## 3 Safety Management Strategy
### Temperature Management of the Reactor  
There is no doubt that the facility with the highest temperature in an SCWO system is the reactor, which is the place that oxidizing agents and organic materials mix and oxidation reaction occurs. The over-heating of a reactor is a severe accident that may cause a fast-corrosion failure of a reactor. Moreover, the allowable stress of the alloy used as component materials for a reactor decreases with the increasing of temperature. The most widely used material for the reactor is Ni-based alloy 625. The allowable stress of Alloy 625 at 650 \({}^{\circ}\)C is about 140 MPa, but it rapidly decreases to 80 MPa at 700 \({}^{\circ}\)C. That is to say, over-heating of a reactor is very dangerous because the component materials cannot withstand the high pressure (up to 30 MPa) at the temperature beyond the design value, resulting in swellings of tubes or even bursting.  
In our commercial SCWO plant, two methods are used to prevent the over-heating of a reactor. The first one is oxygen multipoint injection. Figure 1 is a simplified SCWO system diagram, which includes only the core facilities such as the pre-heater, the heat exchanger and the reactor. For a conventional SCWO plant, oxygen is injected only into a reactor. In our new plant, there are many oxygen injection points located at the heater, the heat exchanger and the reactor, as shown in Figure 1. A small amount of oxygen is injected into the heat exchanger to perform the oxidation reaction with a portion of organic material. The injecting point on the heat exchanger is located to the place where the temperature of the feedstock is about 374 \({}^{\circ}\)C, which is considered as the \"large specific-heat region\" [44]. By injecting a portion of oxygen into the heat exchanger and the heater, both the COD of feedstock and the amount of oxygen to be injected into the reactor can be diminished, which effectively reduces the intensity of the oxidation reaction in the reactor. Moreover, as shown in Figure 1, there are many oxygen-injection points that are uniformly distributed from the entrance to the end of the reactor. Different amounts of oxygen are injected from these entrances, which can also decrease the intensity of the oxidation reaction at the beginning of the reactor and reduce the risk of over-heating. Some studies show that the multi-injection of oxygen has better results than the single injection [45; 46]. A novel design mixer was used for the mixing of the oxygen and the feedstock to avoid the formation of hot spots. The structure and the function of the mixer has been introduced in our previous article [20].  
Oxygen multipoint injection is a method for reducing the potential risk of over-heating of the reactor, which cannot be completely avoided. Therefore, a desuperheater is employed for over-heating accidents. As shown in Figure 1, the desuperheater is located on a bypass of the main feedstock pipeline from the heater to the reactor. The valves V1 and V2 are kept closed under the normal operating condition. If the temperature of the reactor increases sharply in a short time, valves V3 and V4 are closed, valves V1, V2 and V5 are opened, and cooling water is injected into the system via the desuperheater. The flux of cooling water is calculated according to the real-time highest temperature of the reactor, aiming to reduce the temperature to a normal value. The bypass is used to avoid the insoluble solid particlesfrom feedstock blocking off the tiny holes on the desuperheater (namely the outlet of cool water) during normal operation.