[ RadSafe ] Fukushima: Now It's a Matter of Trust
Thompson, Dewey L
DThompson3 at ameren.com
Tue Mar 29 14:31:42 CDT 2011
A good read from SmartEnergyPortal.net, article by Ms. von Meier..
E DThompson3 at ameren.com
Text copied for those who don't want to link.
With the crisis unfolding at the Fukushima Daiichi nuclear plant, it is too soon to draw final conclusions. But watching the struggle to bring the site under control, we cannot help but wonder: What exactly is going on, and what lessons might be learned?
Nuclear experts around the world are still trying to piece together a coherent picture of events, from bits of information haltingly released. It seems clear that at least part of the fuel in each of three reactors has been uncovered for some amount of time, causing it to overheat and melt. A similar thing may have happened in one or more of the spent fuel pools. Melting nuclear fuel is bad for three reasons: First, it breaches the layer of cladding that holds the highly radioactive fission products inside the solid fuel rods, allowing them to be swept away. Second, at high temperatures the zirconium cladding reacts chemically to produce explosive hydrogen gas. Third, melting fuel can rearrange itself into a shape that brings enough fissile uranium atoms close enough together to allow pockets of a nuclear chain reaction to restart, producing vastly more heat and radiation. The first two events have already taken place. The third, known as re-criticality, is less likely, but some reports of elevated neutron readings at the plant raise the question if some of this may also have occurred.
The crux of the heat removal problem is the fact that it is not about a fixed inventory of heat to be removed by cooling. Rather, the inventory consists of a continuous rate of heat production from ongoing radioactive decay. This rate does decline over time, meaning that fewer gallons per hour are needed to transport the heat away. But the job is never finished: as soon as the flow of cooling water stops, the temperature rises again. Only after enough years, the heat flow can be dissipated by air alone, and the fuel can be taken out of submersion.
To absorb the heat faster than it is being produced, operators have been furiously forcing seawater into the reactors. Their efforts, especially during the first hours and days, undoubtedly saved countless lives. But the reactors and pools will have to have water pumped through them for years to come. The water, in turn, needs to give off the heat somewhere. There are two options: dump a huge volume of hot water somewhere (most likely, the ocean), or evaporate water to make steam, which then increases the pressure inside the reactor and needs to be vented (into the atmosphere). Both options have been used; how much, exactly, we don't know.
The best-case scenario going forward is that whatever water or steam comes out of the reactors can be effectively filtered to remove most of the contamination, before it is released into the environment. But this scenario presumes that the water circulation can be effectively re-established. Even with off-site electricity restored, this is not yet a certain outcome. Not only may there be damage to pumps and plumbing, but the evaporation of thousands of gallons of seawater must have left behind something every beachgoer knows about: a residue of salt - in this case, literally tons. Where this salt has accumulated and how it will affect the flow of water through the reactors, or the flow of heat across the fuel rods' surface into the water, we don't know.
The best-case scenario also assumes that the outflow of cooling water is well controlled. But we have also heard that at least one reactor has a crack in its steel pressure vessel; another may have a crack in the doughnut-shaped pool below it. Under the given conditions, these can't be fixed; years from now they might be entombed, but for now they will continue leaking contaminated water or steam. The main objective is to keep this water away from people - not to mention out of the workers' boots.
The worst-case scenario is that fuel in any one of the reactors or pools overheats, melts, and causes an explosion or fire that would disperse it around the site - badly enough to prevent workers from servicing the other reactors and pools, which might then have to be abandoned and left to the same inescapable fate. This danger will diminish substantially as automated cooling water circulation is brought back online, although it explains why experts are still on the edge of their seats.
Meanwhile, official reports from Japan have tried to emphasize the good news: water successfully sprayed onto pool; electric cable connected; radiation levels lower today. But there remains a disconcerting lack of coherent information - to the outside world, and worse, presumably, to people working on the site. The most vital resource for the operator of any complex technical system - whether a power plant, an electric transmission grid, or the air traffic around a large airport - is situational awareness. With lights off and instruments broken, the critical challenge is to maintain a big-picture view of events, starting with the essential question: what's the water level on the fuel? Alas, nobody can see.
What we do know is that the workers on the site are struggling with the job of a lifetime. The full reality of their working conditions will only become understood in time, but we can imagine their task is akin to landing an airplane with both engines failed, in zero visibility - or rather, landing several such aircraft side by side, on parallel runways. Even with casualties sustained, their courage deserves our sincere admiration.
Inexcusable, on the other hand, would be anything but the fullest disclosure of any and all information available, to these workers and to everyone in the fallout's path. The mind-boggling reports that workers were allowed to enter a flooded area without complete protective gear, leading them to sustain severe and potentially fatal radiation injuries, cast doubt on whether full disclosure is indeed the modus operandi, even on the site. Moreover, to scientists on the outside trying to calculate the "source term" - that is, the initial amounts of radioactive releases, linking the causative events at the plant to the measured radiation doses elsewhere - the numbers still don't all add up.
One early lesson is this: Safety depends on transparency. Whether it concerns workers trying to maintain an accurate mental picture of the hazards surrounding them, or citizens making informed choices about whether to evacuate or which foods to avoid, prompt and forthright communication is of the essence. Even if authorities themselves don't know all the facts, transparency means accurately reporting data as well as uncertainties. Only then can information be considered trustworthy, or indeed worthy of staking one's life upon.
Trust, ultimately, is also the problem when it comes to the implications of Fukushima for the future of nuclear energy - and this applies to safety as well as economics. For the industry and its proponents, the argument in favor of nuclear power has always been a rational, quantitative one, delivered in the language of probabilistic risk assessment and redundant safety mechanisms. But to those on the outside, the numbers mean nothing if they can't be trusted. Be it failure probabilities or cost estimates, one false report or suppressed incident is all it takes to confirm opponents' darkest suspicions about the establishment's lack of integrity. Statements about safety become irrelevant: "It's safe" contains no information value if we expect that message, regardless of actual circumstances. Likewise, "It's affordable" means nothing if we suspect that the conclusion was foregone, the spreadsheets skewed, liabilities swept under the rug. Without trust, optimism engenders not confidence but further doubt. And trust is not easily won but hard earned.
In the face of events in Fukushima, the nuclear industry has its work cut out. The highest priority is for Tokyo Electric to provide a fully transparent accounting of facts, most urgently for the safety of its workers and citizens, and secondarily for the global nuclear industry to draw lessons for improving the safety of its existing reactor fleet and spent fuel inventories. When it comes to building new reactors or extending operating licenses, trust will be the name of the game - except that, as we are all witnessing, this is not a game. Let us wish for the best possible outcome for the Japanese people, and honor their courage and fortitude by learning everything we can from their ordeal.
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