Fabrication of Zr-2.5Nb Pressure Tubes to Minimize the Harmful Effects of Trace Elements

Abstract

Trace elements can reduce the fracture resistance of Zr-2.5Nb pressure tubes. The effects of hydrogen as hydrides and oxygen as an alloy-strengthening agent are well known, but the contributions of carbon, phosphorus, chlorine, and segregated oxygen have only recently been recognized. Carbides and phosphides are brittle particles, while chlorine segregates to form planes of weakness that produce fissures on the fracture face of test specimens. A high density of fissures is associated with low toughness. With long hold times in the (α + β) region, oxygen partitions into the α-grains; such grains are hard and, if they survive fabrication, may reduce the toughness of the finished tube. Through a cooperative program involving AECL and the manufacturers, a series of manufacturing innovations and controls has been introduced that minimizes these harmful effects. Hydrogen is present in the zirconium sponge as water, can be absorbed at each stage of tube fabrication, and needs to be carefully controlled, particularly during ingot breakdown and subsequent forging. Hydrogen concentrations in finished tubes have been reduced by a factor of three through the optimization of manufacturing processes and the implementation of new technology. Multiple vacuum arc melting, use of selected raw materials, and intermediate ingot surface conditioning have resulted in much improved fracture toughness through the reduction of chlorine and phosphorus concentrations. Optimum distribution of oxygen may be achieved through changes to the extrusion process cycle. An understanding of the Zr-2.5Nb-C phase diagram, particularly the solubility of carbon at low concentrations, has resulted in the specification of a lower carbon concentration.

Stress-reorientation of hydrides and hydride embrittlement of Zr–2.5 wt% Nb pressure tube alloy

“…Tubes made from Zr-2.5 wt% Nb alloy [1][2][3][4][5][6][7] in cold worked and stress-relieved (CWSR) condition, are called pressure tubes, which act as miniature pressure vessels in Pressurized Heavy Water Reactors (PHWR) and operate over the temperature range of 526-566 K under an internal pressure of $10.5 MPa, and neutron flux of the order of 3 · 10 17 n/m 2 /s. The initial hydrogen content of the pressure tubes is kept as low as possible by controlling the manufacturing process parameters [8]. However, part of the hydrogen/deuterium evolved during service from coolant-metal corrosion reaction is picked up by the pressure tubes of the PHWR.…”

Stress field estimation around a semi-constrained inclusion and its validation by interpreting hydride platelet orientation around a blister in a Zr–2.5Nb alloy

“…Either due to garter spring displacement from the designed location or due to creep or both [8], the PT sags, and may contact the CT, resulting in setting-up of thermal gradient across the PT. Though the initial hydrogen content of the PT is kept as low as possible by controlling manufacturing process parameters [9], it can pick up hydrogen/deuterium during service [10]. Hydrogen is known to migrate down the concentration and temperature, and up the tensile stress gradient [11] in zirconium and its alloys.…”

Performance of Pressure Tubes in Candu Reactors

“…Previously, it has been shown that pressure tubes manufactured from ingots of 100% recycled material (RM) have high toughness due to the elimination of particles including the Zr-Cl-C complex carbide, zirconium phosphide, and carbide [3][4][5]. The elimination of these particles reduces the number of void nucleation sites and improves the fracture toughness as shown in Figure 1.…”

“…The CCL ranges from a minimum of ≈41 mm to a maximum of >80 mm. The minimum is from a burst test of a low toughness tube, which had a high concentration of harmful trace elements, Cl and P [5]. This tube was manufactured in the late 1960s and is not considered representative of modern production tubes.…”

“…2. Fracture propensity is controlled by minimizing hydrogen, chlorine, carbon, and phosphorus concentrations [3,5]. 3.…”