Not a single person has been killed from the meltdown of a modern nuclear reactor...

Modern reactors, with negative 'reactivity coefficients', are intrinsically safe.

What is the ‘reactivity coefficient’?

In nuclear energy, the ‘reactivity coefficient’ is a measure of how the reactor's reactivity changes in response to changes in certain conditions, such as the temperature of the coolant or the number of control rods inserted into the core.

There are two main types of reaction coefficients:

• The temperature coefficient: This coefficient measures how the reactor's reactivity changes in response to changes in the temperature of the coolant. A positive temperature coefficient means that the reactor becomes more reactive as the coolant temperature increases. This is because the hotter coolant becomes, the less effective it is at absorbing neutrons, which are the particles that cause nuclear fission.

• The void coefficient: This coefficient measures how the reactor's reactivity changes in response to changes in the void fraction, which is the fraction of the coolant that is in the form of bubbles. A positive void coefficient means that the reactor becomes more reactive as the void fraction increases. This is because the bubbles make the coolant less effective at absorbing neutrons.

The reaction coefficient is an important factor in the design and operation of nuclear reactors. A reactor with a positive reaction coefficient is more likely to become unstable and experience a power excursion, which is a rapid increase in power that can lead to a meltdown. For safety reasons, reactor designs usually aim for these coefficients to be negative, meaning that if the reactor starts to heat up, the resulting changes will naturally push the reactor to be less reactive, providing an inherent safety mechanism. This self-stabilizing behavior is crucial for passive safety, wherein reactors can shut themselves down or reduce their power output without active human intervention in the event of certain anomalies.

This figure shows the power excursion as a result of positive reactivity on a logarithmic scale. There is a curve without feedback and a curve for the same reactivity insertion but for which the effects of negative temperature feedback are included. It can be seen both curves initially follow the same, but as the power becomes larger, the curve with feedback becomes concave downward and stabilizes at constant power. At this point, the negative feedback has completely compensated for the initial reactivity insertion. Source: https://www.nuclear-power.com/nuclear-power/reactor-physics/nuclear-fission-chain-reaction/reactivity-coefficients-reactivity-feedbacks/

Chernobyl Nuclear Reactor and Accident in 1986…