Zone 2A Ventilation Exhaust System

Since the Zone 1 and Zone 2A ventilation exhaust systems are not required to operate under abnormal and accident conditions, the exhaust fans have no safety function. However, these exhaust systems interface with the SCB and the Zone 2A canyon/Room 109 environments, respectively. Therefore, the exhaust ducting and filter plenums constitute extensions of the respective radioactive material confinement boundaries, and thus provide both defense in depth and worker safety functions. 

Furthermore, when the ventilation system is operating (and thus removing radioactive material from the HCF confinement structures) radioactive halogens and radioactive particulate matter are removed from the exhaust gases by charcoal filters in the Zone 1 and Zone 2A ventilation exhaust systems and HEPA filters in the hot exhaust system. Thus,... 

Zone 1 and Zone 2A ventilation exhaust systems (hot exhaust ducting, charcoal filters and plenums, and HEPA filters and plenums) Defense in depth Worker safety 1. Remove iodine and radioactive particulate matter from Zone 1 and Zone 2A ventilation exhaust when the ventilation system is operating 2. Provide a confinement boundary to the HCF stack for ventilation system hot exhaust 1. HEPA and charcoal filters are in service 2. Confinement design (Design Feature)... 

When the ventilation system is operating, the pressure in the Zone 2A canyon and Room 109 must be maintained negative with respect to worker occupied areas of the HCF. The Zone 2A ventilation exhaust system maintains the Zone 2A canyon and Room 109 pressure < -0.76 mm (0.03 in) WG with respect to worker occupied areas (Zone 2) of the HCF. 

Zone 1 and Zone 2A Ventilation Exhaust Systems Safety Function... 

The safety function of the HCF Zone 1 and Zone 2A ventilation exhaust systems is to provide a controlled, filtered path for radioactive releases during normal and abnormal operating conditions. Radioactive particulate matter and iodine are rerhoved from Zone 1 and Zone 2A exhaust air by means of HEPA and charcoal filters, respectively. 

Chapter 2 contains detailed descriptions of the Zone 1 and 2A portions of the ventilation system. Ventilation system ducting provides the necessary confinement pathway during normal ventilation system operation to ensure filtration of exhaust gases prior to atmospheric release. The Zone 1 ventilation exhaust system contains redundant series-parallel charcoal filter banks located in the MER. The Zone 2A ventilation exhaust system contains charcoal filter banks located in the MER. In addition, ventilation system hot exhaust from Zone 1 and Zone 2A is routed through a HEPA filter before entering the HCF stack. These ventilation systems are described in more detail in Chapter 2. 

Table 4.4-4 provides performance criteria needed to demonstrate that the functional requirements for the Zone 1 and Zone 2A ventilation exhaust systems are met No operational events can affect the ability of the HEPA and charcoal filters to perform their safety functions, since these functions ahe only required when the HCF ventilation system is operating. Furthermore, failure of filter bank inlet or outlet dampers to remain open during ventilation system operation will essentially stop the flow of Zone 1/Zone 2A exhaust air to the HCF stack. The only events that could affect the ability of the charcoal filters to perform their safety function are a fire in the MER or an external event such as an earthquake or aircraft crash that would destroy the MER. The only events that could affect the ability of the HEPA filters to perform their safety function are similar events. 

As indicated in Section 5.3, there are no SLs or LCSs associated with the Zone 1 and Zone 2A ventilation exhaust systems. However, since these systems provide defense in depth against off-site radioactive material releases during normal and abnormal operating conditions, the following LCO is established for the processing operation mode (see Table 5.4-1) ... 

The Zone 1 and Zone 2A ventilation exhaust ducting and HEPA and charcoal filters are passive components. Their safety-related function is performed continuously while the HCF ventilation system is in operation. When the ventilation system is not operating, no isotope processing operations are being conducted, and any residual radioactive materials are in nonvolatile states and are confined within the SCBs and Zone 2A canyon. Thus, the functional requirements applicable to the filters are ... 

A TSR requirement to verify that Zone 1 and Zone 2A ventilation exhaust HEPA and charcoal filters are in-service vwll be implemented to assure that exhaust gases are being filtered when the HCF ventilation system is in operation. A TSR requirement to verify the ventilation system fan sequencing interlock is operable vvnil be implemented to ensure that proper building airflow patterns are maintained in the event of exhaust fan failures. The ventilation system exhaust ducting provides only an inherent passive safety function (i.e., confinement) and no specific TSR controls are required to ensure continued performance of this function. 

The Zone 2A canyon boundary serves as a secondary confinement area that surrounds the SCBs (Zone 1). This secondary confinement volume is exhausted by a separate ventilation system. The Zone 2A ventilation system also draws on Room 109 so that any potential contamination from waste is appropriately controlled. When both moveable shield walls are in the up position, a negative pressure differential will exist between Room 109 and the Zone 2A canyon. When either of these shield doors are lowered, this pressure differential will approach zero. However, the airflow through Room 109 into the Zone 2A canyon, which acts to mitigate the undesired migration of contamination, will be maintained. All exhaust from Zone 2A is filtered through a HEPA filter located in the Zone 2A canyon and a charcoal filter located in the MER prior to being combined with the Zone 1 exhaust flow in the MER. 

A ventilation system fan sequencing interlock ensures that Zone 1 confinement boundary integrity is not compromised by either exhaust fan faiiures or operational errors. This interlock automatically starts the backup exhaust fan if the operating exhaust fan fails, thereby maintaining the desired directional airflow control across ventilation exhaust system boundaries. Thus, no operator action is required to maintain Zone 1-to-Zone 2A differential pressure in the event of exhaust fan failures. 

The makeup-air unit supplying Zone 2 receives its air from the building distribution system, the Zone 3 ventilation system (fan 3), boosted by fan 13 and recirculated by fan 11. The volume of makeup air is determined by the amount of air exhausted from the east and west Zone 2 sections, the amount of makeup air supplied to Zone 2A (which includes the Zone 1 supply air), less the amount of infiltration air into Zone 2. In addition to maintaining the correct pressure differential, the Zone 2 system provides at least four air changes per hour in Zone 2 and provides necessary environmental conditioning. The volume of recirculated air is regulated by a manual intake damper located in the intake duct of the Zone 2 filter and cooling coil. 

When the ventilation system is operating, Zone 2A exhaust airflow must be routed to charcoal filters before being exhausted from the HCF stadc. Ductwork from Zone 2A routes airflow generated by venfliation system fans to parallel charcoal filter plenums in the MER. 

Inservice verification and testing of safety-related SSCs is conducted by HCF personnel in compliance with TSR surveillance requirements. These operational activities consist of Zonel-to-Zone 2A and Zone 2A-to-Zone 2 ventilation system differential pressure verification, differential pressure instrumentation channel functional testing, ventilation fan sequencing interlock operability testing, and ventilation system hot exhaust filter operability verification. Details of these TSR surveillance tasks and the frequency with which they are to be performed are formalized in an operational surveillance procedure. 

HCF operators must verify that Zone 2A exhaust charcoal filters are in service during ventilation system operation. 

A ventilation system fan sequencing interlock ensures that the backup exhaust fan is automatically started upon loss of the primary operating fan. An alarm alerts the operations staff upon loss of the operating Zone 2A exhaust fan. 

Ventilation Hot Exhaust (Zones 1 and 2A combined exhaust) Control of off-site radioactive material releases. 1. Hot exhaust confinement boundary integrity must not be compromised by ventilation system fan mis-operation or malfunction. 2. HCF operators must maintain the required functional performance of confinement controls. 3. HCF operators must verify the functional performance of the hot exhaust HEPA filters. 1. A ventilation system fan sequencing interlock ensures that the backup exhaust fan is automatically started upon loss of the primary operating fan. An aiann alerts the operations staff upon loss of the operating stack exhaust fan. 2. Administrative controls are in place to ensure periodic ventilation system fan sequencing interlock testing. 3. Administrative controls are in place to ensure periodic functional performance and efficiency testing of the HEPA filters. 

The primary function allocated to the machine side of this interface involves prevention of adverse pressure differentials. This function is performed by the ventilation system fan sequencing interlock, which establishes a hierarchy of fan operation to ensure that hot exhaust ducting does not get pressurized in the event of exhaust fan failures or mis-operation. An interlock automatically starts the backup fan upon loss of an operating fan. in addition, the interlock automatically shuts down all upstream fans if both of the Zone 1, Zone 2A, or stack exhaust fans fail. The interlock also allows only a sequenced startup of ventilation system fans to prevent pressurization of the hot exhaust ducting because of operator failure to adhere to the required fan startup sequence. 

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