CA1075835A

CA1075835A - Zirconium brazing system

Info

Publication number: CA1075835A
Application number: CA256,078A
Authority: CA
Inventors: James W. Standish; Kenji Koyanagi
Original assignee: Canadian General Electric Co Ltd
Current assignee: General Electric Canada Co
Priority date: 1976-06-30
Filing date: 1976-06-30
Publication date: 1980-04-15
Also published as: JPS534196A

Abstract

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.

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 ~; - 2 -- 10758~S Ca~e 2464 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 . .
_ 3 _ 1075~35 Ca3e 2464 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.

Claims

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.