Impact of Hydrogen on Dimensional Stability of ZIRLO Fuel Assemblies

Abstract

The high-operating-temperature fuel duty, 18-month and longer-cycle lengths, and higher burnups have put increased demands on PWR fuel during recent years. As a result of these demanding operating conditions, the corrosion and dimensional stability of PWR fuel assembly skeletons are being challenged. To understand the in-reactor performance of fuel assembly skeletons, two hot cell programs were initiated on high duty fuel. Specifically, two Zircaloy-4 skeletons and one ZIRLO skeleton were dimensionally characterized and destructively examined to understand the effect of corrosion on the dimensional stability of guide tubes and grids. While previous papers have dealt with the effects of fluence and crystallographic texture on the irradiation growth of zirconium alloys, this paper focuses on the effect of corrosion (oxidation and hydrogen uptake) on the dimensional stability of ZIRLO and Zircaloy-4. Laboratory tests were performed to quantify potential dimensional changes resulting from corrosion. Accelerated autoclave tests of ZIRLO and Zircaloy-4 in 589 K water containing 700-ppm lithium revealed that dimensional changes were well correlated to hydrogen content, not oxide thickness. In addition, the dimensional changes for both guide tube and strip (grid) autoclave specimens showed similar dependency upon hydrogen content and no significant dependency on crystallographic texture. The hot cell evaluation of the irradiated ZIRLO and Zircaloy-4 structural components revealed increasing dimensional changes of grids and guide tubes from the bottom to the top of the assembly. This variation was strongly correlated with increasing hydrogen content as suggested by the autoclave tests. For a given alloy (i.e., ZIRLO or Zircaloy-4), the grid dimensional change is similar to the guide tube diameter change, suggesting that the effects of hydrogen are generic and not related to processing or starting material geometry. Comparison of ZIRLO and Zircaloy-4 dimensional data reveals greater stability of ZIRLO as a result of lower hydrogen uptake, a lower fluence component to growth, and a lower hydrogen component to growth. ZIRLO structural components, thimbles and grids, show an enhanced degree of dimensional stability with the combined effects of irradiation and corrosion/hydriding.

Influence of water vapour on the zircaloy-4 oxidation at high temperature - Stress determination by in situ X-Ray diffraction

“…It appears that the average stress magnitudes are larger for oxygen compared to steam. These results are consistent with the values obtained by synchrotron X-ray diffraction by Béchade et al at 400°C [29,30].As observed in the present work, it has been proposed that under wet atmospheres, the average stress values are at a maximum at the very early oxidation stage and then decreases. This behaviour may be related to stress relaxation phenomena taking place early during the formation of the scale [21].…”

“…This indicates that no important stress relaxation by scale cracking occurred in zirconia scales obtained under wet air conditions during cooling, because cracking has already occurred under isothermal conditions after the early stage oxidation period (about 60 hours). It has also been proposed that the incorporation of hydrogen can occur in the metal in the presence of steam [30]. Hydrogen is located in interstitial positions in the Zr hexagonal lattice; or if the solubility limit is exceeded, it forms Zr hydride precipitates.…”

Influence of Water Vapour on the Zircaloy-4 Oxidation at 700 °C: Stress Determination by In Situ X-ray Diffraction

“…It appears that the average stress magnitudes are larger in dry oxygen than in steam. These results are also consistent with the values obtained by synchrotron X-ray diffraction at 400 °C [31,32]. As observed in the present work, it has been proposed that under wet atmospheres, the average stress values are very low at the very early oxidation stage and becomes less negative.…”

Mechanical and Irradiation Properties of Zirconium Alloys Irradiated in Hanaro

“…Additionally, the dimensional changes of the grid strap, which was irradiated in the plant, were also compared to measure free irradiation growth. The data were acquired through the results of previous studies [5].…”

“…In the plant reactor, according to Steven's study, the growth results of zirconium alloys are well documented and written in Table 3 as one of the representative growths on fluence [5]. For our analysis, the condition of irradiation is thought to be the beginning of the irradiation cycle in the plant.…”