Note: Descriptions are shown in the official language in which they were submitted.
~ 87~9
Fuel rod for a nuclear reactor
Technical Field
This invention relates to a fuel rod for a nuclear
reactor, the fuel rod being of the kind comprising a cladding
tube containing a nuclear fuel, ~or example pellets of
5 uranium dioxide.
Background Art
The cladding tubes normal]y used in fuel rods for nuclear
reactors comprise thin-walled tubes of zirconium-based alloys~ ;
for example the alloys known under the trade name "Zircaloy".
10 These alloys, which contain dissolved alloying materials such
as tin, iron, nickel, chromium and oxygen, become irradiation-
hardened under neutron irradiation. The irradiation hardening
results in a brittling of the material and a considerably
increased sensitivity to rod damage induced by stress corrosion.
15 To counteract this hardening, it is known to provide such
cladding tubes internally with a layer of zirconium. Zircon-
ium is well suited for this purpose, since it has good resist-
ance to hardening when exposed to neutron irradiation, and
since it is a relativel'y soft material. A zirconium layer
c 20 on the internal surface of the cladding tube can therefore be
plastically deformed and protect the cladding tube against
stresses occurring in case of power changes during operation
of the nuclear reactor in which the fuel rods are installed.
In the known cladding tubes with an internal zirconium
25 layer, the zirconium has had from 1,000 parts per million
'(ppm) to 5,000 ppm. Of these impurities, from 200 to 1,200
' ppm consist of oxygen. The amounts of the other impurities
lie within the normal limits for the respective materials in
commercial zirconi'um sponge of reactor quality, namely up
to 75 ppm of aluminum, up to oD4 ppm of boron,'up to 0.4 ppm
of cadmium, up to 270 ppm of carbon, up to 200 ppm of chromium,
up to 20 ppm of cobalt, up to 50 ppm of copper, up to 100 ppm
of hafnium, up to 25 ppm of hydrogen, up to 1,500 ppm of iron,
.. . .
., ~
' ~ ~
l l ~ 9
up to 20 ppm of magnesium, up to 50 ppm of Manganese, up to
50 ppm of molybdenum, up to 70 ppm of nickel, up to lO0 ppm
of niobium, up to 80 ppm of nitrogen, up to 120 ppm of
silicon, up to 50 ppm of tin, up to 100 pp~ of tungsten,
up to 50 ppm of titanium and up to 3.5 ppm of uranium.
The present invention aims to provide a fuel rod of the
kind referred to, in which the cladding tube has a consider-
ably improved resistance to stress corrosion compared with
hitherto known fuel rods.
Disclosure of` the Invention
According to the inven-tion, there is provided a nuclear
reactor fuel rod comprising a cladding tube of a zirconium-
based alloy, and a nuclear fuel within said cladding tube,
wherein said cladding tube is lined internally with a layer
of zirconium which contains at least one substance, selected
from the group consisting of molybdenum, carbon, phosphorus
and silicon, present in an amount, expressed as a percentage
of the weight of the zirconium in said layer, which in the
case of molybdenum is-from about 0.1 to about 3.0 per cent
and in th~ case of carbon, phosphorus or silicon is from
about 0u03 to about l.0 per cent.
C
In addition, the zirconium in said layer may contain
-other impurities included in commercial zirconium sponge of
( reactor quality, for example at least one of the aforemention-
ed impurities in the stated amounts.
A possible explanation of the improved resistance to
stress corrosion of the cladding tube of a nuclear fuel rod
according to the invention is as follows:
. . .
During manufacture of a cladding tube with an internal
zirconium layer, which can be performed by extrusion, the
materials are subJected to high temperatures. This results
in a grain growth taking place in the zirconium, which is
rapid in the case of the hitherto known zirconiu~ layer in
- ::
" .' " ', ' ,
'
,
1 1 6 ~ 9
` ` ', ~
which the alloying materials are dissolved. When one or
more of the additives molybdenum, carbon, phosphorus and
silicon is or are present in the above-stated amounts, a
precipitation of stable compounds takes place, such as inter-
5 metallic compounds, carbides, phosphides, silici~es, in the
form of free particles in the zirconium matrix. This
precipitation prevents the grain growth, so that a structure
with smaller gr~ns is obtained in the zirconium than in the
case of the hitherto known zirconium layer. The different,
( 10 fine-grained structure is responsible for the increasedresistance to stress corrosion. 5
The thickness of the zirconium layer in a fuel rod
in accordance with the invention may be from about 0.005
to about 0.8 mm, and preferably from about 0.05 to about 0.1
15 mm.
The zirconium-based alloy cladding tube, on the internal
surface of which the zirconium layer is provided, pre1er- -
ably consists of a zirconium-tin alloy, for example the
zirconium-based alloys known under the trade names '1Zircaloy
20 2" and "Zircaloy 4", whose contents of alloying materials
are within the limits, by weight, of from lo ? to lo 7 per cent
C of tin, from 0.07 to 0.2~ per cent of iron, from 0.05 to
0.15 per cent of chromium~ from 0.0 to o.o8 per cent of
- nickel, and from 0.10 to 0.15 per cent of oxygen, the balance
c 25 being zirconium and possibly existing impurities of ordinary
kind. The nuclear fuel in the cladding tube preferably
consists of uranium dioxide.
Brief Description of Drawing
The invention will now be described, by way of example,
30 with reference to the accompanying drawing, the single Figure
of which is a cross sectional view of a fuel rod according to
the present invention for a light water nuclear reactor.
Desc~iption of Preferred_Embodime_
. Two par~s by weight of molybdenum are mixed with 98 parts
.
. '
:
L 1687~9
by weight of the aforementioned commercial zirconium sponge
of reactor quality and heated to form a melt. From this
material there is ~ade a tube having a wall thiclmess o~ 1.25 m~
and an external diameter o~ 44 mm. The tube is arranged
inside a tube of "Zircaloy 2" having a wall thickness of
10 mm and an internal diameter of 45 mm. The two tubes are
welded together at the two end surfaces o~ the assembled
tubesO The composite tube thus obtained is extruded at a
temperature of about 600C. The extruded product is then
- 10 cold-rolled in several stages with intermediate recrystalli-
zation annealings, to produce the tubular end product shown
in the Figure~ consisting of a layer 1 of '~Zircaloy 2"
having a thickness of 0.73 mm and an internal diameter of
lOo 65 mm and of a layer 2 of zirconium with alloyed
molybdenum having a thickness of 0.07 mm. The Figure also
shows the nuclear fuel consisting o~ circular cylindrical
pellets 3 cf uranium dioxide arranged end-to-end in the
axial directlon of the cladding tube.
.
Instead of molybdenum in the above described example,there
may be used 0.4 per cent by weight of carbon or o.8 per cent
by weight of phosphorus or o.8 per cent by weight of silicon.
It is also possible to use two or more of the four components
together in the stated amounts.
..~;
