“Being stingy with their maintenance causes unnecessary plant shutdowns and generates waste.”
MANUAL REACTOR SCRAM DUE TO LOWERING CONDENSER VACUUM – Event 48097 – July 12, 2012 – Nine Mile Point Unit 2
“On July 12, 2012, at 0200 EDT, clean steam reboiler ‘B’ failed, causing a loss of sealing steam. This resulted in degrading condenser vacuum and rising off gas system pressure. The main steam backup supply for sealing steam also failed, and condenser vacuum and off gas system pressure continued to degrade.
“In response to rising off gas system pressure and lowering condenser vacuum, reactor power was lowered to 85% in accordance with Special Operating Procedures. With off gas system pressure approaching the procedural limit and condenser vacuum degrading rapidly, a manual reactor scram was inserted at 0220 EDT.
“All control rods fully inserted and all systems functioned as expected on the scram.
“Plant is currently shutdown and parameters are stable.
“The unit is currently implementing post scram recovery procedures and plant cooldown is in progress.
“The cause of the loss of both the primary and backup sources of sealing steam is under investigation.”
The shutdown electrical lineup is normal and decay heat is being removed via steam bypass valves to the main condenser. The licensee has notified the NRC Resident Inspector and the Public Service Commission.
The Loss of Condenser Vacuum removes the normal capability for the BWR to reject heat to the Cooling Towers or Ultimate Heat Sink Water Supply. Eventually this means that Steam would have to be dumped from the Main Steam (Reactor) to the Suppression Pool and alternate pumps Feed Pumps such as RCIC be utilized for Cooling. Loss of Vacuum also places stress on the Turbine which is very costly to repair if damaged.
Overall, this event shows weakness in Plant’s secondary system “health” but was not in itself dangerous. It is interesting to analyze because an unplanned Shutdown for secondary system failure during peak Summer months is adverse to company goals and plant reliability. Further, every Shutdown creates additional Waste which must be treated at great expense. Lastly, it challenges the operators which can lead to more significant events. Since this is the second forced Shutdown in a Month for Unit 2, a performance deficiency should be investigated. For the Gear Heads out there, some Technical Information is provided.
First, some facts about the Main Condenser:
From Nine Mile Point 2 FSAR
10.4.1 Main Condenser
The purpose of the main condenser is to provide a heat sink for the turbine exhaust steam, turbine bypass steam, and other turbine cycle flows and drains. It also provides deaeration, noncondensable gas removal, and storage of condensate which is returned to the condensate system after a period of radioactive decay.
10.4.1.1.2 Power Generation Design Bases
The main condenser design parameters are shown in Table 10.4-1. The main condenser is designed to accept a maximum of approximately 25 percent of the turbine VWO throttle steam flow from the turbine bypass system (Section 10.4.4). This steam flow is accommodated without increasing the condenser backpressure to the turbine trip setpoint or exceeding the allowable turbine exhaust temperature.
Condenser Vacuum is maintained at greater than 26 In-Hg (Mercury) for a variety of reasons. Thermodynamically, this vacuum corresponds to about 80F which provides an excellent differential pressure into which Main Steam is exhausted through the Low Pressure Turbines. It also prevents Turbine Blade damage due to erosion of water droplets on the final stages of blading.
Vacuum is achieved by all the following:
Cooling Water circulating through Condenser Tubes – which cools the tube surfaces in order that steam on the outside of tubes may transfer heat
Air Ejectors or Vacuum Pumps to remove air and other non-condensable gases
Sealing Steam that is introduced around Condenser Penetrations so that instead of Air being sucked in, condensable steam is. This is what failed in this event.
Sealing Steam is produced in the Reboiler, which provides a source of clean steam. When the Reboiler stopped functioning, a back-up source of steam was not available. Normally, Main Steam could be immediately switched over to fulfill this function however, it was not successful due to an undisclosed reason.
From the Nine Mile Point 2 FSAR:
10.4.3 Turbine Gland Sealing System
10.4.3.1 Design Bases
10.4.3.1.1 Safety Design Bases
The turbine gland sealing system is not nuclear safety related. The piping from the main steam system, up to and including the first block valve including the first pipe support thereafter, is designed to a seismic consideration as described in Section 3.2.
The system is designed to provide clean sealing steam from the clean steam reboilers to the turbine seals to minimize the potential for release of radioactive gas to the environment.
Information concerning the Reboilers:
10.4.3.2 System Description
Each of the two full-capacity clean steam reboilers can produce clean steam at 50 psig for the steam seal system and radwaste reboilers. The shellside of each reboiler is supplied with demineralized water from the condensate system. The tubeside of each reboiler is provided with heating steam via the auxiliary steam system, from either the auxiliary boiler (prior to MSL pressurization or after MSIV closure), the main steam system (below approximately 65-percent turbine load), or the extraction steam system (above approximately 65-percent turbine load).
As you can see, BOTH Reboilers are capable of multiple sources of Heating Steam. The weakest link however is the Steam Seal Regulator which apparently failed to deliver Steam from all the above sources.
From the NMP 2 FSAR Safety Analysis for the Reboilers:
10.4.3.3 Safety Evaluation
Turbine gland sealing is not nuclear safety related. The system is designed to provide a continuous supply of clean steam during normal operation. In the event of total failure of normal supply, steam from the Main Steam Line (MSL) to the turbine bypass chest will be used for shaft sealing. This will avoid potential turbine damage caused by pulling cold air across the hot turbine shaft.
This event skirted on VOIDING Constellation’s Safety Analysis because the Main Steam system was not capable of providing Sealing Steam.
According to Constellation, “The cause of the loss of both the primary and backup sources of sealing steam is under investigation.”
The Main Steam System with its Condenser Dump Valves is depicted here;
Loss of Condenser Vacuum also removes the normal Feedwater and Condensate System from service. This necessitates the use of back-up sources of Water from either the Suppression Pool or Condensate Storage Tank. Both are a limited supply of water. RCIC, HPCI or other Emergency cooling pumps are then used to keep water proper water level in the Reactor Vessel. Condenser Vacuum provides a fast easy way to control Primary Coolant Temperature. Other methods are slower acting and challenge operators.
Here you can see that when the Steam is “Turned Off” to the Main Condensers (because of Loss of Vacuum in this case), you Loss the Normal Condensate System Water Supply to Feed the Reactor. You can also see where the Reboiler gets its source of Clean Water for its uncontaminated Steam:
Nine Mile Point Unit 2 has a BWR Mark II type containment as shown here:
Below is a diagram of one way to cool the Suppression Pool after dumping Main Steam into it over a period of time. There are STRICT limits on how high the temperature is allowed to reach. As the water in the Suppression Pool begins to heat up, cooling via the Heat Removal Mode of the LPCI or RHR system is Required. This poses a minor complication over the normal heat removal cycle.
This event is not particularly devastating as long as the Offsite Power and Emergency Power were available as they were in this case. The interesting part of this event is the Loss of a Back Up Sealing Steam causing vacuum loss. This is in conflict with the NMP 2 Safety Evaluation shown above. Although the condenser and Steam Sealing Systems are not Safety Related, their malfunction can complicate reliable operation of the plant.
Every operator prefers a hungry Condenser with plenty of vacuum into which steam is dumped in order to control Primary Coolant Temperature quickly. Other methods of dumping steam are slower and are crude in comparison.
Since the Unit was previously forced off-line due to Fire in a Feedwater Pump in the last 30 days, an investigation into the programs and maintenance of secondary plant systems is indicated.