Prompt neutrons, delayed neutrons, chain reaction control

[etudiant]

Will defer to the experts we benefit from on this site. They will have better insight.
My understanding is that the Chernobyl reactor did go up in power by about a thousand times in an instant as some of the control rods were being reinserted, because the ends of the control rods did not absorb neutrons, but only slowed them. That was enough power to ensure world's most effective steam explosion as all the coolant was flashed into very high temperature steam in a fraction of a second.
A nuclear explosion by itself is just a heat source, the particles streaming from the fission or fusion reactions may be lethal, but are not really that damaging, a reality that has of course stimulated the development of neutron bombs. Here the steam explosion did the damage.
Chernobyl was a nuclear excursion, but not an explosion, because the reactor blew apart before there could be a nuclear explosion. It underscores that the problem in making a nuclear bomb is how to keep the components together long enough for the explosion to occur.

 

[Astronuc]

Ok so I get from what you say and also from what I have read in my life that most latest generation and second generation reactors are built such that under normal operating conditions the reactor is calculated and designed such that it cannot go supercritical , in other words its coolant and solid reactor moderators combined make it impossible to go supercritical to the point of bomb like chain reaction speed? So what happens for example in a PWR or BWR for that matter if the coolant is lost like at Fukushima or TMI ? If I remember correctly even then the design doesn't allow for supercriticality , its only that with no coolant the decay heat generates enough heat to eventually melt the fuel cladding and make the fuel turn into a pound of lava at the base of the reactor vessel from where it makes some gasses in reaction to the metals around which either are ventilated out the vessel or can cause some pressure damage to the vessel or hydrogen formation and a hydrogen gas explosion but still no criticality explosion correct?

So if the reactor has built in moderators like graphite in some reactors and or water/heavy water in others , then why the control rods are needed at all ? are they simply servig the purpose of stopping the reactor once needed and reactivity increase/decrease aka power level up or down ?

There are a number of important technical aspects here. One aspect is normal reactor operation with safeguards to check abnormal situations. The other is abnormal or accident conditions, and how reactors are designed to mitigate adverse consequences.

Here is a reasonable good discussion of criticality, and particularly prompt criticality.
http://www.nuclear-power.net/nuclea.../reactor-criticality/prompt-critical-reactor/

A commercial reactor could go prompt critical if the reactivity increase in the core exceeds the effective delayed neutron fraction (β_eff). When a control rod or controlling neutron absorber is withdrawn or removed from the core, the k_eff increases above 1. The reactor is configured to allow changes in k, or Δk, to be less than β_eff. Operators are concerned about operational errors or mishaps where to much reactivity is added to the core.

There are certain events where you may have localized prompt criticality but the total core is stable or subcritical. For example a control rod drop accident in a BWR can do this. You have localized fuel damage but no gross core damage.

Hiddencamper provided an example of localized prompt criticality which would result in local fuel damage, but still allow the reactor to shutdown while maintaining coolability. The PWR 'control rod ejection' is the PWR analog to BWR control rod drop accident.

We are also concerned about accidents in which large amounts of reactivity (e.g., the reactivity addition is >> β_eff) are inserted in the core. But I probably need to explain how power responds to reactivity insertions.

Control rods are required in order to assure 'shutdown' of the nuclear reactor, and maintain k < 1. In PWRs, control rods typical sit above the core during operation, although some special control rods may be inserted to facilitate power distribution or are used during power maneuvering (See my previous post on load-following); otherwise, reactivity control is maintained with soluble boron in the coolant, in conjunction with burnable poisons (neutron absorbers) in the fuel.. In contrast, BWRs use control rod during operation, since they cannot use soluble boron in the coolant like PWRs, due to the boiling in the core. BWR fuel also uses burnable poisons in the fuel.