Metal treatment – Process of modifying or maintaining internal physical... – Processes of coating utilizing a reactive composition which...
Patent
1997-10-28
2000-03-07
Sheehan, John
Metal treatment
Process of modifying or maintaining internal physical...
Processes of coating utilizing a reactive composition which...
148277, 148281, 148283, 148284, 4284721, 427539, 427253, 376438, 376353, 376457, 376305, 20415743, C23C 2800
Patent
active
060334932
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention relates to a process for coating a substrate made of a passivatable metal or an alloy based on a passivatable metal, such as zirconium, with a metal oxide layer.
BACKGROUND OF THE INVENTION
Fuel rods of pressurized water nuclear reactors consist of pellets of a nuclear fuel material in oxide form stacked inside a tubular cladding, generally made of a zirconium alloy. The claddings of fuel rods must hold the nuclear fuel pellets and they constitute the primary barrier preventing the nuclear fuel from spreading inside the reactor.
In the case of local piercing of the cladding of a fuel rod, the reactor cooling water penetrates inside the cladding of the fuel rod; this water, which boils, reacts with the metal of the cladding to form hydrogen that produces hydridation of the metal of the cladding. This may result in further perforations of the cladding which amplify the results of the initial piercing.
It is therefore necessary to reduce to a minimum the risks of piercing of the fuel rod claddings, in particular at the bottom grid of the assemblies, where the rods are subjected to intense transverse hydraulic flows and are therefore susceptible to wear, because of the "fretting" phenomenon, by vibration against the rod supports constituted by the springs and the dimples.
Furthermore, the bottom grid of a nuclear fuel assembly is a location highly susceptible to trapping of debris which is vibrated by the circulating cooling fluid. This vibrating debris can damage the rods by friction and wear.
It has therefore been proposed to protect the fuel rod claddings by a layer of wear-resistant coating, in particular in the zones of the cladding which are most exposed to wear.
It has, for example, been proposed to improve the wear resistance of the fuel rod claddings made of zirconium alloy by depositing a hard coating on the external surface of the claddings, in particular by zirconium oxide (zirconia) deposits, these deposits being capable of guaranteeing some degree of protection against wear by the debris carried by the reactor cooling water.
In FR-A-89-12920, filed by FRAMATOME and COGEMA, a process was proposed for surface oxidation of a part made of a passivatable metal such as zirconium or a zirconium alloy, which makes it possible to produce a diffused layer of zirconium oxide on this part. This process is applicable in particular to coating the external surface of fuel rod claddings for a nuclear reactor.
In the context of this process for surface oxidation of a passivatable metal part, a gas activated by a cold plasma is used, i.e., a gas containing species activated by supply of energy. Using an activated gas makes it possible to obtain an oxide layer composed of fine and homogeneous grains at the surface of the part at moderate temperatures.
This process makes it possible to obtain oxide coating layers which guarantee a high degree of protection against the corrosion of elements such as cladding tubes; however, the layers obtained, being thin, do not guarantee very efficient protection against damage of mechanical origin, due, for example, to friction of debris against the cladding tubes.
It has also been proposed in WO-A-92 09716 to reduce the wear by fretting of zirconium alloy cladding tubes by producing carbide or oxide coatings on the surface of the cladding tubes, by using a gaseous phase or molten salts at a temperature of less than 500.degree. C. It has thus been possible to form protective layers whose thickness may be up to 2 .mu.m. The layers formed on the surface of the cladding tubes are still too thin to make it possible to guarantee very effective protection against mechanical damage, for example to prevent scratching of the surface of the cladding tubes.
It has also been proposed in U.S. Pat. No. 5,171,520 to produce a local coating of the nuclear fuel rods in those zones most greatly subjected to wear, by spraying ceramic and glass onto the surface to be coated. It is possible in this way to obtain layers with a thickness greater than 100 .mu.m mad
REFERENCES:
patent: 4239819 (1980-12-01), Holzl
patent: 5238524 (1993-08-01), Hertz
Kumashiro et al., Low Temperature Preparation of TiO.sub.2 Thin Films by Plasma-Enhanced Chemical Vapor Deposition, Journal of the Ceramic Society of Japan, 1993, No. 5.
Cao et al. Research on YSZ thin films prepared by plasma-CVD process, Elsevier Science S.A., 1994, pp. 163-167.
Choi et al., Effects of the reaction parameters on the deposition characteristics in ZrO.sub.2 CVD, Journal of Materials Science: Materials in Electronics, Jun. 1992, vol. 3, No. 2, pp. 87-92.
Yasuda et al., Low-temperature preparation of SiO.sub.2 /Si(100) interfaces using a two-step remote plasma-assisted oxidation-deposition process, Applied Physics Letters, 1992, vol. 60, No. 4, pp. 434-436.
Cao et al., Research on YSZ thin films prepared by plasma-CVD process, Elsevier Science S.A., 1994, pp. 163-167.
Belmonte Thierry
Gavillet Jerome
Hertz Dominique
Michel Henri
Cogema Velizy
Framatome
Oltmans Andrew L.
Sheehan John
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