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Running your reactor http://energyfromthorium.com/forum/viewtopic.php?f=2&t=4282 
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Author:  Cyril R [ Jan 09, 2014 3:56 am ] 
Post subject:  Re: Running your reactor 
jaro wrote: It's important to note that there is a significant difference in reactor controllability when using different fuel types in reactors. For example, the delayed neutron fraction of U233 is small relative to the U235 we normally use: 0.67% versus 1.62%. It doesn't appear to be very relevant, looking at different fuel cycles' transients. Much more important is speed and magnitude of the feedback coefficients, and reactivity reserve (in case of control rod withdrawal accidents)/reactivity insertion possibilities. Quote: Besides that, there may also be important sources of delayed neutrons that do not come from fission product decay. For example, when heavy water moderator is used, we get some delayed neutrons from gamma ray hits on deuterium nuclei. MSRs should do well, with light elements like Be, F in the salt and alpha emitters directly inside this light element mixture. After some time the salt becomes a giant neutron source. 
Author:  Lars [ Jan 09, 2014 10:42 am ] 
Post subject:  Re: Running your reactor 
A larger delayed neutron fraction is a positive in the sense that we shorten the required reaction time but negative in the sense that the reactivity insertion due to flow stoppage. Which is the more challenging problem for an MSR? 
Author:  Cyril R [ Jan 09, 2014 11:13 am ] 
Post subject:  Re: Running your reactor 
Lars wrote: A larger delayed neutron fraction is a positive in the sense that we shorten the required reaction time but negative in the sense that the reactivity insertion due to flow stoppage. Which is the more challenging problem for an MSR? These should cancel out right ? 
Author:  Lars [ Jan 09, 2014 1:48 pm ] 
Post subject:  Re: Running your reactor 
The effects of a smaller delayed neutron fraction is both beneficial and harmful. Since different solutions are required for the reactivity insertion (time scale a few seconds) and normal reactivity control (time scale a millisecond) these effects don't really cancel each other out. My sense is that for control of normal reactivity Doppler effects are plenty fast and have sufficient capacity. For flow stoppage though the Doppler effects do not have sufficient capacity so we will also depend on thermal expansion. Even with thermal expansion though I suspect we don't have enough control capacity so a few neutron poison control rods held out of the core by upflowing salt will be needed to ensure we compensate for flow stoppage (due to no longer having delayed neutrons released out of core). 
Author:  Cyril R [ Jan 09, 2014 5:41 pm ] 
Post subject:  Re: Running your reactor 
Lars wrote: The effects of a smaller delayed neutron fraction is both beneficial and harmful. Since different solutions are required for the reactivity insertion (time scale a few seconds) and normal reactivity control (time scale a millisecond) these effects don't really cancel each other out. My sense is that for control of normal reactivity Doppler effects are plenty fast and have sufficient capacity. For flow stoppage though the Doppler effects do not have sufficient capacity so we will also depend on thermal expansion. Even with thermal expansion though I suspect we don't have enough control capacity so a few neutron poison control rods held out of the core by upflowing salt will be needed to ensure we compensate for flow stoppage (due to no longer having delayed neutrons released out of core). The thing is though, the thermalhydraulic conditions aren't very challenging. Let us imagine a 1/3 out of core with a 450 pcm delayed neutron worth. 150 pcm gets added from flow stoppage. At an alpha of 5 pcm/K this causes an increase in the temperature of 30C. Next is a lower delayed neutron worth of 240 pcm. 1/3 out of core means 80 pcm is added, with the same alpha the temp rise is 16C. The lower delayed neutron worth has actually resulted in a lower heatup. But neither heatup is challenging, 16 or 30C in seconds, hardly a transient at all. So while not enough control capacity exists to prevent a heatup from flow stoppage, the heatup is hardly exciting. We can tolerate a great many such flow stoppages (pumps trip in practice) by design. 
Author:  jaro [ Jan 09, 2014 9:13 pm ] 
Post subject:  Re: Running your reactor 
A little "thought experiment" to try: Suppose you have a reactor fuelled with only U234, only Pu240, or only Am241. Criticality is readily achieved with these fuels, but because they have ~zero fission xsection below about 1MeV, their effective delayed neutron fraction is also zero. This has an impact on controlability, since power doubling time is then measured in microseconds. The practical implication is that reduced delayed neutron fractions impact operating safety: In fact, this is the reason why LWR SNF can't be recycled more than once or twice  the operating margin becomes too small, as the evennumber isotopes like Pu240 build up. And while some MSR concepts may have certain mitigating effects, no regulatory authority will buy blanket dismissals as proof of safe operation. 
Author:  Cyril R [ Jan 10, 2014 5:50 am ] 
Post subject:  Re: Running your reactor 
jaro wrote: A little "thought experiment" to try: Suppose you have a reactor fuelled with only U234, only Pu240, or only Am241. Criticality is readily achieved with these fuels, but because they have ~zero fission xsection below about 1MeV, their effective delayed neutron fraction is also zero. This has an impact on controlability, since power doubling time is then measured in microseconds. The practical implication is that reduced delayed neutron fractions impact operating safety: In fact, this is the reason why LWR SNF can't be recycled more than once or twice  the operating margin becomes too small, as the evennumber isotopes like Pu240 build up. And while some MSR concepts may have certain mitigating effects, no regulatory authority will buy blanket dismissals as proof of safe operation. Too extreme an example. Th cycle has less delayed neutrons but far far from zero. Also, solid fuel reactors have thermal limits on fuel and cladding, made worse by the heterogeneity of the fuel (it is only ever in one place, doesn't mix). There are also issues with void coefficients for some higher actinides in LWRs. Those limits are absent from liquid fuelled reactors. With MSRs, any reactivity insertion that is smaller than the delayed neutron worth, will be generally boring. A good paper on multi recycle Pu for LWRs, showing less and less negative void coefficient for multi recycles. http://janleenkloosterman.nl/papers/klooster9803.pdf From the paper, Quote: The þe f f for the UO2 fueled reactor decreases from 0.7% at BOL to 0.5% at EOL, due to the burnup of U235 and the buildup of Pu239. The latter nuclide has a much smaller delayed neutron fraction. For the MOX fueled reactors, the þef f equals about 0.4% and is almost constant as a function of burnup. So there shouldn't be any issues with the lower delayed neutron worth itself. 
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