FROM THE TSUNAMI WRAPPED PLANT AREA FINAL PREPARATIONS ARE BEING MADE INSIDE THE FACILITY.

>> 12 YEARS AFTER THE HISTORIC MELTDOWNS REMAINS A TOXIC WASTE SITE, UPWARDS OF 5000 WORKERS ARE HERE EVERY DAY INNOVATING AND ORCHESTRATING THE MOST COMPLICATED NUCLEAR CLEANUP IN HISTORY.

>> IT'S INTERESTING TO SEE IT CHANGE QUITE A BIT.

>> MY GUIDE IS A NUCLEAR ENGINEER WHO LED THE CLEANUP AFTER THE THREE MILE ISLAND MELTDOWN IN 1979.

HE IS NOW A PAID CONSULTANT FOR FUKUSHIMA.

>> THIS IS VERY SIMILAR.

>> THIS IS A MULTIBILLION DOLLAR A YEAR EFFORT BEING SPENT AT FUKUSHIMA.

IT'S GOING TO TAKE SOME TIME TO COMPLETE THIS WORK.

>> WILL YOU LIVE TO SEE IT?

>> PROBABLY NOT.

>> WE BEGAN OUR TOUR AS CLOSE TO THE MELTDOWN AS HUMANS CAN DARE.

>> THE RUSTY FRAMES OF THE STRUCTURE, THE ORIGINAL STRUCTURE.

THEY HAVE THE SKILL STRUCTURE ALL THE WAY AROUND.

- - STEEL STRUCTURE ALL THE WAY AROUND TO BE ABLE TO DIG DOWN.

THE ULTIMATE GOAL MELTED STEEL AND URANIUM WITH ALL OF THE RADIOACTIVE ISOTOPES.

NOT TOO FAR FROM US.

YOU CAN'T GO IN THERE.

CATCHING EVEN A FLEETING GLIMPSE IS AN EXTRA ORDINARY CHALLENGE.

INTENSE RADIATION RENDERS THEM USELESS.

THE PATH IS A TREACHEROUS LABYRINTH.

DIFFERENT.

WE WERE INSIDE UNIT NUMBER FIVE, UNSCATHED BY THE DISASTER, THE REACTOR IS IN YOUR REPLICA OF THE THREE THAT MELTED DOWN.

>> WE ARE STANDING OUTSIDE THE PEDESTAL.

>> IT WAS LIKE BEING A MOUSE INSIDE OF AN ENGINE.

A THREE-DIMENSIONAL MAZE SUITED BY A CONTORTIONIST.

>> WE ARE UNDERNEATH THE UNIT.

THESE ARE THE CONTROL ROD MECHANISMS.

THAT'S WHERE THE REACTOR IS.

THIS LOCATION -- 20 FEET BELOW IS A CONCRETE FLOOR.

THIS IS WHERE THE PILES SIT, UNDERWATER.

40 OR 50 BEAT ABOVE US.

THIS IS ON THE FLOOR.

>> THE STEADY STREAM OF CONTAMINATED WATER HAS CREATED A PROBLEM.

AT FIRST, THEY WERE GENERATING BETWEEN 130 AND 160,000 GALLONS OF RADIOACTIVE WATER EVERY DAY.

IN 2016, THEY COMPLETED THE ICE WALL, A $300 MILLION SUBTERRANEAN PERIMETER OF PIPES COLD ENOUGH TO FREEZE THE STORY - - SOIL AND KEEP MUCH OF THE GROUNDWATER AT BAY.

IF YOU ASK ME, IT'S NOT EASY AND WATER WILL CONTINUE TO BE CONTAMINATED.

>> ALL ABOUT WATER SITS IN A TANK.

ALMOST 1100 TANKS.

>> HOW LONG WOULD IT TAKE BEFORE THEY ARE NOT RADIOACTIVE?

>> GOING TO BE MANY DECADES, BUT THAT IS NOT POSSIBLE.

>> YOU CAN KEEP TILTING TANKS?

>> THEY NEED TO MAKE ROOM TO SAFELY CONTAIN DEBRIS THAT WILL BE RADIOACTIVE.

>> YOU HAVE TO BUILD A VARIETY OF FACILITIES.

IT'S ESSENTIAL AT THIS STAGE WE ARE NOW.

>> BEFORE THAT, IT FLOWS THROUGH TREATMENT FACILITIES DESIGNED TO REMOVE 100 RADIOACTIVE ISOTOPES.

WE HAD TO ADD MORE PROTECTIVE CLOTHING, IF ONE OF THESE PIPES SPRUNG A LEAK, THE CONTAMINATION RISK WILL BE VERY HIGH.

>> THIS IS CALLED THE ADVANCED LIQUID PROCESSING PLANT.

>> FACILITIES CAN PROCESS ABOUT 300,000 GALLONS OF WATER EACH DAY.

>> IT'S A CHEMICAL PROCESS, NOT A NUCLEAR PROCESS.

>> I LIKE THAT ANALOGY.

IN CHEMISTRY TERMS, THAT'S WHAT IT IS.

HE OUT -- OTHER ISOTOPES.

>> HE SAYS IT REMOVED 99.99%.

TRACE LEVELS WITH ONE EXCEPTION.

AN ISOTOPE CALLED TRITIUM.

>> CHEMICALLY, IT'S WATER.

>> IT'S A MILDLY RADIOACTIVE FORM OF HYDROGEN THAT OCCURS NATURALLY, IT'S LUMINESCENT.

IT REACTS WITH OXYGEN JUST LIKE REGULAR HYDROGEN, CREATING WATER THAT'S RADIOACTIVE.

>> MUST BEING CONSIDERED.

THE JAPANESE GOVERNMENT IS PLANNING TO DO THE SAME.

FISHERMEN NEARBY AND OTHER COUNTRIES THROUGHOUT THE REGION.

MORE ON THAT WE CONTINUE OUR SERIES.

I AM MILES O'BRIEN.