Harvesting Reactor Pressure Vessel Beltline Material from the Decommissioned Zion Nuclear Power Plant Unit 1 ^†

Abstract

The decommissioning of the Zion Nuclear Power Plant (NPP) provided a unique opportunity to harvest and study service-aged reactor pressure vessel (RPV) beltline materials. This work, conducted through the U.S. Department of Energy’s Light Water Reactor Sustainability (LWRS) Program, aims to improve the understanding of radiation-induced embrittlement to support extended nuclear plant operations. Material segments containing the Linde 80 flux, wire heat 72105 (WF-70) beltline weld and the A533B Heat B7835-1 base metal, obtained from the intermediate shell region with a peak fluence of 0.7 × 10¹⁹ n/cm² (E > 1.0 MeV), were extracted, cut into blocks, and machined into test specimens for mechanical and microstructural characterization. The segmentation process involved oxy-propane torch-cutting, followed by precision machining using wire saws and electrical discharge machining (EDM). A chemical composition analysis confirmed the expected variations in alloying elements, with copper levels being notably higher in the weld metal. The harvested specimens enable a detailed evaluation of through-wall embrittlement gradients, a comparison with the existing surveillance data, and the validation of predictive embrittlement models. This study provides critical data for assessing long-term reactor vessel integrity, informing aging-management strategies, and supporting regulatory decisions to extend the life of nuclear plants. This article is a revised and expanded version of a paper entitled, “Current Status of the Characterization of RPV Materials Harvested from the Decommissioned Zion Unit 1 Nuclear Power Plant”, PVP2017-65090, which was accepted and presented at the ASME 2017 Pressure Vessels and Piping Conference, Waikoloa, HI, USA, 16–20 July 2017.

1. Introduction

The decommissioning of the Zion Nuclear Power Plant (NPP) in Zion, Illinois, after approximately 15 effective full-power years of service presented a unique opportunity for obtaining and characterizing the degradation of the in-service reactor pressure vessel (RPV) beltline materials [1] and to assess the currently available models for predicting the radiation embrittlement of RPV steels [2,3,4,5,6,7]. The components and structures in a nuclear power plant (NPP) must withstand a very harsh operating environment, including long periods of time at high temperatures, stress from operational loads, neutron irradiation, and corrosive media. Moreover, extending the reactor’s service beyond 60 years will increase the effects of these stressors and possibly introduce new modes of degradation [8]. Although numerous modes of degradation are complex and vary depending on the location and material, understanding and managing material degradation is key to the continued safe and reliable operation of NPPs. An important element of understanding the aging-related degradation modes of the RPV and associated components is the examination of service-aged materials. The LWRS Zion Harvesting Project (ZHP) [1] provided the first opportunity to examine service-aged beltline RPV materials in the United States. This investigation is critically important because access to materials from active or decommissioned NPPs provides invaluable in-service-degraded material to inform relicensing decisions and assessments of the current degradation models, and can help to further develop the scientific basis for understanding and predicting long-term environmental degradation behavior. Moreover, the RPV beltline materials, if successfully harvested, allow for through-wall thickness attenuation and property distributions to be evaluated and compared with surveillance specimen test data and archival materials.

The ZHP, in cooperation with Zion Solutions, LLC, a subsidiary of Energy Solutions, Inc., an international nuclear services company, was established in 2011 to initiate and coordinate the selective procurement of materials, structures, components, and other items of interest to the LWRS Program from the Zion NPP in support of the extended service and current operations of the US nuclear reactor fleet. The Zion NPP was a decommissioned, two-unit, Westinghouse four-loop PWR facility, with each unit being capable of producing 1040 MWe. The units were commissioned in 1973, permanently shut down in 1998, and placed into SAFSTOR (a method of decommissioning where a nuclear facility is placed and maintained in a condition that allows the facility to be safely stored and subsequently decontaminated to levels that permit its release for unrestricted use) in 2010. Materials of high interest included the through-thickness sections of the RPV, low-voltage cables, and concrete.

The RPV is a potentially life-limiting component in light–water reactors (LWRs) because the replacement of the RPV has not been considered a viable option owing to the significant replacement costs [1]. Researchers studying the effects of radiation on RPV materials have long been interested in evaluating service-irradiated materials to validate physically informed transition-temperature-shift predication models [2,3,4,5,6,7]. Kussmaul and Fohl [9] performed the first postmortem examination of a beltline RPV material from Gundremmingen NPP, a German boiling water reactor. This was followed by a few other studies of in-service-aged RPV materials, such as the French pressurized water reactor (PWR) CHOOZ-A [10]. Valo et al. [11] published an examination of a beltline weld from the first commercial prototype VVER (Soviet-style PWR) RPV, Novovoronesh-1. Viehrig et al. [12] reported the postmortem examination of the VVER-440 RPV beltline weld. They used the Master Curve approach, as described in ASTM E1921-08 [13], to characterize trepans acquired from the Greifswald Unit 1 RPV, and demonstrated that the Master Curve reference temperature T₀ varied depending on the thickness of the weld. Recently, service-aged welds from the reactor head and beltline region harvested from Barsebäck 2 PWR RPV [14] have been studied. The main goal of all these studies was to perform an integrity assessment of the vessel to evaluate the effects of the long-term, low-flux, in-service degradation of beltline RPV materials.

The current project involved acquiring segments of the Zion NPP Unit 1 RPV, cutting the segments into blocks that contain the circumferential beltline WF-70 weld and intermediate shell base metal B7835-1 [15,16,17], and machining the blocks into mechanical (Charpy, compact tension, and tensile) test specimens and coupons for microstructural (transmission electron microscopy and atom probe tomography) characterization and chemical composition. The mechanical properties and microstructural results will be discussed in detail in separate manuscripts. These results will be used for comparison with previously reported surveillance data, to assess current radiation damage models, and to validate the current codes and standards for evaluating transition temperature shifts. Ultimately, the purpose of this work is to understand and manage RPV material degradation, which is essential for ensuring the continued safe and reliable operation of NPPs.

2. Zion Unit 1 RPV

The Zion Unit 1 RPV consisted of the head, three rings or shell sections composed of semicylindrical plates with two vertical welds, and a bottom plate, as shown in a preliminary segmentation plan in Figure 1. Its height, without the head plate, was approximately 419 in. (1064 cm). The vessel wall had an inner diameter of 173 in. (439 cm) and thickness of 8.8 in. (22.4 cm) over the beltline region, including the cladding. The reactor vessel weighed about 700,000 lb. (317,515 kg).

The primary consideration in the evaluation of which RPV segments to harvest is the circumferential fluence. As shown at the bottom of Figure 2 and highlighted in blue, the circumferential fluence varies approximately by a factor of three over a 45° arc from the vertical weld positions to midway between two vertical weld positions. Based on this variation, the optimum region to harvest the beltline weld would be a section midway between the upper (intermediate shell) and lower (lower shell) vertical welds.

3. Segmentation

The Zion Unit 1 RPV was cut into 16 segments using oxy-propane torch over three levels: horizontal cuts were made in the nozzle section just above the intermediate shell and midway into the lower shell above the bottom plate. Level l, which includes the inlet and outlet nozzles, was cut into eight 45° segments of 157.5 in. (400 cm) in height. Level 2 was also cut into eight 45° segments of 157.5 in. (400 cm) height and 72.9 in. (185.2 cm) in length, as measured from end to end of the outer diameter (Figure 1).

Because of the location of the overhead bridge, the vessel could not be rotated 22.5° after the nozzle segments were cut. Therefore, the Level 2 segments, which include the intermediate shell and a portion of the lower shell and the WF-70 beltline weld, were cut along the same vertical lines as the nozzle cuts (i.e., at the two vertical welds of the intermediate shell, directly above the two vertical welds of the lower shell, and in the middle of the peak circumferential fluence) (Figure 1 and Figure 2). Moreover, four segments (including Segments 1 and 2) also contained the intermediate shell base metal B7835-1, which was used to make surveillance specimens (Figure 1 and Figure 3). The beltline pieces cut from the vessel were 8.8 in. (22.4 cm) thick, including a 5/16 in. (7.9 mm) stainless steel cladding on the internal surface. Each piece weighs approximately 28,000 lb. (12,727 kg).

4. Harvesting

Zion Unit 1 Segment 1, with dimensions of 157.5 × 72.9 × 8.8 in. (400 × 185.2 × 22.4 cm) and containing the previously well-characterized WF-70 beltline weld and base metal ASTM A533/A533M type B (C: 0.25 wt% max; Mn: 1.15–1.50 wt%; P: 0.035 wt% max; S: 0.035 wt% max; Si: 0.15–0.40 wt%; Mo: 0.45–0.60 wt%; Ni: 0.40–0.70 wt%) heat B7835-1, Figure 3, was cut from just below the circumferential weld between the nozzle section and the intermediate shell and the circumferential weld between the lower shell and the bottom plate using an oxy-propane torch, Figure 4a. The right edge (as viewed from the outer wall of the RPV) begins at the 0°/WF-4 vertical weld (Figure 2 and Figure 3). Segment 1 was removed using a gripper crane and positioned on the “down-ender” frame to allow for its proper positioning in the steel-shipping box. Segment 5 was loaded face-up, as shown in Figure 4b, and the matching Segment 1 was loaded face-down to provide “clamshell” shielding for shipment to the Energy Solutions Memphis Processing Facility (MPF) before cutting the segment into specimen blocks [1].

5. Cutting Sample Blocks from Segment 1

As shown in Figure 5, plans to cut seven blocks were developed to obtain sufficient through-thickness mechanical test specimens from the beltline weld and base metal to assess radiation embrittlement models and for possible annealing and re-irradiation experiments. The blocks were cut, using a diamond wire saw, into pieces varying in length from approximately 5.7 × 2.0 × 8.8 in. (144.8 × 50.8 × 223.5 mm) to 7.6 × 3.0 × 8.8 in. (193.0 × 76.2 × 223.5 mm) and to 11.25 × 3.0 × 8.8 in. (285.8 × 76.2 × 223.5 mm), and designated “F”, “C”, and “CF”, respectively. The block designations correspond to the type of samples being machined; F blocks are for fracture toughness specimens of base metal, C blocks are Charpy and tensile specimens of base metal, and the CF block contains alternating rows of fracture toughness and Charpy specimens for weld metal. The process of cutting sample blocks from the beltline weld and base metal above the weld sections of Segment 1 began with locating the exact position of the beltline weld and determining the slope across the outer diameter of the segment.

Owing to the high probability that the oxy-propane flame cut along the vertical direction may have obscured the weld, two 1 in. (25.4 mm) holes were drilled approximately 6 in. (152.4 mm) above and below the estimated position of the beltline weld and 2.75 in. (69.8 mm) from either edge of the segment (Figure 6). A wire saw was then placed through the holes to cut out small sections to reveal fresh surfaces that could be etched to locate the exact position of the weld and to determine the slope (if any) in the weld line relative to the horizontal cut of the segment. After the small section was removed, chemical etching, using a 10% Nital (nitric acid/alcohol) solution, revealed the weld cross-section, and its position was approximately 2 in. (50.8 mm) below the estimated position.

Once the position and slope of the weld were determined, a thin metal template was placed over the segment and aligned with the beltline weld (Figure 7a). The template was designed based on the location of the RPV blocks relative to the beltline weld and base metal and the location of the 13 holes used to thread the wire to cut the blocks, including the kerf, along the lines shown in Figure 5. The holes were used to insert the diamond wire to cut two beltline weld blocks and five base metal blocks from Segment 1, as shown in Figure 7b. With the completion of Cut 11 (Figure 7b), the CF block was removed from Segment 1, as shown in Figure 7c, using a lift magnet. Similarly, the remaining blocks were successfully cut and lifted out from Segment 1, as shown in Figure 8.

Upon completion of the cutting phase operation, the seven RPV blocks cut from the Zion Unit 1 RPV Segment 1 at the MPF were packaged and loaded into two 55-gallon drums, along with individual lift magnets. The drums were placed into a B-25 box with 1 in. (25.4 mm) steel shielding beneath and 1 in. (25.4 mm) steel shielding around the side of the drum. The B-52 box was shipped to BWX Technologies, Inc. (BWXT) in Lynchburg, Virginia, to machine the mechanical test specimens, coupons for chemical characterization, and microstructural characterization samples. The cutting waste and unused RPV segments were shipped to the Energy Solutions waste disposal site in Clive, Utah.

6. Machining Samples from Segment 1 Blocks

Before receipt of the seven Segment 1 blocks at BWTX, the machining plan underwent a detailed review. All steps were assessed, including the removal of the cladding, the block orientation for identifying the location of the high-fluence edge, how to best fit the cut plan into the actual dimensions of the cut blocks, unique sample identification numbers, block cutting order (four F blocks followed by C2 and CF blocks), and the estimated schedule for the project.

After receipt of the Zion Unit 1 RPV sample blocks at BWXT, the seven blocks were inspected, their radiation levels were verified, and the stainless-steel cladding was removed. This process was performed with an electrical discharge machining (EDM) using a 0.010 in. (0.25 mm) diameter wire. The cut with the kerf created a straight, approximately 0.1 in. (2.5 mm) cut above the apex of the cladding and approximately 0.35 in. (8.9 mm) into the block from the outside of the block. The cladding thickness was measured manually to verify that no remnant pieces remained on the RPV steel block. A rough sketch of the cut is shown in Figure 9a and the actual WF-70 Beltline weld after removing the cladding is shown in Figure 9b.

Two F blocks, F3 and F4, were dedicated to machining 0.5T compact tension (C(T)) specimens. The base metal F block layout is such that 4 0.5T C(T) specimens were machined from each row, with the notch oriented along the circumferential direction of Segment 1 (parallel to the beltline welding direction), and 14 rows of specimens were machined through the thickness, starting from the inner surface of the vessel. The machining should result in 56 0.5T C(T) specimens for each block. However, machining the anomalies in five rows of Block F3 and Block F4 caused the number of 0.5T C(T) samples to be reduced to 48 and 54, respectively. Moreover, BWXT machined 28 miniature compact tension (mini-C(T)) specimens from Block F3 and 20 from Block F4, as summarized in

Table 1. Summary of samples machined from base metal blocks, F3, F4, and C2.

Specimen Type	Total Samples
0.5T C(T) specimen	102
Mini-C(T)	48
Charpy V-notch specimen	239
Coupons for chemical composition and microstructural characterization	90
SS3 tensile specimen	128

[1].

A total of 255 Charpy-size (10 × 10 × 55 mm) bars were machined from base metal Block C2. From each row, 15 specimen bars were machined with the notch oriented along the circumferential direction of the segment (parallel to the beltline welding direction), and 17 rows were machined through the thickness of the vessel, starting from the inner vessel surface. From those 255 Charpy-sized bars, 239 Charpy V-notch (CVN) specimens (10 × 10 × 55 mm) were machined; see Table 1. The remaining 16 Charpy-size bars were machined into 128 SS3 subsized tensile specimens (Table 1), with eight per bar, with dimensions (25.4 × 5 × 0.76 mm), and 90 coupons (10 × 10 × 0.5 mm) (Table 1), for chemical and microstructural characterization. Those 16 bars were the first and the last bars in each even row.

Before machining the beltline weld Block CF, chemical etching, using a 10% Nital solution, identified the centerline and cross-section of the weld. This step was critical to ensure that all Charpy V-shape notches, the gauge section of all SS3 tensile specimens, the 0.4T C(T) compact tension specimen notches, and coupons for chemical and microstructural characterization were centered on the beltline weld.

Block CF was machined by alternating rows of Charpy bars and 0.4T C(T) specimens through the thickness of the vessel wall. The first and closest to the inner surface row was dedicated to Charpy bars, followed by the next row of 0.4T C(T) specimens, and so on. Each row of Charpy specimens contains 22 Charpy-size bars. A total of nine rows of Charpy-size bars were machined through the thickness of the vessel wall, thus making a total of 198 Charpy bars. From each Charpy row, the first and the last bars were machined into SS3 tensile specimens (eight per bar) and chemical and microstructural characterization coupons (four per bar), as shown in Figure 10. The Charpy V-notch and the notch of 0.4T C(T) specimens were oriented along the welding direction (T-L orientation), whereas the axes of the tensile specimens were oriented perpendicular to the welding direction (T-orientation). Despite the authors’ best efforts, a few Charpy bars and C(T) specimens were machined out of tolerances. Instead of being salvaged, these samples were used to machine mini-CT specimens for future considerations.

Table 2. Summary of samples machined from weld Block CF.

Specimen Type	Total Samples
Charpy V-notch Specimen	174
Coupons for chemical composition and microstructural characterization	72
SS3 tensile specimens	142
0.4T C(T) specimen	77
Mini-CT	9

summarizes all specimens machined from the beltline weld Block CF.

7. Chemical Composition

Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) was used to measure silicon, phosphorus, chromium, manganese, nickel, copper, and molybdenum content. LA-ICP-MS was performed on samples that were mounted and ground with 600-grit sandpaper. The ablation was performed using an ESI NWR213 laser ablation system coupled with a Nu Instruments ATTOM and magnetic sector ICP-MS. The samples were analyzed by first removing the oxide layer with a cleaning pass and then using an analytical pass. Both ablations were performed at laser settings of 45%, 10 HZ, a 40 μm spot size, and 2 μm/s line speed. The ICP-MS method employed was a deflector scan method with 2500 resolution. The elements were bracketed by three NIST standard reference materials (SRMs) (i.e., SRM1134, SRM 1271, and SRM1762). Before each analytical run, the standards were ablated and collected to build the regression calibration curves.

The specific locations at which the composition measurements were taken are detailed in [18]. The results of the analyzed specimen from base metal Block C2 are shown in

Table 3. Block C2 (base metal)’s chemical composition results. The second letter in the ID column indicates the row position from the vessel’s inner surface to the outer surface in alphabetical order; digits indicate the bar positioning in the row.

I.D.	Chemical Composition (wt%)
	Si	P	Cr	Mn	Ni	Cu	Mo
2A15	0.29	0.007	0.11	1.36	0.44	0.12	0.44
2B01	0.22	0.006	0.13	1.34	0.50	0.13	0.48
2C15	0.26	0.008	0.10	1.31	0.46	0.11	0.46
2D01	0.25	0.006	0.10	1.48	0.51	0.13	0.47
2E15	0.21	0.016	0.10	1.34	0.53	0.14	0.47
2F01	0.24	0.008	0.10	1.26	0.52	0.13	0.48
2G15	0.24	0.008	0.10	1.27	0.46	0.11	0.47
2H01	0.26	0.009	0.11	1.52	0.54	0.14	0.51
2I15	0.22	0.006	0.09	1.28	0.49	0.12	0.47
2J01	0.24	0.009	0.09	1.32	0.48	0.11	0.46
2K15	0.30	0.010	0.12	1.39	0.58	0.16	0.53
2L01	0.25	0.008	0.12	1.42	0.56	0.15	0.58
2M15	0.29	0.010	0.10	1.41	0.51	0.12	0.51
2N01	0.30	0.012	0.13	1.31	0.50	0.13	0.44
2O15	0.21	0.006	0.09	1.46	0.50	0.12	0.46
2P01	0.25	0.009	0.10	1.34	0.51	0.12	0.54
2Q15	0.20	0.006	0.13	1.38	0.52	0.13	0.47

. The copper content ranged from 0.11 to 0.15 wt%. A comparison of the results is presented in Table 3, which demonstrates good agreement with the composition reported in [17] for the same material heat, confirming the fidelity of the compositional analysis. The second analysis was of the weld metal in Block CF. The results are reported in

Table 4. Block CF (weld metal)—chemical composition results. The second letter in the ID column indicates the row’s position from the vessel’s inner surface to the outer surface in alphabetical order; digits indicate the bar positioning in the row.

CVN I.D.	Chemical Composition (wt%)
	Si	P	Cr	Mn	Ni	Cu	Mo
FA03	0.66	0.017	0.10	1.59	0.58	0.37	0.41
FA22	0.68	0.018	0.10	1.42	0.55	0.34	0.43
FC01	0.59	0.016	0.08	1.35	0.53	0.34	0.34
FC22	0.64	0.016	0.09	1.59	0.58	0.37	0.38
FE01	0.66	0.018	0.08	1.49	0.57	0.34	0.37
FE22	0.58	0.015	0.07	1.47	0.48	0.27	0.34
FG01	0.63	0.016	0.08	1.43	0.50	0.33	0.33
FG22	0.57	0.019	0.07	1.51	0.54	0.32	0.35
FI01	0.58	0.017	0.06	1.57	0.51	0.28	0.34
FI22	0.68	0.017	0.10	1.62	0.54	0.32	0.39
FK01	0.68	0.019	0.10	1.49	0.54	0.35	0.39
FK22	0.69	0.020	0.09	1.66	0.55	0.33	0.39
FM01	0.63	0.019	0.06	1.65	0.57	0.37	0.38
FM22	0.67	0.020	0.06	1.66	0.59	0.35	0.38
FO01	0.70	0.015	0.10	1.34	0.52	0.36	0.38
FO22	0.61	0.017	0.07	1.47	0.51	0.35	0.37
FQ01	0.71	0.020	0.09	1.60	0.60	0.29	0.42
FQ22	0.59	0.018	0.06	1.47	0.55	0.27	0.32

. The copper content roughly tripled from the content found in the base metal, ranging from 0.27 to 0.37 wt%. Elevated copper content in reactor pressure vessel (RPV) welds is a well-documented characteristic stemming from early fabrication practices, during which higher copper concentrations in the weld flux were intentionally utilized to promote favorable metallurgical properties and ensure the integrity of beltline welds. Based on the NIST standards used to calibrate the BWXT LA-ICP-MS instruments, the elemental measurements from the instrument were within the standard value, with a deviation of less than 0.05 wt%.

8. Summary

This paper documents the acquisition and shipment, by rail, of four segments of the Zion Unit 1 beltline RPV material to the Energy Solutions MPF for cutting into blocks, and shipment to BWXT for machining into various test specimens for laboratory testing. Only Segment 1 of the Zion Unit 1 RPV, containing a section of the well-characterized Linde 80 WF-70 beltline weld (circumferential weld between the lower and the intermediate shells) and base metal A533B Heat B7835-1, were harvested and shipped for cutting into blocks to be machined into mechanical and microstructural characterization samples. Access to service-irradiated RPV beltline weld and plate sections will allow for through-wall attenuation studies to be performed. These studies will be used to assess current radiation embrittlement models [2,3,4].

The testing results in this project will provide significant data for comparison with previously reported surveillance data, to assess current radiation damage models and validate current codes and standards for evaluating transition temperature shifts. Moreover, the ZHP is critically important because access to materials from active or decommissioned NPPs provide an invaluable in-service-degraded material to inform relicensing decisions and assessments of current degradation models to further develop the scientific basis for understanding and predicting long-term environmental degradation behavior, providing a sound basis for informed aging management.