ML23059A428
| ML23059A428 | |
| Person / Time | |
|---|---|
| Site: | PROJ0734 |
| Issue date: | 03/10/2023 |
| From: | Hans Arlt NRC/NMSS/DDUWP/RTAB |
| To: | Chris Mckenney NRC/NMSS/DDUWP/RTAB |
| Alexander G; Arlt, H | |
| References | |
| eConcurrence 20230227-70011 | |
| Download: ML23059A428 (5) | |
Text
March 10, 2023 MEMORANDUM TO: Christepher A. McKenney, Chief Risk and Technical Analysis Branch Division of Decommissioning, Uranium Recovery, and Waste Programs Office of Nuclear Material Safety and Safeguards (NMSS)
FROM: Hans D. Arlt, Sr. Systems Performance Analyst /RA/
Risk and Technical Analysis Branch Division of Decommissioning, Uranium Recovery, and Waste Programs Office of Nuclear Material Safety and Safeguards
SUBJECT:
TECHNICAL INFORMATION FROM U.S. NUCLEAR REGULATORY COMMISSION FEBRUARY 1, 2023, ONSITE OBSERVATION VISIT TO THE SAVANNAH RIVER SITE SALTSTONE DISPOSAL FACILITY During the Onsite Observation Visit conducted on February 1, 2023, the U.S. Nuclear Regulatory Commission (NRC) staff toured construction of Saltstone Disposal Structure (SDS) 8 and SDS 10. Specifically, the NRC staff observed the SDS 8 sump, perimeter, and high-density polyethylene (HDPE) seam welding and the placement and welding of the SDS 10 HDPE between the lower and upper mud mats. Additional information regarding the February 1, 2023, Onsite Observation Visit is available in the corresponding Onsite Observation Report.
SDS 8 Sump and Perimeter The NRC staff was able to observe close up one of the four HDPE SDS-8 sumps before it was installed into the concrete lower mud mat, as shown in Figure 2.2-2 of DOE document SRMC-CWDA-2022-00025, Rev. 3. The disposal structure is designed so that any excess water from the structure will flow down towards one of the four sumps unless a defect or hole in the HDPE/
geosynthetic clay liner (HDPE/GCL) composite barrier allows the water to escape elsewhere.
Sheets of HDPE and of GCL surrounding the sump had already been installed. The overlaps of HDPE sheets had been welded together by double hot-wedge fusion welding while HDPE pieces with more complex configurations (e.g., the sump or footing HDPE embedment) were welded together by extrusion welding. The process for footing HDPE embedment includes the following steps: heat tacking to hold the HDPE in place, grinding the surface down and then cleaning the surface (many geomembrane areas were partially covered with soil and pebbles) to prepare for the extrusion welding, and then the extrusion welding. The process also included workers stapling copper wires along those locations where extrusion welding was to occur. The NRC staff observed workers performing all these steps while onsite.
C. McKenney After workers completed the extrusion welding, they tested the geomembrane for HDPE seam defects with a high-voltage spark test. In the spark test, a spark would be generated with the help of the copper wire (to provide a conductivity pathway below the geomembrane) if a hole was present in the area being tested. However, the SRS workers preferred method for detecting holes or defects in the HDPE after extrusion welding was the vacuum box method, although if the surfaces were too uneven, the spark method would be used. If double hot wedge welding had been used, NRC staff was informed that the pressurized air channel method was used to check for holes or other defects along the seam. The air pocket between the two parallel double hot wedge fusion welds would be pressurized and a drop in pressure would indicate that a defect was present.
The SDS 10 HDPE/GCL Composite Barrier Between the Lower and Upper Mud Mat The NRC staff observed that the SDS 10 HDPE/GCL composite barrier had been installed on top of the lower mud mat and part of the upper mud mat had already been poured. The upper mud mats typically are poured onto the composite barrier in sections over multiple nights. The NRC staff was not able to observe any part of the pouring of the upper mud mat for SDS 10 since the pouring had been delayed due excessive precipitation. However, the DOE did provide videos of the pouring of the upper mud mat on top of the HDPE several days after the Onsite Observation Visit. Screen captures of that pouring are provided in Figures 1 through 5 below.
NRC staff found the HDPE geomembrane sheets not to be resting flat and smooth on top of the GCL and lower mud mat (NRC staff assumes that the GCL sheets lie flat), but rather was uneven and had many waves, ripples, and bubbles. The DOE informed the NRC staff that this unevenness increased during the day with sunlight and HDPE warping decreased during the night and darkness. It was for this reason that the pouring happened at night. On average, the composite barrier, once installed, was exposed to the environment for two to four weeks. The DOE staff indicated that not much consideration was given to temperature while the composite barrier was exposed nor while the upper mud mat cementitious material was poured on top of the HDPE. The DOE staff informed the NRC staff that there was a method for pouring the cementitious material onto the composite barrier so as to eliminate the waves and bubbles in the HDPE geomembrane. The pouring started at the center of the floor and moved outward while workers would straighten and move the waves and unevenness away from the pouring.
Eventually the bubbles and waves would reach the perimeter of the disposal structure and air trapped under the HDPE could escape.
The DOE also informed the NRC staff that good contact was provided between the HDPE and the GCL by the weight of the cementitious material on the HDPE and GCL. The sheets for both the HDPE and GCL overlapped by approximately 15 to 20 cm (6 to 8 in) for the HDPE and 15 to 61 cm (6 to 24 in) for GCL. DOE staff further stated that workers applied bentonite powder to the GCL overlaps, which the DOE expects to be pressed together by the weight of the cementitious material. The sheets of the HDPE and GCL are of different sizes, so that an HDPE seam does not regularly match up with a seam of the GCL and one seam would not lie directly on top of the other seam.
Puddles of water were observed on top of the HDPE and people were allowed to walk on the HDPE geomembrane. An extrusion weld repair of an apparent defect was noticed by NRC staff.
The NRC staff asked if there was a systematic process for finding such construction-related defects, such as a sharp heavy object falling of the HDPE. The DOE staff on location responded that they knew of no such process to detect non-seam related defects. For example, the DOE staff said no Electrical Leak Location (ELL) tests were used to detect rips and tears since this
C. McKenney would be difficult to do with the cementitious material being both above and below the geomembrane. Although the observed repair of the HDPE was completed with the extrusion welding method, the entire HDPE geomembrane for the mud mat is welded with the double hot-wedge fusion method. A demonstration was given of this welding device whereby it automatically moved over two approximately 2 m (7 ft) long overlapping HDPE sheets and welded them together with two parallel seams. Small samples of the double seams were cut from the sheets and given to the NRC staff.
Figure 1: Pouring of the SDS 10 Upper Mud Mat on Top of the HDPE (1 of 5)
C. McKenney Figure 2: Pouring of the SDS 10 Upper Mud Mat on Top of the HDPE (2 of 5)
Figure 3: Pouring of the SDS 10 Upper Mud Mat on Top of the HDPE (3 of 5)
C. McKenney Figure 4: Pouring of the SDS 10 Upper Mud Mat on Top of the HDPE (4 of 5)
Figure 5: Pouring of the SDS 10 Upper Mud Mat on Top of the HDPE (5 of 5)
Docket No.: PROJ0734 ADAMS NO.: ML23059A428