Reply To: YouTube Focus Fusion Video

[jamesr]

Alchemist32 wrote: I just wanted to clarify a few points. Boric acid is not used in reactor water, this would create a major radiation problem from the production of approx 10^18 to 10^20 prompt gamma photons per second. It would also cause major problems regarding the conductivity of the water (causing corrosion). Instead, in some reactors, boric acid is added to the concrete surrounding the core. This boric acid is normal abundance boron since the 20% B-10 is more than sufficient for this purpose.

Unfortunately, the separation of B-10 and B-11 is not done on an industrial scale. It hypothetically could be, it just isn’t. The major use of isotopically enriched boron is in semiconductor doping, where B-11 is used to produce neutron-hardened ICs for the military. A secondary and much smaller use is for B-10 enriched boron compounds utilized in boron neutron capture therapy (BNCT) (I am a professor in this area). B-11 enriched decaborane would be exorbitantly expensive, ~ $10,000 per gram, although, this cost could be brought down to about $5k per gram.

From my masters last year we had several lectures on PWR water chemistry and they went through a number of operating scenarios used in the past and how they have improved the corrosion rates by carefully adjusting the pH, and adding small amounts of things like zinc. But they all use boric acid. Initially all at natural isotopic ratios, but as it stays in there longer the proportion of B10 drops slightly as it is used up.

The pH condition is normally kept at ~7.4 (neutral pH at 300C is 5.71) but varies over the fuel cycle as the concentration of boron is reduced from ~1200ppm to 0 by diluting it as the fuel is used up over an 18month or so period, before refueling again. So it is overall basic, due in part to the lithium, which is carefully controlled and kept at around 2ppm concentration. The large change in boron concentration over a fuel cycle to maintain criticality means it cannot be built into the structure and has to able to be gradually reduced to compensate.

Advanced operating cycles are now using enriched boron of around 30% B10 in order to work with higher enrichment MOX fuels, and longer cycles to achieve higher burn-up rates. This has been done at Gosgen and other Seimens plants since 1999. The B-10 concentration again drops normally during a fuel cycle but is topped up with 98% B-10 solution.

At 1200ppm considering the many tonnes of water in the system this corresponds to several kg of boron in the primary circuit and CVCS (chemical & volume control system) at any one time. So I assumed if they are using kg quantities of B-10 enriched boron for some PWRs, then there must be the equivalent amount of B-11 by-product lying around somewhere.

I don’t doubt the price though – I’m sure they spend millions on the boron systems for PWRs

For BWRs the water chemistry is complicated further due to the concentrating effect of the boiling action. But the principle of the neutron absorption rate having to change over a fuel cycle is still the same.