A long letdown curve is the ultimate objective of burnable poison rod especially for long cycle core design, or maybe for soluble boron free operation small modular reactor core (SBFO SMR) design - anyone done burnable poison design for this reactor? I found Duplex design by KAERI Korea. But still, there are not so many papers on this one.
Hi, Mohamad,
Not being involved in power reactor design, I can answer only theoretically. If you consider a thick foil out of a strong neutron absorber (Cd, Gd), most of the impinging neutrons are going to be absorbed close to the surface. Therefore a double layered poison rod will have (if the layers are thick), practically the same effect on the reactor kinetics (reactivity) than a single layered rod. Now, if you were to take the two layers and make two single layered poison rods and insert them into the reactor core, the effect on the reactivity would be greater.
I hope this helps. Cheers, Vladimir
Thank you Vladimir..interesting. I think, maybe the double layered design, purposely to hold the effect longer..? For example..if we have a double layered burnable poison rod with the outer layered is Erbia and inner layered is Gadolinia (stronger absorber than Erbia but burn faster), so most of letdown curve effect at the early stage of cycle would be from Erbia. While Gadolinia (inner layered) effect, which is suppose to be burnt faster and stronger, will last a little bit longer, because of most of the impinging neutrons are going to be absorbed close to the surface, the Erbia layered first..so we can get longer letdown curve of burnable absorber, maybe.
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I'm doing design using MCNPX, I will try to proof my statement as above..maybe the flux distribution inside the double layered rod can explain those claim..
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Thank you again Vladimir Radulovic..send my regards to Dr Luka Snoj and Dr Andrej Trkov, I really admire their work on MCNP and reactor physics.
HI all,
I fully agree with the former comments. However just to add something more: the use of burnable poisons has dramatically improved performance in fission reactors in the last 30 years (burn up increase). It's basically to control and balance (i.e. flatten) the core radial neutron flux, over reactor operating time, so to get a better and more uniform burn out of fuel elements (mainly in the core outer region) and to better control reactor long term reactivity. That being said, the use of poison rod, inserted inside proper positions of some fuel element matrix is not only due to how effective is the capture cross section in the thermal region by materials like boron, gadolinium, erbium, etc, but also the contribution in the resonance region. Therefore the value of the resonance integral plays an important role as well. So the combination of capture cross section by the resonance energy region and by the thermal region can produce a combined poison effect having larger time duration and performance.
It is an upcoming design to demonstrate the feasibility of using atomic layer
deposition (ALD) to apply ultrathin neutron-absorbing, corrosion-resistant layers
consisting of ceramics, metals, or combinations thereof. The manufactured composite particles will ideally be used to address materials issues with Generation IV nuclear power plants, using scaled-up powder coating techniques developed at the University of Colorado