Worse than Fukushima Daiichi?

Japan Strains to Fix a Reactor Damaged Before Quake

TSURUGA, Japan — Three hundred miles southwest of Fukushima, at a nuclear reactor perched on the slopes of this rustic peninsula, engineers are engaged in another precarious struggle.

Source

Please link the story. Do you have any comments on it or is this just an FYI?

reply to post by jadedANDcynical


Source please.

Well isn't this just great.
Japan can't catch a break. waiting for more info

Thanks.

ETA i think this is the source: Linky

i found your link for you

www.nytimes.com...

Originally posted by BobbyShaftoe
i found your link for you

www.nytimes.com...

edit on 1818/6/11 by BobbyShaftoe because: (no reason given)

And this from the link. Apparently something huge crashed into the reactor last August, 2010 and they've been trying ever since to remove it. So I'd say no... IT"S NOT WORSE THAN FUKUSHIMA. The title of this thread is misleading.

The Monju prototype fast-breeder reactor — a long-troubled national project — has been in a precarious state of shutdown since a 3.3-ton device crashed into the reactor’s inner vessel, cutting off access to the plutonium and uranium fuel rods at its core.

Engineers have tried repeatedly since the accident last August to recover the device, which appears to have gotten stuck. They will make another attempt as early as next week.

reply to post by jadedANDcynical


This is the plant in question
en.wikipedia.org...
It's on the west side of Japan, if this goes wrong we have Japan firmly sandwiched between 2 nuclear disasters.

This reactor had an accident before, but they tried to cover that up(sounds familiar doesn't it)

An accident in December 1995, in which a sodium leak caused a major fire, forced a shutdown. A subsequent scandal involving a cover-up of the scope of the accident delayed its restart until May 6, 2010, with renewed criticality reached on May 8, 2010.

I don't know if it's common practice to build a reactor on a 'slope' but that doesn't seem like a good idea


Let's keep an eye on this.

Even if the device can be removed, restarting the reactor will be risky, given its safety record and its use of highly toxic plutonium as fuel

There's that plutonium again

“Let’s say they make this fix, which is very complicated,” Mr. Ban said. “The rest of the reactor remains highly dangerous. And an accident at Monju would have catastrophic consequences beyond what we are seeing at Fukushima.”

Sounds like it has the potential to be worse that Fukushima

Critics have been fighting the project since its inception in the 1970s. “It’s Japan’s most dangerous reactor,” said Miwako Ogiso, secretary general of the Council of the People of Fukui Prefecture Against Nuclear Power. “It’s Japan’s most nonsensical reactor.”

This is just one bad design after another :shk:

Monju was reopened in May 2010, and just three months later, the 3.3-ton fuel relay device fell into the pressure vessel when a loose clutch gave way. In the two decades since the reactor started tests in 1991, the atomic energy agency has managed to generate electricity at the reactor only for one full hour.

It seems that the use of nuclear power was never properly thought out, all the things that can and do go wrong, and how to fix them afterwards.

We're wiping ourselves out as a species

Just how many reactor accidents have happened?

This one certainly sounds as if it is quite serious:

The plant, a $12 billion project, has a history of safety lapses. It was shuttered for 14 years after a devastating fire in 1995, one of Japan’s most serious nuclear accidents before this year’s crisis at the Fukushima Daiichi Nuclear Power Station.

How bad was that fire if it had to be closed for over a decade?

The plant is 60 miles from Kyoto, a city of 1.5 million people, and the fast-breeder design of the reactor makes it more prone to Chernobyl-type runaway reactions in the case of a severe accident, critics say.

Looks like I'm going to be researching Fast Breeder Reactor design next.

Monju was reopened in May 2010, and just three months later, the 3.3-ton fuel relay device fell into the pressure vessel when a loose clutch gave way. In the two decades since the reactor started tests in 1991, the atomic energy agency has managed to generate electricity at the reactor only for one full hour.

That has to be the most expensive hour of electricity production in history!

And what do we have here? How much more classic a case of NIMBY could there be?

The commitment to Monju is rooted in the way Japan has sold its nuclear program to local communities, experts say. In persuading towns and villages to provide land for nuclear power stations, Japan has promised that the spent nuclear fuel — which remains highly radioactive for years — will not be stored permanently on site, but used as fresh fuel for the nuclear fuel cycle.

This is an entrenched policy and part of the way business as usual is conducted. At Fukushima Daiichi we are witnessing just how dangerous and short-sighted that policy is and it seems as though it is also biting others in the backside as well.

Giving up on any part of the fuel cycle would mean the government would have to find communities willing to become the final resting ground for the spent fuel.

“Of course, no community would accept that, and suddenly Japan’s entire nuclear program would become unviable,” said Keiji Kobayashi, a retired fast-breeder reactor expert formerly at the Kyoto University Research Reactor Institute.

So we have a classic example of the national government cajoling the local governments into accepting something that they otherwise would not have considered due to underhanded tactics and less-than-complete disclosure of the risks of the project being promoted.

Speaking of risks:

But the technology comes with risks. Instead of water, which is used in commercial nuclear reactors, the prototype reactor uses 1,600 tons of liquid sodium, a hazardous material that reacts fiercely with water and air, to cool its fuel. The presence of an estimated 1.4 tons of highly toxic plutonium fuel at the reactor makes it more dangerous than light-water reactors, which use mainly uranium fuel, critics charge.

Liquid sodium (over 3 million pounds) and plutonium (2800 pounds) are illustrative of highly toxic substances used in this process. And all we are doing is boiling water...

The atomic energy agency hopes the extraction will be complete by the end of the month. The agency says it will conduct extensive safety checks, and bolster its earthquake and tsunami defenses, before the reactor is eventually restarted.

“The device will definitely come out this time,” said Toshikazu Takeda, director at the University of Fukui Research Institute of Nuclear Engineering, and head of a government panel that approved the latest repair plans. He said that engineers had recreated removal procedures at a lab and perfected their handling of the crane that will lift the device from the reactor vessel.

This all sounds oddly familiar.

Multiple attempts at fixing a problem none of the engineers ever foresaw; all of which that have yet to work or bring about an acceptable conclusion to the aforementioned issue.

Source for above quotes

Apparently our savvy is too inadequate to continue trying to use nuclear reactors.

Originally posted by Iamonlyhuman


snip

And this from the link. Apparently something huge crashed into the reactor last August, 2010 and they've been trying ever since to remove it. So I'd say no... IT"S NOT WORSE THAN FUKUSHIMA. The title of this thread is misleading.

snip

From the article:

“Let’s say they make this fix, which is very complicated,” Mr. Ban said. “The rest of the reactor remains highly dangerous. And an accident at Monju would have catastrophic consequences beyond what we are seeing at Fukushima.”

It is not currently worse (or even in the same league), but it does have the potential to be. More research into this particular plant is needed to be able to quantify that statement, so maybe the title is misleading but it is possible that it is entirely accurate...

reply to post by jadedANDcynical


Loved this:

Japan badly needs sources of energy. By closing the loop on its nuclear fuel cycle, Japan aims to reuse, recycle and produce fresh fuel for its 54 reactors.

“Monju is a vital national asset,” said Noritomo Narita, a spokesman here in Tsuruga for the reactor’s operator, the government-backed Japan Atomic Energy Agency. “In a country so poor in resources, such as Japan, the efficient use of nuclear fuel is our national policy, and our mission.”

Like... Did They learn ANYTHING!?! "Gee. Nuclear plants are trouble. And gee. Putting them on fault lines is an order of magnitude more trouble if there is a good sized earthquake. Heck. Let's ignore all that and push forward with this nuclear power effort!"

[sigh]

Looking into older news articles we find the following:

Japan restarted a costly fast-breeder nuclear reactor Thursday for the first time since it was shut down 14 years ago because of a major accident and cover-up.

Yet another cover-up? Why am I not surprised?

The experimental reactor Monju, which means wisdom, uses plutonium fuel instead of conventional uranium and produces radioactive substances that can be reused as fuel.

emphasis mine

From dictionary.com:

wisdom (ˈwɪzdəm) — n 1. the ability or result of an ability to think and act utilizing knowledge, experience, understanding, common sense, and insight 2. accumulated knowledge, erudition, or enlightenment 3. archaic a wise saying or wise sayings or teachings 4. obsolete soundness of mind

I don't think they know what that word means...

Monju's initial start-up in August 1995 lasted only four months. It was shut down on Dec. 8 of that year when more than a ton of volatile liquid sodium leaked from a secondary cooling system. No one was hurt and no radioactivity escaped, but Monju's operators came under fire for concealing videotape that showed extensive damage to the reactor.

Ok let's say that no radioactive materials were leaked and all we had to deal with was the liquid sodium, just how dangerous is that?

Sodium and NaK don't corrode steel to any significant degree and are compatible with many nuclear fuels, allowing for a wide choice of structural materials. They do however ignite spontaneously on contact with air and react violently with water, producing hydrogen gas. Neutron activation of sodium also causes these liquids to become intensely radioactive during operation, though the half-life is short hence their radioactivity doesn't pose an additional disposal concern.

source-wikipedia

Spontaneous ignition on contact with air? Violent reaction with water to produce hydrogen gas?

Wow

Let's see what else is in the article:

The U.S., Britain, France and Germany, which were former leaders in fast-breeder projects, have all abandoned their attempts because they are not safe, are not economically viable and pose nuclear proliferation risks, the civil group Citizens' Nuclear Information Center said recently.

So four other countries abandoned this type of reactor precisely because of possible dangers.

The nuclear energy industry in Japan has been plagued by safety violations, reactor malfunctions and accidents.

The Fukui region also was the scene of Japan's deadliest-ever nuclear-plant accident, when a corroded cooling pipe -- carrying boiling water and superheated steam -- burst at a plant in Mihama in August, 2004, killing five workers. No radiation was released in that accident.

Again, all I have to say is wow...

Source for all quotes except the Wiki excerpt

ETA: video of workers assessing damage



Thanks to Purplechive for finding this video!

reply to post by jadedANDcynical


Excellent posts. Thank you for taking the time to bring this to everyone's attention. The mounting evidence suggests that we all need to be paying attention to these nuclear emergencies.

Having followed the Fukushima Japan Nuclear Emergency Thread (anyone know the record for the longest thread in ATS history?) I appreciate more light being shone on these disasters. We are at a crossroads. Fukushima is way beyond the worst case scenario anyone has even fully comprehended. All current plans on how to handle the contaminated water have ceased because radiation levels they expected to accumulate in a MONTH, accumulated in JUST FIVE HOURS of operation of the latest plan. Operators at the plant are saying that within one week the tons of highly radiated water in the lower levels of this facility will overflow above ground into the plant grounds and Pacific Ocean. Minds around the world are scrambling, with inadequate analysis from the ground, on how to possibly stop these breathing Blobs of no-end-in-sight horrors.

Nuclear, as humanity knows it, is a failure. The safety hurdles with current technology are not reachable. Nuclear accidents can too easily become catastrophic and will contaminate our planet with deadly radiation for a minimum of more than 26,000 years already.

We need a new map on how to shut down all nuclear in the world now. Use of nuclear power plants must end now. They are far beyond our present abilities to control safely.

If we intend to have a tomorrow as a species to figure out nuclear's potential, we need to stop playing with the matches now because # is popping off down and up range. And, it's a fire we don't know how to put out. It's burning and it's spreading.

Thank you again for bringing this news here. Well done OP.

Looking back into the history of this plant reveals a few things of interest. Among them is this:

Japanese Suicide Linked To Nuclear Plant Leak
Reuters Published: January 14, 1996

The official in charge of investigating a possible cover-up of Japan's worst nuclear power accident committed suicide today by jumping from the roof of a Tokyo hotel, the police said.

A police spokesman identified the suicide as Shigeo Nishimura, 49, deputy general manager of the general affairs department of the Power Reactor and the Nuclear Fuel Development Corporation, the Government concern that runs the country's prototype fast-breeder reactor.

A cover-up at yet another nuclear power plant. And this one is not operated by Tepco.

Officials of the corporation said that Mr. Nishimura was not involved in the cover-up but was distressed by evidence he had unearthed.

I wonder what it was that was so unsettling that he felt the need to throw himself off of the hotel. I wonder maybe if he had help going over the side...

At the news conference, the officials acknowledged that a video of the accident had been heavily edited before it was given to news media to make the leak appear less serious.

I really am at a loss here. I spent a good amount of time on the mega thread researching past Tepco cover ups and now see that it is a much more widespread issue inside the entire nuclear industry throughout all of Japan.
Source for above quotes.

Elsewhere, I find this:

April 8 (Bloomberg) -- Japan's Atomic Energy Agency may not be able to restart the Monju reactor by October as planned after the government ordered all detectors be checked, the Yomiuri newspaper reported, without saying where it obtained the information.

The order yesterday came after five sodium detectors were found bent in pipes since March 26, the Yomiuri said. The detectors may have been out of position since Monju was built in February 1990, the report said. Inspecting all detectors may take a few months, it said.

Source-2008 article

So the leak may have been the result of shoddy initial construction.

Now, I'm a restaurant manager and when our building was complete, but before we opened (or even started hiring people) we did a walk through the the general contractor in which we went through a thorough punch-list of items. Everythig on the list had to be complete and a further list was made during the process in which things not originally listed were able to be addressed.

I can only imagine that a nuclear reactor (and an experimental one at that) would have an even more rigorous checklist to adhere to prior to any dangerous activity taking place.

The more I look the more incredulous I become at the deceitful practice and absolute total lack of regard for safety systems.

Looking at Fast Breeder Reactor (FBR) design and basic explanations I have found this .pdf (206KB) posted on the IAEA website. It has a few specifics about the Monju reactor and several others around the world.

The fission process is based on the fact that when a neutron is captured by the nucleus of an atom of fissile material, that atom splits or fissions The energy released as a result of this process is used in power reactors to produce steam, which can then be made to drive a turbine and generate electricity

The energy release mentioned in the quote above is the driving force used in reactors today to boil water.

Materials which become fissile upon absorbing neutrons are known as "primary fuel" materials or "fertile" materials. In the case of fast breeder reactors it is uranium-238 which is the most interesting fertile material, and it is converted into the fissile isotope plutonium-239 through neutron absorption. Natural uranium contains more than 99% uranium-238, while in depleted uranium, which accumulates at plants that enrich uranium for existing nuclear power stations, the proportion is nearly 100%

So the U238 captures a neutron becoming P239 in the process.

A liquid metal fast breeder reactor is so named because during conversion of the fertile material into fissile material use is made of high-energy ("fast") neutrons and the coolant employed is sodium, which remains in the liquid state ("liquid metal") at the prevailing high working temperatures.

In many respects fast breeder reactors are similar to the power reactors in operation at the present time. However, the core of a fast breeder has to be much more compact than that of a light-water reactor. Plutonium or more highly enriched uranium is used as fuel, the fuel elements are smaller in diameter, and they are clad with stainless steel instead of Zircaloy.

So there are some similarities to the Boiling Water Reactor (BWR) we are familiar with at Fukushima Daiichi, but there are are also key differences. The main one being that it is liquid sodium that is the heat transfer mechanism in the FBR whereas water is used in the BWR. The fuel assemblies still use the "drinking straw" with pellets inserted as the structure of the assembly, but a different material is used for the cladding.

So, why liquid sodium? Pressurized helium is mentioned as well as different liquid metals, the case for sodium being:

Sodium exhibits the best combination of required characteristics as compared with other possible coolants, namely excellent heat transfer properties, a low pumping power requirement, low system pressure requirements (one can use virtually atmospheric pressure), the ability to absorb considerable energy under emergency conditions (due to its operation well below the boiling point), a tendency to react with or dissolve (and thereby retain) many fission products that may be released into the coolant through fuel element failure, and finally, good neutronic properties.

There are, however, several issues with using this as a cooling/heat transfer mechanism, those are enumerated here:

Among sodium's unfavourable characteristics are its chemical reactivity with air and water, its activation under irradiation, its optical opacity and its slight neutron decelerating and absorption properties, but these disadvantages are considered in practice to be outweighed by the merits of sodium as a coolant

See below

The sodium in the primary circuit (i.e. in direct contact with the core) is not used in any of the fast breeder reactor designs to produce steam. Instead, use is made of an intermediate sodium circuit (secondary circuit), which makes it possible to avoid a release of radioactive sodium in the event of a steam generator failure This necessitates the use of intermediate heat exchangers as an interface between the primary and secondary sodium circuits. The use of a secondary sodium circuit isolates the primary circuit, and hence the sodium-filled reactor, from any contact with water.

Good so far, we have the primary sodium circuit segregated through a heat exchange mechanism so the irradiated sodium is not the material driving the heat into the water. We do have this one final qualifying statement to the paragraph above:

But this, of course, does not make it any easier to design steam generators which are able to keep the sodium and water effectively segregated.

Why is it that safety concerns are given as an afterthought?

It was a sodium leak and subsequent fire, I believe, that has been the cause for the plant being shut down for over a decade.

[atsimg]http://files.abovetopsecret.com/images/member/73b488f75237.jpg[/atsimg]
image source

More information on Monju specifically in future posts will be forthcoming.

As mentioned in my prior post, here is more on the Monju Liquid Metal Fast Breeder Reactor.

In this article dated 07 May 2010 we have some interesting information.

OSAKA — Monju, a nuclear reactor designed to generate more plutonium than it burns, resumed operation Thursday morning in Tsuruga, Fukui Prefecture, 14 years and five months after a sodium coolant leak and subsequent fire inside the plant shut it down.

Well known and established, let's see if there is anything else.

The Japan Atomic Energy Agency, which operates Monju, said the reactor was started up at 10:36 a.m. The reactor is expected to reach criticality — the point when a nuclear reaction becomes self-sustaining — by Saturday.

Now we have the beginnings of a timeline around which to base events.

"When operating the plant, it should be 'safety first,' and that should make the people of Fukui proud," Fukui Gov. Issei Nishikawa told reporters in the morning.

I think we can all agree with that. Safety should be of prime concern. Safety. Not profit.

Nishikawa gave his approval to restart Monju late last month after getting reassuring signals from the central government over his requests for assistance on local transportation projects — particularly in regard to an extension of the Hokuriku bullet-train line that is expected to link Tokyo with Kanazawa in Ishikawa Prefecture by spring 2015.

Here in America we call that Pork Barrel Spending.

Monju, which can generate up to 280 megawatts, is the prototype for a nationwide chain of commercial fast-breeder reactors that would burn a special mixture of uranium and plutonium oxides known as MOX. The fuel is designed to create more plutonium than consumed when burned, leading to a theoretically endless supply of nuclear fuel.

By the way, this plutonium can be fairly easily adapted for weapons use.

Monju's restart comes less than two weeks after safety concerns were raised again when a faulty coolant detector was discovered in the reactor's auxiliary building. The detector was quickly repaired and officials said no leak was detected.

It seems to me that if you close a plant for nearly a decade and a half, you would have had plenty of time to conduct and ungodly thorough inspection of each and every square millimeter of the system you had. Problem with, but what do I know?

"Monju has been shut down since December 1995. Over the ensuing 14 years, equipment and piping have aged. The Japan Atomic Energy Agency says there are no problems with Monju's equipment, but visual inspections were carried out on only a small fraction of the inside of Monju's extensive piping," said Hideyuki Ban, codirector of the Tokyo-based NGO Citizens' Nuclear Information Center.

Wait, what?

Only a small fraction?

I guess I don't know as much as nuclear engineers.

Since 1995, two seismic fault lines near Monju have been discovered, while the plant was built to earthquake standards established three decades ago.

In July 2007, a magnitude-6.8 earthquake struck the Niigata area, forcing the closure of the Kashiwazaki-Kariwa nuclear power plant. The magnitude exceeded predictions and forced the government to adapt new standards when evaluating seismic safety. In 1948 as well, an earthquake in Fukui, about 60 km from Tsuruga, struck with a magnitude of 7.1, killing nearly 3,700 people.

Hmm, this all sounds oddly familiar.

New faults discovered near a nuclear plant well after it had been constructed, in an area that had a history of previous earthquake activity. My teenagers are smarter than these people!

Yep, this plant is totally insane. They are using liquid sodium to cool the reactor... and guess what... if it comes in contact with air, it ignites... so if you have any leaking from this, the whole plant could burn. REAL BRIGHT...

Workers Remove Device From Damaged Japanese Reactor

A 3.3-ton device that bedeviled the troubled Monju prototype fast-breeder reactor for nearly a year was removed on Friday morning, Japan’s Atomic Energy Agency said.

The in-vessel transfer machine that crashed into the reactor’s inner vessel last August had cut off access to the plutonium and uranium fuel rods and left the reactor in an uncertain state. Engineers had tried several times to retrieve the device, which was apparently jammed inside the reactor.

On Thursday night, the operators of the plant, which is in Fukui Prefecture about 300 miles west of Tokyo, finally removed the device along with a sleeve. The recovery work took more than eight hours, ending at 4:55 a.m. on Friday.

Well this is good news at least.

Originally posted by Vitchilo
Yep, this plant is totally insane. They are using liquid sodium to cool the reactor... and guess what... if it comes in contact with air, it ignites... so if you have any leaking from this, the whole plant could burn. REAL BRIGHT...

Workers Remove Device From Damaged Japanese Reactor

A 3.3-ton device that bedeviled the troubled Monju prototype fast-breeder reactor for nearly a year was removed on Friday morning, Japan’s Atomic Energy Agency said.

The in-vessel transfer machine that crashed into the reactor’s inner vessel last August had cut off access to the plutonium and uranium fuel rods and left the reactor in an uncertain state. Engineers had tried several times to retrieve the device, which was apparently jammed inside the reactor.

On Thursday night, the operators of the plant, which is in Fukui Prefecture about 300 miles west of Tokyo, finally removed the device along with a sleeve. The recovery work took more than eight hours, ending at 4:55 a.m. on Friday.

Well this is good news at least.

Good news indeed that the equipment in question has managed to be retrieved.

The JAEA planned to resume power generation tests by next April, but an agency spokesman said that may be pushed back given the protracted crisis at Tokyo Electric Power Co's Fukushima Daiichi nuclear plant and a need to check the project's own safety standards.

I hope that safety inspections are actually conducted in a thorough and complete manner. Preferably by a third,-party that has no incentives provided by the nuclear power industry, but that is like too much to expect. We shall see.

Quote soure

As to the specific piece of equipment that has been referred to in this accident, I present the following pages from a .pdf (869KB in Japanese) indicating the problem faced.

If anyone can read Japanese, translations would be most gratefully welcome.

This is a simplified side elevation showing the reactor building.

Zoomed closer still, this is the reactor. The blue box in the bottom of the unit with the "X" through it is, I believe, the core.

The orange "goldfish-looking" in-vessel transfer machine is what got lodged within the sleeve.

This image has a closer view of the in-vessel transfer machine as well as indicating areas of interest within the reactor. I've got page 3 uploaded also, but am not including it in this post as it doesn't really add much more unless we are able to get it translated.

Here we have some detailed views of portions of the IVTM with specified areas of interest that seem to indicate wear marks, again a translation would clarify this.

This is the final image in the series and the last page of the report, it shows how the IVTM was lodged askew within the sleeve it was designed to traverse.

It looks like the left side of the device got caught on a burr or some other flaw in the side of the sleeve which resulted in the IVTM becoming canted off plumb.

reply to post by jadedANDcynical


Saw your post about suicide of nuclear industry member. You know why he jumped right? The uppers made him lie to the public( or at least keep secret) about the details of what is really happening over there. Probably being a man of conscience he just could no take it any more. I don't know.

I do know that after TMI was all said and done, Industry workers and spokespersons came forward and detailed how they had struggled with the official policy of non disclosure of information to the public about leaks and levels, etc. Many of them resigned and others would just blurt stuff during the press conferences during the emergency.

This contributed overall to conflicting reports that were forthcoming and the general mistrust of the industry and the government by the public at large. Can see some of this during the recent crisis in Japan, where reports of leaks and seriousness seem to bounce back and forth. Officials, workers and spokespersons are struggling with their own demons in trying to cope with the situation and at the same time keep the public safe.

Fissile nuclei are far more likely to absorb a neutron and be split if it is slowed. Something that slows the neutrons is called a moderator. In almost all civilian power reactors this is done by water. Steam is a poor moderator, thus in a normal reactor if reactor power rises, more water is turned to steam, less moderation occurs and the reaction will slow itself. Basically, if there's no water, there's no nuclear reaction but of course there's still decay heat that needs to be taken care of.

Fast reactors on the other hand have no moderator, the neutrons are fast, so the coolant is instead typically liquid metal. Since each atom is less likely to be split by a neutron, fast reactors compensate by having more fissile material in the same space. Since, as far as I know, liquid metal tends to absorb a small amount of neutrons, if it boils and turns to a gas then rather than the nuclear reaction slowing as in a conventional reactor, the reaction will get much faster.

This isn't a problem under normal operation because the reactor can instead rely on expansion of the core due to heat, and doppler broadening. But if for some reason in an accident the control rods are not inserted and the heat isn't taken away, then the sodium could begin to boil leading to something known as a Hypothetical Core Disruption Accident (HCDA), or basically an out of control reaction. If core geometry is compressed (extremely severe earthquake or something) then the same thing could occur. If the control rods are inserted but decay heat isn't taken away, then the fuel could melt and pool at the bottom of the reactor, away from the control rods, again the same thing could occur.

Fast reactors have a greater proportion of prompt neutrons which means the rate of reaction can change much more quickly.

It's not all bad news though. Sodium, unlike water, can be used at atmospheric pressure and also boils are very high temperatures. Since in most sodium fast reactor designs, the sodium in the reactor vessel does not leave the reactor vessel, the possibility of a loss of coolant accident has essentially been eliminated. Sodium is also very conductive and it has a fairly high heat capacity, so reactors can be cooled under emergency situations by natural convection of air (no electrical power required). They can be designed in such a way that the fuel melting will not cause a nuclear reaction or HCDA. They can also be designed so that any potential HCDA can be accommodated. I don't think Monju had this level of safety yet, but it is what Monju was designed to help develop and is what Japan is (or was?) aiming for.

Sodium can be difficult to deal with, especially since it's opaque, reactive and with a high melting temperature. I'm not 100% convinced sodium is safe or if it can be dealt with easily operationally yet, however water should never come close to the reactor (heat is transferred to the steam to spin turbines through an intermediate sodium loop, which is what leaked at Monju) so sodium-water reactions in the reactor should be impossible. Most (and I suspect all) sodium reactors are also surrounded by an inert cover-gas

GE has their own fast reactor design, PRISM, which the NRC partially reviewed, has a lot of these safety features. Quoted section is about sodium boiling, skim through the conclusions part to get a lot more information:

The positive sodium void worth is a concern in the passive safety argument. Because of it, one must qualify any characterization of the PRISM reactor response as ‘passively safe’ by pointing out that this is conditional on the sodium remaining below the boiling temperature. Should sodium boiling begin on a core-wide basis under failure-to-scram conditions, the reactor would be likely to experience a severe power excursion and a potential HCDA. GE states that the PRISM reactor vessel and its closure can safely accommodate the anticipated HCDA without loss of structural integrity, disengagement of the rotatable plug from the reactor closure, or expulsion of sodium. Due to the highly diverse reactor shutdown systems and the reactive feedback-based passive reactor runback mechanism, wide-scale sodium voiding is highly unlikely, though not impossible.

Due to the highly diverse reactor shutdown systems and thre reactive feedback-based passive reactor runback mechanism, wide-scale sodium voiding is highly unlikely,though not impossible. The loss of all EM pump flow without adequate EM pump coastdown has the potential to lead to sodium boiling and will require further study before the acceptability of the PRISM design can be determined. Mitigation of this event using the submitted design details for the GEMs system needs further study.

...

Because the EM pumps have no moving parts and, therefore, no stored kinetic energy, a synchronous coastdown machine is required for each pump to provide coastdown flow of the coolant upon loss of power.


NUREG-1368
www.osti.gov...

NUREG-1368 was a length review of a fast reactor design, PRISM, it also included a preliminary risk assessment which showed it would probably be much safer than existing reactors.

In the end, fast reactors can be made safe although they also have the potential to be very dangerous relative to existing reactors. Fast reactors are alluring because they have extremely high theoretical potential. Nuclear waste becomes fuel and fuel efficiency is increased by a factor of over 100 if combined with reprocessing (and so on).

Where Monju sides in this I don't know. It seems silly to place a reactor, especially a prototype fast reactor without many of the aforementioned safety features in an earthquake (and possible tsunami) zone, then again practically all of Japan is an earthquake zone yet that hasn't stopped people from living their. Plus Japan doesn't have a whole lot of resources or a lot of space to put renewable sources. Maybe they should focus more on passive (no power required for cooling) light water reactors (with better risk assessment) and as much renewable energy as they can.

(also without a lot of specialization, reactor grade plutonium is very difficult to make into weapons. Making bomb grade plutonium can be done if it's operated on a non-standard cycle but japan probably wouldn't do that).