ML20005B512
| ML20005B512 | |
| Person / Time | |
|---|---|
| Site: | Fort Calhoun  |
| Issue date: | 07/01/1981 |
| From: | William Jones OMAHA PUBLIC POWER DISTRICT |
| To: | Clark R Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML20005B513 | List: |
| References | |
| RTR-NUREG-0737, RTR-NUREG-737, TASK-2.B.1, TASK-TM TAC-44374, NUDOCS 8107080321 | |
| Download: ML20005B512 (6) | |
Text
Omaha Public Power District 1623 MARNEY a OMAHA, NEBRASMA 6J102 a TELEPHONE S36 4000 AREA CODE 402 July 1, 1981 Mr. Robert A. Clark, Chief U. S. Nuclear Regulatory Commission Office of Nuclear Reactor Regulation Division of Licensing Operating Reactors Branch No. 3 Washington, D.C.
20555
Reference:
Docket No. 50-285
Dear Mr. Clark:
The Commission's letter dated October 31, 1980, forwarded addi-tional clarification of the TMI task action plan requirements (NUREG-0737). Task II.B.1 of NUREG-0737 detailed the requirement for a reactor coolant gas vent system.
In response to the task II.B.1 requirements, the design description of the Fort Calhoun Station reactor coolant gas vent system is attached.
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Response to Document Requirements of Section II.B.1 of NUREG 0737 Reactor Coolant System Vents 1.
RESPONSE TO NRC POSITION POSITION:
Submit a description of the design, location, size, and power supply for the vent system along with results of analyses for loss-ot-coolant accidents initiated by a break in the vent pipe.
The results of the analyses should demonstrate compliance with the acceptance criteria of 10 CFR 50.46.
RESPONSE
A description of the design, location, size, and power supply for the Reactor Coolant Gas Vent System (RCGVS) along with results of analyses for loss-of-coolant accidents initiated by a break in tne vent pipe may be found in enclosure (1) Combustion Engineering's transmittal to Omaha Public Power District, number 18074-940.
This' system description is enciesed for NRC staff review.
POSITION:
Submit procedures and supporting analysis for operator use of the vents that also include the informaen available to the operator for initiating or terminating vent usage.
RESPONSE
Procedural guidelines and supporting analyses for operator use of the RCGVS may be founo in enclosure (2) Combustion Engineering's transmittal to Omaha Public Power District, number 18074-940.
The procedural guidelines are enclosed for NRC :taff review.
l 2.
DISCUSSION OF DESIGN WITH RESPECT TO NRC " CLARIFICATIONS".
l a)
The important safety function enhanced by this venting capa-bility is core cooling.
For events beyond the present design basis, this venting capability will substantially increase the plant's ability to deal with large quantities of nonconden-sible gas which could interfere with core cooling.
l Procedures addressing the use of the reactor coolant system vents should define the condition under which the vents should be used as well as the conditions under which the vents should not be used.
The procedures should be directed toward achiev-ing a substantial increase in the plant bef al able to maintain core cooling without loss of containrrw integrity for events beyond the design basis.
The ues ef terJs for accidents within the normal design basis N. act esult in a violation of the requirements of 10 CFR SL &.37
, CFR 50.46.
Discussion - At present a procedural guidelir.e has been drafted and included in this response.
Detailed procedures will be based on these guidelines.
4 b)
The size of the reactor coolant vents is not a critical issue.
The desired venting capability can be achieved with vents in a fairly broad spectrum of sizes.
The criteria for sizing a vent can be developed in several ways.
One approach, which may be considered, is to specify a volume of noncondensible gas to be vi.nted and in a specific venting time.
For contain-ments particularly vulnerable to failure from large hydrogen releases over a short period of time, the necessity and de-sireability for contained venting outside the containmtit must be considered (e.g., into a decay gas collection ar1 storage system).
c)
The reactor coolant vent system shall be operable from the control room.
A positive indication of valve position should be provided in the control room.
Discussion - The system is designed to permit th.e operator to vent the reactor vessel head or pressurizer steam space from the control room with positive open/close position indication for all remotely operated solenoid valves.
d)
The probability of a vent path failing to close, once opened, should be minimized; this is a new requirement.
Each vent must have its power supplied from an emergency bus.
A single failure within the power and control aspects of the reactor coolant vent system should not prevent isolation of the entire vent system when required.
Discussion - The solenoid valves are powered from emergency bus.
Parallel valves assure a vent flow path to containment in the event of single active failure.
A single failure within the power or control aspects of the system will not prevent isolation of the entire vent system when required.
This has been accomplished by having two electrical operated valves in series (powered from different power services) for isolation. Valves are qualified for LOCA environment.
e)
Vant paths from the primary system to within containment snould go to those areas that provide good mixing with con-tainment air.
Discussion - The vent path to containment is located in an open area of containment where good mixing with containment air is assured.
f)
Provisions to test for operability of the reactor coolant vent system should be a part of the design.
Testing should be performed in accordance with subsection IWV of Section XI of the ASME Code for Category B valves.
Discussion - The valves will be full-stroke exercised during cold shutdowns Valves shall be leak-tested at the same (or greater) frequency as scheduled refueling outages, but not less than once every two years.
A pressure transmitter has been provided to facilitate leak testing.
3
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g)
It is important that the displays and controls added to the control room as a result of this requirement not increase the potential for operator error.
A human-factor analysis should be performed taking into consideration:
(a) the use of this information by an operator during both normal and abnormal plant conditions; (b) integration into emergency procedures, Discussion - The basic purpose of the vent system is to remove non-condensible gases from the RCS in a timely manner.
Since the system may be required to operate under a variety of post-accident conditions to remove gases from the RCS, the system is designed to vent non-condensible gas from the RCS in a reasonable period of time without reference to a specific bubble size or to reactor coolant temperature and pressure conditions.
Over the range of conditions considered, (pres-sure from 250 psia to 2250 psia and temperatures form 200 F to 700 F), the system is designed to remove 5000 standard cubic feet of gas-roughly the size of the entire reactor vessel-in a maximum of approximately 2\\ hours.
Stated somewhat diffe-rently, the system is designed to vent one-half of the vessel volume in one hour with the vented volume expressed in stand-ard cubic feet of gas over the range of venting conditions considered.
h)
Where practical, the reactor coolant system vents should be kept smaller than the size corresponding to the definition of LOCA (10 CFR 50, Appendix A).
This will minimize the chal-lenges to the amergency core cooling system (ECCS) since the inadvertent opening of a vent smaller than the LOCA definition would not require ECCS actuation, although it may result in leakage beyond technical specification limits.
On PWRs, the use of new or existing lines whose smallest orifice is larger than the LOCA definition will require a valve in series with a vent valve that can be closed from the control room to ter-minate the LOCA that would result if an open vent valve could not be reclosed.
I Discussion - Coolant liquid loss through the vent shall not exceed makup capacity.
This limits the mass loss to below the definition of a LOCA in 10 CFR 50, Appendix A.
This has been accomplished by placing two flow restricting orifices of 7/32" t
diameter by 1" long.
These are located at both vent points for the reactor vessel and pressurizer.
1)
Since the reactor coolant system vent will be part of the reactor coolant system pressure boundary, all requirements for the reactor pressure boundary must be met, and, in addition, sufficient redundancy should be incorporated into the design to minimize the probability of an inadvertent actuation of the system.
Administrative procedures, may be a viable option to meet - the single-failure criterion.
For vents larger than the LOCA definition, an analysis is required to demonostrate compliance with 10 CFR 50.46..
Discussion - The vent path shall be safety grade meeting the same or better qualifications as were accepted for the RCS at time of licensing.
The power operated valves shall be class 1E powered uith key locked valve control switches and admini-strative controls on valve; operation to minimize the possi-bility of inadvertent operation of the system.
(c) integration into operator training, and (d) other alarms during emergency and need for prioritization of alarms.
Disc: sion - To reduce potential for operator error a human fact; analysis has been performed on the location and place-ment of displays and controls added to the control room as a result of this design.
j)
The reactor coolant vent system (i.e., vent valves, block valves, position indication devices, cable terminations, and piping), shall be seismically and environmentally qualified in accordance with IEEE 344-1975 as supplemented by Regulatory Guide 1.100, L92, and SEP 3. 92, 3.43, and 3.10.
Environ-mental qualifications are in accordance with the May 23, 1980 Commission Order and Memorandum (CLI-80-21).
Discussion - Electrical equipment required for the operation of this system is qualified for IEEE 323-1974 and IEEE 344-1975.
k)
Each PWR licensee should provide the capability to vent the reactor vessel head.
The reactor vessel head vent should be l
can?ble of venting noncondensible gas from the reactor vessel hot legs (to the elevation of the top of the outlet nozzle) and cold legs (through head jets and other leakage paths).
I Discussion - Capability has been ptovided.
1)
Venting of the pressurizer is required to assure its avail-ability for system pressure and volume control.
These are important considerations, especially during natural circula-tion.
l Discussion - Capability has been provided.
l m)
Additional venting capability is required for those portions of each hot leg that cannot be vented through the reactor vessel head vent or pressurizer.
It is impractical to vent each of the many thousands of tubes in a U-tube steam gene-rator; however, the staff believes that a procedure can be l
developed that assures sufficient liquid or steam can enter l
the U-tube region so that decay heat can be effectively re; i
moved from the RCS.
Such operating procedures should incor-potate this consideration.
Discussion - Venting of the hot legs will be accomplished by j
this system. L
3.
ENCLOSE DRAWINGS AND DIAGRAMS a)
Reactor Coolant Gas Vent System Nuclear Structures 2002-04-1 Rev. 1 Piping Isometric and Plan b)
Pipe Supports Nuclear Structures 2002-04-2 Rev. 1 c)
Pipe Supports Nuclear Structures 2002-04-3 Rev. 1 d)
Schematic, Elementary, OPPD 11405-E-144 Rev. O Wiring Diagram and
.3 Switch Development e)
Reactor Coolant Gas Vent OPPD 11405EM 176/182 Rev. O System.
Instrumentation and Control Equipment List.
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