End fitting treatment and end fitting

Abstract

An end fitting for a pressure tube in a nuclear reactor system includes an inboard end configured to surround and be joined to the end of the pressure tube in a fluid tight manner by a rolled joint. The rolled joint includes a grooved region having at least one annular groove. The end fitting also incudes a wettable inner surface that extends axially outboard from an outboard-most annular groove of the grooved region. The end fitting also incudes a corrosion barrier applied to form a coated portion of the wettable inner surface. The coated portion extends from a first position that is at or outboard of a coating reference position at the outside edge of the outboard-most annular groove, to a second position. An axial extent of the coated portion, measured axially from the coating reference position to the second position, is at least about 20 mm.

Description

END FITTING TREATMENT AND END FITTINGCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This disclosure is related to and claims the benefit of priority to United States Provisional Patent Application No. 63 / 520,127, filed on August 17, 2023 and entitled END FITTING TREATMENT AND RELATED END FITTING, and United States Provisional Patent Application No. 63 / 571 ,294, filed on March 28, 2024 and entitled END FITTING TREATMENT AND RELATED END FITTING, the entirety of which are incorporated herein by reference.TECHNICAL FIELD

[0002] This disclosure relates generally to a method for inhibiting or preventing the ingress of hydrogen into metallic materials, and more specifically to a method for inhibiting or preventing the corrosion of a stainless-steel component adjacent a zirconium alloy component in a nuclear reactor, thereby reducing the ingress rate of hydrogen into the stainless-steel component and reducing the inventory of hydrogen able to diffuse through the stainless-steel component to the adjacent zirconium alloy component.BACKGROUND

[0003] During service, pressurized components in the fuel channel of a CANadian Deuterium Uranium (hereinafter CANDU™) reactor, including the pressure tubes and their end fittings, are exposed to heavy water at high temperature and pressure. Corrosion is known to occur on the inner diameter surfaces of CANDU™ fuel channel components that are wetted by heavy water.

[0004] The pressure tubes are typically made from zirconium alloys, in part due to their relatively low neutron-capture cross section. The end fittings are typically made from stainless steel, and more specifically AISI type 403 stainless steel alloy. The alloy is used at least in part due to its strength, corrosion resistance, and impact resistance.

[0005] To provide a diffusion barrier to hydrogen ingress from the end fitting to the pressure tube through a rolled joint connection, White et al. investigated the use ofchromium plating in the rolled joint between the stainless steel end fitting and zirconium alloy pressure tube in a CANDll™ reactor. See “Plated end fittings to reduce hydrogen ingress at rolled joints in CANDll reactors” presented by A. J. White et al. at the International Conference on Expanded and Rolled Joint Technology, Toronto, Ontario, 1993.SUMMARY

[0006] The following introduction is provided to introduce the reader to the more detailed discussion to follow. The introduction is not intended to limit or define any claimed or as yet unclaimed invention. One or more inventions may reside in any combination or sub combination of the elements or process steps disclosed in any part of this document including its claims and figures.

[0007] In accordance with one broad aspect, there is provided an end fitting for a pressure tube in a nuclear reactor system, the end fitting comprising: an inboard end configured to surround and be joined to the end of the pressure tube in a fluid tight manner by a rolled joint between an inner surface of the inboard end of the end fitting and an outer surface of the end of the pressure tube, wherein the rolled joint comprises a grooved region having at least one annular groove; a wettable inner surface that extends axially outboard from an outboard-most annular groove of the grooved region, the wettable inner surface comprising the portion of the end fitting that, during operation of the nuclear reactor system, is contacted by coolant water flowing through the pressure tube and the end fitting; and a corrosion barrier applied to form a coated portion of the wettable inner surface, the coated portion extending from a first position that is at or outboard of a coating reference position at the outside edge of the outboard-most annular groove of the grooved region, to a second position, wherein an axial extent of the coated portion, measured axially from the coating reference position to the second position, is at least about 20 mm.

[0008] In some embodiments, the axial extent of the coating is at least about 35 mm.

[0009] In some embodiments, the axial extent of the coating is at least about 40 mm.

[0010] In some embodiments, the axial extent of the coating is at least about 90 mm.

[0011] In accordance with another broad aspect, there is provided an end fitting for a pressure tube in a nuclear reactor system, the end fitting comprising: an inboard end configured to surround and be joined to the end of the pressure tube in a fluid tight manner by a rolled joint between an inner surface of the inboard end of the end fitting and an outer surface of the end of the pressure tube, wherein the rolled joint comprises a grooved region having at least one annular groove; a wettable inner surface that extends axially outboard from an outboard-most annular groove of the grooved region, the wettable inner surface comprising the portion of the end fitting that, during operation of the nuclear reactor system, is contacted by coolant water flowing through the pressure tube and the end fitting; and a corrosion barrier applied to form a coated portion of the wettable inner surface, the coated portion extending along the wettable inner surface from a first position that is at or outboard of a coating reference position at the outside edge of the outboard- most annular groove of the grooved region, to a second position, wherein a path length of the coated portion, measured along the wettable inner surface from the coating reference position to the second position, is at least about 20 mm.

[0012] In some embodiments, the path length of the coated portion is at least about 45 mm.

[0013] In some embodiments, the path length of the coated portion is at least about 100 mm.

[0014] In some embodiments, the first position is within 5 mm of the coating reference position.

[0015] In some embodiments, the first position is within 2.5 mm of the coating reference position.

[0016] In some embodiments, the coated portion extends axially outboard substantially continuously from the first position to the second position.

[0017] In some embodiments, the grooved region has three axially spaced apart annular grooves.

[0018] In some embodiments, the grooved region is uncoated.

[0019] In some embodiments, the end fitting is made from ANSI 403 stainless steel.

[0020] In some embodiments, the corrosion barrier comprises at least one of: chromium, zirconium, zirconium oxide, gold, titanium, titanium oxide, and FeCrAI.

[0021] In some embodiments, the corrosion barrier has a thickness of between 5 and 20 pm.

[0022] In some embodiments, the end fitting further comprises a diffusion barrier applied to the inner surface of the inboard end of the end fitting at the grooved region or portions thereof.

[0023] In accordance with another broad aspect, there is provided an end fitting for a pressure tube in a nuclear reactor system, the end fitting comprising: a generally tubular inboard end configured to surround and be joined to the end of the pressure tube in a fluid tight manner by a rolled joint between an inner surface of the inboard end of the end fitting and an outer surface of the end of the pressure tube, the inboard end of the end fitting having an average radial thickness across the rolled joint; and a corrosion barrier that extends axially outboard along an axial length of an inner surface of the end fitting located outboard of the rolled joint, wherein the axial length is at least three times the average radial thickness.

[0024] In some embodiments, the axial length is at least four times the average radial thickness.

[0025] In some embodiments, the axial length is at least five times the average radial thickness.

[0026] In some embodiments, the axial length is at least ten times the average radial thickness.

[0027] In some embodiments, the corrosion barrier extends axially outboard substantially continuously from the rolled joint.

[0028] In some embodiments, the corrosion barrier extends axially outboard substantially continuously from an outboard end surface of the end of the pressure tube.

[0029] In some embodiments, the end fitting is made from ANSI 403 stainless steel.

[0030] In some embodiments, the corrosion barrier comprises at least one of: chromium, zirconium, zirconium oxide, gold, titanium, titanium oxide, and FeCrAI.

[0031] In some embodiments, the nuclear reactor system is a pressurized heavy water nuclear reactor system.

[0032] In some embodiments, the nuclear reactor system is a CANDll™ nuclear reactor system.

[0033] In accordance with another broad aspect, there is provided a method for treating an end fitting for a pressure tube in a nuclear reactor system to inhibit hydrogen ingress into the end fitting and thereby inhibit subsequent hydrogen migration into a pressure tube joined to the end fitting by a rolled joint, the method comprising: identifying a target inner surface that extends axially outboard from a first position that is outboard of an expected location of an outboard-most shoulder of an annular groove of the rolled joint; and applying a corrosion barrier to the target inner surface.

[0034] In some embodiments, the target inner surface extends axially at least about 20 mm from the expected location of the outboard-most shoulder of an annular groove of the rolled joint.

[0035] In some embodiments, the target inner surface extends axially at least about 35 mm from the expected location of the outboard-most shoulder of an annular groove of the rolled joint.

[0036] In some embodiments, the target inner surface extends axially at least about 40 mm from the expected location of the outboard-most shoulder of an annular groove of the rolled joint.

[0037] In some embodiments, the target inner surface extends axially at least about 90 mm from the expected location of the outboard-most shoulder of an annular groove of the rolled joint.

[0038] In some embodiments, the target inner surface has a path length that extends at least about 20 mm from the expected location of the outboard-most shoulder of an annular groove of the rolled joint.

[0039] In some embodiments, the path length of the coated portion extends at least about 45 mm from the expected location of the outboard-most shoulder of an annular groove of the rolled joint.

[0040] In some embodiments, the coated portion extends axially at least about 100 mm from the expected location of the outboard-most shoulder of an annular groove of the rolled joint.

[0041] In some embodiments, the corrosion barrier comprises at least one of: chromium, zirconium, zirconium oxide, gold, titanium, titanium oxide, and FeCrAI.

[0042] In some embodiments, the corrosion barrier is applied at a thickness of between 5 and 20 pm.

[0043] In some embodiments, the method further comprises applying a diffusion barrier to at least a portion of the inner surface of the end fitting that is inboard of the expected location of the outboard-most shoulder of an annular groove of the rolled joint.BRIEF DESCRIPTION OF THE DRAWINGS

[0044] For a better understanding of the described embodiments and to show more clearly how they may be carried into effect, reference will now be made, by way of example, to the accompanying drawings in which:

[0045] Figure 1 is a schematic example of a fuel channel assembly suitable for use in a CANDll™ nuclear reactor;

[0046] Figure 2 is a schematic cross-section of a rolled joint of a pressure tube - end fitting assembly;

[0047] Figure 3 is an example cross-section and enlarged section of a region of a pressure tube showing prior art chromium plating in a rolled joint;

[0048] Figure 4 is a schematic diagram of a rolled joint conceptually showing different possible modes of hydrogen ingress;

[0049] Figure 5 is a schematic diagram of a rolled joint showing details of grooved regions and hydrogen ingress paths;

[0050] Figure 6 is a section profile view of an example end fitting geometry;

[0051] Figure 7 is a section profile view of another example end fitting geometry;

[0052] Figure 8 is the schematic cross-section of Figure 2, illustrating the location of a coating origin point;

[0053] Figure 9 is a schematic section profile view of a rolled joint and grooved region, illustrating boundary conditions as investigated by a modelled example;

[0054] Figure 10 is a graph showing the impact of end fitting design differences on resultant hydrogen flux to the pressure tube as demonstrated by a modelled example;

[0055] Figure 11 is a graph showing the effect of a coating gap adjacent the grooved region of the rolled joint on the hydrogen flux to the pressure tube for a modelled example;

[0056] Figure 12 is a graph showing the effect of the axial extent of a corrosion resistant coating on the hydrogen flux to the pressure tube for a modelled example;

[0057] Figure 13 is a graph showing the effect of the path length of a corrosion resistant coating on hydrogen flux to the pressure tube for a modelled example;

[0058] Figure 14 is a graph showing the effect of the surface area of a corrosion resistant coating on the hydrogen flux to the pressure tube for a modelled example

[0059] Figure 15 is a schematic representation of a CANDll™ end fitting - pressure tube region showing a target treatment region;

[0060] Figure 16 is a schematic representation of another CANDll™ end fitting - pressure tube region showing a target treatment region; and

[0061] Figure 17 is a flow diagram for an example method for applying a corrosion barrier to an end fitting in accordance with an embodiment.

[0062] The drawings included herewith are for illustrating various examples of apparatus and methods of the teaching of the present specification and are not intended to limit the scope of what is taught in any way.DETAILED DESCRIPTION

[0063] Various apparatuses and methods are described below to provide example embodiments and implementations of the technology. The technology includes apparatuses and methods that relate to CANDll™ nuclear reactors, and may be applicable to other nuclear reactor types, including pressurized heavy water reactors.

[0064] It will be appreciated that for simplicity and clarity of illustration, where considered appropriate, reference numerals may be repeated among the figures to indicate corresponding or analogous elements. In addition, numerous specific details are set forth to provide a thorough understanding of the example embodiments described herein. However, it will be understood by those of ordinary skill in the art that the example embodiments described herein may be practiced without these specific details. In other instances, well-known methods, procedures, and components have not been described in detail so as not to obscure the example embodiments described herein. Also, the description is not to be considered as limiting the scope of the example embodiments described herein.

[0065] During operation of nuclear reactors, such as pressurized heavy water reactors and in particular CANDll™ nuclear reactors, hydrogen and / or its isotopes (such as deuterium), can accumulate within certain metal components. Such accumulation can include interstitially dissolved hydrogen and hydrogen that has formed relatively brittle metal-hydrides. This accumulation can be undesirable under some conditions and in some applications. For example, this accumulation can have an adverse effect on the mechanical or physical properties of affected components and / or systems containing affected components.

[0066] In the present disclosure, hydrogen (i.e. protium) and its isotopes (deuterium and tritium) may be collectively referred to as hydrogen material for simplicity and easeof explanation. That is, term hydrogen should be understood to also refer to the isotopes of hydrogen unless stated otherwise.

[0067] Hydrogen can accumulate in a metal component due to numerous factors, including environmental service conditions, various corrosion mechanisms, gradients in chemical potential, diffusion phenomena, etc. Under some conditions, brittle metal- hydrides may form, which can adversely affect the mechanical properties of the component. Such changes in mechanical properties may limit the service life of a component, and / or limit the service life of one or more systems that rely on the affected component.

[0068] For example, hydrogen accumulation in a CANDU™ reactor pressure tube, particularly in the vicinity of the rolled joint (at both inlet and outlet ends), may negatively affect the structural integrity of the pressure tube, and can limit the service life of the fuel channel.

[0069] Specifically, it has been reported that when hydrides are present in a CANDU™ reactor pressure tube, the fracture toughness of the pressure tube may be reduced and there may be increased susceptibility to delayed hydride cracking (see e.g., C.E. Coleman, J.F.R. Ambler, “Delayed Hydride Cracking in Zr-2.5 wt% Nb alloy”, Reviews on Coatings and Corrosion, Vol. Ill, (1979), 105-157). As a result, reactor pressure tubes may need to be replaced and / or reactor operating restrictions may need to be applied.

[0070] Figure 1 is a schematic example of a portion of a CANDU™ type nuclear reactor fuel channel assembly. In the illustrated example, the fuel channel assembly includes a pressure tube 400 that extends along a tube axis 418. Pressure tube 400 is located within a surrounding calandria tube 402 and is supported via a plurality of spacers 412. Multiple fuel bundles 404 (not all shown) can be positioned within the pressure tube 400 while the reactor is in use, and a coolant liquid (such as heavy water in a CANDU™ type nuclear reactor) can flow through the interior of the pressure tube 400 and enter / exit the fuel channel assembly via feeders 408.

[0071] In this example, pressure tube 400 is formed from a zirconium alloy (such as Zr-2.5Nb). The ends of the pressure tube 400 are each connected to respective end fittings 406 which are formed from a different material to the pressure tube. Typically, end fittings 406 are made of Type 403 stainless steel.

[0072] The pressure tube 400 and fittings 406 may be joined together using any suitable technique that provides a fluid tight seal. In the illustrated example, the pressure tube end is mechanically joined with the end fitting using a rolled joint, indicated schematically at 410.

[0073] Figure 2 is a (simplified) schematic at a location of a rolled joint 410 in the fuel channel assembly. At the interface between the pressure tube 400 and the end fitting 406, the rolled joint 410 extends axially between a burnish mark 420 on the pressure tube caused during the rolling of the joint at the inboard-most extent, and the end of the pressure tube 400 at the outboard-most extent. Within the rolled joint, the pressure tube may be deformed such that it has an appreciably larger inner diameter to that in the remainder of the tube. It should be noted that full contact mechanical interfacing between the pressure tube 400 and end fitting 406 may not extend for the full length of the rolled joint 410. For example, a crevice may expose wettable surfaces between the end fitting and the pressure tube, particularly on the outboard end of the rolled joint. Within the rolled joint there may exist a grooved region 421 containing keying grooves for achieving mechanical interlocking. This region may be bounded by the respective outside corners or lands of both the inboard-most groove and outboard-most groove of the rolled joint.

[0074] One region in which hydrogen can accumulate within the pressure tube 400 is in the portions of the wall of the pressure tube 400 that are adjacent an end fitting 406. The present disclosure is directed to inhibiting, limiting, and / or preventing the ingress of hydrogen into the pressure tube wall at such locations.

[0075] Prior attempts at reducing or preventing hydrogen ingress into the zirconium alloy pressure tube have focused on modifying the nature of the rolled-joint region, or modifying the pressure tube itself with coatings, treatments, or the addition of an additional component composed of a hydride getter material.

[0076] For example, “Plated end fittings to reduce hydrogen ingress at rolled joints in CANDll reactors" (presented by A. J. White et al. at the International Conference on Expanded and Rolled Joint Technology, Toronto, Ontario, 1993) discloses the use of a chromium plating primarily in the grooved region of the rolled joint at the interface between the zirconium alloy pressure tube and Type 403 stainless steel end fitting to reduce hydrogen ingress into the pressure tube. Figure 3 conceptually illustrates the location of this chromium plating layer. White et al. report that with 18 pm of chromium plated onto the end fittings primarily in the grooved region of the rolled joint, hydrogen ingress was observed to be reduced by 30% to 60% in the short term (extrapolated to be up to 95% in the long term) when compared to that observed with standard (i.e. un-plated) end fittings.

[0077] Accordingly, providing a plated chromium layer in this region of the rolled joint has proven to be relatively effective at reducing hydrogen ingress into the pressure tube. However, providing such a plated chromium layer may also be considered problematic in certain respects. For example, the plating disclosed in White et al. appears to interfere with the intimate contact and mechanical keying necessary between the pressure tube and end fitting in the grooved region of the rolled joint, which has resulted in leakage issues at the rolled joint. Any leakage at this joint is considered highly undesirable as it is designed to function as a critical pressure boundary.

[0078] It is therefore considered beneficial to inhibit, limit, and / or prevent the ingress of hydrogen and its isotopes into metal reactor components (such as zirconium alloy pressure tubes) to help maintain desirable material characteristics and mechanical properties of the components, without negatively impacting on the overall performance of the system.

[0079] Several potential routes of hydrogen ingress into the pressure tube in the vicinity of the rolled joint have previously been identified. (See e.g. Sawatzky, A. and Ells, C. E., 2000, “Understanding Hydrogen in Zirconium”, Zirconium in the Nuclear Industry: Twelfth International Symposium, STP 1354, ASTM International, West Conshohocken, PA, June 15-18, pp. 32-48.) With reference to Figure 4, these hydrogen ingress routes (numbered accordingly) include hydrogen:(1 ) entering the pressure tube directly from the inside surface of the pressure tube due to corrosion;(2) migrating along the outboard crevice (A-B), between the pressure tube and the end fitting, and entering the pressure tube directly;(3) forming due to corrosion on the surface of the end fitting and migrating through the steel into the pressure tube;(4) in the annulus gas migrating through the steel end fitting into the pressure tube;(5) in the annulus gas migrating along the inboard crevice, entering the pressure tube directly.

[0080] Experiments conducted on simulated rolled joint assemblies in an annulus gas environment with up to 20 vol% hydrogen added have shown that hydrogen in the annulus gas is not a contributing factor (i.e. that routes (4) and (5) are not significant contributors to hydrogen ingress into the pressure tube).

[0081] Route (1 ) is thought to be a dominant means of hydrogen ingress into the main body of the pressure tube (away from the end fittings). However, although this route contributes to ingress at the rolled joint, it does not account for the increased amount of hydrogen which is specifically observed in the rolled-joint region compared with the main body of the tube.

[0082] Prior to recent work carried out by the inventors, the hydrogen sources for ingress routes (2) and (3) were believed to be primarily associated with generation of hydrogen within crevices and galvanic effects, followed by diffusion of hydrogen along the crevice to points of intimate dissimilar metal contact between the end fitting and pressure tube. Also, hydrogen ingress into the pressure tube via route (2) was thought to be more significant than ingress via route (3). Without wishing to be bound by theory, the inventors believe that hydrogen ingress via route (3) is more significant than previously thought, and inhibiting corrosion along the inner surface of the end fitting may significantly reduce the inventory of hydrogen able to diffuse through the stainless-steel component to the adjacent zirconium alloy component.

[0083] Figure 5 includes several enlarged views of a rolled joint and the grooved region within it. Without wishing to be bound by theory, recent improvements in theunderstanding of ingress mechanisms at the rolled joint have established that hydrogen ingress is not uniform across the rolled joint. Appreciable diffusion into the pressure tube from the end fitting is limited to areas of intimate metal-to-metal contact. This occurs dominantly in the shoulders or corners 435 of the grooves 430, where surface oxides are cracked or otherwise damaged in the act of forming the rolled joint. This has been supported by metallography of rolled joint samples which shows regions of direct metal- to-metal contact at the rolled joint groove comers i.e. perpendicular to the axis of the pressure tube. In contrast, the contact between the surfaces parallel to the pressure tube exhibits less metal-to-metal contact and more intact oxide layers. Figure 5 also illustrates an example crevice 437 between pressure tube 400 and end fitting 406.

[0084] Recent work by the inventors has identified a significant hydrogen source contribution that has previously been underestimated. This work has shown that the Type 403 stainless steel of the end fitting may undergo considerable corrosion in any areas where it has an exposed, wetted surface (i.e. a surface exposed to heavy water at high temperature and pressure). These areas of the end fitting may be remote of the rolled joint, and yet still contribute appreciably to resultant hydrogen in the pressure tube wall surrounding the rolled joint. This results when appreciable levels of hydrogen generated by corrosion of the end fitting diffuse into the bulk of the end fitting, and may then be available to diffuse through the rolled joint interface into the zirconium alloy pressure tube. This may happen even in the absence of crevices and galvanic effects at the interface between the pressure tube and the end fitting at the rolled joint.

[0085] In one example, the effect of applying a corrosion-resistant or corrosioninhibiting coating on end fitting surfaces in a target treatment region has been investigated using a finite-element model of hydrogen transport in the end fitting to predict the hydrogen flux into the rolled joint as a function of operating conditions and the addition of the corrosion-inhibiting layer. The hydrogen transport can be modelled with Fickian diffusion, as described in A. Turnbull, M. W. Carroll, and D. H. Ferriss, “Analysis of hydrogen diffusion and trapping in a 13% chromium martensitic stainless steel,” Acta Metall., vol. 37, no. 7, pp. 2039-2046, Jul. 1989.

[0086] This treatment region may be adjacent to the grooved region of the rolled joint, but it may be largely remote from it. Generally speaking, any wetted surface of an end fitting may be considered a target treatment region, as wetted surfaces - if not protected - may be able to serve as a bulk source of hydrogen through corrosion. The impact on hydrogen flux of applying such a corrosion inhibiting coating may not (and likely will not) be uniform across the treatment region. Characterising this effect according to coating extent and location may therefore be considered useful.

[0087] CANDll™ end fitting geometry can vary between reactors and even between channels within a particular reactor (e.g. inlet end vs outlet end, fixed face vs free face). Figures 6 and 7 illustrate two different end fitting geometries (designated A and B, respectively) in cross-section and adapted as representative from several sources. Their structures were simplified and truncated ~50 cm nearest the rolled joint as the distant parts of the end fitting are not expected to significantly impact the hydrogen flux outcomes. These two geometries were used to assess the impact of end fitting geometry differences on the potential effectiveness of corrosion inhibiting coatings. Also shown in Figures 6 and 7 are regions 440 of the end fitting 406 where the rolled joint with the pressure tube would be positioned.

[0088] Figure 8 is a schematic cross-section of the rolled joint and identifies a coating reference position 820, that is defined at the outside edge (e.g. the outside or outboard- most corner or land) of the outboard-most groove of the grooved region of the rolled joint. For the purposes of describing the physical parameters of a corrosion inhibiting coating, this position shall be taken as the zero position or origin, with coating axial extent, path length, or surface area measured from this position in an outboard direction.

[0089] Figure 9 is a schematic cross-section of the as-modelled inner surface of the end fitting in the region of the rolled joint (which would contact the pressure tube on assembly).

[0090] The modeling was based on several assumptions, including:• In the region inboard of location 830 (i.e. the outboard edge or corner of the second rolled joint groove), there is no wetting of the end fitting surface and hence no corrosion;• The region outboard of location 820 (i.e. the outboard edge or corner of the outermost rolled joint groove), is assumed to be fully wetted and therefore subject to corrosion unless a coating has been applied;• In the region 825 between 820 and 830, the possibility of wetting and hence corrosion of the end fitting surfaces parallel to the pressure tube is provided for in the following discussion;• Throughout the grooved region 440 of the rolled joint, all of the groove corners or edges (i.e. the surfaces largely perpendicular to the axis of the pressure tube) are assumed to have full metal-to-metal contact. They are therefore assumed to be a perfect sink for the diffusion of hydrogen from the end fitting into the pressure tube;• Throughout the grooved region 440, the end fitting surfaces that are parallel to the pressure tube are assumed to be adjacent a pressure tube surface with intact oxide layers which prevent metal-to-metal contact. It is therefore assumed that there is no diffusion of hydrogen from the end fitting into the pressure tube at these surfaces; and• Uncoated surfaces of the end fitting that are wetted in use are a source of hydrogen as they react with coolant water; and this hydrogen is then absorbed by the end fitting. The corrosion rate of 403 stainless steel under CANDU-like temperature, flow, and chemistry conditions was taken from corrosion and hydrogen absorption experimental test data.

[0091] Non-destructive examination of in-service rolled joints indicates that detectable water is sometimes present in between the first and second groove of the joint, that is in region 825. It is therefore possible that these wetted parts of the end fitting may also undergo corrosion, producing hydrogen. The rate of corrosion in this region is uncertain for a variety of factors and may vary from channel to channel. However, to fully assess the impact that corrosion in this region could have on hydrogen flux, the model includes a flag to enable or disable the contribution of corrosion on the surfaces parallel to the pressure tube in region 825. In the result discussions that follow, when this contribution is enabled in the modelled hydrogen uptake, the overall hydrogen flux result is designated “Wet” and when it is not enabled, it is designated “Dry”.

[0092] The effectiveness of the corrosion resistant or corrosion inhibiting coating (e.g. a chromium coating), could vary based on any number of parameters including local coolant chemistry, temperature, and flow conditions. To more fully assess the impact ofthis variability, the model was used to parametrically explore the impact of the efficacy of the corrosion inhibiting layer on resulting hydrogen flux. This was achieved by selecting appropriate upper and lower values for corrosion inhibition efficacy based on historical experimental observation. In the result discussions that follow, when the corrosion inhibition effect is at its upper value, the resulting overall hydrogen flux result is designated “High” and when it is at its lower value, it is designated “Low”.

[0093] In the following discussion of hydrogen flux result outcomes as a function of coating placement or extent, various maxima and minima for these contributing variables and combinations thereof are reflected as individual data sets. Wet / Dry reflects the inclusion / exclusion of a contribution for wetting (and hence a contribution to resulting corrosion and hydrogen flux) in the region between the first and second groove of the rolled joint. High / Low reflects the maximum and minimum values for corrosion inhibition coating efficacy used in the model. For each combination of these, the impact on hydrogen flux generation is reported as relative hydrogen flux, where the resulting hydrogen flux has been normalised against the uncoated version for each case. While the absolute magnitude of the effect on hydrogen flux differs with each variable combination selection, it is evident that the impact of coating placement or extent trends consistently, i.e. that optimal coating placement or extent is consistent between data sets.

[0094] Figure 10 is a graph showing the impact of coating axial extent on relative hydrogen flux for the two different end fitting designs shown in Figures 6 and 7 for a specific Dry / High variable combination. The axial extent of the coating is the distance that the corrosion-resistant coating extends axially in an outboard direction from the coating reference position 820. It is evident from this graph that while end fitting geometry may have some impact on the relative hydrogen flux response to coating placement or extent, the optimal thresholds for this placement or extent remain consistent.

[0095] It is expected that the coating of the end fitting surface in or closest to the grooved region of the rolled joint will have the greatest impact, as the diffusion path for hydrogen within the end fitting to the pressure tube (via metal-metal contact in the grooves) is shortest here. However, as previously discussed, prior art observations indicate that allowing the coating to extend into the rolled joint grooves can lead to sealingand performance problems. The model was therefore used to investigate the impact of allowing a small gap at the edge of the grooved region of the end fitting (defined previously as the coating reference position) before beginning the coating. In Figure 9, this gap 815 is indicated between the outboard edge of the grooved region of the end fitting (i.e. coating reference position 820) and the inboard-most extent of the coating (which may be characterized as a first coating position), illustrated as position 810. It will be appreciated that the location of first position 810 relative to the coating reference position 820 will vary based on the length of the gap 815.

[0096] Figure 11 is a graph showing the effect of such a coating gap on the hydrogen flux of a modelled example, where the rolled joint coating gap is zero at the previously defined coating reference position. The predicted relative hydrogen flux values are shown in Figure 11 as a function of the gap size. As expected, the largest hydrogen flux reduction corresponds to the smallest gap. In balancing optimal coating efficacy outcomes with the performance risk arising if the coating extends into the first groove, a maximum coating gap of 2.5 mm is proposed. In some embodiments, the gap 815 may be 5 mm or less, 4 mm or less, 3 mm or less, and preferably is 2.5 mm or less, and may be 2 mm, 1 .5 mm, 1 mm or 0.5 mm or less in some embodiments.

[0097] The axial extent of the coating is defined herein as the distance that the corrosion-resistant coating material extends axially in an outboard direction from the coating reference position 820. Returning to Figure 9, the axial extent of a coated portion is the distance from 820 to 840, while the coated portion itself extends from first position 810 to second position 840. Put another way, the coated portion in Figure 9 extends from first position 810 to second position 840, and therefore the axial length of the coated portion (810 to 840) will be less than the overall axial extent of the coated portion (820 to 840) by the axial length of the gap (815).

[0098] Figure 12 is a graph showing the effect of the coating axial extent on the resulting hydrogen flux to the pressure tube. As demonstrated here, to minimise hydrogen flux to the pressure tube, the corrosion-resistant coating should extend axially in the outboard direction at least about 35 to 40 mm from the coating reference position. At a coating axial extent of between about 90 to 110 mm, the coating approaches its maximumefficacy in reducing hydrogen flux for the end fitting designs investigated, with minimal additional benefit expected for extending the coating axially in the outboard direction beyond this. For alternative, thicker-walled end fitting designs, there may be a benefit to extending the coating axial extent further.

[0099] The path length of the corrosion inhibiting coating differs from the axial extent of the coating in that it describes the path length of the actual surface, measured from the coating reference position 820. In colloquial terms, the axial extent can be characterized as an ‘as the crow flies’ distance from the coating reference position in the outboard direction, and the path length can be characterized as an ‘as the ant walks’ distance from the coating reference position along the surface in an outboard direction. While the path length is more complex to quantify, it more easily reflects the impact of specific geometric features of the end fitting surface. With reference to Figure 9, the path length of a coated portion is the distance along the surface from 820 to 840, while the coated portion itself extends along the surface from first position 810 to second position 840, and therefore the true length of the coated portion (810 to 840) will be less than the path length of the coated portion (820 to 840) by the length of the gap (815).

[0100] Figure 13 is a graph showing the effect of the path length of a continuous corrosion resistant coating on the hydrogen flux to the pressure tube. As demonstrated here, to minimise hydrogen flux to the pressure tube, the corrosion-resistant coating path length should extend in the outboard direction at least 45 mm from the coating reference position. At a coating path length of approximately 100 mm, the coating approaches its maximum efficacy in reducing hydrogen flux for this type of end fitting design, with minimal additional benefit expected for extending the coating in the outboard direction beyond this. For other, thicker-walled end fitting designs, there may be a benefit to extending the coating path length further.

[0101] Figure 14 is a graph showing the effect of the surface area of a continuous corrosion-resistant coating on the hydrogen flux to the pressure tube. As demonstrated here, to significantly minimise hydrogen flux to the pressure tube, a continuous corrosion- resistant coating, applied outboard from the coating reference position, should have a surface area of at least 0.018 m2. The coating approaches its maximum efficacy inreducing hydrogen flux, as it is extended in an outboard direction until it has a surface area of approximately 0.04 m2Minimal additional benefit is expected for extending the coating axially in the outboard direction beyond this.

[0102] Despite being applied largely remote (i.e. outboard) from the grooved portion of the rolled joint, such a corrosion barrier may act to substantively reduce or prevent initial hydrogen ingress into the end fitting. By inhibiting or preventing initial hydrogen ingress into the end fitting, such a coating may limit subsequent hydrogen migration into the pressure tube via the end fitting.

[0103] This novel approach may avoid the deleterious leakage observed when using chromium plating in the rolled joint region. For example, a corrosion resistant coating may not need to be applied in the grooved region of the rolled joint where intimate mechanical contact between the pressure tube and end fitting is required for effective sealing purposes.

[0104] Furthermore, such a coating, being applied remote from the grooved region of the rolled joint, may be more effective at reducing hydrogen ingress into the pressure tube than similar coating layers introduced directly in the grooved region. For example, the tight tolerances required in the grooved region of the rolled joint may limit the maximum thickness (and hence efficacy) of a chromium plating provided as a diffusion barrier layer. In contrast, a coating or surface treatment applied to a wetted surface of an end fitting may primarily function as a corrosion barrier, rather than as a diffusion barrier. As a result, treatment coatings or barriers may be as or more effective at lower thicknesses.

[0105] The coating material may be of a material suitable to act as an effective corrosion barrier between a Type 403 stainless steel end fitting and the ‘wetted’ environment in a target treatment region (e.g. exposed to heavy water at high temperatures and pressures). Suitable coating materials that are expected to inhibit or prevent corrosion of the end fitting include: chromium, zirconium, zirconium oxide, gold, titanium, titanium oxide, and iron-chromium-aluminium (FeCrAI) alloys. Table 1 includes examples of several suitable coating materials, techniques that may be used, possible coating thicknesses for each technique / coating, and potential post-processing that may be required after an initial coating step.Table 1 : Examples of suitable coating materials, application techniques, optimal coating thickness and potential post-processing requirements

[0106] In one aspect, a coating may be provided to act as a corrosion barrier or corrosion inhibitor in a target treatment region between the end fitting and the heavy water coolant of a CANDll™ type reactor in the region of the end fitting internal surface that is adjacent and outboard to the grooved portion of the rolled joint.

[0107] Figure 16 illustrates an example of a CANDll™ fuel channel assembly, referred to generally as 500, that includes a pressure tube 501. The pressure tube 501 can be formed from a zirconium alloy. In the illustrated example, the end of the pressure tube 501 is connected to an end fitting 502 made of dissimilar material to the pressure tube 501 , such as Type 403 stainless steel. The pressure tube 501 and end fitting 502 are joined together using a rolled joint, indicated schematically at 503. A corrosion-resistant coating 505 (indicated schematically on the upper portion of the Figure by a thickened line between (C) and (E), and omitted from the lower portion of the Figure for clarity) has been applied to the interior surface of the end fitting 502 from the edge of the grooved region of the rolled joint region 503 in an outboard direction (i.e. moving further away fromthe fuel bundles in the fuel channel) such that it protects a surface that would be wetted by the heavy water coolant from corrosion.

[0108] In the example illustrated in Figure 16, coating 505 extends axially outboard substantially continuously from the annular connection region 503 - more specifically from an outboard-most extent of the mechanical interfacing of the pressure tube and end fitting (e.g. location (C), referred to as 820 above) - over substantially all of the wettable inner surface, to an interface between the end fitting 502 and the tube liner 504 (e.g. location (E)).

[0109] In some embodiments, corrosion barrier 505 may be applied to at least a majority of the wettable inner surface, or to at least 75% of the wettable inner surface, or to at least 95% of the wettable inner surface.

[0110] In some embodiments, corrosion barrier 505 may extend axially outboard substantially continuously from an outboard end surface of the end of the pressure tube (e.g. location (D)) towards an interface between the end fitting 502 and the tube liner 504 (e.g. location (E)).

[0111] With continued reference to Figure 16, in some embodiments a coating 505 may be applied to the interior surface of the end fitting 502 over an axial length LBARRIER that is at least 3 times the average thickness TACR of the wall of the end fitting component across the annular connection region 503.

[0112] In some embodiments, corrosion barrier 505 may be applied to the interior surface of the end fitting 502 over an axial length LBARRIER that is at least 5 times the average thickness TACR of the wall of the end fitting component, or over an axial length LBARRIER that is at least 10 times the average thickness TACR of the wall of the end fitting component.

[0113] Referring to Figure 17, there is illustrated a method 600 for applying a corrosion barrier to an end fitting.

[0114] Method 600 may be performed on an end fitting 502 as disclosed herein, or other end fittings of components of a CANDU™ reactor system.

[0115] At 610, a target region of the end fitting to be protected from corrosion is identified. For example, the target region may include between a majority of and substantially all of an interior surface that will be exposed to heavy water at high temperature and pressure during operation of the pressure tube assembly. Additionally, or alternatively, the target region may extend axially outboard along an axial length of an inner surface of the end fitting that is at least three times the average radial thickness of the end fitting across the annular connection region.

[0116] Additionally, or alternatively, the target region may extend axially outboard at least about 35 to 40 mm from a reference position that is positioned at or near the outboard-most extent of the mechanical interfacing of the pressure tube and end fitting. Additionally, or alternatively, the target region may extend at least about 45 mm along the surface of the end fitting from the reference position. Additionally, or alternatively, the target region may cover at least about 0.018 m2the surface of the end fitting outboard from the reference position. The inboard extent of the coating material (i.e. first position 810) is preferably no more than about 2.5 mm from the outboard-most extent of the mechanical interfacing of the pressure tube and end fitting (i.e. reference position 820).

[0117] At 620, a corrosion barrier is applied to the target region. For example, a coating material may be electroplated, sprayed, fused, deposited, or otherwise applied to the target region.

[0118] Optionally, at 630, a diffusion barrier may applied to the inner surface of end fitting at the annular connection region. For example, the diffusion barrier may be selected to inhibit or prevent hydrogen diffusion into the pressure tube at the rolled joint.

[0119] As used herein, the wording “and / or” is intended to represent an inclusive - or. That is, “X and / or Y” is intended to mean X or Y or both, for example. As a further example, “X, Y, and / or Z” is intended to mean X or Y or Z or any combination thereof.

[0120] While the above description describes features of example embodiments, it will be appreciated that some features and / or functions of the described embodiments are susceptible to modification without departing from the spirit and principles of operation of the described embodiments. For example, the various characteristics which are describedby means of the represented embodiments or examples may be selectively combined with each other. Accordingly, what has been described above is intended to be illustrative of the claimed concept and non-limiting. It will be understood by persons skilled in the art that other variants and modifications may be made without departing from the scope of the invention as defined in the claims appended hereto. The scope of the claims should not be limited by the preferred embodiments and examples, but should be given the broadest interpretation consistent with the description as a whole.

Claims

CLAIMS1. An end fitting for a pressure tube in a nuclear reactor system, the end fitting comprising: an inboard end configured to surround and be joined to the end of the pressure tube in a fluid tight manner by a rolled joint between an inner surface of the inboard end of the end fitting and an outer surface of the end of the pressure tube, wherein the rolled joint comprises a grooved region having at least one annular groove; a wettable inner surface that extends axially outboard from an outboard-most annular groove of the grooved region, the wettable inner surface comprising the portion of the end fitting that, during operation of the nuclear reactor system, is contacted by coolant water flowing through the pressure tube and the end fitting; and a corrosion barrier applied to form a coated portion of the wettable inner surface, the coated portion extending from a first position that is at or outboard of a coating reference position at the outside edge of the outboard-most annular groove of the grooved region, to a second position, wherein an axial extent of the coated portion, measured axially from the coating reference position to the second position, is at least about 20 mm.

2. The end fitting of claim 1 , wherein the axial extent of the coating is at least about 35 mm.

3. The end fitting of claim 2, wherein the axial extent of the coating is at least about 40 mm.

4. The end fitting of claim 3, wherein the axial extent of the coating is at least about 90 mm.

5. An end fitting for a pressure tube in a nuclear reactor system, the end fitting comprising: an inboard end configured to surround and be joined to the end of the pressure tube in a fluid tight manner by a rolled joint between an inner surface of the inboard endof the end fitting and an outer surface of the end of the pressure tube, wherein the rolled joint comprises a grooved region having at least one annular groove; a wettable inner surface that extends axially outboard from an outboard-most annular groove of the grooved region, the wettable inner surface comprising the portion of the end fitting that, during operation of the nuclear reactor system, is contacted by coolant water flowing through the pressure tube and the end fitting; and a corrosion barrier applied to form a coated portion of the wettable inner surface, the coated portion extending along the wettable inner surface from a first position that is at or outboard of a coating reference position at the outside edge of the outboard-most annular groove of the grooved region, to a second position, wherein a path length of the coated portion, measured along the wettable inner surface from the coating reference position to the second position, is at least about 20 mm.

6. The end fitting of claim 5, wherein the path length of the coated portion is at least about 45 mm.

7. The end fitting of claim 6, wherein the path length of the coated portion is at least about 100 mm.

8. The end fitting of any one of claims 1 to 7, wherein the first position is within 5 mm of the coating reference position.

9. The end fitting of claim 8, wherein the first position is within 2.5 mm of the coating reference position.

10. The end fitting of any one of claims 1 to 9, wherein the coated portion extends axially outboard substantially continuously from the first position to the second position.11 . The end fitting of any one of claims 1 to 10, wherein the grooved region has three axially spaced apart annular grooves.

12. The end fitting of any one of claims 1 to 11 , wherein the grooved region is uncoated.

13. The end fitting of any one of claims 1 to 12, wherein the end fitting is made from ANSI 403 stainless steel.

14. The end fitting of any one of claims 1 to 13, wherein the corrosion barrier comprises at least one of: chromium, zirconium, zirconium oxide, gold, titanium, titanium oxide, and FeCrAI.

15. The end fitting of any one of claims 1 to 14, wherein the corrosion barrier has a thickness of between 5 and 20 pm.

16. The end fitting of any one of claims 1 to 15, further comprising a diffusion barrier applied to the inner surface of the inboard end of the end fitting at the grooved region or portions thereof.

17. An end fitting for a pressure tube in a nuclear reactor system, the end fitting comprising: a generally tubular inboard end configured to surround and be joined to the end of the pressure tube in a fluid tight manner by a rolled joint between an inner surface of the inboard end of the end fitting and an outer surface of the end of the pressure tube, the inboard end of the end fitting having an average radial thickness across the rolled joint; and a corrosion barrier that extends axially outboard along an axial length of an inner surface of the end fitting located outboard of the rolled joint, wherein the axial length is at least three times the average radial thickness.

18. The end fitting of claim 17, wherein the axial length is at least four times the average radial thickness.

19. The end fitting of claim 18, wherein the axial length is at least five times the average radial thickness.

20. The end fitting of claim 19, wherein the axial length is at least ten times the average radial thickness.21 . The end fitting of any one of claims 17 to 20, wherein the corrosion barrier extends axially outboard substantially continuously from the rolled joint.

22. The end fitting of any one of claims 17 to 20, wherein the corrosion barrier extends axially outboard substantially continuously from an outboard end surface of the end of the pressure tube.

23. The end fitting of any one of claims 17 to 22, wherein the end fitting is made from ANSI 403 stainless steel.

24. The end fitting of any one of claims 17 to 23, wherein the corrosion barrier comprises at least one of: chromium, zirconium, zirconium oxide, gold, titanium, titanium oxide, and FeCrAI.

25. The end fitting of any one of claims 1 to 24, wherein the nuclear reactor system is a pressurized heavy water nuclear reactor system.

26. The end fitting of claim 25, wherein the nuclear reactor system is a CANDll™ nuclear reactor system.

27. A method for treating an end fitting for a pressure tube in a nuclear reactor system to inhibit hydrogen ingress into the end fitting and thereby inhibit subsequent hydrogenmigration into a pressure tube joined to the end fitting by a rolled joint, the method comprising: identifying a target inner surface that extends axially outboard from a first position that is outboard of an expected location of an outboard-most shoulder of an annular groove of the rolled joint; and applying a corrosion barrier to the target inner surface.

28. The method of claim 27, wherein the target inner surface extends axially at least about 20 mm from the expected location of the outboard-most shoulder of an annular groove of the rolled joint.

29. The method of claim 28, wherein the target inner surface extends axially at least about 35 mm from the expected location of the outboard-most shoulder of an annular groove of the rolled joint.

30. The end fitting of claim 29, wherein the target inner surface extends axially at least about 40 mm from the expected location of the outboard-most shoulder of an annular groove of the rolled joint.31 . The end fitting of claim 30, wherein the target inner surface extends axially at least about 90 mm from the expected location of the outboard-most shoulder of an annular groove of the rolled joint.

32. The end fitting of claim 27, wherein the target inner surface has a path length that extends at least about 20 mm from the expected location of the outboard-most shoulder of an annular groove of the rolled joint.

33. The end fitting of claim 32, wherein the path length of the coated portion extends at least about 45 mm from the expected location of the outboard-most shoulder of an annular groove of the rolled joint.

34. The end fitting of claim 33, wherein the coated portion extends axially at least about 100 mm from the expected location of the outboard-most shoulder of an annular groove of the rolled joint.

35. The method of any one of claims 27 to 34, wherein the corrosion barrier comprises at least one of: chromium, zirconium, zirconium oxide, gold, titanium, titanium oxide, and FeCrAI.

36. The method of any one of claims 27 to 35, wherein the corrosion barrier is applied at a thickness of between 5 and 20 pm.

37. The method of any one of claims 27 to 36, further comprising applying a diffusion barrier to at least a portion of the inner surface of the end fitting that is inboard of the expected location of the outboard-most shoulder of an annular groove of the rolled joint.

38. The method of any one of claims 27 to 37, wherein the nuclear reactor system is a pressurized heavy water nuclear reactor system.

39. The method of claim 38, wherein the nuclear reactor system is a CANDU™ nuclear reactor system.

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