v3 Chapter 535： Safety experiment and accident cause (2/2)

It must be admitted that compared with the pressurized water reactors used in most countries in the world today, rmbk has natural defects at the beginning of the design. It sacrifices part of the safety for faster construction speed and more Low operating costs, refueling without shutdown, and the production of military plutonium and other advantages.

In the eyes of the people at that time, if there were any shortcomings, it was probably that the emission of the rmbk reactor was slightly more radioactive, and there was no cleaning from the VVER type pressurized water reactor.

But in fact, the more serious problem is the design of the control rod. Djatrov has previously said that the control rod is used to adjust the nuclear fission reaction, because the boron that constitutes the control rod can absorb neutrons, which can also reduce the reactor. Power, but the problem is that there is a small piece of graphite at the lower end of the control rod. The role of graphite as a neutron moderator is exactly the opposite of that of boron. It is used to reduce fast neutrons to slow neutrons to increase reactor power.

Of course, the reason why the earlier designers did this is not to let the reactor operators feel the ”surprise” brought by the reversal. The graphite at the bottom also has its effect, because the graphite stack has a significant deceleration effect on neutrons, which can be appropriate Reduces the purity of uranium fuel, thereby saving costs.

From the economic point of view, there is no problem, but the low-abundance uranium fuel is not stable during the reaction, especially when the reactor is running at low power, then the graphite at the lower end of the control rod can play a role, which can make the reactor Get better control.

However, this design will bring potential risks. When the control rod is completely pulled out of the reactor and then inserted back, the first contact with the core is graphite instead of boron. At this time, the reaction power will not only decrease but will increase. High, although the duration of the whole process is very short.

What surprised Zhang Heng was that according to the information he collected, before the accident at the Chernobyl nuclear power plant, other nuclear power plants had discovered that the power of the control rod was inserted into the reactor and then increased and then decreased, and reported to The relevant departments, but because it did not cause any security incidents, did not get enough attention.

On the other hand, it must also be recognized that the designers of the rmbk reactor have made a lot of efforts to ensure the safety of the reactor ~~ For example, it is clearly stated in the safety manual that the core of the reactor must be guaranteed during the operation of the reactor At least 28 to 30 control rods are inserted.

In addition, the reactor is also equipped with an emergency core cooling system. When the cavitation coefficient in the reactor increases, the water tank of the emergency core cooling system will be opened immediately and injected into the reactor. However, in order to prevent cooling water from entering the high temperature reactor , Causing thermal shock, in the experiment, this device that can save lives at critical moments was disconnected from multiple forced circulation circuits.

After seeing Fuming ’s first failed experiment, Zhang Heng was almost certain to have exploded that night. The people in the central control room did not fully comply with the safety manual as they said before.

The situation on the night of the explosion was also more complicated than in the first experiment, because the safety experiment failed to proceed as planned. The reactor had been operating in a low-power state for a considerable period of time before falling into the iodine pit, in order to climb out In the iodine pit, the operator withdrew too many control rods, which eventually caused the reactor power to climb rapidly. Later, because the safety system was cut off and could not take effect immediately, the people in the central control room could only hurriedly insert all the control rods back into the core. , Want to suppress the fission reaction.

Unexpectedly, the design defects of the trigger rate reactor eventually led to the entire nuclear power plant GG.