HCF Ventilation System

When used without a clarifying adjective or phrase and when not specifically associated with the HCF ventilation system, the term Zone 2A" identifies the hot cell canyon as described... 

Design barriers to solid and liquid hazardous material releases include the buildings themselves as well as the process and storage hardware. Design barriers to gaseous and volatile hazardous material releases include the HCF ventilation system and associated charcoal filters. 

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 ... 

The safety function associated with providing a confined pathway for hot exhaust gases is performed continuously while the HCF ventilation system is in operation. The applicable functional requirement is ... 

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. 

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. 

TSR selection Criterion 1 is not considered applicable for the HCF. HCF ventilation system differential pressure instrumentation functions primarily to monitor control of radioactive contamination migration across confinement barrier boundaries. Although this instrumentation provides a positive indication of Zone 1 and Zone 2A canyon confinement barrier integrity, the absence of the appropriate differential pressure gradient does not necessarily indicate a significant degradation of a confinement barrier. Therefore, application of a Limiting Condition for Operation to this instrumentation is not warranted. 

The release point for the HCF ventilation system exhaust is the HCF exhaust stack. Gaseous effluent discharged in the ventilation system is continuously monitored. The monitor electronically records selected beta-gamma emitting radionuclides as well as noble gases. The system is calibrated and maintained by representatives of the SNL ES H Center and is discussed in Chapter 7. Liquid effluent from the LECS is sampled prior to release into the sanitary system. The samples are analyzed by gamma spectroscopy and liquid scintillation. No discharges to the sanitary system are allowed until analyses show that levels are within the established release limits.. ... 

It is evident from Table 13.4-1 that the HCF ventilation system is the safety-related SSC whose human-machine interfaces are the most numerous and also the most critical with respect to preventing uncontrolled off-site radioactive material releases. This system thus warrants close operator attention. Ventilation system fans and dampers are operated from the EMCS keyboard located in the Operations Center. The following HCF ventilation system displays and alarms are input to the EMCS display monitor ... 

Process operators control SCB ventilation flowrate by means of SCB exhaust valve controls located in a local control panel at each processing station. The following HCF ventilation system displays and alarms are also located in these panels ... 

Identify possible safety-related systems, structures, or components Radiation shielding of the hot cell laboratory shielded support area and steel confinement boxes. HCF ventilation system, shielded glove box and ventilation hoods of the analitv control laboratory, and shield doors of the process radioactive waste storage area. ... 

The HCF ventilation system provides a safe operating environment for workers within the... 

Blast pressures caused by the closest potential explosion to the facilities are not anticipated to cause damage to the HCF or its associated radioactive material storage areas. The effect the explosion through the HCF stack on internal HCF filters would be to blow the filters and any radiation freed from them back into the HCF ventilation system and not out of the facility to the public and environment (Restrepo 1995). 

The effect of a ioss of off-site power wouid be to temporarily shutdown the HCF ventilation system if standby diesei generator power were not available. Thus, no releases to the environment would occur and slow local contamination within limited areas the HCF might occur if power were not restored to the ventilation system for an extended period. 

Stack exhaust fans 4 5 (Transports the ventilation system filtered exhaust out of the HCF stack and controls flow rate through the stack HEPA filter.) Shutdown due to loss of electrical power, fen electrical failure, or plugged HEPA filter Onsite or offeite electrical or mechanical malfunction Control and monitoring system indications and periodic maintenance inspections Two parallel fens provide redundant flow. Complete loss of ventilation flow if both fens stop. Loss of flow forces shutdown of HCF ventilation system. 

Of the various scenarios evaluated, the most serious consequences resulted from the spill of volatile process materials or a fire in an SCB. Based on these evaluations, structures, systems and components with the potential to mitigate this hazard were identified. While many features in the HCF control or mitigate the hazards for both workers and the public, the most significant mitigative feature for the protection of the public is the filtered HCF exhaust ventilation system. 

B6580C, located at the base and to the west of the HCF stack, houses ventilation system HEPA filters and exhaust fans 4 and 5. This building and the stack are located directly north of B6580. 

Portions of the HCF structure, along with the steel confinement boxes (SCBs), gloveboxes, fume hoods, and the ventilation systems perform confinement functions. These confinement systems provide defense in depth by ensuring that hazardous materials are retained in specific designated areas within the HCF. They accomplish this function by maintaining an air pressure differential hierarchy from regions of greater contamination to those of lesser contamination within the facility. These differentials are described later in this section. This pressure differential controls the movement of contamination by diffusion and by adverse airflows. The Identified contamination zones in the HCF are as follows ... 

In the MER, a separate fan exhausts 400 cfm from the Zone 1 Filter Room and discharges it into the ventilation exhaust. The purpose of this system is to exhaust any airborne contamination that could result from Zone 1 ventilation system filter replacements or fan maintenance to the HCF stack. 

Thermostats in each of the 13 areas served by the Zone 3 ventilation system control the temperature of the supply air (by automatic operation of the preheat coil, the air washer, associated dampers, and the reheat coil). Return air from areas served by the Zone 3 ventilation system is collected and eventually exhausted through redundant prefilters, HEPA filters, and fan units (Fans No. 1 2) located at the base of the HCF stack. 

Compressed air to operate ventilation-system dampers comes from two redundant, industrialsized compressors [40 standard ft /min (SCFM) at 125 Ib/in. (psi)], located in Room 222 of B6581. The two compressors operate as a primary supply and as a backup. Downstream of the normal air compressors is miscellaneous support equipment including an air receiver and filters. The air supply is then split into two separate pathways (1) general building compressed air, and (2) dry Instrument air, which is used in the HCF for ventilation system damper positioning. 

While fire protection systems are typically considered important to safety, fire protection In the HCF does not provide a major contribution to defense in depth due to the limited quantity of flammable materials permitted in the process extraction SCB s and the form and containerization of the hazardous process materials. A fire in an SCB does not threaten the ventilation system filters and would normally not result in any significant release of hazardous materials. The fire scenario has been evaluated and included in the accident analyses described in Section 3.4, which indicates that the release is bounded by the consequences of a process spill in the SCB. 

The ventilation system purpose described above, that of cleanliness, does not meet the criteria specified in DOE-STD-3009-94 to be identified as safety-significant. Ventilation failure analyses (Mitchell and Naegeli, 1999), summarized in Section 3.4, indicate that without the ventilation system operating, the expected dose to the public is negligible, and the contamination levels which would build up in normally occupied areas of the HCF are several orders of magnitude below that necessary to result in serious injury or death. Of course, normal operations could not continue to be accomplished with an inoperative ventilation system, in that contamination levels would eventually build up in occupied areas so as to result in worker doses which exceed SNL radiation protection administrative control limits and would be operationally unacceptable. Thus, the... 

The release of radiological materials in any specific accident scenario is usually less than the total inventory present, depending on the postulated confinement barrier failure in the DBA and the limitations to release Inherent In the physical form of the hazardous material. The source term released by initial confinement barrier failure may be reduced even further by mitigative effects which occur during transport or leakage from the facility to the environment. For example, filtration in the ventilation system will normally trap a large fraction of the fission products but would still allow the volatile noble gas fission products to be released from the HCF stack. 

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