RAIO-1217-57528, LLC Response to NRC Request for Additional Information No. 250 (Erai No. 9191) on the NuScale Design Certification Application
| ML17341B188 | |
| Person / Time | |
|---|---|
| Site: | NuScale |
| Issue date: | 12/07/2017 |
| From: | Rad Z NuScale |
| To: | Document Control Desk, Office of New Reactors |
| References | |
| RAIO-1217-57528 | |
| Download: ML17341B188 (8) | |
Text
RAIO-1217-57528 NuScale Power, LLC 1100 NE Circle Blvd., Suite 200 Corvalis, Oregon 97330, Office: 541.360.0500, Fax: 541.207.3928 www.nuscalepower.com December 07, 2017 Docket No.52-048 U.S. Nuclear Regulatory Commission ATTN: Document Control Desk One White Flint North 11555 Rockville Pike Rockville, MD 20852-2738
SUBJECT:
NuScale Power, LLC Response to NRC Request for Additional Information No.
250 (eRAI No. 9191) on the NuScale Design Certification Application
REFERENCE:
U.S. Nuclear Regulatory Commission, "Request for Additional Information No.
250 (eRAI No. 9191)," dated October 13, 2017 The purpose of this letter is to provide the NuScale Power, LLC (NuScale) response to the referenced NRC Request for Additional Information (RAI).
The Enclosure to this letter contains NuScale's response to the following RAI Questions from NRC eRAI No. 9191:
06.02.05-7 06.02.05-8 06.02.05-9 This letter and the enclosed response make no new regulatory commitments and no revisions to any existing regulatory commitments.
If you have any questions on this response, please contact Marty Bryan at 541-452-7172 or at mbryan@nuscalepower.com.
Sincerely, Zackary W. Rad Director, Regulatory Affairs NuScale Power, LLC Distribution:
Gregory Cranston, NRC, OWFN-8G9A Omid Tabatabai, NRC, OWFN-8G9A Samuel Lee, NRC, OWFN-8G9A : NuScale Response to NRC Request for Additional Information eRAI No. 9191 Za Zackary W. Rad Director Regulatory Affairs
RAIO-1217-57528 NuScale Power, LLC 1100 NE Circle Blvd., Suite 200 Corvalis, Oregon 97330, Office: 541.360.0500, Fax: 541.207.3928 www.nuscalepower.com :
NuScale Response to NRC Request for Additional Information eRAI No. 9191
NuScale Nonproprietary Response to Request for Additional Information Docket No.52-048 eRAI No.: 9191 Date of RAI Issue: 10/13/2017 NRC Question No.: 06.02.05-7 10 CFR Part 50.44, Combustible gas control for nuclear power reactors subpart (c),
Requirements for Future Water-Cooled Reactor Applicants and Licensees, requires in part that all equipment required to establish safe shutdown and ensure containment function must have the capability to withstand a hydrogen burn and detonation resulting from an amount of hydrogen up to and including that generated following a fuel clad-coolant reaction involving 100 percent of the fuel cladding, unless such events can be shown to be unlikely to occur. The equipment must retain its function during and after exposure to the conditions resulting from the hydrogen burn during design-basis and significant beyond design basis accidents.
Equipment must be demonstrated to survive environmental conditions including but not limited to temperature and pressure resulting from a hydrogen burn. Although the temperature effects of combustion are explored in Section 3.3.5.6 of the combustible gas control technical report (TR-0716-50424), no provisions or discussion related to the effect of short duration temperature pulses for equipment other than steel exist. The equipment required to function includes both the steel containment and the containment penetrations, which vary in material composition.
Additionally, the calculation performed by NuScale examines the effect of temperature increases due to hydrogen combustion events at only a very high level. Preliminary calculations performed by the staff indicate that mixtures closer to stoichiometric than those examined by NuScale in the TR can produce highly detonable conditions capable of producing a very short term temperature spike. The current equipment qualification envelope defined by NuScale is either a combustion temperature increase of 75 F (as defined in FSAR Tier 2 Section 19.2.3.3.8) or 300 F (as defined in Section 3.3.5.6 of the TR), and both are stated to remain within the bounds of the containment design parameters. Accordingly, the staff requests that NuScale provide a justification or additional discussion in the TR for equipment survivability in the event of a hydrogen combustion (up to a potential detonation) due to temperature effects for components required to maintain containment integrity, including penetrations, and to reconcile the existing equipment survivability limits for temperature in the FSAR/TR.
NuScale Response:
Section 3.3.5.6 of TR-0716-50424 describes an analysis that considers the standard net heat of
NuScale Nonproprietary combustion of hydrogen. The analysis examines the steel of the RPV, CNV, and pipes in the containment because this material dominates the energy absorption from the containment atmosphere after a combustion event due to its large surface area and heat capacity. Small components of different compositions will not significantly affect the distribution of temperature in the containment.
NuScale's basis for not analyzing these small components is that they will see a heat-up no greater than the comparable mass of material on the surfaces of the materials included in the analysis. For example, evaluating 1% of the mass of the steel is akin to modeling the heat-up of a thin skin of the steel components while assuming an adiabatic interior of the steel structures.
The thickest large components are the RPV shells, which are approximately 5 thick. A skin that is 1% of the thickness (1% of the mass) on one side of the shell is therefore about 0.05 thick.
Heat transfer coefficients on all metallic surfaces inside the CNV are similar and the containment atmosphere is well mixed, meaning energy deposition from a CNV atmosphere with elevated temperature is approximately proportional to surface area. The components in the CNV are all metallic; no organic materials are permitted. This implies that thermal conductivities are likewise similar to the steel that is evaluated. Therefore, no component in the CNV will experience an average temperature increase in an outer 0.05 skin that significantly exceeds 300°F for the combustion event discussed in Section 3.3.5.6 of TR-0716-50424, Combustible Gas Control. This event bounds all combustion events within 72-hours of event initiation and the characteristics of the combustion front (detonation vs. deflagration) do not affect this finding.
The smallest and thinnest components in the CNV are much thicker than 0.05, meaning their average temperature increase through the thickness will be lower than 300 °F and thus remain below the design temperature for the component (550°F or 650°F). Some examples of small components in the CNV are mineral insulated cable, valve bonnets, hydraulic tubing for the ECCS, and nuts for pressure bolting of small components (such as valves). All of these components are substantially larger than 0.05 and thus will see average temperature increases that are a small fraction of 300°F. With starting temperatures of 250°F, a 300°F increase in temperature remains below the design temperature for components in the CNV.
Impact on DCA:
There are no impacts to the DCA as a result of this response.
NuScale Nonproprietary Response to Request for Additional Information Docket No.52-048 eRAI No.: 9191 Date of RAI Issue: 10/13/2017 NRC Question No.: 06.02.05-8 10 CFR Part 50.44, Combustible gas control for nuclear power reactors subpart (c),
Requirements for Future Water-Cooled Reactor Applicants and Licensees, requires in part that all equipment required to establish safe shutdown and ensure containment function must have the capability to withstand a hydrogen burn and detonation resulting from an amount of hydrogen up to and including that generated following a fuel clad-coolant reaction involving 100 percent of the fuel cladding, unless such events can be shown to be unlikely to occur. The equipment must retain its function during and after exposure to the conditions resulting from the hydrogen burn during design-basis and significant beyond design basis accidents.
The calculated deflagration-to-detonation transition (DDT) pressure in the combustible gas control technical report (TR-0716-50424) is based on a calculated base pressure assuming the entire containment free volume is available. In the event of a severe accident of the nature examined in the TR, containment is unlikely to have no water in it, which would confine the available volume and therefore increase the base containment pressure. Staff recognizes that the water that leaves the reactor will allow for gas and vapor space within the reactor vessel, but based on initial scoping studies performed by the staff there does not appear to be a proportional relationship between the water in the vessel and the additional vapor space in the reactor vessel on the resulting calcuated pressure. Staff requests that NuScale justify why the base pressure calculated as an input using an empty containment vessel is representative of expected conditions in containment, rather than partially filled with condensed water.
NuScale Response:
The volume used to determine the initial pressure for combustion events is equivalent to the CNV internal volume, however it is not intended to be considered as the actual containment with no liquid. Rather, it is an estimate of the total available vapor space volume for the two vessels.
Initially, the total gas phase volume for the two vessels (the RPV plus the CNV) is equal to the containment volume and the pressurizer gas space volume combined. As the event progresses, the liquid cools. This results in a reduction of the liquid phase volume due to increased liquid phase density of 22% (for a reduction in temperature from the initial RCS temperature to 250°F).
NuScale Nonproprietary At 250°F with an initial pressurizer level of 68%, the total vapor space volume for the two vessels is about 1.1 times the containment free volume. The limiting combustion events (i.e., the events with the highest resulting pressure and temperature) analyzed in TR-0716-50424, Combustible Gas Control, occur at relatively high pre-combustion pressure with a small differential pressure between the RPV and CNV. Using the containment free volume as opposed to the predicted total vapor space (1.1 times the containment free volume) conservatively compensates for pressure in the CNV being lower than pressure in the RPV, as the differential pressure is less than 10% of the absolute pressure for the limiting combustion cases.
For combustion events that initiate at low pressure, the containment volume does not represent the available gas volume as well as it does in the higher pressure cases because the non-condensable gases are not as evenly distributed between the two vessels. However, such low pressure cases have not been found to be limiting with regard to the impulse load applied by the combustion front, so the higher pressure cases discussed above conservatively bound the effects of low pressure cases.
Impact on DCA:
There are no impacts to the DCA as a result of this response.
NuScale Nonproprietary Response to Request for Additional Information Docket No.52-048 eRAI No.: 9191 Date of RAI Issue: 10/13/2017 NRC Question No.: 06.02.05-9 10 CFR 50.44(c) requires that applicants for design certification applications demonstrate that equipment required to maintain safe shutdown and containment structural integrity remain capable of performing its function following the burning of hydrogen, and that a structural analysis be performed that demonstrates containment structural integrity following an accident that releases hydrogen generated from a 100 percent fuel clad-coolant reaction and subsequent hydrogen burn.
In TR-0716-50424, NuScale states the scope of the TR is limited to the first 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> because accumulation of combustible gases beyond 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> can be managed by licensee implementation of severe accident management guidelines. This is a reasonable summary of the NRC position with respect to combustible gas control, but that view is in the context that for traditional reactor designs, long term hydrogen concentrations are generally no worse than those considered early in the accident (as stated in FR Vol 68, No. 179, pertaining to 50.44 rulemaking). For NuScale, the inventory of combustible gases increases in a near-linear fashion as the transient progresses, instead of being largely dependent on the hydrogen release from the fuel clad-coolant reaction. From that perspective, staff requests that NuScale provide a justification why the implementation of mitigating actions after 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> will not result in a combustible gas inventory more severe than that analyzed during the first 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />.
NuScale Response:
The risk-informed revision of 10 CFR 50.44 was aimed at reducing the risk of combustion challenges leading to radiological release up to "approximately 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />" following a core damage event, in order to implement the Commission's Safety Goal Policy (see SECY-00-0198,, Feasibility Study for a Risk-Informed Alternative to 10 CFR 50.44, Standards for Combustible Gas Control System in Light-water-cooled Power Reactors, page 2-3). In recommending that the rulemaking proceed, NRC Staff recommended that the postulated "combustible gas source term...would only address challenges to the containment that could potentially result in a large radionuclide release within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />." (SECY-00198, pg. 7.)
Accordingly, Staff also recommended "that long-term (more than 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />) control of combustible gas be included as part of the licensees Severe Accident Management Guidelines
NuScale Nonproprietary (SAMG) since combustible gases still pose a challenge to containment integrity in the long term." (Id., pg. 8.) Therefore, NuScale's approach is consistent with the overall intent of 10 CFR 50.44 for all designs.
As noted in the Statements of Consideration, BWR Mark I and Mark II containments were specifically considered under the risk-informed 10 CFR 50.44 rulemaking (68 FR 54126).
Those designs are required to be inerted, and are thus oxygen-limited, to prevent hydrogen combustion. The rulemaking concluded that, though these designs "can be challenged beyond 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> by the long-term generation of oxygen through radiolysis," additional mitigating systems were not justified based on low risk significance. For these designs, "combustible gases beyond 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> can be managed by licensee implementation of the severe accident management guidelines (SAMGs) or other ad hoc actions because of the long period of time available to take such action." Likewise, the rulemaking study noted that for existing large and subatmospheric containments, which were considered likely to withstand hydrogen combustion events without severe accident combustible gas control, "the possibility exists for the accumulation of significant quantities of combustible gases (H2 and CO) in the long term (i.e., after several days), which should be considered during implementing accident management strategies. (SECY-00-0198, Attachment 2, Feasibility Study for a Risk-Informed Alternative to 10 CFR 50.44, Standards for Combustible Gas Control System in Light-water-cooled Power Reactors, page 4-12).
The NuScale design is a small containment design in which combustion is oxygen-limited rather than hydrogen-limited. In this regard, the NuScale contaiment is similar to BWR Mark I and Mark II containments. In hydrogen-limited designs the combustible gas concentrations do not significantly increase after the initial clad-coolant reaction stage of a severe accident. In oxygen-limited designs, the combustible gas concentrations continue to increase indefinitely.Though combustion events are evaluated in Chapter 6 of the NuScale FSAR, Chapter 19 demonstrates that these events are rare and are not significant contributors to plant risk, including beyond 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />.
Design features that support potential mitigating actions of venting and nitrogen addition are described in Section 2.8 of TR-0716-50424, Combustible Gas Control. Because the analysis methodology calculates the maximum optimized combustion load for the amount of oxygen present in the reactor, only the addition of oxygen could result in a combustible gas inventory more severe than analyzed. The Severe Accident Management Guidelines have not been developed, but they are expected to make use of these design features, which do not include the addition of oxygen. Therefore, mitigating actions are not expected to result in a combustible gas inventory more severe than that analyzed.
Impact on DCA:
There are no impacts to the DCA as a result of this response.