## 4 Accident Control Strategy
### The Movement toward Accident Countermeasures
#### 4.3.2 Level 2 Accident  
The strategy for dealing with a Level 2 accident is to make the system shut down immediately, by a so-called \"fast shutdown procedure\". There are some differences between the normal shutdown procedure and the fast shutdown procedure. For the normal shutdown process, the feedstock of the system is changed from sludge to water gradually, by adding water into the sludge tank (1). The temperature of the reactor (7) is gradually decreased with a decreasing rate of 100 \({}^{\circ}\)C per hour, which is achieved by interlocking the temperature signal from the reactor (7) and the power of the electrical heater (5). According to our estimation, it takes about 6 to 8 h for the system to shut down by the normal procedure. Obviously, there is not so much time during the occurrence of a Level 2 accident. The \"fast shutdown procedure\" is aimed at stopping the system and replacing the sludge in the system in one hour. The sludge tank (1) is directly cut off from the system, and the water from the water tank (2) is directly transported into the system. The heater (5) is turned off, and the decreasing rate of the reactor temperature is not controlled anymore. The effluent is discharged into the underground tank (16) for storing.  
Although the \"fast shutdown procedure\" can be applied to handle every Level 2 accident, there are some special operations for different accidents.  
1. Overpressure accident: if overpressure occurs in the electrical heater (5), the first step is to change the location of the heating module where salt precipitation forms, as introduced in Section 3.2. If the overpressure occurs in the heat exchanger (4) or the reactor (7), the fast shutdown procedure should be carried out directly.
2. Over-heating accident: the first step in handling an over-heating accident is to cool down the place where the temperature is exceeding the normal. If it occurs in the heater (5), the heating module of the heater (5) should be turned off immediately, and all the dismountable thermal-insulation material should be removed to help cooling. If over-heating occurs in the heat exchanger (4) or the reactor (7), the desuperheater (6) is used for fast cooling, and then the fast-shutdown procedure will be carried out.
3. Opening of pressure relief valves: the opened relief valve will re-seat when the pressure decreases, back to the set-point. During the period of the relief valve opening, the feedstock pump (3) should be stopped. The insoluble solid particles from the feedstock may stick to the sealing face of the relief valve when it closes, leading to the unstoppable discharging of the feedstock. Therefore, the valves from the inlet and the outlet of the overpressure equipment should be closed once the relief valve opens. For example, if the relief valve S3 opens, the valves V6 and V9 should be closed immediately to isolate the reactor (7) from the system. When the relief valve closes, the fast-shutdown procedure will be carried out.
4. Breaking of the heater (5): there is no backup for the heater (5) in the system, and the heater (5) functions by adjusting the reaction temperature during normal operation and the normal shutdown procedure. If the heater (5) cannot work anymore, the system should enter into the fast-shutdown procedure.