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The reactor itself had several serious design flaws. The three most serious were 1) a positive void coefficient 2) control rods tipped with graphite instead of boron and 3) a design that allowed prompt criticality. This meant that during any event where you saw a sudden increase in reactivity, causing water to boil more rapidly, reactor power went up rather than down. Further, any insertion of the rods from the fully withdrawn position initially increased reactivity rather than reducing it.
The third flaw was by far the most serious. All US reactors are designed so that they cannot go prompt-critical (i.e. explode via a chain reaction like grade school diagrams of an atomic bomb depict.) Without a moderator they do nothing at all; there is not enough nuclear fuel in the core to sustain a nuclear reaction. Only when you add a moderator (like water) do you slow the neutrons down enough that they become reactive enough to sustain a nuclear reaction. Chernobyl was not designed like that; it COULD go prompt critical, and enter into a rapid runaway chain reaction similar to that in a nuclear weapon.
Another factor was a factor common to most reactors - poisoning. During normal operation, isotopes of uranium decay to isotopes of iodine to isotopes of xenon. Xenon absorbs neutrons very strongly, and during normal operation, the xenon thus produced is almost immediately burned up, But at extended low power levels, the process is uranium -> iodine -> xenon and it stops there; xenon accumulates and can almost completely stop the reaction by absorbing all the neutrons. After a long enough period of low power operation, you could end up in a state where all the control rods are completely withdrawn but there's not enough reactivity for a chain reaction.
The operator errors were equally as lengthy. They did not realize that they were poisoning the reactor by operating it at a low power level. They did not realize that the reactor had become unstable, even when they had to manually retract almost all the control rods to get the power level back to even 20% of nominal. They ignored dozens of alarms going off that indicated uneven neutron flux, uneven heating and general core instability. They then ran a test that gradually shut down the circulating pumps, resulting in lower flow rates and steam voids in the core.
The last mistake they made was scramming the reactor as a means to shut it down. The control rods, most of which had been completely removed, began to re-enter the core. The tips, composed of graphite, entered first, increasing reactivity. The remaining water boiled off almost immediately, resulting in a prompt-critical chain reaction. The resulting nuclear explosion would have leveled hundreds of square miles if the core had remained together. Fortunately, the core almost immediately disintegrated. The last reading before the instruments were destroyed was that the core was producing 33 gigawatts of power. The resulting explosion completely dispersed the core and stopped the chain reaction.
The reactor itself had several serious design flaws. The three most serious were 1) a positive void coefficient 2) control rods tipped with graphite instead of boron and 3) a design that allowed prompt criticality. This meant that during any event where you saw a sudden increase in reactivity, causing water to boil more rapidly, reactor power went up rather than down. Further, any insertion of the rods from the fully withdrawn position initially increased reactivity rather than reducing it.
The third flaw was by far the most serious. All US reactors are designed so that they cannot go prompt-critical (i.e. explode via a chain reaction like grade school diagrams of an atomic bomb depict.) Without a moderator they do nothing at all; there is not enough nuclear fuel in the core to sustain a nuclear reaction. Only when you add a moderator (like water) do you slow the neutrons down enough that they become reactive enough to sustain a nuclear reaction. Chernobyl was not designed like that; it COULD go prompt critical, and enter into a rapid runaway chain reaction similar to that in a nuclear weapon.
Another factor was a factor common to most reactors - poisoning. During normal operation, isotopes of uranium decay to isotopes of iodine to isotopes of xenon. Xenon absorbs neutrons very strongly, and during normal operation, the xenon thus produced is almost immediately burned up, But at extended low power levels, the process is uranium -> iodine -> xenon and it stops there; xenon accumulates and can almost completely stop the reaction by absorbing all the neutrons. After a long enough period of low power operation, you could end up in a state where all the control rods are completely withdrawn but there's not enough reactivity for a chain reaction.
The operator errors were equally as lengthy. They did not realize that they were poisoning the reactor by operating it at a low power level. They did not realize that the reactor had become unstable, even when they had to manually retract almost all the control rods to get the power level back to even 20% of nominal. They ignored dozens of alarms going off that indicated uneven neutron flux, uneven heating and general core instability. They then ran a test that gradually shut down the circulating pumps, resulting in lower flow rates and steam voids in the core.
The last mistake they made was scramming the reactor as a means to shut it down. The control rods, most of which had been completely removed, began to re-enter the core. The tips, composed of graphite, entered first, increasing reactivity. The remaining water boiled off almost immediately, resulting in a prompt-critical chain reaction. The resulting nuclear explosion would have leveled hundreds of square miles if the core had remained together. Fortunately, the core almost immediately disintegrated. The last reading before the instruments were destroyed was that the core was producing 33 gigawatts of power. The resulting explosion completely dispersed the core and stopped the chain reaction.
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