PSI - Issue 14

Ramesh Kumar et al. / Procedia Structural Integrity 14 (2019) 577–583 R. Kumar / Structural Integrity Procedia 00 (2018) 000–000

579

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Fig. 2 Flow chart of DHC initiation evaluation of volumetric flaws

2.1. Evaluation of bulk hydrogen equivalent concentration (H th ) for DHC during sustained hot condition

The presence of flaw-tip hydride determines whether DHC can occur at a given operating condition. Temperature changes during reactor operation can result in the dissolution of previously existing flaw tip hydride or precipitation of new flaw-tip hydride. The bulk hydrogen equivalent concentration is equal to the initial hydrogen concentration, Hi, plus the upper-bound estimate of deuterium, D, as follows:

2 H H D i eq = +

Flaw-tip hydride is predicted to exist at sustained hot conditions when the hydrogen equivalent concentration at the flaw tip is greater than TSSD. The hydrogen equivalent concentration at the flaw tip may be assumed to be the equilibrium concentration. Due to the hysteresis in the terminal solid solubility for hydrogen, the hydrogen equivalent concentration in solution at the flaw tip is dependent on thermal history. Calculation of threshold bulk hydrogen equivalent concentration, H th at which equilibrium concentration at flaw tip is equal to TSSD at sustained hot condition was evaluated using the CSA N285.8 (A.6.3.3.2) procedures.

   

   

V

(

)

−

−

σ

σ

H

hyd

hyd

2

1

= H TSSD T

op ( ) exp

th

RT

op

Where V H = 1.67 × 10

–6 m 3 /mole (partial molar volume of hydrogen in zirconium), R = 8.314 J/(K-mole)

T = temperature under evaluation, K