Why not just pour a ton of boron into the core?

[Kidphysics]

In case of a meltdown wouldn't large amounts of boron dust dumped over the core eat up all the free neutrons?

 

[gmax137]

Yes but eating up all the neutrons doesn't stop the heat being generated by the decay of the previously created fission fragments. That decay heat, not an inability to shut the reactor down, is what (generally) results in core melt.

 

[Hiddencamper]

Typically a reactor is shutdown well before melting. Automatic safety systems will insert control rods when low water or a loss of coolant is detected.

What causes meltdowns is decay heat. There are two heat sources in a nuclear reactor, fission and radioactive decay.

The control rods and/or boron completely shut down fission. But you still have decay heat. While decay heat isn't a lot of heat, there is enough of it that if it is not continuously removed you will boil off your cooling water and melt your fuel cladding.

Tmi and Fukushima were all shut down well before the plants had melting.

 

[Simon Bridge]

Off the top of my head ... have you checked the cost-benifit?

i.e.
How much would you need? What would it cost?
Where would it be stored and how? (What are the problems with storing Boron dust?)
How fast does it have to be deployed?
How would it work in with other damage limitation measures?
What is the melting/boiling point compared with the temperature of a core in meltdown?

Boron is already used for shielding though.
Are you thinking of the cases where the containment has been breached?

Then there's considering what it is supposed to achieve (the benefit).
Is the idea to make a full meltdown less likely of just to reduce radiation exposure from neutrons, or something else?

 

[Astronuc]

In case of a meltdown wouldn't large amounts of boron dust dumped over the core eat up all the free neutrons?

Boron (and specifically B-10) would not do much outside (on the periphery) of the molten core, if that molten core attains a critical state. The boron would have to be distributed in the core mass. One would have to determine if the molten core is porous such that water could infiltrate and provide moderation. Molten cores may be undermoderated.

As others indicated, the problem with damaged/molten cores is the decay heat, which comes from the decay of fission products and transuranics. Radionuclide decay is not affected by boron, or any other chemical element.

The idea of boron or other neutron absorber is to prevent a mass of fissile material or molten core from achieving criticality.

Otherwise, the objective, as others have stated, is to remove the decay heat in a closed loop. The decay heat is then dissipated to the environment. Loss of heat removal was the problem at TMI and Fukushima.

Two key design objectives in nuclear reactor design are reactivity control (ability to shutdown the fission reaction) and coolability (the ability to keep the core fuel and structures well below the temperature which would cause damage or severe failure of the fuel). The goal is to retain the fission products and prevent them from entering the environment.