Is or was nuclear chain reaction possible in damaged Fukushima reactors?


The Fukushima nuclear reactors, No.1 through No.3, were shut down at the earthquake completely, which was done by inserting all the control rods from the bottom of the nuclear reactor core.  Immediately, the ECCS system (emergency Core Cooling System) started working with the electric power generated by the emergency power generator. However, ECCS stopped at each plant 45 minutes after the earthquake because the emergency electric generators were damages by the tsunami.


After the shutdown of the reactors, nuclear chain reactions that involve fission of uranium 235 and plutonium 239 have never been possible. The first reason is the control rods in the core consists of a very strong neuron absorbers that never allow enough neutrons to bombards fissionable material such as uranium 235 or plutonium 239 on a sustainable level.  Nuclear chain reactions are somewhat similar to human or animal population cycle. With control rods, the population dies down very fast.  The second reason is that the ECCS has injected water into the core together with boric acid, which is a strong neutron absorber and has the same effect as the control rods.


However, if the fuel rod claddings of zirconium alloys break and the fuel pellets inside start falling in the reactor, then they would be stuck somewhere at the bottom of the core, below the core but above the bottom, or totally on the bottom of the pressure vessel.


If the water in the pressure vessel does not contain boric acid, formation of critical mass of uranium oxide pellet for nuclear chain reaction is a possibility.  In reality, however, this is impossible because boric acid has been injected by the ECCS, and also the sea water was mixed with boric acid when injected into the core.


Hypothetically, what would happen if no boric acid has been mixed in the water in the reactor pressure vessel and a critical mass has been reached?     Then heat is generated, the rate of which would be increased quickly, which would generate steam bubbles around the fuel pellets.  The steam bubbles have a strong negative power coefficient, that means the nuclear chain reactions will be suppressed.  This suppressing effect of the bubbles on the chain reaction is the great advantage of water cooled reactors such as BWR and PWR.  Thus, it is unconceivable that any sustainable nuclear chain reaction can occur during the core melt-down accident. If such a nuclear chain reaction would occur, a fast and acute pressure rise with some fluctuation would be detected, which has never been observed in Fukushima reactors.


In conclusion, nuclear chain reactions during shut down of a nuclear reactor, even when fuel rods are damaged, have been impossible.