STEAM GENERATOR CONTROLS | BALANCE OF PLANT


Main feed water


themometer 1


Main feed water


themometer 2


Main feed water


themometer 3


Acquisition mudule 4


Acquisition mudule 5


Processor module 2A


Acquisition mudule 6


Processor module 2B


Left, figure 1: System structure diagram of the public part of feed water flow regulation system


ADG


Conditioning signal


Acquisition mudule 7


Defining the BDMP model The BDMP model is mainly composed of top events, bottom events, logic gates, trigger links and logic links. According to the basic fault tree concept, the top node in the BDMP model is the root node, while the bottom nodes are leaf nodes. In the BDMP model, the trigger link is from the trigger node to the triggered node. Value, relevant pattern and required pattern are the three most important attributes of each node in the BDMP model, which together determine the dynamic behavior of the BDMP model. The failure mechanism of each leaf node in the BDMP model is different. Leaf nodes of type f can fail only when they are in both relevant pattern and required pattern. Leaf nodes of type i can fail only when the required pattern changes. When leaf nodes of sf type are in the relevant pattern, their required pattern and non-required pattern correspond to two different failure rates.


Steam Generator Level Control System The steam generator level control system is composed of the steam generator feed water flow regulation system and the main feed water pump speed regulation system. The main function of the former is to adjust the feed water flow for each steam generator; the main function of the latter is to maintain the differential pressure between the feed water main and the steam main at the set value. Each of the three SGs in nuclear plants with three loops


has an independent steam generator feed water flow regulation system, which controls the feed water flow by controlling the opening of the main feed water regulating valve and bypass feed water regulating valve installed on the inlet side of the steam generator. Finally, to achieve the purpose of controlling the water level of the SG, at the same time, there is a public part within the feed water flow control of the three SGs. The main function of the public part is to obtain the load signal of the SG and to select the highest temperature among the main feed water temperatures of the three SGs to obtain the water level control gain to improve stability under low load. The system structure diagram of the public part of feed water flow regulation system is shown in Figure 1, above.


Main feed water pump speed regulation The main function of main feed water pump speed regulation system is to maintain the differential pressure between the steam main and feed water main at a preset value, which increases with the increase in load. The system input signal is the main steam flow signal and the mismatch signal of the main steam main and main feed water main differential pressure signal, and the output is the pump speed setting value. The feed water system of the nuclear power generation unit is equipped with three electric water feed pumps for each unit, and the actual pressure drop between the feed water main and the steam main is measured by three differential pressure gauges. The system structure diagram of main feed pump speed regulation system is shown in Figure 2; overleaf.


Failure analysis of steam generator controls The BDMP model file described by the defined script language is input into the conversion program, and that obtains the equivalent Markov model described in the PRISM script language through the conversion program. Finally, the PRISM software is used to perform a quantitative analysis. The results are then compared with the analysis results of EDF’s KB3 and YAMS (due to the limitation of the KB3 modeling node, the YAMS calculation result is taken as the sum of the three parts of the steam generator water level control system) to verify the correctness of the proposed method. As shown in Figure 3, the YAMS analysis result is 1.45176E- 3, and the PRISM analysis result is 1.42581E-3. Taking the YAMS analysis result as the reference value, and the error is about 1.7% of the reference value. It can be seen from Figure 4 that the unavailability rate


of the feed water flow regulation system (independent part) accounts for about 73.8% of the unavailability rate of the entire SGLCS, and the unavailability rate of feed water flow regulation system (common part) accounts for about 24.4%, and the main feed water speed regulation system is less than 2.0%. The results show that the failure of the feed water flow regulation system is the main cause of the failure of SGLCS in NPPs.


www.neimagazine.com | August 2023 | 33


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