ML24198A093
| ML24198A093 | |
| Person / Time | |
|---|---|
| Site: | Kemmerer, 99902100 File:TerraPower icon.png |
| Issue date: | 04/25/2023 |
| From: | Joshua Borromeo NRC/NRR/DANU/UAL1 |
| To: | TerraPower |
| References | |
| EPID L-2023-TOP-0027 | |
| Download: ML24198A093 (1) | |
Text
TERRAPOWER, LLC - FINAL SAFETY EVALUATION OF TOPICAL REPORT NAT-3 226, AN ANALYSIS OF THE POTENTIAL VOLCANIC HAZARDS AT THE PROPOSED NATRIUM SITE NEAR KEMMERER, WYOMING (EPID L -2023 -TOP -002 7)
SPONSOR AND SUBMITTAL INFORMATION
Sponsor: TerraPower, LLC
Sponsor Address: 15800 Northup Way Bellevue, WA 98008
Project No.: 99902100
Submittal Date: April 25, 2023
Submittal Agencywide Documents Access and Management System (ADAMS)
Accession No.: ML23115A387
Brief Description of the Topical Report: TerraPower, LLC (TerraPower) topical report (TR)
NAT-3 226, An Analysis of Potential Volcanic Hazards at the Proposed Natrium Site near Kemmerer, Wyoming, Revision 0A, provides an analysis of potential volcanic hazards that may impact the proposed site near Kemmerer, Wyoming. The TR follows the guidance in NRC Regulatory Guide (RG) 4.26, Volcanic Hazards Assessment for Proposed Nuclear Power Reactor Sites, Revision 0 ( ML20272A168). The results of the U.S. Nuclear Regulatory Commission (NRC) staffs review of this TR methodology are summarized in the following sections of this safety evaluation (SE).
By email dated June 28, 2023 (ML23167B309), the NRC staff informed TerraPower that the TR provided sufficient information for the NRC staff to conduct a detailed technical review. On September 1, 2023, the NRC staff transmitted an audit plan to TerraPower (ML23248A199),
and subsequently conducted an audit of materials related to the TR from September 11, 2023, to October 19, 2023. The audit summary was issued on December 4, 2023 ( ML23338A051).
REGULATORY EVALUATION
Title 10 of the Code of Federal Regulations (10 CFR) 50.34(a)(3)(i) requires construction permit applicants to include principal design criteria (PDC) as part of the preliminary safety analysis report for a proposed facility. As noted in 10 CFR 50.34(a)(3)(i), the General Design Criteria (GDC) in Appendix A, General Design Criteria for Nuclear Power Plants, to 10 CFR Part 50, Domestic Licensing of Production and Utilization Facilities, outline the minimum requirements for the PDC for water-cooled nuclear power plants similar in design and location to plants for which construction permits have previously been issued by the Commission. Further, 10 CFR 50.34(a)(3)(i) notes that the GDC provide guidance to construction permit applicants in
Enclosure establishing PDC for other types of nuclear power units, which would include the Natrium design.
The GDC address the characterization of geologic and seismic information as it relates to natural hazard phenomena, including volcanic hazards. As relevant here, GDC 2 requires
[a]ppropriate consideration of the most severe natural phenomena historically reported for the site and surrounding area[.] TerraPower cites this requirement in its volcanic hazards TR. The NRC staff is reviewed a separate TR from TerraPower establishing PDC for the Natrium design (ML24197A226). In establishing PDC 2 for the Natrium design, TerraPower s PDC TR does not propose to change the GDC 2 language quoted above. Therefore, consistent with 10 CFR 50.34(a)(3)(i), the volcanic hazards TR, and the PDC TR currently under review, the NRC staff considered the GDC 2 requirement regarding [a]ppropriate consideration of the most severe natural phenomena historically reported for the site and surrounding area in its review of the volcanic hazards TR. If the PDC ultimately proposed in the application for the p roposed Natrium site near Kemmerer, Wyoming, differs from the GDC 2 language quoted above, that difference would need to be addressed in the application. 1
Additional requirements for site characterization information to be considered, including the need to consider volcanic activity at a proposed site, is described in 10 CFR 100.23, Geologic and seismic siting criteria. In particular, 10 CFR 100.23(c) relat es to the assessment of volcanic activity and 10 CFR 100.23(d)( 2) and 10 CFR 100.23(d) (4) relate to the consideration of surface deformation and siting factors for other design conditions, respectively.
RG 4.26 provides guidance on the performance of the volcanic hazards assessment for a proposed nuclear power reactor site to meet the applicable regulatory criteria. This RG was initially issued in 2021 and revised in 2023 (ML23167A078) to include administrative updates.
TerraPower indicated that the TR was developed consistent with RG 4.26, Revision 0. The NRC staff performed its review in accordance with RG 4.26, Revision 0, but noted that the TR is consistent with RG 4.26, Revision 1.
TECHNICAL EVALUATION
The subject TR provides the volcanic hazards assessment performed for the proposed TerraPower Natrium site near Kemmerer, Wyoming. The TR described the analysis methodology and criteria used by TerraPower in the assessment of volcanic sources, development of the tectono-magmatic model for the site region, the numerical modeling performed to assess the screened-in volcanic hazards and sources, and the physical characteristics and potential mitigation measures for these volcanic hazards.
Section 1, Introduction, of the TR gives the introduction to the general site characteristics and the applicable regulations. Section 2, Characterization of Quaternary Volcanic Systems, of the TR describes the Quaternary volcanic systems in the site region while Section 3, Regional Tectono-Magmatic Model, of the TR explains the regional tectono-magmatic model developed for the site region. Section 4, Screening of Volcani c Hazards, and Section 5, Evaluation of Initial Risk Ins ights, of the TR descr ibe the screening of volcanic hazards and application of initial risk insights, respectively, consistent with the methodology outlined in RG 4.26. Section 6, Analysis of Mafic Tephra-Fall Hazards, Section 7, Analysis of Silicic Tephra-Fall Hazards,
1 TerraPower, on behalf of US SFR Owner, LLC, a wholly owned subsidiary of TerraPower, submitted the CP application for the Kemmerer Power Station Unit 1 site on March 28, 2024 (ML24088A059), and the NRC staffs review is ongoing.
2 Section 8, Tephra-Fall Hazards at the Proposed Natrium Site, and Section 9, Characteri stics of Tephra Falls, of the TR describe the analysis of the screened in tephra fall hazards in greater detail, including the numerical modeling used to assess these hazards and the physical properties of tephra hazards at the site. Finally, Section 10, Considerations for Potential Tephra-Fall Events at the Proposed Facility, of the TR considers the potential actions to mitigate the effect of these tephra fall hazards on the proposed site.
The NRC staff focused their evaluation on the technical content provided in Sections 1 through
- 8. Section 9 describes the characteristics at tephra fall hazards but does not go into detail of how these characteristics would impact safety -related structures, systems, and components (SSCs) at the Natrium site. The information in Section 10 provides a good overview of potential mitigating actions to consider but does not propose specific mitigating actions to ensure safety -
related SSCs continue to perform their intended safety functions during a tephra fall event.
Accordingly, while the NRC staff evaluated the physical characteristics of tephra fall hazards at the site as provided in Section 9 of the TR, the NRC staff is not evaluating the effect of tephra hazards on safety-related SSCs at the Natrium site or the content of Section 10 of the TR in this SE.
- 1. Characterization of Quaternary Volcanic Systems
Section 2 of the TR characterizes the Quaternary volcanic systems within the 320 kilometers (km ) radius site region surrounding the proposed site near Kemmerer, Wyoming. These systems include the Yellowstone Volcanic Field, Upper Wind River Basin Volcanic Field, Leucite Hills Volcanic Field, Northern Black Rock Desert Volcanic Field, Curlew Valley Volcanic Field, Blackfoot Reservoir Volcanic Field, and Eastern Snake River Plain Volcanic Field. The TR also considered volcanoes located outside the site region but determined that there are no Quaternary deposits from these sources in the 40 km radius site vicinity. For each of the sources listed above, TerraPower provided the g eneral volcanic explosivity index (VEI), number and type of vents, age, extent of tephra, proximal hazards, pyroclastic density currents, and potential for new vent opening. This information is summarized in Table 9, Summary of Quaternary volcanoes in the site region, of the TR.
The VEI i s a commonly used measure t o characterize volcanic eruptions based on the volume of tephra produced by the eruption 2. Additionally, VEI also includes estimates of tephra fall volume and eruptive column height, from which magma density, deposit density, deposit volume and deposit mass can be determined. VEI is a logarithmic scale from 0 to 8, ranging from low ejecta volume and column height effusive eruptions, classified as VEI 0, up to VEI 8 supervolcano events characterized by greater than 1,000 km3 of ejecta and column heights exceeding 25 km.
The NRC staff reviewed the information provided as well as published literature and the United States Geological Survey volcanic hazard monitoring maps. The NRC staff determined that TerraPower adequately considered all Quaternary volcanic systems within the site region in accordance with RG 4.26.
- 2. Regional Tectono-Magmatic Model
2 Newhall, C.G., and Self, S., 1982, The volcanic explosivity index (VEI) an estimate of explosive magnitude for historical volcanism: Journal of Geophysical Research: Oceans, v. 87, p. 1231-1238, doi:10.1029/JC087iC02p01231.
3 Section 3 of the TR summarizes the regional tectono-magmatic model for the p roposed Natrium site. TerraPower stated that the immediate site vicinity is u nderlain by sedimentary rock. Apart from the Tertiary 3 age basalt at Black Mountain, Utah, 62 km west -northwest of the site, the next closest igneous rocks occur over 100 km from the site. Furthermore, TerraPower concluded that the lack of volcanic activity in the site vicinity over the last 16 Ma is consistent with current tectonic processes and volcanism in the site region. In reaching this conclusion, TerraPower considered the regional sources of Quaternary volcanism to determine if tectonic processes or geophysical conditions exist similar to those that resulted in past volcanic activity and could therefore result in volcanism occurring in the site vicinity during the operational period of the proposed facility (i.e., on the order of 100 years).
TerraPower followed the process in RG 4.26 to consider potential sources of Quaternary volcanism within the site region in the development of the regional tectono-magmatic model.
Figure 14, Regional patterns of faulting and seismicity, and distribution of Quaternary volcanic rocks, of the TR, is reproduced as Figure 1 of this SE. This figure provides a regional overview of the Quaternary volcanic sources as well as regional locations for faulting and seismicity. T he Eastern Snake River Plain, Yellowstone Volcanic Field, and Upper Wind River Basin Volcanic Field all resulted from the passage of the North American plate over the Yellowstone h otspot, but the proposed site is located to the south of the hotspot track. Accordingly, TerraPower concluded that the potential for volcanism in the site vicinity from the passage of the Yellowstone hotspot is negligible.
Volcanism occurring in the Basin and Range Physiographic Province, as at the Curlew Valley Volcanic Field, Northern Black Rock Desert Volcanic Field, and Blackfoot Reservoir Volcanic Field, is characterized by crustal extension inducing mantle melting and basaltic volcanism in a distributed volcanic field typical of the province. In contrast, the proposed site is located within the Sevier overthrust belt and is generally characterized by compression, not extension.
Accordingly, TerraPower concluded that there is negligible potential for the formation of volcanism in the site vicinity due to extension in the Basin and Range Physiographic Province during the operational period of the proposed facility.
3 The Tertiary period of geologic time, from the end of the Mes ozoic (~66 millions of years old (Ma)) to the beginning of the Quaternary (~2.6 Ma) is no longer used by the International Commission on Stratigraphy (ICS). Instead, the ICS recognizes the Paleogene (66-23 Ma) and the Neogene (23-2.6 Ma) periods.
Tertiary remains in use for publications prior to the ICS change and was used by TerraPower in the TR and is therefore used in this SE.
4 Figure 1: Reproduced from TR Figure 14 showing the location of the proposed site relative to faults, regional seismicity, and Quaternary volcanic rocks in the site region 4
The NRC staff reviewed the information provided by TerraPower and found its conclusion that the drivers of volcanism in the site region do not occur within the site vicinity, acceptable. As such, the potential for the formation of new sources of volcanism in the site vicinity is negligible during the operating life of the facility. According, the NRC staff concludes that the approach used by TerraPower is consistent with the guidance in RG 4.26 and provides sufficient information to characterize the regional tectono-magmatic model for the proposed Natrium site.
- 3. Screening of Volcanic Hazards
Section 4 of the TR describes the screening of volcanic hazards to determine whether additional analysis was warranted. TerraPower screened out proximal hazards, lava flows, pyroclastic density flows, and tephra fall hazards from volcanic sources outside the site region from further
4 Yellow dots are earthquakes of magnitude greater than 2.5 from the United States Geological Survey.
Silicic and mafic shaded areas reflect the chemistry of the volcanic rocks. Holocene rocks were erupted within the last 11,700 years.
5 analysis. TerraPower used the PVHA_YM code 5 to analyze the potential for new vent opening in the Leucite Hills Volcanic Fiel d, the closest Quaternary volcanic source to the proposed site, to confirm the tectono-magmatic model. The PVHA_YM code uses a kernel density function to calculate the vent density for points on a grid. Although originally developed to assess the potential for new vent opening at the Yucca Mountain site in Nevada, the only component in PVHA_ YM that is specific to Yucca Mountain is the geographic coordinates. Accordingly, TerraPower modified the PVHA_YM code to transpose the geographic coordinates to reflect the location of the proposed Natrium site. TerraPower clarified that it made no additional changes to the PVHA_YM code for this analysis. During the audit, TerraPower provided higher quality images of the PVHA_YM results and demonstrated the use of the PVHA_YM code. Based on the results from the PVHA_YM code, TerraPower determined that new vent opening can be screened out from further consideration due to the lack of Quaternary volcanism i n the site vicinity, the greater than 100 km distance to the nearest Quaternary volcanic feature, the low probability of volcanism occurring at the nearest volcanic source, and the lack of evidence that the Quaternary volcanic systems in the site region could extend into the site vicinity.
Although located in the same drainage basin, TerraPower screened out lava flow hazards from the Leucite Hills Volcanic Field based on distance and topography between the source and the proposed site, as well as the extent of previous flows from the source, which were limited to within 60 km of the source. Similarly, TerraPower screened out pyroclastic density currents from the Yellowstone caldera due to the distance from the source to the proposed site and the extent of previous pyroclastic density currents from the source, which was limited to less than 120 km from the caldera. TerraPower concluded that the only volcanic hazards that screen in for additional analysis are tephra fall from volcanic sources within the site region and debris flow of remobilized tephra on the North Fork of the Little Muddy Creek.
The NRC staff reviewed the results of the volcanic hazards screening and determined that it is reasonable to screen out proximal hazards, lava flows, and pyroclastic density flows based on the distance to the sources and topography between the sources and the proposed site.
Similarly, it is reasonable to screen out tephra hazards from sources outside the site region (200-mile (mi) radius), given the absence of tephra deposits in the site vicinity (25 mi radius) from sources outside the site region. The NRC staff also reviewed the inputs and results of the PVHA_YM analysis and concludes that the likelihood of a new vent forming that would impact the proposed site was negligible. The NRC staff further concludes that screening in tephra hazards from volcanic sources within the site region and debris flows of remobilized tephra deposits on the North Fork of the Little Muddy Creek are consistent with the tectono-magmatic model and local topography. The NRC staff concludes that the approach used by TerraPower to screen for volcanic hazards is consistent with the approach outlined in RG 4.26.
- 4. Evaluation of Initial Risk Insights
In Section 5 of the TR, TerraPower considered certain risk insights with respect to the screened-in hazards. TerraPower determined that because the Natrium design relies on air circulation systems for cooling and ventilation and volcanic tephra deposits can linger for weeks to months, a more detailed analysis of tephra fall hazards on the proposed site is warranted.
This analysis of tephra fall hazards is included in Section 6 and 7 of the TR and evaluated in Section 5 of this SE.
5 Nuclear Regulatory Commission, 2002, PVHA_YM Version 2.0: U.S. Nuclear Regulatory Commission, Software Release Notice, 32 p., (ML040780533).
6 The NRC staff reviewed these initial risk insights and concluded that the evaluation of these risk insights is consistent with the approach described in RG 4.26. Furthermore, because tephra fall hazards have the potential to adversely affect the performance of certain safety -related SSCs,
additional analysis of tephra fall hazards and local debris flows is appropriate.
- 5. Analysis of Tephra Fall Hazards
Sections 6 and 7 of the TR consider the analysis of tephra fall hazards at the proposed Natrium site from both mafic and silicic sources. To analyze these hazards, TerraPower used the ASHPLUME code with slight modifications referred to as ASHPLUME2 in the TR 6.
ASHPLUME2 is a code developed in support of the volcanic hazard analysis at the Yucca Mountain site and uses parameters derived from physical eruption characteristics to determine a probabilistic distribution of tephra fall thicknesses at a specific location. TerraPower stated that the use of ASHPLUME2 is appropriate for the proposed site due to the previous NRC approval for use of the code in licensing reviews and the code being developed consistent with the requirements in 10 CFR Part 5 0, Appendix B, Quality Assurance Criteria for Nuclear Power Plants and Fuel Reprocessing Plants.
TerraPower clarified the ASHPLUME2 modifications in response to several questions raised during the audit and provided a demonstration of the code to the NRC staff. To run the model, TerraPower used wind data from the National Oceanic and Atmospheric Administration and conservatively directed the plume to the site for each realization. Due to the computational limits in ASHPLUME2, a maximum of 1,000 realizations can be calculated per location. Therefore, TerraPower selected run intervals across the axis for each volcanic source to maximize these realizations, which resulted in different interval spacings for individual volcanic fields or by type of eruption (i.e., mafic or silicic). During the audit, TerraPower clarified that the selection of the different interval spacings for individual volcanic fields or by type of eruption ( i.e., mafic or silicic) was sufficient to capture the range of uncertainty and that changing the interval does not affect the results.
5.1 Tephra Hazards from Mafic Sources
For mafic tephra fall hazards, Table 12, Summary of tephra volumes for mafic eruptions in the site region, of the TR summarizes the mafic tephra volumes for the four potential mafic sources of tephra: Upper Wind River Basin, Leucite Hills Volcanic Field, Blackfoot Reservoir Volcanic Field, and the Eastern Snake River Plain. Table 25, Summary of conditional tephra-fall hazards at the proposed site, from mafic volcanoes in the site region, of the TR summarizes the overall results of the mafic tephra analyses performed for the four source fields and Table 26, Summary of exceedance probabilities for mafic tephra-fall hazards at the proposed site, from potential mafic volcanic eruptions in the site region, summarizes the exceedance probabilities for each source field. TerraPower determined that the average thickness for tephra fall hazards at the proposed site from mafic sources ranges from 0.001 to 0.019 c entimeters (cm) (0.0004 to 0.0075 inches). The largest t ephra hazard from mafic sources, just over 2 cm for the 95 th percentile thickness, comes from the Blackfoot Reservoir Volcanic Field more than 115 mi northwest of the proposed site. TerraPower determined a probability for this event of 6.0x10 -08 but did not consider the effect of this event on the overall probabilistic risk assessment (PRA) as that is beyond the scope of this TR.
6 Nuclear Regulatory Commission, 1997, ASHPLUME Version 1.0: U.S. Nuclear Regulatory Commission, CNWRA Software Control Documentation, 61 p., (ML040020193).
7 TerraPower also analyzed the potential impact of a debris flow from remobilized tephra fall deposits along the North Fork of the Little Muddy Creek drainage. TerraPower considered the frequency of extreme rainfall events, potential flow characteristics of flood events, the amount and source of particles suspended in typical floods, and potential for tephra to initiate debris flow conditions. From this information, TerraPower determined that mafic tephra falls within the basin do not have the potential to induce debris flow hazards at the proposed site.
5.2 Tephra Hazards from Silicic Sources
For silicic tephra fall hazards, Table 34, Estimates of tephra -fall volumes for silicic volcanoes in the site region, based on assumed relationship to dome and lava flow volumes, of the TR summarizes the silicic tephra volumes for the four potential silicic sources of tephra: the Blackfoot Reservoir Volcanic Field, Eastern Snake River Plain, Curlew Valley Volcanic Field, and the Yellowstone Volcanic Field. Table 50, Summary of conditional tephra-fall hazards at the proposed site, from silicic volcanoes in the site region, of the TR summarizes the overall results of the silicic tephra analyses performed for the silicic sources in the site region and Table 51, Summary of exceedance probabilities for silicic tephra-fall hazards at the proposed site, from potential silicic eruptions in the site region, of the TR summarizes the exceedance probabilities for each source. For tephra fall hazards from eruptions ranging from VEI 2 to 5, average deposits range from 0.001 to 0.034 cm. The largest tephra fal l hazard from a silicic source modeled in ASHPLUME2 was for the VEI 4 to 5 eruptions from the Yellowstone Volcanic Field, which yields a 95th percentile thickness of 11.9 cm with a probability of 1.7x10 -08. As with tephra hazards from mafic sources, although TerraPower calculated probabilities of event occurrence, TerraPower did not consider the effect of this event on the overall plant PRA.
To assess the tephra fall hazards from silicic events in the Yellowstone Volcanic Field with a VEI of 5 or greater, TerraPower referred to Mastin et al. 7, which used the Ash3D model to assess large VEI events occurring in the Yellowstone Volcanic Field. TerraPower reported that the average deposit thickness would be 84 cm with an annual exceedance probability of less than 5.0x10 -07 from a VEI 8 event occurring in the Yellowstone Volcanic Field.
TerraPower also assessed the potential for debris flows on the North Fork of the Little Muddy Creek from remobilized tephra deposits from silicic sources using the same approach used for mafic sources of tephra. TerraPower determined that following the deposition of 54 to 113 cm of tephra from an eruption in the Yellowstone caldera, the potential debris flow would not overtop the channels during a flooding event.
5.3 Tephra Hazard Conclusions
The NRC staff reviewed the methods used by TerraPower to assess the potential for tephra fall hazards from both mafic and silicic sources and the NRC staff concludes that the methods used by TerraPower are appropriate. Additionally, the NRC staff performed confirmatory calculations using the ASHPLUME2 code and input files. The NRC staff obtained similar results to those of TerraPower for the thickness of tephra deposits from the sources within the site region. The NRC staff concludes that the methods used by TerraPower to calculate tephra fall hazard thicknesses are consistent with the guidance in RG 4.26. Further, the results are appropriate
7 Mastin, L.G., Van Eaton, A.R., and Lowenstern, J.B., 2014, Modeling ash fall distribution from a Yellowstone supereruption: Geochemistry, Geophysics, Geosystems, v. 15, p. 3459-3475, doi:10.1002/2014GC005469.
8 inputs to any subsequent consideration of mitigating actions or design improvements to address the potential effects of these hazards on the proposed facility.
Although TerraPower assessed the probabilities associated with the tephra fall hazards in the site region, the plant-specifi c PRA information and the impacts of these events on the plant risk were not provided by TerraPower in the TR. Accordingly, the NRC staff are not making any conclusions related to the impacts of the volcanic hazard event probabilities on the plants cumulative risk metrics in this SE.
- 6. Tephra Fall Hazards at the Proposed Natrium Site
Section 8 of the TR presents the results for tephra fall hazards at the proposed site. Table 54, Combined mafic (Table 26) and silicic (Table 51) tephra-fall hazards from all potential volcanic sources in the proposed site region, of the TR summarizes the combine d mafic and silicic hazards from all potential sources of tephra hazards in the site region. The average tephra fall thickness is 0.12 cm with a probability of exceedance of 5.6x10 -06 /year. Except for a VEI 8 eruption from Yellowstone, the average deposit thickness ranges from 0.001 to 0.034 cm with average probabilities ranging from 2.1x10 -07 to 8.3x10-09. TerraPower determined that the tephra fall hazards at the site are a potential beyond-design-basis event.
The NRC staff reviewed TerraPowers summary of the tephra fall hazards at the proposed site and determined that the method used to determine the deposit thicknesses is consistent with RG 4.26 and the results are reasonable for the volcanic sources that screen in for further consideration at the Natrium site. Additionally, although the probabilities of occurrences were provided, the e ffects of these hazards on the overall plant risk were not discussed by TerraPower in the TR. Therefore, the NRC staff will defer conclusions related to the impact of volcanic hazard events on the plant risk until the PRA is available for the NRC staff to review.
Furthermore, although the NRC staff determined that the thickness of tephra deposits at the site is reasonable given the physical characteristics of the source volcanic systems, TerraPower did not provide information on the effect of a deposit of these thicknesses on the Natrium facility.
The NRC staff will defer its review of the overall effect of volcanic hazards on the performance of safety-related SSCs to a later date when this information is provided for NRC staff review.
- 7. Characteristics of Tephra Fall Hazards
Section 9 of the TR describes the characteristics of tephra fall hazards that may be of importance when considering the effects of the hazard on Natrium facility SSCs. TerraPower summarized the composition of volcanic tephra, particle sizes, thermal conductivity, thermal emissivity, deposit densities, tephra eruption characteristics, and post-eruption characteristics.
The composition of volcanic tephra can range in size from one micron up to six cm and is generally characterized as either mafic (46-58 weight percent (wt%) silica) or silicic (59-76 wt%
silica). TerraPower noted that due to the distance traveled from volcanic source to the proposed site, tephra deposited at the site is likely to have median particle sizes of less than 0.1 millimeter (mm) for mafic eruptions and between 0.02 to 0.4 mm for silicic tephra. The l argest VEI 8 eruptions from Yellowstone would yield median diameters between 0.1 and 0.35 mm.
TerraPower stated that the thermal conductivity of volcanic tephra will vary depending on composition and vesicularity but generally ranges from about 0.1 to 2 Watts per meter-Kelvin (W/(m K)), with higher conductivity for more silicic compositions.
9 TerraPower stated that the thermal emissivity can assist in the interpretation of eruption plumes in satellite imagery focused on infrared wavelengths from 4 to 30 micrometers ( µm) and will be higher for finer particle sizes. For the proposed site, TerraPower stated that the range of thermal emissivity will be higher for finer particle sizes. For wavelengths between 4 and 8 µm, the thermal emissivity increases from about 0.95 to 0.97, and decreases to about 0.75 for silicic and 0.9 for mafic in the wavelength range of six to 12 µm.
TerraPower stated that deposit densities are typically not measured at the distances at which tephra hazards would travel to the proposed site from the potential sources in the site region.
Additionally, due to the vesicularity of tephra deposits, the deposit density will increase when wet. For the proposed site, TerraPower estimated a dry density of 600 kilogram per cubic meter (kg/m 3) for silicic deposits, increasing to 1,280 kg/m 3 when wet for silicic deposits, and a dry density of 1,500 kg/m 3, increasing to 2,180 kg/m3 when wet for mafic deposits.
TerraPower also described tephra eruption characteristics, which will generally be preceded by a period of unrest in the volcanic system. This period of unrest may include increased seismic activity, ground deformation, or changes in water composition. TerraPower also discussed arrival time for tephra falls at the site, which are dependent on the eruption column height, and speed and direction of prevailing winds.
The NRC staff reviewed TerraPowers characterization of tephra fall hazards and the NRC staff determined that TerraPower provided sufficient information to characterize the tephra fall hazards at the proposed site near Kemmerer, Wyoming. The NRC staff further concludes that the characteristics of volcanic hazards as described in the TR are appropriate to use as inputs to further engineering analysis of the potentially affected SSCs in the Natrium reactor design and for consideration in assessing the potenti al need for mitigating actions in the event of an impending volcanic hazard arriving at the proposed site. Finally, the NRC staff concludes that the methods used by TerraPower in the volcanic hazards assessment to characterize the volcanic hazards at the proposed site are consistent with the guidance in RG 4.26 and GDC 2 and the requirements of 10 CFR 100.23(c), 10 CFR 100.23(d)(2) and 10 CFR 100.23(d)(4) with respect to the adequate consideration of hazards resulting from volcanic activity at a proposed site.
Although TerraPower described in general the characteristics of tephra fall hazards at the site in Section 9 and identified potential design improvements or mitigating actions in Section 10, TerraPower did not provide specific design enhancements or mitigating actions to be taken in the event of these volcanic hazards. Accordingly, the NRC staff will defer its conclusions regarding the adequacy of any design improvements or mitigating actions based on the potential impacts of volcanic hazards on the proposed facility to a time when this information is made available.
LIMITATIONS AND CONDITIONS
The NRC staff identified the following limitations and conditions applicable to any licensee or applicant referencing this TR:
- 1. The conclusions reached in this SE do not address the content provided in Section 10 of the TR. Thus, any licensee or applicant referencing this TR must evaluate specific design, mitigation, or monitoring actions required to mitigate the effects of volcanic
10 hazards at the site, including any monitoring requirements for notification of impending volcanic events.
- 2. The conclusions reached in this SE do not address the impacts of the calculated probabilities of volcanic hazard events on the cumulative plant risk. Thus, any licensee or applicant referencing this TR should evaluate the effect of volcanic hazards on the overall plant risk.
CONCLUSION
The NRC staff has completed its review of TR NAT-3226, Revision 0A. Based on its evaluation of the TR,the NRC staff finds that the TR, subject to the limitations and conditions discussed above, provides an acceptable characterization of the potential volcanic hazards at the proposed site near Kemmerer, Wyoming, to demonstrate compliance with the applicable regulatory requirements. Accordingly, the NRC staff concludes that volcanic hazards and associated characteristics determined for the proposed Natrium site near Kemmerer, Wyoming, are acceptable for use in subsequent considerations of the effect of these hazards on SSC performance and development of mitigating actions, should design enhancements or mitigating actions be necessary to ensure adequate performance under the volcanic hazard conditions.
Project Managers: Mallecia Sutton, NRR Stephanie Devlin-Gill, NRR Roel Brusselmans, NRR
Principal Contributors: Jenise Thompson, NRR Hanh Phan, NRR
Date:
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