ML18355A806

Safety Enhancements After Fukushima

ML18355A806
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Issue date:	12/31/2018
From:	NRC/EDO/AO
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1 l Davis Besse Reactor Pressure Vessel Head DegradationSAFETY ENHANCEMENTS AFTER FUKUSHIMAHardened Vents Seismic Defense FLEX EquipmentFLEX Offsite EquipmentEmergency Preparedness Flooding Defense Mitigation Strategies Spent Fuel Pool Instrumentation 2 POST-FUKUSHIMA SAFETY ENHANCEMENTSSafe Facilities Made SaferOn March 11, 2011, a 9.0-magnitude earthquake, followed by a 45-foot tsunami, heavily damaged the nuclear power reactors at Japan's Fukushima Dai-ichi facility. Following this accident, the NRC required 1 The enhancements included: adding capabilities to maintain key plant safety functions following a large-scale natural disaster; updating evaluations to better handle potential reactor core damage events; and strengthening emergency preparedness capabilities. Combined, these actions ensure that the nuclear industry and the NRC are prepared for the unexpected.

1 Although this brochure focuses on commercial nuclear power plants, the NRC considered the lessons of the accident for other types of nuclear facilities and required enhancements as appropriate.Hardened Vents Seismic Defense FLEX EquipmentFLEX Offsite EquipmentEmergency Preparedness Flooding Defense Mitigation Strategies Spent Fuel Pool Instrumentation 3 MITIGATION STRATEGIESAbility to Handle Unanticipated Events In order to maintain key safety functions, the NRC ordered every

long-term loss of standard safety systems. Instead of speculating on which and diversity in responding to extreme natural phenomena, such as cool, preserve the containment barrier that prevents or controls radiation Plants with multiple reactors should be able to implement these measures at all reactors simultaneously. Each plant installed new emergency response equipment, stored onsite and protected from natural hazards. nuclear power plants. Additional equipment and resources are stored at two National Response Centers, ready to be deployed to a plant during an emergency.

4 FLEX EQUIPMENTMaintain Key Functions with Backup and Redundant Equipmentstrategy, called Diverse and Flexible Mitigation Capability, or FLEX. FLEX maintains long-term core and spent fuel cooling and containment integrity with installed plant equipment that is protected from natural hazards, as well as backup portable onsite equipment. If necessary, similar equipment can be brought from offsite.Portable equipment is stored in areas protected from potential natural hazards, such as the robust FLEX building in the background.

(Photo of FLEX building outside the Hatch nuclear power plant, Southern Company.)Portable pumps and generators provide water and power to maintain key safety functions.(Photos of pumps and generators at the Diablo Canyon nuclear power plant.)

5 weep holeBoric acid depositsStandard mechanical and electrical connections for the portable equipment were installed at all U.S. plants. (Photo of FLEX Jumper connection panel at St. Lucie nuclear power plant and mechanical connection at Millstone nuclear power plant.)Backup equipment can be brought from o~site to any U.S. nuclear power plant within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />. (Photos of equipment at National Response Center.)Debris removal equipment and alternative plans ensure access to the site.

(Photos of support equipment used by U.S. Forest Service in California and the Diablo Canyon nuclear power plant.)

6 CONTAINMENT VENTING SYSTEM Improved Fukushima-Style DesignsThe Fukushima accident disabled the plant's reactor core cooling systems, causing heat and pressure to build within the concrete and steel containment buildings that surround the reactors. This buildup eventually damaged the buildings and made it easier for radioactive material to plants similar in design to the Fukushima plants to improve or install a reliable, hardened vent to remove heat and pressure before potential reactor core damage. This helps preserve the integrity of the containment building, while delaying reactor core damage or melting. NRC updated the order to ensure those vents would function in the conditions following reactor core damage. completed at all affected plants and inspectors. These upgrades were not necessary for other designs of nuclear power plants, so the order did not apply to them.Reactor VesselSuppressionPoolSuppressionPoolSteel ContainmentReactorCoreConcrete BasematIsolation ValvesRelief ValveVenting ValveVent PipeVent NOT TO SCALECONTAINMENT VENTING PHASE 1 7 SPENT FUEL POOL INSTRUMENTATION Ability to Monitor Level of Cooling Water During the accident at Fukushima, the plant operators lost both their ability to cool the spent fuel pools and determine the amount of water in the pools. If enough water boiled away or was otherwise lost, the spent of radiation. Weeks later, it was learned that the spent fuel was always covered, but the information gap diverted attention and limited resources nuclear power plants to install water level instrumentation in their spent fuel pools. The instrumentation currently in place reports at least three distinct water levels: (1) normal level; (2) low level but still enough to shield workers above the pools from radiation; and (3) a level near the top of the spent fuel rods where more water should be added without delay. EMERGENCY PREPAREDNESSEnhanced Disaster Response CapabilitiesThe accident at Fukushima highlighted the complexity of emergency response when multiple reactors on the same site are affected at the same time and electrical power is unavailable. In response, the NRC asked they will need to respond to a large accident that may affect multiple reactors at their site and make changes to their emergency plans as necessary. The NRC also asked the plant operators to ensure that they can power the communications equipment staff will need to effectively respond to such an accident. This included power for response team radios, cellular telephones, and satellite telephones. Licensees purchased based on the insights from these evaluations. NRC inspectors reviewed these enhancements.

8 FLOODING AND SEISMIC DEFENSEIn light of the Fukushima plant damage from the extreme earthquake to perform detailed "walkdown" inspections of their installed seismic degraded conditions. NRC inspectors performed follow-up reviews.Based on advances in the knowledge and understanding of seismic and Dai-ichi, the NRC requested the licensees of operating reactors to used updated information and methodologies to inform plant operators of potential impacts to their sites. As a result, several nuclear power FLOOD PROTECTION PUMPS SITE DRAINAGE PLANT ELEVATIONWATERTIGHT DOORSINFLATABLE BERMS PERMANENT FLOOD WALLS 2 1 5 3 4 6 DIESEL GENERATORSSAND BAGSAddressed Potential Impacts from Natural Disasters 9 Examples of Flooding Protection Features

1. Temporary watertight barriers dry.2. Temporary and permanent pumps can alleviate the effects of 3. systems can be either man-made (drains) or natural (grading).4. Man-made structures primarily designed to 5. Permanently increasing the elevation of a nuclear power plant's site during construction helps to ensure potential 6. equipment, such as emergency diesel generators.Examples of Seismic Protection Features

1. Mechanically attaching equipment to a surface to prevent movement during an earthquake. This minimizes potential damage to the restrained equipment as well as nearby equipment.2. Physical separation of equipment or structures to minimize their interactions during an earthquake.3. decouple equipment or structures from the base substructure impacted by an earthquake. This protects the structural integrity of the isolated equipment or building.

10 POST-FUKUSHIMA SAFETY ENHANCEMENTS RESOURCEShttps://www.nrc.gov/docs/L1205/ML12054A735.pdf(https://www.nrc.gov/docs/ML1205/ML12056A044.pdf)(https://www.nrc.gov/docs/ML1314/ML13143A321.pdf)(https://www.nrc.gov/docs/ML1205/ML12053A340.pdf)(https://www.nrc.gov/reactors/operating/ops-experience/japan-dashboard.html)

(https://www.nrc.gov/docs/ML1118/ML111861807.pdf)(https://www.nrc.gov/docs/ML1810/ML18101B396.pdf) 11 12 l Davis Besse Reactor Pressure Vessel Head DegradationDecember 2018