Beese Davis Reactor to Restart in July After Repairs to Nozzles.

[NNadir]

FirstEnergy's Davis-Besse nuclear power plant will be ready to restart in July now modifications to reactor head control rod drive mechanism nozzles have been completed, but it will have to work at a lower core temperature and on a shorter operating cycle.

Signs of cracking were spotted around several of the control rod drive mechanism (CRDM) nozzles, during a scheduled refuelling and maintenance outage at the Ohio plant earlier this year. FirstEnergy Nuclear Operating Company (Fenoc) carried out extensive tests and has now announced that modifications have been carried out to 24 of the 69 nozzles, each of which penetrates the reactor vessel head, using approved methods including robotic welding.

The nozzles are made of material now known industry-wide to be susceptible to stress cracking, and Fenoc says its investigations have indicated that such cracking is accelerated by elevated reactor head temperatures. The company intends to address this by lowering the reactor core temperature by reconfiguring the fuel assemblies, and also shortening the next operating cycle. The nozzles will then undergo more intensive inspections and testing during the unit's planned 2012 outage. Finally, a new reactor head with nozzles made of a different alloy that is less susceptible to cracking is due to be installed in 2014.

http://www.world-nuclear-news.org/RS-US_nuclear_plant_good_to_go_after_repairs-0706107.html">US nuclear plant good to go after repairs.

It seems the reactor was repaired much more quickly than the dangerous fossil fuel industry, which funds the anti-nuke industry, has been able to repair the leaky well, and infinitely faster than they will be able to repair the Gulf of Mexico.

Have a nice evening.

[kristopher]

1. Move along, nothing to see here

The reactor core at the Davis-Besse nuclear plant sits within a metal pot designed to withstand pressures up to 2,500 pounds per square inch. The pot -- called the reactor vessel -- has carbon steel walls nearly six inches thick to provide the necessary strength. Because the water cooling the reactor contains boric acid that is highly corrosive to carbon steel, the entire inner surface of the reactor vessel is covered with 3/16-inch thick stainless steel.

But water routinely leaked onto the reactor vessel's outer surface. Because the outer surface lacked a protective stainless steel coating, boric acid ate its way through the carbon steel wall until it reached the backside of the inner liner. High pressure inside the reactor vessel pushed the stainless steel outward into the cavity formed by the boric acid. The stainless steel bent but did not break. Cooling water remained inside the reactor vessel not because of thick carbon steel but due to a thin layer of stainless steel. The plant's owner ignored numerous warning signs spanning many years to create the reactor with a hole in its head.

Workers repairing one of five cracked control rod drive mechanism (CRDM) nozzles at Davis-Besse discovered extensive damage to the reactor vessel head. The reactor vessel head is the dome-shaped upper portion of the carbon steel vessel housing the reactor core. It can be removed when the plant is shut down to allow spent nuclear fuel to be replaced with fresh fuel. The CRDM nozzles connect motors mounted on a platform above the reactor vessel head to control rods within the reactor vessel. Operators withdraw control rods from the reactor core to startup the plant and insert them to shut down the reactor.

The workers found a large hole in the reactor vessel head next to CRDM nozzle #3. The hole was about six inches deep, five inches long, and seven inches wide. The hole extended to within 1-1/2 inches of the adjacent CRDM nozzle #11. The stainless steel liner welded to the inner surface of the reactor vessel head for protection against boric acid was at the bottom of the hole. This liner was approximately 3/16-inch thick and had bulged outward about 1/8-inch due to the high pressure (over one ton per square inch) inside the reactor vessel.

What could have happened?

A loss-of-coolant accident (LOCA) occurs if the stainless steel liner fails or CRDM nozzle #3 is ejected. The water cooling the reactor core quickly empties through the hole into the containment building. The containment building is made of reinforced concrete designed to withstand the pressure surge from the flow through the break.

To compensate for the reactor water exiting through the hole, water inside the pressurizer (PZR) and the cold leg accumulators flows into the reactor vessel. This initial makeup is supplemented by water from the Refueling Water Storage Tank (RWST) delivered to the reactor vessel by the high, intermediate, and low pressure injection pumps. The makeup water re-fills the reactor vessel and overflows out the hole in the reactor vessel head. Approximately 30 to 45 minutes later, the RWST empties. Operators close valves between the pumps and the RWST and open valves between the low pressure injection (RHR) pumps and the containment sump. Water pouring from the broken reactor vessel head drains to the containment sump where the RHR pumps recycle it to the reactor vessel. A cooling water system supplies water to the RHR heat exchanger shown to the left of the RHR pump to remove heat generated by the reactor core.
On paper, that's how the safety systems would have functioned to protect the public. But the following examples suggest that things might not have gone by the book:

-The Three Mile Island nuclear plant experienced a loss of coolant accident in March 1979. Emergency
pumps automatically started to replace the water flowing out the leak. Operators turned off the pumps
because instruments falsely indicated too much water in the reactor vessel. Within two hours, the reactor
core overheated and melted, triggering the evacuation of nearly 150,000 people.

-At the Callaway nuclear plant in 2001, workers encountered problems while testing one of the emergency
pumps. Investigation revealed that a foam-like bladder inside the RWST was flaking apart. Water carried
chunks of debris to the pump where it blocked flow. The debris would have disabled all the emergency
pumps during an accident.

-At the Haddam Neck nuclear plant in 1996, the NRC discovered the piping carrying water from the RWST
to the reactor vessel was too small. It was long enough but it was not wide enough to carry enough water
during an accident to re-fill the reactor vessel in time to prevent meltdown. The plant operated for nearly 30
years with this undetected vulnerability.

-At several US and foreign nuclear power plants, including the Limerick nuclear plant 8 years ago, the force
of water/steam entering the containment building during a loss of coolant accident has blown insulation off
piping and equipment. The water carried that insulation and other debris into the containment sump. The
debris clogged the piping going to the emergency pumps much like hair clogs a bathtub drain. According to
a recent government report, 46 percent of US nuclear plants are very likely to experience blockage in the
containment sumps in event of a hole the size found at Davis-Besse opens up. For slightly larger holes, the
chances of failure increase to 82 percent.<1>

Thus, events at Davis-Besse may have gone by the book had the stainless steel failed it would have become the subject of many books on the worst loss of coolant accident in US history...
UCS -- Aging Nuclear Plants -- Davis-Besse: The Reactor with a Hole in its Head

http://www.ucsusa.org/assets/documents/nuclear_power/acfnx8tzc.pdf







Scapegoating of Davis Besse by NRC
http://www.ucsusa.org/news/press_release/federal-agency-scapegoating-0141.html

Retrospective
http://www.ucsusa.org/nuclear_power/nuclear_power_risk/safety/davis-besse-retrospective.html

[Dogmudgeon]

2. That's way more ironic than you probably intended

--d!

[Name removed]

3. Deleted message

Message removed by moderator. Click here to review the message board rules.

[Greyskye]

4. Nice spin from the 'world nuclear news'. NYT headline:"Old Nuclear Problem Creeps Back"

"An Old Nuclear Problem Creeps Back"

The American nuclear industry, primed to begin new construction projects for the first time in 30 years, is about as eager for an operating problem at an old reactor as the oil industry was for a well blowout on the eve of opening the Atlantic coast to oil drilling.

Nonetheless, a nuclear reactor where a hidden leak caused near-catastrophic corrosion in 2002 has experienced a second bout of the same problem.

In 2002, the plant, Davis-Besse, in Oak Harbor, Ohio, developed leaks in parts on the vessel head, allowing cooling water from inside the vessel, at 2,200 pounds per square inch of pressure, to leak out.

(snip)

The plant was shut for 14 months. First Energy Nuclear Operating Company, which owns it, eventually brought in a replacement head of similar design from a reactor in Midland, Mich., that had been abandoned during construction.

The company assumed it had solved the problem. But recently the new vessel head showed the same leakage pattern. Once again, the parts prone to leaking are nozzles through which the control rods for the reactor pass. When the rods are inserted, they choke off the flow of neutrons that sustains the reaction; when they are withdrawn, the reactor starts up. But the nozzles are prone to a problem called “stress corrosion cracking,’’ leading to the leaks.

It is not clear why Davis-Besse’s problem is more serious than other plants have had, although it surfaced in 2002 that First Energy had won approval to delay inspections that the Nuclear Regulatory Commission wanted. (When the problem became clear, those approvals set off a crisis of confidence for the Nuclear Regulatory Commission.)

Another problem may be the metal used in the original nozzles — the same metal used in the nozzles on the Midland reactor. While the vessel head from Midland “didn’t have any hours on it,’’ said Todd Schneider, a spokesman for Davis-Besse, it is of an older design.

(snip)

So by 'good to go', they mean the following.
1) Run at "2 to 3 degrees" lower operating temperature.
2) Instead of refueling and inspecting every 24 months, they will refuel and inspect every 24 months minus 100 days. Cutting off 100 days out of a 2 year cycle. Hey, maybe the vessel head will have more than 1/4" of steel remaining after 24 months minus 100 days, than it did after 24 months.
3) Oh, and they are still using the original nozzle design and material. New nozzles won't be ready to install until 2014.

Not defending the gulf catastrophe by any stretch of the imagination. Just thought it appropriate to put the nuclear industry's spin in context.

I await in anticipation being told how ignorant I am for pointing this out. :) Of course, that would draw more attention to the dis-ingenuousness of the OP, so... :shrug: