Influence of Hydrogen Content on the α/β Phase Transformation Temperatures and on the Thermal-Mechanical Behavior of Zy-4, M4 (ZrSnFeV), and M5™ (ZrNbO) Alloys During the First Phase of LOCA Transient

Abstract

To simulate the end-of-life behavior of cladding tubes during the first phase of a LOCA transient, one may assume that the main effect of a long service exposure on the cladding deformation behavior during LOCA arises from the hydrogen uptake associated with the cladding oxidation at high burn-up. Thus, the recent metallurgical studies and EDGAR [2] tests performed on pre-hydrided Zr-base alloys are presented. The influence of hydrogen has been studied for concentrations ranging from ∼ 100 up to ∼1000 (weight) ppm on FramatomeANP low-tin Zy-4, M4 (ZrSnFeV), and M5™(ZrNbO) alloys. The decrease of the α/β phase transformation temperatures with the increase of the hydrogen content is noticeable and has been quantified, and then modeled, for both quasi-equilibrium (calorimetry) and dynamic (dilatometry) conditions for heating rates up to 100°C/s. Some complementary microstructural examinations on hydrided samples, beforehand partially transformed into β phase, have been performed to get a better insight of the metallurgical features associated with the hydrogen effects. Finally, the EDGAR thermal-mechanical test results are presented and discussed. The alloys have been tested under steady state conditions of pressure and temperature, on the one hand, and with continuous heating (thermal ramps) on the other. The results show that the mechanical behavior cannot be explained solely by the effect of hydrogen on the shift of the α/β phase transformation temperatures, but that hydrogen modifies also the creep behavior and the burst criterion, especially in the a domain, and in the lower α+β temperature range. As a result, hydrogen decreases the creep strength and the ductility of the materials, the effect being greater for higher hydrogen content. All these data are used to model the thermal-mechanical behavior of the hydrided cladding tubes in order to simulate the LOCA behavior of the clad after long-term in-service exposure. Finally, preliminary thermal ramp tests under uniaxial loading performed on irradiated Zy-4 are presented and compared to the behavior of non-irradiated as-received and hydrided Zy-4. These last experiments were made to validate the assumption that the main effect of a long service exposure on the cladding deformation behavior during the first phase of LOCA is mainly linked to the hydrogen uptake associated with the cladding oxidation.

Cladding embrittlement during postulated loss-of-coolant accidents.

“…The 38 s cooling time for the ANL tests is long enough for oxygen redistribution during the partial phase change from beta-to-(alpha + beta) in this temperature range. Based on the work of Brachet et al [30], the beta-to-alpha phase transformation is expected to be ≈50% complete at 800ºC for Zry-4 with asfabricated levels of oxygen and with 300-600 wppm hydrogen. The ANL and CEA post-oxidation ductility data are also consistent for samples that were slow cooled from 1200ºC to RT, even though the total cooling time from 1200ºC to 150ºC was much greater for the CEA samples.…”

High-temperature steam-oxidation behavior of Zr-1Nb cladding alloy E110

“…As such, extensive studies have been conducted over the past decades on the hightemperature steam-oxidation of fuel-rod cladding made of traditional zirconium alloys, such as Zircaloy-2 (Zry-2) and Zircaloy-4 (Zry-4) [1][2][3][4][5][6][7][8][9]. Advanced zirconium alloys, such as Zr-Sn-Nb and Zr-Nb, exhibit better corrosion resistance than Zry-4 under the operating conditions of pressurized water reactors (PWRs) [10][11][12][13][14][15][16][17][18]. Cladding alloys such as ZIRLO TM (Zr-1 wt.% Sn-1 wt.% Nb) and M5 TM (Zr-1 wt.% Nb) have replaced Zry-4 in most US PWRs, and E110 (Zr-1 wt.% Nb) is used in PWRs designed by the Russian nuclear vender (TVEL).…”

Oxygen segregation in pre-hydrided Zircaloy-4 cladding during a simulated LOCA transient

“…(2)) considering the oxygen in the "n" phase is an ideal mixture. The motion of layer boundaries is derived from the mass balance of oxygen at the interface between two layers according to equation (3). The oxygen diffusion coefficient is extracted from literature data for each of the a(O) phase and b-phase.…”

“…It was frequently reported that oxygen and hydrogen contents in the prior-b-phase or in the a+b-phase had a combined influence on the cladding post-quenching embrittlement [2][3][4][5][6][7]. It is thus necessary to understand the mechanisms influencing the motion of these two chemical elements to fully describe the post-quench embrittlement.…”