The NRC Allowed Dominion to use any means necessary to restart the reactors at North Anna, including allowing the utility to only perform visual inspections, use the unconfirmed EPRI report as means for not fully investigating the damage, and bind the damage found at Unit 2 to Unit 1.
No damage to the Unit 2 fuel or inserts was identified by the visual inspections and tests that were performed, so it was concluded that the North Anna 1 fuel and inserts were similarly not damaged as a result of the August 23, 2011 earthquake and they would continue to perform their design function.
By binding Unit 1, to the inspection at Unit 2, the utility was allowed to restart despite complaints that there may be unforseen problems with control rods, and fuel rod integrity after the plant was subjected to a earthquake which exceeded the design basis for the plant.
BWR reactors may have many spring-loaded safety valves, pilot operated valves and/or spring-loaded mechanical assist valves, depending on plant design and steam capacity.
The 1-CH-PCV-1145 is an inverse acting valve, which controls the RCS
When the output drifts down,the valve is actually opening.
When the valve opens, the letdown steam begins to cool off.
The main reason for reducing steam pressure is rather fundamental. Every item of steam using equipment has a maximum allowable working pressure (MAWP). If this is lower than the steam supply pressure, a pressure reducing valve must be employed to limit the supply pressure to the MAWP. In the event that the pressure reducing valve should fail, a safety valve must also be incorporated into the system.
This is not, however, the only occasion when a pressure reducing valve can be used to advantage.
Most steam boilers are designed to work at relatively high pressures and should not be run at lower pressures, since wet steam is likely to be produced. For this reason, it is usually more economic in the long term to produce and distribute steam at a higher pressure, and reduce pressure upstream of any items of plant designed to operate at a lower pressure.
This type of arrangement has the added advantage that relatively smaller distribution mains can be used due to the relatively small volume occupied by steam at high pressure.
Since the temperature of saturated steam is closely related to its pressure, control of pressure can be a simple but effective method of providing accurate temperature control. This fact is used to good effect on applications such as sterilisers and contact dryers where the control of surface temperature is difficult to achieve using temperature sensors.
Plant operating at low steam pressure:
- Can tend to reduce the amount of steam produced by the boiler due to the higher enthalpy of evaporation in lower pressure steam.
- Will reduce the loss of flash steam produced from open vents on condensate collecting tanks.
|Power Reactor||Event Number: 47497|
|Facility: NORTH ANNA
Region: 2 State: VA
Unit:  [ ] [ ]
RX Type:  W-3-LP, W-3-LP
NRC Notified By: DON TAYLOR
HQ OPS Officer: MARK ABRAMOVITZ
|Notification Date: 12/05/2011
Notification Time: 09:35 [ET]
Event Date: 12/05/2011
Event Time: 08:47 [EST]
Last Update Date: 12/05/2011
|Emergency Class: NON EMERGENCY
10 CFR Section:
OTHER UNSPEC REQMNT
BINOY DESAI (R2DO)
JEFFERY GRANT (IRD)
MARK KING (NRR)
|Unit||SCRAM Code||RX CRIT||Initial PWR||Initial RX Mode||Current PWR||Current RX Mode|
|1||N||Y||100||Power Operation||100||Power Operation|
|DISCOVERY OF AFTER-THE-FACT EMERGENCY CONDITION – UNUSUAL EVENT”At approximately 0847 EST, Unit 1 Letdown Pressure Control Valve, 1-CH-PCV-1145, began acting erratically which resulted in the Letdown Relief Valve, 1-CH-RV-1203, lifting and flowing to the Pressurizer Relief Tank.”At 0848 EST, the relief valve reseated and the leakage stopped. Approximately 42 gpm leakage resulted from the relief valve lifting.”This identified flow rate exceeded the threshold for entry into a Notice of Unusual Event under EAL tab SU6.1 due to identified leakage greater than 25 gpm.”
The licensee is troubleshooting the Letdown Pressure Control Valve. The licensee notified the NRC Resident Inspector and will notify State and local agencies.
Source: NRC Event Notifications
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