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Neutron embrittlement is a big problem in reactor design, as high energy neutrons cause lattice defects in materials like steel by slamming into iron nuclei. This limits the lifetime of the reactor, because nobody likes a pressure vessel failure.

In order to combat this, why don't we coat the inside of a pressure vessel in boron? The boron would absorb the neutrons and thus the steel would avoid embrittlement.

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  • $\begingroup$ How thick does the boron need to be? Is it going to mix into the water? What happens as it absorbs neutrons over time? How many other engineering problems can you come up with? $\endgroup$ – Jon Custer Jun 9 at 22:51
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While you can do that, you can also just take a thicker piece of steel as it's typically one of the cheapest materials you can get. Energy spectrum is typically not that hard as neutrons are usually moderated (if it's not fast-neutron reactor).

But embrittlement in pressure vessel is not a major concern during reactor design. Neutron economy is the most important as it has direct impact on nuclear reactor economy. Every neutron = $ (you can ether trade it for fuel regeneration or lower fuel enrichment = lower costs).

In the nuclear reactor design it is preferred to use as much neutrons as possible, and hence try to reflect neutrons back to the core rather than dump them into the pressure vessel. Without reflector you will have very high non-uniformity of neutron flux in the volume of the core and it will be hard to operate.

In VVER PWR reactors reflector is made from water-steel sandwich. In graphite-based reactors reflector is also made from graphite.

You can see reactor composition example here: https://www.researchgate.net/figure/Horizontal-Top-and-vertical-Bottom-cross-section-of-3D-60-0-symmetric-two-energy_fig8_309576283

enter image description here

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  • $\begingroup$ If neutrons = $, then what is preventing us from using stupidly high neutron fluxes (I read somewhere that the typical neutron flux is around 1E13 n/cm^2, why can't we do 1E16 and therefore minimize core size) - is it thermal problems? $\endgroup$ – Nikhil Murali Jun 10 at 16:35
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    $\begingroup$ @NikhilMurali 1) Small core size = higher enrichment needed to achieve criticality = increased cost / risk of proliferation 2) There is a limit on how much energy you can extract from a given area of fuel assembly. It is very hard to scale it safely. 3) Small core with high flux = fast burnout, you'll have to reload fuel every week/month instead of every 18 month (as there is a limit how much energy you can extract until fuel is unstable mechanically and cause significant changes in reactor physics). Reload = time where you don't make money = money lost. $\endgroup$ – BarsMonster Jun 11 at 0:27
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    $\begingroup$ @NikhilMurali 4) And surely, components that are inside the core - will suffer from this increased flux (fuel assembly, control rods, instrumentation), but not pressure vessel. $\endgroup$ – BarsMonster Jun 11 at 0:28
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Fast neutrons are responsible of neutron embrittlement in the pressure vessel , so boron is not the best material to acts on fast neutrons .

In the new PWR EPR french reactor , a multi sections (12) heavy reflector in stainless steel is fitted between the polygonal core and the cylindrical core barrel .

This reflector reduces the number of neutrons escaping from the core , flattens the power distribution and reduces the exposure of the vessel to fast neutron fluxes slowing down their energy .

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