## 1 Introduction  
Human error has been the main cause of severe nuclear accidents, such as Three Mile Island in 1979 and Chernobyl in 1986, rather than device malfunction and poor operating environment given the rapid development of technology. An investigation conducted by major nuclear power producing countries illustrated that human failures account for 60%-80% of accidents [1]. Acute stress is an important contributory factor to human error. In the field of human reliability analysis (HRV), acute stress was firstly defined as a physical or mental tension state elicited by various stressors [2]. [PERSON] et al. [3] refined the definition and considered stress to be an appraisal process evoked by certain environmental demands, such as noise, time pressure, and task load, which exceed the mental and physical resources of operators and leads to undesirable physiological, psychological, behavioral, and social outcomes. Tasks in the master control room may become stressful, overwhelm operators, and make them feel extremely confused and anxious when unexpected circumstances, such as rapid signal changes, a sudden increase in workload, complicated decision-making conditions, and limited operating time occur in the nuclear power plant. In [PERSON]'s study [4], he examined the relationship between vital exhaustion, and temperament during experimentally induced stress. He found that participants felt anxious, tense, fearful, sad, depressed, angry, irritated, and disappointed when experiencing stress. Acute stress elicits not only negative emotions in individuals, such as fear, irritation, and anxiety, but also impairs their cognitive functioning. For instance, [PERSON] found a greater amplitude of SPCN (the sustained posterior contralateral negativity) in the stress group than in the control group, which indicates that it is more difficult for individuals to disengage their attention from threat stimuli. These results illustrated that attention disengagement toward threat stimuli is impaired under acute stress situations [5]. Other psychological studies have shown that individuals with high math anxiety stress demonstrated reduced working memory capacity, which led to increased reaction times and errors in their tasks [6]. In a simulated study, the experiment was carried out in a training simulator that matches the actual control room of a nuclear power plant [7]. The result illustrated that information-seeking performance decreases and reaction time decelerates under stressful situations. These stress responses can lead to fatal and serious accidents. Therefore, immediately recognizing the stress state of nuclear power plant operators and then inhibiting the occurrence of human errors is crucial. Acute stress can be measured and quantified in a variety of ways. Subjective scales mainly used in psychology measure the process of cognitive appraisal and affective responses under stress, including stress appraisal measures (SAM) [8], the perceived stress scale (PSS) [9], and the positive and negative affect scale (PANAS) [10]. However, most scales assess the chronic stress state caused by life events or catastrophes, such as earthquakes and wars. Otherwise, self-reported measures rely on inaccurate recall or memory and fail to assess stress levels continuously without interrupting the task process. Therefore, these approaches are impracticable for monitoring or predicting the stress state of nuclear power plant operators. Acute stress physiologically activates the hypothalamic pituitary adrenal (HPA) axis and the autonomic nervous system (ANS). The chronic activation of HPA results in the increase of adrenocorticoph hormone (ACTH) and cortisol levels [11]. Meanwhile, the rapid activation of ANS can lead to changes in heart rate, blood pressure, skin temperature, and skin conductivity. [PERSON] found that activity in the autonomic nervous system, such as heart rate and skin temperature, distinguished among negative emotions [12].