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Terrapower, LLC - Transmittal of Responses to NRCs Request for Supplemental Information HYD-10, HYD-13, and HYD-14
	ML25076A001
Person / Time
Site:	Kemmerer File:TerraPower icon.png
Issue date:	03/16/2025
From:	George Wilson; TerraPower
To:	; Office of Nuclear Reactor Regulation, Document Control Desk
References
	TP-LIC-LET-0408
	Download: ML25076A001 (1)
	v • d • e

15800 Northup Way, Bellevue, WA 98008 www.TerraPower.com P. +1 (425) 324-2888 F. +1 (425) 324-2889 TP-LIC-LET-0408 Docket Number 50-613 March 16, 2025 U.S. Nuclear Regulatory Commission Washington, DC 20555-0001 ATTN: Document Control Desk

Subject:

Transmittal of Responses to NRC's Request for Supplemental Information HYD-10, HYD-13, and HYD-14 TerraPower, LLC (TerraPower) on behalf of US SFR Owner, LLC (USO), a wholly owned subsidiary of TerraPower, submits this letter to provide responses to NRC's request for supplemental information for Kemmerer Unit 1 Environmental Report. The NRC request for supplemental information is in (ML24311A168). Enclosure 1 contains supplemental information for Environmental Audit questions HYD-10, HYD-13, and HYD-14.

This letter and enclosure make no new or revised regulatory commitments.

If you have any questions regarding this submittal, please contact Nick Kellenberger at nkellenberger@terrapower.com.

I declare under penalty of perjury that the foregoing is true and accurate. Executed on March 16, 2025.

Sincerely, George Wilson Vice President of Regulatory Affairs TerraPower, LLC

March 16, 2025 Page 2 of 2

Enclosures:

1.

Transmittal of Responses to NRC's Request for Supplemental Information HYD-10, HYD-13, and HYD-14 cc:

Mallecia Sutton, NRC Josh Borromeo, NRC Patricia Vokoun, NMSS Daniel Barnhurst, NMSS Nathan Howard, DOE Jeff Ciocco, DOE

TP-LIC-LET-0408 Transmittal of Responses to NRC's Request for Supplemental Information HYD-10, HYD-13, and HYD-14

NRC Question HYD-10:

Provide a description of post-construction groundwater-surface water interaction.

TerraPower's Reponse:

The North Fork of Little Muddy Creek behaves as a losing stream. This is based on interpretation of North Fork of Little Muddy Creek water levels, groundwater levels, and water chemistry results from both monitoring wells and the North Fork of Little Muddy Creek. This evaluation is explained in Section 7.1.5 of NAT-7267, Rev. 1, Groundwater Flow Model.

NAT-7267, Rev.1 was provided for NRC review on the electronic reading room.

A post-construction simulation using a calibrated groundwater model is documented in NAT-7267, Rev.1. This simulation illustrates the post-construction groundwater response to the following modifications at the Kemmerer Unit 1 (KU1) site:

Changes in recharge because of land-use modification.

Inclusion of building foundations affecting groundwater flow pathways.

Given that the North Fork of Little Muddy Creek is a losing stream, it will remain a losing stream during construction of KU1 as the groundwater table is lowered, and also as the groundwater table recovers following construction.

CPA REVISION:

Section 5.2.2.1 of the ER will be updated with the results of the post construction groundwater-surface water interaction.

NRC Question HYD-13:

The groundwater flow model calculation provided in response to safety information needs 2-3 and 2-4 includes contour maps of groundwater head in each model layer during construction dewatering (Figures 27-31). Provide a map (or maps) of groundwater drawdown resulting from construction dewatering.

TerraPower's Response:

Figures of drawdown in each model layer at the end of the construction dewatering simulation are shown in NAT-7267, Rev. 1 Groundwater Flow Model and are reproduced as Figures 1 through 5 below. NAT-7267, Rev.1 was provided for NRC review on the electronic reading room.

As expected, these figures indicate a drawdown center around the nuclear island where construction dewatering of the excavations will occur. In addition, these figures show a drawdown center to the northwest of the nuclear island near the northern boundary of the model domain in layers three through five. This is a result of using a variable conductance value for the northern general-head boundary (GHB) cells during model calibration to match the groundwater contours. The hydraulic conductivity values used in the GHB cells vary from 5 ft/d at the western end, reducing to 3 ft/day and down to 0.5 ft/day adjacent to where the drawdown center is located. The value then increases to 3 ft/day to the eastern end of the northern boundary.

The groundwater flow model calculation will be updated to incorporate the full two-years of groundwater monitoring data collected as part of the baseline monitoring program. During this update, the boundary conditions at the northern extent of the model domain will be revisited.

CPA REVISION:

Figure 1 - Drawdown in Layer 1 during construction dewatering simulation - Base case Model 1. Contours in feet TP-LIC-LET-0408 Page 3 of 9 SUBJECT TO DOE COOPERATIVE AGREEMENT NO. DE-NE0009054 Copyright © 2025 TerraPower, LLC. All Rights Reserved

Figure 2 - Drawdown in Layer 2 during construction dewatering simulation - Base case Model 1. Contours in feet SUBJECT TO DOE COOPERATIVE AGREEMENT NO. DE-NE0009054 Copyright © 2025 TerraPower, LLC. All Rights Reserved TP-LIC-LET-0408 Page 4 of 9

Figure 3 - Drawdown in Layer 3 during construction dewatering simulation - Base case Model 1. Contours in feet SUBJECT TO DOE COOPERATIVE AGREEMENT NO. DE-NE0009054 Copyright © 2025 TerraPower, LLC. All Rights Reserved TP-LIC-LET-0408 Page 5 of 9

Figure 4 - Drawdown in Layer 4 during construction dewatering simulation - Base case Model 1. Contours in feet SUBJECT TO DOE COOPERATIVE AGREEMENT NO. DE-NE0009054 Copyright © 2025 TerraPower, LLC. All Rights Reserved TP-LIC-LET-0408 Page 6 of 9

Figure 5 - Drawdown in Layer 5 during construction dewatering simulation - Base case Model 1. Contours in feet SUBJECT TO DOE COOPERATIVE AGREEMENT NO. DE-NE0009054 Copyright © 2025 TerraPower, LLC. All Rights Reserved TP-LIC-LET-0408 Page 7 of 9

The groundwater flow model calculation provided in response to safety information needs 2-3 and 2-4 includes mass balance results from the simulation (Figures 16 and 32). Provide the units for the numbers included in these figures. Provide an explanation for why the construction dewatering results in a decrease in the discharge from North Fork Little Muddy Creek to the groundwater.

TerraPower's Response:

The mass balance results for the calibrated model and dewatering model are now shown in Figures 16 and 36 of NAT-7267, Rev. 1 Groundwater Flow Model. NAT-7267, Rev.1 was provided for NRC review on the electronic reading room. The units for these two figures are ft3/day.

A visual comparison of the stream flux to groundwater between the calibrated model and dewatering model is shown in Section 7.6.1 of NAT-7267, Rev. 1, Groundwater Flow Model, along with the difference between the two models as shown in Figure 1.

The decrease between the two models is related to use of the stream (STR) package within MODFLOW. As dewatering occurs and the upper layer dries out adjacent to the North Fork of Little Muddy Creek, the stream cells become inactive and flow from the creek to the aquifer ceases. This represents a conceptual shortcoming of the STR package, because the water lost from the stream should be applied as recharge to the uppermost active layer. A future revision to the groundwater model will replace the STR package with the streamflow-routing (SFR) package to represent streamflow losses and recharge of the underlying groundwater system.

CPA REVISION:

None

Figure 1 - Surface Water Flow (leakage) from North Fork of Little Muddy Creek to Groundwater

ML25076A001

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