Some questions about neutron shielding in fast reactors

[sf1001]

I've been reading about fast reactor designs lately (not necessarily ones that have actually been built). A few articles I've come across < https://drive.google.com/folderview?id=0B-MNeRy61MgpTVExcUQ3T3ZWWG8&usp=sharing > discuss the possibility of building fast reactors, fueled with molten U, TRU chlorides diluted in Alkali/Alkaline chlorides (not including Li or Be), w/ and w/o a possible Th/U chloride/fluoride breeding blanket. It is claimed, in some of these articles, that fast molten salt reactors (FSMSRs) would not have the need for constant on line removal of fission products (other than more easily removed volatile and noble metal fission products), like thermal molten salt reactors would. One technical problem discussed in these articles that FSMSRs would face (at least for FSMSRs w/o a fertile blanket) is the need to shield the reactor vessel (RPV) from high fast neutron flux; it is suggested in at least one of the articles that ferritic/martensitic steels, which cannot stand the high temperatures of FSMSRs (or any molten salt reactor), coated w/ a nickle based alloy resistant to corrosion (but not fast neutron flux) at high temperatures, are perhaps the only suitable neutron shield for the RPV of a FSMSR w/o a fertile blanket, and that the neutron shields would need to be replaced, perhaps multiple times, during an FSMSR's life time. In one article, document titled: ANL-6792.pdf, w/ most references from 1960s or earlier, it is claimed that INOR (a nickle based alloy?) would be a suitable separator of the fissile core from the fertile blanket. If INOR (or some other nickle based high temperature alloy) were used only as a separator of the fertile blanket from the fissile core, and not as neutron shielding for the RPV, would the embrittlement of the material from the high neutron flux not be a problem (i.e. not prevent the material from chemically separating the fertile blanket from the fissile core)? Also, for a FSMSR w/ a molten salt breeding blanket, would the need for solid material to shield the RPV from fast neutrons be greatly reduced or perhaps eliminated?

 

[Astronuc]

Fast reactors would likely use a blanket/reflector, or perhaps a moderating shield in order to drop the fast flux in the vicinity of the core baffle/shroud. Liquid metal fast reactor fuel typically uses shrouds/channels on the individual fuel assemblies.

The blanket/reflector fuel would be of low enrichment in order to mitigate the fission reactions, which is the source of fast flux.


INOR alloys are apparently Ni-Mo-Cr alloys

INOR-8 is Ni, 17% Mo, 7% Cr, 5% max Fe, with traces of Mn, Si, W, Co, Ti/Al, Cu

http://web.ornl.gov/info/reports/1961/3445606041761.pdf

See page 183 of this text for other INOR compositions.
http://books.google.com/books?id=tfDwOe7xWeQC&pg=PA183&lpg=PA183&dq#v=onepage&q&f=false


An article on Chemical Considerations for the Selection of the Coolant for the Advanced High-Temperature Reactor

https://inlportal.inl.gov/portal/server.pt/gateway/PTARGS_0_2_3310_277_2604_43/http%3B/inlpublisher%3B7087/publishedcontent/publish/communities/inl_gov/about_inl/gen_iv___technical_documents/chemical_considerations_4.pdf

 

[raining Pete]

As an internet forum user, I find the concept of fast reactor designs and molten salt reactors (MSRs) very intriguing. I have also come across some articles discussing the potential benefits of using fast MSRs fueled with molten uranium and transuranic (TRU) chlorides diluted in alkali/alkaline chlorides, with or without a thorium/uranium chloride/fluoride breeding blanket.

One interesting claim made in these articles is that fast MSRs would not require constant online removal of fission products, unlike thermal MSRs. This is because fast MSRs would only produce easily removable volatile and noble metal fission products. However, one potential technical challenge for fast MSRs without a fertile blanket is the need to shield the reactor vessel (RPV) from high fast neutron flux. It has been suggested that ferritic/martensitic steels coated with a nickel-based alloy resistant to corrosion at high temperatures may be the only suitable neutron shield for the RPV. However, these neutron shields would need to be replaced multiple times during the lifetime of the reactor.

In one article, ANL-6792.pdf, it is claimed that INOR, a nickel-based alloy, would be a suitable material for separating the fissile core from the fertile blanket. However, there is concern about the embrittlement of this material from the high neutron flux. If INOR (or another nickel-based alloy) were used only as a separator and not as a neutron shield for the RPV, would the embrittlement still be a problem and prevent the material from effectively separating the fertile blanket from the fissile core?

Another question that comes to mind is whether the use of a molten salt breeding blanket in a fast MSR would greatly reduce or eliminate the need for solid material to shield the RPV from fast neutrons. It would be interesting to see if there have been any studies or experiments done on this topic.

Overall, I find the concept of fast MSRs and their potential advantages very intriguing and I look forward to reading more about this topic. Thank you for sharing these articles and sparking this discussion.