The Nasty Neutron

[Meredith Trimble]

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<span style=”font-family: UICTFontTextStyleBody; font-weight: bold; caret-color: #000000; color: #000000; font-size: 28px; -webkit-text-size-adjust: auto;”> The Nasty Neutron</span>
<p class=”p2″ style=”margin: 0px; font-stretch: normal; font-size: 17px; line-height: normal; caret-color: #000000; color: #000000; -webkit-text-size-adjust: auto;”><span class=”s2″ style=”font-family: UICTFontTextStyleBody;”>Many of those concerned with carbon emissions and climate change have asked, what happened to the nuclear option? <span class=”Apple-converted-space”> </span>Conventional reactor designs kept getting bigger and bigger to take advantage of economies of scale. <span class=”Apple-converted-space”> </span>The concept changed little from the late ‘50s with water as the moderator of neutron energies. <span class=”Apple-converted-space”> </span>When nuclear fission happens in U235, it emits high energy neutrons. <span class=”Apple-converted-space”> </span>To trigger the next reaction the neutron energy must be absorbed by light atomic weight particles like hydrogen (protons) before entering the next U235 nucleus. <span class=”Apple-converted-space”> </span>Some of the high energy neutrons are not moderated enough, however. <span class=”Apple-converted-space”> </span>Over time when end of economic life occurs, the components of the reactor may have induced radioactivity from exposure to these energetic neutrons. <span class=”Apple-converted-space”> </span>This requires special handling for disposal as low level nuclear waste. In economic terms, the components have negative salvage value.</span></p>
<p class=”p3″ style=”margin: 0px; font-stretch: normal; font-size: 17px; line-height: normal; min-height: 22px; caret-color: #000000; color: #000000; -webkit-text-size-adjust: auto;”></p>
<p class=”p2″ style=”margin: 0px; font-stretch: normal; font-size: 17px; line-height: normal; caret-color: #000000; color: #000000; -webkit-text-size-adjust: auto;”><span class=”s2″ style=”font-family: UICTFontTextStyleBody;”>Technology has improved and what has been proposed are what are called “Fast “ nuclear reactors. <span class=”Apple-converted-space”> </span>These operate at higher temperatures and use more energetic neutrons. <span class=”Apple-converted-space”> </span>Higher temperatures allow higher thermal efficiency, but at a price. </span></p>
<p class=”p2″ style=”margin: 0px; font-stretch: normal; font-size: 17px; line-height: normal; caret-color: #000000; color: #000000; -webkit-text-size-adjust: auto;”><span class=”s2″ style=”font-family: UICTFontTextStyleBody;”>From: What is a </span><span class=”s3″ style=”font-family: UICTFontTextStyleItalicBody; font-style: italic;”>fast</span><span class=”s2″ style=”font-family: UICTFontTextStyleBody;”> reactor? <span class=”Apple-converted-space”> </span>Whatisnuclear.com</span></p>
<p class=”p2″ style=”margin: 0px; font-stretch: normal; font-size: 17px; line-height: normal; caret-color: #000000; color: #000000; -webkit-text-size-adjust: auto;”><span class=”s4″ style=”font-family: UICTFontTextStyleBody; text-decoration: underline;”>There are also significantly more free neutrons in fast reactors. Since the probability of fission is lower for faster energies for every actinide, the neutron density is higher in fast reactors than it is in most reactors of the same power (since power is effectively the neutron density multiplied by the fission probability). Structural materials inside fast reactors thus undergo higher radiation damage rates than those in slow-neutron reactors.</span></p>
<p class=”p3″ style=”margin: 0px; font-stretch: normal; font-size: 17px; line-height: normal; min-height: 22px; caret-color: #000000; color: #000000; -webkit-text-size-adjust: auto;”></p>
<p class=”p2″ style=”margin: 0px; font-stretch: normal; font-size: 17px; line-height: normal; caret-color: #000000; color: #000000; -webkit-text-size-adjust: auto;”><span class=”s2″ style=”font-family: UICTFontTextStyleBody;”>Fusion energy was promised by saying we would use the same process as occurs in the sun, ie hydrogen hydrogen fusion. <span class=”Apple-converted-space”> </span>It soon became obvious that man made fusion would need to be done with isotopes of hydrogen containing one or two neutrons, along with the proton. <span class=”Apple-converted-space”> </span>This complicates things since now the bulk of the energy of fusion(which is large) is contained in these neutrons. <span class=”Apple-converted-space”> </span>Some of the energetic neutrons can be used to “breed” tritium from lithium, but most are a nuisance.</span></p>
<p class=”p3″ style=”margin: 0px; font-stretch: normal; font-size: 17px; line-height: normal; min-height: 22px; caret-color: #000000; color: #000000; -webkit-text-size-adjust: auto;”></p>
<p class=”p2″ style=”margin: 0px; font-stretch: normal; font-size: 17px; line-height: normal; caret-color: #000000; color: #000000; -webkit-text-size-adjust: auto;”><span class=”s2″ style=”font-family: UICTFontTextStyleBody;”>From:Fusion is not what it is cracked up to be, Bulletin of Atomic Scientists</span></p>
<p class=”p3″ style=”margin: 0px; font-stretch: normal; font-size: 17px; line-height: normal; min-height: 22px; caret-color: #000000; color: #000000; -webkit-text-size-adjust: auto;”></p>
<p class=”p2″ style=”margin: 0px; font-stretch: normal; font-size: 17px; line-height: normal; caret-color: #000000; color: #000000; -webkit-text-size-adjust: auto;”><span class=”s4″ style=”font-family: UICTFontTextStyleBody; text-decoration: underline;”>To produce usable heat, the neutron streams carrying 80 percent of the energy from deuterium-tritium fusion must be decelerated and cooled by the reactor structure, its surrounding lithium-containing blanket, and the coolant. The neutron radiation damage in the solid vessel wall is expected to be worse than in fission reactors because of the higher neutron energies. Fusion neutrons knock atoms out of their usual lattice positions, causing swelling and fracturing of the structure. Also, neutron-induced reactions generate large amounts of interstitial helium and hydrogen, forming gas pockets that lead to additional swelling, embrittlement, and fatigue. These phenomena put the integrity of the reaction vessel in peril.</span></p>
<p class=”p3″ style=”margin: 0px; font-stretch: normal; font-size: 17px; line-height: normal; min-height: 22px; caret-color: #000000; color: #000000; -webkit-text-size-adjust: auto;”></p>
<p class=”p2″ style=”margin: 0px; font-stretch: normal; font-size: 17px; line-height: normal; caret-color: #000000; color: #000000; -webkit-text-size-adjust: auto;”><span class=”s4″ style=”font-family: UICTFontTextStyleBody; text-decoration: underline;”>Bombardment by fusion neutrons knocks atoms out of their structural positions while making them radioactive and weakening the structure, which must be replaced periodically. This results in huge masses of highly radioactive material that must eventually be transported offsite for burial. Many non-structural components inside the reaction vessel and in the blanket will also become highly radioactive by neutron activation. While the radioactivity level per kilogram of waste would be much smaller than for fission-reactor wastes, the volume and mass of wastes would be many times larger. What’s more, some of the radiation damage and production of radioactive waste is incurred to no end, because a proportion of the fusion power is generated solely to offset the irreducible on-site power drains.</span></p>
<p class=”p3″ style=”margin: 0px; font-stretch: normal; font-size: 17px; line-height: normal; min-height: 22px; caret-color: #000000; color: #000000; -webkit-text-size-adjust: auto;”></p>
<p class=”p2″ style=”margin: 0px; font-stretch: normal; font-size: 17px; line-height: normal; caret-color: #000000; color: #000000; -webkit-text-size-adjust: auto;”><span class=”s2″ style=”font-family: UICTFontTextStyleBody;”>So what should be done? <span class=”Apple-converted-space”> </span>One hopeful development has occurred more recently with discovery of other fusion reactions beside hydrogen hydrogen which don’t produce neutrons. <span class=”Apple-converted-space”> </span>The problem is that these reactions are almost as difficult. <span class=”Apple-converted-space”> </span>Government investing this “aneutronic” fusion research is not significant. <span class=”Apple-converted-space”> </span>One private company researching this is LPP Fusion who are investigating the B11-hydrogen fusion. <span class=”Apple-converted-space”> </span>It remains to be seen if this reaction can be scaled up to sizes for base load generation. <span class=”Apple-converted-space”> </span>High energy conversion efficiency is promised by avoiding thermal processes used in most generating systems, instead using direct conversion along with x-ray photocells. <span class=”Apple-converted-space”> </span>Direct conversion harnesses the energy of charged particles emitted from fusion (note that neutrons are not charged). <span class=”Apple-converted-space”> </span>This may be in the near future.</span></p>

[Meredith Trimble]

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