Why Chernobyl nuclear reactor accident happend?
Chernobyl Nuclear Disaster: Why Chernobyl nuclear reactor accident happened?
On 26th of April 1986, massive explosion happened in unit 4 of Chernobyl nuclear reactor. This accident damaged the reactor components, reactor buildings and resulting in the spread of huge amount of radioactive substances in the environment. Salient features of Chernobyl RBMK reactor has been discussed earlier.
- An experiment was planned in unit 4 before its planned maintenance related shutdown. Aim of the experiment was to make the reactor more safe if the station power fails, when core cooling is must to remove decay heat from the fission products, otherwise core meltdown can happen. It was planned to investigate whether slowing down turbine can be used to generate sufficient power for running vital components like cooling pumps.
Planned power level for the experiment was 700-1000 MWth. Series events on 25-26.04.1986 has been explained earlier.
During full power (3200MWth), the reactor configuration was as Fig.1. As per the plan, power reduction started and was brought down to 50% of designed power of 3200 MWth.
Fig.1: Chernobyl RBMK reactor during normal operation
Beyond this reduction was not allowed by Grid, as the requirement was high at the time. With 1600 MWth power, reactor operated for about 10h (25.04.1986, 13:00h-23:10h).
As per the test procedure, at14:00h, the emergency core cooling system (ECCS) was switched off.
- Xenon poisoning
Power reduction initiated from 23:10h. Instead of planned power level of 700-1000MWth, suddenly power got reduced upto 30MWth. Reason was due to either 'hold power at required level' was not given by operator or the command was not registered in the system or due to 'Xenon poisoning'.
Efforts started to increase the power level by pulling most of the absorber/ control rods. Power increased upto 200MWth only. This is due to the phenomena called 'Xenon poisoning'. Xe-135 is a fission product with extremely high absorption cross section of 2.6 million barns.
Nuclear properties of 135Xe is as follows
235^U(n,f)--> 135^I (T1/2: 6.57h)
235^U(n,f)--> 135^Xe (T1/2: 9.10h)
235^U(n,f)--> 135^I (T1/2: 6.57h) --> 135^Xe (T1/2: 9.10h) --> 135^Cs (2.3x10^6y)
135^Xe (n,g) (σ: 2.65(11)x10^6 barn)--> 136^Xe (σ: 0.26(2) barn) (T1/2: >8x10^20y)
- During reactor operation, due to 235U fission, both 135I and 135Xe forms. 135Xe forms by decay of 135I as well as by direct fission of 235U. Due to extremely high absorption cross section, 135Xe plays an important role for the control of reactor.
- During reactor operation four phenomena happens simultaneously, a) 135I forms from fission, b) decays to 135Xe and c) 135Xe gets burnt by absorbing neutron and becomes 136Xe and d) 135Xe decays to 135Cs.
- During shutdown, two phenomena happens simultaneously, 135I continues to decay to 135Xe and 135Xe gets burnt by absorbing neutron and becomes 136Xe.
- During reactor operation with reduced power, four phenomena happens simultaneously, a) 135I forms from fission at reduced rate, b) decays to 135Xe and c) 135Xe gets burnt by absorbing neutron and becomes 136Xe but at a slower rate and d) 135Xe decays to 135Cs.
So at reduced power of reactor operation, Xenon concentration builds up high and it creates major problem in reactor operation especially while increasing the reactor power.
The solution is delay and decay. The parent I-135 has half life of 6.57h, eventually that will decay to Xe-135. Let the Xe-135, with half life of 9.1h decay to a considerable extent.
Another way of increasing the reactor power can be by pulling the absorber/ control rod up. But the risk associated in this process is that, it may increase reactor power suddenly, when the Xe-135 is burnt off considerably. However, online reactivity calculation by keeping Xe-135 concentration and an automated and fast compensation using absorber rod/ control rod can be used to increase the reactor power fast.
In Chernobyl the absorber/ control rod was pulled up (Fig.2), to increase the reactor power quickly from 30 MWth. It reached only upto 200 MWth.
Fig.2: Chernobyl RBMK reactor before the planned experiment
At this point, 2 reserve circulating pumps were put additionally
into operation to increase the total no. of pumps in operation to 8. So, the flow rate of water through core increased than normal and hence reduced the steam generation. Decrease in cavitation (vapor droplet in water) observed, finally lead to negative reactivity effect generated.
Since the reactivity started decreasing, the automatic control rods were fully withdrawn within 30s and attempts were made to withdraw manual control rod also. But the reactivity started overcompensation, power of the reactor started decreasing, the automatic rods began to move back into the core. The reactor became therm-hydraulically unstable.
At 01:22:30h since control rods were withdrawn, especially at the lower portion of the core was filled with water. Water has very high neutron absorption cross section. In normal state, the nominal temperature of the coolant at the inlet of the reactor is 265–270°C and the outlet temperature 284°C , at pressure in the drum separator and reactor of 69 bar. Boiling point of water is 296°C at 69bar. Any increase in the temperature or decrease in pressure might initiate boiling of water and can change the steam to water ratio and hence composition of the coolant. Increase in vapor content means less water molecule density, less neutron absorption. But here graphite is the moderator. Graphite shall moderate the neutron and decrease the energy of fast neutron to thermal (slow) neutron. Fission cross section for 235U is high for thermal neutron. Very high positive reactivity is the result.
- Test begins
By 01:22h reactor was stable to carry out the planned experiment. The test begins at this point at 01:23:04h of 26.04.1986. Automatic emergency shutdown system and other safety systems were switched off to carry out the experiment. Feed water flow rate was brought back to the initial level. Feed water flow rate was brought back to the initial level. Coolant inlet temperature increased, unstable situation created.
As part of the experiment, the turbine stop valve was closed. Stop valve is used to stop the flow of liquid or gas in a system. Four of the primary circulation pumps that had been connected to the test turbo generator were disconnected from their supply. Steam to the turbine was shut down. Experiment began. Power of the reactor was 200 MWth. Operation reactivity margin (ORM) was only 6-8 absorbers rod equivalent instead of minimum 15.
Reactor power started to increase.
At 1:23:40h SCRAM or 'Safety Control Rod Axe Man' (emergency button (AZ-5)) was pressed which is used for manual shut down of the reactor. Shutdown happens by sending all control rods and safety rods to the core (Fig.3).
Fig.3: Chernobyl RBMK reactor, if emergency AZ-5 command would have worked properly.
Instead of decreasing reactivity power started increasing.
01:23:43: Power reached 530MWth.
Absorber rods stopped on its way and could not reach to its lower stop (Fig.4). Layer current was also cut off to the sleeves of the servo drives to enable the rods to fall into the reactor under their own weight, which was also most likely failed.
The reactor power reached about 3,20,000 MWth i.e. 100 times the full power (3200 MWth), fuel disintegrated, and excess steam pressure broke the pressure tubes.
Fig.4: Chernobyl RBMK reactor, when emergency AZ-5 command worked with malfunction.
- Reactor explodes.
Two explosion was seen from outside observer. 1st one at 01:24h and 2nd one within 3-4s, blows the 1,000-ton roof right off the reactor.
1st explosion
1st explosion was due to super heated steam explosion, rupture of fuel channel.
2nd explosion
Upon reaction of this steam and Zirconium clad and hot graphite, resulted in production of hydrogen and carbon mono oxide, which upon reaction with air caused 2nd explosion.
- Main Causes for explosion
It is important to understand why instead of decreasing the power and finally shutting down the reactor, why the power shoot up.
The Chernobyl RBMK reactor normal operation is shown in Fig.1. Graphite displacer is placed in front of Boron carbide rod. Advantages and problems of the graphite displacer rod in Chernobyl RBMK reactor is already discussed in our earlier posts.
To increase the power from 30MWth, most of the control rods were withdrawn (Fig.2), as a result power could be brought to 200MWth only.
Instead of pulling the control rod, 135Xe should have been allowed to decay.
Since control rods were withdrawn, especially at the lower portion of the core was filled with water. Water has very high neutron absorption cross section. To perform the experiment at 200MWth, four of the primary circulation pumps that had been connected to the test turbo generator were disconnected from their supply. Temperature of waster increased, pressure of the water decreased, water started boiling. Water vapor content was increased. In presence of graphite moderator instead of decrease, fission increased. Very high positive reactivity was generated. In water cooled and water moderated reactor system, reactivity would have decreased and reactor would have shut down.
In this scenario, emergency shutdown button (AZ-5) was pressed. All of a sudden 179 no. of control rods pulled back to the core. Its speed was slow, moreover operation stopped in between and could not be inserted fully even after manual intervention. Since control rods had graphite displacers, these graphite displacers displaced the water from the channel. Since graphite has less neutron absorption cross section than water, even though due to water's boiling, no. density of water molecule was less, when huge no. of graphite reached in by displacing water, suddenly there was more thermal neutron, more thermal fission. Availability of thermal neutron was increased in presence of graphite, instead of boiling water i.e thermal flux increased. Thermal fission cross section of 235U is more than fast fission. Higher the flux, higher is the reaction (fission) rate, higher release in energy.
Hence, reactor's power increased very fast, 10x, 100x, which finally resulted in two massive explosions.
Lack of containment building resulted into spread of the massive amount of radioactive waste in the atmosphere.
Lack of transparency resulted into the spread of radioactivity to the neighboring countries and the people staying nearby the Chernobyl RBMK reactor.
Lack of proper safety culture allowed to conduct such a crucial experiment in the night shift at such a unstable low level of 200MWth power, bypassing the emergency shutdown system, mismanagement of xenon poisoning issue and by violating minimum operation reactivity margin (ORM).
Reference
Highlighted in the textRead other posts of this website
Chernobyl Nuclear Disaster: Series of events on 25-26 April 1986
Problems of Chernobyl RBMK reactor
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