Energy From Thorium Discussion Forum

In ORNL's MSBR about half a percent of the neutrons are produced from n,2n reactions, almost all from Be9 n,2n. This is from about 17 percent BeF2 in the fuel salt.

This seems a considerable amount of neutrons. How does this affect reactor control? The reaction only occurs with fast neutrons, so that a hardening of the spectrum increases reactivity from n,2n. In a well moderated reactor this should have a negative effect on reactivity, right?

Another issue is breeding, if n,2n makes neutrons that adds to the breeding ratio. Does it scale simply with the amount of Be in the core (all other things being equal)? So that doubling Be in the core doubles n,2n?

I don't follow. Do you just mean well moderated has less n,2n? Or do you mean some sort of reactivity coefficient? If the spectrum somehow suddenly got harder, yes you might have a slight n,2n increase but the negative effect of the fission cross section dropping would swamp that out.



Beryllium's n,2n is a nice bonus and just about makes Be absorption neutral. However, if you are replacing LiF with more BeF2 you are adding extra Fluorine atoms and this might offset any advantage of having less Li. As well of course the BeF2 content has big effects on both viscosity and trifluoride solubility.

David L.

Well moderated, assuming the moderator is not largely Be9, means fewer neutrons in fast range which means less n,2n. Fluorine is annoying in this sense because it quickly gets neutrons away from the n,2n fast region.

Regarding breeding ratio. According to table 2 from this reference:

http://hal.archives-ouvertes.fr/docs/00 ... 021911.pdf

Be9 captures a total of 0.05 n/f in a MSBR like system. It makes 0.012 n/f. With a total of 2.5 n/f this means Be9 makes a net 0.3 percent of the neutrons.

Even if we attribute all of Li6 captures to Be9 source,the Be9 capture rate becomes 0.09 n/f with a net 0.03 n/f production rate or 0.12 percent of the neutrons. Ok, so that extra fluorine will steal more neutrons. But if you look at Jaro's design and put in BeO moderator you get around 8x more Be9 in the core as an MSBR of equivalent thermal rating. In such a reactor, do we get 1% extra neutrons or is the scaling more complicated (the spectrum in the moderator will be more thermal on average than the spectrum in the salt)?

Also the graphite steals 0.049 n/f in a MSBR, 0.2 percent of the neutrons.

What is the time constant for the 9Be(n,2n) reaction? (I hope it is long)

Good point Lars. If the time constant is long it adds effective delayed neutrons.

Probably though this won't be the case. The n,2n reaction occurs when a neutron slams hard enough into a nucleus to penetrate it and upset it. Most likely this happens very fast. For sure the fission into two alphas happens very fast, but that might happen after the neutron emission which will be much slower.

Googling turned up nothing.

If you used Be2C in place of BeO, Absorption effect of F will not be there and moderation effect of C will be added. Be2C is a solid like graphite but much harder. Will the n,2n reaction cause deterioration of Be2C crystals?

Generally any solid with carbon in it will swell under neutron irradiation, and oxygen is much less.

BeO, unlike the carbide, also doesn't burn, important public relations item.

I found this nice reference on a gas fuelled reactor. Crazy idea, but lots of good information, including the effect of a thick BeO reflector on neutron production:

http://www.osti.gov/bridge/servlets/pur ... 799231.pdf



I wonder what it will look like for Jaro's MSR with D2O replaced with BeO...

Yes, this excellent research was published in many papers, in the journals of the AIAA (American Institute of Aeronautics & Astronautics), as well as in Nuclear Science & Technology.
I have copies of several of them.

However, one of the authors - the manager who took credit for much of his colleagues' work - ended up in jail for defrauding the US gov't.

While that fact certainly doesn't invalidate the quality of the VCR-MHD project as a whole (since it was peer-reviewed by other scientists), this Forum has blacklisted anything to do with this information -- so be prepared for your post to be deleted in the very near future.

Well, 10% extra neutrons does seem fantastically high, given the MSBR only gets 0.5 percent extra neutrons from its Be9, and only 0.12 percent if we consider the eventual captures (neutron alpha on Be9 and neutron gamma on Li6 decay product).

But then again the MSBR doesn't have that much Be (9 tonnes if I've calculated it correctly). With your MSR design there would be a lot of BeO moderator and reflector. The ARE used BeO but I haven't seen any mention of the n,2n effect on breeding probably because they didn't care about breeding so much as performance.

This confirms what is going on with a big n,2n bonus from BeO: fast leakage in a thick BeO reflector. 3-6% reactivity bonus is quoted, seems like a lot to me.

http://www.sciencedirect.com/science/ar ... 4982900640

...leakage being presumably pretty high for a UF4/KF vapour core. Makes sense.

Leakage may also be high for your HW-MSR. Especially the axial leakage. Leaked neutrons at the bottom will tend to go in the uranium in the dump tank. Leakage at the top will need a reflector. If BeO is used as radial reflector and BeO as top reflectors, it could be a nice n,2n bonus. If BeO is used as moderator it could be even better.

What happened to the BeO of the Aircraft Reactor Experiment?

Maybe it was re-used in the Project Pluto ramjet reactor ? (Tory-2C, 500MWth)

(not likely.... just wanted to point out that there were other reactors using BeO moderator)