Execution guidelines

Execution involves actions related to preparing the test coupons and racks, immersion/withdrawal of the coupons, and corrosion evaluation of the coupons, as described in the paragraphs below.

7.2.3.1. Preparation of coupons and racks

CSPs are designed to help evaluate the effect of water parameters on the corrosion of fuel assemblies and other reactor pool or spent fuel basin structures. In this context, it should be emphasized that both MTR type fuel assemblies and Russian origin reactor fuels are always stored vertically, with the fuel plates or tubes also vertical.

Most CSP use racks in which coupons are assembled horizontally and not vertically as fuel plates are within fuel assemblies. Experience gained in the IAEA supported CRP Corrosion of Spent Aluminium Clad Fuel in Water revealed that horizontal coupons are more susceptible to corrosion than vertical coupons. It is therefore recommended that CSP use horizontal coupons as it is a much more severe condition and therefore provide an early and conservative indication of the fuel corrosion.

Different types of stainless steel racks were used in the two phases of the IAEA coordinated CRP and in a regional CSP for Latin America (RLA) [7, 8]. The models used in the CRP and the RLA evolved from other designs, and are recommended. The insulating separator should be non-porous and resistant to radiation, like dense alpha-alumina. A typical rack is composed of coupons, in the form of disks, assembled using insulating washers as separators. Figure 31 shows a typical rack with the coupons. The coupons are made of materials that effectively represent the materials used in the research reactor and exposed to the reactor water environment or the spent fuel storage basin that also contains primary circuit quality water. Experience gained from the use of racks and coupons similar to those shown in Fig. 31 revealed that one of the parameters that affect coupon corrosion is settled solids from the reactor or spent fuel basin water; i.e. the top surfaces of coupons higher up in the rack are more prone to corrosion compared to similar coupon surfaces further down in the rack. Hence, it is recommended that for conducting CSP in waters where settled solids have a marked influence, the thickness of the separators be about

FIG. 31. A typical corrosion test rack with coupons.

2.5 cm. Such thick separators would reduce the shadowing effects of coupons on those immediately below it. If thick separators are not available, it is recommended that more than one separator be used to increase the space between successive coupons in a rack.

The coupon assembly should also reproduce bimetallic contacts and crevices. Figures 32 and 33 show typical coupon configurations to simulate bimetallic contacts and crevices. Figure 34 shows a sketch of a crevice washer (separator), and Fig. 35, a typical coupon assembly, or test rack, containing 70 coupons.

The following is a list of suggested coupons that should be considered in a CSP for research reactors or spent fuel storage basins using aluminium and/or aluminium alloys for fuel cladding and as structural materials:

(a) Individual coupons:

Coupons made of 32 mm φ aluminium alloy discs (include fuel clad and storage rack materials including discs with weld beads).

(b) Bimetallic coupled coupons:

— Coupons made of 32 mm φ aluminium alloys (include fuel clad and storage rack materials) disks in contact with larger 70 φ mm disk coupons that would be the cathode in a galvanic couple (e.g. AISI 304 SS coupons);

FIG. 32. Schematic diagram of a galvanic coupon subassembly showing the crevice between the coupons and between the coupons and washers.

FIG. 33. Schematic diagram of (A) bimetallic coupled coupons and (B) an individual coupon, with insulating separators. Separator

‘a’ prevents bimetallic contact between the coupled coupons and the metal rod as well as with other coupons above or below it.

Separator ‘b’ can be either flat or serrated as shown in Fig. 34.

— Coupons made of 32 mm φ aluminium alloys (include fuel clad and storage rack materials) disks in contact with larger 70 mm φ coupons of a different aluminium alloy (include fuel clad and storage rack materials) (for example, 70 mm φ AA 1100 in contact with a 32 mm φ AA 6061 and/or another 70 mm φ AA 1100 in contact with a 32 mm φ AA 6063).

(c) Crevice coupled coupons:

— Coupons made of 32 mm φ aluminium alloys (include fuel clad and storage rack materials including disks with weld beads) disks next to a coupon of the same alloy and of the same size;

— Coupons made of 32 φ mm aluminium alloy (include fuel clad and storage rack materials) disks next to a coupon of the same size and alloy, but with a serrated TFE washer on one side of the coupon, as shown in Fig. 34.

(d) Other coupons:

Inclusion of coupons of other alloys is recommended, as long they are representative of materials used in the research reactor or storage basin facility.

FIG. 34. Schematic diagram of a serrated separator to create crevices on coupons.

FIG. 35. A coupon assembly used in a research reactor CSP with 32 and 70 mm φ coupons. The white area between the coupons is a PTFE separator, used to isolate the central stainless steel rod from the coupon and the coupons from one another.

7.2.3.2. Immersion location

The test coupon rack should be positioned or placed at a region where the water flow conditions are minimal.

(i.e. with the most unfavourable environment conditions, especially in terms of corrosion). In the case of storage basins, the test coupon racks should be located at or near the fuel in the pool. One or more locations should be selected if some locations have stagnant water conditions or are otherwise distinct. The test coupon racks should be positioned several meters below the surface of the pool, and those that are part of a CSP campaign should be positioned together in order to enable evaluation of corrosion data as a function of exposure duration. Figure 36 shows some typical coupon assemblies in storage pools.

7.2.3.3. Immersion/withdrawal frequency

The number of test coupon assemblies or racks to be used in a CSP must be consistent with the expected life of the facility, and with the duration of interim storage of spent fuel. The programme should enable surveillance throughout the desired service life of the reactor water system or spent fuel basin. In the case of an initial three years

FIG. 36. Coupon assemblies in fuel storage pools.

programme, scheduled annual withdrawals are recommended for reactor, decay and storage pools. After the first three years, the withdrawal period can be extended to two, three and five years, followed by five years intervals until decommissioning of the facility.

Test coupons in a separate rack can also be immersed after the programme has been initiated. For example, if measures have been taken to improve water quality based on prior results from the CSP, it is recommended that a new test coupon rack be included in the CSP to re-evaluate the effect of the new water parameters on coupon corrosion.

7.2.3.4. Coupon evaluation

Coupons may be radioactively contaminated in the pool and should be handled in contamination facilities.

Withdrawn test coupon racks are typically placed inside a chamber fitted with adequate facilities to handle contaminated coupons and racks. The following are recommended steps for a full evaluation of aluminium coupons withdrawn from reactor or spent fuel storage pools.

Coupons are identified (alloy and coupon ID number), dried, weighed and their weights recorded. The two sides of the coupons are photographed. If necessary, all these activities are done inside the chamber. Based on a visual inspection, selected coupons are cleaned using 16M nitric acid for a minimum of 30 minutes, rinsed with deionized or distilled water, inspected for residual surface oxide and in the case of retained surface oxide, cleaned again with nitric acid, taking care to avoid acid attack of the coupon. After drying, the coupons are again weighed, photographed and the information recorded.

Pit locations are identified and examined visually. Pit measurements are carried out with an optical microscope as per ASTM G-46 [74] (determining the average and maximum pit depths and pit density over a 0.58 mm² area) or with automated imaging equipment, if available. The number of pits in areas of 0.762 × 0.762 mm is counted, and the pit density determined over the 0.58 mm² area. The average pit depth is determined considering the ten deepest pits. If the average pit depth is determined with less than ten pits, this should indicated in the records. The coupon weights and pit data are compiled and reported in the form of graphs.