Energy From Thorium Discussion Forum

It is currently Feb 23, 2018 11:07 am

All times are UTC - 6 hours [ DST ]




Post new topic Reply to topic  [ 42 posts ]  Go to page 1, 2, 3  Next
Author Message
 Post subject: Protactinium-233
PostPosted: Apr 12, 2012 9:19 pm 
Offline

Joined: Oct 06, 2011 1:34 pm
Posts: 81
As I understand it, a LFTR starts out with a charge of fissile material and thorium dissolved in flibe salt. Let us say the fissile material is uranium-235, and there is enough to initiate criticality, so there is a flux of neutrons, moderated by graphite, which heat the core and bombard thorium nuclei, transmuting them to Th-233, which beta decays with a half-life of 22 minutes to Pa-233, which has a half-life of ~27 days, beta decaying to U-233. At startup, all of the energy comes from the U-235, but over the course of weeks or months, the reactor progresses toward a point of equilibrium at which the quantity of U-233 bred replaces the U-235 being consumed. Eventually, the quantity of U-235 approaches zero and the reactor is running entirely on U-233 bred from thorium which is continuously added to replace that which is transmuted and burned. Please correct any inaccuracies so far.

I have a few questions. First, does the Pa-233 capture a significant number of neutrons, such that this must be taken into account in the design of the reactor? Can the Pa-233 be left in the core, or does it interfere with the fission chain reaction to the extent that it has to be removed from the core and segregated until it has decayed to U-233? What happens when a Pa-233 nucleus gets hit by a neutron? My last question, do the beta decay of Th-233 to Pa-233 and the beta decay of Pa-233 to U-233 release significant amounts of energy, contributing to the output of the reactor?


Top
 Profile  
 
 Post subject: Re: Protactinium-233
PostPosted: Apr 12, 2012 11:09 pm 
Offline

Joined: Jul 28, 2008 10:44 pm
Posts: 3069
Steve Brown wrote:
As I understand it, a LFTR starts out with a charge of fissile material and thorium dissolved in flibe salt. Let us say the fissile material is uranium-235, and there is enough to initiate criticality, so there is a flux of neutrons, moderated by graphite, which heat the core and bombard thorium nuclei, transmuting them to Th-233, which beta decays with a half-life of 22 minutes to Pa-233, which has a half-life of ~27 days, beta decaying to U-233. At startup, all of the energy comes from the U-235,

More precisely when the u235 fissions.
Quote:
but over the course of weeks or months,

a few years
Quote:
the reactor progresses toward a point of equilibrium at which the quantity of U-233 bred replaces the U-235 being consumed.

The 233U replaces the235U before equilbirum.
Quote:
Eventually, the quantity of U-235 approaches zero and the reactor is running entirely on U-233 bred from thorium which is continuously added to replace that which is transmuted and burned. Please correct any inaccuracies so far.


I have a few questions. First, does the Pa-233 capture a significant number of neutrons, such that this must be taken into account in the design of the reactor?

Yes. In particular for the long term evolution of the fuel.
Quote:
Can the Pa-233 be left in the core, or does it interfere with the fission chain reaction to the extent that it has to be removed from the core and segregated until it has decayed to U-233?

Depends on whether you want to maximize breeding. Pulling the 233Pa out you will save some neutrons and get a bit more 233U. You can actually breed in a thermal reactor but you have to be very careful. If you goal is iso-breeding (making just enough 233U to make up for the stuff that fissions) then you can afford to be a little bit less extreme with saving neutrons. The first place I'd spend that neutron budget would be to avoid separating the 233Pa.
Quote:
What happens when a Pa-233 nucleus gets hit by a neutron?

It converts to 234Pa which rapidly decays to 234U. The 234U won't fission but when it captures a neutron it becomes 235U which will.
Quote:
My last question, do the beta decay of Th-233 to Pa-233 and the beta decay of Pa-233 to U-233 release significant amounts of energy, contributing to the output of the reactor?

About 5MWatts for a 1GWe reactor.


Top
 Profile  
 
 Post subject: Re: Protactinium-233
PostPosted: Apr 13, 2012 6:50 am 
Offline

Joined: Apr 19, 2008 1:06 am
Posts: 2238
If you have a fast chloride reactor, PaCl4 will boil off at working temperature. After resting for 180 days, 99% of it will get converted to 233UCl4, which will be a solid or liquid (MP 590C) and can be returned to reactor to replace any further losses along with the thorium feed. You could even opt to return 90% after 90 days.


Top
 Profile  
 
 Post subject: Re: Protactinium-233
PostPosted: Apr 13, 2012 12:42 pm 
Offline

Joined: Aug 15, 2011 2:16 pm
Posts: 40
Location: Pickering, Ontario
jagdish wrote:
If you have a fast chloride reactor, PaCl4 will boil off at working temperature. After resting for 180 days, 99% of it will get converted to 233UCl4, which will be a solid or liquid (MP 590C) and can be returned to reactor to replace any further losses along with the thorium feed. You could even opt to return 90% after 90 days.


The only problem with this is you run into big proliferation issues if you can produce this high quality fissile material. I would side with Lars and shy away from removal of Pa from the system.


Top
 Profile  
 
 Post subject: Re: Protactinium-233
PostPosted: Apr 13, 2012 12:52 pm 
Offline

Joined: Oct 06, 2011 1:34 pm
Posts: 81
It just dawned on me, I had completely forgotten that thorium is not put into the core but into a blanket surrounding the core. That is where the Pa-233 evolves, so it cannot poison the fission going on in the core.

It also occurred to me that if the half-life of Pa-233 were much shorter, say 27 minutes or even 27 hours, it might have been practical to put the thorium into the core, where it would quickly breed into U-233. However, at 27 days, Pa-233 would be dwelling in the core rather long, possibly interfering with fission, so it is much better to have the thorium in an outer blanket. But what if the half-life of Pa-233 were longer, say 27 weeks? I imagine the blanket might still be feasible, but if the half-life were 27 months, the breeding of U-233 becomes a long-term project, and a steady supply of fissile material from an outside source will be required for years. That the half-life of Pa-233 is 27 days, not longer, seems to be a blessing for humankind.


Last edited by Steve Brown on Apr 13, 2012 1:26 pm, edited 1 time in total.

Top
 Profile  
 
 Post subject: Re: Protactinium-233
PostPosted: Apr 13, 2012 1:22 pm 
Offline

Joined: Jul 28, 2008 10:44 pm
Posts: 3069
Steve Brown wrote:
It just dawned on me, I had completely forgotten that thorium is not put into the core but into a blanket surrounding the core. That is where the Pa-233 evolves, so it cannot poison the fission going on in the core.

Whether thorium is in the core or not is dependent on the design specifics. I prefer to have thorium in the core. Yes having the thorium in the blanket helps. Half the neutron flux is then in the core and the Pa doesn't see it so you save half there. Second, you can increase the thorium content to your hearts desire making it more likely that a neutron hits a thorium rather than the Pa. There is little disadvantage to increasing the thorium concentration in the blanket until you get up to the 25% range. If you want to reduce your losses to Pa still further you can make the blanket thicker - spending some money because you buy more salt and have a slightly larger reactor - but saving neutrons twice over (less Pa absorption and fewer neutrons manage to leak out to hit the outer wall).

Losses to Pa absorption need to be part of the engineering tradeoffs when deciding on design details. The big starting question is whether you are trying to maximize breeding or are you content with iso-breeding. If you are content with iso-breeding then skipping Pa removal is a practical choice.


Top
 Profile  
 
 Post subject: Re: Protactinium-233
PostPosted: Apr 13, 2012 9:20 pm 
Offline

Joined: Oct 06, 2011 1:34 pm
Posts: 81
Steve Brown wrote:
What happens when a Pa-233 nucleus gets hit by a neutron?

Lars wrote:
It converts to 234Pa which rapidly decays to 234U. The 234U won't fission but when it captures a neutron it becomes 235U which will.


So, by that route, it should take 3 neutrons to get a fission reaction which produces on average 2.5 neutrons, for an average loss of half a neutron for each time a 233Pa absorbs a neutron while in the core.


Top
 Profile  
 
 Post subject: Re: Protactinium-233
PostPosted: Apr 13, 2012 9:40 pm 
Offline

Joined: Jul 28, 2008 10:44 pm
Posts: 3069
approximately. Actually because the 235U doesn't fission all the time it is somewhat worse.


Top
 Profile  
 
 Post subject: Re: Protactinium-233
PostPosted: Apr 13, 2012 10:49 pm 
Offline

Joined: Oct 06, 2011 1:34 pm
Posts: 81
I thought of another possible configuration. Instead of a blanket of thoriated salt surrounding the core, thorium could be dissolved in flibe coolant circulating through a heat exchanger in the core. This irradiated salt could be circulated through an external heat exchanger which would transfer thermal energy to gas to drive turbines. Wait for the ratio of 233Pa to 232Th to exceed a certain value, then pump the coolant to a holding tank while replacing it with coolant containing a fresh charge of thorium. The decay of 233Pa to 233U in the holding tank should keep the salt in the liquid phase. After 90 to 180 days, extract the 233U from the holding tank and reuse the remaining thoriated salt.


Top
 Profile  
 
 Post subject: Re: Protactinium-233
PostPosted: Apr 14, 2012 12:41 am 
Offline

Joined: Jul 28, 2008 10:44 pm
Posts: 3069
The point of the blanket is to absorb the neutrons that would otherwise escape the core and beat up on your vessel. Putting thorium in the secondary salt won't help. The point of the secondary salt is to provide one of the multiple barriers between the fission products and the environment. Putting 233Pa into the secondary salt would defeat that purpose.


Top
 Profile  
 
 Post subject: Re: Protactinium-233
PostPosted: Apr 14, 2012 1:52 am 
Offline

Joined: Oct 06, 2011 1:34 pm
Posts: 81
OK, but the same principle could apply to managing the blanket. Instead of continuous processing, wait for the Pa-233 to Th-232 ratio to exceed a certain value, then flush the blanket to a holding tank, simultaneously replacing it with a fresh load of thoriated salt. Let most of the Pa-233 decay to U-233 in the holding tank, remove the U-233, and reuse the remaining thoriated salt.


Top
 Profile  
 
 Post subject: Re: Protactinium-233
PostPosted: Apr 14, 2012 2:44 am 
Offline

Joined: Jul 28, 2008 10:44 pm
Posts: 3069
Pretty close to the general idea. People have proposed having an external tank where additional blanket salt flows through to reduce the chances of Pa capture without adding Pa extraction. But I expect if you work the numbers you will find it is best to flow the blanket salt through the extra hold up so that it contains as concentrated Pa as the blanket itself. So rather than doing things in batches one should pump the blanket salt through the external holdup at a decent pace. The blanket itself will always have higher concentration but the two will be almost the same concentration if the volume of the holdup+blanket gets exchanged once per day.

But better still would be to simply make the blanket thicker by whatever amount of blanket salt you are willing to buy. That way not only do you reduce Pa capture but you also reduce neutron leakage to the wall.


Top
 Profile  
 
 Post subject: Re: Protactinium-233
PostPosted: Apr 14, 2012 7:26 am 
Offline

Joined: Apr 24, 2008 4:54 am
Posts: 491
Location: Columbia, SC
Lars wrote:
... That way not only do you reduce Pa capture but you also reduce neutron leakage to the wall.


The bigger concern IMHO. The helium production in the vessel and neutron embrittlement of the vessel are far worse problems to solve than the Pa-233 parasitic neutron issue.


Top
 Profile  
 
 Post subject: Re: Protactinium-233
PostPosted: Apr 14, 2012 10:46 am 
Offline

Joined: Jul 28, 2008 10:44 pm
Posts: 3069
I'd agree that the Pa233 absorption isn't a killer problem.

We should give folks a little background. We are talking about the wall that contains the blanket salt. Some neutrons generated in the core will reach the boundary between the core and the blanket without being absorbed. These travel into the blanket. Some of those neutrons will travel through the blanket without being absorbed. At the outer edge of the blanket is a layer (typically 0.4m thick) of B4C. Boron is a particularly good neutron absorber. But still some percentage of neutrons will travel through the B4C layer and strike the outer wall. The outer wall is composed primarily of nickel, and 68% of which is 58Ni. For thermal neutrons (which is primarily what you would have by the time a neutron got this far) there is a two stage process to generate an alpha particle (an Helium atom stripped of its electrons). First, 58Ni absorbs a neutron to become 59Ni. Then the 59Ni absorbs a neutron and about 15% of the time undergoes an (n,alpha) reaction releasing an alpha particle. The alpha particles fly out and after a bit steal some electrons to become helium atoms. The helium atoms are somewhat mobile inside the metal. In the original composition of Hastalloy the helium atoms would migrate to a grain boundary. The concentration of helium atoms at the grain boundary would cause the metal to become weak there. The solution was to add some carbide to the metal. The carbide disperses throughout the metal and the helium atoms collect by the carbide atoms. The result is that the helium does not get concentrated and the metal can tolerate much higher levels of helium before weakening.

IIRC MSBR planned on a layer 0.4m thick to reduce the neutron flux to the point that the outer wall is good for 30 years with the Hastalloy variant they had at the time.

My impression is that the cost of the B4C is incidental so making it thicker should not be an issue. I don't think the lifetime of the second wall (the one surrounding the blanket) is an engineering challenge.

I'd also be curious if it would be valuable and feasible to use it as an insulating layer to reduce the temperature of the outer wall.


Top
 Profile  
 
 Post subject: Re: Protactinium-233
PostPosted: Apr 14, 2012 2:59 pm 
Offline

Joined: Oct 06, 2011 1:34 pm
Posts: 81
This article (http://energyfromthorium.com/essay3rs/) explains that U-233 is removed from the blanket salt by fluorination, converting uranium tetrafluoride to gaseous uranium hexafluoride which is easily removed from the blanket salt. Exposing the uranium hexafluoride to hydrogen converts it back to uranium tetrafluoride which can be put into the core salt. There is no mention of separating protactinium, so I presume this means that Pa-233 is left in the blanket salt until it decays with a half-life of 27 days to U-233. During that time, the unwanted capture of neutrons by Pa-233 can occur. My idea is to pump the blanket salt to a holding tank when the ratio of Pa-233 to Th-232 exceeds a certain value, mechanically removing Pa-233 from the blanket. At the same time, fresh thoriated salt is pumped into the blanket. Let the salt sit in the holding tank for 3 to 6 months, after which most of the Pa-233 has transmuted to U-233, without capturing any neutrons from the reactor core. Then extract the uranium by fluorination and cycle the remaining thoriated salt back into the blanket.

The steps of extracting uranium from the holding tank, recycling residual thoriated salt from the tank back into the blanket (along with a fresh charge of thorium), and refilling the holding tank with irradiated salt from the blanket, can all be coordinated and done on a periodic schedule, say every 120 days.


Top
 Profile  
 
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 42 posts ]  Go to page 1, 2, 3  Next

All times are UTC - 6 hours [ DST ]


Who is online

Users browsing this forum: No registered users and 2 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
Powered by phpBB® Forum Software © phpBB Group