Effect of irradiation on failure mode during creep
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I.
INTRODUCTION
FUEL pins in commercial fast breeder reactors will be exposed to conditions of elevated temperature, stress, and fast neutron flux. Performance prediction of fuel pins under these conditions requires knowledge of the stress to rupture behavior of the fuel cladding material. Prior to 1976, failure criteria in the United States were based on stress to rupture data generated by postirradiation testing. 1,2In postirradiation tests, the material is irradiated in the unstressed condition, then removed from the reactor and tested. These tests showed a serious degradation of both the stress to rupture and strain to failure properties of the material?-]3 During analyses and comparison of thermal and in-reactor creep of 20 pct cold worked AISI 316 stainless steel (316 CW), it was realized that the creep processes were dynamically altered by irradiation. ]4:5 Irradiation during testing resulted in microstructural and microchemical changes in the material which could not be duplicated in the postirradiation testing procedure. Therefore a program was initiated to investigate the in-reactor creep rupture properties of 316 CW in Experimental Breeder Reactor II (EBR-II). This is the second report of a series which extends the in-reactor stress rupture results to 11,000 hours and corresponding fluences greater than 1023 n / c m 2 (E > 0.1 MeV).]6 Results of metallographic examination of rupture specimens and analyses of failure strains are reported.
II.
EXPERIMENTAL METHODS
In-reactor creep rupture data described in this report were generated using the pressurized tube specimen. Details of manufacture and measurement of these specimens have previously been described. 16'17'18 The specimens were constructed from cladding material lots used to fabricate fuel pins of the Fast Test Reactor (FIR) located in Richland, Washington. Accuracy of thermal control test temperatures was +-5 ~ Irradiation was conducted in two different experimental vehicles designated as the YY07 biaxial creep capsule and
the AAII heat pipe canister. 18'19 The first vehicle (YY07) was fully instrumented with on-line temperature control. This experiment consisted of three canisters which operated at nominal temperatures of 600, 650, and 700 ~ Two thermocouples were located in each canister and temperature time plots for periods of irradiation were maintained. A typical temperature time plot for the 650 ~ canister is shown in Figure 1. Although during steady reactor operation the temperature in the center canister (650 ~ was controlled to - 15 ~ the two end canisters (600 and 700 ~ were subject to larger fluctuations resulting from changes in the axial distribution of reactor power. For purposes of data correlation, an effective temperature was computed for the temperature time history of each specimen. The effective temperature Te was computed using the Dorn parameter 0 where
0 = ~ ti exp(-Q/RTi) i
ti = time at temperature Ti (K) Q = 65,000 cal/gatom R = 1.987 cal/gatom-~ and
TE = - Q / R ln(O/t) t =
Eti i
Intermittent rises in the canis
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