0.5 mol %, has a RMS surface roughness value less than 40 nm, and has a thermal conductivity of the layer ranges from 1000 to 1900 W.Math.m.sup.1.Math.K.sup.1; wherein, when the nuclear reactor is operated at a temperature above 850 C., the polycrystalline diamond layer undergoes a phase change and is transformed into a mixture of crystalline graphite, graphene, amorphous carbon, and atoms from the zirconium alloy tube, wherein the transformation is configured to consume environment energy to lower environment temperature, operating the nuclear reactor so that the polycrystalline diamond layer is still present and shows partial graphitization and amorphization.

5. The method as set forth in claim 4, comprising operating the nuclear reactor at a temperature above standard operating conditions so that the polycrystalline diamond layer undergoes a phase change and is transformed into a mixture of crystalline graphite, graphene, amorphous carbon, and atoms from the zirconium alloy tube.

6. The method as set forth in claim 5, wherein the mixture includes a carbide.

7. The method as set forth in claim 5, wherein the temperature above standard operating conditions is above 850 C.

Description

EXPLANATION OF DRAWINGS

(1) The presented solution is illustrated by FIG. 1 and FIG. 2 showing the Raman spectrum of the homogenous polycrystalline diamond layer covering the sample of the fuel element made of zirconium alloy both in the basic condition and after simulation of standard as well as emergency conditions of the nuclear reactor.

(2) In FIG. 1 peaks of Raman spectra show vibrational states of various phases of carbon protective layer. FIG. 2 shows that after ion implantation, simulating material interaction with particles in a nuclear reactor, the polycrystalline diamond layer underwent partial graphitization but the diamond crystalline phase in the layer was still present. After simulation at accident conditions, i.e. exposure to hot water steam, a phase change in the protective layer occurred, during which the crystalline diamond transformed into a mixture of graphite, graphene and amorphous carbon.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(3) The proposed solution and the subject of this patent is the protection of the surface of zirconium alloys used as materials in nuclear reactors by polycrystalline diamond layers. Diamond features high thermal conductivity and stability, low chemical reactivity, it does not degrade over time and has a suitable effective cross-section for interaction with neutrons. The surface of elements made of zirconium alloys are coated with homogenous polycrystalline diamond layers prepared by chemical vapor deposition method, abbreviated as CVD, with typical columnar nature of diamond crystallites. The CVD method means that diamond is prepared by decomposition of a mixture of methane (or other carbon containing species) and at pressures from 0.01 mbar to 100 mbar and at substrate temperatures of 250 C. to 1000 C.

(4) Polycrystalline diamond layers suitable for the protection of zirconium alloys surface is 100 nm to 50 m thick and with crystalline cores in the layer ranges from 10 nm to 500 nm in size. From the chemical composition point of view the layer can be specified based on the maximum content of non-diamond carbon, which is a maximum 25 mol %, and by the total content of non-diamond impurities with a maximum value of up to 0.5 mol %. Surface roughness of the polycrystalline diamond layer must not exceed the RMS roughness value of 40 nm. Thermal conductivity of the layer ranges from 1000 to 1900 W.Math.m.sup.1.Math.K.sup.1.

(5) Crystalline diamond has a strong and rigid isotropic structure due to its cubic crystal symmetrycarbon atoms are bound by strong covalent bonds. On the contrary, carbon atoms in graphite are bound by different and bonds in the hexagonal crystalline system. In this specific configuration, one electron is weakly bounded and thus contributes to a significantly higher electrical conductivity of graphite compared to diamond. Stable planar structures of graphite are mutually bonded by Van der Waals forces, thereby forming the soft, malleable and also resistant material.

(6) Under standard operation conditions in nuclear reactors the polycrystalline diamond layer will maintain its original properties and will participate both in dissipation of heat released during the reactor's operating mode, and also will protect the coated surface against undesirable chemical reactions and changes of the structure composition related to diffusion of the hydrogen atoms from dissociated molecules of water into the zirconium alloy. After long-term interaction with elementary particles released from the nuclear reactions the polycrystalline diamond layer shows partial graphitization and amorphization but the diamond crystalline phase is still present in the layer. Polycrystalline diamond layer will further limit the undesirable high-temperature chemical reactivity of the zirconium alloys surface and therefore also the high-temperature dissociation of water steam molecules and subsequent formation of the zirconium oxide and explosive hydrogen. In the case of temperature-induced changes of the zirconium tubes volume, the layer will benefit from the mixed nature of the protective carbon layer, which besides crystalline diamond cores with sp.sup.3 of hybridized carbon contains also flexible amorphous phase sp.sup.2 from hybridized carbon capable of good adaptation to the volume changes/expansion of the metal substrate without disrupting the protective layer integrity.

(7) If under accident conditions in the nuclear reactor the system gets heated above temperature of 850 C., the protective polycrystalline diamond layer undergoes a phase change. Crystalline diamond transforms into a mixture of crystalline graphite, graphene and amorphous carbon. Non-diamond carbon material, or its selected components feature a high melting temperature of 3642 C. The process of the crystalline diamond thermal transformation to graphite, graphene and non-s crystalline carbon consumes part of the energy from the environment, thus also at least slightly lowering its temperature. This protection, carbon-containing layer further worsen the conditions for the high temperature degeneration of the surface, including the passivated layer, also any quenching of zirconium alloy and further reduces the probability of explosion of hydrogen.

(8) Below is an example showing the practical impact of the use of protective homogeneous polycrystalline diamond layer on zirconium samples, FIG. 1 and FIG. 2.

(9) Raman spectra of the sample of the fuel cell made from a zirconium alloy, homogeneously coated by 300 nm thick polycrystalline diamond film by vapor deposition is shown in FIG. 1. All the Raman spectra were measured at different locations on the surface to prove the regularity of the sample state. Raman peak positions in spectra were same at different locations on the sample surface. The vibration peak at 1332 cm.sup.1 corresponds to the sp.sup.3 hybridized carbon, that means the diamond phase in layer. Vibrations in the range of 1450-1650 cm.sup.1 correspond to the sp.sup.2 hybridized carbon, thus non-diamond phase represented in the polycrystalline diamond layer.

(10) The partial graphitization of polycrystalline diamond films was obtained after ion implantation, simulating the load of material interactions with elementary particles in a nuclear reactor for the 3 MeV Fe ions, the dose of 1.9510.sup.16 cm.sup.2, as corresponding to 10 dpa damage. But diamond crystalline phase was still present in the layer, see the Raman spectrum, FIG. 2. FIG. 2 shows the Raman spectra of a homogeneous polycrystalline diamond layer covering a portion of the fuel cell of the zirconium alloy in a basic condition, after ion implantation, and heated in a steam environment at a temperature of 1100-1200 C. The vibration peak at 1332 cm.sup.1 corresponds to the sp.sup.3 hybridized carbon, thus the diamond phase of carbon, the vibration peak at 1355 cm.sup.1 to the crystalline graphite, vibrations in the range of 1450 to 1650 cm.sup.1 to the sp.sup.2 hybridized carbon, thus non-diamond carbon phase

(11) The Raman spectrum line of the sample coated with the polycrystalline diamond layer is shown in FIG. 2 by the full line.

(12) Dashed line shows the spectrum of the sample coated with the polycrystalline diamond layer after ion implantation simulating the load of material by interaction with fundamental particles in a nuclear reactor. Ion implantation causes partial graphitization of the polycrystalline diamond layer but the diamond crystalline phase remains still present in the layer.

(13) Dotted line shows the Raman spectrum of the sample coated with the polycrystalline diamond layer after simulation of accident conditions by steam oxidation when the phase change of the crystalline diamond in the protective layer occurs and the crystalline diamond transforms into the mixture of graphite, graphene and amorphous carbon.

(14) It is evident, that after the simulation of accident conditions in a nuclear reactor, therefore heating in a steam environment at a temperature in the range 1100 to 1200 C., the phase change occurs in the protective layer. Crystalline diamond has transformed into a mixture of graphite, graphene and amorphous carbon.

(15) Elemental analysis, of the substrate and the protective layer in its initial state and after the thermal stress in the steam chamber, where the simulated environment of a nuclear reactor accident at 1100-1200 C. was made by ESCA, Electron Spectroscopy for Chemical Analysis. It was found that thermally transformed carbon layer comprises a mixtures of carbon, oxygen and atoms of substrate. The newly composed layer thus absorbed atoms from surroundings and separated zirconium alloy surface from the surrounding environment so that its status under the protective layer from the base material Zr alloys atomic composition differed minimally.

INDUSTRIAL APPLICABILITY

(16) The above mentioned protection of zirconium alloys surfaces by homogenous polycrystalline diamond layers may be applied to a wide range of functional elements for nuclear reactors, such as fuel rod elements. This involves, in particular, parts of commercially operated power producing light-water reactors PWR, BWR, VVER and heavy-water reactors CANDU. The described protective layer significantly increases such reactors' operation safety.