EMMA ALBERICI, PRESENTER: It's supposed to be the future of nuclear power generation, a reactor that produces its own fuel in a self-sustaining cycle. Known as Monju, the reactor on the country's west coast is held up as the saviour of a nation without energy resources. But Monju has been plagued with problems and many call it the most dangerous reactor in the world. In part two of his series on Japan's so-called nuclear alley, North Asia correspondent Mark Willacy was given an exclusive look inside Monju.

MARK WILLACY, REPORTER: People frolic in its shadow, a reactor its critics call the most dangerous in Japan. The name Monju comes from one of Buddha's chief attendants, a purveyor of enlightenment depicted resting on the back of a lion, a beast whose phenomenal powers are controlled only by Monju's wisdom.

But opponents of this prototype reactor fear its operators do not have the wisdom to harness its enormous energy.

KEIJI KOBAYASHI, FAST-BREEDER REACTOR EXPERT (voiceover translation): If a meltdown happens, it will get out of control very quickly. If the reactor core was to melt, the explosive energy would produce a blast like a nuclear bomb.

FUKIKO IKEJIMA, 'STOP MONJU' GROUP (voiceover translation): If a big accident were to happen, the impact would not stop in Japan, but spread around the world. It is our most dangerous reactor.

MARK WILLACY: And this is one of the reasons many Japanese fear Monju, because it uses sodium to cool a reactor, the substance that can ignite upon contact with oxygen. In 1995, a sodium leak at Monju caused a serious fire, one that resulted in the plant being out of operation for 15 years.

Lateline was given an exclusive tour of Monju, including an interview with the plant's director-general, Satoru Kondo.

SATORU KONDO, MONJU DIRECTOR-GENERAL (voiceover translation): In 1995, 700 kilograms of sodium leaked out. It started to burn when it was exposed to air. It put our plan to operate Monju 15 years behind.

MARK WILLACY: So why are the Japanese so determined to push ahead with Monju? The answer lies in the country's lack of energy resources and Monju's ability to generate its own fuel.

SATORU KONDO (voiceover translation): Japan is resource poor. Monju is what we call a fast-breeder, which can use uranium fuel to produce plutonium. With the plutonium, we can generate power for more than 1,000 years without relying on energy resources from overseas.

MARK WILLACY: But some experts like retired fast-breeder researcher Keiji Kobayashi believe the technology is too dangerous.

KEIJI KOBAYASHI (voiceover translation): Fast-breeders are impractical, they're unsafe, they're economically unviable and they produce high-quality plutonium, which is ideal for nuclear weapons.

MARK WILLACY: To its proponents, Monju represents the future of energy generation. To its critics, it could make Chernobyl look like a walk in the park. What is undeniable though is that Monju is pretty much a $12 billion flop because since it started testing almost two decades ago, it's generated just one hour of electricity.

SATORU KONDO (voiceover translation): Monju's purpose isn't to generate electricity. It's a prototype for research and development.

MARK WILLACY: This is the headquarters of the community group fighting the multi-billion-dollar Monju project, a cluttered office at the back of a second-hand shop. The founder of the Stop Monju movement, Fukiko Ikejima, has even produced a worst-case scenario simulation should a big quake strike.

This hellish simulation predicts a massive sodium leak, leading to a huge fire and explosion, spreading radiation across Japan.

That may be pure conjecture, but what isn't in dispute is that Monju has been plagued by avoidable accidents.

FUKIKO IKEJIMA (voiceover translation): In the most recent accident two years ago, an in-vessel transfer machine which weighs more than three tonnes fell into the reactor vessel and got stuck. It took them nearly a year to get it out.

MARK WILLACY: There's another reason some want Monju stopped. This year Japan's nuclear safety agency surveyed an active fault just 500 metres west of the Monju reactor.

MITSUHISA WATANABE, TECTONIC GEOMORPOLOGIST (voiceover translation): The problem with Monju is that there are many faults in the site directly underneath the reactors. There's a big chance those faults could move if a big fault nearby shifts.

MARK WILLACY: Even former supporters are now openly sceptical of Monju. In a recent interview with the ABC, former Japanese prime minister Naoto Khan questioned whether it was a case of throwing good money after bad.

NAOTO KAN, FORMER JAPANESE PRIME MINISTER (voiceover translation): I think the time has come to make a decision over Monju which has cost an immense amount over many years. And it seems it has no prospect of attaining its goals, despite constant extensions to the project.

SATORU KONDO (voiceover translation): So far $12 billion have been used for research and development at Monju. If we stop now, we're wasting 50 years of effort.

MARK WILLACY: For anti-nuclear activist and Buddhist monk Tetsuen Nakajima, Monju is supposed to be a figure of enlightenment and wisdom. He denounces this prototype reactor as an abomination, both to science and to his religion.

TETSUEN NAKAJIMA, BUDDHIST ABBOT (voiceover translation): As a Buddhist, it's unforgiveable to use such a holy name as Monju. Monju serves the Buddha. They should use a name that explains that this reactor is a super dangerous beast.

MARK WILLACY: After half a century of development, Monju's future could be determined within a month, and the volatile beast may be tamed for good.

Mark Willacy, Lateline.