ML19171A009
| ML19171A009 | |
| Person / Time | |
|---|---|
| Site: | NuScale |
| Issue date: | 06/20/2019 |
| From: | NRC |
| To: | NRC/NRO/DLSE/LB1 |
| References | |
| Download: ML19171A009 (4) | |
Text
NuScaleDCRaisPEm Resource From: Cranston, Gregory Sent: Thursday, June 20, 2019 7:38 AM To: Request for Additional Information Cc: Lee, Samuel; Chowdhury, Prosanta; Lupold, Timothy; Hansing, Nicholas; NuScaleDCRaisPEm Resource
Subject:
Request for Additional Information No. 524 eRAI No. 9691 (3.9.4)
Attachments: Request for Additional Information No. 524 (eRA No. 9691).pdf Attached please find NRC staffs request for additional information (RAI) concerning review of the NuScale Design Certification Application.
Please submit your technically correct and complete response by August 19, 2019, to the RAI to the NRC Document Control Desk.
If you have any questions, please contact me.
Thank you.
1
Hearing Identifier: NuScale_SMR_DC_RAI_Public Email Number: 575 Mail Envelope Properties (SN6PR09MB289673AFED47D4A3B94B41E690E40)
Subject:
Request for Additional Information No. 524 eRAI No. 9691 (3.9.4)
Sent Date: 6/20/2019 7:37:31 AM Received Date: 6/20/2019 7:37:35 AM From: Cranston, Gregory Created By: Gregory.Cranston@nrc.gov Recipients:
"Lee, Samuel" <Samuel.Lee@nrc.gov>
Tracking Status: None "Chowdhury, Prosanta" <Prosanta.Chowdhury@nrc.gov>
Tracking Status: None "Lupold, Timothy" <Timothy.Lupold@nrc.gov>
Tracking Status: None "Hansing, Nicholas" <Nicholas.Hansing@nrc.gov>
Tracking Status: None "NuScaleDCRaisPEm Resource" <NuScaleDCRaisPEm.Resource@nrc.gov>
Tracking Status: None "Request for Additional Information" <RAI@nuscalepower.com>
Tracking Status: None Post Office: SN6PR09MB2896.namprd09.prod.outlook.com Files Size Date & Time MESSAGE 357 6/20/2019 7:37:35 AM Request for Additional Information No. 524 (eRA No. 9691).pdf 111676 Options Priority: Standard Return Notification: No Reply Requested: No Sensitivity: Normal Expiration Date:
Recipients Received:
Request for Additional Information No. 524 (eRA No. 9691)
Issue Date: 06/20/2019 Application
Title:
NuScale Standard Design Certification 048 Operating Company: NuScale Power, LLC Docket No.52-048 Review Section: 03.09.04 - Control Rod Drive Systems Application Section: 3.9.4 QUESTIONS 03.09.04-13 10 CFR 50 Appendix A, General Design Criterion (GDC) 4, "Environmental and dynamic effects design bases,"
requires, in part, that structures, systems, and components important to safety (including the control rod drive system) be designed to accommodate the effects of and to be compatible with the environmental conditions associated with normal operation, maintenance, testing, and postulated accidents, including loss-of-coolant accidents.
GDC 26, "Reactivity control system redundancy and capability," requires, in part, a reactivity control system using control rods which shall be capable of reliably controlling reactivity changes to assure that under conditions of normal operation, including anticipated operational occurrences, and with appropriate margin for malfunctions such as stuck rods, specified acceptable fuel design limits are not exceeded.
GDC 27, "Combined reactivity control systems capability," requires, in part, the control rod drive system be designed to reliably controlling reactivity changes to assure that under postulated accident conditions and with appropriate margin for stuck rods the capability to cool the core is maintained. NuScale has proposed an exemption to this GDC, but their proposed PDC also contains this language.
GDC 29, "Protection against anticipated operational occurrences," requires, in part, the control rod drive system be designed to assure an extremely high probability of accomplishing its safety functions in the event of anticipated operational occurrences.
During the April 17, 2019 Advisory Committee on Reactor Safeguards (ACRS) subcommittee meeting (ML19114A107), several members inquired about unique environmental conditions for the control rod drive mechanisms, which are very similar in configuration to existing PWRs, but operate in different environmental conditions. While operating experience exists for existing PWR control rod drive mechanisms (CRDMs), they typically operate in a water solid environment, so NuScale's unique design, where the mechanisms operate in a borated steam environment and are cooled by cooling coils , introduce additional uncertainties. Specifically, an ACRS member inquired about the potential for chemical buildup formed by substances evaporating off the top of the pressurizer water level and this "goo" (as the ACRS member put it) preventing the rod from inserting into the core. Significant accumulation of particulates such as boric acid crystals around the moveable elements of the CRDM latch mechanism could inhibit the ability of the latches to release the control rod drive shafts and scram the reactor. This accumulation need not be limited to a single CRDM, so this may lead to a common cause failure of all control rod drive mechanisms.
RAI 8930, Question 15-27 considers a boron dilution event after Emergency Core Cooling System actuation, where boron volatility causes boron to plate out on the cooler metal surfaces of the in-vessel structures and upper portions of the containment vessel. Experimental studies have demonstrated that boron can volatize and be transported by steam, which can later be deposited on metal surfaces. See "Conclusions on Boron Precipitation Following a Large Break Loss of Coolant Accident," by K. Umminger, B. Schoen, and S.P. Schollenberger; "About the Volatility of Boron in Aqueous Solutions of Borates with Vapour in Relevance to BWR-Reactors," by S. Bohlke, C. Schuster, and A. Hurtado, and "Experimental Study of Solubility of Boric Acid in Steam at Boiling," by A V Morozov, et. al. Westinghouse has also studied the behavior of boric acid solutions at high temperatures in WCAP-3713, "Some Physicochemical Studies of Boric Acid Solutions at High Temperature," and LOCA-75-127, "Post LOCA Boric Acid Mixing Experiment." A similar phenomenon of boron plate-out could be postulated for the metal surfaces of the CRDM, which are cooled by the CRDM cooling coils. These cooler metal surfaces would be a preferential site for solid boron accumulation. The volatility of boric acid increases with temperature, so the high operating temperature in the pressurizer during normal operation could magnify this effect.
During the ACRS full committee meeting, held on 5/2/19, a NuScale representative indicated that the presence of noncondensable gases in the CRDM would displace any steam which could carry the previously mentioned "goo" into the CRDM for deposition. While noncondensable gases may be present, the necessary volume of these gases to displace this steam would be rather significant, given that the NuScale normal operating conditions of 1850 psia, 625 F (3.9.4.3) would compress the gases by nearly a factor of 70 when compared to conditions of 1 atm and 140 F (assumed ultimate heat sink temperature) using the ideal gas law. Additionally, because of the temperature differential between the components cooled by the CRDM cooling coils and the rest of the reactor coolant system, it is feasible that a driving force could push steam up into the housing, where it will mix with the noncondensable gases, condense, and recirculate back into the pressurizer. Additional justification is requested to support the mitigating effect of the noncondensable gases, as described in the ACRS full committee meeting. The applicant should provide information regarding how the presence of these noncondensable gases will prevent the accumulation of boric acid on the CRDM internals. Should this effect of noncondensable gases not prevent the accumulation of boric acid, the staff seeks information regarding additional mitigating factors to prevent this potential challenge to the safety-related function of the CRDMs or to demonstrate that the safety-related function will not be impacted by any such accumulation.
The staff seeks this additional information to provide assurance that the CRDMs will meet the applicable aspects of GDCs 4, 26, 27, 29, as discussed above. Specifically, the applicant should provide appropriate justification that the ability of the CRDMs to perform their safety-related function of dropping the control rods will not be adversely impacted by the accumulation of boric acid crystals.