ML20202F315
| ML20202F315 | |
| Person / Time | |
|---|---|
| Site: | Portsmouth Gaseous Diffusion Plant, Paducah Gaseous Diffusion Plant |
| Issue date: | 02/09/1998 |
| From: | Toelle S UNITED STATES ENRICHMENT CORP. (USEC) |
| To: | Pierson R NRC OFFICE OF NUCLEAR MATERIAL SAFETY & SAFEGUARDS (NMSS) |
| References | |
| GDP-98-0017, GDP-98-17, TAC-L32012, NUDOCS 9802190175 | |
| Download: ML20202F315 (11) | |
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L y USEC A Gloted Energy Company February 9,1998 Mr. Robert C. Pierson SERIAL: GDP 98-0017 Chief, Special Projects Branch Division of Fuel Cycle Safety and Safeguards, NMSS United States Nuclear Regt2atory Commission Washington, D.C. 20555-0001 Paducah Gaseous Diffusion Plant (PGDP)
Portsmouth Gascous Diffusion Plant (PORTS)
Docket No. 70-7001 and 70 7002 Compliance Plan issue 3, Action 9, ASME Code Interpretation and Assessment Results, (TAC NO. L32012)
Dear Mr. Pierson:
As required by the Plan for Achieving Compliance with NRC Regulations at the Paducah Gaseous DifTusion Plant (Compliance Plan), Issue 3, Action 9, PGDP has obtained an ASME Code Interpretation on the need for pressure relief on the transfer manifold of the C-360 parent-daughter transfer system. The ASME Code Interpretation and USEC's assessment of the need for system modification as a result of this interpretation have been submitted for NRC review and approval (Reference 1). In references 2 and 3, tla NRC requested additional information relating to USEC's assessment of the need to provide relief protection during transfer operations at the PGDP Toll Transfer and Sampling Facility (C-360). USEC's response to these additional information requests is provided in Enclosure 1.
The Plan for Achieving Compliance with NRC Regulation at the Portsmouth Gaseous Diffusion Plant (Compliance Plan) requires PORTS to obtain a similar ASME Code Interpretation on the need for pressure relief protection for the UF, cylinders. Due to the similarities in the design of the autoclaves and associated systems, as well as similarities in the Compliance Plan Issues for PORTS and PGDP and in anticipation that ASME would provide a similar, if not identical, Code Interpretation for PORTS, USEC has not requested a PORTS specific ASME Code Interpretation.
Instead, USEC intends to rely on the ASME Code Interpretation obtained for PGDP to satisfy that portion of PORTS Compliance Plan Issue 3, Action 9, requiring PORTS to obtain a Code 9902190175 980209 "
- 6903 Rockledge Drive. Bethesda, MD 20817-1818
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l Mr. Robert C. Pierson February 9,1998 GDP 98-0017, Page 2
~ Interpretationi PORTS has reviewed the ASME Code Interpretation, obtained for PGDP, and has concluded that no modifications to the system or system operation are required to complete PORTS Compliance Plan issue 3, Action 9. He PORTS assessment of the need for modification is provided in Enclosure 2.
If you have any questions or require additional information, please contact Mark Smith at (301) 564-3N4, There are no new commitments contained in this submittal.
Sincerely, A
S. 4. L Steven A. Toelle :
Nuclear Regulatory Assurance and Policy Manager
Enclosures:
As stated cc:
- NRC Region 111 Office NRC Resident inspector-PGDP NRC Resident inspector-PORTS DOE Regulatory Oversight Manager
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l Mr. Robert C. Pierson February 9,1998
~ GDP 98-0017, Page 3
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. References 1.
Letter from Mr. Robert L. Woolley (USEC) to Mr. Robert C. Pierson (NRC), "Paducah -
Gaseous Diffusion Plant, Compliance Plan issue 3, Action 9, ASME Code Interpretation and.
Assessment Results," dated December 31,1996.
2.
_ Letter from Mr. Robert C. Pierson (NRC) to Mr. Steve Toelle (USEC), "Paducah Oaseous -
Diffusion Plant, Compliance Plan issue 3, Action 9, ASME Code Interpretation and Assessment Results," dated October 10,1997, 3.
- Letter from Mr. Robert C. Pierson (NRC) to Mr. Steve Toelle (USEC), "Paducah Gaseous Diffusion Plant, Compliance Plan Issue 3, Action 9, ASME Code Interpretation and Asse:sment Results," dated November 10,1997.
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GDP 98-0017 Page1of.i Resporse to Request for AdditionalInformation Compliance Plan issue 3, Action 9 Paducah Gaseous Diffusion Plant Automatic, Positive Steam Pressure Control in Reference 1, a question was raised about the 90 psig pressure control in the steam supply header, whether the design used "...is sufEciently positive to meet the requirements of a pressure relief device, if you can show that the pressure control device that prevents steam pressure greater than 90 psig from entering the autoclave is an automatic, non reclosing device (UG-126 and 127),
then pressure relief valves on individual cylinders would not be required." Reference I also noted that "ASME Code paragraph UG-125(g) sta:es that protective devices need not be installed directly on a pressure vessel when the source of the pressure is external to the vessel and is under such positive control that the pressure in the vessel cannot exceed the maximum allowable working pressure (200 psig). The note to this paragraph clarifies that pressure reducing valves are not sufficiently positive in action to prevent excess pressures from being developed."
' PGDp rovides positive pressure control of the extemal pressure source (steam piped to the p
4 autoclave for heating) by means ofdual, automatic isolation valves on the steam supply. The ASME Code does not specifically identify that the " positive control" must be a preswre relief device per UG-126 and UG-127. T..mfore, it is not required that the controls be "sufficiently positive to meet the requirements of a pressute relief d:vice." Even though the footnote to paragraph UG-125(g) specifically states that pressure reducing valves are not adequate for posit've pressure control, it does not state that only pressure relief valves are adequate nor does it state that isolation valves are inadequate to provide sufliciently positive controls. The PGDP design provides automatic, positive control of the external pressure source as described below.
The autoclave temperature is controlled to approximately 220 F and a maximum steam pressure of 4 psig by the temperature control loop as described in SAR Section 3.6.1. Should the temperature control loop fail to control the steam pressure, the autoclave steam pressure control system (TSR 2.1.3.3) automatically controls the maximum steam pressure during heating to 8 psig for Category A cylinders and 6 psig for Category B cylinders. This system closes the steam inlet black valves at a steam pressure at or below the Limiting Control Setting values noted above.
There is an additional safety system, the Autoclave High Pressure Isolation System (TSR 2.1.3.1), which automatically actuates when the steam pressure reaches 15 psig and places the autoclave in the containment mode as described in SAR Section 3.6.7.5. It is these systems (i.e.,
the Autoclave Steam Pressure Control System and the Autoclave High Pressure Isolation System) which provide the automatic, positive control of the external pressure source.
4 GDP 98-0017 Page 2 of 3 in additio.., TSR Safety Limit 2.1.2.2 specifies that the cylinder temperatures shall not exceed 250T for Category A Cylinders and 245 F for Categary B cylinders. These temperatures correspond to steam pressures of 15 psig and 12.5 psig respectively. Compliance with this Safety Limit is ensured by the Autoclave Iligh Pressure Isolation System in reference 1, a statement is made concerning the need for the steam pressure controls to meet UG-126 and UG 127 and to be "non-reclosing." This language in the Code relates to requirements for a pressure relief device. The Paducah safety system discussed above employs two isolation valves (block valves) in series which are automatically closed (and automatically locked in the closed position upon safety system actuation) to provide a greater measure of positive control over steam pressure allowed in the autoclave. By design they are not "non reclosing", but they perform the same function by not allowing the isolation valves to re-open upon safety system actuation.
Therefore, the automatic control system at PGDP (Autoclave High Pressure Isolation System) provides automatic, positive control to ensure that steam entering the autoclave is less than 15 psig, which is much less than the concem of 90 psig expressed in Reference 1.
I The discussion in Reference 3, Enclosure 2, Item 4 related to the 90 psig steam limitation was provided to demonstrate that in the non-credible case ofloss of positive control, the cylinder pressure is self limiting and wou'd still only result in cylinder pressures of 207 psig (cylinder differential l
pressure of i17 psi). However, the steam header relief valve is not a safety system and is not taken credit for la the accident analysis. The safety system taken credit for is the Autoclave High Pressure isolation System, which is a safety system (i.e., a "Q" system) and is required to be operable for all autoclave modes of operation except for the Autoclave Open or Out of Service Modes.
The MAWP for Thin Walled Cylinders is not Challenged Reference 2 raised a concern that evaluation of the need for pressure relief on the parent-daughter transfer operation contained in Reference 3 only addressed cylinders designed for 200 psig
- (i.e.,100 psig " thin walled" cylinders were not addressed). As discussed above, Reference 3,
~. Item 4, which discussed the 90 psig steam limitation, was provided to demonstrate that in the non-credible case ofloss of positive control, the cylinder pressure is selflimiting and would still only result in cylinder pressures of 207 psig (cylinder differential pressure of 117 psi).
However, the steam header relief valve is not a safety system and is not taken credit for in the accident analysis. The safety system taken credit for is the Autoclave High Pressure Isolation System, which is a safety system (i.e., a "Q" system) and is always operable as required by the TSR.
The sentence in Reference 3, Enclosure 2 which stated that " cylinders used are designed for 200 psig" is true for the thick walled cylinders which are typically the type of cylinder heated at C-360.
However, thin-walled cylinders are not prohit ited from being heated. The analysis did address both types of cylinders. Ilowever, the discussion of the 90 psig steam selflimiting scenario demonstrates
o GDP 98-0017 Page 3 of 3 that s fer pressure remains below the MAWP only for the thick walled cylinders. (For the thin wallet,ylinders, the i 17 psi differential pressure is greater than 110% of MAWP by 7 psi.)
Upon review of Enclosure 2, reference 3, it is evident that the discussion of that point was not clear. The t.utomatic safety systems (Autoclave liigh Pressure Isolation System and Steam Pressure Control System) provide positive controls which limit the steam pressure in the autoclave to either 15 psig or 8 psig, and temperatures of 250 or 235 *F, respectively. The corresponding pressure in the parent cylinder is less than 62 psig (cylinder differential pressure of 54 psi) and 84
- psig (cylinder differen'lal pressure of 69 psi), respectively. Similar pressures (with slight variations due to piping resistance and head dilTerential) would be observed in the daughter cylinders.
Therefore, the MAWP of the thin walled cylinder is not challenged.
References 1.
Letter from Mr. Robert C. Pierson (NRC) to Mr. Steve Toelle (USEC), "Paducah Gaseous Diffusion Plant, Compliance Plan issue 3, Action 9, ASME Code Interpretation and Assessment Results," dated October 10,1997.-
2.
Letter from Mr. Robert C. Pierson (NRC) to Mr. Steve Toelle (USEC), "Paducah Gaseous Diffusion Plant, Compliance Plan Issue 3, Action 9, ASME Code Interpretation and Assessment Results," dated November 10,1997, 3.
Letter from Mr. Robert L. Woolley (USEC) to Mr. Robert C. Pierson (NRC), "Paducah Gaseous DilTusion Plant, Compliance Plan Issue 3, Action 9, ASME Code Interpretation and Assessment Results," dated December 31,1996.
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GDP 98-0017 Page 1 of 5 ASME Code Interpretation and Assessment Results Compliance Plan Issue 3, Action 9 Portsmouth Caseous Diffusion Plant The Plan for Achieving Compliance with NRC Regulatiens at the Portsmouth Gaseous Diffusion Plant (Compliance Plan), issue 3, Action 9 states:
A code interpretation from the ASME Code Committee will be obtained regarding the need for pressure relief for the UF cylinders. Based on this interpretation, the need for modifications to the affected system operations will be assessed. Both the ASME Code interpretation and the assessment results will be submitted to NRC for review and approval.
The Compliance Plan for the Paducah Gaseous Diffusion Plant contains a similar requirer :nt for obtaining an ASME Code Interpretation. The PGDP ASME Code interpretation has been obtained and submitted, along with the assessment of the need for modification, to NRC for review and approval. The PGDP ASME Code Interpretation indicated that, based on the information I
provided in the code interpretation request, the requirements of ASME Code, Division 1, Section Vill, Paragraph UG-125 (a) would appear to be invocable to the PGDP parent-daughter transfer opcotion. This PGDP interpretation would appear to require pressure relief protection for the transfer manifold. A review of the ASME Code interpretation provided for PGDP and the j
assessment of the need for modifications at PGDP determined that no modification to the system or i
system operation was required (Reference 1). Due to the similarities in the design of the autoclaves i
and associated systems, as well as similarities in the Compliance Plan Issues for PORTS and PGDP and in anticipation that ASME would provide a similar, if not identLal, Code Interpretation for PORTS, USEC has not requested a PORTS specific ASME Code Interpretation to satisfy the PORTS Compliance Plan Action. Instead, USEC will rely on the ASME Code Interpretation obtained for PGDP to satisfy that portion of PORTS Compliance Plan issue 3, Action 9, requiring a PORTS specific ASME Code Interpretation.
PORTS has reviewed the PGDP MME Code Interpretation which appears to indicate that the requirements of UG-125(a) in Section Vill, Division I are invocable to UF. cylinders. However, due to the number of positive controls provided to ensure steam supply is isolated well in advance of the point where cylinder pressure relief would be required, no modifications at PORTS are required in support of the completion of Compliance Plan Issue 3, Action 9. The justification for concluding that no modification is required is provided below. Note that in providing this assessment, PORTS has considered, and appropriately included, the concems identified in the NRC's requests fbr additional infonnation contained in Reference 2 and 3. To provide a foundation for further discussion related to this PORTS assessment, References 2 and 3 are summarized below.
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GDP 98-0017 Page 2 of 5 Reference 2 requests additional information to demonstrate that the device which limits steam pressure within the autoclave is sufficiently po itive to meet the requirements of a pressure reliefdevice.
Reference 3 requests additional information to demonstrate that the 100 psig, thin walled cylinders were considered in the PGDP Engineering Evaluation provided to NRC in Reference 1.
As a result of these additional information requests, it is necessary to demonstrate that, for PORTS, steam pressure is positively controlled at a safe level for the 100 psig UF. cylinders, in order to avoid the need for pressure reliefdevices on individual cylinders. The following discussion
-will demonstrete that this is specifically the case for PORTS and will also provide additional information to demonstrate an overall high level of safety associated with autoclave operations, PORTS provides positive pressure control of the external steam source (steam piped to the i
autoclave for heating), by means of dual, automatic isolation valves on the steam supply. There are l
several TSR systems which actuate the steam isolation valves to ensure automatic, positive steam pressure control such that the intemal cylinder pressure, when the cylinder is heated in an autoclave, will not exceed the pressure values permitted by the AShiE Code. These TSR systems, which are required to be operable during cylinder heating operations are, as follows.
l UF. Cylinder liigh Pressure Steam Shutoff System UF, Cylinder High Temperature Steam Shutoff System
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Autoclave Shell liigh Pressure Containment Shutdown System As described in SAR Section 3.2.1.1.1, these TSR systems provide isolation of the steam supply by monitoring three different process parameters (steam pressure, cylinder skin temperature and cylinder internal pressure) and fail to a safe condition (i.e., steam isolation) in the event ofloss of plant air and/or electrical power which support these systems. These systems provide automatic, positive steam pressure control as described below:
The set points and Limiting Control Settings for the above noted TSR systems have been established to ensure that neither the thin nor heavy walled cylinders would be pressurized above the values permitted by the AShiE Code. The ash 1E Pressure Vessel Code, UG-125, states that pressure reliefdevices provided for pressure vessels shall prevent the pressure from rising more than 10 or 21 percent of the hiaximum Allowable Working Pressure (MAWP), depending on the conditions described in UG-125. Therefore, a thin walled cylinder could be allowed by the Code to reach a pressure of 110 psig or, under certain conditions 121 psig. The heavy wall cylinders, with an MAWP of 200 psig, could be ellowed to reach 220 psig or 242 psig. The set point values, including associated tolerances, for the UF Cylinder High Pressure Steam Shutoff System and the UF. Cylinder liigh Temperature Autoclave Steam Shutdown System are 120 psia (approximately o
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GDP 98-0017 Page 3 of S 105.5 psig) and 240"F, for Category A Cylinders (which equates to a UF. pressure of 70.5 psig). The set point value, including associated tolerance, for the Autoclave Shell liigh Pressure Containment Shutdown System is current r set at 8 psig (which equates to a cylinder temperature of 240 F, and i
i an internal cylinder pressure of 70.5 psig). It is these TSR requ: red systems which provide the automatic, positive control of the external pressure source to ensure that the internal UF cylinder pressure does not exceed the values allowed by the ASME Code.
Dased on this assessment, the systems and their associated controls described above maintain the cylinder pressure below the maximum pressure allowed by the ASME Code and provide a level of protection comparable to any pressure relief device specified by the ASME Code. Consequently, modifications to the system or system operation are not required to complete PORTS Compliance Plan Issue 3, Action 9.
llaving discussed the specific Compliance Plan Action (Issue 3, Action 9),it is beneficial to discuss additional information which contributes to the safety associated with cylinder heating within the autoclaves. The remainder of this enclosure will discuss the fact that the actual cylinder pressure capacity is far greater than the Maximum Allowable Working Pressure, emphasize the secondary containment function of the autoclave and review implementation of prior NRC recommendations regarding cylinder heating.
Prior to 1988, a series of hydrostatic rupture tests were performed on 5 inch through 48 inch cylinders, to include both models of 30 inch cylinders, as well as heavy and thin wall 48 inch cylinders. Water pressure was increased to each cylinder and the pressure monitored until failure.
The lowest documented rupture pressure was 870 psig for the 48 inch, thin walled cylinder.
Therefore, based on these rupture tests, a significant margin exists between the Maximum Allowable Working Pressure (MAWP) and the failure point of the thin walled cylinders. The details of the testing and the test results are found in Reference 4.
Prior to and during the heating process, the cylinders are placed within autoclaves which are ASME Section Vill, Division 1, pressure vessels. These autoclaves provide containment should there be a failure of the cylinder / pigtail boundary during heating. Therefore, once the cylinder is placed within the autoclave, which is designed to contain the affects of a cylinder breach due to over pressurization, the pressure relieving function required to protect personnel is accomplished by the autoclave pressure relief &vice, in the unlikely event of a cylinder rupture, the containment function of the autoclave minimizes the release of material.
There has never been a rupture of a 5 inch through 48 inch cylinder due to UF over pressure 6
at the Portsmouth Gaseous Diffusion Plants. Such an event did occur at the Sequoyah Fuel.s Facility.
In this case, an overfilled UF cylinder was heated in a steam chest and ruptured, resulting in a 6
release of UF,, and liF. Additional information related to this event, including recommendations on
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GDP 98-0017 i
Page 4 of 5 system and hardware modifications, can be found in References 5 and 6. Of particular interest to this discussion, are the following recommendations contained within NUREG 1198 (Reference 6).
l 1.
Pressure sensing instrumentation should be connected to UF. cylinders and cold traps any time heat is applied to them. Ileat should not be applied to UF. cylir.ders or cold traps unless there is verification that a vent path is open to the associated pressure-sensing instrumentation. The pressure-sensing instrumentation should provide both alarm and visual display functions.
2.
Provisions should be made for over pressure relief or automatic heat termination upon over pressurization any time heat is applied to UF cylinders or cold traps.
3.
The use of autoclaves for heating UF cylinders should be evaluated in terms of providing an additional margin of safety.
As discussed within this enclosure, the Portsmouth Gaseous DilTusion Plant has implemented all three recommendations.
In summary, PORTS has demonstrated that there is positive steam pressure control necessary to protect 100 psig, thin walled UF. cylinders. In addition, PORTS has demonstrated that NRC recommendations regarding cylinder heating resulting from the Sequoyah Fuels Facility incident have been implemented and that the worst case,100 psig, thin walled cylinder, exhibits an actual i
burst pressure over eight times the MAWP. Collectively, this information demonstrates a significant level of safety related to cylinder heating within the autoclaves. Consequently, modifications to include pressure relief for UF cylinders, as addressed in Compliance Plan issue 3, Action 9, are not required.
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o GDP 98 0017 Page 5 of 5 References 1.
Letter from Mr. P.obert L. Woolley (USEC) to Mr. Robert C. Pierson (NRC), "Paducah Gascous Diffusion Plant, Compliance Plan issue 3, Action 9, ASME Code Interpretation and Assessment Results," dated December 31,1996.
2.
Letter from Mr. Robert C. Pierson (NRC) to Mr. Steve Toelle (USEC),"Paducah Gaseous Diffusion Plant, Compliance Plan Issue 3, Action 9, ASME Code Interpretation and Assessment Results," dated October 10,1997, 3.
Letter from Mr. Robert C. Pierson (NRC) to Mr. Steve Toelle (USEC), "Paducah Gaseous Diffusion Plant, Compliance Plan issue 3, Action 9, ASME Code Interpretation and Assessment Results," dated November 10,1997.
4.
Ziehlke, K T. and C. R. Barlow, " Rupture Testing of UF Transport and Storage Cylinder",
Uranium llexa0uoride-Safe llandline. Processine and Transportine, CONF-880558.
5.
NUREG-1179, Volume 1, Runture of Model 48Y UF, Cylinder and Release of Uranium llexaGuaride t,
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6.
NUREG 1198, Release of UF from a Runtured Model 48Y Cylider at Sconoyah Fuels Cornoration Facility: I essons-Learned Renort.
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