ML23345A111
| ML23345A111 | |
| Person / Time | |
|---|---|
| Issue date: | 02/08/2024 |
| From: | Martinez-Navedo T Plant Licensing Branch II |
| To: | Saporito T Nuclear Energy Oversight Project |
| Buckberg P | |
| References | |
| EPID L-2023-CRS-0006 | |
| Download: ML23345A111 (1) | |
Text
February 8, 2024
Thomas Saporito Executive Director Nuclear Energy Oversight Project 6526 S. Kannar Hwy. Unit 235 Stuart, FL 34997
Dear Thomas Saporito:
Your petition dated August 27, 2023 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML23264A854), regarding hydrogen mitigation at NRC licensed nuclear reactors was referred to the Office of Nuclear Reactor Regulation pursuant to Title 10 of the Code of Federal Regulations (10 CFR) Section 2.206, Requests for action under this subpart, of the U.S. Nuclear Regulatory Commissions (NRC) regulations. Your petition requested that the NRC take enforcement action against all licensees of commercial boiling water reactors and pressurized water reactors including specific actions against licensees of the AP1000 reactors under 10 CFR 2.206.
In your petition, you requested the following:
Require the licensees to develop and validate computer safety models capable of conservatively predicting rates of hydrogen generation in severe accidents Require the licensees to assess the safety of existing hydrogen re-combiners and discontinue the use of passive autocatalytic recombiners (PARs)
Require licensees to significantly improve existing oxygen and hydrogen monitoring instrumentation.
Require licensees to upgrade current core diagnostic capabilities in order to better signal to plant operators the correct time to transition from emergency operating procedures to severe accident management guidelines Require licensees of AP1000 reactors to ascertain and calculate the probability that the phenomenon of hydrogen deflagration to detonation transition could occur below the hydrogen concentrations of 10.0 volume percent Require licensees to conduct actual and real-time testing of the AP1000 reactor passive cooling system
You provided the following concerns as basis and justification:
The licensees' and NRC's existing computer safety models appear to under predict the rates of hydrogen generation that occur in severe accidents.
The problem of passive autocatalytic recombiners incurring ignitions in elevated hydrogen concentrations remains an unresolved safety issue.
T. Saporito
After the onset of a severe accident, hydrogen monitors should be functional within 1 minute of the injection of coolant water into the reactor vessel.
The probabilistic risk assessment for the AP1000 erroneously claims that it would not be possible for a hydrogen detonation to occur in the AP1000s containment if the hydrogen concentration were less than 10.0 volume percent. A hydrogen detonation could compromise the containment and thus cause a large radioactive release.
To the extent that the AP1000 reactor passive cooling system is a new and unproven technology, it is paramount that licensees conduct actual and real-time testing of this passive cooling system to provide reasonable assurance to the NRC and to the public that the design is valid and can be operational for extended periods of time.
Consistent with NRC Management Directive 8.11, Review Process for 10 CFR 2.206 Petitions (ML18296A043), the NRC staff informed you that the petition screened in on October 25, 2023, and the NRC established a petition review board (PRB) to evaluate your petition. The PRB consists of NRC staff who are knowledgeable of the NRCs evaluations of hydrogen control and AP1000 engineering safety features. In evaluating your petition, the PRB reviewed the NRCs records regarding the issues you raised in your petition.
On November 30, 2023, the petition manager informed you by email of the PRBs initial assessment to not accept your petition for review (ML23334A199) and offered you an opportunity to meet with the PRB to clarify or supplement your petition. The November 30, 2023, email included the following responses to the concerns in your petition.
In SECY-16-0041, Closure of Fukushima Tier 3 Recommendations Related to Containment Vents, Hydrogen Control, and Enhanced Instrumentation, March 31, 2016 (ML16049A079) the NRC provided a high-level summary of the studies and evaluations related to hydrogen control, including studies issued in September of 2003, Federal Register notice for the final rule, Combustible Gas Control in Containment, (68 FR 54123) that supported requirements found in 10 CFR 50.44, Combustible gas control for nuclear power reactors. In SECY-16-0041, the NRC discusses hydrogen-related issues that have been addressed in major studies, such as those documented in NUREG-1150, Vol 1, Severe Accident Risks: An Assessment for Five U.S. Nuclear Power Plants, (ML120960691) and NUREG-1935, State-of-the-Art Reactor Consequence Analyses (SOARCA) Report (ML12332A053).
The NRC, in SECY-16-0041, documented that existing NRC requirements and programs undertaken by licensees addressed the risks to public health and safety from hydrogen generation during severe accidents; therefore, additional requirements would not provide a substantial safety improvement.
Westinghouse AP1000 Design Control Document Rev.19 - Tier 2, Chapter 6 - Engineering Safety Features, June 13, 2011 (ML11171A458) addresses your concerns regarding a hydrogen detonation to occur in the AP1000s containment with hydrogen concentration less than 10.0 volume percent, the PARs, and AP1000 reactor passive cooling system. AP1000 Design Control Document Containment System includes:
T. Saporito
Section 6.2.4, Containment Hydrogen Control System, discusses the hydrogen concentration monitoring and hydrogen control during and following a degraded core or core melt scenarios (provided by hydrogen igniters).
Section 6.2.4.1.1, Containment Mixing, states that passive autocatalytic recombiners act to recombine hydrogen and oxygen on a catalytic surface in the event of a hydrogen release to the containment. The enthalpy of reaction generates heat within a passive autocatalytic recombiner, which further drives containment mixing by natural circulation. Catalytic recombiners reduce hydrogen concentration at very low hydrogen concentrations (less than 1 percent) and very high steam concentrations and may also promote convection to complement passive c ontainment cooling system natural circulation currents to inhibit stratification of the containment atmosphere. The implementation of passive autocatalytic recombiners has a favorable impact on both containment mixing and hydrogen mitigation.
Section 6.2.4.1.2, Validity of Hydrogen Monitoring, discusses monitoring hydrogen concentrations of various locations within the containment.
Section 6.2.4.1.3, Hydrogen Control for Severe Accident, states that the containment hydrogen concentration is limited by operation of the distributed hydrogen ignition subsystem. Ignition causes deflagration of hydrogen (burning of the hydrogen with flame front propagation at subsonic velocity) at hydrogen concentrations between the flammability limit and 10 volume percent and thus prevents the occurrence of hydrogen detonation (burning of hydrogen with supersonic flame front propagation).
Section 6.2.4.2.2, Hydrogen Recombination Subsystem, states that it is designed to accommodate the hydrogen production rate anticipated for a loss of coolant accident. The hydrogen recombination subsystem consists of two non-safety-related PARs installed inside the containment above the operating deck at approximate elevations of 162 feet and 166 feet respectively, each about 13 feet inboard from the containment shell. The locations provide placement within a homogeneously mixed region of containment. The location is in a predominately up flow natural convection region. Additionally, the PARs are located azimuthally away from potential high up flow regions such as the direct plume above the loop compartment. Normally, oxygen and hydrogen recombine by rapid burning only at elevated temperatures (greater than about 1100 °F [600 °C)]. However, in the presence of catalytic materials such as the palladium group, this catalytic burning occurs even at temperatures below 32°F (0°C). Adsorption of the oxygen and hydrogen molecules occurs on the surface of the catalytic metal because of attractive forces of the atoms or molecules on the catalyst surface. Passive autocatalytic recombiner devices use pall adium or platinum as a catalyst to combine molecular hydrogen with oxygen gases into water vapor. Passive autocatalytic recombiners have been shown to be effective at minimizing the buildup of hydrogen inside containment following loss of coolant accidents.
They are provided in the AP1000 as defense-in-depth protection against the buildup of hydrogen following a loss of coolant accident.
T. Saporito
Section 6.2.4.2.3, Hydrogen Ignition Subsystem, discusses that this system is provided to address the possibility of an event that results in a rapid production of large amounts of hydrogen such that the rate of production exceeds the capacity of the recombiners. This massive hydrogen production is postulated to occur as the result of a degraded core or core melt accident (severe accident scenario) in which up to 100 percent of the zirconium fuel cladding reacts with steam to produce hydrogen. The hydrogen ignition subsystem consists of 64 hydrogen igniters strategically distributed throughout the containment. The primary objective of installing an igniter system is to promote hydrogen burning at a low concentration and, to the extent possible, to burn hydrogen continuously so that the hydrogen concentration does not build up in the containment. The igniter assembly is designed to maintain the surface temperature within a range of 1600°F to 1700°F in the anticipated containment environment following a LOCA [loss-of-coolant accident]. A spray shield is provided to protect the igniter from falling water drops (resulting from condensation of steam on the containment shell and on nearby equipment and structures).
Section 6.2.4.4, Design Evaluation (Severe Accident), discusses that the hydrogen monitoring subsystem has sufficient range to monitor concentrations up to 20 percent hydrogen. The hydrogen ignition subsystem is provided so that hydrogen is burned off in a controlled manner, preventing the possibility of deflagration with supersonic flame front propagation which could result in large pressure spikes in the containment. It is assumed that 100 percent of the active fuel cladding zirconium reacts with steam. This reaction may take several hours to complete. The igniters initiate hydrogen burns at concentrations less than 10 percent by volume and prevent the containment hydrogen concentration from exceeding this limit.
On December 1, 2023, you declined the offer to meet with the PRB (ML23335A109).
Given no additional information, the PRBs final determination is that the hydrogen control concerns in your petition do not meet the DH 8.11 acceptance criteria in Section III.C.1(b)(ii) which includes, The issues raised have previously been the subject of a facility-specific or generic NRC staff review and the petition does not provide significant new information that the staff did not consider in the prior review.
The regulations in 10 CFR 2.206 provide an opportunity for the public to petition the NRC to take enforcement-related action, and, while the PRB determined that the issues raised do not T. Saporito
warrant further review, the NRC understands that this process takes time, resources, and energy by petitioners. Accordingly, I thank you for taking the time to raise your concerns.
Sincerely,
Tania Martinez Navedo, Acting Director Division of Engineering and External Hazards Office of Nuclear Reactor Regulation
ML23264A850 (Package); ML23345A111(Letter) NRR-106 OFFICE NRR/DORL/LPL1/PM NRR/DORL/LPL2-2/PM NRR/DORL/LPL1/LA RES/DSA/FSCB NAME JKim PBuckberg KEntz MSalay DATE 12/11/2023 12/11/2023 12/12/2023 12/13/2023 OFFICE NRR/DSS/SCPB OGC - NLO NRR/DORL/LPL2-2/BC NRR/DORL/DD NAME NKaripineni RCarpenter DWrona JHeisserer DATE 1/3/2024 1/12/2024 1/17/2024 1/23/2024 OFFICE NRR/D NRR/DEX/D(A)
NAME AVeil (MKing for) TMartinezNavedo DATE 2/7/2024 2/8/2024