Imagine you are inside a nuclear reactor, a UK design. Not only are you inside it, but you are part of it; a carbon atom inside the graphite core which houses the control rods and fuel rods (the ‘moderator’). Around you the environment is glowing with heat and radiation, all given off in the splitting (fission) of uranium-235 nuclei. The temperature of 450°C is no problem, and you remain tightly bound in a lattice arrangement with your fellow carbons.
However, when the uranium nuclei split, they spit out more neutrons which pelt towards you at high speeds. One slams into you, and you slow it down, as is your job, so it travels at a suitable speed to cause more fission events. In this process you absorb the neutron’s energy, and get knocked out of your slot in the lattice. You whiz towards your fellow carbon atoms, knocking more out of their spaces like a billiard ball, wreaking havoc in the strict order of the graphite crystal. Eventually you transfer all of your extra energy to your neighbours and come to rest, filling a vacancy left by another displaced carbon or squeezing in between the orderly lattice layers (as an ‘interstitial’). Here you wait, ready to absorb the excess energy of the next neutron. The upheaval is routine to you, as during your life in the reactor you may switch places up to 30 times.
Well, the effects are unpredictable. The radiation barrage that the graphite endures can cause it to change its material properties; its thermal expansion, strength and even its dimensions, in strange ways. Even to the human eye, these changes would be noticeable. The moderator can change shape by up to 2%, depending on the grade of graphite; a surface that started smooth may finish rough. The dimensions may warp so that the control rods used to restrain the nuclear reaction may no longer fit into their channels. It is clearly important to completely understand how the graphite will change when designing new reactors or maintaining the existing ones. The problem is that we don’t.
However, when the uranium nuclei split, they spit out more neutrons which pelt towards you at high speeds. One slams into you, and you slow it down, as is your job, so it travels at a suitable speed to cause more fission events. In this process you absorb the neutron’s energy, and get knocked out of your slot in the lattice. You whiz towards your fellow carbon atoms, knocking more out of their spaces like a billiard ball, wreaking havoc in the strict order of the graphite crystal. Eventually you transfer all of your extra energy to your neighbours and come to rest, filling a vacancy left by another displaced carbon or squeezing in between the orderly lattice layers (as an ‘interstitial’). Here you wait, ready to absorb the excess energy of the next neutron. The upheaval is routine to you, as during your life in the reactor you may switch places up to 30 times.
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| A finite element model of a graphite sample and how the model behaves when irradiated or heated. Image credit: Dr Graham Hall. Manchester University |
This is just one atom, but what are the consequences of millions jumping around like this?
Well, the effects are unpredictable. The radiation barrage that the graphite endures can cause it to change its material properties; its thermal expansion, strength and even its dimensions, in strange ways. Even to the human eye, these changes would be noticeable. The moderator can change shape by up to 2%, depending on the grade of graphite; a surface that started smooth may finish rough. The dimensions may warp so that the control rods used to restrain the nuclear reaction may no longer fit into their channels. It is clearly important to completely understand how the graphite will change when designing new reactors or maintaining the existing ones. The problem is that we don’t.
