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braxx20
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I thought that control rods were raised and lowered to control nuclear reactions. If so, why wouldn't they simply be lowered in a crises to stop the reaction?
braxx20 said:I thought that control rods were raised and lowered to control nuclear reactions. If so, why wouldn't they simply be lowered in a crises to stop the reaction?
In Pressurized Water Reactors (PWRs), the control rod drives are located on the upper head of the reactor vessel, and control rods are withdrawn during operation (except for certain reactors using 'grey' power shaping rods) into the region above the core.braxx20 said:I thought that control rods were raised and lowered to control nuclear reactions. If so, why wouldn't they simply be lowered in a crises to stop the reaction?
braxx20 said:I thought that control rods were raised and lowered to control nuclear reactions. If so, why wouldn't they simply be lowered in a crises to stop the reaction?
Bandit127 said:What I didn't know was that a significant part of the heat generated in the Earth's interior is also due to decay heat.
There are two functions - control and coolability - and both are required/mandatory for a nuclear reactor per the General Design Criteria, 10 CFR 50, App. A.Caniche said:So you can insert as many control rods as you like ,but if you lose control of cooling functions those rods will end up as part of corium minestrone,cos control rods melt. but I may be wrong,then again ,whatever happened to "controlled shutdown"?
No. Water is the only moderator in an LWR. If the temperatures got to the melting point of cladding, there is no water, only superheated steam. Stainless steel melts at a lower temperature than Zircaloy-2 (BWR) or Zircaloy-4/Zr-1Sn-1Nb/Zr-1Nb (PWR), but Zr and Fe do form eutectics.Caniche said:When cooling is lost and the molten control rods dissolve within the molten fuel rods and their molten casements , is it possible that that the molten control rod material acts as an extremely efficient moderator?
Astronuc said:No. Water is the only moderator in an LWR. If the temperatures got to the melting point of cladding, there is no water, only superheated steam. Stainless steel melts at a lower temperature than Zircaloy-2 (BWR) or Zircaloy-4/Zr-1Sn-1Nb/Zr-1Nb (PWR), but Zr and Fe do form eutectics.
Basically the molten fuel would exclude water, which would exclude moderation.
The Zr-alloys would react with water in an oxidation reaction of which ZrO2 and H2 are products. Actually, Zr-alloys and water react rapidly well below the melting temperatures, so the Zr-alloys would tend to oxidize with available water well before melting. ZrO2 and UO2 can also form solutions at high temperatures.
Same. BWR and PWR control rods use much the same structural material, SS304L, and in some cases, SS316L, of high purity (i.e., low S and P). PWR control rods typically use more Ag-In-Cd for absorber, but can use B4C. BWR control rods use B4C and perhaps some Hf.Caniche said:Cheers, How about the control rods in a BWR entering the mix?
Dmytry said:Caniche "is it possible that that the molten control rod material acts as an extremely efficient moderator":
Molten control rod material is no moderator, but neutron absorber. The terminology may be confusing - control rods sure can be said to 'moderate' the reaction by absorbing neutrons, but in nuclear engineering the moderator is specifically the material that slows down the neutrons so that they become more likely to be captured by U235 versus U238, increasing the reactivity - not the neutron absorber. The control rods simply absorb the neutrons, decreasing the reactivity - while they do slow down some neutrons in process, that effect is insignificant.
Ahh. Well, the slowing down of neutron happens by scattering of the neutrons on the nuclei of moderator (exchange of momentum, much like bounce). The control rod material's nuclei (in particular, boron nuclei in boron carbide) instead absorb the neutrons that collide with them, so whenever dispersed or not, boron does not work as moderator. Though, that makes me wonder what happens to boron carbide when it all cooks together. Carbon does not capture neutrons and thus can by itself serve as moderator. But i'd not expect the carbon to become separated and the stuff to form into a shape that'd sustain chain reaction.Caniche said:Thank you ,most informative.
My question was probably poorly phrased though. I was aware that the control rod function was to absorb rather than moderate.
I was postulating that perhaps , in molten form ,with fairly uniform dispersion throughout the corium mass ,the control rod material might slow down many more neutrons than it would in it's original configuration.
Caniche said:Thank you ,most informative.
My question was probably poorly phrased though. I was aware that the control rod function was to absorb rather than moderate.
I was postulating that perhaps , in molten form ,with fairly uniform dispersion throughout the corium mass ,the control rod material might slow down many more neutrons than it would in it's original configuration.
braxx20,braxx20 said:I thought that control rods were raised and lowered to control nuclear reactions. If so, why wouldn't they simply be lowered in a crises to stop the reaction?
Control rods are designed to regulate the nuclear reaction in a nuclear reactor by absorbing excess neutrons. However, in the event of a meltdown, the control rods may not be able to stop the reaction due to various factors, such as extreme heat and damage to the reactor.
In most cases, control rods are able to completely shut down the reactor. However, if the reactor has already reached a critical state and the fuel rods have started to melt, the control rods may not be able to stop the reaction completely.
During a meltdown, the extreme heat and pressure within the reactor can cause the control rods to warp, bend, or even melt. This can prevent them from being able to be inserted or removed from the reactor, making it impossible to control the reaction.
Scientists and engineers continuously work to improve the design and materials used in control rods to make them more resistant to extreme heat and pressure. They also have emergency systems in place, such as cooling systems and backup control rods, to try and prevent a meltdown from occurring.
While control rods are essential for regulating the nuclear reaction in a reactor, they are not the only factor that can cause a meltdown. Other factors, such as human error, natural disasters, and equipment failure, can also contribute to a meltdown even if the control rods are functioning properly.