2013 IR&D Annual Report

Develop Method for Hydriding Fuel Cladding and Characterize Influence of
Hydriding on Mechanical Behavior, 20-R8269

Principal Investigators
Xihua He
Yi-Ming Pan
Kwai Chan

Inclusive Dates: 11/01/11 – 6/27/13

Background — Zirconium-based cladding material exposed to coolant water during nuclear reactor operations could produce hydrogen, and part of the hydrogen could be absorbed into the cladding at concentrations from less than 100 to up to 600 ppm. During extended dry storage, cladding plays an important role in spent nuclear fuel retrievability and confinement and in thermal performance and subcriticality. As the cladding cools during extended storage, the hydrogen inside the cladding may precipitate as hydrides; furthermore, under hoop stresses induced by thermal gradient, both existing and newly formed hydrides may reorient. Depending on their size, distribution, and orientation, these hydrides may lead to premature fracture as a result of hydride embrittlement or delayed hydride cracking. Because the United States is actively considering extended dry storage as an alternative approach for managing spent nuclear fuel and has an increased amount of high burnup fuel as a result of changes in plant operating conditions, there is a strong need for data that can be used to predict the lifetime of cladding. The objectives of this project were to develop methods and identify parameters controlling hydride formation at various hydrogen concentrations, identify conditions when hydrides reorient under stress, and characterize the influence of hydrides and their orientation on mechanical properties.

Approach — The primary objectives of this project were:

  • Develop methods and parameters to prepare specimens with various hydrogen concentration levels
  • Identify conditions when hydrides reorient under stress
  • Conduct mechanical tests to characterize the influence of hydriding on cladding mechanical behavior

Accomplishments — Major accomplishments are highlighted in the following areas.

Four methods have been used to hydride the material: electrochemical method — cathodic charging followed by diffusion annealing; hydrogen charging in a tubular reactor with continuous flow of a mixture of hydrogen-argon gas; hydriding in pure hydrogen in a pressurized vessel; and hydriding in supercritical water at 350°C. All these methods need to be operated for tens of hours and at elevated temperatures. A new method for accelerated hydriding at lower temperature was developed, which involves surface activation by some metal salts, hydrogen storage on surface, hydrogen migration and diffusion, as illustrated in the process in Figure 1.

Diagram of the Newly Developed Method for Accelerated Hydriding At Lower Temperatures
Figure 1. Diagram of the newly developed method for accelerated hydriding at lower temperatures.

Hydride reorientation heat-treatment was performed on hydrogen charged Zircaloy-2 three-point bend specimens at 320 to 350°C for one to two hours, followed by cooling to 200°C. Hydride reorientation occurs in Zircaloy-2 at K levels ranging from 5.5 MPa(m)1/2 to 27.4 MPa (m)1/2, and reoriented zone sizes are consistent with a critical hydride reorientation stress in excess of 90 MPa. Reoriented hydrides formed in Zircaloy-2 ranged from submicron-sized to as large as 22 µm.

Fracture testing was conducted on hydride reoriented three-point bend specimens at 200°C in the scanning electron microscope. Reoriented hydrides formed in Zircaloy-2 ranged from submicron-sized to as large as 22 µm. Fracturing of reoriented hydrides of larger sizes (> 10 µm) is more prevalent than that of smaller sizes (< 5 µm). The reoriented hydrides reduced fracture resistance through a void nucleation, growth, and coalescence process at the crack tip, as shown in Figure 2. The resulting crack resistance curves for Zircaloy-2, with reoriented hydrides, decreased from 38 MPa(m)1/2 to 21 MPa(m)1/2, with increasing hydrogen contents from 51 wt. ppm to 1,265 wt. ppm, as shown in Figure 3.

Images: Crack tip fracture process in one specimen after hydride orientation
Figure 2. Crack tip fracture process in one specimen after hydride orientation: (a) K = 20.6 MPa(m)1/2 and (b) K = 24.7 MPa(m)1/2, showing hydride fracture ahead of the crack tip to form voids that link with the main crack tip.
Graph: A comparison of KR curves
Figure 3. A comparison of KR curves of some hydrided Zircaloy-2 specimens tested at 200°C after hydride reorientation and corrected for the forked crack geometry.
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04/15/14