ML22033A245
| ML22033A245 | |
| Person / Time | |
|---|---|
| Site: | North Carolina State University |
| Issue date: | 02/02/2022 |
| From: | North Carolina State University |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| EPID L-2017-RNW-0026 | |
| Download: ML22033A245 (6) | |
Text
RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION RE: LICENSE RENEWAL APPLICATION FOR FACILITY OPERATING LICENSE NO. R-120 FOR THE NORTH CAROLINA STATE UNIVERSITY PULSTAR RESEARCH REACTOR (EPID NO. L-2017-RNW-0026); DATED AUGUST 23, 2021 NORTH CAROLINA STATE UNIVERSITY LICENSE NO. R-120; DOCKET NO. 50-297 FEBRUARY 2, 2022 The Pool Inlet Break and Pool Outlet Break Loss-of-Coolant Accident scenarios performed by NCSU for their PULSTAR reactor safety analysis drain the reactor pool and result in air cooling of the fuel.
The safety analysis did not consider the effect of air oxidation in the calculations. Breakaway oxidation of Zirconium alloys in air is a significant effect and causes thermal runaway from lower cladding temperatures than oxidation in steam. The thermal runaway leads to severe fuel damage.
(a) Perform calculations and safety analysis of the Pool Inlet Line break that include the effect of oxidation in air (including breakaway oxidation) with and without any mitigative measures or assumptions to restrict operations (and decay heat) that demonstrate that the fuel does not suffer damage that exceeds the assumptions used in the maximum hypothetical accident.
(b) Alternately, explain why an analysis of oxidation in air is not required to be considered and describe the mitigative features that preclude the effect of oxidation in air (including breakaway oxidation), and update the model and safety analysis for steam oxidation.
Response
This response addresses item (b) above and details proposed mitigative features (i.e. the use of an Emergency Core Cooling System, i.e., ECCS) that will preclude the effect of oxidation in air, as well as the presentation of an updated model and safety analysis for steam oxidation.
- 1. RELAP ECCS Model Description This model details the results of the most limiting loss of coolant accident (pool inlet case) assuming the installation of ECCS capability. In accordance with the requirements of 10 CFR 50.46 the peak cladding temperature (PCT) cannot exceed a maximum of 2200°F. The 2 MW PULSTAR ECCS system model was developed and simulated using RELAP5/Mod3.3 [1].
This model tests the following cases: a) Verification of the 2 MWth Pool Inlet LOCA results obtained by NRC with steam oxidation/water metal reaction activated; and b) Pool Inlet LOCA results given the installation of an ECCS spray system and the reactor is operating at 2 MWth. To scope the design parameters for the ECCS spray system, a series of parametric studies have been performed for a pool inlet LOCA to develop appropriate performance criteria including i) spray system coolant insertion location, ii) spray system initiating time, and iii) spray system mass flow rate. Initial design parameters for an ECCS are presented in Section 2.
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a) Pool Inlet LOCA Verification Results with Water Metal Reaction Figure 1 provides the RELAP models results, including steam oxidation effects, which are consistent with the NRC confirmatory results for the clad surface temperature at 1 MWth and 2 MWth during a pool inlet LOCA. The results show that the cladding surface temperature exceeds the PCT limit of 2200°F when operating at 2 MWth with the water metal reaction option activated, however it does not in the 1 MWth case.
Figure 1: Verification of the results obtained by NRC with the water metal reaction activated.
b) Parametric Studies of an Emergency Core Cooling System for a Pool Inlet LOCA A spray safety system is modeled in the RELAP5 input deck to push water at a specified mass flow rate into the reactor pool when the pool water level goes below 5 meters (196.85 inches). The simulation is run for 10,000 seconds. Figure 2 shows a simplified nodalization as implemented in the PULSTAR RELAP5 input deck where the spray system is shown connected to pipe 120, which represents the upper middle part of the pool.
Figure 2: PULSTAR RELAP5 input simplified nodalization.
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Results of the Parametric Study on Spray System Initiating Time for a Pool Inlet LOCA The spray system was tested after initiating a pool inlet LOCA at time 0.0 sec and reaching a pool water level of 5 meters (196.85 inches). As an example, Figure 3 shows pool water level and spray system flow rate being initiated after 1000.0s from reaching the 196.85 inch water level, thus validating the function of the modeled spray system.
Figure 3: Water level and spray system flow rate initiated after 1000.0s.
Figure 4 gives the results for the clad surface temperature for the hottest pin with the initiation of a 40 GPM spray system after 60, 200 and 800 second delays following reaching the 5m water level. The results show that the spray system can successfully cool the reactor in all cases. Based on conservative initiating conditions, the spray system may be actuated up to 500s following event initiation and the clad surface temperature will reach nearly 1000 °F, which is well below the 820°C (1508°F) limit for air cooling.
Figure 4: Clad surface temperature results for initiating the spray system following delays of 60, 200 and 800 seconds at a flow rate of 40 GPM.
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Results of the Parametric Study on Spray System Mass Flow Rate with the Spray System Available Indefinitely for a Pool Inlet LOCA After verifying the initiation of the spray system, a parametric analysis was performed at different ECCS flow rates. Figure 5 gives the results for the clad surface temperature as a function of ECCS flow rate. The results indicate that the PCT limit of 2200°F is not exceeded at all flow rates of greater or equal to 20 GPM.
Cooling the fuel down to ambient temperatures may be achieved rapidly with ECCS flow rates of greater than 30 GPM.
Figure 5: Clad surface temperature at different flow rates.
ECCS Design Study Conclusions This preliminary analysis demonstrates a spray system delivering water to the PULSTAR core with mass flow rates in the range of 20 to 50 GPM following initiation times of 60 to 800 seconds after the pool inlet break. This spray system would be capable of maintaining the PULSTAR fuel PCT below the 10CFR50.46 limit of 2200°F under conservative assumptions and with considerable margin.
In addition, it was confirmed that the Maximum Hypothetical Accident (MHA) for the NCSU PULSTAR remains fuel damage associated with a cladding failure accident (CFA) postulated to occur from mishandling or mechanical shock to a fuel assembly. The design basis accident for the release of fission products is the failure of all 25 pins in one fuel assembly with a maximum burnup of 20,000 MWd/MTU.
This fuel cladding failure accident results in the greatest potential for significant radiation doses to personnel and the public and remains the MHA for the PULSTAR Reactor. The worst case pool inlet LOCA as mitigated by an ECCS precludes fuel damage and is therefore bounded by the CFA MHA.
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- 2. Implementation and Design Aspects of an Emergency Core Cooling System (ECCS) at the NCSU PULSTAR Based on the results of the model discussed above, an ECCS will be designed with the following assumptions and inputs:
a) The campus potable water supply will be utilized as the primary source for suppling the required coolant. The campus water supply will be configured to provide greater than or equal to 30 GPM for the ECCS system.
b) A secondary backup to the campus potable water supply will be the City of Raleigh Fire Departments (RFD) ability to provide a 500 gallon tanker pump truck to the reactor site within 5 minutes, and an 1800 gallon tanker truck to the reactor site within less than 10 minutes. Additional tanker trucks (a Tanker Task Force) would be continuously supplied at one every ten minutes as long as the backup water supply was required.
c) The ECCS core spray system will be manually initiated if the pool level drops greater than 4 feet below full (269.5 inches or 6.85 meters from the pool bottom) and is dropping at a rate that is greater than the maximum service water make up rate (10 GPM).
d) Based on the response times for operator actions and RFD response, it is assumed that ECCS coolant will be provided within 500 seconds of an indication of an Emergency Pool Level at -4 feet below full.
e) The ECCS system will provide adequate cooling to maintain the fuel cladding temperature below the limits given in 10 CFR 50.46.
The PULSTAR ECCS will consist of a water supply routed to a spray header located within the reactor pool above the reactor core with spray distribution nozzles directing coolant into the top of each fuel assembly (see Fig. 6). The spray header would be located at an elevation within two to four feet above the top of the core, depending on the spray pattern and alignment of the coolant nozzles. The coolant water will be supplied with redundancies based on the assumptions stated above. Connections to the campus water supply will be supplemented with an auxiliary fire department connection allowing RFD pumper and tanker trucks to connect and supply water to the ECCS.
Figure 6: Schematic of an ECCS Spray Distribution Header at top of PULSTAR Core.
The reactor currently scrams automatically if the Pool Level Channel indicates a water level of 3 feet below full, causing the primary pump to trip off and an alarm to annunciate on the reactor console. The Pool Level Channel would be configured with a new trip so that if the level dropped to 4 feet below full (16 feet above the core), an Emergency Pool Level alarm would trip, sounding an annunciation on the reactor console alerting reactor personnel to verify the loss of pool water and to manually initiate ECCS flow. As an additional protective feature, the new Emergency Pool Level trip actuation would also automatically Nuclear Reactor Program 5 l6
cause pool isolation valves P-1 and P-5 to close via new motor actuators. Immediate operator actions following receipt of an Emergency Pool Level alarm would be to a) immediately request tanker water supply service from the RFD via calling 911, and b) manually start ECCS flow from the campus supply within 100 seconds by opening two supply isolation valves to be located in the reactor bay.
Automatic initiation of the ECCS is being explored as a redundant safety feature in combination with manual operator activation. Automatic initiation of the spray system would occur in the event operator action was delayed due to human factor considerations. Aspects being considered include redundant pool level measurement channel initiating trips and solenoid actuated coolant supply valves, and mitigation of single channel failure modes.
- 3. References
- 1. NUREG/CR-5535, RELAP5/MOD3.3 Code Manual Volume I: Code Structure, System Models, and Solution Methods, August 1995, NRC Accession Number: ML110330200 https://www.nrc.gov/docs/ML1103/ML110330200.pdf Nuclear Reactor Program 6 l6