atw 2017-06

More documents

Recommendations

Info

atw Vol. 62 (2017) | Issue 6 ı June 392 ENVIRONMENT AND SAFETY Retrofitting a Spent Fuel Pool Spray System for Alternative Cooling as a Strategy for Beyond Design Basis Events Christoph Hartmann and Zoran Vujic Due to requirements for nuclear power plants to withstand beyond design basis accidents, including events such as happened in 2011 in the Fukushima Daiichi Nuclear Power Plant in Japan, alternative cooling of spent fuel is needed. Alternative spent fuel cooling can be provided by a retrofitted spent fuel pool spray system based on the AP1000 plant design. As part of Krško Nuclear Power Plant’s Safety Upgrade Program, Krško Nuclear Power Plant decided on, and Westinghouse successfully designed a retrofit of the AP1000® plant spent fuel pool spray system to provide alternative spent fuel cooling. 1 Introduction Following the tsunami and resulting events in 2011 at the Fukushima Daiichi Nuclear Power Plant in Japan, the Western European Nuclear Regulators Association (WENRA) updated the safety reference levels in its report “WENRA Reactor Safety Reference Levels,” [1] to incorporate lessons learned from the event. The update includes establishing an independent heat removal system for the spent fuel pool to maintain the integrity of used fuel assemblies being temporarily stored there in the unlikely event of a beyond design basis accident. The AP1000® nuclear power plant design foresees provisions for beyond design basis events. This includes failure of the spent fuel pool walls or floor, which would result in the spent fuel pool draining and fuel assemblies being uncovered. This design is also in agreement with the Nuclear Energy Institute (NEI) issue of NEI 06-12, Revision 2, “B.5.b Phase 2 & 3 Submittal Guideline” [2], where an external spent fuel pool makeup and spray strategy is recommended. For events with extended loss of AC power, that is, station blackout, and/ or loss of heat sink due to the spent fuel pool draining or partially draining, spent fuel cooling can be provided by a spent fuel pool spray system. A spent fuel pool spray system based on the AP1000® plant design can be retrofitted for existing nuclear power plants. In the case of an uncontrolled spent fuel pool water level drop to such an extent that the spent fuel pool would be completely dried out, an emergency spray system is the best practical solution that can be applied for sufficient cooling of the spent fuel assemblies. 2 AP1000 Plant Spent Fuel Pool Cooling The AP1000® plant design features multiple, diverse lines of defense to ensure spent fuel cooling can be maintained for design basis and beyond design basis events. During normal and abnormal conditions, defense-in-depth and duty systems provide highly reliable spent fuel pool cooling. These systems are driven by offsite AC power or the onsite standby diesel generators. For unlikely events with extended loss of AC power, that is, station blackout, and/or loss of heat sink, passive systems provide spent fuel pool cooling. These passive systems require minimal or no operator action and are sufficient for at least 72 hours under all possible loading conditions. After 72 hours, several different means are provided to continue spent fuel pool cooling using installed plant equipment, as well as off-site equipment. Even for beyond design basis events with postulated spent fuel pool damage and multiple failures in the passive safety-related systems and active defense-in-depth systems, the AP1000® plant spent fuel pool spray system provides an additional line of defense to prevent spent fuel damage. The spent fuel pool is located in a hardened section of the Auxiliary Building and contains used fuel that has been removed from the nuclear reactor core. Typically, 64 fuel assemblies are removed from the reactor | | Fig. 1. AP1000® Plant Spent Fuel Pool Spray System. Spray headers and nozzles (left) and section view of spray pattern from nozzle (right). Environment and Safety Retrofitting a Spent Fuel Pool Spray System for Alternative Cooling as a Strategy for Beyond Design Basis Events ı Christoph Hartmann and Zoran Vujic
atw Vol. 62 (2017) | Issue 6 ı June | | Fig. 2. Pipe routing of spent fuel pool alternate cooling with mobile heat exchanger (MHX) and spray system. during refueling every 18 months and stored in the spent fuel pool. The AP1000® plant’s spent fuel pool has the capacity to cool up to 889 spent or used fuel assemblies, which are continuously submerged beneath approximately 7.6 m of water. The spent fuel assemblies continue to generate decay heat naturally even when they are removed from the reactor and are placed in the spent fuel pool. This decay heat will decrease significantly over time so that older spent fuel produces less heat than spent fuel that has recently been removed from the reactor. The spent fuel in the spent fuel pool is cooled by transferring the decay heat from the used fuel to the water in the spent fuel pool. The spent fuel pool water is, in turn, pumped through a loop with a heat exchanger where it is cooled and decay heat is transferred to a second water cooling system. The cooled water is then returned from the second water cooling system to the spent fuel pool and the decay heat is transferred to the environment. There are two identical spent fuel pool cooling trains, though only one pump and heat exchanger in one of the two trains are in operation in most circumstances. 3 AP1000 plant spent fuel pool spray system The AP1000® spent fuel pool spray system is designed to cool the spent fuel during a beyond design basis event in accordance with the B.5.b guideline [2]. The AP1000® plant spent fuel pool spray system has two redundant spray headers located on either side of the spent fuel pool. There are 16 spray nozzles on each header (Figure 1, left). One header receives water through either gravity-fed draining of the passive containment cooling water storage tank, which is located on top the Shield Building, or from a flanged connection located in the truck bay, which is used with an onsite portable pump. The other header receives water from the fire protection water tanks and the diesel-driven or electric motor-powered fire protection system water pumps. Spray nozzles distribute water spray in the form of a hollow spray cone over the fuel assemblies. Only one spray header is required to assure sufficient cooling of the exposed spent fuel due to sensible heat and latent heat from water spray vaporization (Fig. 1, right). The spray system used to cool the spent fuel pool during a postulated loss-of-large-area event is sized to provide an adequate amount of spray to the hottest fuel assembly that will enter the spent fuel pool. The analytical basis for determining the minimum amount of spray needed to cool a fuel assembly is adapted from the calculation used in Section 3.3 of the Sandia report, “Mitigation of Spent Fuel Pool Loss-of-Coolant Inventory Accidents And Extension of Reference Plant Analyses to Other Spent Fuel Pools” [3]. Further, to prevent pressurization inside the Fuel Handling Building, the system includes a relief panel to release steam that is produced during the cooling process. 4 Krško nuclear power plant safety upgrade program Krško Nuclear Power Plant was already in the process of making significant upgrades as a result of applying for a license extension in 2009 to operate beyond 2023. The Krško Safety Upgrade Program was designed in response to the Slovenian Nuclear Safety Administration’s re gulations and interpretation of reference safety levels from the report, “ WENRA Reactor Safety Reference Levels” [1], concerning reasonable measures to prevent and mitigate severe accidents in preparation for the possibility of extending original plant operating licenses. The reference safety levels within the report were updated in 2014 to incorporate lessons learned from the event at the Fukushima site. The measures defined in the frame of the Krško Safety Upgrade Program are in agreement with the nuclear industry’s response to the Fukushima accident and the resulting update of the safety reference levels proposed by WENRA. This includes plant upgrades and design changes to address design extension conditions defined in the report and beyond design basis accidents. Krško’s Safety Upgrade Program is divided into various projects being carried out during three phases. The Spent Fuel Pool Alternative Cooling Project is in the scope of Phase 2. The project is scheduled to be completed by the end of 2017. The Spent Fuel Pool Alternate Cooling Project shall assure alternate cooling of used fuel assemblies by using a mobile heat exchanger or spray system (see Figure 2). Furthermore, it shall assure depressurization of the Fuel Handling Building by using relief panels to release steam produced during the cooling process. The systems of the Spent Fuel Pool Alternate Cooling Project are designed to assure that heat is removed from the spent fuel during Design Extension Conditions A and B and to mitigate spent fuel damage. The operational conditions for the systems of the Spent Fuel Pool Alternate Cooling Project are classified according to the plant’s severe accident scenarios, following the WENRA guidance document “ Issue F: Design Extension of Existing Reactors” [1]. ENVIRONMENT AND SAFETY 393 Environment and Safety Retrofitting a Spent Fuel Pool Spray System for Alternative Cooling as a Strategy for Beyond Design Basis Events ı Christoph Hartmann and Zoran Vujic
Page 1 and 2: nucmag.com 2017 6 ISSN · 1431-5254
Page 3 and 4: atw Vol. 62 (2017) | Issue 6 ı Jun
Page 5 and 6: Kommunikation und Training für Ker
Page 27: atw Vol. 62 (2017) | Issue 6 ı Jun
Page 67 and 68: PROGRAMME OUT NOW VGB CONGRESS 2017

atw 2017-06

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?