Sampath Ranganath

An Examination of the Role of the Assumed Young’s Modulus Value at the High Cycle End of ASME Code Fatigue Curve for Stainless Steels

When the ASME Code fatigue curves (S-N curves) are used in the assessment of high frequency cyclic stresses (such as those produced by flow-induced vibrations), the question arises as to the need for an E correction (i.e., multiplying the calculated cyclic stress by the ratio of the E value at the room temperature and the E value at the temperature used in the stress analysis). This question becomes significant for materials such as stainless steels when the two sets of S-N curves up to the 2007 Edition of the Code [1] are specified: i) the first curve covered the cyclic range of 10 to 106 cycles and specified an E value. This curve covered mostly the strain controlled fatigue data for which the correction for E is required. ii) The second curve covered the cyclic range of 106 to 1011 cycles and didn’t provide a specific E value. This curve covered mostly the load controlled fatigue test data for which the correction for E is not required since the stress was independent of E (stress was either P/A for axial loading or Mc/I for bending). However the 2010 and subsequently the 2013 Editions of the Code [2] combined the two curves into a single curve with a cycle range of 10 to 1011 cycles with E value specified at room temperature. This means that the E correction applies across the board for the entire cyclic range of 10 to 1011 cycles including the high cycle end where the test results are independent of E. The inclusion of the E correction for the high cycle fatigue range presents a problem for the evaluation of components with vibratory loading. The present paper describes the results of a thorough review of the past technical basis papers for the ASME Code S-N curves and examines the necessity for E correction at the high cycle end of the Code S-N curves for stainless steels.Copyright

Thermal Aging and Neutron Embrittlement Evaluation of Cast Austenitic Stainless Steels

NUREG-1801, Rev. 1, Section XI.M.13, states that an ASME Code Section XI VT-3 examination is required to be performed of reactor internal components. In addition, the NUREG report specifies that for the license renewal period, these inspections shall be augmented by an aging management program to assess the synergistic effects of thermal aging and neutron embittlement of cast austenitic stainless steel (CASS) components. This aging management program consists of (a) identifying susceptible components; and (b) either performing additional inspections of these components, or performing a component-specific evaluation to confirm that the stresses in the components are sufficiently low such that augmented inspections are not warranted. This paper presents the results of evaluations conducted to assess the potential synergistic effects of thermal aging and neutron embrittlement of CASS components in BWR internals and recommend augmented inspections if needed. The evaluation shows that all the BWR CASS components have ferrite levels below the level for which aging embrittlement is a concern. Furthermore, for the Control Rod Guide Tube Base and Core Spray Sparger Nozzle Elbows, the end of life fluence is less than the threshold value for toughness loss. The end-of-life fluence levels for the orificed fuel support, the jet pump assembly castings and the Low Pressure Core Injecion (LPCI) Couplings exceed the threshold, but the toughness data for irradiated austenitic stainless steel show that these components will have sufficient fracture toughness at end of the license renewal period so that augmented inspection is not required. It is concluded that augmented inspections are not required for the BWR CASS internals.Copyright