Why did they use Boron at Chernobyl?

[Jimmy87]

Hi,

Please could someone explain the reason for putting Boron onto a compromised reactor core like they did (or more specifically tried to do) at Chernobyl? This article on the topic is written by a nuclear reactor engineer:

www.livescience.com/amp/65515-chernobyl-in-modern-times-nuclear-emergency.html

It says that the boron stops further fissioning (like it does in a functioning reactor). However, other sources (Wiki being one of them) say that when the reactor exploded and the fuel channels ruptured, the chain reaction stopped. With the core material exceeding 1200 degrees Celsius wouldn’t the neutrons be going too fast anyway to cause any further fissions? Also with the fuel now spread out surely there would be much less chance of a neutron hitting a uranium nucleus? So how would the boron help?

Thanks for any insights offered

 

[.Scott]

First, there is a difference between the chain reaction stopping and the chain reaction going sub-critical.
After the explosions, the chain reaction certainly went sub-critical (that is, on average for each neutron emitted, less than one additional neutron was directly generated), because if it hadn't there would have been yet another explosion.

But the remain were still very radioactive and thermally hot. And even sub-critical reactions can add to the heat being generated. Also, you don't want to dump anything onto the pile without thought - you have to ask if what your doing could recreate criticality.

The objective of the dumps was to stop the fires, improve containment, and to improve the working environment. When the boron was dumped, the reactor was still venting large amounts of radioactive material into the air.

 

[mfb]

They didn't know much about the state of the core and there was no way to control it properly. Was it still near criticality? Could it become critical again? It was hot, that reduces fission cross sections, but the reactor was also missing water as neutron absorber. Dumping boron on it was the safe option.

 

[.Scott]

Someone with more knowledge than I can verify this - but 1200C doesn't sound very hot as neutrons go. Anything that was moderating the neutrons to sustain the chain reaction would be moderating them down to a much higher temperature than 1200C.

 

[mfb]

1/8 eV vs. 1/40 eV, a factor 5, leading to a factor ~3 or so in fission cross section. Here is a plot.

 

[anorlunda]

there is a difference between the chain reaction stopping and the chain reaction going sub-critical.

I agree except that the subcritical reactions need not be chain reactions. Just remove the word chain.

You can have a neutron cause an isolated fission event even if the new neutrons do not produce more fissions. The boron can help suppress that.

 

[DEvens]

Shortly after a reactor stops being critical you still have fission products.

Fission products typically have too many neutrons to be stable. This is because as you move along the period table to larger number of protons, the number of neutrons required to produce a stable nucleus rises faster than the number of protons. For smaller number of protons it tends to be an equal number of protons and neutrons. For larger number of protons you tend to need extra neutrons. Lead, for example, has 82 protons, and the stable isotopes all have 122 to 126 neutrons.

So fission products tend to have too many neutrons. So they tend to emit these neutrons to get stable. This is in addition to just bunches of other radioactive decays. They are doing beta plus and minus, gamma, alpha, electron capture, even a small number of spontaneous fission reactions.

So the area around recently-operating fuel is heavy with neutrons as well as lots of other forms of radiation.

Boron has a big capture cross section for neutrons. Boron is relatively cheap and fairly safe to carry around. You probably have some borax soap in your house right now.

 

[willem2]

First, there is a difference between the chain reaction stopping and the chain reaction going sub-critical.

There really isn't a significant difference. If the reaction becomes sub-critical it will stop, and if it becomes critical again, the reaction will start up again in a few seconds, because there will always be some neutrons around.(spontaneous fission of U238).
The Boron is added to make sure the reactor stays sub-critital. The reactivity can increase again if the reactor cools down, or if a pool of fuel collects at the bottom and water gets into it, or if neutron poisons decay.

 

[Jimmy87]

There really isn't a significant difference. If the reaction becomes sub-critical it will stop, and if it becomes critical again, the reaction will start up again in a few seconds, because there will always be some neutrons around.(spontaneous fission of U238).
The Boron is added to make sure the reactor stays sub-critital. The reactivity can increase again if the reactor cools down, or if a pool of fuel collects at the bottom and water gets into it, or if neutron poisons decay.

Thanks. So I am right in thinking that the reactor melted down mainly because of decay heat of the daughter isotopes since, as you say, fission reactivity decreases with increasing temperature? What is the reason for decreasing fissions with increasing temperature?

 

[Rive]

It says that the boron stops further fissioning (like it does in a functioning reactor). However, other sources (Wiki being one of them) say that when the reactor exploded and the fuel channels ruptured, the chain reaction stopped.

Boron obstructs chain reaction by absorbing free neutrons. It has nothing to do with (natural) fission or decay.

In case the core is damaged and its status is unknown it is a priority that no further chain reaction could happen, by any chance. To ensure that, to choke the remains of the core in boron is the traditional solution.

If there is any moderator around (water, or in that case: graphite) then there is a slight chance that even a pile of low enrichment debris might go critical (since the size and composition of the pile is unknown, as well the presence of any control rod remains).

 

[mfb]

Thanks. So I am right in thinking that the reactor melted down mainly because of decay heat of the daughter isotopes since, as you say, fission reactivity decreases with increasing temperature? What is the reason for decreasing fissions with increasing temperature?

Typically the chain reaction stops because the material stops being in a critical configuration - but who wants to rely on that?
The neutron cross section decreases with increasing energy (in that range). A classical analogy that works surprisingly well: At low speed neutrons are close to the nucleus longer.

 

[DEvens]

Thanks. So I am right in thinking that the reactor melted down mainly because of decay heat of the daughter isotopes since, as you say, fission reactivity decreases with increasing temperature?

The reactor first exploded. Then some of the remaining fuel melted. Also, non-trivial portions of the reactor were on fire for some time after the explosion. The burning parts included a lot of graphite.

In a more whimsical moment, a coworker described this as "reactor shutdown through spontaneous disassembly."

Some parts of the fuel were dispersed far enough and into small enough chunks that they were cool enough to not melt. These chunks were hazardous in their own special ways. Emergency workers would be searching along with radiation detectors, and find a lump of fuel partially buried or some such. Then they would have to collect it for disposal. There were enough dangerous jobs to go around.

Some of the heating came from the graphite fire. Graphite, being carbon, burns in a manner not completely different from a similar amount of coal.

Some parts were large and heated by the decay of fission products sufficiently to melt. Especially after the graphite burned away enough for the remaining fuel to start settling. This was primarily fuel that was left near the bottom of the core after the explosion. Note that this heat comes both from the isotopes themselves getting hot when they release radiation, and from them catching the radiation of nearby fission products. So it's very dependent on how densely packed the irradiated fuel is. Once it melts, if it forms a puddle, the puddle can be just amazingly efficient at holding heat.

What is the reason for decreasing fissions with increasing temperature?

The primary one has been discussed. Higher temperatures mean faster moving neutrons. Faster moving neutrons see a part of the cross-section-vs-energy curve where the fission cross section is smaller. Very roughly, extremely very roughly, a faster moving neutron has less time near a nucleus, and so it has less probability to interact. But be aware there are huge amounts of complications that come with this notion. Resonances just being one.

There are indirect effects such as changing density of various materials. That's getting way very complicated.