ML19200A208
| ML19200A208 | |
| Person / Time | |
|---|---|
| Site: | NuScale |
| Issue date: | 07/19/2019 |
| From: | Rad Z NuScale |
| To: | Document Control Desk, Office of New Reactors |
| References | |
| RAIO-0719-66334 | |
| Download: ML19200A208 (8) | |
Text
RAIO-0719-66334 July 19, 2019 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.
524 (eRAI No. 9691) on the NuScale Design Certification Application
REFERENCE:
U.S. Nuclear Regulatory Commission, "Request for Additional Information No.
524 (eRAI No. 9691)," dated June 20, 2019 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 Question from NRC eRAI No. 9691:
03.09.04-13 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-8H12 Samuel Lee, NRC, OWFN-8H12 Marieliz Vera, NRC, OWFN-8H12 : NuScale Response to NRC Request for Additional Information eRAI No. 9691 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. 9691 NuScale Power, LLC 1100 NE Circle Blvd., Suite 200 Corvalis, Oregon 97330, Office: 541.360.0500, Fax: 541.207.3928 www.nuscalepower.com
Response to Request for Additional Information Docket No.52-048 eRAI No.: 9691 Date of RAI Issue: 06/20/2019 NRC Question No.: 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 NuScale Nonproprietary
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 NuScale Nonproprietary
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.
NuScale Response:
Non-condensable gas in the CRDMs and pressurizer The NuScale pressurizer is located at the top of the integrated reactor pressure vessel (RPV).
The control rod drive shafts pass through the pressurizer. The sixteen control rod drive mechanisms (CRDMs) are located at the top of pressurizer in the gas phase. The control rod drive shafts also pass through the baffle plate at the bottom of the pressurizer. The CRDMs have a housing that provides additional volume for the gas phase above the pressurizer. Part of this additional volume above the gas phase is cooled by heat exchangers cooled by the reactor component cooling water system, to ensure proper function of the CRDM electromagnetic coils.
The NuScale reactor coolant system utilizes dissolved hydrogen in the reactor coolant during power operations in accordance with the EPRI Primary Water Chemistry Guidelines. In order to start the plant, a liquid phase concentration of dissolved hydrogen of 25 cc/kg must be achieved. To add hydrogen to the reactor coolant, the CVCS adds gaseous hydrogen into the CVCS injection line. This hydrogen either dissolves in the liquid phase or rises to the pressurizer gas phase upon arriving in the reactor pressure vessel. A closed vessel with a gas phase and a liquid phase, like the RPV, obeys Henrys Law equilibrium for hydrogen and other dissolved NuScale Nonproprietary
gases. This means that to achieve a specific concentration of dissolved hydrogen in the reactor coolant, a specific partial pressure of hydrogen in the gas phase must also be maintained.
Steam can only exist at a partial pressure that is consistent with the local temperature or else it condenses. The cooled section of the CRDMs could be as cold as 100°F, which can only sustain a steam partial pressure of around one psia. Condensation of steam in this section of the CRDM lowers the pressure, drawing in more gas from the pressurizer. This process accumulates non-condensable gas, especially the lighter-than-steam hydrogen entrained in the pressurizer gas phase, in the CRDM volume. This process continues until either the CRDM volume is pure non-condensable gas (in which case flow into the CRDM from the pressurizer stops), or until the pressurizer gas phase has no non-condensable gas remaining (in which case flow continues as condensation falls out of the CRDMs to be replaced by steam). The latter option is not possible because if the pressurizer gas phase is depleted of non-condensable gas (hydrogen in particular), then the liquid phase is also depleted of hydrogen (following Henrys Law equilibrium). NuScale has committed to the EPRI Primary Water Chemistry Guidelines, which ensures that the operator must add hydrogen to the RCS until the minimum dissolved liquid phase concentration is achieved. This requires that the cooled portion of the CRDM volume has achieved a pure non-condensable gas state, such that it no longer extracts non-condensable gas from the pressurizer gas phase. This requires a significant addition of hydrogen gas prior to power operations.
Accumulation of Deposits at a Phase Interface A well-known phenomenon, visible in many industries and even in nature, is accumulation of dissolved solids at a phase interface. Some examples are a bathtub ring, or a salt deposit at the shore of a briny lake. In this phenomenon, a liquid-gas interface exists where the liquid contains dissolved solids and the gas phase is not saturated. Both of these conditions are necessary, because deposition requires evaporation at the phase interface and a substance to deposit. Air in an open space is almost never at 100% humidity (saturated with water vapor), so nearly any liquid exposed to air will see this effect.
The pressurizer has a phase interface between saturated liquid and saturated water vapor.
There is no evaporation at the surface of the pressurizer, only boiling at the pressurizer heaters.
There is no heat flux from the CRDM to the pressurizer liquid, so there is no driving force for evaporation. Even though the pressurizer liquid contains dissolved material, there will be no deposition or accumulation without sustained evaporation or boiling. Because of this arrangement, the control rod drive shafts in the pressurizer are not expected to accumulate deposits at the phase interface.
NuScale Nonproprietary
Transport of Boric Acid in the Gas Phase It is the NuScale position that there is no mechanism for volatilized boron contained in a saturated steam to be deposited on a cooled metal surface. It is documented that when water containing boric acid is vaporized, a fraction of the boric acid in the vaporized liquid becomes volatilized. The weight concentration of boric acid contained in the steam is typically less than 10% of the weight concentration of the boric acid solution prior to vaporization. See "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. Assuming the saturated steam comes in contact with a cooled surface, the vapor condenses to a liquid, and the boric acid previously volatilized in the condensing steam becomes dissolved in the liquid condensate. The condensate has a boron concentration much less than the original liquid, and thus no solidification occurs as the condensate is well below the boron solubility limit. While at power, the primary boron concentration is limited to less than 1800 parts per million (ppm). Assuming a boron volatility fraction of 10%, when the steam condenses the condensate would have a boron concentration of 180 ppm. The solubility limit for boron is greater than 4500 ppm for water at 32 degrees F and only increases with temperature. Given the operational limit on boron concentration, condensate would freeze on the cooling coils prior to solidifying, due to reaching the boron solubility limit.
Of the references provided in the RAI question, only "Conclusions on Boron Precipitation Following a Large Break Loss of Coolant Accident," by K. Umminger, B. Schoen, and S.P.
Schollenberger and LOCA-75-127, "Post LOCA Boric Acid Mixing Experiment." have documented examples of solid boron deposition occurring. From these tests, for boron solidification to occur, the local concentration must be at or above the solubility limit. The mechanisms for boron solidification are discussed at length in WCAP-17047-NP Phenomena Identification and Ranking Tables (PIRT) for Un-Buffered/Buffered Boric Acid Mixing /Transport and Precipitation Modes in a Reactor Vessel During Post-LOCA Conditions which summarizes the phenomena observed in LOCA-75-127 and other tests. Additionally, "Conclusions on Boron Precipitation Following a Large Break Loss of Coolant Accident," by K. Umminger, B. Schoen, and S.P. Schollenberger specifically mentions that the solidification observed in the steam generator tubes was the result of entrained borated liquid droplets vaporizing on a heated surface.
There is no mention in either document of a boron solidification mechanism from the condensation of borated steam, and instead there is discussion of condensate re-dissolving solidified boron that came into contact that had plated out from other mechanisms. Furthermore, NuScale Nonproprietary
it is well understood that the boron in the steam is retained in newly formed liquid when the steam condenses. This premise is the basis for a group of methods for removing boron from radioactive waste streams as described in IAEA-TECDOC-911 Processing of nuclear power plant waste streams containing boric acid.
Summary The staff has documented several issues regarding mechanisms for deposition of boric acid on the control rod drive mechanisms or shafts in the gas space. Deposition of solid boric acid at the phase interface is not expected because there is no driving force for surface evaporation in the saturated pressurizer environment. Deposition of solid boron on cold surfaces, such as the pressurizer walls of the cooled CRDM portions, is not expected because gas phase transport and deposition of solid boric acid can only occur when saturated droplets of boric acid solution are entrained in the gas phase and encounter a surface. In the NuScale design, there is no entrainment of droplets, and the source boric acid solution is well below saturation conditions.
Volatilized boric acid in the gas phase condenses on cold surfaces in the presence of steam, producing a boric acid solution that is less than one tenth the concentration of the source solution. Accordingly, boric acid deposition on the CRDMs or shafts that could impede the safety function of the control rods is not expected.
Impact on DCA:
There are no impacts to the DCA as a result of this response.
NuScale Nonproprietary