The Depth in the “Defence in Depth”

 - Surendra Gadekar

Nuclear Power technology is inherently dangerous. Here is a process of boiling water that inevitably produces tonnes of poisons. Some of these poisons are extremely poisonous. Nanogramme quantities if inhaled or ingested can quite often prove fatal. To successfully manage to isolate thousands of kilogrammes of the stuff essentially for ever from the environment seems a superhuman endeavour. And sometimes, far too often for our survival as a race, it does prove to be so.

This is not to say that the people involved in the nuclear industry are careless or incompetent or corrupt, though sometimes, as is inevitable in any society, some are. But generally they do take great care and are very meticulous in following safety procedures, probably more so than in other hazardous industries. But nuclear technology is totally unforgiving. It does not allow for even a single error. And to err is human.

The story of the rise of Japan from the ashes of the great war to becoming the second  largest economy in the world is one of incredibly hard work and discipline. Even today when we see how patiently they wait in orderly queues to get scarce daily necessities from department stores, one can only admire their forbearance and discipline and a sense of common purpose. Can one imagine to see such scenes in a similar situation in India? The large number of people dying in stampedes with distressing regularity, or the way car owners will try to jump the queue and drive to be in the first position at every railway crossing, will deter any such hope.

In the process of rebuilding the nation, Japan was faced with a cruel dilemma. It is a country with a very high population density but few natural energy resources: no large coal or oil or gas deposits. Since 1868, when the Japanese elites decided to ape the West on path to 'Modernity' they have dreamed of energy independence. Many wars have been fought. But the dream remained elusive. The second 'World War' put a full stop to thoughts of achieving this through conquest. Hiroshima and Nagasaki just rubbed this in. At the end Japan lay broken and battered.

But ironically, in the very process of atomic destruction lay a gleam of achieving the elusive dream of energy security. Japan could create its own energy mines through plutonium – the man made metal from Hell. The conquest of the atom would make the “Co-prosperity Sphere” unnecessary. The Arab oil shocks of the seventies just reinforced this belief. Japan would go for the whole works.

Large number of light water reactors of both the pressurised water (PWR) and the boiling water (BWR) kind were built in profusion (54 by the latest count). Reprocessing the spent fuel and creating a vast stockpile of plutonium was the key to elusive energy security. However, many a proverbial slip between the cup and the lip. The reprocessing spent fuel plant at Rokasho suffered series of 'teething troubles': just one damn problem after the other. Many problems still have not been fully overcome. As a stop-gap measure, Japan  started sending spent fuel for reprocessing to France and getting back the recovered plutonium as Mixed Oxide (MOX) fuel despite all the potential risk of a maritime disaster. Many an earthquake were near misses especially the July 2007 Chūetsu offshore earthquake that forced a two year closure of Kashiwazaki-Kariwa Nuclear Power Plant reactors. Thus, despite the radiological suffering inflicted by Hiroshima and Nagasaki, despite innumerable protest by large number of citizens and activist groups; despite the many near misses
with potential catastrophes; Japan became the poster child of nuclear industry. Come hell or highwater the Japanese elite was determined to nuclearize Japan. Unfortunately this time they have got both hell and high-water together.

Nuclear proponents put forth the argument that a lot of industrial activity is full of hazards. There are many processes that produce poisons and toxins that need careful handling. Are we to forego the benefits of nuclear energy just because it is hazardous. However they forget that nuclear energy production is uniquely hazardous in one way. It doesn't shut down when it shuts down. In other industries, one can be fairly sure that if the hazardous plant has been shut down then barring an act of sabotage or terrorism, or a natural disaster, its poisons will remain contained and isolated from the environment. They will not on their own and spontaneously begin to cause havoc. In contrast a nuclear power plant needs active intervention in the form of continuous cooling to remain shut off. Any disruption in the cooling, for whatever reason, can cause temperatures to rise and subsequent damage to fuel elements and dispersal of deadly radioactive substances in the environment.

Fukushima Daichi

This is essentially what has happened at Fukushima Daichi. When the earthquake struck, just three of the six reactors were working. Reactors 4, 5, and 6 were in a shut-down state.  As the earthquake struck, the sensors signalled the reactor control rods to fall and shut down the nuclear chain reaction in the first three. Due to the earthquake the normal A.C. power was interrupted. But the diesel generators automatically took over and all was well. After about an hour, the tsunami struck. Anticipating the tsunami (after all tsunami is one of the very few Japanese words that have an international resonance), a six metre high wall had been constructed around the plant. This proved totally inadequate and the tsunami sliced through it like a knife through butter and knocked of not only the diesel generators that supplied the power for the reactor but also the diesel supply. But, even this had been anticipated: there was an eight hour battery supply that automatically took over and continued to supply electricity to the plant. However, this too proved inadequate and the station suffered a “station blackout”.

At this stage the reactor operators were confronted with two separate set of problems (amongst many others). One was to cool the reactor cores in reactors 1, 2, and 3 and the other was to provide cooling to the spent fuel pools of all the six reactors. Why was this cooling required if the nuclear chain reaction had successfully shut down? That is because, although the chain reaction has shut down the various radionuclide that it has spawned continue to produce heat (See Box 1) and in the absence of cooling, this would lead to a meltdown and dispersal of radioactivity to the surroundings.

The Boiling-water reactor (BWR) design has the spent fuel bundles tucked in a room just above though not directly above the reactor. While the reactor core is encased in a steel vessel inside the primary containment, (another defence in depth safety feature), the spent fuel is outside this containment. All that shields it from getting dispersed into the environment and becoming part of the food chain are the thick outer walls of the reactor building; the so called secondary containment. While the core cooling system has a lot of redundant safety (diesel generators and battery systems), the spent fuel cooling system does not have all these defence in depth features. All it has is lots of water. In normal operation, this water is kept below 250 C. The reasoning being that since spent fuel is not very hot to begin with, it would take many hours in the absence of cooling for all this water to boil off. This would give adequate time for remedial measures to be undertaken.

Two things need to be understood about spent fuel. First, there is a lot of it. (See Box 2) This is especially true of countries like Japan and India that don't consider spent fuel as nuclear waste and thus a headache but rather as a resource with such wonderful material as plutonium that can be extracted through reprocessing. So every time, uranium fuel is considered adequately 'burnt' it is taken out of the reactor and stored in the spent fuel pool. After years of cooling, it would get sent for reprocessing. However, reprocessing technology hasn't managed to reprocess fuel fast enough and so there are large amounts of spent fuel assemblies still patiently lazing in the pool awaiting their day in the reprocessing plant. As nuclear power proceeds on its way to energize the nation, more  and more of the stuff accumulates.

The second point is that spent fuel is potentially far more harmful to the environment than the material in the core. This is because all the short lived radio-isotopes have already decayed away and what is left are isotopes with long half-lives that will stay in the environment for centuries and millennia.

At Fukushima as the batteries gave up the ghost and there was no power to run the cooling systems, temperatures began to rise. As they rose, more and more water started boiling off. As long as the fuel assemblies were covered in water, all was well, but eventually the tops of the fuel assemblies became bare and their temperatures passed the take-off stage. Fuel assemblies are composed of pellet sized slightly enriched uranium fuel enclosed with a zirconium alloy cladding. Zirconium is used as cladding material since it has good thermal properties like any metal but more importantly it does not capture too many neutrons and thus helps the neutron economy inside a reactor. But it has a serious drawback. At high temperatures it starts reacting with water to produce hydrogen. Thus, in the interior of the core containment, there was a build up of hydrogen and the pressure inside the reactor vessel increased. At this stage the operators were faced with a 'yaksha prashna'. They had to cool the reactor somehow to keep the pressure down to prevent an explosion that would rip open the containment but did not have adequate quantities of fresh water to do so. In their desperation, they decided to use sea water. This decision must have taken some doing with a lot of back and forth between TEPCO corporate bosses in  Tokyo and the operators in Fukushima. After all using sea water meant the writing-off of the reactor since sea water is highly corrosive and with boric acid added to it (to capture neutrons and prevent any possibility of chain reaction again taking off) even more so. The TEPCO bosses must have wrung their hands and decided not to take the risk of Chernobyl repeat since this was after all a forty year old reactor. (Reactor # 1)

However, with high pressure inside the core vessel, and the pumps used to pump seawater into the vessel operating at low pressure, this water just wouldn't go in or at least not in adequate enough quantities. Periodically, workers opened valves to vent steam and gas from the reactor vessel to into the primary containment. This flow in turn increased the pressure inside the containment. When the pressure in the primary containment rose too high, workers vented the containment to the atmosphere. The vent piping passed through the reactor building, but discharged well outside of it, and should not have led to a hydrogen build-up inside the building. However, hydrogen did build-up inside the reactor building (the so-called secondary containment). Nobody knows for sure why this happened, but happen it did not only in unit-1 but also in unit-3. When containment pressure rose too high, operators vented the containment to the atmosphere. They very properly sought to minimize the amount of gas they vented from containment to the atmosphere to lessen the amount of radiation released. They did this by not having too frequent venting.

It is possible that by not having frequent venting, the containment pressures rose high enough to allow hydrogen and other gases to leak into the reactor building. If so, hydrogen could build up to an explosive mixture.

There were huge explosions that demolished the roofs of the reactor buildings at both unit 1 and later at unit 3. With an inevitability that resembled a Greek tragedy, unit 2 followed and there was a fire in the spent fuel pool of unit 4. At present, (22.3.'11) cooling has been restored to the pools at unit 5 and 6 and efforts are continuing to do the same at pools of reactor 1 and 2. The pools at units 3 and 4 are the cause of the greatest worry since the explosions have already damaged the buildings and in case there is any further fuel damage and radioactive gasses released, they will go directly into the environment.

The spent fuel in the pool at unit 4 is far hotter than the rest. This is because it was put in the pool in December 2010 during the refuelling outage at the plant. Therefore, a lot more water needs to be pumped into it to keep the bundles under water, than into others. The water not only evaporates due to the heat, but is also lost because there is probably a leak in the pool as has been reported. Unit 3 has special problems since it uses MOX fuel that has much greater proportion of plutonium and is thus a greater risk to human health if dispersed.

The restoration of power has led to a certain amelioration in the acute emergency nature of the calamity. It has certainly given the operators a little more time to think. But the emergency is not over and the situation can flare up again into a further disaster any time. Nuclear engineers say some of the most difficult and dangerous tasks are still ahead — and time is not necessarily on the side of the repair teams.

Some thoughts on “Defence in Depth.”

Defence in Depth is the mantra of nucleocrats all over the world. According to IAEA, this is a five levelled scheme to ensure that there are multiple barriers to confine radioactive materials. Should one level fail, the subsequent level comes into play. “The correct implementation of defence in depth ensures that a failure whether mechanical or human, at one level of defence and even a combination of failures at more than one level of defence, will not propagate to jeopardise defence in depth at subsequent levels.” Anyone who has had the misfortune of having to listen to a nucleocrat propounding on this would attest to the fact that this concept and its implementation is often sold to the public as the near impossibility of a reactor accident involving large radioactivity releases. Hence, Chernobyl is castigated as a near impossible combination of operator error and design faults. No doubt Fukushima will be characterized as a once in a millennium natural disaster that no human agency could have foreseen. However, just to remind ourselves, Japan is part of the Pacific “Ring of Fire” - a region of very high seismic activity. In fact on this ring there have already been three big earthquakes of intensity greater than 9 at Kamchatka (9.0,1952), Chile (9.5, 1960) and Alaska (9.2, 1964). In fact, the Alaska quake was followed by tsunami of height of 67 meters in Shoup Bay and 31.7 meters at Passage Canal.

Japanese are leagues ahead of most others as far as technological sophistication is concerned. To have nuclear authorities in countries like India claiming that their reactors are safer would be hilariously funny, if the potential consequences of such hubris were not so tragic. Hiding behind the severity of the earthquake also just would not do because if earthquake obsessed Japan cannot make plants “Where no act of God can be  permitted,” nobody can and all that remains to be done is to take concrete measures so that there is one less method to boil water.

While nuclear power is on its way out there are some steps that need to be thought through immediately. The least this disaster should do is to raise questions against the wisdom of locating many reactors at one site. What we had in this case was a natural disaster that overwhelmed all the depth in the defence and then became a hydra-headed technological monster just because so many reactors were located together. Due to economies of scale, nuclear plants all over the world have a number of units together. In Fukushima, with so many individual disasters combining as one, it made the logistics of response well nigh impossible.

While this accident was precipitated by an earthquake and tsunami, the direct cause seems to have been a loss of power supply and the inability to restore it quickly leading to a station blackout. There are many other types of initiating events that could cause such a situation, including war and terrorist attacks. Various regulatory agencies require plants to have the capability to cope with a station blackout for no more than four to eight hours. This needs to re-evaluated. The severity of problems encountered at Fukushima Daichi reactors has allowed the Japanese nuclear establishment to completely wash their hands of problems at other reactors, especially at Fukushima Daini, but also at Onigawa and Tokai plants. While these did escape without the serious consequences of the former, but at least at Fukushima Daini there was some problem that was characterized as level 3 on the INES scale of nuclear accidents. Complete silence on these other problems will not lead to a better understanding.

The INES scale of nuclear disasters has been shown to be a disaster in itself. It is another handle to confuse the public and the change in the characterization of the accident from a level 4 to a level (is it a 5 or is it a 6 or a ….) Again funny if not so tragic.

The storing and piling of spent fuel (high level nuclear waste) in pools at reactor sites requiring active cooling for safety has been shown to be the foolish idea that it is. Saying that eventually it would be reprocessed is not an acceptable solution. Humanity has been waiting since the dawn of the nuclear age for Godot to arrive and find some solution to the intractable problem of storing nuclear waste. Fukushima has shown that one is actually waiting for a disaster.

Surendra Gadekar
A well-known Indian anti-nuclear activist and physicist, Gadekar lives in the remote tribal village of Vedchhi near the Kakrapar atomic power plant in the western Indian state of Gujarat. There, with his wife, a physician, he runs a Gandhian school for young activists and monitors the Indian nuclear industry, conducting surveys of power plants, uranium mines, and nuclear-testing facilities to determine the effect on the public's health. In 1987, he founded Anumukti, a journal devoted to establishing a non-nuclear India.

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