Chernobyl: Part 2. CHRONOLOGY OF ACCIDENT DEVELOPMENT.

Information on the Chernobyl accident and its consequences prepared for the IAEA Report No. 1 (INSAG-1)


C O R E R A N E E
Flash drive
0. Introduction
1. Description of the Chernobyl NPP with RBMK-1000 reactors.
2. Chronology of the development of the accident.
3. Analysis of the process of development of the accident on a mathematical model.
4. Causes of the accident.
5. Preventing the development of an accident and reducing its consequences.
6. Control over radioactive contamination of the environment and public health.
7. Recommendations for improving the safety of nuclear power.





2. CHRONOLOGY OF ACCIDENT DEVELOPMENT


The fourth unit of the ChNPP was put into operation in December 1983. By the time the block was stopped for medium repairs, which was scheduled for April 25, 1986, the active zone contained 1659 TBC with an average burn-up of 10.3 MW day / kg, 1 additional absorber and 1 unloaded channel. The main part of the fuel assemblies (75%) consisted of assemblies of the first load with burn-out of 12-15 MW day / kg.

Before stopping, tests of the turbogenerator (TG) No. 8 in the run-out mode with the load of own needs were planned. The purpose of these tests is to test experimentally the possibilities of using the mechanical energy of the rotor of a turbo-generator that is disconnected in pairs to maintain the performance of the auxiliary equipment of the unit under conditions of de-energizing.

Similar tests have already been carried out earlier at this station. Then it was found out that the voltage on the generator tires drops much earlier than the mechanical energy of the rotor is consumed during the run-out. In the tests scheduled for April 25, 1986, it was envisaged to use a special regulator of the magnetic field of the generator, which was supposed to eliminate this drawback. However, the "Work Program for Testing the Chornobyl NPP Turbine Generator No. 8", according to which they were to be carried out, was not properly prepared and was not coordinated with the responsible services of the NPP.

The program in essence did not provide for additional security measures, it prescribed the disabling of the ECCS. This meant that during the whole test period (~ 4 h) the safety of the reactor would be significantly reduced, which was not allowed by the operating procedure.

Due to the fact that the safety of these tests was not paid due attention, the staff was not ready for them, did not know about the possible dangers. In addition, as will be seen from the following, the staff allowed deviations from the program, thereby creating the conditions for an emergency situation.

On April 25, at 1:00 am the personnel began to reduce the power of the reactor operating at rated parameters, and at 13:55 hours TG # 7 was disconnected from the grid at the thermal power of the 1600 MW reactor. Power supply for own needs (four MCP, two electric pumps, etc.) was transferred to TG # 8 tires.

At 14 hours 00 minutes in accordance with the test program from MCPC was disconnected ECCS. However, at the request of the controller, the withdrawal of the unit from work was delayed. In violation of the regulations, the operation of the unit at this time continued with the disabled ECCS.

At 23 hours 10 minutes the power reduction was continued. In accordance with the test program, the run-off of the generator with the load of own needs was supposed to be carried out at a reactor thermal power of 700-1000 MW. However, when the LAR system was shut down, as stipulated in the reactor's low-power operation regulations, the operator could not quickly eliminate the imbalance in the measuring part of the AP. As a result, the heat output fell below 30 MW. Only by 1:00 am on April 26, 1986 it managed to stabilize at a level of 200 MW. Due to the fact that the "poisoning" of the reactor continued during this period, a further increase in capacity was hampered by the small operative reserve of reactivity, which by that time was significantly lower than the regulatory one.

And yet it was decided to conduct the tests. At 1 h 03 min and 1 h 07 min, in addition to six working MCPs, one more MCP was added on each side so that after the end of the experiment, in which four MCP had to operate, four MCPs remained in the MCC for reliable cooling of the active Zone.

Since the reactor power, and therefore the hydraulic resistance of the core and the MCC were significantly lower than the planned level, and all eight MCPs were in operation, the total flow of the coolant through the reactor increased to (56-58) 103 m3 / h, and in some MCPs to 8000 M3 / h, which is a violation of the operating procedure. This mode of operation is prohibited due to the danger of pump disruption and the possibility of vibrations of the loop lines due to cavitation. Connection of additional MCPs and the resulting increase in water flow through the reactor led to a decrease in vapor generation, a drop in vapor pressure in the BS, and a change in other parameters of the reactor. Operators tried to manually maintain the main parameters of the reactor - steam pressure and water level in the BS - but it was not possible to fully do this. During this period in the BS, there were dips in the vapor pressure of 0.5-0.6 MPa and dips along the water level below the emergency set point. To avoid reactor shutdown under such conditions, the personnel blocked the A3 signals by these parameters.

Meanwhile, the reactivity of the reactor continued to fall slowly. At 1h 22min 30s, the operator on the printout of the rapid reactivity assessment program saw that the operational reactivity margin amounted to a value that required an immediate shutdown of the reactor. Nevertheless, this personnel did not stop and the tests began.

In 1h 23min 04c the stop-control valves (SSC) of TG No. 8 were closed. The reactor continued to operate at a thermal power of ~ 200 MW. The existing A3 for the closure of the SSC of two TGs (TG No. 7 was turned off on April 25, 1986) was blocked in order to be able to repeat the test if the first attempt fails. Thus, one more deviation was made from the test program, in which there was no provision for blocking the reactor A3 to disconnect the two TGs.

Some time after the test began, a slow increase in power began.
At 1h 23min 40s, the block change supervisor gave the command to press the AZ-5 button, at the signal from which all the control rods and rods A3 entered the active zone. The bars went down, but after a few seconds, the blows were heard and the operator saw that the absorber rods had stopped before they reached the lower endpoints. Then he de-energized the couplings of the servo drives so that the rods fell into the active zone under the action of their own gravity.

According to eyewitnesses who were outside the fourth block, approximately one hour and 24 minutes there were two explosions in succession, over the fourth block, some burning pieces and sparks flew up, some of which fell to the roof of the engine room and caused a fire.