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Discussion Starter · #1 ·
US tornadoes force shutdown of two nuclear reactors in Virginia

http://www.guardian.co.uk/world/2011/apr/18/us-tornadoes-shutdown-nuclear-reactors

"...A US nuclear power company has disclosed that one of the tornadoes that hit the US at the weekend, killing at least 45 people and causing widespread damage, forced the shutdown of two of its reactors.

The series of tornadoes that began in Oklahoma late last week barrelled across the country, with North Carolina, where 22 people died, the worst-hit state.

The US nuclear safety regulator said on Mondayit was monitoring the Surry nuclear power plant in Virginia. Dominion Virginia Power said the two reactors shut down automatically when a tornado cut off power to the plant. A backup diesel generator kicked in to cool the fuel. The regulator said no radiation was released and staff were working to restore electricity to the plant..."
 

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US tornadoes force shutdown of two nuclear reactors in VirginiaThe US nuclear safety regulator said on Mondayit was monitoring the Surry nuclear power plant in Virginia. Dominion Virginia Power said the two reactors shut down automatically when a tornado cut off power to the plant. A backup diesel generator kicked in to cool the fuel. The regulator said no radiation was released and staff were working to restore electricity to the plant..."
So what happens when the backup generator is damaged or malfuctions? Oh my!
 

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Discussion Starter · #3 ·
It's curious why these power plants aren't simultaneously self-powered, as well as fed by the outside for redundancy. A power plant that loses power, but needs power to properly shut down, is more than one paradox -- and another Fukushima waiting to happen.
 

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Interesting. I wonder if there is a backup to the backup, to the backup??
And, if this is a power plant.. why do they rely on power from an external source, even if it's 100 yards away? Wouldn't it have protected, internally produced power to sustain itself?
I would have thought they would be self contained and sustained.
Just curious.
 

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They do have protected, internally produced power... its called diesel generators for when the reactors shutdown.

Here's a little easy reading... it's section 3.16 of Surry-1's technical specification, taken from:

http://pbadupws.nrc.gov/docs/ML0529/ML052910358.pdf

3.16 EMERGENCY POWER SYSTEM
Applicability
Applies to the availability of electrical power for safe operation of the station during an
emergency.
Obiective
To define those conditions of electrical power availability necessary to shutdown the
reactor safely, and provide for the continuing availability of Engineered Safeguards when
normal power is not available.
Specification
A. A reactor shall not be made critical nor shall a unit be operated such that the reactor
coolant system pressure and temperature exceed 450 psig and 350'F, respectively,
without:
1. Two diesel generators (the unit diesel generator and the shared backup diesel
generator) OPERABLE with each generator's day tank having at least 290 gallons
of fuel and with a minimum on-site supply of 35,000 gal of fuel available.
2. Two 4,1 60V emergency buses energized.
3. Four 480V emergency buses energized.
Amendment Nos.yl.q and 220
- -
TS 3.16-2
4. Two physically independent circuits from the offsite transmission network to
energize the 4,160V and 480V emergency buses. One of these sources must be
immediately available (i.e. primary source) and the other must be capable of being
made available within 8 hours (i.e. dependable alternate source).
5. Two OPERABLE flow paths for providing fuel to each diesel generator.
6. TNwo station batteries, two chargers, and the DC distribution systems OPERABLE.
7. Emergency diesel generator battery, charger and the DC control circuitry
OPERABLE for the unit diesel generator and for the shared back-up diesel
generator.
B. During POWER OPERATION or the return to power from HOT SHUTDOWN, the
requirements of specification 3.16-A may be modified by one of the following:
l.a. With either unit's dedicated diesel generator or shared backup diesel generator
unavailable or inoperable:
I. Verify the operability of two physically independent offsite AC circuits
within one hour and at least once per eight hours thereafter.
2. Within 24 hours, determine that the OPERABLE diesel generator is not
inoperable due to common cause failure or demonstrate the operability of
the remaining OPERABLE diesel generator by performing Surveillance
Requirement 4.6.A.I.a. For the purpose of operability testing, the second
diesel generator may be inoperable for a total of two hours per test
provided the two offsite AC circuits have been verified OPERABLE prior
to testing.
3. If this diesel generator is not returned to an OPERABLE status within
7 days, the reactor shall be brought to HOT SHUTDOWN within the next
6 hours and COLD SHUTDOWN within the following 30 hours.
L.b. One diesel fuel oil flow path may be "inoperable" for 24 hours provided the
other flow path is proven OPERABLE. If after 24 hours, the inoperable flow
path cannot be returned to service for reasons other than buried fuel oil storage
tank inspection and related repair, the diesel shall be considered "inoperable."
When the emergency diesel generator battery, charger or DC control circuitry is
inoperable, the diesel shall be considered "inoperable."
AmendmentNos. 241 and 240
TS 3.16-3
2. If a primary source is not available, the unit may be operated for seven (7) days
provided the dependable alternate source can be OPERABLE within 8 hours. If
specification A4 is not satisfied within seven (7) days, the unit shall be brought to
COLD SHUTDOWN.
3. One battery may be inoperable for 24 hours provided the other battery and battery
chargers remain OPERABLE with one battery charger carrying the DC load of the
failed battery's supply system. If the battery is not returned to OPERABLE status
within the 24 hour period, the reactor shall be placed in HOT SHUTDOWN. If the
battery is not restored to OPERABLE status within an additional 48 hours, the
reactor shall be placed in COLD SHUTDOWN.
4. One buried fuel oil storage tank may be inoperable for 7 days for tank inspection
and related repair, provided the following actions are taken:
a. prior to removing the tank from service, verify that 50,000 gallons of
replacement fuel oil is available offsite and transportation is available to deliver
that volume of fuel oil within 48 hours, and
b. prior to removing the tank from service and at least once every 12 hours, verify
that the remaining buried fuel oil storage tank contains 2 17,500 gallons, and
c. prior to removing the tank from service and at least once every 12 hours, verify
that the above ground fuel oil storage tank contains 2 50,000 gallons.
If these conditions are not satisfied or if the buried fuel oil storage tank is not
returned to OPERABLE status within 7 days, both units shall be placed in HOT
SHUTDOWN within the next 6 hours and COLD SHUTDOWN within the
following 30 hours.
C. The continuous running electrical load supplied by an emergency diesel generator
shall be limited to 2750 KW.
Basis
The Emergency Power System is an on-site, independent, automatically starting power
source. It supplies power to vital unit auxiliaries if a normal power source is not
available. The Emergency Power System consists of three diesel generators for two
units. The Unit I diesel generator and the Unit 2 diesel generator are dedicated to
emergency buses I H and 2H, respectively. A third diesel generator is provided as a
"swing diesel" and is shared by Units I and 2. Upon receipt of a safety injection signal
on a unit, the shared diesel generator automatically aligns to either emergency bus IJ
(Unit 1) or 2J (Unit 2) as a backup power supply for the accident unit. The shared
diesel is configured to preferentially load to the Unit 2 emergency bus on a loss of
offsite power without a safety injection signal. The Unit 1 and Unit 2 diesel generators
also supply power for certain common or shared plant systems/components. The diesel
generators have a cumulative 2,000 hour rating of 2750 KW. The actual loads using
conservative
Amendment Nos. 241 and 240
I
TS 3.16-4
ratings for accident conditions, require approximately 2,320 kw. Each unit has
two emergency buses, one bus in each unit is connected to its exclusive
diesel generator. The second bus in each unit will be connected to the backup
diesel generator as required. Each diesel generator has 100 percent capacity
and is connected to independent 4,160 v emergency buses. These
emergency buses are normally fed from the reserve station service
transformers. The normal station service transformers are fed from the unit
isolated phase bus at a point between the generator terminals and the low
voltage terminal of the main step-up transformer. The reserve station service
transformers are fed from the system reserve transformer in the high voltage
switchyard. The circuits which supply power through either system reserve
transformer are called "primary source." In the event a system reserve
transformer is inoperable, the remaining one may be cross-tied by a 34.5 bus
to all three reserve station service transformers. Thus, a primary source is
available to both units even if one of the two system reserve transformers is
out of service. Verification of primary source operability is performed by
confirming that the reserve station service transformers are energized.
In addition to the "primary sources," each unit has an additional off-site power
source which is called the "dependable alternate source." This source can be
made available in eight (8) hours by removing a unit from service,
disconnecting its generator from the isolated phase bus, and feeding offsite
power through the main step-up transformer and normal station service
transformers to the emergency buses.
The generator can be disconnected from the isolated phase bus within eight
(8) hours. A unit can be maintained in a safe condition for eight (8) hours with
no off-site power without damaging reactor fuel or the reactor coolant pressure
boundary.
Verification of the dependable a!!ernate source operability is accomplished by
verifying that the required circuits, transformers, and circuit breakers are
available.
Amendment Nos. 167 and 166
MAR 2 1992
TS 3.16-5
The diesel generators function as an on-site back-up system to supply the
emergency buses. Each emergency bus provides power to the following
operating Engineered Safeguards equipment:
A. One containment spray pump
B. One charging pump
C. One low head safety injection pump
D. One recirculation spray pump inside containment
E. One recirculation spray pump outside containment
F. One containment vacuum pump
G. One motor-driven auxiliary steam generator
feedwater pump
H. One motor control center for valves, instruments, control air
compressor, fuel oil pumps, etc.
I. Control area air conditioning equipment - four air recirculating
units, two water chilling units, one service water pump, and two
chilled water circulating pumps
J. One charging pump service water pump
Amendment Nos. 199 and 199
MAY 3 1 1995
TS 3.16-6
The day tanks are filled by transferring fuel from any one of two buried tornado missile
protected fuel oil storage tanks, each of 20,000 gal capacity. Two of 100 percent capacity
fuel oil transfer pumps per diesel generator are powered from the emergency buses to
assure that an operating diesel generator has a continuous supply of fuel. The buried fuel
oil storage tanks contain a seven (7) (lay supply of fuel, 35,000 gal minimum, for the full
load operation of one diesel generator; in addition, there is an above ground fuel oil
storage tank on-site with a capacity of 210,000 gal which is used for transferring fuel to
the buried tanks.
One of the two buried fuel oil storage tanks may be inoperable to permit inspection and
related repair of that buried fuel oil storage tank. While one tank is removed from service,
the remaining buried fuel oil storage tank supplies fuel oil to the EDGs of both units. Prior
to removal of one buried tank from service and while it is inoperable, verification of the
volume in the remaining buried fuel oil storage tank and the above ground fuel oil storage
tank is required to ensure an adequate source of fuel oil remains available onsite. In
addition, verification of the offsite replacement fuel oil supply is also required. While one
buried tank is out of service, the verification of the onsite and offsite fuel oil sources
continues to support full load operation of one diesel generator for seven days.
If a loss of normal power is not accompanied by a loss-of-coolant accident, the safeguards
equipment will not be required. Under this condition the following additional auxiliary
equipment may be operated from each emergency bus:
A. One component cooling pump
B. One residual heat removal pump
C. One motor-driven auxiliary steam generator feedwater pump
The emergency buses in each unit are capable of being interconnected under strict
administrative procedures so that the equipment which would normally be operated by
one of the diesels could be operated by the other diesel, if required.
The electrical power requirements and the emergency power testing requirements for the
auxiliary feedwater cross-connect are contained in TS 3.6.C.4.c and TS 4.6 respectively.
Amendment Nos. 246/245
TS 3.16-7
TS action statement 3.16.B. .a.2 provides an allowance to avoid unnecessary testing of an
OPERABLE EDG(s). If it can be determined that the cause of an inoperable EDG does
not exist on the OPERABLE EDG(s), operability testing does not have to be performed. If
the cause of the inoperability exists on the other EDG(s), then the other EDG(s) would be
declared inoperable upon discovery, and the applicable required action(s) would be
entered. Once the failure is repaired, the common cause failure no longer exists and the
operability testing requirement for the OPERABLE EDG(s) is satisfied. If the cause of the
initial inoperable EDG cannot be confirmed not to exist on the remaining EDG(s),
performance of the operability test within 24 hours provides assurance of continued
operability of those EDG(s).
In the event the inoperable EDG is restored to OPERABLE status prior to completing the
operability testing requirement for the OPERABLE EDG(s), the corrective action
program will continue to evaluate the common cause possibility, including the other unit's
EDG or the shared EDG. This continued evaluation, however, is no longer under the
24-hour constraint imposed by the action statement.
According to Generic Letter 84-15 (Ref. 6), 24 hours is reasonable to confirm that the
OPERABLE EDG(s) is not affected by the same problem as the inoperable EDG.
 

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Discussion Starter · #6 ·
And how long does it take to cool rods? Can any particular design difference explain why Fukushima's rods will appear to be self-cooking for months?

It seems that the paradox is that power is necessary to fully shut down the reactors (circulate cooling water, run lights/sensors/controls/etc.) long after power at the plant is no longer available -- but the rods are still hot. So the irony may be that a nuclear power plant can't be shut down safely without power. That is, there's no safe way to shut it down if no power source is available?
 

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The reactors' systems are designed to produce a large amount of electricity for grid use. When a weather event happens, a damaged grid should be shut down for safety. If the grid is down, there is no purpose for the large electrical output. The generators power the operations of the cooling systems until the reactors are ready to produce.

For safety, can new plants have a small nuclear generator/reactor to supply power in house to the main unit's cooling system during a shut down? How many different back-up is needed?
 

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Discussion Starter · #8 ·
In this case, if there's ever a supply interruption with gas/diesel, ruptured tanks, a broken generator or two, you evidently end up Fukishima'd.

Maybe this is the worst aspect with nuclear. With pretty much any other source of electricity, you can bleed valves, shut down turbines, etc. and be done with it almost immediately. But evidently with nuclear you need power in order to power-down. And there's no simple "off" switch.
 

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Long term need for power at a nuke

And how long does it take to cool rods? Can any particular design difference explain why Fukushima's rods will appear to be self-cooking for months?

It seems that the paradox is that power is necessary to fully shut down the reactors (circulate cooling water, run lights/sensors/controls/etc.) long after power at the plant is no longer available -- but the rods are still hot. So the irony may be that a nuclear power plant can't be shut down safely without power. That is, there's no safe way to shut it down if no power source is available?
The nuclear cores ARE indeed shutdown in seconds when all the control rods are inserted. The power is needed after shutdown to remove decay heat from the fuel. Decay heat is produced for a very long time, however, the amount of decay heat produced diminishes very quickly. Without reliable power (via the grid or onsite diesels) to provide power to the cooling pumps, things heat up very quickly.....as we have seen in Japan. Nuclear plants do not handle station blackout very well.
 

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Discussion Starter · #10 ·
Does it take more power to shut down a reactor fully (running cooling pumps for as long as needed) than the decay heat itself could provide in residual energy? Why can't the residual reaction power its own shutdown mechanism? If that's impossible physically, that's a dangerous engineering limitation.

The nuclear cores ARE indeed shutdown in seconds when all the control rods are inserted. The power is needed after shutdown to remove decay heat from the fuel. Decay heat is produced for a very long time, however, the amount of decay heat produced diminishes very quickly. Without reliable power (via the grid or onsite diesels) to provide power to the cooling pumps, things heat up very quickly.....as we have seen in Japan. Nuclear plants do not handle station blackout very well.
 

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Does it take more power to shut down a reactor fully (running cooling pumps for as long as needed) than the decay heat itself could provide in residual energy? Why can't the residual reaction power its own shutdown mechanism? If that's impossible physically, that's a dangerous engineering limitation.
I think you are confusing something here. The nuclear fission process is stopped in seconds when the control rods are inserted into the core. At that point, the reactor is shutdown (no longer producing enough neutrons to produce power through fission). However, the byproducts of the fission process continue to produce a lot of thermal energy (heat) that must be removed via cooling systems. The cooling requirement is long-term and requires electricity to run the cooling pumps.
Decay heat can produce steam for awhile and can power steam driven turbines in some plant designs, but is inadequate to produce electricity.

Hope this helps....
 

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Discussion Starter · #12 ·
I'm not confusing anything -- there's is a "technical" shut-down (fission process), and there is a genuine shut-down (cool, safe, no power needed, lights out).

The simple stopping of the fission process, as we learned with Fukushima, is almost irrelevant in the event of a large-scale disaster. If you can't completely achieve a sustainable/cool state, you have a potential problem. So it sounds like most nuclear reactors today will go Fukushima in the event of a nearby EMP or something that knocks out the powergrid and backup diesel systems. I assume EMP will knock out power, and control circuitry to backup diesel. So even if you can stop the fission process through sheer mechanical means, the heat will continue building on its own accord.
 

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I'm not confusing anything -- there's is a "technical" shut-down (fission process), and there is a genuine shut-down (cool, safe, no power needed, lights out).

The simple stopping of the fission process, as we learned with Fukushima, is almost irrelevant in the event of a large-scale disaster. If you can't completely achieve a sustainable/cool state, you have a potential problem. So it sounds like most nuclear reactors today will go Fukushima in the event of a nearby EMP or something that knocks out the powergrid and backup diesel systems. I assume EMP will knock out power, and control circuitry to backup diesel. So even if you can stop the fission process through sheer mechanical means, the heat will continue building on its own accord.
Okay, my mistake regarding your definition of "shutdown". Decay heat in a nuclear fuel must be removed for a very long time. Now there are several ways to remove that heat or make up water to the reactor or spent fuel pool-
1-With the pumps and heat exchangers built in to the plant (preferred). This requires power, either from the grid, the main generator (when the station is online) or from the emergency diesel generators.
2-Emergency diesel driven pumps that can be brought it and "hooked up" to plant piping systems with special spool pieces.

Either way, nukes need long-term care. You cannot just shut them down and walk away from them.
 
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