Abstract
Neutron absorbing materials are being considered within commercial spent nuclear fuel disposal canisters to maintain nuclear subcriticality. To select candidate alloys for the canisters, both neutron absorption and corrosion resistance must be considered for this application. This work examines corrosion resistance of Ni-Cr-Mo-Gd alloys developed specifically for neutron absorption. The addition of Gd results in a secondary gadolinide phase (Ni5Gd) that significantly changes the corrosion properties. Testing was performed primarily in seawater at 30 °C. Seawater was selected as the most prevalent terrestrial brine and is characterized by a high chloride concentration. Various electrochemical corrosion techniques were carried out to evaluate Ni-Cr-Mo-Gd alloys with different Cr compositions and investigate the role of Ni5Gd phase on corrosion behavior. C22 was included as a benchmark material, due to the similarity in composition and the significant corrosion data available. Test results showed a tendency to passivate over time which is attributed primarily to dissolution of surface exposed Ni5Gd phase particles leaving primary phase exposed. Cross-sectional analysis indicated that dissolution could penetrate up to hundreds of micrometers deep under aggressive conditions. It was found that higher Cr variant (21.01%) showed much shallower impact, suggesting Cr prevented primary phase corrosion and thus reduced deeper Ni5Gd phase dissolution. Acid pickling of the specimens showed much less dissolution for the higher Cr material and suggested some primary phase dissolution for the low Cr specimen. Acid pickled specimens showed positive shifts in the repassivation potential, suggesting increased surface passivation.
| Original language | English |
|---|---|
| Article number | 154581 |
| Journal | Journal of Nuclear Materials |
| Volume | 584 |
| DOIs | |
| State | Published - Oct 2023 |
Funding
Spent nuclear fuel (SNF) management, including storage, transportation, and eventual disposal, is an important issue in the nuclear industry. The U.S. Department of Energy (DOE) has been tasked to plan for transportation and eventual disposition of spent fuel. One method for this management is to place the SNF into waste packages that will be dispositioned in a deep geologic repository. The purpose of the canisters is to prevent water (neutron moderator) from contacting the SNF. In canister designs, a material to absorb neutrons may be needed to avoid criticality [1–5]. This neutron absorbing material (NAM) requires corrosion resistance should the waste package be breached, and natural water ingress occurs. Idaho National Laboratory (INL) led a metallurgical development program sponsored by DOE's National SNF Program (NSNFP) as part of a larger effort to develop the DOE standardized canister for storage and disposal of highly enriched, DOE-owned SNF [6,7]. The program resulted in the development of Gd-containing Ni-Cr-Mo-Gd alloy documented in ASTM-B 932–04 and has been assigned UNS N06464 [8]. Ni-Cr-Mo-Gd composition was based on Hastelloy C4 (14.5–17.1% Cr). Gd addition was selected due to its high thermal neutron absorption cross section and low solubility in the expected repository environment. The Ni-Cr-Mo alloy family was chosen for the corrosion performance, mechanical properties, and weldability of Ni-Cr-Mo based alloys. The microstructural investigation of these alloys showed that the Gd addition was not soluble in the primary austenite metallurgical phase and is present in the alloy as a Gd-rich secondary phase [3,9]. This secondary phase (Fig. 1 A-B) is comprised of 1–10 μm particles that elongate during the ingot rolling process [7]. Spent nuclear fuel (SNF) management, including storage, transportation, and eventual disposal, is an important issue in the nuclear industry. The U.S. Department of Energy (DOE) has been tasked to plan for transportation and eventual disposition of spent fuel. One method for this management is to place the SNF into waste packages that will be dispositioned in a deep geologic repository. The purpose of the canisters is to prevent water (neutron moderator) from contacting the SNF. In canister designs, a material to absorb neutrons may be needed to avoid criticality [1–5] . This neutron absorbing material (NAM) requires corrosion resistance should the waste package be breached, and natural water ingress occurs. Idaho National Laboratory (INL) led a metallurgical development program sponsored by DOE ’s National SNF Program (NSNFP) as part of a larger effort to develop the DOE standardized canister for storage and disposal of highly enriched, DOE-owned SNF [ 6 , 7 ]. The program resulted in the development of Gd-containing Ni-Cr-Mo-Gd alloy documented in ASTM-B 932–04 and has been assigned UNS N06464 [8] . Ni-Cr-Mo-Gd composition was based on Hastelloy C4 (14.5–17.1% Cr). Gd addition was selected due to its high thermal neutron absorption cross section and low solubility in the expected repository environment. The Ni-Cr-Mo alloy family was chosen for the corrosion performance, mechanical properties, and weldability of Ni-Cr-Mo based alloys. The microstructural investigation of these alloys showed that the Gd addition was not soluble in the primary austenite metallurgical phase and is present in the alloy as a Gd-rich secondary phase [ 3 , 9 ]. This secondary phase ( Fig. 1 A-B) is comprised of 1–10 μm particles that elongate during the ingot rolling process [7] . This manuscript has been authored by Battelle Energy Alliance, LLC under contract No. DE-AC07-05ID14517. This work was supported by the U.S. Department of Energy office of Nuclear Energy.
Keywords
- Corrosion
- Neutron absorbing material
- Ni-Cr-Mo-Gd alloy
- Spent nuclear fuel storage
