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Japan Earthquake: nuclear plants

by gmax137
Tags: earthquake, japan, nuclear
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rogerl
#73
Mar13-11, 11:55 PM
P: 239
Guys. Supposed the fuel rods melted and become molten, do fission still occur in molten state or not anymore?
PietKuip
#74
Mar14-11, 03:21 AM
P: 184
Quote Quote by rogerl View Post
Guys. Supposed the fuel rods melted and become molten, do fission still occur in molten state or not anymore?
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.
rogerl
#75
Mar14-11, 03:55 AM
P: 239
Quote Quote by PietKuip View Post
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.
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?
Borek
#76
Mar14-11, 04:34 AM
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For fission it doesn't matter whether the fuel is solid or melted.
Astronuc
#77
Mar14-11, 08:37 AM
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Quote Quote by rogerl View Post
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?
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.
LanceV
#78
Mar14-11, 08:43 AM
P: 13
Quote Quote by Astronuc View Post
Subsequently, Units 1 and 3 were flooded with seawater to maintain the cooling of the reactors.
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?
nlsherrill
#79
Mar14-11, 08:45 AM
P: 322
Quote Quote by Astronuc View Post
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.
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.
Reno Deano
#80
Mar14-11, 09:32 AM
P: 128
Hypothetical People external to the plant and workers (real) near or within the plant would be the things being exposed to radiation. Eventhough the melted fuel losses its favorable geometry for substained criticality, some neutrons and high energy gammas will continue to prolong the fission process, but at a significantly lower pace and would be considered subcritical. The attached link discusses the fission fragment spectrum of radioisotopes.

http://hyperphysics.phy-astr.gsu.edu...e/fisfrag.html
Astronuc
#81
Mar14-11, 09:50 AM
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Quote Quote by nlsherrill View Post
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.
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.

Xe, Kr, I, Br are fission products produced when U-236 (U235+n) fissions. There are other fission products such as Cs, Ba, La, . . . . and Rb, Sr, Y, . . . which are essentially in solid form, and the unused U, Pu, . . . which is in the fuel. Normally these are surrounded by a metal alloy of Zr, but that alloy has probably corroded/oxidized, and no longer performs its function, which is the keep the fuel (UO2 and fission products) separated from the coolant.

The fuel and fission products can oxidize into particles on the order of several microns, and this can be then dispersed in the coolant.

The noble gases can readily escape into the steam, and it is hoped that much of the fuel will remain intact.

There are also core components, e.g. control rods, and other structures that are made of stainless steel, typically SS304. The control rods contain boron carbide (B4C) and perhaps Hf, which are neutron absorbers used to control/limit the fission reaction or shutdown the reactor. If the temperatures in the core got to ~1300-1400 C, then the control rods could have melted. Above 1000 C, they could have gotten soft and deformed.

When people are exposed to radiation, it is usually beta and gamma radiation, or possibly alpha particles if isotopes of heavy elements, e.g., Rn, Ra, U or tranuranics were ingested or inhaled. Alpha particles are stopped by clothing or skin. Beta particles are more penetrating, and gamma photons are the most penetrating.

Ionizing radiation harms cells by radiolysis of the water (which forms peroxide and hydrogen), which can then chemically react with the complex molecules like DNA, RNA, proteins, vitamins, enzymes, coenzymes, . . . . which are necessary for cells to function.

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.
TCups
#82
Mar14-11, 09:53 AM
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Quote Quote by Borek View Post
For fission it doesn't matter whether the fuel is solid or melted.
Borek: I think the question is (at least mine is. . . ) this: if the core melts and becomes a molten mixture of fissile materials + melted control rods, even if the reactor's fuel has been "killed" with sea water and boron, is it still possible that the molten mixture might become "critical"?

And even if not, how long will the ongoing heat of decay (at 10% of the level of an operational reactor mentioned) persist in the absence of cooling water?

Finally, are either the primary steel containment vessel or the reinforced concrete secondary containment, or both up to the task of containment of a core meltdown and if so, for how long?
Astronuc
#83
Mar14-11, 09:55 AM
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Quote Quote by LanceV View Post
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?
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.

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.

How they are cooling the seawater once it's heated is not clear.

If they pump seawater into the RPV/primary system, and it is flowing out into containment, that could mean an open valve or pipe break. HOWEVER, there is no information about the integrity of the RPV or piping within containment. We can only wait for further information from the site.
misnderstudge
#84
Mar14-11, 10:23 AM
P: 40
well as a civil defence member (non physicist member) i don't think the risk is high at all, but in its history now is the closest to a risk there has been as far as i am aware. they will properly have to cold start it (i think no facts to back that up) which is very expensive and take months. i am surprised World Agency of Planetary Monitoring and Earthquake Risk Reduction,(WAPMERR) has said nothing at all. oh and the chance even if it was to go of a thermal explosion is so small it is practically 0
LanceV
#85
Mar14-11, 10:29 AM
P: 13
Quote Quote by Astronuc View Post
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.
Yes, but the question is can they inject water into the core? Maybe the pressure is to high. Or there is a danger of explosion when water hits the hot fuel rods.

Quote Quote by Astronuc View Post
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.
At first I was under the impression that the sea water is circulated through the reactor. Maybe by extracting it via the suppression pool and then back into the sea. If the core is not molten this would probably result in only a minor pollution.

But it is hard to find any definite information. It seems that there is nobody twittering from the control room. Maybe they are busy atm.
misnderstudge
#86
Mar14-11, 10:39 AM
P: 40
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 cant remember the real name for it but i think you may understand
DrDu
#87
Mar14-11, 11:26 AM
Sci Advisor
P: 3,557
I just noticed that in Ibaraki which is maybe 50 km south of Fukushima, ambient dose rates are rising (to about 180 nGy/h) for the first time since the earthquake. Apparently, the wind is turning.

See:
http://www.bousai.ne.jp/eng/

For Fukushima itself, values are still not available on that site.
Astronuc
#88
Mar14-11, 11:38 AM
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Quote Quote by misnderstudge View Post
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 cant remember the real name for it but i think you may understand
Maybe one is thinking of reflood or quenching (rapid cooling).

The details for each unit are different and unknown.

Apparently Units 1 and 3 lost EDGs and/or electrical buses between EDGs and ECCS due to tsunami. The tsunami did take out some of the fuel systems for the EDGs, but also may have damaged the electrical equipment.

As far as I know, they are pumping water through the piping in order to cool the core, and/or they are flooding the primary containment.

We lack the details, so we don't know what systems are available.


Apparently the US 7th Fleet has detected radiation at sea and are moving out of the area.

Any sustained activity offsite is a bit worrisome because it means radioactivity is getting of site in significant (not quantified) amounts.

Quote Quote by WorldNuclearNews
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.

. . . .
The increase in radioactivity at Onagawa has been attributed to releases from Fukushima.

Quote Quote by WorldNuclearNews
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.
. . . .
Source: http://www.world-nuclear-news.org/RS...s-1303114.html

Some colleagues indicated that World Nuclear News is one of the best sites for information on the ongoing situation.
Reno Deano
#89
Mar14-11, 11:54 AM
P: 128
"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.
Reno Deano
#90
Mar14-11, 11:59 AM
P: 128
Japanese asking for help: http://www.nrc.gov/reading-rm/doc-co...011/11-047.pdf


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