rogerl said:Guys. Supposed the fuel rods melted and become molten, do fission still occur in molten state or not anymore?
PietKuip said:Immediately after the quake, the control rods were automatically inserted, and the fission chain reaction stopped. The heat production now is due to decay of the radioactive fission products, and is only about 10 % of the power when the reactor is on.
The fission reaction would have eventually stopped but more slowly since the core configuration would have been disrupted. During operation of a BWR, some control rods are in the core for control of reactivity, i.e., to keep the core critical at steady state. The control rods are gradually withdraw during the cycle as the enriched uranium is depeleted (consumed) in a carefully devised sequence such that the core is always just near critical, otherwise the power is slowing increasing or decreasing depending on the need for power increase or decrease. Toward end of cycle, when much of the fissile material has been used, the control rods are all out (all rods out, ARO). The reactor can continue for some days afterward, and then it is shutdown for refueling and maintenance.rogerl said:I know. But supposed the controls rods were not inserted and the fuel rods melt and become molten and there is a tough cement underneath that prevent further falling underneath the plant. Would the uranium still fission when it's already in the molten state or would fission only occur when the uranium is solid?
Astronuc said:Subsequently, Units 1 and 3 were flooded with seawater to maintain the cooling of the reactors.
Astronuc said:The fission reaction would have eventually stopped but more slowly since the core configuration would have been disrupted. During operation of a BWR, some control rods are in the core for control of reactivity, i.e., to keep the core critical at steady state. The control rods are gradually withdraw during the cycle as the enriched uranium is depeleted (consumed) in a carefully devised sequence such that the core is always just near critical, otherwise the power is slowing increasing or decreasing depending on the need for power increase or decrease. Toward end of cycle, when much of the fissile material has been used, the control rods are all out (all rods out, ARO). The reactor can continue for some days afterward, and then it is shutdown for refueling and maintenance.
If the fuel melted it would displace the water and reduce the moderation, which would make the core slightly subcritical at some point in time, and the fission reaction would decrease.
However, we know that the control rods did insert in the Fukushima reactors, and the fission reactions ceased. The problem is that fission products continue to decay after the fission reaction is stopped, and that heat must be removed from the core/fuel following shutdown. The heat removal did occur for sometime after shutdown, first using power from emergency diesel generators, and then on batteries. However, at some point the cooling capacity was reduced or lost and some part of the core overheated.
Subsequently, Units 1 and 3 were flooded with seawater to maintain the cooling of the reactors.
From the continuity of matter property, what goes into a system, must come out elsewhere if mass in the system is constant. So, if some seawater goes in, it must come out somewhere as water or steam. It is the steam that is occasionally vented to keep the pressure down. However, this means that the system is no longer closed to the environment, and some fission products, mostly gases Xe and Kr, and perhaps some volatiles, e.g., I, will escape to the atmosphere.
The objective now is to cool the reactor core and minimize the release of radioactive substances to the environment.
Steam from the primary system would carry Xe and Kr (noble) gases, and perhaps some I and Br which are volatiles. It is expected that I and Br would form oxides, and react chemically to form iodates and bromates, if not iodides and bromides.nlsherrill said:So, when the news reports that there is a danger of radiation exposure...what exactly is being exposed? I think I know the very basics of the core, which is essentially uranium fuel rods that are bombarded by free neutrons right? Where does Xe, Kr, and I come from? Are these what uranium decays too? When people get radiation sickness/exposure, what is harming them? electromagnetic radiation or something else?
Clearly I have no clue what I'm talking about, but I would like too.
Borek said:For fission it doesn't matter whether the fuel is solid or melted.
As far as I know, they were trying to inject seawater into the pressure vessel, ostensibly through the piping system used by the ECCS. Flooding containment is also a possibility, but that's mostly outside of the RPV which contains the core, and which is where the cooling water must go.LanceV said:I still haven't found out what exactly they flooded with sea water.
Do they just replace lost water in the RPV by sea water? Or are they trying to cool the RPV from the outside by flooding the containment? Or both?
The use of boron suggests the former but I keep hearing about the latter.
Does anybody know more?
Astronuc said:Flooding containment is also a possibility, but that's mostly outside of the RPV which contains the core, and which is where the cooling water must go.
Astronuc said:The steam is venting somehow, through pressure relief valves, and then the personnel have to vent the containment. So the seawater goes in, and some steam comes out, and there has to be a mass balance.
Maybe one is thinking of reflood or quenching (rapid cooling).misnderstudge said:if it was they would circle the rods not by direct contact but by going around it as such, and when it cools down then flood it or there would be a large cloud were they stand. slow cool then rapid cool can't remember the real name for it but i think you may understand
The increase in radioactivity at Onagawa has been attributed to releases from Fukushima.WorldNuclearNews said:Onagawa 'emergency'
A technical emergency was declared at 12.50pm today at the Onagawa nuclear power plant after radiation levels in the plant site reached 21 microSieverts per hour. At this level plant, owner Tohoku Electric Power Company is legally obligated to inform government of the fact. Within just ten minutes, however, the level had dropped to 10 microSieverts per hour.
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Source: http://www.world-nuclear-news.org/RS-Contamination_found_on_evacuated_residents-1303114.htmlWorldNuclearNews said:Potential contamination of the public is being studied by Japanese authorities as over 170,000 residents are evacuated from within 20 kilometres of Fukushima Daini and Daiichi nuclear power plants. Nine people's results have shown some degree of contamination.
Japan's Nuclear and Industrial Safety Agency (Nisa) – part of the Ministry of Economy, Trade and Industry (Meti) – said that out of about 100 residents evacuated from Futaba by buses, nine people were found to have been exposed. The pathway of their exposure is currently under investigation.
One person was measured to have an exposure of 18,000 counts per minute (cpm); another had a measurement of between 30,000 and 36,000 cpm; while a third evacuee had an exposure of 40,000 cpm. A fourth person initially gave a reading of over 100,000 cpm, but a second measurement taken after the person had removed their shoes was just under 40,000 cpm. Another five people were said to have "very small counts".Radiation levels have been monitored across the Fukushima Daiichi and Daini sites. As of 10.00pm today, Tepco said that radiation levels were lower and stable. The maximum level detected on the 12 March was at 3.29pm when levels reached 1015 microsieverts per hour.
On-site casualties
At Fukushima Daiini unit 3 one worker received a radiation dose of 106 mSv. This is a notable dose, but comparable to levels deemed acceptable in emergency situations by some national nuclear safety regulators.
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I have also been exposed to radiation well beyond what the average person in the general population would receive, and I'm fine.Reno Deano said:"A little radiation is not necessarily bad. Cells can be repared, or dead cells are simply discarded and replaced. The more radiation, the more cellular necrosis, the more one can become seriously ill. Some damaged cells may mutate into cancers. Nerve cells are particularly sensitive to radiation, and they are not so easily replaced."
Considering human evolution occurred during higher levels of terrestrial radiation in the far past, More than a little radiation exposure is not that concerning, except for the weak. I and many of my fellows Nukes have life-time whole body doses exceeding 40 Rem, and are relatively healthy at our retirement ages.
Apparently the US 7th Fleet has detected radiation at sea and are moving out of the area.
Is the contamination distribution denser at high altitudes (and up into the jet stream) or do the planes come back with greater contamination because of their path length through the contaminated air being longer?
Reno Deano said:Its the path length and time immersed. There is long lived 1950's & 1960's weapons testing contamination circulating around up there.
I have been looking for info on the risks of a dry spent fuel pool. Unfortunately, there is little middle-of-the road info out there. Lots of the worst-case scenarios, though many of them have the fingerprints of Fairewinds Associates on them. Phrases like "Chernobyl on steroids" etc. The NRC materials I found were comforting about the level of safety required, but very light on the risks.Astronuc said:Is one referring to Onagawa plant? What is meant by enhanced?
There is some concern about the spent fuel pool at FK-I, Unit 1 and whether or not it went dry. I would hope they have checked that.
No, the radiation source is independent of geometry, and only dependent on the fission products or nature of the radionuclides decaying. If one looks at the decay heat curve as a function of time, one sees that it is decreasing, and the radionuclides decay to long-lived radionuclides, or inert (non-radioactive) nuclides.falcon32 said:Layman's question...doesn't the rate of radiation greatly increase when a core melts into a blob at the bottom of the reactor because of the inverse square law? So wouldn't a lone melted rod give off much less radiation than a bunch of them?
And at what time is the maximum amount of radiation released? Pre-core meltdown when all coolant is gone? Post?
Thanks for the help!
In a BWR, there is one control rod for four fuel assemblies. Most modern BWRs use a 10x10 array of fuel rods, but some fuel rods are part-length rather than full-length (core height), and there are 'water rods' or 'water channels' within the assembly in order to introduce water for moderation in the interior rods of the assembly. So while a 10x10 fuel assembly has 100 lattice positions, some designs have 96 rods, some 91 rods, and other 92 rods, and some of those fuel rods might be 2/3 of the full-length or core height.rogerl said:In a nuclear reactor, there are as many as a thousand fuel rods. But there only appears to be a dozen or so control rods which are put amongst the thousands of fuel rods. How does this stop nuclear reaction within each rod? Or in a nuclear reactor. Do neutrons from different fuel rods hit other fuel rods at a distance and the control boron rods are supposed to block or absorb them? But how can the few control rods absorb the neutrons from thousands of fuel rods?