While a nuclear plant is operating, the fuel rods are carefully spaced and control rods between them absorb the neutrons bouncing around that cause the fuel rods to continue the reaction.
In a normal SCRAM, all of the control rods drop, they absorb all the neutrons bouncing around and the atomic reaction stops – and the remaining problem is the rods are very hot and will take a couple weeks to cool down.
But if the rods catch on fire (Cesium release confirms that) and the uranium rods melt, the uranium no longer is in its safe, well planned configuration including knowing how long each rod has been used. If enough uranium collects without the control rods and the space separation, the mass of molten uranium can reach “criticality”, which means that the nuclear reaction restarts itself spontaneously – so instead of just being hot, the uranium begins to produce new heat and massive radiation.
Boron (boric acid) absorbs neutrons. They are trying to pump in the boric acid to prevent the criticality – if the uranium starts to generate new heat, no amount of sea water is going to cool it down.
So you might be wondering “What does it mean that Boron absorbs a neutron?”…. energy and matter don’t just vanish – that Neutron being absorbed causes something else to happen. We’re beyond my schooling in physics, so take this with much more skepticism.
When the neutron is first absorbed, it emits gamma radiation. Gamma Radiation is a really bad thing – it’s pure electromagnetic energy like visible light – it travels as far as it needs to to dissipate its energy – in air, that means several kilometers. It’s ionizing radiation, which means it causes cellular damage. Lead will stop gamma rays.
The unstable boron atom then decays into Lithium-7 and Helium-4 – both non-radioactive harmless elements
True. But the important thing to remember is that when a neutron is absorbed it’s no longer available to hit another uranium atom and create a LOT more energy (including gamma rays) and more neutrons. A little boron goes a long way. BTW, the reason that boron is being talked about so much on the news is because relatively high concentrations of boron are normally included in all the emergency core cooling water supplies, and in the supplemental coolant used to cool the reactor during a any normal cooldown. Seawater doesn’t have enough natural boron so they’re having to add it. 2200 ppm is the usual concentration.
My cousin also mentioned a little boron goes a long way. He was involved with nuclear sub reactors in the 1960s I think, and worked at Batavia when I was a kid (he’s a generation older than me)
Some info:
Boron has a large capture cross-section, but only for neutrons in the thermal energy spectrum. Once the core is compromised, the flux distribution becomes indecipherable, so boron would be best used where the core is intact. The good news is that neutrons lose energy most often from collisions as neutrons have no net charge (thus the name). Maximum energy transfer occurs when the incident item is about the same size (mass) as the neutron. Hydrogen has one proton and one neutron in its nucleus (choose your favorite atomic model here) and this leads to a fairly rapid reduction in neutron energy from fast to thermal and then to slow. Water in any form is mostly hydrogen (H2O – light water) and provides a fairly high probability for collision. Sea water has both lots of hydrogen as well as the ability to cool those things it comes in contact with. Boron is good – water is better.
It’s hard to decipher things from the media – it can be hard to tell if they are recycling news.
The last hour is dominated by the report that the fuel rods were exposed for 2 hours. They’re making it sound like this just happened, but it may well be they are reporting a news conference describing what happened days ago.
The Fukushima reactor problem is consuming hundreds of tons of Boric Acid. Word has gone out to the world’s nuclear power plant operators begging for them to send any spare Boric Acid to Japan ASAP.