Friday, March 18, 2011

Energy Industry Mechanical Engineer on Fukushima

Here's an expert's opinion on the nuclear situation in Japan. In his note at the end he talks a little about his background and credentials. I've been reading his stuff for years, which I get in a paid e-mail newsletter, but I found a permanent link to it online .

He feels the same way I do: the lives lost and the cost of cleanup of even the worst case imaginable (indeed, imaginary) nuclear accident are nothing in comparison to the lives lost and cost of cleanup of the real disaster that already happened.

Real Disaster, Fake Nuclear Panic
Japan's Nuclear Accident Pales Next to its Natural Disasters

by Jack Wakeland

The press coverage of the nuclear accident in Japan consists of panic over a vast radiological accident that isn't happening. The press has stampeded the people of Tokyo, who are emptying store shelves of emergency supplies, going about with masks over their mouths, or leaving the city to stay with friends to the south. Some foreigners are leaving the country. They've even got people buying potassium iodide tables on the West Coast of the United States (for which the Surgeon General should be fired). The weird juxtaposition of nuclear meltdown news with orders for the evacuation of 200,000 residents within 30 km of the plant and video footage of millions of displaced and dispossessed Japanese people shivering in the cold and patiently waiting for food suggests that the nuclear accident at Fukushima Daiichi has something to do with the nation-wide misery.
It doesn't.
The sequence of the partial meltdowns at Fukushima Daiichi Units 1-3 is straight out of the nuclear safety analysis textbook for what will happen to a nuclear power plant during an extended station blackout.
A 10-meter tsunami inundated the four-unit Fukushima No. 1 site, destroying connections to the national electrical grid, tripping the operating reactors (Units 1-3 were in full power operation), and damaging nearly all of the emergency diesel generators on the site beyond repair. After a reactor trip, heat must still be removed from the nuclear fuel in the reactor core. Four hours after a reactor trip, the heat of radioactive decay is 2% to 3% of the rated thermal power; four days afterwards, it falls to about 0.2% or 0.3%. The heat that normally boils water in the core to power a steam turbine cannot be turned off. It continues to boil water in the reactor at a reduced rate.
The problem, in a station blackout (the loss of all AC power to the nuclear power plant), is that the emergency core cooling pumps required to keep the fuel covered with coolant are available only for a limited period. After several hours of operation, steam to drive the pumps and batteries to power the control systems in the blacked-out Fukushima Daiichi units ran low. On at least two of the units that first day, the inventory of coolant in the reactor also ran low, partially uncovering the reactor core and allowing the fuel to overheat.
In addition to causing fuel in the reactor core to overheat, the loss of all AC power disabled all of the pumps needed to remove the decay heat vented from the reactor vessel to the suppression pool at the base of the containment. (For the parts of this kind of reactor and their relationships, see this diagram.) The suppression pool provides a thermal buffer for heat rejected during and after an emergency reactor shutdown. The heat is rejected from the reactor coolant via steam flow from reactor safety relief valves which are automatically or manually opened to relieve pressure built up from water boiling in the core. In addition to venting steam, the reactor safety relief valves at Fukushima Daiichi Units 1-3 also vented hydrogen gas which was generated from overheated fuel assemblies. The large quantities of hydrogen vented into the suppression pool at Fukushima plant were the product of oxidation of zirconium metal fuel cladding in contact with high temperature (1600 - 1900 F) steam after the reactor cores were uncovered several times.
The excessive boil-off of suppression pool water—there are about 500,000 gallons of it in a torus at the base of the containment vessel—led to the pressurization of the free-standing steel torus located inside a reinforced concrete structure to levels that were probably more than twice the 58 psig design rating.
Emergency procedures in the industry direct operators to perform emergency containment venting when containment pressure reaches 200% of design pressure, but if the ducting or piping downstream of the vent valve isn't designed for it, it will burst, releasing the contents of the containment atmosphere into the mechanical penetration rooms. In a boiling water reactor, the penetration rooms are located just below the refueling deck near the top level of the reactor building.
At Fukushima Daiichi Units 1-4, the Tokyo Electric Power Company joined many in the industry who decided not to harden the emergency containment vent line. So when operators opened the vent valve on Unit 1 at 210% of containment design pressure—pop!—the vent line burst, dumping vented containment atmosphere (steam, nitrogen, and hydrogen) into the top floor of the reactor building, just below the refueling deck. Low density hydrogen rises, and it made its way immediately to the top of the secondary containment structure over the refueling deck, where it found the oxygen it needed to burn. Concentrations of hydrogen above the refueling deck reached more than 8% in an air environment. The hydrogen mixture exploded, injuring four workers. The roof and steel structure of the secondary containment house over the refueling floor can be seen falling from a height of about 800 feet in the air after the blast.
At Unit 3, reactor operators began containment venting at less than 200% containment pressure (they did it at about 85 psig). But that pressure wasn't low enough, and the emergency vent line broke on Unit 3, just as it had on Unit 1, and an even larger explosion occurred above the refueling floor, which injured 11 workers. This explosion was probably larger simply because the Unit 3 reactor is larger, with 60% more fuel elements than Unit 1.
The powerful explosion of hydrogen vented from the Unit 3 containment may have caused the damage to pumps at neighboring Unit 2 which led to 140 continuous minutes of full core uncovery at Unit 2, one of the units that had, before Monday, escaped severe core damage. It may also have cracked or weakened structures in the neighboring Unit 2 and Unit 4 reactor buildings.
The hydrogen "explosion" that has been reported at Unit 2 may not have been any kind of explosion at all. It may have been the sound made by the suppression pool torus cracking after being pressurized to 102 psig (175% of the 58 psig containment design pressure), plus another 30 psig in water pressure from flooding the reactor core and containment vessel up to the top of the active fuel, an operation which is a standard long-term accident recovery procedure for the boiling water reactor.
Following recovery of reactor coolant level in Unit 2, work crews heard noises from the lower levels of the Unit 2 containment building and evacuated the unit. Unit 2 containment pressure immediately dropped to atmospheric level after the noises, indicating a rupture of the torus. This failure opens a path for the leakage of highly contaminated reactor coolant to the oceanfront ground water and to the sea. It also opens a continuous vent path for highly contaminated vapors.
The only good news in this kind of release is that it may end up being a relatively benign release mechanism. In the ocean, tides and currents will rapidly dilute the contaminated coolant.
The release of steam into the local atmosphere, steam containing radioactive iodine and small amounts of radio-cesium and strontium, will continue. Any radioactive xenon and krypton that could be released was released during containment venting. The good news is that the winds during most of these releases carried the vast majority of the airborne radioactive material, offshore, away from all humans (except the servicemen of the US Navy), where it has precipitated harmlessly into the vast Pacific Ocean—an ocean which, I assure you, already contains many hundreds of thousands of tons of highly diluted radioactive compounds.
The accident has now entered a slow-motion phase, in which skeleton crews battle to fill spent fuel pools.
Unit 4 was in refueling mode with a full core offload when the tsunami struck, so that the spent fuel pool, where the fuel assemblies are stored in 30 feet of water, had a relatively high heat load. The bad news is that spent fuel pool temperatures at Units 4, 5, and 6 have risen to 185 F, 145 F, and 140 F, respectively. The spent fuel pool liner and concrete structure are not designed for temperatures above 140 F and may develop leaks. According to a statement the chairman of the US Nuclear Regulatory Commission made to Congress, there may be little or no water left in the spent fuel pool for Unit 4, and he is undoubtedly correct. A day and a half ago, there was a hydrogen explosion on the refueling floor of Unit 4. The only source for hydrogen in the reactor building would be the product of zirconium alloy fuel cladding oxidizing in the air of the spent fuel deck because the fuel is partially or fully uncovered and the fresh offloaded fuel is slowly "baking off" the cladding at 1200 or 1400 F. This process also becomes a release mechanism for airborne radioactivity, which has previously made it impossible for emergency crews to approach the pool in order to run a fire hose into it.
The good news is that the site is becoming more accessible. The radiation field at the site boundary has dropped from 10 to 6 rad/hour on Tuesday, to 0.3 to 0.07 rad/hour last night, to 0.002 rad/hour this afternoon. This is very good news. Tokyo Electric and the Japanese government have, for the first time this afternoon, been able to expand the onsite workforce attending to the ongoing reactor cooling and fuel pool refill operations.
As for the radiation levels braved by the workers at the plant, they are indeed very dangerous, if a misstep leads to an accidental overexposure—but not if onsite workforce can stay within the exposure limits for emergency workers. This fact didn't stop London's Daily Mail from claiming that work on the site is "a suicide mission." One worker's boast that "we're not afraid to die" should actually have been: "we're not afraid to risk an additional 1.25% chance of death 20 or 30 years from now." But that would take the wind out of the sails of this hyperbolic story (one that has been repeated in various forms by several press organizations).
The chance of dying from cancer is increased by approximately 5% for every 100 rad of acute exposure and the Japanese health ministry just raised the limit on radiation exposures allowed for emergency crews working on the Fukushima Daiichi site from 10 to 25 rad. Therefore those workers are risking a 1.25% additional chance of death by cancer over the next 20 or 30 years. The other risks these workers are taking—from fire, explosion, falls from tall buildings damaged by hydrogen explosions—are far greater than the risk from radiation.
Overall, the consequences of the Fukushima meltdowns—including the $4 to $8 billion in corporate losses for Tokyo Electric due to the destruction of Units 1,2, and 3—fade into the background in the midst of a panorama of death and destruction along the shores of eastern Honshu, with 15,000 drowned and $100 to $250 billion in losses, about 4% of this year's GDP for Japan. There is a real disaster in Japan, a vast natural disaster caused by an earthquake and tsunami.
Back in the 1980s, a new branch fault was found near the Diablo Canyon nuclear power plant when construction was nearing completion. Fanatical environmentalists—who mounted riots, charged the fences, and vandalized the construction site—demanded that the Nuclear Regulatory Commission increase the magnitude of earthquake that had to be withstood in the design requirements, from 7.5 to 8.0. Pacific Gas and Electric responded that if there were an 8.0 earthquake in California, a nuclear reactor meltdown would be the least of anyone's worries. The NRC agreed and issued them an operating license for the two-unit plant based on its being able to withstand a magnitude 7.5 quake. Events in eastern Honshu are proving the wisdom of that decision.
The yelping from the anti-industrial left is growing to a crescendo, but in today's better political culture the results will not be severe. We can expect the NRC to take new regulatory action on the issue of the potential for vented hydrogen to explode in the secondary containment areas. We can expect them to require containment vent and purge piping to be re-built to withstand pressures of 150% or 200% or 250% of containment design pressure. We can expect them to require all licensed operating reactors to do a study on containment design margins, and we can expect them to look into forcing some types of modifications for plants in which some or all parts of the containment are free-standing steel vessels.
This will hurt the very strong economic performance of stations that are already operating, but it will not alter the profitability or economic practicality of the units because they've already amortized their gigantic construction costs. Units currently targeted by anti-nuclear activists to prevent license extensions will be under threat of losing their application battles for 20-year license extensions. These stations include Indian Point and Diablo Canyon.
Another place where the anti-industrial political forces may leverage the disaster in Japan (so that we get a small taste of that disaster here) is in the new construction projects for three new twin-unit plants that are going ahead in the US. I don't see how they can get traction against Summer 2 and 3 (in North Carolina) or Vogtle 3 and 4 (in Georgia), because these are AP1000 plants which require zero onsite AC power to safely shut down for a period of 72 hours. Furthermore, these four units are not being built as a purely economic proposition. Their owners are state-regulated utility companies who have already gained approval to recoup their investment from utility rates. Vogtle, in Georgia, is particularly invulnerable. In addition to the $7 or $8 billion in guaranteed federal loans each project is getting, one of the four owners of Vogtle, Georgia's state-owned electrical generating authority, got $2.3 billion in "Build America" bonds.
The South Texas Project 3 and 4 plant, however, is another question. The Fukushima incident has created political and contractual head winds for the primary owner, NRG, a for-profit merchant generating company.
The nuclear panic is going to have a long half-life, poisoning our political system for months or years. People will make repeated calls, with expressions of great and sober concern, for a "national energy policy" that is technologically invalid and will cost the energy industry tens or hundreds of billions of dollars in lost progress, development, and organization. And those of us with knowledge in the field have been sentenced to ten or twenty years of rolling our eyes and patiently explaining that the consequences of the Fukushima Daiichi meltdowns have been greatly exaggerated.
But first we have to endure weeks of surreal panic. The panic over Japan's nuclear disaster is absurd and contemptible. It is a panic that turns its back on a real disaster in preference for one that is imagined. The press is projecting onto nuclear power an imaginary man-made disaster which could never actually result in mass death—even total disintegration of the reactor at Chernobyl didn't kill as many people as a plane crash—while all around them thousands of bodies are being recovered from muddy heaps of garbage which only last week were the clean streets of the towns and cities of the industrialized world's most orderly people.
Author's Note: I'm a mechanical engineer doing analysis and systems engineering working in the nuclear power industry, and seven or eight of my 25 years' experience in the business have been at boiling water reactor plants of the same type as the damaged units at Fukushima Units 1 and 2. I am not the top industry expert you could reach on such matters, but I understand the different potential modes of failure for such a plant. The earthquake and tsunami on the east coast of Japan and the total failure of onsite AC power supply systems are the types events for which we all have done our best to prepare. I work for a prominent architect-engineering firm but will not use the name of my company because I do not represent them in any way in making these remarks. These are my personal observations.—JFW

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