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Why Not Build Thermal and Fast Neutron Reactors Adjacently?

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posted on Aug, 28 2008 @ 10:16 AM
0. An understanding for non experts: There’s two major categories of reactor…
The fast neutron:
And: Thermal Reactor:

In normal Thermal Reactors low enriched fuel (about 2%) has it’s neutron slowed by a moderator. This slower speed increases the probability of them hitting another (say uranium fellow fuel atom) and therefore produces enough collisions to keep the reactor going. The process is normally kept stable using control rods, they absorb surplus neutrons.

The Fast Neutron Reactors use fuel that’s so enriched that there’s enough naturally produced neutrons (of the right speed) to sustain the reaction without a moderator. This process is still normally kept stable with control rods.

1. A Problem That Effects Both Reactors…
They both need their fuel either reprocessed or replaced. To begin with I thought this was because the fuel gets used up over time. In fact only about 1% of the fuel gets used up in about 5 years of the fuels rods operation.
The real reason for reprocessing-replacing is actually because of something called Neutron Poisons.
These are elements that absorb far more neutrons than they ever produce. They “get into” the fuel rods not because of any kind of leakage, but because nuclear reactions can change one element into another (even lead into gold; though very radioactive gold!!). Therefore some of the original fuel gets Transmuted into neutron poisons like Xe 135, and that’s what causes the fuel rods to eventually cause more harm than good.

2. My idea…
1. Build a fast neutron reactor; feed it enriched fuel. The good news is that this can be typically worthless stuff like plutonium left over from old nuclear warheads.
2. After x number of years the neutron poisons build up making the rod quite useless for a fast neutron reactor.
3. At this point it the fuel gets transferred into a Thermal Reactor 1.
The thermal reactor should be able to use this fuel because of the way it works differently. I.e. because it slows the neutrons it uses, it uses radioactivity far more efficiently for producing energy.
4. (Again) neutron poisons continue to build up; so let’s say after bout 10 years it’s “only” got about 98% of it’s original (almost free) plutonium fuel. So at this stage it’s transferred into Thermal Reactor Two. The difference between it and the first is that Thermal Reactor Two has even more moderator; thereby guaranteeing the radioactivity from the fuel (now in its third stage) would be used more efficiently still. Thereby the effect of the neutron poisons is bypassed-ignored. In fact a name for my idea might be “The Ignore Reactor Complex” since it’s all about ignoring-bypassing the effects of these poisons.
5. Eventually: The neutron “poisons” will have built up so much that even very powerful moderatoring reactors will be unable to use it.

A Summary of Advantages so far…
1. Cheap (essentially waste) plutonium like fuel was always used. So no need at all for expensive uranium mining (or primary enrichment).
2. No need for nuclear fuel enrichment during this first (and incredibly longer) part of the process. Nuclear Fuel Enrichment is a very messy process since almost everything you use to enrich the fuel becomes radioactive (robots, acid, water ect). And it’s not just a little bit radioactive either; it’s extremely. Therefore all this stuff needs to go in more waste barrels.
Furthermore: Fuel enrichment is also the same process as you use to get weapons grade material. Fast Neutron reactors need more heavily enriched fuel; but it needn’t ever be delivered to them at weapons grade. Therefore you could trust a country that says it just wants the stuff for peaceful purposes (providing it doesn’t have the enrichment facilities).
3. The neutron poisoned fuel waste you end up with is about as radioactive as the neutron poisoned waste you would get from a thermal reactor. This is quite self-evident because it’s the fact the stuff is incapable of producing much heat (due to all it’s neutron poisons) that it eventually has to be thrown out.

6. Stage Two…
It’s a well known fact that all spent nuclear fuel is so radioactive it’s quite hot.
You can see actual figures here:
Which gave me the idea of using this heat to make electricity without enrichment

It turned out China has been thinking about this…

China to utilise spent fuel for heating and desalination.
A Chinese group has agreed to build a 200 MWt nuclear reactor run on used fuel from nuclear power stations to provide district heating and desalination. The US$ 42 million project at Yingkou in Liaoning province will heat 5 million square metres of buildings over 4-6 months each year and initially desalinate 3000 tonnes (3 megalitres) of seawater per day in warmer months, rising to 80,000 tonnes (80 megalitres) later. The deep-pool reactor will operate at atmospheric pressure, which will reduce the engineering requirements for safety.
Comtex 25/6/02 (via Wilmington).

However I’ve been unable to find almost anything about how they’re planning to build. And I doubt whether it is as crude, safe, and cheap as the (sand insulation based) idea my given ATS thread.
Meanwhile someone else has been thinking about directly using the radioactively
Basically even after the fuel becomes completely neutron poisoned (by any reactors standards) you can still make free heat out of it by placing it in a well thermally insulated pool of water (which I would achieve by putting about a meter of sand in a cavity in it’s wall, ceiling and floor.
Who cares if you only make about 1000watts per second, rather than the 1000,000 you would get with expensive enrichment? The only difference is that the 1000 watt stuff would produce heat a thousand times longer than the million watt stuff (or whatever the ratio might be).

Stage Three-Two…
Perhaps in 500 years time the world might develop a shortage of high level nuclear waste?
So the neutron poisoned waste you’ve got (still highly radioactive) could be taken out of the decay room and: A: finally be reprocessed; B: it perhaps even be transmuted
In the first Fast Neutron Reactor into less dangerous waste (though whether it would need to be unpacked to remove it’s neutron poisons is something I don’t know) (would be worth looking into since unpacking the stuff is (like enrichment) a messy business.
C: Or if there is no shortage of radioactive material, and your decay room is getting short of space, then the waste could be dumped like normal.

So far the waste cycle has been extended to (roughly) a 500 year period, and they’ve still had many times more heat from it than would have been produced by a standard single use (before reprocessing) in a typical Fast Neutron Reactor, or indeed Thermal reactor.

How Not To Build My Idea…
In my original idea I would have the reactors built vertically like a block of flats. I.e. the Fast Neutron Reactor would be on top, and the different Thermal Reactors below (with the one with the greatest moderation on the bottom).
This would save almost all contamination through transportation because the whole thing would be gravity fed making it, even more cost effective. Brilliant?
Not quite.

Because I was thinking that if (somehow) the fast neutron reactor went into meltdown (like Chernobyl) then any molten highly enriched nuclear fuel would trickle downwards into the more moderated sections.
This is a holy c*** scenario since in addition to making an unstoppable hole inside the roof of every single reactor below; also putting highly enriched nuclear fuel into a well moderated environment is called a nuclear bomb!!! (Not good PR).
In all probability the stuff wouldn’t actually go super critical as the enriched stuff could lack the purity for a nuclear explosion (even when new) or could pick up so much concrete-steel and stuff in the process of trickling down that by the time it reaches the first reactor it certainly is not rich enough.
Even so: The effect of enriched fuel landing inside the well moderated reactors would cause them to overheat (which in addition to hole in the roof) is a nuclear bonfire mess.

To avoid This: Either build the “block of flats” layout, but don’t
1. Use (theoretically) weapons grade material anywhere
2. And build each reactor with a clay, combed, melt proof roof so that if there is a meltdown upstairs the mess falls sideways.
3. This is ok if all the reactors are built inside a containment dome-building.
4. Maybe include additional safety measures to prevent a meltdown in the first place

Or: Build the reactors in a straight row. The new safety problem here is what happens if some idiot puts the wrong fuel rods in the wrong reactor (since gravity can no longer guarantee the right order).
So I would recommend building these reactors with there own fuel transport vehicles of diminishing physical sizes, and linking the reactors with a “corridor” that has gateways of diminishing sizes to match. This would ensure it’s not physically possible to directly transfer fuel from the first reactor to the last (well unless a brand new delivery is taken to the wrong point; but hopefully someone would either work that out, or the last reactors entrances could also be designed to be too narrow-shallow to allow this to happen.

Therefore by either using the right size door ways, or using virtually melt proof materials like clay (with the containment dome) this safety problem can be totally eliminated (even with rats rather than people running the reactor).
Hopefully clay does the trick as I quit like the gravity fed vertical design since it reduces both costs and unnecessary equipment contamination.

So ATS what are you’re thoughts?

posted on Aug, 28 2008 @ 10:42 AM
Sounds like you have some good ideas, you should pass them on to the right people, if you haven't already.

posted on Aug, 29 2008 @ 07:03 PM
sounds like your plan is flawed.

You can't have even more moderator in the second thermal reactor, please clarify your logic behind this. Poisons are poisons, they reduce the reactivity in the fuel...

your argument about enrichment is wrong.

your argument about little heat generation from the waste because of 'poisons' is wrong. Nuclear heat is generated by radioactive decay of fission products (mainly gamma/beta radiation), light elements and actinides. Neutron poisons (aka radionuclides with large neutron absorption cross sections) have nothing to do with heat generation via gamma/beta decay.

[edit on 8/29/2008 by porky1981]

posted on Sep, 1 2008 @ 05:17 PM

sounds like you’re plan is flawed

What are you on about?

You can't have even more moderator in the second thermal reactor, please clarify your logic behind this.

Why? You read this bit of the Wikipedia link:
Or how about just this Google search:

And you’ll discover that fast neutron reactors have no moderator!
My logic is that since they have no moderator you can add one in the second reactor (i.e. the first thermal reactor)

Poisons are poisons, they reduce the reactivity in the fuel...

Yes; that’s also why I said use the neutron moderator in the 2nd reactor. You have to remember that these poisons are less than 1% of the fuel. The reason why they have such a detrimental effect on power output is because all nuclear reactor designs are an extremely fine balance between almost no heat, and too much heat (well too much heat for most materials we can actually build).

So my logic is that by using a moderator in the 2nd reactor (which slows down the neutrons, and increases their probability of reacting) this delicate balance is restored!!!

Extra Detail Here

It’s why in thermal reactor No…
1. There is just light water
2. There is just light water and graphite
Or: optionally just heavy water
3. There is just heavy water and graphite
Or: There is just heavy water and graphite and a Neutron reflector (which by the way can be more graphite i.e. just more graphite than there was in 3).
4. Is exactly the same as 3 but with neutron generators such as Polonium and Beryllium introduced.

P.S By increasing factors like the amount of neutron reflector-generator it should be possible to go on ignoring the effects of neutron poisons almost (but not quite) indefinitely.
However the number of times the effects of neutron poisons could be ignored would of course (primary) depend on the original purity-disability of the nuclear fuel.

So I am left wondering: “PLEASE EXPLAIN YOUR “LOGIC”!!!
Or did you simply not read what I wrote correctly?

posted on Sep, 1 2008 @ 05:19 PM
Originally posted by porky1981

your argument about little heat generation from the waste because of 'poisons' is wrong.

No darling; plenty of heat is still produced.
Here’s why: Neutron poisons only affect the speed of decay; not it’s inevitability (yep even uranium 238 will decay one day, matter how much iodine-xenon you surround it in). This is due to something weird called “spontaneous decay”

Spontaneous decay causes things to have half lives (“half lives” are different in nuclear reactors since it’s an accelerated process much like an atom bomb is)
The splitting of atoms still creates heat by
1. Displacing electrons that orbited it before decay
2. A neutrons impact of fusion displaces the nucleus of the receiving atom; this displaces all the electrons around it (as well as those of nearby atoms).
3. In the event of a neutron escaping a weird type of “friction” (I can’t remember the technical name of) still comes into play.

Bottom line is that in worse case scenario neutron poisons can only ever halve the amount of heat produced (by radioactive decay; as opposed to fusion) since what they have done is prevent another fuel atom receiving the neutron. But they have not: Prevented the spontaneous decay of unstable atoms, nor have they prevented the fusion of their neutrons (even if the fusion has occurred with the poison rather than a fuel atom).

Anyway Here’s An Extremely Crude Idea of How Much Heat Can Be Produced…
Plutonium 239 has a half life of 24, 100 years.
The U.K has a plutonium stockpile of about 100 tonnes
So that’s like 0.5 tonne decaying every 241 years
Or 2.074685Kg per year

Given that most nukes are only 1 to 20% efficient
That’s a heck of a lot of energy every year, for the next 24,100 years!!! And of course what I’ve written is so crude it says absolutely nothing about the energy released by any short lived isotopes produced (in just this particular reaction).
For example plutonium 238 has a half life of only 87.7 years. So it would be interesting to know how many Kg per year our 100 tonne (100,000Kg stockpile) is (quite naturally) generating. Never mind other things constantly produced by decay like Radon (half life 3.8 days: )

posted on Sep, 1 2008 @ 06:03 PM
My problem with all forms of neutron radiating reactors is that neutron flux brittlizes materials over time which ends up in reactor breaches. Also the thermal transfer is often accomplished with molten alkaline metals which are very corrosive and cause problems in the long run. There are holes in reactor vessels right now in Canada that are leaking radioactive materials into the ground water as a result of what I am talking about. There is no way to get to the leaks without completely decommissioning the reactor and digging up the ground and doing a thorough remediation of the contaminated soil.

It is happening far more than anyone is willing to talk about because the cause is a matter of a flaw that was not foreseen in the reactor design and this problem exists in so many reactors that the authorities do not want it known that so many of them will end up leaking in the same manner at some point in the future. This problem does not have to remain in future reactors so your idea can be done in future reactor designs but all reactor cores should have full access to the bottom of the reactor vessel. None of them should be built on a cement slab without access underneath in my opinion. Of course this is like adding an extra floor to a building. That means the cost for the reactor will be far higher as a result of requiring a full access decontamination ready sub basement in all future reactors.

[edit on 9/1/2008 by UFOTECH]

posted on Sep, 2 2008 @ 01:08 PM
Originally posted by UFOTECH

There is no way to get to the leaks without completely decommissioning the reactor

Yes it’s a problem I’ve heard about…
Thanks for pointing out there are solutions; primarily: Building the reactor core in “kit form” (to aid its maintenance-dismantling), designing it to take very little of its own weight (and especially tension), also many materials can resist radiations damaging effects far better than steel.

But in the older generations obsolete designs…
I doubt very much the reactor core floor will be doing any leaking. This is because they’re extremely water tight as it is.

Instead it’ll be the pipes!!!
In which case the best solution that comes to mind is shut the reactor down. You won’t need them anymore. So they’ll no longer carry radiation into the environment.

But If: Getting electricity from dangerous, ignorant designs is more important than public safety (apparently it often is) I believe the current solution is to store irradiated water until its “safe” enough to discharge into the river-sea. However its steam coming out of the cooling towers which probably causes the real problem. That’s why it’s getting into your ground water; because it’ll be coming down whenever it rains!!!
It would be interesting to take a Geiger Counter to one of those cooling towers!! Not to measure the wall; but the steam.

In fact (barring nearby water extraction) irradiated water tends to move surprisingly slowly in the ground. It’s this phenonium which has kept the fallout from Chernobyl relatively stable. So if your getting poisoned it’s not from the reactors construction, instead it be from it’s continued operation (which shouldn’t be happening because of it’s ridiculous construction).

Oh: I wonder what happens if there's an earth quake with the core being brittle? (Especially since they only discoverd this effect in the 70's).
Well at least one good thing that came from Chernobyl was Europes largest wildlife park) (animals with short life spans than humans are quite happy in irradiated environments).

[edit on 090705 by Liberal1984]

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