Energy From Thorium Discussion Forum

In opposite to the well-known reactor types that are run with solid fuel in a molten salt reactor with external cooling the fuel is running in a cycle reactor – piping - heat exchanger – pump – piping – reactor. Only a fraction of the emitted delayed neutrons are available in the reactor for reactor control.

To have a sufficient number of delayed neutrons available for reactor control is more challenging as for a solid plutonium fuel fueled reactor. According to a rule of thumb at least ½ of the delayed neutrons created are required for reactor control. Hence the requirement of having sufficient delayed neutrons for reactor control limits the share of fuel outside the reactor. In other words it requires a minimum share of fuel in the reactor which might limit the minimization of the reactor and thus the power density of the reactor.

Calculating the flow of fuel and the period of neutron emissions about 68% of the delayed neutrons emitted are emitted in the reactor.
All in all 0.25% of the total emitted neutrons are delayed neutrons available in the MCFR for reactor control.

Please don`t hesitate to have a look in the attached study

Holger

Is this your paper Holger? I think it needs a discussion of results section.

It strikes me that the MCFR has too much fuel salt in "pipes & pumps of the primary circuit". There's an engineering trade-off between higher pump speed versus larger piping. Faster pumps will need more frequent replacing, but that may not be a problem in a well designed system.

Dear Alex,

I`m very pleased to have this kind of discussion.

I`m working since 2011 on the design with a couple of optimization loops.

The reactor consists of 5 loops including 1 loop for the fertile zone. The thermal power is 4732MW, The mass flow per loop is 5365 Kg/s, Volume flow Salt/m3/s: 1.95 outlet, 1.62 inlet, pipe inner diameter each 0,6m. The flow velocity in the main pipes is limited to < 6m/s a number proposed by a couple of MSR studies. The pressure loss in the piping is about 40000 Pa.

In 2017/2018 I plan to make a more proper CAD design/drawing and I fear that I have to increase the distances reactor - hx - pump and hence pipe lengths as I have to include proper flanges for installation, maintenance and dismantling of components (mo - alloy).

What is your idea Alex?

Holger

I'm not too familiar with dual fluid fast designs.

9.4m3 for a 4732MWt core seems very small. Is there a way of having a larger core - hence more salt, with low power density?

I've heard that one of the things the regulators will look at for a fast design is what happens if an event changes the shape of the core, and can it send the reactor supercritical? I assume the challenge there is to have enough delayed neutrons to ensure that can't happen.

You might be able to avoid increasing the distances if you can move the heat exchangers away for "installation, maintenance and dismantling".

Why is the big tank labelled "5" so far away from the reactor?



Take heart from what Ian Scott of Moltex said: something along the lines: "Most engineering challenges get more complex as you get into the detail. The [Moltex Design] was the opposite. As we analysed the problems, they got easier or turned out to not be problems".

Dear Alex,

the "tank" with number 5 in the sketch is a symbol for the lower area of one of the primary pumps. Please find the pump basic data attached.

Acutally PWR have for ex. an average power density of about 100 w/cm3. Fast sodium cooled reactors have for ex. an average power density in the fissile zone of 300 W/cm3. The limitation for these reactor types is the heat conductivity within the ceramic fuel (avoid melting) and the heat transition fuel rod water.

A power density limitation in a MCFR is the evaporation of fuel in the upper center of the reactor vessel. The evaporation within this design at 12 bar reactor pressure starts at 1800°C. This seams comfortable.
Another limitation of the power density is the embrittlement of the structure material of the reactor wall by neutrons. This was a major reason to increase the reactor size within the MSFR project (based on nickel alloys). The MCFR requires molybdenum alloys as structure material. The knowledge about the allowable max. neutron flux on molybdenum alloys and its embrittlement is limited. There are a few studies about using mo-alloys for fusion reactors (Zinkle, Tanaka). It seems that brittleness for mo-alloys is not as critical as it is for nickel based materials (59Ni n->alfa, 58Ni n-> alfa reaction). It is one of the topics that needs research in depth.

The power density in the MCFR fissile Zone of the reactor is 383 W/cm3. The power density in the breeding zone is 17 W/cm3.

The reactor vesel itself is a simple tank. A potential reduction of the power density ... increase in the reactor volume is not a big challenge for this concept.

Best regards

Holger

Holger,

The situation is actually a bit more complicated (and worse) than this. Physically, it is not really enough to account for the neutrons emitted inside/outside, because there is also a "neutron importance" effect, just by the fact that the delayed neutron will be emitted somewhere else than where it was produced (typically, a lower neutron importance region, for example when you have a delayed neutron precursor created in the center of the core that is moved by with the fuel to the top).

Dr. Aufiero (and co-authors) did some outstanding work on this:
https://www.politesi.polimi.it/bitstrea ... s_2014.pdf

(Chapter 3)

Dear Boris,

the dissertation of Manuele Aufiero is indeed very helpful. Thank You very much!

There is indeed a correction factor <1.

The temperature of the fuel in an MCFR increase from bottom to top and as well in axial direction. The density of the fuel decrease from bottom to top.

- That means the weight center of the fuel is below the volumetric center
- That means center of fissions is below the weight center of the fuel (238U neutron capture increase with temperature)
- The center of delayed neutron emissions is above the center of fissions.

What I did as a single person was to show the volumetric distribution of the emissions of delayed neutrons in a MCFR.

Holger