Hydrogen, the Combustible: Not a gas to take lightly

Information is coming out now about the Fukushima Dai-ichi building blasts. Engineers and executives struggled about when, and how to vent the flammable mixture from the reactor buildings.

People might well wonder why Units 1, 3, and 4 all suffered big hydrogen explosions. How could hydrogen achieve the conditions necessary for explosion so easily? Many accidental, combusible mixtures of fuel gas and air in everyday life never catch fire or blow up.

It helps to know that hydrogen-air mixtures are remarkable in their ability to ignite. Here's a NASA hazard analysis for hydrogen as a gas and a cryogenic liquid.  

Flammability range: Its lower and upper combustibility limits, when mixed with air at standard pressure, are 4% and 75% by volume. Compared to other fuel gases like propane and methane, the lower limit isn't that remarkable, but the upper limit is. Among common fuel gases, only acetylene has a higher upper flammable limit. Hydrogen mixtures with air are explosive in the 18-59% range. 

Energy of ignition: The minimum ignition energy for hydrogen-air mixtures is only a tenth of that needed to start gasoline, methane and propane fires. A spark not visible to the human eye -- less than you feel after walking across a carpet on a winter's day and touching a doorknob -- is enough. 

Permeability through flaws in pipes and fittings: High.

Is it burning? It can be hard to tell. Often the human eye can't detect a hydrogen fire in daylight, if nothing else like plastic or grease is burning. Industrial firefighters use infrared flame detectors.

What's it add up to? The fact that hydrogen needs special attention doesn't mean it's "unsafe" compared to other fuel gases. It's got a safety edge, in fact, when it comes to hazards posed by leaks in open air. As readers of Hindenpunk literature know, hydrogen has the lowest density of all gases. It rises and therefore disperses rapidly into the open air when released from an outside source like a leaking pipe flange. That's safer than a gas that spreads along the surface, like chilly methane boiling off a liquefied natural gas tanker spill. 

The Big Muddy in flood: When loose ships can sink cities

The Coast Guard is off again, then on again, when it comes to allowing ship traffic through the lower Mississippi. The USCG reopened the river near Natchez to a trickle of the usual traffic volume yesterday: vessels must be spaced and stick to the middle.

Most reports explain the traffic slowdown and occasional shutdown as driven by the need to keep wakes down, because ripples can overtop the levees and start erosion. Right now the river level at New Orleans's Carrollton Gauge is 17.23 feet, four feet below the record and up a bit from yesterday.

Not mentioned as prominently in news reports, but equally worrisome, is the danger that a vessel will bash a hole through a levee. That would be particularly ironic to the New Orleans Hyatt, which is to reopen in October after being closed ever since Katrina. The water is so high that in some locations a ship's hull could override the bank and strike a levee. This can happen if a ship loses power or if a barge breaks loose from a tow following a collision or wire break.

During the New Orleans flood of 2005, a storm surge traveling up the Industrial Canal threw a barge against a levee, helping to flood the Lower Ninth Ward.  This from the Wall Street Journal, September 8, 2005:

"As storms approach New Orleans, owners of ships, tugboats and freight barges that populate the city's port and waterways attempt to secure their craft.... As the hurricane rolled into New Orleans, scores of boats broke free or sank. In the Industrial Canal, the gush of water broke a barge from its moorings. It isn't known whose barge it was. The huge steel hull became a water-borne missile."

Other allisions of interest:

In 1996 the outbound MV Bright Field lost power while laden with 56,000 tons of corn and drifted into the Riverwalk Marketplace, injuring dozens of mall-goers. Here tugs are taking it away from the scene:

An NTSB report identified a clogged oil filter, and unheeded engine alarms, as the main causes. (As word-lovers know, the verb for crashing into a stationary object is allide, not collide.) Here's a link to a short video clip of the crash. 

In 1998 and again in 2004, St. Louis saw runaway barges breaking loose and threatening other vessels along the riverbank. This academic paper looked at the lower Mississippi River and identified stretches most likely to have collisions, allisions, and groundings.

This recent report from the Times-Picayune described risks from tied-up barges and tugs: 

"Barges and tugboats are required to stay 180 feet from the levees, for fear that they will crash into the banks and cause damage. But after the Coast Guard prohibited ships from navigating the river at night because of the flooding, barges and tugboats began mooring themselves to the levees so they would not have to keep their engines running at night … .Inspectors on their night-time rounds are on the lookout for the ships. They found nine tied up on Tuesday night and seven on Monday, including some triple-wide barges that could have caused massive damage if they slammed into the levees … Some were close enough that inspectors standing on top of the levees could grab the side of the boat."

Helicopter, Hush Thyself

We've seen many articles the last two days about the stealthy helicopter allegedly used by the 160th SOAR in the Bin Laden raid, said to have been wrecked against a wall in a mishap called settling with power, and left behind as an intentionally demolished wreck with a mostly intact tailboom. Remarkable to me is the fact that the tail rotor blades are essentially undamaged ... normally in a tail-strike incident these are turned into small fragments. So the tail rotor must have been at a full stop before it encountered any hard object. 

Taking it at face value, two qualities are claimed in the Abbottabad stories: low radar cross section, and low noise.

How do such noisy machines hush themselves? I wrote about the original low-noise helo in this article for Air&Space. It was the 500P, the "Quiet One," a highly modified variant of the OH-6A Cayuse. It was financed by the CIA and built by the aircraft division of Hughes Tool. 

 

The 500P went into action to place a wiretap on an enemy telephone line near Vinh, North Vietnam, in December 1972. It flew out of a secret base called PS-44 near Pakse, Laos. The operation was successful, and I interviewed the participants.

The story begins in 1968, when Hughes Tool Aircraft Division sold two piston-powered Model 269 helicopters to an affluent Los Angeles suburb for police patrols. Citizens soon called to complain about the noise, and the city told Hughes to either make them quieter or take them back. An emerging market for police patrols was at stake. Engineers at Hughes identified the tail rotor as one of the biggest noisemakers. By doubling the number of blades to four, Hughes was able to cut the speed of the rotor in half, which reduced the helicopter's noise.

Coincidentally, the Advanced Research Projects Agency was hunting for contractors who could cut noise from military helicopters of all sizes. After hearing about Hughes' work on the police helicopters, ARPA offered the company $200,000 in 1968 to work similar magic on a Hughes OH-6A light helicopter. Hughes Tool made a short movie about the modifications, which included a new set of gears to slow the tail rotor, and showed it to ARPA.  ARPA approved money for an all-out quietizing effort, Phase II, and gave the project the code name Mainstreet. Even before work was fully under way, the CIA ordered two (later registered as N351X and N352X) for use in the field. Test flights began at Culver City, California, in 1971, followed by a brisk training program for the U.S. instructor-pilots who would later train mission pilots.

Flights of the Quiet One included low-level work at the secret Air Force base Area 51 in Nevada and touchdowns on peaks in California to familiarize pilots with close-quarters maneuvering and landing in darkness. Pilots needed at least eight hours to get comfortable with steering by sole reference to the comparatively narrow view of the forward-looking infrared (FLIR) camera, which was mounted just above the skids.

The slapping noise that some helicopters produce, which can be heard two miles away or more, is caused by "blade vortex interaction," in which the tip of each whirling rotor blade makes tiny tornadoes that are then struck by oncoming blades. The Quiet One's modifications included an extra main rotor blade, changes to the tips on the main blades, and engine adjustments that allowed the pilot to slow the main rotor speed, making the blades quieter. The helicopter also had extra fuel tanks in the rear passenger compartment, an alcohol-water injection system to boost the Allison engine's power output for short periods, an engine exhaust muffler, lead-vinyl pads to deaden skin noise, and even a baffle to block noise slipping out the air intake.

The extensive alterations were not to eliminate all noise -- an impossibility -- but to damp the frequencies of noise that people associate with a helicopter. 

How quiet was it? I'm told that because of the quieting gear, the helicopter couldn't be heard from the porch of the PS-44 main building unless it was flying overhead. Even then, at night, it sounded like a far-off airplane. The helicopter had its own hangar so Soviet spyplanes and satellites could not get a look at the peculiar profile produced by the extra main rotor blade, a tail rotor with blades in an odd scissored configuration, and big muffler on the rear fuselage. More technical information is available in this unclassified report. 

Did the remarkable 500P continue to fly by night? Officially, the Vinh wiretap was the first and only wartime mission of the original black helicopter. Its remarkable qualities were offset by the reduced flight performance.

Hunting for Bin Laden? Take a SOAR helicopter

News reports have it that Seal Team 6 and CIA operatives rode from Afghanistan to Abbottabad on helicopters of the Night Stalkers, aka 160^th Special Operations Regiment, or SOAR. 

Those reports make no mention of using the MH-6 Little Bird, which is interesting because old hands at SOAR consider the single-engine, egg-shaped helicopter well-suited to putting operatives on urban rooftops. The other two machines that SOAR uses, variants of the Black Hawk and Chinook, have their good points if a large team is needed, but are extremely loud and offer bigger targets for ground fire. 

While researching The God Machine I visited the 160^th SOAR headquarters at Fort Campbell, Kentucky, talked to pilots, trainers, and old hands, and hopped aboard an MH-60 for a night-time training flight. (Yes, it was a black helicopter ... or at least a very dark gray.) The following is adapted from my book.
 

The origins of SOAR can be found in the southwest corner of Fort Campbell. It's a tan, concrete-walled building and in 1981 housed the super-secret Task Force 160, the helicopter unit preparing for a second try at spiriting 53 American hostages from hostile Iran.

The reason for forming Task Force 160 was the humiliating failure of an earlier helicopter-dependent rescue attempt in 1980. It was code-named Operation Eagle Claw, and popularly known as the “Desert One” mission. The origin of Eagle Claw was a time of high tension between the United States and the new Islamic regime over the fate of Shah Reza Pahlevi, the deposed ruler. Less than two weeks after the United States allowed the shah to enter the country for surgery, armed revolutionaries broke into the U.S. embassy in Tehran and took dozens of prisoners.

A few Americans were released but that still left 53 hostages. President Jimmy Carter authorized an elaborate plan in April 1980 in which forces from all branches of the armed services would go deep into Iran to retrieve them. As part of this eight Navy RH-53 Sea Stallion helicopters, normally used for minesweeping, would depart from an aircraft carrier and fly 600 miles to a remote landing strip called Desert One. Forty-four aircraft would participate in six elaborate phases.

Things started going wrong early, when three of eight helicopters dropped out with real or suspected mechanical problems. That violated minimums and the rescue was scrubbed. Still the story was not over. While one helicopter was hovering in a dust cloud, its pilot struggling to maintain his visual fix on the ground, its rotor blades cut into a C-130 transport. The shrapnel ignited fuel and ammunition on both aircraft. At that point it was a scramble for survival. In the departure, the task force left eight bodies and much wreckage behind.

Eagle Claw generated many official recriminations and reports. it also led to a high-risk program, code-named Honey Badger, to go in one more time with more helicopters and commandos, but different tactics.

For Honey Badger the Army gathered new H-60 Black Hawks, Vietnam-era Loach OH-6 helicopters, and troops from the 101st Airborne. The task would be enormously difficult. The Iranians had dispersed the hostages across multiple locations and kept them moving.

The Loaches trained by night, and evaded notice during daylight hours by sliding into garage stalls in the back of the building each dawn.  “The Huey and OH-58 didn’t suit,” recalled Clif O’Brien, a retired command sergeant-major, and a participant in the preparations. “The MH-6 is easier to work on, rapidly deployable, and crash-worthy. The deployability is excellent. You can put six MH-6’s off a C-141 [transport], and in six or seven minutes you can have them running.” The troops of SOAR call the MH-6 and its armed cousin, the AH-6, the Little Bird.

Preparations for Honey Badger stood down after the release of all hostages in 1981, but the Army decided to retain a permanent, commando-style helicopter force. The existence of Task Force 160 (now the 160th Special Operations Aviation Regiment) remained secret until its cover was blown during Persian Gulf operations in 1987. By then Army helicopter pilots had come to regard it as the prime billet among combat units.

Night-time training accidents were the first obstacle. After four helicopters crashed during training in 1983, a regimen was set up to train pilots about navigating and approaching a target with the early model of night-vision goggles then available; these were originally intended for use by truck drivers and had a narrow field of view.

Task Force 160 did battle for the first time in the island nation of Grenada, supporting an American action to oppose a Marxist movement that was receiving Cuban support and to evacuate American citizens. Instead of its preferred night approach, however, the helicopters had to come in during the day. They took heavy anti-aircraft fire on approaching one target, a prison. One helicopter crashed, for the unit’s first combat fatality. Later the unit shot up Iranian gunboats and a minelayer in the Persian Gulf, used two of its MH-47 Chinooks to haul back a Soviet Mi-24 helicopter gunship abandoned in Chad, and fought troops loyal to Gen. Manuel Noriega in Panama.

The latter action led to SOAR’s second and third combat fatalities, when an armed Little Bird was shot down with a rocket-propelled grenade at the Colon harbor. The helicopter had been covering the exit of SEALs following a commando raid on “high value targets” at a beach house.

Little publicity arose from that, but the reclusive regiment became headline news on October 3, 1993, because of a battle in the narrow streets of Mogadishu, Somalia. That afternoon a U.S. special forces raid arrived via SOAR helicopters at a building across from the Olympic Hotel. The action initially captured two dozen of Mohammad Farah Aidid's assistants but went bad after a rocket-propelled grenade hit the tail of one orbiting Black Hawk, which spun out of control and crashed a few hundred yards away. A second Black Hawk caught an RPG round in the cockpit and crashed a mile further off. The battle to recover bodies and wounded men lasted well into dark, then resumed before dawn. In one of the most dramatic moments, an MH-6 "Little Bird" helicopter with the codename of Star 41 made a high-risk landing in a narrow street in the midst of the gun battle, gathered up Sergeants Daniel Busch and Jim Smith from the crashed Super 61 Black Hawk, and launched safely. The toll for SOAR aviators was five killed and one captured.

The copilot on Star 41, the Little Bird that touched down in the Mogadishu alleyway, was Chief Warrant Officer 3 Karl Maier. At the time I interviewed him, Maier’s job at Fort Campbell was operations officer for the training arm of the SOAR unit, known as the Green Platoon. In warrior style, Maier made no claim to heroism that day: “We were unarmed and afraid,” he said, noting that the combined action of all the gunfire from armed Little Birds overhead was so fearsome that the Somali fighters stayed back and made the rescue possible.

The process to prepare Army helicopter pilots for SOAR work takes three months, followed by two years of additional preparation for those who want the authority to plan and lead a mission. Trainees in Green Platoon stay very busy. After two weeks of individual combat training, pilots spend three weeks planning and flying low-level night missions to unmarked landing zones at least 60 miles from the base. All navigation must be done with map, clock, and compass; no other gadgets are permitted.

The MH-6 and AH-6 helicopters now used in battle by SOAR pilots are modified MD530F models. The MH-6 helicopter weighs 2,100 pounds empty; fully loaded and fueled it weighs more than twice as much. Four fully armed soldiers can ride on the outside of a Little Bird, seated on fold-down planks attached to the landing-skid struts.

“The guys in the MH-6 work close in, up to the front doors and to the top of the building,” said Maier. “It’s very good at urban warfare. Compare that to regular army aviation – their urban guys get you to the outside of town. They’re not dropping you off downtown.” The Little Bird is preferred for dropping off troops because it small and nimble, which makes it hard to hit from the ground. “They [the enemy fighters] don’t know where we’re going to land, and at night we’re all blacked out,” Maier said. “They’re shooting at the noise and that’s behind us.”

After graduating from the Green Platoon, pilots selected for the AH-6 gunship learn to use the trusty, 2.75-inch folding-fin rocket. “This is direct fire on a target, not standoff like the Apache [helicopter],” Maier said. “The good guys identify themselves and you shoot around ‘em.” At a distance of 200 yards Little Bird pilots can put a full load of rockets into an standard garage door.

By the time an AH-6 Little Bird pilot is ready to graduate from Fort Campbell he has fired a small mountain of rockets, and a truckload of machine-gun rounds. Most of all, he's had the benefit of learning from the world’s best assault-helicopter instructors.

Atlantis on STS-46: An alert crew saves the day

In honor of Endeavour's upcoming launch, here's a link to a fine time-lapse video of launch preparations for STS-131: raising the orbiter Discovery, mating it to the external tank and solid rockets, and rolling it out for launch.

While researching a piece on the redesigned solid rocket motor for Air&Space Magazine, I visited the Vertical Assembly Building during operations and had the chance to watch an overhead crane hoist a segment for stacking into a full solid rocket motor. Slow was the word.

Normally visitors aren't allowed into the VAB when the solid rocket booster sections are being moved. The reason is that solid fuel propellant acts like an explosive if dropped -- see this 1990 account of a fatality when a Titan 4 solid fuel booster hit the ground during a crane lift at Edwards AFB. Because my article was specifically about the booster, I got permission to watch the lift in the company of an escort. Movements of the booster segment were most amazingly slow ... much less than walking speed.

The Shuttle program has reached its 30th year and the last flight will be Atlantis, numbered STS-135, a cargo flight to the International Space Station. After that, it's off to the museums. 

The plans for Atlantis reminds me of one of that ship's earlier missions, STS-46. Within that mission, is a story of how an alert crew caught a potentially dangerous situation from causing damage. Disasters hardly ever come like a bolt from the blue: they develop from a combination of weak spots that join up over days, months, and even years.

The close call involved EURECA, short for European Retrievable Carrier satellite. It was like a truck for long-duration orbital experiments, to be retrieved by another shuttle later. Here's a photo of the Canada Arm lifting EURECA out of the payload bay.

So far, so good. The satellite took up a position 300 meters away while controllers at a command center in Germany ran through a series of checks. The crew waited several hours, then broke for dinner, except astronaut Andy Allen. He stood guard on the flight deck, the crew members all aware that having a powered satellite so close was a factor that needed watching.

It was a good thing that this guard was kept, as Atlantis passed into the night side of the Earth and the shuttle went into a communications blackout zone -- an area between relay stations.

Next, the shuttle's proximity radar set off a "range rate" alarm, indicating that EURECA was heading for the shuttle, later determined to be a closing speed of four to five feet per second. Unable to reach the satellite controllers in Germany, Allen fired the reaction control thrusters to get out of the way, though there was a risk the exhaust could damage the satellite's solar panels. The booms of these thrusters roused the other astronauts, who hustled up from dinner. "Where's the satellite?" they wanted to know. Allen couldn't say, other than it was so close the radar couldn't pick it up. The astronauts looked out the windows with flashlights.

The prompt action had avoided a collision. After conversation with Earth was possible, the crew learned that a glitch in Germany had fired the satellite's thrusters in such a way to send it hustling toward Atlantis.

Afterward, an astronaut took this wonderful photo of EURECA, now safely back in its station-keeping position, seemingly scooting among the clouds east of Cape Canaveral (the coastline visible at bottom).

This photo shows vividly how clouds come in many flavors, and at many altitudes.

 For more such photos, see JSC's image archive. Great stuff!

Fukushima Dai-ichi Unit 1: Risky piece inside a wicked problem

Tepco has released a set of flow charts and simple diagrams called the Roadmap, which lay out a broad summary of its plan to get control of Fukushima Dai-ichi sometime in 2011.

In tackling this wicked problem Tepco plans to fill the reactor pressure vessel (RPV) of Unit 1 with enough water to cover the fuel assembly fully. It's considered a priority because the core of Unit 1 is in the worst shape of all the Dai-ichi reactors, with more than two-thirds of its fuel rods damaged.

See this WSJ article on an engineering dispute about whether the damaged structure of Unit 1 is strong enough to take the full weight of water planned.

(Also, as I mentioned in this post, Unit 1 is unique because the pressure has steadily been rising in its RPV. It now registers 1.12 megapascals -- that's 162 pounds per square inch -- on Pressure Gauge B, which is five times the reading in mid-March and much higher than the pressure in the other damaged reactors. It doesn't mean Unit 1's RPV is in a dangerous state but it ranks as an anomaly.)
 
Since a major roadblock to human intervention is radioactive water in the basements and nothing much can be done until the source is cut off and water is pumped out, Tepco's hope is that repairs in the near future can stop this coolant from dumping into the basement; Tepco wants to keep the reactor coolant in a closed circuit, with heat transferred to the air rather than was water going into basements or the ocean.

Meanwhile, Tepco has been posting still and video shots on this website. The pace of added material has slowed since April 20 but there is good material, lately from cameras on board the T-Hawk ducted-fan drone and ground robots.

Putting Unit 1 in the limelight requires separating it from the others. First, here's the big picture, from the Wikipedia website on the Fukushima crisis. Before the damage they looked like this, from above:

From AsiaCorrespondent.com, an airplane view:

From Wikipedia, the post-explosion view. Unit 1 is the boxlike structure on the far right of the four shown in Reactor Row.

It can be hard to keep them straight in news clips -- I've seen captions that confuse Unit 4 with Unit 1 -- so it helps to remember that Unit 1 has a different appearance from the rest. From most angles,  the lower walls of Unit 1 appear to be mostly intact while the upper walls are missing, leaving only the structural steel showing.

By contrast, Unit 2 still has its walls, and Units 3 and 4 look mostly like skeletons.

Next is a T-Hawk drone-cam view of Unit 1's roof. Presumably the explosion wrecked the deck's trusswork and wall connections and it collapsed onto the upper levels of the massive concrete structure. The roof will be one of many complications in dealing with the spent fuel pool. I'm not sure what the big brown cylinder is on the right -- maybe a heat exchanger?

Here's a clearer view of Unit 1, with Unit 2 on the left:

Here's the control room of Unit 1. Something,  either the earthquake or a later blast, caused all the panels in the false ceiling to come loose. The control room is in a location other than the reactor building.

Fukushima's Sea-Salt Problem, Continued

In previous blogs I fretted about the use of seawater to cool Fukushima, because these are boiling water reactors. As hundreds of tons of seawater boiled away, a heavy load of minerals remained around the core and below. While most experts agree seawater injection via the feedwater lines was the right thing to do given the alternative (no cooling at all, and prompt meltdown), there is good reason to believe that salt buildup is blocking effective cooling of the fuel assembly in Unit 1 and perhaps the others. 

Unit 1 is what I've been following, because it's an old model with fewer protections, because the upper containment walls and roof blew off, and because the main trend of pressure readings in the reactor pressure vessel (RPV) has been steadily upward for three weeks, to nearly one megapascal. That's still well below the manufacturer's pressure rating, but does make one wonder what it means, and why the rising trend hasn't leveled off.

Late in March NRC staff considered the scant evidence, ran models, and advised the Japanese that salt deposits in Unit 1 most likely were jammed into the lowest part of the reactor pressure vessel (RPV) up to the bottom of the fuel assembly. That's a lot of salt.

There is some hope that the tonnage of salt could be less now, given that Tepco began injecting freshwater on March 25. And it makes sense that fresh water could dissolve salt and carry it out in solution.

Tepco could estimate the salt-removal rate by sampling the heated water for mineral content as it leaves the RPV. Are they? Who knows! Very little operational data is on line.

The pressure rise in Unit 1's RPV could be a good thing, or a bad thing. Maybe it hints that Tepco is more confident of its improvised plumbing now. But it would be worrisome if it means that a chunk of mineral is blocking water flow at an outlet and the only way Tepco has been able to maintain a critical flow rate (called the minimum debris retention injection rate, or MDRIR) has been to dial up the pressure. 

MDRIR sounds like just another metric in a long list but it means "enough emergency cooling water to keep the wrecked fuel -- the debris -- from melting through the reactor pressure vessel." So maintaining flow at or above the MDRIR does matter. The latest official guesstimate is that crumbled fuel has gathered at the bottom of Unit 1's RPV, but has not melted the steel wall and escaped into the primary containment. Pressure readings appear to support that conclusion.

Getting the salt out from the bottom of the RPV will depend on jet pumps and recirculation pumps that can force water to the bottom of the vessel.

The emergency cooling water has been entering the middle part of the vessel via the feedwater lines. Therefore the bottom section, where wrecked portions of fuel assembly have piled up, isn't being cooled well. ( But there's less decay heat now, so that's one bit of good news.)

Restoration of proper cooling will depend on successful completion of some very difficult tasks in and around the bottom of the semi-wrecked and highly radioactive building: replacing seals, repairing motors, calibrating instruments and transducers, checking for cracked pipes and hangers, and freeing up stuck valves. 

Notice how congested it is in the lower reaches, below compartments of reinforced concrete that prevent work by overhead cranes.

No workers can carry out such heavy-duty repairs until highly radioactive water is removed and surfaces are cleaned. So until flow is restored to the bottom part of the RPV through the recirculation circuit, Unit 1 is going to continue suffering from blocked arteries.