Japan Earthquake: Nuclear Plants at Fukushima Daiichi

[tsutsuji]

For radiation levels inside the building to go up without venting there must have been some leaks. But in a way it is not surprising that the unit 1 containment was leaking in 2011 (when it was 40 years old and stressed to the max by a melting core) when it had already leaked unacceptably in 1992 during routine inspections when tested at 3 bar:

Faked pressure test

Yet in the most serious case of all, Tepco officials are alleged to have faked a pressure test designed to test the integrity of the containment building. The test involves pumping nitrogen gas into the building to increase the pressure to about three times atmospheric pressure, then taking pressure readings to measure the leak rate.

Regulations state that the leak rate must be less than 0.45% per day. However, at Fukushima I-1 in 1992, the company conducted its own tests before the government inspectors turned up, and discovered that the building might not pass the test. One source quoted in the Daily Yomiuri said that leak rates fluctuated from 0.3% to 2.5% per day.

Documents found at Hitachi by Tepco’s own investigative team describe a method to fake the test by secretly pumping in extra air from the main steam isolation valve. At the time, Hitachi had a contract to check Tepco equipment. It is alleged that Tepco officials followed this procedure when the government inspectors were checking the leak rate.

http://www.klimaatkeuze.nl/wise/monitor/574/5441

2.5% of several thousand cubic meters of nitrogen at 3 bar is several hundred cubic meters that would have leaked per day.

The following NISA report, written in December 2002, contains a time-line. Here are a few translated excerpts :

September 25 (Wednesday). The Yomiuri Shimbun evening edition reports that fraud took place, during leak rate tests conducted in 1992.
[...]
November 06 (Wednesday). Start of legally required on-site inspection regarding the leak rate of the concerned unit.

November 29 (Friday). A one-year shut-down of the concerned unit is ordered.
[...]
December 05 (Thursday). Tepco announces that, regarding the concerned unit, it obtained a leak rate measurement result of 0.092% / day which satisfies the standard criteria.

http://www.meti.go.jp/report/downloadfiles/g21224d0122j.pdf p. 15-16

The 28 May 2004 Tepco press release announces the following :

28 May (Friday) 10:00～16:00 : 0.122% / day (below the 0.348% / day standard criteria)

27 May (Thursday) 10:00～16:00 : 0.123% / day (below the 0.348% / day standard criteria)

http://www.tepco.co.jp/fukushima1-np/bi4509-j.html

The 15 December 2010 press release about regular inspection No. 26 (March 2010 -December 2010) says :

13 July 08:00～14:00 : 0.166% / day※ (below the 0.4% / day standard criteria) (※ 95% confidence limit - upper limit)
http://www.tepco.co.jp/nu/f1-np/press_f1/2010/pdfdata/bi0c06-j.pdf page 5 (pdf page number 7)

http://www.tepco.co.jp/nu/f1-np/press_f1/2009/pdfdata/bi9714-j.pdf (page 5) 17 February 2009 : 0.176%
http://www.tepco.co.jp/nu/f1-np/press_f1/2007/pdfdata/bi8116-j.pdf (page 7) 12-13 September 2007 (24 hour test) : 0.101%

 

[tsutsuji]

An interesting (1) theory is proposed to explain the March 20~24 radiation peak in the Kanto area:

A second meltdown likely occurred in the No. 3 reactor at the Fukushima No. 1 nuclear power plant, a scenario that could hinder the current strategy to end the crisis, a scientist said.
[...]
One factor used by Tanabe in speculating that a second meltdown occurred is the increase in radiation levels from the morning of March 21 in areas downwind from the Fukushima No. 1 plant, such as the Fukushima No. 2 nuclear power plant as well as the Kanto region municipalities of Kita-Ibaraki, Takahagi and Mito.

Initially, officials of the Nuclear and Industrial Safety Agency explained that the higher radiation levels were caused by radioactive materials falling to the ground with the rain.

But there is also the possibility that additional radioactive materials emitted from the second meltdown may have been blown by the wind.

Between 1 a.m. and 3 a.m. on March 21, the pressure within the pressure vessel of the No. 3 reactor core increased sharply to about 110 atmospheres, likely caused by an explosion within the pressure vessel due to a lack of cooling of the fuel. That was probably the start of the second meltdown, Tanabe said.

http://www.asahi.com/english/TKY201108080276.html "Report suggests second meltdown at reactor at Fukushima plant" by Tomooki Yasuda staff writer

See also the diagrams on the Japanese language article page : http://www.asahi.com/national/update/0807/TKY201108070330.html

(1) I discussed the 21 March radiation peak in Mito City in April on https://www.physicsforums.com/showthread.php?p=3258064&highlight=Mito#post3258064 and again in May in relation with the 21 March 8~12 MPa unit 3 pressure data on https://www.physicsforums.com/showthread.php?p=3308800&highlight=Mito#post3308800

http://www.nikkei.com/news/category...39797E3E2E2E2;at=DGXZZO0195165008122009000000 The unit 1 SFP cooling system will be launched on 10 August. The remote-controlled construction of the steel frame of unit 1's cover structure will be achieved by mid-September. SARRY will be launched next week.

 

[joewein]

Thank you, tsutsuji!

From 0.101% to 2.5% leakage per day seems like a huge spread. I wonder what measures they took to reduce the leakage and if this is tested while the reactor is in cold shutdown or powered up. Temperatures could hugely alter tolerances due to thermal expansion and contraction.

I checked the operating records and for example the 2007 figure for unit 1 was measured more than 6 weeks before the reactor went on the grid again and after 9 months of shutdown.

The Mark I containment in unit 1 seems to have a free volume (dry+wet) of 5800 m3, of which 2100 m3 is water in the suppression chamber and 3700 m3 is space for nitrogen. A permitted leak of 0.4% per day of 3700 m3 is 14,800 liters of containment gas per bar of internal pressure. In a station blackout the SGTS could not take care of cleaning up any contamination from that.

 

[tsutsuji]

Thank you, tsutsuji!

From 0.101% to 2.5% leakage per day seems like a huge spread.

You're welcome, Joewein. Note also the 0.092% rate I mentioned above, measured by Tepco 15 days after the 20 November 2002 shutdown.

I wonder [...] if this is tested while the reactor is in cold shutdown or powered up.

Sealing tests are usually carried out at the last phase of government inspections
http://www.thefreelibrary.com/Agency+begins+probe+into+TEPCO+data+manipulation+case.-a093437939

Which perhaps leaves plenty of time if the company wants to perform its own informal tests beforehand. So It seems that the tests are usually performed during cold shutdown. Wouldn't it be dangerous to perform tests while the plant is in full operation ?

 

[HowlerMonkey]

I think the brown/red stuff is from rainwater that gets between the insulation and the outside of the pipe itself that is flowing to the bottom and out the gaps as rust.

You can see similar deposits near that railing and this facility is bordering the ocean which causes things to rust at unbelievable rates.

 

[westfield]

I have a nasty suspicion that the rain water simply is passed to the normal storm water system and then to the ocean and/or ground water. The rationale being that on the "design basis" only filtered gases are exhausted through the stack. Remember that the hardened vent was a retrofit.

or you could go and look at some drawings of say Oyster Creek as an example that show the sump pumps in the base of their stack are indeed fed to rad waste treatment.

http://pbadupws.nrc.gov/docs/ML0522/ML052220603.pdf"


Even when things are normal at these plants the stack emissions are still not 100% clean by any measure so it would be a given any fluid in the stack sump\s should be sent to treatment not to normal stormwater.

 

[tsutsuji]

http://mainichi.jp/select/today/news/20110810k0000m040046000c.html Struck by a lightning at 8:20 PM on 8 August, the water treatment facility was shut down for two hours as a wrong signal was emitted by some storage tank's water level gauge and a fuse was blown at another tank. The facility is not equipped with lightning countermeasures. Junichi Matsumoto said "if long term use is considered, countermeasures are needed".

http://www.nikkei.com/news/category...39797E3E2E2E2;at=DGXZZO0195165008122009000000 The utilization rate for the 3 August - 9 August week is 77%. This is short of the 90% goal for August. Tepco admits that the plan to treat all the accumulated water by the end of the year will be "a little delayed". On the one hand, the bypass lines which have been used since 4 August have enabled to greatly recover from the flow rate decline, but on the other hand, the facility stopped for 7 and a half hours on 7 August. A gas sample from unit 2 containment vessel was analysed, finding a radiation level lower than expected, and Xe and Kr among the radioactive substances.

http://www.tepco.co.jp/cc/press/betu11_j/images/110810g.pdf 3 August - 9 August water treatment weekly report (Japanese)

http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_110810_03-e.pdf "Results of Gas Sampling inside the Primary Containment Vessel of Unit 2 Fukushima Daiichi Nuclear Power Station"

http://www.tepco.co.jp/en/press/corp-com/release/11081003-e.html [unit 1] "At 9:00 am on August 10, we started to assemble steel frame for the Reactor Building Cover"

http://www.tepco.co.jp/cc/press/betu11_j/images/110810l.pdf Update of the worker radiation exposure statistics (Japanese)

http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_110810_02-e.pdf Water level gauge problem at the desalination facility, causing a shutdown from 1:50 to 9:35 AM.

 

[rmattila]

http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_110810_03-e.pdf "Results of Gas Sampling inside the Primary Containment Vessel of Unit 2 Fukushima Daiichi Nuclear Power Station"

The detection of Kr-85 (if the results are correct) is interesting. As far as I am aware, Kr-85 is not produced in the long decay chains, so all of it has been around ever since the fissions stopped. If it is still found in the containment, my first impression is that either

(a) the containment has somehow been able to contain the noble gas Kr-85 ever since the fuel failures occurred in spite of the leaks and the suspected hydrogen explosion early on during the accident
(b) Kr-85 has been released to the containment atmosphere more recently, which means that some fuel rod claddings have lost their integrity only recently

 

[zapperzero]

c) corium is still outgassing

 

[rmattila]

c) corium is still outgassing

As far as I am aware, noble gas releases from fuel would have reached 100 % before it even starts to melt. Therefore, the only way I see new release of Kr possible is that part of the fuel would have not overheated to 100 % release levels (=would possible have maintained their cladding integrity and would continue to slowly release noble gases). This is exactly the point I found interesting about the results: to me it seems that either the containment is able to contain noble gases for a long period of time or part of the core must have remained unmelted.

 

[SpunkyMonkey]

As far as I am aware, noble gas releases from fuel would have reached 100 % before it even starts to melt. Therefore, the only way I see new release of Kr possible is that part of the fuel would have not overheated to 100 % release levels (=would possible have maintained their cladding integrity and would continue to slowly release noble gases). This is exactly the point I found interesting about the results: to me it seems that either the containment is able to contain noble gases for a long period of time or part of the core must have remained unmelted.

The http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_110810_03-e.pdf" says the Kr-85 was in liquid samples. The air sample doesn't list it.

 

[Gary7]

I think the English handout is mistaken. On the Japanese original it says the KR-85 was found in the air sample. The liquid sample doesn't mention KR-85.

http://www.tepco.co.jp/nu/fukushima-np/images/handouts_110810_04-j.pdf

 

[joewein]

The http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_110810_03-e.pdf" says the Kr-85 was in liquid samples. The air sample doesn't list it.

I think the English handout is mistaken. On the Japanese original it says the KR-85 was found in the air sample. The liquid sample doesn't mention KR-85.

http://www.tepco.co.jp/nu/fukushima-np/images/handouts_110810_04-j.pdf

Gary7 is correct. In the English version both tables were accidentally labeled as "Liquid samples", but actually only the first table (that only mentions cesium and iodine) is about liquids ("ekitai"), the second is about gaseous ("kitai") samples.

 

[causeceleb]

we only need steam at 300 degrees Fahrenheit
to make electricity; so, nuclear fission is
a magnificent way to boil water.
but, it is just a never ending mystery
to me why and how the engineers have
turned the whole thing into such an ordeal:
i mean, what the hell can be so difficult
about maintaining an atomic pile to boil
water to 300 degrees fahrenheit and keep
the whole process under control, huh?

<disparaging remark deleted>

 

[Joe Neubarth]

we only need steam at 300 degrees Fahrenheit
to make electricity; so, nuclear fission is
a magnificent way to boil water.
but, it is just a never ending mystery
to me why and how the engineers have
turned the whole thing into such an ordeal:
i mean, what the hell can be so difficult
about maintaining an atomic pile to boil
water to 300 degrees fahrenheit and keep
the whole process under control, huh?

<disparaging remark deleted>

I would not go that far in a statement. I have known a few design engineers and they were highly intelligent. Smarter men than I, and I always stand in awe of how beautifully their brains work.

Nuclear plant designs are well thought out. Site designs are another thing.

When I look at Fukushima, I do not see a sea wall. That is a site engineering problem in a country that knows that the run up from a four meter tsunami can exceed 40 feet. They needed a sea wall built for that possibility and they did not do so.

In addition to not having a sea wall, they put their Diesel Generators in positions where they could be douched by a tsunami that they did not build a sea wall for.

In addition to that they had motor controllers (for cooling pumps that needed to be used in a meltdown like crisis) located where they, too, were douched by the tsunami that the did not build a sea wall for.

The list of site engineering screwups at Fukushima is endless, but site engineering is not nuclear engineering. It is obvious to me that the site engineers did not have a clue what they were building for and against.

 

[etudiant]

we only need steam at 300 degrees Fahrenheit
to make electricity; so, nuclear fission is
a magnificent way to boil water.
but, it is just a never ending mystery
to me why and how the engineers have
turned the whole thing into such an ordeal:
i mean, what the hell can be so difficult
about maintaining an atomic pile to boil
water to 300 degrees fahrenheit and keep
the whole process under control, huh?

<disparaging remark deleted>

As has already been pointed out, this disaster arose because of poor site design rather than nuclear engineering.
Separately, a steam plant working at 300 degrees F will be quite inefficient, as basic thermodynamics limit the efficiency to the absolute temperature swing percentage, a little under 20% ( versus a bit over 30% for the Fukushima reactors) using 420 degrees Kelvin ( 300 * F) for the steam and 340 degrees Kelvin ( 150 * F) for the condenser. So for the same electric output, the reactor would need to be at least 50% bigger, which makes it correspondingly more challenging to keep cool in the event of a power failure. So a lower steam temperature may not help much in terms of risk reduction.

 

[HowlerMonkey]

Is it possible to build an efficient reactor that does not require large amounts of cooling when it's shut down?

I would guess the spacing required to fit enough "control rods" or "control media" to be inserted would impact efficiency badly.

 

[Joe Neubarth]

Is it possible to build an efficient reactor that does not require large amounts of cooling when it's shut down?

I would guess the spacing required to fit enough "control rods" or "control media" to be inserted would impact efficiency badly.

Nuclear Water Reactors are extremely wasteful of energy, but since there is so much of it waste conservation never was a real consideration. Reactor fuel rods will continue to give off heat for days after a scram. It is just the physics of the beast to do that.

IF you are talking about another means to generate electricity from radiation, most of them are experimental and are not being tried on a commercial basis that I know of.

 

[robinson]

Nuclear reactors are built and designed to make money, or power, not for safety. Like the site design of having 6 reactors right next to each other. It was cheaper, not safer, to design like that.

 

[HowlerMonkey]

Nuclear Water Reactors are extremely wasteful of energy, but since there is so much of it waste conservation never was a real consideration. Reactor fuel rods will continue to give off heat for days after a scram. It is just the physics of the beast to do that.

IF you are talking about another means to generate electricity from radiation, most of them are experimental and are not being tried on a commercial basis that I know of.

Then it's the design of the rods as well as the reactors that are responsible for heat production after a reactor is shut down since the rods themselves are producing heat the requires cooling even when not packed to the density level of a reactor.

I was implying in my question whether fuel rods and reactors are able to be designed such that they don't require cooling once they are shut down or whether the mechanism for a "non-cooling needed shut down" is even possible without making a reactor completely inefficient.

 

[rmattila]

Then it's the design of the rods as well as the reactors that are responsible for heat production after a reactor is shut down since the rods themselves are producing heat the requires cooling even when not packed to the density level of a reactor.

I was implying in my question whether fuel rods and reactors are able to be designed such that they don't require cooling once they are shut down or whether the mechanism for a "non-cooling needed shut down" is even possible without making a reactor completely inefficient.

It's not in the design of the reactor or the rods: it's in the physical process of nuclear fission, which splits a nucleus in two pieces, releasing energy as a result, but these pieces have always too many neutrons and are therefore radioactive (=produce heat). It is this radioactivity of the products of the very fission process that lead to the decay heat, and it can not be helped by any design considerations, as long as the fission reaction is used as the energy source.

 

[htf]

It is not a technical problem ...

The real problem is that NPPs are run by profit oriented organizations who try to make as much profit as possible out of their investment. Less safety means more profit and higher bonuses for the top management.

Fucushima is a typical example. The risks that eventually caused the disaster were all known for a long time and technical solutions were also known. It simply was an economical decision not to fix the deficiencies.

It was the same story with Chernobyl.

And I am convinced that economical reason will be the reason when the next NNP blows up. I don't know where this will happen but it will happen.

 

[Astronuc]

The real problem is that NPPs are run by profit oriented organizations who try to make as much profit as possible out of their investment. Less safety means more profit and higher bonuses for the top management.

Fucushima is a typical example. The risks that eventually caused the disaster were all known for a long time and technical solutions were also known. It simply was an economical decision not to fix the deficiencies.

It was the same story with Chernobyl.

And I am convinced that economical reason will be the reason when the next NNP blows up. I don't know where this will happen but it will happen.

Less safety may mean loss of profit.

In Tepco's case, they lost a multibillion dollar plant.

Chernobyl had nothing to do with profit. It was a incredibly bad experiment that took the plant outside the design basis. That was negligence.

 

[Astronuc]

Then it's the design of the rods as well as the reactors that are responsible for heat production after a reactor is shut down since the rods themselves are producing heat the requires cooling even when not packed to the density level of a reactor.

As rmattila indicated, the decay heat from fission products and transuranics is inherent in the fission process. There is no way to get around it.

I was implying in my question whether fuel rods and reactors are able to be designed such that they don't require cooling once they are shut down or whether the mechanism for a "non-cooling needed shut down" is even possible without making a reactor completely inefficient.

Fuel rods (cores of the nuclear reactors) produce heat which is their principal purpose. The fission process can be shutdown, which is one mandatory functional requirement.

The other main mandatory functional requirement is to maintain core coolability during operation and after shutdown. This is the basic requirement in which TEPCO failed.

Related to those other requirements is another mandatory requirement to protect the plant and its safety systems, which include the emergency core cooling system(s) (ECCS). That is another area in which TEPCO failed.

TEPCO and the government did not forsee the possibility of a 14 m tsunami. Had they recognized this, then they would have ensured protection against such a natural event. Somehow, they convinced themselves that protection against a smaller tsunami (5.7 m seawall) was sufficient. Obviously it wasn't - and the rest is history.

 

[jim hardy]

I suppose it's natural to lash out at the designers.

Anyone familiar with the "General Design Criteria" for nuclear plants recognizes the profoundly straight thinking and safety conciousness of the old timers (early 50's) who wrote them into US law.


""Criterion 2—Design bases for protection against natural phenomena. Structures, systems, and components important to safety shall be designed to withstand the effects of natural phenomena such as earthquakes, tornadoes, hurricanes, floods, tsunami, and seiches without loss of capability to perform their safety functions. The design bases for these structures, systems, and components shall reflect: (1) Appropriate consideration of the most severe of the natural phenomena that have been historically reported for the site and surrounding area, with sufficient margin for the limited accuracy, quantity, and period of time in which the historical data have been accumulated, (2) appropriate combinations of the effects of normal and accident conditions with the effects of the natural phenomena and (3) the importance of the safety functions to be performed."" emphasis mine - jh
http://www.nrc.gov/reading-rm/doc-collections/cfr/part050/part050-appa.html

Somebody somewhere wasn't quite diligent enough about that one. I'd hate to be him now.

old jim

 

[htf]

Dear Astronuc,

your answers are typical for an rational engineer. However top management thinks different.

Less safety may mean loss of profit. In Tepco's case, they lost a multibillion dollar plant.

... if an accident happens. But money spent on safety will impair the profit now. Top management is in charge for a few years at best. Do you think they care what happens afterwards? This thinking worked perfectly for generations of TEPCO CEOs. Bad luck for the present TEPCO CEO!

Chernobyl had nothing to do with profit. It was a incredibly bad experiment that took the plant outside the design basis. That was negligence.

Wrong. The reactor design was chosen for economical reasons. It was an advancement of an existing design - this saved time and costs. The known design faults were ignored and concealed - for economical reasons. The test was shifted into the night and hence carried by an inexperienced team because the reactor was needed for power production during the day - a decision made because of economical reasons. And so on ...

 

[Astronuc]

Dear Astronuc,

your answers are typical for an rational engineer. However top management thinks different.

... if an accident happens. But money spent on safety will impair the profit now. Top management is in charge for a few years at best. Do you think they care what happens afterwards? This thinking worked perfectly for generations of TEPCO CEOs. Bad luck for the present TEPCO CEO!

I work with many utility managers, and they are always concerned about safety as well as cost.

I know of two cases where plants managers were sacked because of safety issues at their plants. In some cases, some managers can face criminal prosecution for safety violations.

Wrong. The reactor design was chosen for economical reasons. It was an advancement of an existing design - this saved time and costs. The known design faults were ignored and concealed - for economical reasons. The test was shifted into the night and hence carried by an inexperienced team because the reactor was needed for power production during the day - a decision made because of economical reasons. And so on ...

Please provide the evidence to support this claim. I don't believe that there was concern about profit in the Soviet Union. They were certainly lax about safety and probably overconfident about their capabilities and technology. Note that they stopped construction or canceled additional RBMKs, as they had started building VVERs. The VVER-440 is not as safe as VVER-1000, and the EU and US have been encouraging the phase out of VVER-440s.

http://en.wikipedia.org/wiki/RBMK#Status

 

[Astronuc]

I suppose it's natural to lash out at the designers.

Anyone familiar with the "General Design Criteria" for nuclear plants recognizes the profoundly straight thinking and safety conciousness of the old timers (early 50's) who wrote them into US law.


""Criterion 2—Design bases for protection against natural phenomena. Structures, systems, and components important to safety shall be designed to withstand the effects of natural phenomena such as earthquakes, tornadoes, hurricanes, floods, tsunami, and seiches without loss of capability to perform their safety functions. The design bases for these structures, systems, and components shall reflect: (1) Appropriate consideration of the most severe of the natural phenomena that have been historically reported for the site and surrounding area, with sufficient margin for the limited accuracy, quantity, and period of time in which the historical data have been accumulated, (2) appropriate combinations of the effects of normal and accident conditions with the effects of the natural phenomena and (3) the importance of the safety functions to be performed."" emphasis mine - jh
http://www.nrc.gov/reading-rm/doc-collections/cfr/part050/part050-appa.html

Somebody somewhere wasn't quite diligent enough about that one. I'd hate to be him now.

old jim

The GDCs are what I had in mind.

EBASCO was a leader in the initial unit. Toshiba and Hitachi came in on the subsequent units.

I'd like to see their rationale behind the seismic and tsunami risk.

 

[Astronuc]

The detection of Kr-85 (if the results are correct) is interesting. As far as I am aware, Kr-85 is not produced in the long decay chains, so all of it has been around ever since the fissions stopped. If it is still found in the containment, my first impression is that either

(a) the containment has somehow been able to contain the noble gas Kr-85 ever since the fuel failures occurred in spite of the leaks and the suspected hydrogen explosion early on during the accident
(b) Kr-85 has been released to the containment atmosphere more recently, which means that some fuel rod claddings have lost their integrity only recently

Kr-85 has a long half-life (t_1/2 = 3916 d), but Kr-85m (IT) has a short half-life (t_1/2 = 4.48 hours).

http://www.nndc.bnl.gov/chart/reCenter.jsp?z=36&n=49 (Use Zoom 1)
http://www.chemicalelements.com/elements/kr.html (see isotopes)

I don't believe all of any of the cores melted, so it's possible some incipient failures ruptured. I do expect that a lot of the Zr-2 (and Zr-liner) reacted with steam, and much of it oxidized to powdery ZrO₂. The UO₂ would have oxidized to higher order oxides e.g., U₄O₉ and U₃O₈, and fission gas diffusivities are rather high compared to those of UO₂.

 

[Cire]

Nuclear reactors are built and designed to make money, or power, not for safety. Like the site design of having 6 reactors right next to each other. It was cheaper, not safer, to design like that.

Reactors are not built for safety? Really? Did you really just type that?

 

[Astronuc]

Nuclear reactors are built and designed to make money, or power, not for safety. Like the site design of having 6 reactors right next to each other. It was cheaper, not safer, to design like that.

This is quite false. Reactor are designed for mandatory safety considerations. All engineered technology must involve safety considerations. However, there are sometimes deficiencies.

 

[nikkkom]

Is it possible to build an efficient reactor that does not require large amounts of cooling when it's shut down?

"LFTR+online reprocessing" scheme comes to mind. In it, decay heat is seriously reduced.

 

[robinson]

Safety would mean isolating each reactor, so that if a catastrophic failure occurs, you don't risk losing all the reactors.

 

[Most Curious]

Virtually EVERY system, plant or single object is designed with profit versus safety considerations. Be it a nuke plant, a bomb, an airplane, your car or a toothbrush that balance will always exist. NPP are designed more toward safety than profit but NONE would ever be built if ONLY safety were considered. ALL ACTIVITY carries some risk and nukes are no exception. The trick is balancing ACCEPTABLE RISK against cost in such a manner that the plant CAN be built and operated. Why would it EVER be built if it could not even break even? Only governments do such foolish things as can be seen in ethanol as fuel or various "green" energy fiascos that can NEVER be economically viable and are directly subsidized because they are losers.

Every design by man has flaws. Either we learn from those flaws and improve - or else we cease to move forward at all. The BWR has gone through a series of improvements and will, hopefully, go through even more once the lessons of this accident are learned and understood. Standing still is not an option - we MUST improve, move forward. To stand still is to fall behind and FAIL. We are becoming far too risk averse which will stop ALL innovation and therefore progress. Stopping all progress does not serve humanity.

The strong anti-nuclear sentiment, often by those of questionable scientific or engineering background, has actually CAUSED increased risk in some ways. The defacto moratorium on new nukes in the USA has made it necessary to extend the operating lives of old, less advanced designs such as the early BWRs or Russia's AWFUL RBMK.

The Chernobyl accident had many facets - poor reactor design, poor operator training, a HORRIBLY flawed test, unbelievable violations of written safety criteria and POLITICS in the old USSR that covered up known flaws in design and operations. The only real surprise to me is they got away with it all for as long as they did! At least in US nuclear industry there is exchange of ideas, knowledge of other operator's findings and problems. In the old USSR such things were state secrets and they paid a terrible price for it.

Do not take the above as a defense of TEPCO or the many designers of Fukushima. I suspect design errors will become rather obvious as time goes by and hopefully result in improvements in existing plants and future plants. In hindsight, some of those errors may look rather foolish. I also hope all of the glaring management errors are corrected as well. Coverups are NEVER acceptable and should be harshly punished. I would hope, however, careful consideration is made of the facts, thorough NON POLITICAL investigations are made of every item involved in this event so REASONED action can be taken - neither cover-up and white wash nor kneejerk anti-nuclear hysteria. We must keep in mind too that this event was set off by a natural disaster of nearly unprecedented severity. Hind sight is always 20-20......

rant off

 

[etudiant]

Safety would mean isolating each reactor, so that if a catastrophic failure occurs, you don't risk losing all the reactors.

While it is true that a catastrophe such as Fukushima would have been much less had the site only had one reactor, the offset is that the likelihood of more than one site getting hit in a large earthquake would be greatly increased.
I'm not sure that the effect of two or more reactors in trouble at widely separated sites would be any less damaging than what we have now. Also, it is not implausible that widely dispersed stand alone sites will dilute the available technical and human resources, possibly making accidents more likely.
That said, it may be that reactor design needs changing. Perhaps a site should comprise 50 much smaller reactors, each of which can be easily cooled, ganged together to power the turbines. That would allow for walkaway type shutdown. The question is whether such an approach would perform reliably or just guarantee 50x as many small nuclear incidents.