Without water—lost in the steam production—the negative temperature coeff. Is diminished or lost completely.
The rate at which reactivity is added to the reactor is important in startup to avoid super criticality and an uncontrolled –run-away—reaction. Any positive temperature coeff. resulting in an increase in fast neutron flux is unacceptable and needs to be avoided. Loss of liquid water would be a problem if it happened fast and added reactivity and loss of the negative temperature coeff. Bob Cook Bob Cook Sent from Mail<https://go.microsoft.com/fwlink/?LinkId=550986> for Windows 10 ________________________________ From: mix...@bigpond.com <mix...@bigpond.com> Sent: Tuesday, April 30, 2019 3:51:31 PM To: vortex-l@eskimo.com Subject: Re: [Vo]:Thorium breeding now? In reply to bobcook39...@hotmail.com's message of Tue, 30 Apr 2019 20:57:40 +0000: Hi Bob, [snip] >As suggested, steam would reduce the flux of thermalized neutrons in the >reactor and shut it down. To maintain criticality, reactivity would need to >be added, probably by the movement of control rods to increase the flux of >fast neutrons upon which criticality depended. Did you mean "...to increase the flux of [slow] neutrons"? >Control systems, including the mechanical portion of the system for insertion >of negative reactivity—the control rods, need to be very fast to avoid super >critical conditions. I think that would only be true is the feedback were positive, however negative feedback is by definition self-regulating. IOW a power increase leads to more boiling reducing the slow neutron flux and therefore slowing the reactor down again. Inversely, a power decrease reduces the number & size of steam bubbles, resulting in more moderation, and an increase in thermal neutron triggered fission events and the power increases. IOW a boiling water reactor would be self-regulating and tend to stabilize at a fixed power level. The actual level at which it stabilized could be regulated by control rods, the degree of insertion thereof determining the current stable power level. Of course the maximum power level would be determined by the overall size of the reactor and the concentration of fuel nuclei in the solution. Radiolysis is a potential problem, though I think that can largely be overcome by using recombiners. Note that bubbles created by radiolysis have the same negative feedback characteristic that steam bubbles have. > The design of fast control systems with high reliability is a problem IMHO > given the complex analysis associated with knowing the local variety of > materials at any given place. > > > >The “nice” thing (RELATIVELY SPEAKING) about normal fission reactors is the >reliable knowledge of the location of the various isotopes in a dynamic >reactor with pressure, temperature and dimensional change occurring rapidly. > I think such variability "might" be reduced to acceptable levels by pumping the working fluid through a many densely packed parallel tubes in the reactor core, rather than having it is a single large container. Individual tubes tend to minimize convection currents, especially if they run horizontally. A rapid fluid flow also helps. Control rods can be inserted between the tubes, and parallel with them. Regards, Robin van Spaandonk local asymmetry = temporary success
