Brown et al49 compared the swelling behavior of STA Nimonic PE16 and two cold-worked austenitic steels (M316 and Nb-stabilized FV548) which were irradiated in DFR as fuel pin cladding. Two PE16 clad pins were examined, which were irradiated to burn-ups of 6.1% and 21.6% of heavy atoms, corresponding to peak damage levels of about 17 and 80 dpa, respectively. Void concentrations and swelling were lower in PE16 than in the austenitic steels. Swelling data, void concentrations, and void diameters for the two PE16 pins examined by Brown et al. are shown in Figure 7. Note that Brown etal49 only showed trend lines for void concentration and void size in the less highly irradiated pin and com­pared the swelling tendencies of the two pins; the individual data points were not plotted and those shown in Figure 7 are previously unpublished data obtained by Sharpe. Brown et al. stated that the void concentration in PE16 decreased with increasing irradiation temperature but did not alter greatly with an increasing dose above ~17 dpa. It should be noted, however, that swelling measurements for the higher burn-up pin were restricted to temperatures

below 525 °C, so that a direct comparison of void concentrations in the two pins cannot be made at higher temperatures. Although there were fewer voids in PE16 than in the two steels, the voids

appeared to be homogeneously distributed and to have developed during the early stages of irradiation; once nucleated, the growth rate of voids in PE16 remained low. These observations are clearly contrary to early models which suggested that low swelling rates result from incomplete void nucleation and extended tran­sient regimes. Rather, in agreement with the more recent observations of Muroga et a/.,45,46 it appears that the swelling resistance ofPE16 is due to a combi­nation of a comparatively low saturation void concen­tration, which is reached at a relatively low displacement dose, and a low void growth rate. There does not appear to be any evidence of an accelerated swelling rate in PE16 once void nucleation is complete.

Additional data on void concentrations in neutron — irradiated PE16 are available from Cawthorne eta/.,8 Sklad et a/.,50 and Boothby.28 The results presented by Cawthorne et a/. for PE16 fuel pin cladding irra­diated in DFR to a peak fluence of 5.6 x 1026nm~2 (^28 dpa) differ from those shown in Figure 7 in that, although void number densities are similar for irradiations at ^380-520°C, void concentrations are about an order of magnitude higher at 350°C and 600-630 °C. Such discrepancies might arise from uncertainty and/or variability in irradiation tempera­tures. Another possibility is that void nucleation was incomplete at the higher irradiation temperatures in the lower burn-up pin examined by Brown et a/. Data from Sklad et a/. show an increase in void numbers in unstressed PE16 specimens irradiated in EBR-II at 500 °C from an average (for two differently heat trea­ted conditions) of about 4 x 1019 to 1.2 x 1020m~3 with increasing fluence from 1.2 x 1026 to

1. x 1026nm~2 (E > 0.1 MeV), that is, from ^6 to 20 dpa. In this case, the void concentration and overall swelling of ^0.2% at ^20 dpa remain below the levels shown in Figure 7 for the DFR-irradiated pin at ~ 17 dpa; this may reflect the effect of stress on swelling for fuel pin cladding.

Void swelling data determined from Transmission electron microscope (TEM) examinations of three heats of PE16 which were irradiated in the UK-1 rig in EBR-II are shown in Figure 8, which includes previously unpublished results for the low boron (4ppm) heat Z184 as well as data for heats DAA766 and Z260D (with 18 and 70 ppm boron, respectively) which were reported by Boothby.28 Data are shown for all three heats in the STA condition (ST 1020 °C and aged 4h at 750 °C) and for DAA766 in the OA condition (a multistage heat treatment that included aging at 900 °C, slow cooling to 750 °C, and then aging for 16 h at that temperature, resulting in the

O STA DAA766 □ OA DAA766 л STA Z260D ° STA Z184

д

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1019

О

1018

300 350 400 450 500 550 600 650

Temperature (°C)
100

90

80

70

60 ф

50

га

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>

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Figure 8 Swelling data, void concentrations, and void diameters for Nimonic PE16 samples irradiated in UK-1 rig in Experimental Breeder Reactor-II. Adapted from Boothby, R. M. J. Nucl. Mafer.1996, 230, 148-157; Unpublished data for Boothby, R. M. The Microstructure of EBR-II Irradiated Nimonic PE16; AEATRS 2002 (FPSG/P(90)23), with permission from AEA Technology Plc.

precipitation of TiC and an overaged g structure). Swelling data derived from the density measure­ments of STA PE16 heat DAA 766 from the same experiment are shown in Figure 4. An example of the

void distribution in the OA condition is shown in Figure 9. Note that the voids in neutron-irradiated PE16 tend to be cuboidal and that enhanced growth of voids attached to TiC precipitates (located at the site of a prior grain boundary) has occurred.

Neutron fluences and irradiation temperatures in the UK-1 experiment were similar to those for the first withdrawal of the AA-1 rig for which data is shown in Figure 2. Void concentrations for heats DAA766 and Z260D shown in Figure 8 appear to be less temperature-dependent than for the fuel pin clad­ding data shown in Figure 7. Void numbers are gener­ally lower than in the cladding at temperatures up to ^550 °C, but are intermediate between the results of Brown et a/.49 and Cawthorne et a/.8 for irra­diations at ^600 °C. Void concentrations for PE16 irradiated to fast neutron fluences (E >0.1MeV) of 9.4-12.3 x 1026nm~2 at 477-513 °C in the UK-1 ex­periment were very similar to those determined by Sklad et a/.50 for 4.0 x 1026nm~2 at 500 °C. The low boron heat Z184 showed atypical behavior, with a very high concentration of small voids and low swelling at 438 °C, but high swelling owing to increased void sizes at normal void concentrations at temperatures above 513 °C. It is probable that the effect of boron on swelling is related to the formation ofboron-vacancy complexes, which can give rise to the nonequilibrium segregation of boron in the presence ofquenched-in thermal vacan­cies as well as to radiation-induced effects.51

Some variability in the swelling response of Nimonic PE16 in PFR (Prototype Fast Reactor) components was reported by Brown and Linekar.52

Increased swelling in PE16 subassembly and guide tube wrappers in PFR compared to expectations based on the performance of DFR pin cladding appeared to be related to temperature fluctuations, particularly at temperatures below 400 °C during the early operation of PFR. Void concentrations were reported to be higher in the PFR components, and it was suggested (by Cawthorne, unpublished data) that this may have been due to the release of vacan­cies from vacancy loops which had formed during lower temperature excursions. In fact, the void con­centration reported by Cawthorne eta/.8 for DFR pin cladding irradiated at 350 °C was higher than the highest value reported for the PFR components by a factor of about 3, but this comparison was not made by Brown and Linekar. There were also indications of heat-to-heat variability and effects of the fabrication route on the swelling of PE16 wrappers in PFR. Nevertheless, swelling of PE16 wrappers, although higher than expected, remained low in absolute terms and did not give rise to any operational problems.

Although PE16 was originally selected as the ref­erence wrapper material for PFR and as an alterna­tive to cold-worked M316 steel for fuel pin cladding, PE16 was favored as a cladding material with 12%Cr ferritic-martensitic steel wrappers in subsequent subassembly designs.53 The 12%Cr steel was chosen as a wrapper material because of its superior swelling resistance, but its use was limited to relatively low temperatures owing to inadequate strength at the higher operating temperatures experienced by pin cladding. Design calculations for PE16 fuel pin clad­ding made by Cole54 indicated that cladding hoop stresses, which arise from the internal pressure from the gaseous fission products released from the fuel, were much lower than the yield stress of the material and were generally expected to remain below about 70 MPa. In addition, the void swelling and irradiation creep behavior of PE16 were considered to be well matched to the fuel swelling, so that fuel-clad inter­action stresses also remain low. Fuel pins with PE16 cladding successfully attained high burn-ups in PFR, with some 3500 pins exceeding dose levels of 100 dpa and 265 pins reaching maximum doses of 155 dpa.55 Very few failures of PE16 clad pins were recorded — three failures occurred in pins which had reached burn-ups over 17at.%, with one failure at 11.3 at.% burn-up which was believed to have resulted from a fabrication defect.56 In addition to the four PE16 cladding failures in PFR, Plitz et a/.57 recorded 14 failures in austenitic steel cladding, all at lower burn — ups than in PE16. The failures in PE16 cladding were regarded as benign and permitted continued opera­tion, with no significant loss of fuel into the primary circuit coolant. A peak burn-up of 23.2 at.%, corresponding to a peak dose in the PE16 cladding of 144 dpa, was achieved in PFR in an experimental fuel cluster. Postirradiation examinations of pins from this cluster and a high burn-up subassembly (18.9 at.%, with a peak cladding dose of 148 dpa) were carried out by Naganuma et a/.58 Maximum diametral strains of less than 1% were measured, attributable to the combined effects of void swelling, creep deformation arising from internal gas pressure in the pins, and small contributions from mechanical interactions between the fuel and cladding in the lower part of the pins.