Posted on 05/19/2008 5:59:37 AM PDT by Straight Vermonter
A Nuke Bump!
Otherwise, what a great idea!
Too bad a president got rid of the National helium Reserve, though. Who needs Zeppelin in the 21st century?
Pump in more helium and you get more reactivity. Remove helium and it moderates. Dump all the helium, and it stops reacting.
There are still issues in making the pebbles, but it is a very sound concept.
We need to start building now, with a target of having at least 80% of our electricity be nuke within 10 years, and at least a doubling of our electric capacity (to accommodate electric cars, and a switchover from oil heat to electric heat in the Northeast)
The real beauty of the PBMR is its modularity.
A utility can start with a small unit and add on, as power demand increases.
PEBBLE BED REACTOR PING!!!!!...............
It is constant. The uranium and the "moderators" are mixed together in the pebbles, kinda like ingredients in a cake. You can't see the eggs, but they are there................
It's at the point in the article where they mention Doppler broadening
At the higher temperature, the more plentiful uranium-238 nuclei absorb more neutrons (due to an effect called Doppler broadening) and the reactor output decreases, lowering the reactor temperature until an equilibrium is reached.If the temperature goes up, the fission rate decreases, which is a nice negative-feedback mechanism. So you modulate reactivity by modulating core temperature, and you modulate core temperature my modulating coolant flow
A lot of homes up here were built with electric heat but oil or gas heat was added later as electric became too expensive. This winter lots of folks turned on their electric heat for the first time in ages.
Not exactly..........the helium is the "working fluid". From the article: The pebble bed modular reactor uses helium gas, rather than water, as its working fluid. At a PBMR power output of 165 MW, helium gas flows at 190 kilograms per second down through the reactor in the interstices formed by the spherical carbon surfaced pebbles. The gas is heated by convection and radiation from the nuclear fission heat generated in the kernels and conducted to pebble surfaces. The PBMR design does not put the helium in direct contact with radioactive substances, since the fissile material and fission products are sealed in the kernels and pebbles. What's more, helium is inert, so neutron activity does not cause it to become radioactive. Thus, the helium may be used directly as the working fluid in gas turbine turbomachinery, without the need for an isolating heat exchanger and another, separate fluid loop. The part load characteristics of a closed-cycle gas turbine are remarkably good, quite unlike open-cycle operation. If the PBMR gas turbine is at the design output of 165 MW, load reduction is achieved by bleeding helium from the closed loop. This reduces the helium mass flow rate, reducing power output, and lowering the mean gas density, but maintaining constant gas velocities at constant rpm. In gas turbine designer terminology, the turbomachinery velocity triangles remain the same, so that the PBMR design efficiency of 41 percent will remain the same over a wide range of loads...
And one of the biggest costs for a nuke plant is all the environmental paperwork and approvals process. Once you have a nuke site approved, the incremental cost of adding more nuke modules is much lower than the cost of the site preparation for the first module
Control rods of neutron absorbing material---just like any other nuke plant.
I doubt that. Certainly it is true that the neutron moderators are part of the fuel pebbles, but those are NOT the control elements (which is what I take him to be referring to). There still has to be some kind of reactivity control device, and those are most likely to be regular old "control rods" made of a neutron absorber. The precise design and arrangement will obviously be different from a standard LWR or PWR, but there will still be a control system in place
Why does this name leave me feeling uneasy?
The reactor is reactivity-limited by a phenomenon called Doppler broadenining. Basically, the hotter the reactor runs, the more the U-235 atoms of the fuel vibrate around, thus “broadening” their virtual capture cross-section to the thermalized neutrons being emitted by other,surrounding fuel that sustain the chain reaction. As the reactor heats up, the rate of neutron capture goes up, and the total flux of thermalized neutrons needed to sustain criticality falls. Once a certain temperature is reached, the fuel is soaking up enough of the excess t-neutrons to take the core subcritical, and the heating stops. Of course, once the heating stops, the capture x-section of the fuel drops again, allowing more neutron flux, and the core goes critical again. To control reactivity, therefore, one need only control the temperature inside the core by manipulating coolant flow. If the flow of helium coolant running through the core is decreased, the core heats up, capture x-section increases, and reactivity decreases. If coolant flow is increased, core temp drops, capture x-section decreases, and reactivity increases.
It really is a clever system.
I was going to bump for later reading but noted that it uses helium as the working fluid.
I read a few years ago that there was going to be a helium shortage in a few years. One of the major sources is in Kansas and it is predicted to run dry in a few years.
Where Has All the Helium Gone?
http://www.blm.gov/wo/st/en/info/newsroom/2007/january/NR0701_2.html
I’ve been a big fan of PBRs for a long time. Wish I could get one in my backyard!
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