The present invention relates to the field of nuclear fuel cladding, in particular
nuclear fuel rod cladding, and to the manufacturing process thereof.
The nuclear fuel including the fissile material is generally contained in a sealed
cladding which prevents the dispersion of the nuclear fuel.
Nuclear fuel assemblies used in light water reactors generally comprise a bundle of
10 nuclear fuel rods, each nuclear fuel rod comprising a cladding containing nuclear fuel, the
cladding being formed of a cladding tube closed by a plug at each of the two ends thereof.
The cladding tubes of the nuclear fuel assemblies are made e.g. of zirconium or of
an alloy containing zirconium. Such alloys have high performance under normal conditions
of use in nuclear reactors. However, they can reach the limits thereof in particular in terms
15 of temperature during severe accident conditions, such as e.g. during a Loss of Coolant
Accident (or LOCA).
During such an event, the temperature can reach more than 800°C and the cooling
fluid is essentially in the form of water vapor. This can cause a rapid degradation of the
cladding tube, in particular a release of hydrogen and a rapid oxidation of the cladding tube
20 leading to the weakening thereof or even to the bursting thereof, and thus to the release of
nuclear fuel out of the cladding. During oxidation, part of the hydrogen produced is absorbed
(hydriding) by the cladding, entailing the weakening of the latter.
WO2016/042262A1 proposes a nuclear fuel cladding comprising a substrate made
of zirconium or zirconium alloy and covered with a protective coating made of chromium or
25 chromium alloy, the protective coating having a columnar microstructure.
One of the aims of the invention is to propose a nuclear fuel cladding element which
has satisfactory resistance to hydriding and/or oxidation.
To this end, the invention proposes a nuclear fuel cladding element, the cladding
element comprising a substrate made of a material containing zirconium and a protective
30 coating covering the substrate on the outside, the protective coating being made of a
material containing chromium, wherein the protective coating has a columnar microstructure
composed of columnar grains and has on the outer surface thereof a microdroplet density
of less than 100 per mm2
.
The columnar microstructure allows obtaining a ductile protective coating which can
35 resist deformation, which limits the risk of occurrence of cracks in the event of deformation
of the cladding element. The occurrence of a crack would be likely to expose the substrate
2
to the outer environment, which could cause the degradation thereof and the weakening
thereof, and ultimately lead to an opening of the cladding element.
Limiting the density of the microdroplets present on the surface of the protective
coating further improves the resistance of the cladding element. Indeed, the presence of
5 microdroplets limits the protection provided by the protective coating by allowing the cooling
fluid to infiltrate along the boundaries of the microdroplets, reducing the corrosion and
oxidation resistance of the cladding element, in particular, at high temperatures.
Microdroplets are discontinuities in the microstructure of the protective coating, which form
points of weakness and are likely to initiate cracks in the protective coating. Furthermore,
10 the presence of microdroplets affects at least locally, the microstructure of the protective
coating, the columnar grains generally having a larger mean diameter under the
microdroplets.

I/We Claim:
1. A nuclear fuel cladding element, the cladding element comprising a
5 substrate (16) made of a material containing zirconium and a protective coating (18)
covering the substrate (16) on the outside, the protective coating (18) being made of a
material containing chromium, wherein the protective coating (18) has a columnar
microstructure composed of columnar grains (20) and has on the outer surface (18B)
thereof a microdroplet density of less than 100 per mm2
.
10 2. The cladding element according to claim 1, wherein next to and/or at the
interface between the cladding element and the protective element, the columnar grains
have a mean diameter less than or equal to 1 m, preferentially less than or equal to 0.5 m.
3. The cladding element according to claim 1 or claim 2, wherein next to and/or
on the outer surface (18B) of the protective element, the columnar grains (20) have a mean
15 diameter between 0.05 m and 5 m, preferentially between 0.1 m and 2 m.
4. The cladding element according to any of the preceding claims, wherein the
microdroplets have a diameter less than or equal to 20 m.
5. The cladding element according to any of the preceding claims, wherein the
protective coating (18) has a thickness comprised between 5 m and 25 m.
20 6. The cladding element according to any of the preceding claims, wherein the
protective coating is made of a material containing chromium, e.g. pure chromium or an
alloy containing zirconium, e.g. a binary chromium alloy, in particular a binary chromiumaluminum alloy, a binary chromium-nitrogen alloy or a binary chromium-titanium alloy.
7. The cladding element according to any of the preceding claims, wherein the
25 cladding element is a cladding tube, in particular a cladding tube of a nuclear fuel rod.
8. A nuclear fuel element comprising nuclear fuel disposed within a cladding
consisting of at least one cladding element according to any preceding claim.
9. A nuclear fuel rod comprising nuclear fuel arranged within a cladding consisting
of a tubular cladding member according to any of claims 1 to 7, closed at by plugs at the
30 ends thereof.
10. A method of manufacturing a cladding element according to any of claims 1
to 7, comprising obtaining the substrate (16) and then depositing the protective coating (18)
onto the substrate (16) by physical vapor deposition by sputtering of a target (24) or by
physical deposition by cold spraying.
35 11. The manufacturing method according to claim 10, wherein the deposition is
carried out by physical vapor deposition by magnetron sputtering.
15
12. The manufacturing method according to claim 11 or claim 12, wherein the
substrate (16) has the shape of a plate and the deposition step is carried out in such a way
that the rate of deposition of the protective coating (18) onto the substrate (16) is comprised
between 1 µm/h and 30 µm/h.
5 13. The manufacturing method according to claim 11 or claim 12, wherein the
substrate (16) is a tube which has a central axis, the deposition step is carried out by rotating
the substrate (16) about the central axis thereof and in such a way that the rate of deposition
of the protective coating (18) onto the substrate (16) is comprised between 1/π µm/h and
30/π µm/h.
10 14. The manufacturing method according to any of claims 10 to 13, wherein the
deposition is carried out by physical vapor deposition by supplying the target with pulsed
current with current peaks.
15. The manufacturing method according to claim 14, wherein the deposition is
carried out with an average power density comprised between 1 W/cm2 and 5 W/cm2
;
15 16. The manufacturing method according to claim 14 or 15, wherein the deposition
is carried out with a peak power density comprised between 30 W/cm2 and 100 W/cm2
;
17. The manufacturing method according to any of claims 14 to 16, wherein the
deposition is carried out with a frequency of the current pulses comprised between 50 Hz
and 5000 Hz;
20 18. The manufacturing method according to any of claims 14 to 17, wherein the
deposition is carried out with a current pulse duration comprised between 10 µs and 50 µs;
19. The manufacturing method according to any of claims 14 to 18, wherein the
deposition is carried out under a pressure comprised between 0.1 Pa and 0.4 Pa.
20. The manufacturing method according to any of claims 14 to 19, wherein the
25 deposition is carried out with a distance between the substrate (16) and the target (24)
comprised between 50 mm and 200 mm, more preferentially between 80 mm and 140 mm.
21. The manufacturing method according to any of claims 10 to 20, wherein the
deposition is carried out by physical vapor deposition by supplying the target with direct
current so as to obtain a current density comprised between 0.0005 A/cm2 and 0.1 A/cm2
on the target (24), preferentially between 0.0005 A/cm2 and 0.05 A/cm2 30 , or in pulsed current
with current peaks so as to obtain a current density comprised between 0.01 A/cm2 and 5
A/cm2 on the target (24) during current peaks, preferentially between 0.01 A/cm2 and 0.5
A/cm2
.
22. The manufacturing method according to any of claims 10 to 21, wherein the
35 deposition of the protective coating (18) is carried out by supplying the target (24) with a
direct current so as to obtain a power density comprised between 0.5 W/cm2 and 100 W/cm2
16
for the target, preferentially a power density comprised between 0.5 W/cm2 and 50 W/cm2
or with a pulsed current with current peaks so as to obtain a power density comprised
between 10 W/cm2 and 50.000 W/cm2
(i.e. the peak power density) at the target,
preferentially a power density comprised between 10 W/cm2 and 5000 W/cm2
.
5 23. The manufacturing method according to any of claims 10 to 22, wherein the
deposition is carried out by physical vapor deposition with an electrical bias voltage of the
substrate (16) with respect to the target (24) during physical vapor deposition, which is
negative and comprised between – 10 V and – 200 V.
24. The manufacturing method according to any of claims 10 to 23, wherein the
10 deposition is carried out in an atmosphere consisting of an inert gas.