One error in the column was the statement that “Reactors enriched uranium don’t require a moderator.”
You CAN run a reactor with natural uranium and heavy water. The Canadians do it with their CANDU reactors.
“Fast reactors” don’t require a moderator at all since they don’t want the neutrons to slowdown. But there are only a couple of them in the world (none in the U.S. now that FFTF is shut down).
Virtually every other reactor in the world uses enriched uranium AND needs a moderator. Enriching the uranium does NOT eliminate the need for a moderator.
You are correct. What I actually should have said is that enriched uranium reactors do not require a moderator other than ordinary light water. Light water is a rather poor moderator, but in enriched uranium reactors, it is sufficient. To use natural, unenriched uranium, one needs a better moderator, such as heavy water or graphite.
Welcome to the Straight Dope Message Boards, sfrantz, glad to have you with us.
When you start a thread, it’s helpful if you provide a link to the Column or (in this case) Staff Report that you’re commenting on. Yes, it’s on the front page today, but in a week or so it will be buried in the Archives. So, to save readers’ time in searching, we recommend providing a link. Helps keep us all on the same page.
No biggie, I’ve edited a link into your post, and you’ll know for next time… and I hope there will be many “next times.”
Q - (Are we on a first name basis yet?) – Let TubaDiva know if you’d like your Report edited to reflect this.
A few other quibbles with this otherwise on-track article:
“Heavy water (D2O) is one of several commonly-used moderators found in nuclear reactors (others include graphite, beryllium and light–i.e., ordinary–water).”
Graphite was used a lot in early plutonium-making reactors and the infamous Soviet RBMK (Chornobyl) reactors, and the early British Windscale reactors, but it’s not used in any current design in the USA that I know of. Graphite has too many hazards (flammability and Wigner’s disease) which make it less than desireable as a moderator.
Beryllium isnt used as a moderator in any reactor design that I know of. It does have some use as a neutron REFLECTOR, placed in the periphery.
“A moderator slows down fast-moving neutrons released by nuclear fission so they have more time to react with the nuclear fuel.”
“More time to react” is a bit off. The goal is to lower the average neutron energy to match the resonance absorption bands of the fissile material.
“Reactors using enriched uranium don’t require a moderator.”
This isnt true. A reactor without moderator needs almost pure fissile material. The usual 1-3% enrichment is not enough.
Enriched uranium reactors still require substantial moderation.
“Obtaining a significant amount is damn near impossible for the average Joe”
I wouldnt say “near impossible”.-- as the OP noted, heavy water is sufficiently different in gross physical characteristics. You can separate it out with something as simple as a carefully controlled distillation or fractional crystallization. Granted you don’t get very much heavy water, but if you boil off enough water you’ll eventually get your 1/6700’s worth. With a little more cleverness you can run higher-yield processes such as simple electrolysis (the electrodes in water kind, not the bikini-area kind), or one of the many other not too complicated well-known methods.
BTW there was a fascinating cover story on this in an old (1961?) Scientific American. Lots of mildly yucky color pictures of heavy-watered mice with nasty skin lesions.
As soon as I read this statement, I headed over to this forum to rebut it, only to find that I’m late for the party.
So just to chime in: Even a highly enriched uranium reactor requires a moderator.
Light water pressurized reactors use ordinary water for the moderator. One inherent safety factor of such a reactor is that in the event of a complete loss of coolant (CLOC) casualty, the moderator is also lost. This tends to shut down the nuclear reaction. While there may then be significant decay heat to deal with, at least the fission reaction will stop.
In contrast, the Chernobyl-type reactors use graphite for their moderator which (along with a positive temperature coefficient of reactivity) makes them much less inherently safe. The Chernobyl-4 reactor that had the accident in 1986 continued to fission until the core blew apart. The Three Mile Island reactor in 1979, on the other hand, was successfully shut down. It was the residual decay heat that melted part of the core.
Google “beryllium nuclear reactor moderator” without quotes. It may not be used in any current reactor designs, but it definitely has good moderator properties, and is mentioned in every list of reactor moderators I’ve seen so far.
True, but my original statement was borrowed from several simplified articles on the subject, and is sufficiently close to reality that I’ll allow it to stand, as it’s not central to the material of the Report.
This has already been addressed and will be corrected in the Report.
While you’re technically absolutely right, I maintain such procedures are beyond the ability of the average person. You and I could do it, but the average guy off the street wouldn’t know where to begin. And it would take an effort. To get one ounce of heavy water, you’d have to distill 6700 ouces, or 52.3 gallons of water, assuming 100% recovery efficiency. I’d put the real figure closer to 20% efficiency, just from guesstimating, which means processing over 260 gallons.
I hope this isn’t a hijack, but grg88’s post mentions obtaining heavy water from ordinary water. It’s complicated and as Q.E.D. mentions in his follow-up post, it can be inefficient.
So, I’ve heard that one can increase the efficiency by starting with water that already has a higher-than-average concentration of heavy water.
And my brain recalls that water off the coast of Norway is deuterium rich. I seem to remember reading somewhere that the Nazis were “harvesting” water in that area during the latter days of WW II. Is my brain recalling correctly?
There was certainly a heavy-water plant in Norway; it was the world’s only commercial heavy-water plant. Heavy allied raids convinced the Germans to evacuate, and an Allied saboteur destroyed the entire inventory in transit.
But my information is that the plant was used because it was possible to produce heavy water as a by-product of the Haber-Bosch process for producing ammonia for nitrogen fertilizer, which was already in place at Norsk Hydro, and the Haber-Bosch process was there, in turn, because Norsk Hydro was chiefly a hydroelectric power plant.
Yes, the manufacturing side of Norsk Hydro was primarily geared towards producing ammonia. The idea was that they had a cheap, plentiful supply of electricity to hand and lots of water, so it was easy to generate hydrogen by electrolysis. Since that was also one way of generating deuterium-enriched hydrogen, this process could be set up to produce heavy water as a by-product. The 1930’s market for heavy water was limited to it as a specialist laboratory item, so this was originally never seen as anything other than an opportunistic sideline. German wartime demand for it in much larger quantities did make it more of a priority at the plant.
The ammonia production from the plant was of major strategic significance in itself and this has been suggested as one reason why the Germans largely failed to react to what was actually Allied interest in heavy water. (I recently discussed what little reaction there was in this thread.)
Did a bit more research about Beryllium moderators. I could only find one reactor that uses it as a moderator, and it’s a small reasearch reactor. Apparently it’s not widely used because:
(1) It costs about $15,000 a kilo! A typical reactor would need over $100M just in beryllium. Water is much cheaper.
(2) It’s hazardous to grind or machine it.
(3) It breaks apart after about 15 yrs in a reactor, requiring something like a 2 year hiatus to replace the moderator.
So while encyclopedias may list Beryllium as a moderator, it’s not a economical, safe, or trouble-free moderator.
Re: “average Joe”, I suspect the average Joe could figure out how to boil water, albeit carefully, and lots of it.
Sure, but controlling the temperature, and properly capturing the distillate is fairly critical to efficiency. It’s this part I suspect he’d bobble. Of course, your average hillbilly moonshiner might manage it ok.
