Hi Brian,
To address your initial question, I've done a summary of some information I found at the International Atomic Energy Agency.
http://www.iaea.org/newscenter/focus/fukushima/
I'm not sure about the copyright/legal issues involved if I upload a document that summarises and uses directly material from a website, so I leave off doing that for now. If it isn't a copyright issue I'd be glad to upload it...
I'll try to sum up what I've been able to find out...
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First, some terminology. Ive used this document to learn about Boiling Water Reactors:
http://www.ansn-jp.org/jneslibrary/npp2.pdf
Ill use these terms:
RPV - Reactor Pressure Vessel. As I understand it, this is where water comes into direct contact with the nuclear material. This generates steam which drives turbines, thus generating power. This water is highly radioactive.
RCV Reactor Containment Vessel. This encloses the Reactor Pressure Vessel (RPV), other primary components and piping. Reinforced concrete, designed to prevent the release of radioactive substances.
SFP - Spent Fuel Pool. The spent fuel is stored in a pool on-site, and needs to be kept covered in water and cooled at all times to prevent possible heat buildup and reactivity.
TEPCO - Tokyo Electric Power Company, operators of the Fukushimi Daiichi Nuclear Power Plant.
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So, it seems the main issues are with the Fukushimi Daiichi Nuclear Power Plant. There are 4 reactors there.
There are 5 main criteria used to describe the situation at the plant:
1. CONTROL OF REACTIVITY.
This is achieved when the reactor is subcritical, and this can be demonstrated and maintained. This has been achieved in all 4 reactors.
2. RESIDUAL HEAT REMOVAL
This function requires keeping the coolant temperature sufficiently below boiling point at atmospheric pressure, covering the damaged core with water, having off-site and backup power available and having long term closed-loop heat removal capability.
This has not been achieved in Reactors 1,2 and 3.
Reactor 1:
- Changes in Temperature and pressure are not significant.
- Provisional analysis shows that fuel pellets melted and fell to the bottom of the RCV. TEPCO also reported that leakage of cooling water from the RPV is likely to have occurred.
If I understand this correctly, it means the fuel pellets have melted through the RPV, and are now on the floor of the RCV. This has resulted in the leaking of highly radioactive water into the RCV. It is likely this water has leaked from the RCV.
Reactor 2:
-Changes in temperature and pressure are not significant.
-Water level of reactor core is about 1500mm and 2100mm from the top of active core. Fresh water injection is provided, however closed loop heat removal is not yet established.
I understand this to mean the upper part of the nuclear fuel is exposed. This is not good, as it needs to be covered in water to keep the temperature under control.
Reactor 3:
There have been increases in temperature noted. How significant this is, I don't know.
3. Containment Integrity
This criteria is met when the containment is leak-tight, containment pressure maintained below design limits and hydrogen explosion prevented.
This has not been achieved in reactors 1, 2 and 3.
Unit 1
Pressure stabilised below design limits, but safe state not achieved.
Unit 2
Containment believed to be damaged.
I take this to mean the RCV is damaged.
Unit 3
(quoting directly from IAEA document)
"On 20 March a sudden significant drop in pressure in the reactor pressure vessel and a decrease in the containment pressure occurred. The reasons for this are unknown. One possible explanation is a loss of containment integrity; however, the pressure in the containment was decreasing slowly and at present remains stable at around atmospheric. In addition water on the turbine building floor of Unit 3 does not show high level radioactivity. Images showed a crack in the primary containment and steam being released from the reactor building."
4. Confining radioactive material
This is achieved when RPV doesn't leak, or leak is confined. Leakages from containment should be prevented or controlled.
This has been partially achieved in reactor 1, and not achieved in reactors 2 and 3.
Reactor 1:
RPV is assumed to be leaking. The location of the leak is not clear as the level in the reactor is not known. TEPCO reports that leaking of cooling water from RPV due to pellets melted and falling to the bottom of the RPV is likely to have occurred.. TEPCO considers the actual damage to RPV is limited.
Reactor 2
RPV is assumed to be leaking. Location of leak is not clear as the level in the reactor is not known. Containment is believed to be damaged. Construction of a cover above the reactor building has to be pursued.
Reactor 3
RPV is assumed to be leaking. Location of the leak is not clear, as the level in the reactor is not known. Containment is believed to be damaged. Construction of a cover above the reactor building should be pursued.
5. Limiting Effects of Releases
No additional releases shall be anticipated, and radiation monitoring effects shall be available.
Reactor 1
Intermittent releases have been observed. RPV and RCV are assumed to be leaking. Opening of the airlock in reactor building did not lead to measurable increase in releases to environment.
Reactor 2
Intermittent releases have been observed. Samples of water in the turbine building floor show high radioactivity releases from the reactor and the containment to the environment.
Reactor 3
Intermittent releases have been observed. RPV and pressure containment vessel are assumed to be leaking. Highly contaminated water flew out into the sea from a pit near the intake channel of Unit 3 (NISA report May23).
Again, a lot, close to all of this actually, is copied directly from the IAEA site and documents.
One thing I have noticed is that they are only able to assume certain conditions, as the site is too dangerous to be observed directly.
Regarding the Spent Fuel Pools, in all cases periodical fresh water injection serves to reduce the SFP temperature. However the closed loop cooling system is not operative, and this must be reinstated.
Now some information from
http://www.slideshare.net/iaea/radio...nt-2-june-2011
Again, I am quoting directly and summarising...
RADIOLOGICAL MONITORING AND CONSEQUENCES OF FUKUSHIMA NUCLEAR ACCIDENT
This document contains lots of data regarding emission monitoring that I cant comment about as I don't understand it.
Key points from document:
Deposition of I‐131 and Cs‐137 in 47 prefectures
I‐131 (Radioactive Iodine?) Not detected since 18 May
Cs‐137 (Radioactive Caesium?) Detected in a few prefectures over a few days. Low levels ranging from 2.2 to 91 Bq/m2
Protective actions
Drinking water: All restriction lifted
The Government of Japan has announced evacuation measures:
Beyond distances of 30 km from Fukushima Nuclear Power Plants
NISA: evacuation of the Planned Evacuation Zones within Iitate village and Kawamata town commenced on 15 May.
Food Monitoring and Food Restrictions (Reported 19 31 May)
818 samples from 18 prefectures
Over 40% of monitoring is in Fukushima prefecture
Over 93% of samples indicated Cs‐134 /Cs‐137 or I‐131 were not detected or were below the Japanese regulation values
Less than 7% were above the Japanese regulation values for Cs‐134 /Cs‐137 and/or I‐131
Restrictions on the distribution and/or consumption of specific foods in certain areas of Fukushima and Ibaraki prefectures remain in place
So, there seems to be limited impact on food supplies. However, I do remember hearing somewhere that the Japanese regulation values were changed after this incident, raising the minimum exposure guidelines. This is not something I am sure of though, and needs further research to confirm or otherwise.
Finally, I'll again summarise and quote directly from the following document:
http://www.slideshare.net/iaea/marin...nt-2-june-2011
MARINE ENVIRONMENT MONITORING
General Comments:
The contamination of the marine environment has occurred both through atmospheric fallout or washout with precipitation, and through discharges of contaminated water into the sea
Discharge to the marine environment decreased significantly over time since end of March; concluding from the near field concentration measurements, up to now, more than 99 % of the activity was discharged between 28th of March and 11th of April.
There is a further continuous discharge of contaminated water into the marine environment with variable activities and activity ratios between I-131 and the two dominating radio-caesium nuclides.
TEPCO and MEXT are continuing to conduct programmes for sea water sampling and to perform measurements. Also marine food and several sediment stations are now monitored.
Conclusions on the impact to the marine environment
The highest levels of radioactive substances are still measured close to the Nuclear Power Stations Fukushima, namely at the screen of Unit 2, 30 m, 330 m and 10 km near-shore. The levels showed a decreasing trend until beginning of May but remained relatively constant since then.
Higher activities are also found in surface sediments near the discharge areas of the NPPs.
Concentration data from about 30 km off-shore are lower and most of the analyses were below the limit of detection at the applied methods (about 10 Bq/L).
There is a continuous outflow of contaminated water from the site, which keeps the levels on the monitoring stations of TEPCO near the shore relatively constant.
Conclusions
In recent days, a significant increase of I-131 and - to a less extent
Cs-134/Cs-137 near the discharge area was observed
Further dilution in the Pacific will lead to lower concentrations of longer lived radionuclides. Traces from the releases from Fukushima NPPs will be taken up by the Kuroshio-current system in the north Pacific and transported across the Ocean.
So, it seems there is continuing leaking of contaminated materials into the ocean, which is entering the oceanic current systems and being transported across the ocean. How significant this is I cannot comment on, but as there is a continuous outflow from the site I think there will be increasing residual contamination until this flow is stopped. Whether this is of a level to significanty impact the ecosystem, or alternatively register above background levels, is something I don't know.
I hope this helps Brian. I wanted to learn for myself more about what was exactly happening. I've tried to find sources that are as impartial as possible. It seems there are a lot of vested interests and agendas that I don't understand going on here, and it's kind of hard to get through all the noise. Hopefully by being as factual as possible we can move our understanding forward a bit.
I hope if anyone can interpret the data and offer more expert analysis than my own humble efforts they will chip in.
Cheers
Stephen