Safe zero base reactivity level nuclear fission reactor?

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Summary
Is it possible to create a safe nuclear fission reactor, that - when turned off - immediately falls down to a reaction level close to natural decay?
There are already 4 generations of nuclear fission reactors. (Compare https://en.wikipedia.org/wiki/Nuclear_reactor#By_generation.)

However among these, there does not seem to be a (maximum) safe nuclear fission reactor design, which immediately stops the nuclear chain reaction when the reactor is turned off. I guess this is related to the following:
  1. Large amount of nuclear fuel.
  2. Usual impossibility to remove the nuclear fuel element out of the reactor, when the reactor is operation. (Because the fuel element is very hot and highly radioactive.)
Effectively, this leads to risk of meltdown, namely the risk nuclear fuel can continue the nuclear fission chain reaction, when the reactor is already turned off, especially also when the nuclear has already become molten. To prevent the meltdown risk high efforts for cooling the reactor and backup power have to be taken in place.

Q1: Is it possible to create a nuclear fission reactor, that has a zero base reactivity level, so that the nuclear decay chain will stop more or less immediately when the reactor is turned off, respectively a meltdown-risk free nuclear fission reactor?

I am thinking about a nuclear fission reactor, that
  • contains less nuclear fuel
  • is based on nuclear fuel in liquid form
  • has a more effective neutron moderator than water
  • would only process significant chain reaction, when the nuclear fuel ions are concentrated in a small volume by an electrical field (and will fall down to natural decay immediately, when the electrical field is switched off)

Q2: What would be necessary to create such zero base reactivity level nuclear fission reactor?
 

anorlunda

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You are a little off base. Inserting the control rods stops the chain reaction.

The problem with fission reactors is what we call decay heat, which can be as high as 15% of rated power immediately after the chain reaction is stopped.

Decay heat is produced by the radioactive decay of unstable isotopes produced after splitting the atoms of the fuel. i.e. the so-called fission-products.
That is not a chain reaction.

So you're correct identifying decay heat as the origin of the safety problem, but wrong in calling it a chain reaction.

Nuclear fusion, not fission, is the usual silver bullet that people look towards as a safer form of nuclear power.

There are fission reactor designs that claim to be safer. For example.
https://en.wikipedia.org/wiki/Pebble-bed_reactor said:
This type of reactor is claimed to be passively safe;[1] that is, it removes the need for redundant, active safety systems. Because the reactor is designed to handle high temperatures, it can cool by natural circulation and still survive in accident scenarios, which may raise the temperature of the reactor to 1,600 °C.
 

etudiant

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The challenge of course is that a different design such as the pebble bed reactor will have different failure modes, not all of which are readily predictable. So I'd take any claims of superior safety by a new design with a grain of salt.

For instance, the Julich experimental pebble bed reactor wound up contaminating much of its environment because the pebble feed jammed at 3 am and the operators tried to clear the blockage with a broomstick.
They broke over 40 pebbles, releasing fissionable material all over the site.
 

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