ML20195J663
| ML20195J663 | |
| Person / Time | |
|---|---|
| Site: | Portsmouth Gaseous Diffusion Plant |
| Issue date: | 06/16/1999 |
| From: | Yawar Faraz NRC OFFICE OF NUCLEAR MATERIAL SAFETY & SAFEGUARDS (NMSS) |
| To: | Pierson R NRC OFFICE OF NUCLEAR MATERIAL SAFETY & SAFEGUARDS (NMSS) |
| References | |
| NUDOCS 9906210077 | |
| Download: ML20195J663 (27) | |
Text
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June 16, 1999 MEMORANDUM TO: Robert C. Pierson, Chief Special Projects Branch
- Division of Fuel Cycle Safety and Safeguards FROM
- Yawar H. Faraz, Pvtsmouth Project Manager Enrichment Section d SM Special Projects Branch Division of Fuel Cycle Safety and Safeguards
SUBJECT:
MEETING
SUMMARY
- PORTSMOUTH GASEOUS DIFFUSION PLANT (PORTS) MODERATION CONTROL TECHNICAL SAFETY REQUIREMENT (TSR)
On May 19,1999, Nuclear Regulatory Commission (NRC) staff met with United States Enrichment Corporation (USEC) staff to discuss USEC's intent to amend the PORTS TSRs which deal with moderation control of greater-than-safe-mass UO2 F2 deposits (also known as planned expeditious handling (PEH) deposits) in cascade equipment. A list of attendees is ,
contained in Attachment 1. '
At the meeting, USEC staff provided a handout describing the intended changes and the reasons and justifications for the changes. This handout is contained in Attachment 2. USEC staff indicated that the amendment would positively impact USEC's efforts to remove all PEH j deposits from cascade equipment. NRC staff suggested that this and other positive safety impacts as well as any negative safety impacts of the amendment should be described in the j application USEC staff indicated that the certificate amendment request would likely be ;
submitted by the end of June of this year.
Attachments: (1) List of Attendees I (2) USEC's Handout cc: Steve Toelle, USEC /
Docket 70-7002 l G
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3 22 DisTRIBuTloN:
Docket 70-7002 09#tC FHe Center ' PuBLIC NMss r/f Ko'Brien, Rtil CBlanchard. Rill FCSs r/l sPB r/l KWinsberg, oGC " DHartland, Rill WTroskoski, FCoB PHiland, Rill YChen CCox CTitpp LBerg JDavis
'see previous concurrence oFC sPB b SPB _sPB NAME YFaraz , edley y DATE 6/ l 7/99 6/ /b /99 & k /99 / /99 / /99 C = COVER E = COVER la ENCLOSURE N = No COPY G:\sPB\MTsM.ModCntri.PoR.wpd olTICIAL RECORD COPY 9906210077 990616 PDR ADOCK 07007002 C PDR I
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NRC ATTENDANCE SHFET MEETING: fu FsmJ N h4 (bb( r.IR DATE: s-/ t9 :' 19 Name Affiliation Phone faaut. -fgc7- N&(- FDS -3P6 JoI tlI T-BII3 fcl 0ynof (NE(.' - Poet 1 7YO-897-Y70; kl-Eb V -34y AA/(' M 17// VSEC - & Htson- __
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d HANDOUT l NRC AND USEC MEETING PORTSMOUTH GASEOUS DIFFUSION PLANT TECHNICAL SAFETY REQUIREMENT MODERATION CONTROL FOR URANIUM DEPOSITS MAY 19,1999 9:00 A.M.
U.S. NUCLEAR REGULATORY COMMISSION TWO WHITE FLINT NORTH BUILDING .
ROOM T-7C1 Attachment 2
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1 Specific Changes to PORTS Moderation Control TSRs Attached TSR 2.2.3.15 Redline / Strikeout Version Example !
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- 1. Condition A The wording in the Condition A was revised to clarify that a i fluorinating environment includes chemical treatments which utilize fluorinating agents CIF3/F2- l
- 2. _ Required Action A.! This change adds "for the deposit" to the required action to clarify and support the condition statement.
- 3. Required Action A.2 The need to determine each deposits significance at the point of discovery has been eliminated. The significance of a deposit in the cascade has already been evaluated from an " envelope" perspective in the SAR criticality scenarios (Reference 1) and the appropriate site NCSA/NCSE(s). The commensurate ;
actions from the evaluations are reflected in the Moderauon i Control TSRs. I
- 4. Required Action A.4 The surveillance action was added to provide a cross reference action to the required TSR surveillance which supports the ,
developed plan of action. The completion time is consistent l with the surveillance frequency.
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- 5. Condition B The reference to "with the deposit", "in Mode VI", and
" chemical treatments are not in progress" are redundant information to the condition "UO2F2 > safe mass not in a fluorinating environment" and are removed.
- 6. Required Action B.I.1 The requirement to have a dry cover gas blanket when the deposit is not in a fluorinating environment is still maintained.
The change is intended to reflect that it is not feasible to perform the functions necessary to remediate a deposit and still maintain the dry cover gas blanket at all times. These dry cover gas blanket requirements minimize the time available for wet air inleakage while taking into consideration the time that is required to conduct transition activities associated with deposit remediation, such as equipment removal, leakrates, evacuations, cell startup, etc.
- 7. Required Action B.I.2 Reference to surveillance provides added assurance of compliance.
- 8. Required Action B.2 "> safe mass" was removed due to its redundancy to the LCO statement.
- 9. Required Action B.3 This new action reflects one of three options that exist in regards to the handling of a deposit. The three basic options are to 1) maintain or place the deposit in a fluorinating
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environment 2) buffer the deposit and 3) physically remove the deposit. It should be noted that a fluorinating environment can also include the performance of chemical treatments which is I the optimal method for remediation of a deposit.
- 10. Required Action Note This action re-enforces that upon completion of action B.3, the l condition that would then apply to the deposit is condition A. l 1
- 11. Condition B Completion Times The referenced (8,9,10) NCSA/NCSE(s) for remediation of deposits recognize that it is impossible to prevent certain levels of wet air inleakage while removing equipment from the {
cascade or performing maintenance in which the containment
)
boundary of the cascade is breached. This inleakage is minimized to the greatest extent possible. Technical studies and Engineering evaluations as discussed in the amendment justi0 cation, have determined that minimal amounts of water via moist-air inleakage will be introduced during periods when the buffer is not maintained, even during extended periods of time (e.g.,1.6 or longer years). This amount of moist air is less than required to cause a criticality, even in a large deposit.
Furthermore, the re-application of a fluorinating environment or a dry cover gas blanket tends to have a drying effect on the deposit and will return the deposits H/U ratio to its originally low value. The completion time for B.I.2 is consistent with the surveillance.
- 12. Condition C This new condition was added to address the situation where !
there are at least two deposits greater than a safe mass in the same proximity with only one of the deposits being physically removed. The specific actions are designed to ensure the deposits are not moderated while at the same time allowing for necessary maintenance work to proceed. These changes recognize that the transition from deposit conditions of either a fluorinating environment or a dry cover gas blanket to the performance of chemical treatments (which can include the re-installation of removed equipment necessary to support the chemical treatment process) is not instantaneous but requires actions which temporarily discontinue the dry cover gas blanket and allows inleakage of atmospheric air due to " breathing" The performance of these transitional actions i essential to the ultimate removal of the greater than safe mass deposit. As discussed in item 11 of the justification section, this inleakage i
is not significant from a nuclear criticality safety perspective.
- 13. Condition D This condition which was condition C in the existing TSRs removed " system is in a shutdown mode" which has no significance to the moderation control issue of providing added
. assurance that liquid water could not leak into the deposit area l from the RCW condenser. The addition of " system" only
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provides a more descriptive adjective for the coolant pressure.
- 14. Required Action D.l.1 Based c,n the intent of Condition D, this change to add l
" condenser" serves to clarify which RCW pressure is essential to perform the action. At PORTS, the cascade building RCW pressure is read and then the head loss to the RCW condenser is subtracted to obtain the condenser RCW pressure. The added surveillance reference increases the assurance level that the required TSR surveillance will be performed as required.
- 15. Required Action D.I.2 Reference to surveillance provides added assurance of compliance. l
- 16. D.I.2 Completion Time The completion time for D.I.2 is consistent with the ,
surveillance. I
- 17. D.2 Completion Time The change from 16 to 20 hours2.314815e-4 days <br />0.00556 hours <br />3.306878e-5 weeks <br />7.61e-6 months <br /> was made to provide the time necessary to coordinate and complete the draining effort. The additional 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> will not invalidate the NCSA/NCSE(s) that support double contingency for the prevention of liquid RCW entering a cell via the condenser.
- 18. Condition E This condition was added to address the situation in which a buffer is not applied or re-applied as required. The E.1 action and the completion time are consistent with the existing surveillances 2.2.3.15.2/2.7.3.14.2 which address ensuring a proper buffer on a shiftly basis (TSR section 1.3, Use and Application).
- 19. SR 2.2.3,15.1/2.7.3.14.1 The surveillance was changed to provide a more explicit cross ;
reference to the related Condition D and Required Actions.
- 20. SR 2.2.3.15.2/2.7.3.14.2 The change to this surveillance frequency is made to cross tie the surveillance to the appropriate Condition B or C, where this surveillance is invoked.
- 21. SR 2.2.3.15.3/2.7.3.14.3 The change to this surveillance frequency is made to cross tie the surveillance to the appropriate Condition A, where this surveillance is invoked. Also the surveillance was reworded to eliminate redundant wording which did not change the surveillance intent.
- 22. Basis Changes to the Basis were editorial and restructuring in nature.
The justification associated with technical issues discussed in the Basis to support TSR changes can be found in the corresponding TSR amendment justification section.
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l . .TSR-PORTS PROPOSED NOVEMBER 21,1997 i
RAC97X0422 -
SECTION 2.2 SPECIFIC TSRs FO.R X-330 AND X-333 FACILITIES
! 2.2.3 LIAIITING CONTROL SETTINGS, LISIITING CONDITIONS FOR i OPERATION, SURVEILLANCES 2.2.3.15 Moderation Control Applicability: Cascade Operational Modes I, II, III. IV, V, VI LCO: Moderation Control shall be maintained when the UO2F2 mass is > safe inass.
ACTIONS: Note: TSR 1.6.2.2(d) does not apply Condition Required Actions Completion Time A. UO:F3deposit > safe mass in a A.! Contmue to maintain a fluorinating Irnmediately fluorinating (including chemical environtneht for the deposit treatment) environment. ANV A.2 Irunate accons to determme the cause of Immediately deposit ..-J .m ,,,mu~mnce.
ASV A.3 Establish and document a plan of action 30 days ASV A.4 Initiate SR 2.2.3.15.3 90 Davs B. UO,Fideposit > safe mass with the B.I.! Establish a dry cover gas blanket at Within-e 72 Hours after deposit not in a fluormatmg " .x x ;pr .-d p' c "Ma 14 entering Mode VI1rnd environment i M i * .J ,,. .J
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psia cacept when performing maintenance :p"-.= "F, o,a. ,..,.<,,c.v,,.m., or operational activities associated with negarve remediation of the deposit. equipment removal or leak repair.
ASV B.I.2 Initiate SR 2.2.3.15.2 12 Ilours AND B.2 Remove equipment containing the UO:F3 +00 366 days deposit ; a.L ou.5; from the cascade OR Note: Upon completion of B.3, Condition A is re-entered.
B.3 Initiate re-fluorination activities Within 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> of removal of dry cover gas blanket C. Installed equipment containing known C.! Apply TSR 2.2.3.16 as appropriate to immediately or previously unknown deposit of equipment removed UOaF deposit > safe mass opened to AND atmosphere. C.2 Cover opening (s) that expose UO3 F, Immediately after ,
deposit to atmosphere when maintenance determining acceptable evolutions are agt impacting equipment. UF/IIF conditions AND C.3 Maintain dry cover gas blanket a 14 psia Within 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> after when cascade system maintenance completing REQUIRED evolutions are act impacting equipment. ACTION C.2 AND Note: Upon completion C.4, Condition D is re-entered.
C.4 Maintain dry cover gas blanket a 14 psia Within 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> after following completion of cascade system completing system maintenance on affected equipment and maintenance UO(, deposit is not in a fluorinating environment.
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2.2-27 l
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. TSR-PORTS PROPOSFS NOVEMBER 21,1997 RAC97XO422 SECTION 2.2 SPECIFIC TSRs FOR X-330 AND X-333 FACILITIES 2.2.3 LIMITING CONTROL SETTINGS, LIMITING CONDITIONS FOR OPERATION, SURVEILLANCES D. Uo,F deposit > safe mass, not in a D.I .I Increase coolant system presmre to > 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> fluorinanng environmen;. w ... ,, , . RCW condenser pressure.
J.a-.. me and coolant systern AND pressure s RCW condenser pressure. D.1.2 initiate SR 2.2.3.15.1 12 Ilours OR D2 Drain RCW from coolant condenser 20 M hours E. UGaF deposit > safe mass with the E.1 Re-establish a dry cover gas blanket a 14 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> deposit not in a fluorinatmg psia.
environment and required dry cover gas blanket < 14 psia.
SURVEILLANCE REQUIREMENTS:
Frequency Surveillance Each shift when not in a fluorinating SR 2.2.3.15.1 Verify coolant system environment, deposit > safe mass and pressure > RCW condenser pressure.
RCW not drained l Each shift when in dry cover gas blanket is SR 2.2.3.15.2 Monitor the system pressure l required by Condition B or C and adjust pressure to a 14 psia.
Quarterly when in Condition A SR 2.2.3.15.3 Monitor UO F, deposit >
safc nia55 for size of the deposit when in a fluorinating cnvironnicnt. j Quarterly SR 2.2.3.15.4 Perform routine qualitative radiation surveys of bypass housings to check for deposits and initiate "NDA" quantitative measurements based on
" radiation reading trending" 2.2 28
TSR-PORTS PROPOSED NOVEMBER 21,1997 I
RAC97X0422 SECTION 2.2 SPECIFIC TSRs FOR X-330 AND X-333 FACILITIES 2.2.3 LIMITING CONTROL SETTINGS, LIMITING CONDITIONS FOR OPERATION, SURVEILLANCES 2.2.3.15 Moderation Control (continued) 1 BASIS:
As used in this TSR, the term " safe mass" is defined as being 43.5% of the minimum l fissionable mass for systeni conditions (enrichment, geometry, H/U, reflection , etc.). Cascade l
deposits of UO F 2(and deposits of other compounds resulting from wet air inleakage) and freeze-out of UF6are an expected result of normal operation. It is considered non-credible for a dry criticality to occur in the Cascade. Therefore, for a freeze-out condition, criticality l would not result and the UF 6freeze-out may be remediated at the discretion of the operating organization. Any deposit that has a uranium mass less than the "always safe mass (i.e.
l optimally moderated material) and may be remediated at the discretion of the operating organization. In regards to those situations in which a loss of moderation control could result in criticality, it has been determined that NCSA specified controls provide double contingency against the inleakage ofliquid water into the cascade. Based on additional technical evaluations it is not possible to hydrate a deposit of uranyl flu' o ride above a H/U ratio of 4 by I
exposure to ambient air within the proce'ss buildings. Therefore, there is no potential for l criticality when a cascade deposit is less than the safe mass at a H/U ratio of 4 due to exposure l to atmospheric water vapor in the ambient process building air.
UF6, F 2, and CIF 3react with available water more readily than UO2 F absorbs water. For
- instance, water entering onstream cascade equipment will preferentially react with UF to form l more UO F
- rather than react with UO2 F2 to form hydrates (moderated forms) of UO 2 2F . IIF gas formed as a byproduct of the water-UF6 reaction cannot liquefy to moderate a deposit at the pressures encountered in the cascade. A UO F2 deposit cannot become moderated ifit is being continuously fluorinated and moderation is not a concern until the equipment is taken off-stream and evacuated of UF , Continued fluorination of the deposit provides nuclear I criticality safety by preventing moderation of the deposit.
Chemical treatment processes which involve the addition of CIF 3 and/or F 2 (i.e. fluorinating agents) provide the same level of moderation control as when the deposit is exposed to UF6 .
Fluorinating gas treatments have been used as a means of drying out equipment after exposure to atmospheric air and for removing / reducing uranium deposits since the enriclunent plants were placed into service. It has been demonstrated that these fluorinating agents will react vigorously and preferentially with any available moisture. The presence of excess l
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2.2-29 u
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TSR-PORTS PROPOSED NOVEMBER 21.1997 i
RAC97X0422 SECTION 2.2 SPECIFIC TSRs FOR X-330 AND X-333 FACILITIES 2.2.3 LIAIITING CONTROL SETTINGS, LI51ITING CONDITIONS FOR OPERATION, SURVEILLANCES 2.2.3.15 Moderation Control (continued) j l
fluorinating agents will not only prevent further hydration of a deposit but will over time effectively remove any free moisture and dehydrate the exposed deposit to an H/U ratio as low as when the deposit was exposed to the UF6 process. In addition, use of fluorinating agents ,
will convert UO F deposits to UF ,6thereby reducing the deposit mass. Repeated use of the 2 2 I fluorinating agents (i.e. chemical treatment) will proceed to reduce / eliminate the deposit which is the safest condition. Therefore, a deposit that has been hydrated to some extent due to
" breathing" or during the times necessary to expose the deposit to atmosphere when I maintenance functions are performed can be dehydrated by the presence of a fluorinatina agent. The sustained liberation of UF 6from the deposit during a chemical treatment is the l proven indicator that the deposit has been dehydrated. Once a deposit has been dehydrated, re- i entry into Condition B establishes a new initiating time for required actions. After having been i exposed to a fluorinating environment in which there has been the sustained liberation of UF 6, the re-entry to the buffered condition for one year will not decrease the assumed safety margin '
for this condition. Chemical treatment activities as discussed in this LCO may include preparation activities such as evacuation, leakrate, seal checks and cell startup.
UO2F2deposits in onstream operating equipment are not a nuclear criticality safety concern due to continuous fluorination of the deposit. Over time, sustained or large wet air inleakage in operating equipment (active process area) will readily announce itself in the form of changing motor loads, compressor ' surging, line recorder readings , stage control valve positions A-suction pressures, etc.. Additionally, deposit formation in operating equipment will be dispersed by the gas flow. This dispersion of UO 2F2can occur on the inside of process piping, across barrier tubing, on cooler fins and inside compressors on the rotor and stator. Due to this dispersion, the formation of deposits in unsafe geometries in active process areas where there is UF6gas flow is not likely, given the above indicators. However, the above mentioned indicators and continuous gas flow are not always available for wet air inleakage in bypass / auxiliary piping, expansionjoints and valves (inactive process areas). Operational experience indicates that quarterly surveillances by NDA methods for UO2 F2 deposits in inactive process areas is appropriate for early detection and prudent remediation of the deposit. I Follow up ' surveys are conducted to assure that the deposit does not become sufficiently large to become an operational problem or a cascade structural concern.
Routine NDA surveillance methods are of limited value (e.g. with respect to quantification of deposit size) for active process areas which include compressors, converters, process gas coolers and freezer /sublimers. However, sustained or large wet air inleakage in active process areas will readily announce itself which will prompt corrective actions by operating personnel.
Also, the formation of UO F2 deposits 2 in unsafe geometries in active process areas is not likely given the above corrective actions. The limited ability to hydrate a deposit in in-place process 2.2-30
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[ . TSR-PORTS PROPOSED NOVEMBER 21.1997 RAC97X0422 SECTION 2.2 SPECIFIC TSRs FOR X-330 AND X-333 FACILITIES 2.2.3 LIMITING CONTROL SETTINGS, LIMITING CONDITIONS FOR (
OPERATION, SURVEILLANCES I l
i 2.2.3.15 Moderation Control (continued) l equipment assures that these deposits will remain critically safe after shutdown. Thus the primary concern for the formation of UO F2 deposits 2 in unsafe geometries in operating equipment is if this equipment trips or is shutdown while containing UF6 and massive wet air I inleakage occurs. In this event, the wet air inleakage will be obvious from the equipment leak l rate which will prompt corrective actions to limit the size of the deposit.
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For shutdown equipment, moderation control can be provided by a fluorinating environment or ,
by a dry cover gas (plant air or nitrogen) blanket over the deposit even if significant wet air )
inleakage has occurred. Once a system has been isolated from the cascade and filled to 214 psia with dry gas blanket, normal atmosphere pressure fluctuations may cause minor in and out I flow through any existing system leaks. Analyses have demonstrated that this " breathing" of {
the cell or even the exposure to atmospheric air (diffusion) when the system is opened to allow I for necessary maintenance will not significantly affect deposit moderation. Even for periods much longer than the one year limitation, moderation above an H/U ratio of 4 would not be experienced. The daily surveillance demonstrates that the gas blanket is maintained as assumed in the analyses. The LCO requirements of this TSR assure nuclear criticality safety for equipment with UO 2F 2deposits greater than a safe mass.
Maintenance evolutions or cascade system maintenance terminology, used in the Required Action statements, include other related tasks such as decontamination and sampling.
Condition C is considered to be met when the UF primary system is first breached. Also the potential for moderation from RCW system water is controlled by NCSA requirements and demonstrated to meet the double contingency principle.
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in the gaseous diffusion process, significant quantities of gaseous uranium hexafluoride (UF,) l are processed through thousands of feet of process piping and thousands of equipment l components. Considering a system of this size and complexity, it is anticipated that equipment failures of various types will lead to inleakage of air, and therefore moisture, into the cascade, particularly in those areas that operate at below atmospheric pressure. Since the diffusion process employs uranium in the form of gaseous UF which has a propensity for reacting with water to form solid uranium compounds, the formation of solid deposits within the system is npected. These deposits generally occur, in the case of airborne moisture, as a result of the j following chemical reaction. '
UF6 + 2H 2O - UO2 F + 41IF Moist air can enter the operating cascade through leaks in expansion joints, flanges, compressor seals, welds, etc. The moisture in the air will react with UF, to form intermediate i uranium oxyfluoride compounds and HF, along with UO:F2 . When an excess of UF is l present at a deposit location, the reaction will form other intermediate uranium oxyfluoride ;
compounds, such as UOF 4 , U2 03 F6 , and U3 03F , with associated HF, that are not fully hydrolyzed. Once these compounds are formed, if UF is removed the intermediate compounds can react with any additional moisture to form UO:F 2 and HF. The deposits will l begin to accumulate on the piping, converter shell, barrier tube-sheet and barrier etc.
depending on the location of the inleakage. As leakage continues, deposits will continue to i I
accumulate at a rate dependent on the inleakage rate, which is a factor of a number of variables including hole size, differential pressure at the hole and the moisture content of the ambient air. These deposits cannot be hydrated while in operating equipment in the cascade. The H/U ratio will remain low (i.e. H/Us 0.33, SAR C.1.2.3.2) and water will not be available to serve as a moderator as long as there is UF6 gas flow that continues to react with the moisture and allows the relatively light HF gas to quickly move up the cascade and away from the deposit.
There are two types of uranium deposits that exist in the cascade. The first type is UQF2 deposited in a very diffuse and reasonably uniform film or layering on inner surfaces; usually formed in the presence of UF gas flow and less than or equal to an inch thick. The second type is larger, localized deposits of UO 2F2which are normally formed in reduced / restricted UF flow areas and also are usually thicker and have less surface area. Deposits are ultimately detected through routine surveys or by changes in cascade performance.
1 The cascade NCSA/NCSEs(References 8,9 & 10) place controls on the enrichment and moderation parameters to ensure, where applicable, double contingency (SAR Section 5.2) is i
met for the uranium deposit condition. The referenced NCSA/NCSEs support that double contingency against the loss of moderation control due to the inleakage of liquid water into the cascade is met. The referenced NCSAs/NCSEs still contain the current TSR requirements and l will be revised as appropriate to reflect the final results of this request. The loss of moderation
! control due to the exposure of a UO F2 deposit to moist air is determined to not meet double l contingency for all deposits and is therefore the basis for these TSRs. It should be noted that a l deposit whose mass is less than or equal to the " safe mass" which is defined as 43.5% of the minimum fissionable mass for system conditions (enrichment, geometry, H/U, reflection, etc.),
can not become critical and therefore does meet double contingency. TSRs are required for single contingent events, i.e. when a single event can directly result in a criticality. This single contingent condition does not occur until there is a deposit that equals or exceeds the minimum critical mass at an H/U ratio of 4, for example: at 3% enrichment the minimum fissionable mass equals 951 pounds of UO 2F2, at 5% enrichment the minimum fissionable mass equals 513 pounds of UO2 F and for a 10% enrichment it is 285 pounds of UQF 2-Correspondingly, the respective safe mass values at which the TSR controls are invoked, are as follows: 414,223 and 124 pounds of UO:F 2-The proposed changes to these TSRs and the associated justification are intended to bring the TSR controls into perspective with the analyzed criticality risk due to wet air exposure.
As long as a deposit exists in a fluorinating environment (UF., F2 , CIF3 or a mixture there of) there is no potential for a UQF: deposit to become hydrated or moderated. In fact, the Duorinating gases will react with any water vapor and the hydrate to " dry out" the UQF2 deposit. Fluorinating gas treatment using F 2and/or CIF ,3 sometimes referred to as " chemical treatment", has been used as a means of drying equipment after exposure to atmospheric air and for " moving deposits since the initial operation of the enrichment plants. These Duorinatmg agents react vigorously and preferentially with any moisture in the gas phase and then with any hydrate water that is present. Once the moisture has been consumed, the fluorinating agents are then available to react with the UQF2 deposit to convert it back to UF.. Therefore, the exposure of a deposit that contains some moisture, introduced during those periods in which the deposit was open to the cell Door ambient air, to a Duorinating environment will remove moisture from the deposit. Removal of the deposit by fluorination reduces the need to process solid uranium material through a liquid decontamination process, thereby eliminating the need for mechanically removing the deposit with the associated risks to workers and the increased risk of exposing the deposit to liquid water once removed from the cascade. The preferred method for remediation of a deposit is via chemical treatment.
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The chemical reactions for fluorinating agents and moisture and/or UO:F2 water of hydration i in deposits have extremely high equilibrium constants. Equilibrium constants indicate how favorable and fast a reaction will proceed. The reactions of the fluorinating agents with moisture are shown below along with their equilibrium constants @ 77'F (Reference 6):
UFu + 2H20g3 - UO 2F 3,3 + 411Fq3Equilibrium Const. = 2.6x10" l
l ' 2CIFu + 2H O 2 >
g - ClO Fa3 2 + CIFg3 + 4HF g3 Equilibrium Const. = 4.0x10' 3CIF% + 3H2033 - ClO 3Fq3 + 2CIFq3 + 6HFe ) Equilibrium Const. = 7.5x10"'
2Fu + 2H O > - Ou + 4HF , Equilibrium 2 g g Const. = 1.7x10" The equilibrium constant is a function of the ratio of the concentration of reaction products to the concentration of the reactants present. A large equilibrium constant would mean that the reactants would form reaction products until the reactant with the lower concentration is l essentially used up. By maintaining an excess of fluorinating agents, it is possible to make water the lower concentration reactant and reduce the water concentration to levels equivalent to those experienced in the cascade by converting the water to HF. Equilibrium constants are therefore a means of evaluating how favorable a reaction is. The equilibrium constants for all the reactions shown above indicate that free water will be consumed. Based on the l
hygroscopic studies (References 4&S) and plant experience exposing the deposits to temperatures above the 77 F will improve the removal of water from the deposits above that indicated by the equilibrium constants. Re-fluorination of a deposit will react directly with the moisture adsorbed on the deposit forcing additional dehydration of the deposit according to the following reactions:
UFu + 1.25(UO F2 *1.6H 2 O)<,3 2 - 2.25UO 2F 2 + 4HFg3 Equilibrium Const. = 4.6x10 2 1 i
2ClFu + 1.25(UO F2 *1.6H 2 O)(,3 2 - ClO2Fu, + CIFg3 + 4HFg, + 1.25UO 2F 3,3 Equilibrium Const. = 7.0x10" l Fu + 1.25(UO F2 *1.6H 2 O)<,3 2
1.25UO2 F 2 + 4HF g > + Og Equilibrium Const. = 2.0x10*
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. 1 The high reactivity of these fluorinating agents with free water vapor and hydrated uranium compounds explains why exposure of the deposit to a fluorinating agent will redry the deposit back to its original low H/U condition (operating cascade). Since the deposits under discussion have been formed under cascade operating conditions, they are essentially dry while they remain in operating equipment where the UF. continues to react with any moisture and the resultant HF flows up the cascade. Should such a deposit become exposed to ambient wet air outside of the cascade operating conditions and there is no fluorinating agent present, the surface of the deposit can begin to hydrate. However, due to d e fact that during this period the deposit is going to be cycled through chemical treatments ani/or dry gas buffering,
! hydration level remains on the surface. Even if the deposit was again exposed to UF, such that !
the deposit could become larger, the UF. would again react with the available moisture as it
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formed additional deposit material, thereby maintaining a dry deposit. Regardless of how many times this cycle is repeated the hydration of a deposit whether thick or thin would be confined to the immediate surface area and would be accessible to the fluorinating agents that would react with the water and produce a dry deposit. The use of chemical l
treatments / fluorinating agents to transition back to a fluorinating environment poses no '
additional risk and supports the concept of resetting the " clock" or re-establishment of the ;
time frame for which a deposit in a buffered environment can safely remain in the cascade.
l During the time a deposit is in condition A and is exposed to UF. there is a potential for the l
deposit to become larger. It is documented in NCSEs (References 8,9&l0) that an ,
i unmoderated deposit cannot become critical. TSR surveillance 2.2.3.15.3/2.7.3.14.3 provides for monitoring of the deposit to alert personnel to any potential structural or operational I concerns. For the other conditions, UF is isolated from the deposit and will not become ;
larger.
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Studies (References 4 & 5) of the hygroscopic properties of uranyl Duoride (UO;F 2) have been conducted and provide a basis for extending the time frames that a greater than safe mass I deposit can be exposed to wet air without a significant increase in the probability of a l criticality. It is known that UO F2 will 2 absorb moisture and form hydrates. The degree of hydration is a function of temperature, the partial pressure of the water vapor and the deposit surface area available to be contacted with the moisture present in the gas phase. In the cascade fluorinating environment, the UF 6and other fluorinating gases present are much better i
desiccants than UO2 F2 and will preferentially react with any water that enters the process equipment. The hydrates range from 0 to 4 moles of H 2O per mole of UO 2F2depending on the temperature and relative humidity (RII) of the ambient air. Correspondingly the II/U ratio ranges from 0 to 8. The studies concluded that deposits when exposed to RII in the range of 50 to 60 % formed stable hydrates that contained 1.6 to 2.0 moles of H2 O per mole of UO F2 or li/U ratios of 3.2 to 4.0. The test material did not experience deliquescence until the RH was at 100%(entrained liquid droplets) and the temperature was below 95 F. The associated experiments revealed that deliquescence / hydration did not occur when the source of water was separated from the deposit in a static environment. Apparently, where the transfer of moisture must occur by pure gaseous diffusion, the diffusion rates are not suf ficient to hydrate past a H/U=3.2 and deliquesce the deposit. The existence of 100% RH and/or the presence of a constant moisture laden air flow over the deposit is not credible for a functioning gaseous diffusion facility. It should be noted that the associated experiment utilized a thin layer of fine UO2F2powder which is not characteristic of a cascade uranium deposit that exceeds a safe mass. Regardless of the deposit geometry, i.e. relatively thin slab or as a larger mass the hydration occurs principally at the surface and diffuses inward. As the deposit is exposed to Duorinating agents and the water is reacted, there is a point reached when the water is consumed and the liberation of UF6 occurs. The liberation of UF 6is therefore the indicator that a deposit is dry. This conclusion is derived from the many cell treatments that have been performed. Relative humidity on the process building cell Door is normally considerably less than the RH available from the ambient outdoors. Cell Door temperatures are typically 20 to 80 degrees above the summer / winter outdoor temperatures. Relative humidities will decrease with a rise in temperature without any corresponding addition or removal of moisture. Based on local meteorological RH data the cell Hoor humidity will be normally less than or equal to 60%. Should there be periods of time in which the cell floor relative humidity is above 60%,
it will not significantly affect the hydration of uranium deposits in buffered equipment nor will it invalidate the calculations discussed bdow(Reference 7) which assumed a RH of ambient air through out the course of a year. It is therefore concluded that the maximum H/U ratio of a hydrated UO2F2deposit in the cascade due to exposure to wet air is 4. It is further evident that there is no potential for loss of moderation control of a cascade deposit which is less than the minimum critical mass at an H/U ratio of 4 from inleakage of atmospheric water vapor.
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The criteria used to identify deposits exceeding " safe mass" for application of TSR moderation control for wet air inleakage, is based on a minimum critical mass at an II/U ratio of 4. The criteria is conservative, since the minimum critical mass value of a UQF 2 deposit at an II/U of 4 assumes optimum geometry (i.e. sphere) and full renection (i.e. I foot of water or equivalent encapsulation) which has never been observed for applicable cascade deposits. SAR Section C.1.2.3.1 provides the relationship, based on modeling calculations for various cascade conditions, using centrifugal compressor geometry with reflection from the cell floor and i varying enrichment, deposit size and moderation levels. Specifically, Fig. C.1-4 illustrates a realistic cascade case, in which at a H/U=4,406.5 pounds of 10% enriched uranium in a compressor would have a K, of 0.66. Correspondingly, utilizing the current safe mass criteria (43.5% of minimum critical mass at II/U=4 and 10% enrichment) TSR moderation control actions would be taken for a deposit equating to 124 pounds of UQF 2 . Comparing these results show the conservatism between actual cascade conditions and the theoretical basis used (i.e. optimum geometry and full reflection) to determine the need for deposit buffering.
Calculations have been performed (DAC-XSITE-E18892R14-NS-01 and 02 which are included in Reference 7) that indicate it would take 1.6 to 9.6 years, depending on cell size to moderate a deposit with a minimum critical mass (at a H/U=4) to a H/U of 4 by initially pressuring up the cell to atmospheric pressure by' ambient outdoor air and allowing the cell to continue to
" breath" due to changes in the barometric pressure. The influx of moisture from the atmosphere into shutdown equipment is limited based on two mechanisms: 1) " breathing" of the cascade equipment, due to changes in atmospheric pressure and 2) molecular diffusion.
The barometric pressure varies approximately 0.1 psi / day ( the maximum observed daily variation in barometric pressure during 1995 was 0.7 psi). The barometric variation during storms is on the order of 0.2-0.3 psi / day. The mass of the cascade, including the structure, provides a massive heat sink that prevents large short term temperature swings on the cell floor due to ambient outdoor temperatures and therefore will tend to maintain constant cell floor humidity.
Molecular diffusion , with no driving force other than the difference in water vapor pressure from the outside to the inside of the system / equipment, is an extremely slow process and is trivial in comparison to the " breathing" of the container with changes in atmospheric pressure.
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4 4 The studies (References 4 & 5) indicate that a container with a static atmosphere and 100% RH did not cause deliquescence of UO2 F2 as long as the UO2 F2 was physically separated from the source of water. This indicates that the diffusion of water in a static atmosphere does not provide an adequate driving force to hydrate a deposit. At the Oak Ridge Gaseous Diffusion Plant cascade equipment in the K-29 Building was shut down in 1985 (Reference 11). Since shutdown, the process system remained essentially closed and unbuffered(buffering removed in 1991) until the start of the deposit removal project in 1997. However, since discontinuation of l enrichment operations, personnel have on occasion removed some process equipment items for shipment to other enrichment facilities . These equipment removal activities created large openings in the process system which allowed moisture laden air free access to the deposits.
During the 12 years the equipment was shutdown daily fluctuations, in temperature and pressure resulted in the exchange of ambient air into the process system. NDA surveys identified 32 deposits with greater than 550 grams2 "U with a potential enrichment of 12.5%
based on historic data. A 1995 analysis also concluded that several of the K-29 deposits contained a mass greater than the critical mass quantity at a H/U ratio of four. This is significant because UO2 F2 deposits that have been exposed to ambient air are theoretically expected to absorb moisture up to approximately this level of moderation. Visual observations of the deposits to be removed indicated that they were hard and required mechanical methods (i.e. shovels, scrapers, etc.) to physically remove the deposits. Deposit hardness of this degree is consistent with the criticality risk assessment which assumed a maximum H/U of 4. During the removal process it was also identified that in some cases HF would still evolve from the deposit when exposed to ambient air. This is indicative of a deposit that contained intermediate oxyfluoride compounds which have not fully reacted (
hydrolyzed) by the exposure to moisture while in the cascade. The same basic K-29 findings were evident in the deposit removal activity for the K-25 HEU equipment that had experienced many (20 to 30) years of uncontrolled exposure (i.e. equipment removed without coverings) to atmospheric conditions at the Oak Ridge Gaseous Diffusion Plant (Reference 13). These openings allowed the unimpeded diffusion of moist air through the process equipment. As expected, personnel removing additional equipment did not witness any moisture collecting in low points of the process equipment. Furthermore, examination of some deposits revealed that they were not moist but rather the surface of the deposits had hardened, significantly limiting the deposits' solubility or receptivity to hydration. As described, it takes on the order of years for water molecules to contact the UO:F2 surface area to form a hydrated deposit. The same experience would indicate that the penetration of the water molecules into the deposit is self limiting from which it can be concluded that deliquescence is not credible.
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The hydration calculations (Reference 7) are believed to be extremely conservative for the following reasons:
- 1. The calculations as. ame RHs and moisture content associated with the outdoor ambient air and do nut consider the reduction in RH due to cell floor heating and all water entering the system remains.
- 2. The calculations assume that none of the moisture that enters the cell is consumed by the significant barrier area which will adsorb moisture.
- 3. An additional conservatism is that deposits of this size (greater than minimum critical mass at an H/U of 4) are very unlikely to be present in a compact (e.g.
optimum geometry) orientation. In most cases visually inspected to date, such uranyl fluoride deposits have been present in a thin layer (less than 1 inch thick) spread over the interict surfaces of the compressor and converter. In this orientation, it would require significantly more mass to present a minimum critical mass even if moderated to an H/U of 4. In those cases where the deposit is in the form of a solid mass, the surface area exposed or available to be hydrated is significantly reduced. Correspondingly, the hydration by exposure to wet air of the total deposit, that was originally shutdown in a fluorinating environment, to a level that could support a criticality would require many years for the reasons previously discussed.
In June 1990, two cylinders in the depleted UF cylinder storage yards at Portsmouth were discovered to have holes in their walls at the valve-end stiffening ring at a point below the level of the gas-solid interface of the UF.
(Reference 12). The cylinder with the larger hole, which extended under the stiffening ring, was stacked in a top row for 13 years. The cylinder with the smaller hole had been stacked in a bottom row for 4 years. Following the initial mechanical failures, reaction of UF6with the ambient atmosphere (rain and snow) led to the formation of an acid solution containing HF and UQF: and ultimately, to a protective salt layer composed of UF, and iron fluorides. Slow seepage of the acid solution from below the UF, layer gradually corroded and extended the hole surfaces, such that the relative sizes of the openings correlated with the total time that the respective cylinders had been exposed to the elements. For the referenced 13 year old cylinder, the depth of the UF, hydrate layer was about 4 in. And the depth of the yellow hexavalent uranium oxy-fluoride layer was in excess of 8 in. Observations of the damaged depleted UF.
cylinders in which the UF was6 exposed to ambient weather (rain, snow, large temperature variations, etc.) concluded that it would take perhaps as much as 30 years to hydrolyze the cylinder contents which further supports the premise that hydration and associated reactions of solid deposits, even under the most extreme moderation conditions, is a very slow process.
- 4. NDA measurements for determining deposit size and enrichment are conservative. This is in part due to the addition of a 50% uncertainty factor to the final deposit size. PORTS utilizes the same NDA methods and equipment as did the deposit removal project and therefore would expect the same kind of experience. Plant experience at PORTS and documentation from the K-29 removal project (Reference 11) as illustrated below would indicate that the NDA measurements without the 50% factor are also very conservative.
Ex. NDA deposit estimate was 1,190 kg U @ 3.3%. Actual amount of material removed was 450.1 Kg U @ 3.0% with 43.3 Kg U remaining in the pipe for a total quantity of 493.4 Kg U. This represents an approximate 58% over statement of deposit size.
Ex. NDA deposit estimate was 773 kg U @ 2.0%. Actual amount of material removed was 370.6 kg U @ an ave.1.87% with 56.4 kg U remaining in the pipe for a total quantity of 427.1 Kg U. This represents an approximate 44.7% over statement of deposit size.
Ex. NDA deposit estimate was 209 Kg U @ 3.5%. Actual amount of material removed was 84.4 kg U @ an ave. 2.95% with 14.05 kg U remaining in the pipe for a total quantity of 98.5 Kg U. This represents an approximate 52.8% over statement of deposit size. j The time in which a cascade deposit can be exposed to cell floor ambient air can be increased without compromising nuclear safety due to the close correlation between the theoretical / laboratory reaction kinetics associated with the UO2 F:/H2O reaction and field observations which define a safety margin of well over one year. Also keeping in mind that the exposure to ambient cell floor air is only during those periods (not continuous) in which work is being performed to expedite the ultimate elimination of the deposit. Administrative ,
controls exist and will remain in place that should liquid water be present (e.g. sprinklers, l RCW, roof leak, etc.) openings to a deposit above a safe mass will be covered immediately.
The proposed change will not significantly reduce the safety margin as defined in the bases for j TSRs 2.2.3.15/2.7.3.14. The existing basis statements already acknowledge that moderation I control is not a concern until the cell / system that contains the greater than safe mass deposit is taken offstream and evacuated of its UF6. The basis goes on to state that there are analyses that demonstrate that " breathing" of a cell, i.e. wet air exposure, will not significantly affect deposit moderation even over a period much longer than 180 days.