Was it the safety valve that caused the problem at the Three Mile Island nuclear accident?

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This post was last edited by At the foot of Hengshan Slope on 2017-2-15 at 16:41. (The following content is original to me; please contact me if you wish to reproduce it: QQ364772239@qq.com) In many previous Chinese publications, when discussing the importance of safety valves, reference is often made to the stuck \"safety valve\" that occurred during the Three Mile Island nuclear accident. Out of curiosity, I gathered some information and would like to share my own understanding. 1. Description of the NRC accident in the United States: Firstly, certain factors caused the secondary loop feed water pumps to stop operating, which in turn led to the shutdown of the turbine. The secondary loop is unable to remove the heat generated by the primary loop, causing the temperature and pressure in the primary loop to rise gradually. To control the pressure, the pilot operated relief valve (PORV) installed at the top of the pressure regulator opens (note that a safety valve is also installed at the top of the pressure regulator, as shown in Figure 1). Under normal conditions, this valve should close promptly when the pressure drops to the set value, but it has become stuck. The valve appeared to be in the closed position on the display instruments in the control center, so the relevant personnel failed to take any action, resulting in the coolant in the primary loop continuously vaporizing and being discharged through the PORV. The pressure inside the island continued to drop; the primary loop main pump shut down automatically to prevent excessive vibration, and the temperature inside the island kept rising. However, there were no instruments in the equipment to display the level of the coolant inside the reactor, and the operators mistakenly assumed that the liquid level within the reactor was still high, thus slowing down the emergency cooling process. This further exacerbated the rise in temperature inside the core, eventually leading to its melting. Figure 1. Schematic diagram of the equipment at the Three Mile Island (TMI) nuclear power plant. The information provided above comes from: http://www.nrc.gov/reading-rm/doc-collections/fact-sheets/3mile-isle.html. 2. Regarding the pilot-operated relief valve, based on the NRC’s description, we have always referred to this valve as a pilot-operated safety valve or pilot-operated release valve, and classified it as a safety valve. Since API520 refers to pilot-operated safety valves as pilot operated relief valve (PORV) – Figure 2, SEBIM also calls its own pilot-operated pressure regulator safety valves pilot operated safety relief valve (POSRV) – Figure 3. At the same time, some experts abroad believe that PORVs are actually the same as POSRVs (PORV are also called pilot-operated safety valve (POSV), pilot-operated pressure relief valve (POPRV), or pilot-operated safety relief valve (POSRV), depending on the manufacturer and the application. 【From Wikipedia】PS: It seems that there is also some confusion regarding this concept abroad. Figure 2. Names of pilot valves in API520. Figure 3. Pilot for SEBIM pressure regulator. However, after checking relevant information, it seems incorrect to classify this PORV as a type of safety valve. In fact, this valve is also referred to as a power operated relief valve (PORV) in many foreign documents. It is not what we commonly understand by a pressure regulator safety valve; however, its function and principle are similar to those of a pilot-operated pressure regulator safety valve, but it is more akin to an electromagnetic release valve. (Some sources abroad also suggest that a pilot-operated relief valve is actually a type of power-operated relief valve.) Further research revealed that the PORV that became stuck at the Three Mile Island nuclear plant was a 31533VX model manufactured by DRESSER; this valve can still be found in DRESSER’s samples, but the samples do not provide details regarding its internal structure. An accident report indicates that the stuck PORV had the structure shown in Figure 5; however, the image is not clear and provides little information, but it is likely to be the 31533VX. Figure 4: Dresser 31533VX. Figure 5: Schematic of the PORV structure used in the TMI-2 accident. In summary, I believe the valve that got stuck at Three Mile Island should not be classified as a safety valve. Due to the similarities in function and structure with safety valves, after 1979, the possibility of an LOCA accident caused by a blockage in the safety valve or relief valve was taken into consideration during the design of pressure regulator safety valves, especially in the design of pilot-operated pressure regulators. The WeChat personal official account (Zhang Jian Talks about Safety Valves) is as follows; everyone is welcome to share their suggestions and ask questions