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PostPosted: May 03, 2012 10:02 am 
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As with many Small Modular Reactors out there, a potential "dirty little secret" they keep to themselves is the fissile material needed for start up. I've been trying to figure out what the tiny 10 MWe Toshiba 4S might require and unless I'm way off somehow it looks horrendous.
The 4S core is long and narrow to specifically lose lots of neutrons so they can run it 30 years by slowly lowering a neutron reflector. The active core though appears to be 1.5 m wide by 2.5 m high. Otherwise the core seems to be based on the S-Prism and IFR concepts with 90% (U and/or Pu) and 10% Zirconium fuel. This 10 MWe core seems to be about the same volume (maybe even more) than the 311 MWe S-Prism (I think 3.8 m3 of core, not including blankets). the S-Prism 311 MWe needs about 1.24 tonnes fissile Pu (239+241) or 1.7 tonnes U235 (as 15% enriched) to start. So unless I'm out to lunch somehow it would seem the 10 MWe 4S might need on the order of 1.5 to 2 tonnes of U235 (or 150 to 200 tonnes/GWe). At a conservative cost of 30$ a gram for U235 that is at least 50 million$ just for their fuel to be bought on day one (yes, it does last 30 years though). That is already a capital cost of 5$ a watt before you even look at the real capital costs! The trick of course which all reactor designers use is to hide the startup fuel costs in the annual fuel costs. If you do that and assume a 10% discount rate then that is about 5M$ a year or close to 7 cents/kwh (85% capacity factor).
Comments or anyone find an error here or have found more concrete core details of the 4S?

David LeBlanc


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PostPosted: May 04, 2012 9:30 am 
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Not sure about the 4S but generally smaller fast reactors are terrible in fissile startup

http://bravenewclimate.files.wordpress. ... tique2.png

Breeding blankets are not as effective for fast reactors as they are for thermal two fluid lftrs.


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PostPosted: May 04, 2012 3:32 pm 
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Thanks Cyril,

It seems even worse than I thought. I used the value of 5.5 tonnes U235 per GWe for the IFR but that was only for the biggest versions as that graph you posted shows. The S-Prism is actually almost double that (per GWe). The next time fast breeder folks try to tell you they can get a breeding times down to 7 years keep in mind that is only true for the absolute best case scenario and the biggest version they've studied (1400 MWe). Of course by that core size they start running into all sorts of other issues.

David L.


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PostPosted: May 05, 2012 4:53 am 
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Well to be fair to fast reactors, the curve does level off after half a GWe, so the 600 MWe IFR would be a good size to start with, at 6 tonnes fissile startup.

So we can assert that fast reactors must be at least medium sized, small sizes have prohibitive fissile startup needs.

In other words fast reactors are no good for small modular reactors (SMRs).

Also for the same high fissile startup reasons, small fast reactors are not good at eating the existing reactor waste.

The fast reactors could of course be started up on mined uranium, but that's a pretty large amount of mining we'd have to do.

That kind of goes against the "no more mining" and "eat the existing reactor waste problem" arguments we often hear.


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PostPosted: May 06, 2012 9:26 pm 
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Cyril R wrote:
Also for the same high fissile startup reasons, small fast reactors are not good at eating the existing reactor waste.


Why? Plenty of waste, is it the cost of reprocessing the old waste?

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Paul Studier


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PostPosted: May 07, 2012 10:08 am 
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pstudier wrote:
Cyril R wrote:
Also for the same high fissile startup reasons, small fast reactors are not good at eating the existing reactor waste.


Why? Plenty of waste, is it the cost of reprocessing the old waste?


Cost, absolutely. And then there's your other objectives, such as powering a future energy hungry planet that will need 4-8x more electricity than today. Not enough waste to start up enough reactors to do that. With LFTR you only need 1-2 tonnes TRUs startup, so there's enough waste available to startup enough reactors to power the world.


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PostPosted: May 07, 2012 11:22 am 
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Cyril R wrote:
And then there's your other objectives, such as powering a future energy hungry planet that will need 4-8x more electricity than today. Not enough waste to start up enough reactors to do that. With LFTR you only need 1-2 tonnes TRUs startup, so there's enough waste available to startup enough reactors to power the world.

There is enough to start many LFTRs but not enough for all our needs. At startup charge of 1-2 tonnes fissile/GWe implies an efficient, thermal design. At world-wide build out would be around 10,000 GWe. So far, the world has around 500 GWe that has been operating around 30-40 years producing perhaps 250kg fissile (??) for a grand total of 500*40*250 = 5,000 tonnes.

The TRUs we have today are a good start but we will need 2-4 times that amount. By the time we stop building more LWRs and shut down the last one we might have 2-4x the spent fuel we have today.

TRUs are critical for startup if one is targeting fast reactors and is precluded from using HEU. But for a thermal design one could use 20% LEU instead. So, we can consume the existing waste, and likely the future waste as well but we our deployment isn't slowed by lack of TRUs.


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PostPosted: May 07, 2012 11:54 am 
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20% LEU is tough for an isobreeding thermal LFTR. There's that 80% U238 that messes it up. Also TRUs from the U235 nonfission chain are a significant poison (as is evident in HEU startup). If you have a 2 fluid reactor, you can do David Leblanc's trick of starting up on LEU then store the blanket bred U233. After a number of years, swap out the LEU core fuel and replace with U233 that has been bred.

TRU is much nicer than LEU though, with over 50% fissile, and high cross sections in thermal spectrum. You could probably use this to start up the isobreeding thermal LFTR, including a single fluid design, due to it being an otherwise "pure" Th-U233 cycle. Will it isobreed? Well, just barely. If it doesn't though, you can add some makeup TRUs in the first couple of years, then as U233 is bred you'll get into isobreeding, then stop feeding TRUs.

We don't have enough TRUs yet for the future global demand TRUs to startup LFTRs. But that's great, it means Gen III+ is synergistic with LFTR. So we can go ahead and feel good about building loads of AP1000s, displacing dirty coal burners. (no I don't own stock in Westinghouse, but now looks like a good time to buy)


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PostPosted: May 07, 2012 1:33 pm 
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Cyril R wrote:
20% LEU is tough for an isobreeding thermal LFTR. There's that 80% U238 that messes it up.

It does hurt but when DMSR starts it has 110 tonnes 232Th and 14 tonnes 238U. For higher energies these have about the same cross-section but for lower energies thorium has around 2.5x CX. The resonance region starts at higher energy for 238U (20keV) than 232Th (4keV). Net I would expect that 85-95% of the fertile to fissile conversion is 232Th->233U.

The killer for long term isobreeding is if you enforce that the fuel in the core should always be denatured. This is true whether you start with TRUs, 235U or even 233U.


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PostPosted: May 07, 2012 3:30 pm 
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Lars wrote:
Cyril R wrote:
20% LEU is tough for an isobreeding thermal LFTR. There's that 80% U238 that messes it up.

It does hurt but when DMSR starts it has 110 tonnes 232Th and 14 tonnes 238U. For higher energies these have about the same cross-section but for lower energies thorium has around 2.5x CX. The resonance region starts at higher energy for 238U (20keV) than 232Th (4keV). Net I would expect that 85-95% of the fertile to fissile conversion is 232Th->233U.

The killer for long term isobreeding is if you enforce that the fuel in the core should always be denatured. This is true whether you start with TRUs, 235U or even 233U.


ORNL's DMSR never hits CR=1. Not even close. Even with rapid lanthanide removal it's going to be really close. But rapid lanthanide removal means you also ditch some of your bred plutonium. So you then have to go complicated with online liquid metal exchange.


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PostPosted: May 07, 2012 4:22 pm 
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in dmsr they added 238u to denature the core. Follow those rules and it won't matter what you start on you will not get to isobreeding. If you relax that rule (and assume that you can have heu inside the core just like you can have pure 239Pu inside an LWR) then you can get to isobreeding with any fissile, including 20%LEU.


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PostPosted: May 07, 2012 4:29 pm 
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Lars wrote:
in dmsr they added 238u to denature the core. Follow those rules and it won't matter what you start on you will not get to isobreeding. If you relax that rule (and assume that you can have heu inside the core just like you can have pure 239Pu inside an LWR) then you can get to isobreeding with any fissile, including 20%LEU.


Do you have a source for this?


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PostPosted: May 07, 2012 9:05 pm 
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Unfortunately no. TM7207 made a basic assumption of no fuel processing for 30 years and so you lose fuel quality toward the end which even a leisurely reprocessing could help with. But this won't be enough to get to isobreeding. I thought they added a lot of 238U to keep things denatured but on re-reading 7207 I see they only added 347kg of 238U that wasn't 20%LEU. I think I have to retract my statement. Trying to logic my way through neutronics seems to get me in trouble every time.


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PostPosted: May 07, 2012 10:37 pm 
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I thought the Toshiba 4S bred plutonium in place, kinda like the traveling wave reactor (not the best idea, I know, but still interesting). Although, come to think of it, I've never seen any documentation to that effect, it just seems like me the way it would have to work.

I think that the fissile load in a Toshiba 4s would be smaller than a PRISM (for the amount of energy produced), but I don't think I've ever seen any official numbers.


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PostPosted: May 08, 2012 2:55 am 
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Ned Speirs wrote:
I thought the Toshiba 4S bred plutonium in place, kinda like the traveling wave reactor (not the best idea, I know, but still interesting). Although, come to think of it, I've never seen any documentation to that effect, it just seems like me the way it would have to work.

I think that the fissile load in a Toshiba 4s would be smaller than a PRISM (for the amount of energy produced), but I don't think I've ever seen any official numbers.




All reactors on the U-Pu cycle breed plutonium. The issue with small fast reactors is that they leak huge amounts of neutrons. Fast neutron reflectors and fast breeding blankets just don't work as well as thermal reflectors and thermal blankets. Fast neutrons are escape artists.

To compensate for this high leakage, more fissile is needed, OR you need a much bigger core. The 4S is a very small reactor and worse, it is too slender that further increases leakage. This does make small fast reactors safer (coolant voiding results in more leakage) but also greatly increases the initial fissile fuel needed per MWe of capacity. Now if you actually want a good breeding rate you need even more fissile to startup.

This results in 2 fluid LFTR being much more amenable to small modular design than fast reactors. Fast reactors have to be medium sized at least (if blankets are used) and without blankets they must be even bigger.


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