**Site:** Office of River Protection Hanford

**Facility:** Tank Farms

**System:** Double Shell Tank (DST) Primary Tank Ventilation Systems

**System Classification:** Safety Class

**System Safety Function:** 1. Prevention of In-Tank Flammable Gas
Deflagration

(Safety Class Function)

> 2\. Confinement Ventilation (Environmental Protection Function)

**[OBJECTIVE:]{.underline}**

**VSS-1 This vital safety system is operational and personnel and
processes are in place that ensures its continued operational
readiness.**

**VSS-1.1** VSS safety functions are defined and understood by
responsible line managers, and supporting information/documentation is
available and adequate. System testing is adequate to ensure
operability. (Review Approach items 1, 2, 3 and 7)

> [Discussion of Results]{.underline} --
>
> [System Safety Functions]{.underline}
>
> [Prevention of In-Tank Flammable Gas Deflagration]{.underline}: The
> DST and AWF tank primary tank ventilation systems are identified as
> Safety-Class (SC) Systems, Structures, and Components (SSC) for the
> prevention of the Flammable Gas Deflagrations accident described in
> Section 3.4.2.2 of HNF-SD-WM-SAR-067, Rev. 2, *Tank Farms Final Safety
> Analysis Report* (FSAR). The safety function of the systems is to
> prevent the accumulation of flammable gases due to steady-state
> releases, thus decreasing the frequency of the accident. The accident
> analysis identified active ventilation systems as the primary means of
> maintaining DST and AWF tank vapor spaces below the flammability
> limit. Section 4.3.2 of the FSAR describes the system, its safety
> function and functional requirements, and Technical Safety Requirement
> (TSR) controls.
>
> [Confinement Ventilation]{.underline}: Environmental drivers require
> minimization of fugitive radioactive emission. Washington
> Administrative Code (WAC) 246-247, regulated by the Washington
> Department of Health, requires the utilization of As Low As Reasonably
> Achievable Control Technology (ALARACT) for the control of radioactive
> emissions. To satisfy the confinement function, the DST and AWF
> primary tank ventilation systems are intended to maintain a vacuum
> within the tank and provide High Efficiency Particulate Air (HEPA)
> filtration of the emissions.
>
> Additionally, in-series redundancy of HEPA filtration is identified
> (in conjunction with HEPA filter differential pressure interlocks) as
> [safety-significant]{.underline}[^1] for the DST and AWF primary tank
> ventilation systems to prevent releases from exceeding the onsite risk
> guidelines as a result of the accident described in Section 3.3.2.4.2
> of the FSAR, *HEPA Filter Failure -- Exposure to High Temperature or
> Pressure*. The safety function of the ventilation system HEPA filters
> is to provide HEPA filtration of headspace gases before their release
> to the environment, thus reducing the likelihood of unfiltered
> releases due to partial HEPA filter release events associated with the
> accident. However, for the bounding scenario analyzed, the HEPA
> filters would likely fail because of the over-pressurization and,
> therefore, were not credited. Section 4.4.3 of the FSAR describes the
> system, its safety function and functional requirements, and Technical
> Safety Requirement (TSR) controls.
>
> [Conditions Under Which The Systems Are Intended To Perform Safety
> Function(s)]{.underline}
>
> [Prevention of In-Tank Flammable Gas Deflagration]{.underline}: The
> assumption in the analysis of the Flammable Gas Deflagration accident
> is that the ventilation systems are operable and operating during
> normal waste tank operations and will remain operable and operating
> during a tank waste Gas Release Event (GRE) that may occur in a tank.
> Specific controls (and their bases) are in place to protect the
> operability assumptions. Credit is not taken for the safety function
> performance during abnormal or accident situations.
>
> [Confinement Ventilation]{.underline}: Both DST and AWF primary tank
> ventilation and system exhaust HEPA filtration are intended to be
> operable during normal tank operating conditions; that is, while waste
> storage and waste transfers may be occurring. For the bounding
> scenario analyzed in the accident described in section 3.3.2.4.2 of
> the FSAR, the HEPA filters would likely fail as a result of an
> over-pressurization condition and, therefore, were not credited.
> However, in a less than bounding over-pressurization or
> over-temperature accident where the conditions are less severe,
> partial or full HEPA filter integrity may continue to exist, thus
> reducing the likelihood of unfiltered releases associated with the
> accident
>
> .
>
> [System Functional Requirements and Performance Criteria]{.underline}
>
> [Prevention of In-Tank Flammable Gas Deflagration]{.underline}:
>
> Functional Requirements: The DST and AWF tank primary ventilation
> systems shall maintain vapor space pressures of all tanks \<0 kPa (\<0
> in. WG relative to atmospheric pressure).
>
> Performance Criteria: Operability of the primary tank ventilation
> systems shall be verified daily to ensure compliance with the
> functional requirements. To ensure that the standby systems will
> operate in the event of a primary ventilation system failure, the
> standby systems are functionally tested on a periodic basis.
>
> [Confinement Ventilation]{.underline}:
>
> Functional Requirements:

1.  The DST and AWF tank primary ventilation systems shall maintain
    vapor space pressures of all tanks \<0 kPa (\<0 in. WG) relative to
    atmospheric pressure.

2.  The DST and AWF ventilation system exhaust HEPA filter units shall
    remove at least 99.95% of particles of an approved challenge aerosol
    that are as small as 0.3 m.

> Performance Criteria:

1.  Operability of the primary tank ventilation systems shall be
    verified daily to ensure compliance with the functional
    requirements. To ensure that the standby systems will operate in the
    event of a primary ventilation system failure, the standby systems
    are functionally tested on a periodic basis.

2.  Per Administrative Control (AC) 5.18.2 (e) of HNF-SD-WM-TSR-006,
    Rev.2, *Tank Farm Technical Safety Requirements*, the HEPA filter
    nominal particulate removal efficiency shall be verified to be
    99.95% by performing an aerosol test with the design air flow
    through the filter, according to the guidance in American Society of
    Mechanical Engineers (ASME) standard N510, *Testing of Nuclear Air
    Treatment Systems*, and Energy Research and Development
    Administration (ERDA) 76-21, *Nuclear Air Cleaning Handbook*. The
    aerosol test shall be performed after filter
    installation/replacement and every 12 months thereafter.

> [Surveillance Test Acceptance Criteria]{.underline}
>
> [Prevention of In-Tank Flammable Gas Deflagration]{.underline}:
> Limiting Condition of Operation (LCO) 3.2.1 of HNF-SD-WM-TSR-006,
> Rev.2, *Tank Farm Technical Safety Requirements* (TSR), provides the
> operability requirements for the exhausters for the primary tank
> ventilation system for DST and AWF waste tanks. Surveillance
> requirement (SR) 3.2.1.1 of the TSR requires verification that the
> active primary tank ventilation system is operable.
>
> The preferred method is to confirm that the primary tank vapor space
> pressure is \< 0 kPa (\< 0 in. WG). Alternatively, monitoring of
> ventilation system airflow rate is an acceptable practice. Operability
> of the safety function is confirmed daily through use of the following
> Operator Surveillance Rounds:

-   TF-OR-A-03, Rev. E-71, *AP and AW Tank Farms Daily Rounds.*

-   TF-OR-EF-AN, Rev. C-41, *AN Tank Farm Rounds*.

-   TF-OR-EF-AYAZ-D, Rev. C-67, *AY and AZ Tank Farms Daily Rounds*.

-   TF-OR-EE-702AZ-D, Rev A-21, *702-AZ Primary Tank Ventilation System
    Operational Daily Rounds*.

-   TF-OR-WST-01-D, Rev. C-27, *242-S, 244-S, 241-SY, 241-S and 241-SX
    Day and Night Shift Rounds*.

> The acceptance criteria is confirmation that a vacuum exists in the
> primary tank vapor space or that the primary tank ventilation system
> exhaust fan(s) is operating.
>
> [Confinement Ventilation]{.underline}:

1.  Verification that the confinement ventilation system is operable is
    conducted using operator daily surveillance rounds as described
    above.

2.  Verification of the HEPA filter performance criteria of AC
    5.18.2 (e) is conducted using maintenance procedure 3-VBP-156, Rev.
    B-9, *HEPA Filter In-Place Leak Test (Aerosol Test)* in conjunction
    with location specific datasheets. The test is in accordance with
    the guidance provided in ASME N510, *Testing of Nuclear Air
    Treatment Systems*, and Energy Research and Development
    Administration (ERDA) 76-21, *Nuclear Air Cleaning Handbook*. The
    test must demonstrate filter particulate removal efficiency to be
    99.95% for particles are as small as 0.3m at the design airflow
    rate.

> [Frequency of Surveillance Tests and Identification of
> Requirements]{.underline}
>
> [Prevention of In-Tank Flammable Gas Deflagration]{.underline}:
> Verification of both primary tank vacuum and primary tank ventilation
> exhaust fan operation is conducted once per shift per the operations
> surveillance procedures specified above ("Surveillance Test Acceptance
> Criteria"). This is a Surveillance Requirement required to support LCO
> 3.2.1 of the TSR.
>
> [Confinement Ventilation]{.underline}: Verification of both primary
> tank vacuum and primary tank ventilation exhaust fan operation is
> conducted once per shift per the operations surveillance procedures
> specified above ("Surveillance Test Acceptance Criteria").
>
> Administrative Control (AC) 5.18.2 (e) requires the exhaust HEPA
> filter aerosol tests to be performed after filter
> installation/replacement and every 12 months thereafter. The activity
> is on a 330-day recall schedule in the Preventive
> Maintenance/Surveillance (PM/S) system.
>
> [Drawings That Document the System Configuration]{.underline}
>
> Essential drawings and other design baseline drawings are readily
> available via microfilm at Document Control Centers or on the on-line
> Hanford Document Control System (HDCS). Outstanding and incorporated
> Engineering Change Notices (ECNs) to these documents are identified in
> HDCS, and are readily available in HDCS or at the document centers.
>
> HDCS has the capability of identifying system cognizant engineers and
> system design authority engineers associated with the drawings.
>
> The typical set of drawings used in daily operation of the DST
> ventilation system consist of; Piping and Instrument Diagrams, HVAC
> equipment schedules, electrical one-line diagrams and panelboard
> schedules, and HVAC equipment plans). A list of the drawings for the
> DST and AWF primary tank ventilation systems is included in Attachment
> F. This is not a complete listing of all the drawings for these
> systems, but it is a list of what is considered necessary for safe
> day-to-day operation and maintenance.

**VSS-1.2** The backlog for surveillances, tests, inspections,
maintenance, repair, upgrades, or other work on the system is managed
and kept to an appropriate minimum. (Review Approach item 6)

> [Discussion of Results]{.underline} --
>
> [Current Maintenance, Testing, and Surveillance Backlog]{.underline}
>
> The current backlog of Preventive Maintenance (PM), Corrective
> Maintenance (CM), and modification work packages for the DST and AWF
> primary tank ventilation systems is included in Attachment E. No
> backlog of surveillances, tests, or inspections has been identified.
> Backlog of corrective actions from recovery or corrective action plans
> will be identified separately from this document for all systems.
>
> In the SY Tank Farm the backup exhauster aerosol test is overdue,
> because the motor on the exhauster has failed. A corrective
> maintenance work package is issued and the exhauster motor will be
> replaced. The aerosol test will occur subsequent to the successful
> motor replacement.
>
> The SY Tank Farm Primary exhauster has had a problem with the
> demister/
>
> de-entrainer. Water buildup within the exhaust filter system has
> caused higher differential pressure through the first stage HEPA
> filter. A replacement of the first stage HEPA filter was needed, due
> to this issue. A replacement demister/
>
> de-entrainer for this exhaust unit needs to be procured and installed.

**VSS-1.3** Configuration Management and Maintenance programs
effectively ensure operational availability of the system. (Review
Approach items 5, 8 and 9)

> [Discussion of Results]{.underline} --
>
> [Scheduled Maintenance, Walk downs, Inspections, and
> Assessments]{.underline}
>
> Weekly, and monthly surveillance operator rounds are conducted to
> ensure tank farm operating equipment is functioning properly. These
> surveillances include items that are intended to ensure reliable
> performance of the DST and AWF primary tank ventilation systems.
> Applicable Operator Rounds include:

-   TF-OR-A-03, Rev. E-71, *AP and AW Tank Farms Daily Rounds*.

-   TF-OR-EF-AN, Rev. C-41, *AN Tank Farm Rounds*.

-   TF-OR-EF-AYAZ-D, Rev. C-67, *AY and AZ Tank Farms Daily Rounds*.

-   TF-OR-EE-702AZ-D, Rev A-21, *702-AZ Primary Tank Ventilation System
    Operational Daily Rounds*.

-   TF-OR-WST-01-D, Rev. C-27, *242-S, 244-S, 241-SY, 241-S and 241-SX
    Day and Night Shift Rounds*.

> Additionally, preventive maintenance and surveillance (PM/S)
> activities are regularly scheduled and conducted through the PM/S
> module of the Job Control System (JCS). These activities include the
> calibration, functional testing and other periodic maintenance items
> necessary for system components that assist operators in monitoring
> and controlling system operations, but are not necessarily tied
> directly to a safety-related function.
>
> Regularly scheduled Environmental Surveillances and Compliance
> Inspections are conducted within the DST and AWF tank farms. Included
> in these surveillances are primary tank ventilation systems.
> Attachment C includes surveillance schedules and example checklists.
>
> The Facility Excellence Program (FEP) as described in Section 5.8.3,
> "Identify and Implement Improvement Opportunities" of the "Integrated
> Environment, Safety, and Health Management System Description for the
> Tank Farm Contractor" (RPP-MP-003, Rev. 1), provides regularly
> scheduled self-assessments of different facilities by employees. Walk
> down assessments are informal, but equipment condition issues have
> been addressed in these walk downs. Attachment D contains schedules
> and a sample checklist for these activities.

**VSS-1.4** The system is operable and available to fulfill its safety
function when required. (Review Approach items 4 and 10)

> [Discussion of Results]{.underline} --
>
> [System Acceptance Test Failures: Last Three Years]{.underline}
>
> [Prevention of In-Tank Flammable Gas Deflagration]{.underline}: There
> is no maintenance testing or functional test intended to confirm
> operability. Operability of the safety function is confirmed daily
> through use of the Operator Surveillance Rounds as previously
> described in VSS-1.1 and VSS-1.3. The system is not required to be
> operable during planned shutdowns for maintenance, etc. Unplanned
> primary tank ventilation system shutdowns are typically detected in
> response to tank pressurization alarms or by operator rounds if
> pressurization alarms do not occur. Unplanned shutdowns are typically
> system failure on demand, and these are addressed in the next section
> ("System Failures in Response to Facility Conditions").
>
> [Confinement Ventilation]{.underline}: During the three year period
> from 1998 to 2000, there were two events where HEPA filters failed to
> meet the annual (330 days) functional aerosol test acceptance
> criteria. Both of these failures were determined during the same work
> evolution (work package #2E-00-01475) on 10/26/00 at the 241-AP tank
> farm primary tank ventilation system.
>
> One filter from each parallel exhaust train failed to meet the
> criteria during annual in-place testing of the filters. The filter
> component numbers are AP241-VTP-FLT-302 and -402. Both of these
> filters are the first stage filters of their respective dual-stage
> parallel filter train, so the confinement function of the filtration
> was not compromised because of the redundancy of filtration inherent
> in the design.
>
> [System Failures In Response to Facility Conditions: Last Three
> Years]{.underline}
>
> [Prevention of In-Tank Flammable Gas Deflagration]{.underline}: During
> the three year period from 1998 to 2000, there have been a total of 42
> events where one of the five DST or AWF primary tank exhaust systems
> have experienced unplanned shutdowns (see Attachment A for details).
> Broken down by tank farm primary exhaust system, these are:

-   241-SY 21 Events [Total time down:]{.underline} 99 hours, 51 minutes

-   241-AP 5 Events [Total time down:]{.underline} 37 hours, 33 minutes

-   241-AW 4 Events [Total time down:]{.underline} 16 hours, 51 minutes

-   241-AN 4 Events [Total time down:]{.underline} 13 hours, 11 minutes

-   241-AY/AZ 8 Events [Total time down:]{.underline} 13 hours, 47
    minutes

> [Confinement Ventilation]{.underline}: The confinement ventilation
> function downtime is the same as presented above for the prevention of
> in-tank flammable gas deflagration accident. In other words, the
> exhausters must be operable to provide a vacuum in the primary tank to
> minimize the potential for unfiltered fugitive emissions through leak
> pathways.
>
> There were no failures of HEPA filter integrity during the period,
> with the exception of the two 241-AP tank farm filters that failed the
> criteria of the in-place efficiency test. Because of the redundancy of
> in-series HEPA filters, these failures did not result in a loss of the
> confinement ventilation function. Because of the operability of the
> alternate safety control LCO 3.1.4 (Stack CAM Fan Interlock), the
> safety function to provide redundant in-series filtration in
> conjunction with LCO 3.1.8 (HEPA Filter dP Interlock) was not
> compromised.
>
> [Percentage of Time the System Was Not Capable Of Accomplishing Its
> Safety Function: Last Three Years]{.underline}
>
> [Prevention of In-Tank Flammable Gas Deflagration]{.underline}: When a
> DST or AWF primary tank ventilation system is discovered to be not
> operable to meet LCO 3.2.1 requirements, LCO actions are immediately
> entered to ensure safe operating configuration. While in LCO 3.2.1
> action mode, the systems are not required to be operable, but
> immediate action is taken to restore the primary tank ventilation
> system to operable status as soon as possible.
>
> The systems are not required to be operable during planned shutdowns
> for maintenance (LCO actions are entered). Additionally, should a
> ventilation system fail for any reason (unplanned shutdown), there is
> sufficient time for recovery before flammable gas concentrations in a
> tank might reach the Lower Flammability Level (LFL) necessary for
> minimum combustion.
>
> It is determined in the accident analysis that a loss of active
> ventilation can result in an accumulation of flammable gases to Lower
> Flammability Level (LFL) conditions in 32 days for Tank 241-SY-101.
> The next shortest time to LFL is calculated to be 50 days for Tank
> 241-AN-107.
>
> Twenty-one (21) of the 28 DSTs, based on current contents, cannot
> attain LFL conditions[^2].
>
> Nevertheless, for the purposes of this analysis, an estimate of
> percentage downtime and availability of the safety function for each
> DST/AWF tank farm primary tank ventilation system is provided based
> upon unplanned system shutdowns (see Attachment A for details):

-   241-AP Percentage downtime = 0.14% Availability: 99.86%

-   241-SY Percentage downtime = 0.38% Availability: 99.62%

-   241-AW Percentage downtime = 0.06% Availability: 99.94%

-   241-AN Percentage downtime = 0.05% Availability: 99.95%

-   241-AY/AZ Percentage downtime = 0.05% Availability: 99.95%

> [Confinement Ventilation]{.underline}: The confinement ventilation
> function requires that the exhausters be operable to provide a vacuum
> in the primary tank to minimize the potential for unfiltered fugitive
> emissions through leak pathways. There were no failures of primary
> tank ventilation exhaust HEPA filter systems during the analyzed
> period. Therefore, the estimated percentage of time that the system
> was not capable of providing this function when required to be
> operable is the same as presented above associated with ventilation
> system unplanned shutdown periods.
>
> [Supporting Systems/Equipment For Operation of the Vital Safety
> System]{.underline}
>
> Electrical power is required for operation of the DST and AWF primary
> tank ventilation systems.
>
> Although not essential to its operation (can operate in bypass), the
> 241-AY/AZ primary tank exhauster is supported by a moisture removal
> system (condenser) with cooling system (chiller and cooling tower) and
> an air recirculation module. The purpose of this system is to remove
> moisture from the air stream for return to the tanks, cool the air
> stream, and recirculate part of the air stream back to the tanks for
> waste tank cooling.
>
> This additional system is installed in the 241-AY/AZ primary tank
> exhausters because the four tanks in these two Aging Waste Facility
> tank farms contain waste with a high heat load.

**[CONCLUSIONS:]{.underline}**

The DST and AWF Primary Tank Ventilation Systems serve two primary
functions: (1) dilution and removal of flammable gas buildup for
prevention of a potential in-tank flammable gas deflagration accident
(safety-class function) and (2) provide confinement ventilation to
minimize fugitive unfiltered emission (environmental function) by
maintaining a vacuum within each tank. The systems are operable and
available to perform their safety functions.

The systems are not required to be operable during planned shutdowns for
maintenance. Additionally, should a ventilation system fail (unplanned
shutdown) for any reason, there is sufficient time for recovery before
flammable gas concentrations in a tank might reach the Lower
Flammability Level (LFL) necessary for minimum combustion. It is
determined in the accident analysis that a loss of active ventilation
can result in an accumulation of flammable gases to Lower Flammability
Level (LFL) conditions in 32 days for Tank 241-SY-101. The next shortest
time to LFL is calculated to be 50 days for Tank 241-AN-107. Twenty-one
(21) of the 28 DSTs, based on current contents, cannot attain LFL
conditions[^3].

Without regard to actual ventilation system recovery periods for
prevention of an in-tank gas deflagration accident, the ventilation
systems were analyzed for availability over the past three years based
upon down time as a result of unplanned shutdowns. Availability of the
systems to provide ventilation is greater than 99% for each of the five
systems (ref. VSS-1.4).

The SY Tank Farm Primary exhauster has had a problem with the
demister/de-entrainer. Water buildup within the exhaust filter system
has caused higher differential pressure through the first stage HEPA
filter. A replacement of the first stage HEPA filter was needed, due to
this issue. A replacement demister/de-entrainer for this exhaust unit
needs to be procured and installed.

The safety systems will need to have the role of each unique piece of
equipment defined. These requirements need to be defined in an update to
the RPP Safety Equipment List.

DOE employee who reviewed this assessment \_\_\_[DOE Review
Team]{.underline}\_\_\_\_\_ Date \_\_[2/28/01]{.underline}\_

(\*See below)

Hours required for completing the assessment.

DOE: [40]{.underline} Hrs.

Contractor: [240]{.underline} Hrs.

\*Harkins, BA

Harp, BJ

Harwood, RG

Noorani, YG

Sullivan, RS

Trenchard, GD

Wade, KG

Williamson, BI

Wright, TC

[^1]: NOTE: Safety-Significant (SS) and Defense-in Depth (DID) SSCs are
    to be addressed in a later CRAD submittal. However, confinement
    ventilation systems were to be addressed in this submittal.
    Therefore, periodic functional testing to confirm HEPA filter
    integrity and confinement performance will be presented here, but
    other SS and DID TSR controls (e.g., Stack CAM Interlock and HEPA
    Filter Differential Pressure Interlock) associated with a loss of
    filtration analyzed accident will be part of the next CRAD
    submittal.

[^2]: ^2^ Recent remediation efforts at Tank 241-SY-101 have reduced the
    gas generation rate by a factor of 7. Also, reanalysis of flammable
    gas generation rates in other tanks have extended the period to
    reach LFL concentrations. Approval of these new analyses into the
    FSAR is pending and in process.

[^3]: ^2^ Recent remediation efforts at Tank 241-SY-101 have reduced the
    gas generation rate by a factor of 7. Also, reanalysis of flammable
    gas generation rates in other tanks have extended the period to
    reach LFL concentrations. Approval of these new analyses into the
    FSAR is pending and in process.