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PostPosted: Feb 16, 2013 4:25 pm 
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Cyril R wrote:
jaro wrote:
Cyril R wrote:
....In BWRs, the moderator (coolant) coefficient dominates all others. This will not be the case in a molten salt reactor where the coefficients are expected to be in a similar ballpark.

Finally, I'll note that the thermal response time between fuel and coolant in MSRs is nearly instant, whereas in BWRs with thermally sluggish oxide fuel - would be a rather good insulator in fact, if it weren't radioactive - there's a large delay between fuel and coolant response. This means the BWR has low frequency oscillations where more damaging energy can accumulate.
On the other hand, a graphite-free TMSR or chloride MSR has a much shorter average neutron lifetime than your typical BWR: exponential power ramp is a lot quicker, in response to any reactivity changes.
Looking forward to any sort of realistic analysis - as opposed to blanket dismissal of potentially troublesome instability operation modes.
Got any ?
Thanks.


I'm sorry, I didn't mean to be dismissive, rather I think it important to have a mechanistic undertanding of the processes and differences involved. Perhaps it's my lack of formal training in nuclear physics, but just ranting on about the processes helps me understand much.

The issue of oscillations is certainly complex. But we need to also look at existing reactor systems and their experience. Even PWR and BWR differ much so that oscillations seen in BWR are not seen in PWRs with sufficient subcooling margin. MSR is basically more similar to PWR, except you get a 700 degree Celsius margin to boiling rather than a 15 degree Celsius margin, and smaller % density changes per degree Celsius. Even with that small margin for a PWR, it gets to avoid most of the oscillations that are observed in BWRs.

In practise, the oscillations in BWRs were never found to be damaging. They were found to be rather usefully self-flattening the power profile of the core.

The viscosity of the molten salt, and it's great weight (several times that of pressurized hot water) will also be beneficial as it absorbs energy from pressure waves.

Agreed that it is different for fast spectra MSRs. But I don't find these very convincing for a first build, certainly not the French design that lacks a blanket in one dimension. I'd like to see a start with a simple graphite moderated converter reactor. With lots of little channels, laminar flow. Turbulent just results in pressure drop and is not necessary for heat transfer as the coolant is integrated with the fuel. A simple, orificed, laminar flow and well thermalized spectrum would be perfect.

This paper includes an analysis for the safety case in the event the bubbles collapsed. It does not include oscillatory behavior though.


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