Depressurization Of Reactor Pressure Vessel Research Articles

RPV Depressurization Strategy Analysis for Chinshan Nuclear Power Plant by Using MAAP5

When an accident occurs, operators in nuclear power plants (NPPs) must follow emergency operating procedures (EOPs) or severe accident management guidelines (SAMGs). However, EOPs and SAMGs are symptom-based procedures and guidelines to cope with severe transients and accidents. Operators depend on real-time operating parameters of NPPs to perform each action in EOPs or SAMGs. When a beyond-design-basis accident like the Fukushima Daiichi accident of 2011 occurs, EOPs or SAMGs cannot be performed effectively without adequate information. One lesson learned from the Fukushima accident is that such a situation requires advance preparation regarding the key indicators, the water supply, reactor pressure vessel (RPV) depressurization, and containment venting strategies so actions can be performed with limited manpower and time. After the Fukushima accident, Taiwan Power Company established ultimate response guidelines (URGs) and has implemented them in three operating NPPs. An URG is an event-based guideline developed to manage accidents caused by a compound disaster beyond the design basis. The purpose of this study is to find out the differences of RPV depressurization strategies between EOPs and URGs and to discuss the effect of different RPV depressurization strategies on fuel integrity. The plant responses and accident physical phenomena are simulated using MAAP5. The results show that the RPV water level should be maintained as high as possible and the RPV pressure should be controlled sufficiently low at the beginning of RPV emergency depressurization to avoid core uncovery and assure fuel integrity. The URG provides the better RPV depressurization strategy to respond to a beyond-design-basis accident and mitigate an anticipated severe accident consequence as early as possible.

A Study of the Containment Venting Strategy for the Fukushima Daiichi Accident

The Fukushima Daiichi accident occurred on March 11, 2011. A seismic event and tsunami induced an extended station blackout plus loss of the ultimate heat sink. Three units progressed into a core melt severe accident. The accident occurred in the emergency operation procedure (EOP) domain. However, this situation was already beyond the scope of an EOP. The operator followed the EOP faithfully, and a core melt situation still occurred. An interesting topic is whether it is possible to avoid this type of accident.The purpose of this study is to survey the Fukushima accident progression with respect to the effect of the containment venting strategy for the Chinshan Nuclear Power Plant EOPs. Under the emergency situation, only a small reactor pressure vessel (RPV) injection system was available. This type of accident may be avoided by an early shift from the EOP to the severe accident guideline (SAG), switching from high-pressure injection to low-pressure injection while the reactor core isolation cooling system is available, gradually lowering the RPV pressure, and maximizing the injection flow rate. The plant responses and accident physical phenomena were simulated using MAAP5. The results show that the consequences of an uncovered core and core melt can be avoided by adopting the proper RPV depressurization and containment venting strategy.

Validation of RCS Bleed-and-Feed Strategy for the SAMG of the Kuosheng Nuclear Power Plant

The severe accident management guideline (SAMG) was developed and implemented at the Kuosheng nuclear power plant (NPP) in Taiwan at the end of 2003. The Kuosheng NPP is a Mark-III boiling water reactor (BWR)-6 located in the north of Taiwan. The SAMG of the Kuosheng NPP is developed based on the BWR Owners Group Emergency Procedure and Severe Accident Guidelines. In this paper, MELCOR 1.8.5 is used to investigate the effectiveness of the Kuosheng SAMG and to analyze the reactor coolant system (RCS) bleed-and-feed strategy in a postulated station blackout (SBO) accident since this accident is a dominant sequence that can induce core damage. These simulation results can be applied for the typical BWR-6 NPPs. Based on the simulation results, the major events of an SBO accident without any operator actions, including core uncovery, cladding oxidation, hydrogen generation, fuel relocation, vessel failure, and containment failure, are well presented. The RCS bleed-and-feed strategy can cool down the hot core and bring the vessel into the stable condition in the SBO accident with high-pressure core spray injection according to the Kuosheng SAMG. The reactor pressure vessel depressurization before the reactor liquid level reaches one-fourth of the core liquid level can prevent damage to the core fuel. However, the water temperature of the suppression pool will reach saturation temperature, and the containment pressure may challenge the containment integrity. These phenomena were not considered while developing the SAMG.

Analysis of Severe Accident Management Strategy for a BWR-4 Nuclear Power Plant

The Chinshan nuclear power plant (NPP) is a Mark-I boiling water reactor (BWR) NPP located in northern Taiwan. The Chinshan NPP severe accident management guidelines (SAMGs) were developed based on the BWR Owners Group Emergency Procedure Guidelines/Severe Accident Guidelines and were developed at the end of 2003. The MAAP4 code has been used as a tool to validate the SAMG strategies. The development process and characteristics of the Chinshan SAMGs are described. The T5UtXC sequence, the highest core damage frequency in the probabilistic risk assessment insight of the Chinshan NPP, is cited as a reference case for SAMG validation. Not all safety injection systems are operated in the T5UtXC sequence. The severe accident progression is simulated, and the entry condition of the SAMGs is described. Then, the T5UtXC sequence is simulated with reactor pressure vessel (RPV) depressurization. Mitigation actions based on the Chinshan NPP SAMGs are then applied to demonstrate the effectiveness of the SAMGs. Sensitivity studies on RPV depressurization with the reactor water level and minimum RPV injection flow rate are also investigated in this study. Based on MAAP4 calculation and the default values of the parameters calculating the severe accident phenomena, the result shows that RPV depressurization before the reactor water level reaches one-fourth of the core water level can prevent the core from damage in the T5UtXC sequence. The flow rate of two control rod drive pumps is enough to maintain the reactor water level above the top of active fuel and cool down the core in the T5UtXC sequence without operator action.