SYSTEME DE BRASAGE D'ELEMENTS AU ZIRCONIUM

ZIRCONIUM BRAZING SYSTEM

Abrégé anglais

ABSTRACT OF THE DISCLOSURE
The securing together of zirconium components
in the fabrication of nuclear fuel is effected by brazing,
using substantially cobalt-free braze metal comprising
stainless steel of the AISI type 300 series stainless steel.

Revendications

case 2464




The embodiments of the invention in which an
exclusive property or privilege is claimed are defined as
follows:
1. A nuclear fuel bundle having a plurality of
elongated cylindrical elements formed of a material having
zirconium as its principal constituent and including zirconium
alloy component parts in mutually secured relation to at least
one of said elements by a braze metal bond using as the braze
metal a composition comprising a steel in the 300 series of
stainless steel, substantially free of cobalt.
2. The bundle as claimed in claim 1 wherein said
stainless steel comprises essentially about 6-22% weight nickel;
16-26% weight chromium, not more than about 0.3% carbon; not
more than about 0.045% phosphor; not more than about 2.5% man-
ganese; not more than about 1.00% silicon; not more than about
0.1% cobalt and balance iron.
3.The bundle as claimed in claim 2 wherein said nickel
and chromium content is limited essentially to 8-12% weight
and 18-20% weight respectively.
4. The bundle as claimed in claim 1, claim 2 or claim
3 wherein said component parts comprise cylindrical sheath
portions having localized pad portions protruding from the
outer surfaces thereof, secured thereto by said braze metal bond.

Description

107583S Case 2464

This invention is clirected to an improved nuclear
fuel, and in particular to nuclear fuel havin~ zirconium
components secured in bonded relation with a brazing alloy.
In the Canadian nuclear induqtry widespread use is
presently made of elongated fuel elements having spacer pads
secured as appendages to the zirconium sheath of the fuel,
by way of a berylium braze.
This use of berylium as a brazing material suffers
from the serious drawback that berylium metal is highly toxic,
and its use in brazing has necessitated the adoption of
remote brazing techniques. Furthermore, as the berylium
material must be initially applied to one or other of the
components being brazed, as a localized coating layer, this
initial coating step also must be effected within a highly
protected environment.
Previous work carried out in this field includes
the use as braze material of a zirconium alloy incorporating
elements common to stainless steel, as set forth in Canadian
Patent No. 630,239 - McCuaig et al, October 31, 1961. The
alloys taught by this prior patent lie within narrowly
specified ranges, and do not encompass a type 300 series
stainless steel, in accordance with the present invention.
; One of the problems encountered in using brazed
zircalloy components within a water cooled reactor such as
the Candu type, wherein the heavy water coolant circulates
externally of the reactor calandria through a system of
collection headers to a heat exchanger, is that the presence
of cobalt to any signi~icant extent in the composition of the
components can lead to leaching o~ the cobalt into the coolant,
with subsequent plating-out of the cobalt as a localized
concentration on surface portions of the coolant system most

conducive thereto. This cobalt material, having become radio-
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~LO'75~35 Case 246a,

active due to prolonged exposure within the core of the reactor,
produces a radiation hot-~pot which constitutes a highly
undesired phenomenon.
It has been found that a successful non~toxic
brazi~g process may be applied to joining zirconium components
in brazed relation using as the braze material an alloy
generally of the AISI type 300 ~3eries stainless steel with
substantially no cobalt. The range of analysis of this type
of stainless steel is:
Carbon 0.03% maximum; phosphorous 0.045% maximum;
magnesium 2% maximum; sulphur 0.030% maximum; chromium 18-20%
nickel 8-12% cobalt 0.1% maximum with the balance iron.
The brazing temperature at which satisfactory
brazing can be effected can be restricted successfully
to not more th~n 2100F - thereby avoiding or minimizing
the onset of grain growth, which must be limited.
Thus, an exemplary zirconium product incorporating
such a stainless steel braæe has been provided, utilizing
the stainless steel of the type AISI 300 series stainless,
being a specially formulated alloy having an extremely low
cobalt content, of not more than 0.1% by weight.
Restriction to the lower chromium content of the
series, while giving lower strength, can be effected at
lower temperatures such as AISI 302 and AISI 303, so as to
diminish grain growth~ In the case of type AISI 304 stainless
the higher chromium content gives higher strength but the
temperature requirement rises to 210GF, with conse~uent
formation of larger crystals.
While the structural strength of brazed joints
produced using the alloy have been found to be less than for
corresponding berylium brazes, the strength factor is com-

pletely adequate for the joining of the spacers as well as
the wart-like appendages used as spacers, of the type



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currently in use.
Certain embodiments of -the invention are de~cribed,
reference being made to the accompanying drawings, wherein:
Figure 1 i~ a side view of an end portion of a
fuel bundle according to the present invention;
Figure 2 is an end view of the same bundle; and
Figure 3 is an enlarged view of the juncture of
three of the elements encircled at 3 in Fig. 2, with end
plate removed.
In Figure 1, the end portion of a fuel bundle 10
has an end plate 12 to which are secured a plurality of fuel
elements 14 having zirconium alloy sheaths. Referring to
Fig. 2 each of the outer row of elongated elements 14 is
provided with spacers 16 to ensure spacing of the elements
14 from the inner surface of the calandria pressure tube
(not shown), within which the bundle 10 is inserted. ~-
~he elements 14 are also provided with warts 18,
shown in enlarged detail in Figure 3, in relation to the
specific elements 141, 142, 143.
The present invention is particularly directed to
the brazing of the appendages 16 and 18 to the respective
fuel elements 14, by way of welds 20.
Thus, in accordance with the invention at least
some of the appendages 1~, 18 are secured by brazing to the
respective fuel elements, using as braze material a material
stainless steel of AISI type 300 series, with not more than
~; 0.1% cobalt, being considered to be substantially free of
cobalt, and having a composition in the following ranges:
~i 6 - 22% by weight
Cr 16 - 26% by weight
Fe 78 - 52% by weight
Adoption of this material permits brazing at tem-
peratures not exceeding 2100F, whereby the problems of grain

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growth in the zirconium material of the sheath are mitigated.
Thi5 range oE values compares with the values of
Canadian Patent No. 630,239:
Ni 6 - 9% by weight
Cr 6 - 9% by weight
Fe 0 - 7.5% by weight
The term zirconium as used herein refers in general
to alloys of zirconium as used in general practice and not to
the pure metal.
It is contemplated that the major constituents, iron
chrome and nickel of the 300 series stainless steel might well
be provided in appropriate individual proportions and as separate
constituents, as an alternative to being in combined form, as
a stainless steel.
When berylium or steel is stated as being the
braze alloy, it is understood that the above materials are
constituents of the braze alloy ~berylium-zircaloy or stainless
steel-zircaloy) which îs formed by the alloying of the berylium
or stainless steel and zircaloy in carrying out the braze.