ML15224B029
| ML15224B029 | |
| Person / Time | |
|---|---|
| Site: | Oconee  |
| Issue date: | 09/02/1986 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML15224B027 | List: |
| References | |
| NUDOCS 8609110344 | |
| Download: ML15224B029 (2) | |
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.~REG64 UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, D. C. 20555 SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION EVALUATION OF COMPATIBILITY OF DIFFERENT TYPES OF GREASE IN LIMITOPQUE VALVE OPERATORS (TIA 86-26FM)
DUKE POWER COMPANY OCONEE NUCLEAR STATION, UNITS 1, 2 AND 3 DOCKET NOS. 50-269, 50-270 AND 50-287 By letter dated June 26, 1986, Region II requested the NRR staff to evaluate the information submitted by Duke Power Company covering the use of mixed greases in Limitorque valve operators at Oconee Nuclear Station, Units 1, 2, and 3. This information included a report by a consultant to Duke on the capabilities of the mixed greases. Limitorque motor-operated valves are used at Oconee to isolate the containment building during an accident, i.e. loss of coolant accident (LOCA).
To accomplish this function, the valves must remain operable in the high radiation environment which may exist in the containment after a LOCA. Originally, the gearing mechanisms in these valves were lubricated with Exxon Nebula EP grease which was recommended by the valve manufacturer as an environmgntally qualified lubricant. Nebula grease was qualified to withstand 2x10 rads radiation dose, and 140aF ambient and 340oF transient temperatures. However, during subsequent maintenance of the valve operators, another type of grease was introduced and as a result, a mixture of two greases was formed. The other grease, Texaco Marfak 0, is less resistant to radiation and temperature and is not environmentally qualified for nuclear application.
Two concerns arise with regard to the Nebula/Marfak grease mixture: 1) Are the two greases compatible such that their mixture retains the required lubricating characteristics and (2) will the grease mixture survive the environment in the containment which may exist after an accident in order to permit valve operability? To address these questions, we have developed the following information. The Nebula EP type grease is a calcium soap gelled-mineral oil compound. The Marfak 0 type grease, on the other hand, is a sodium soap gelled-mineral oil product. Although no data could be found in the literature on the properties of the mixtures of these two greases, there is substantial information on similar systems.
In general, we found that sodium based greases are compatible with calcium based greases. This means that the properties for the mixture can be estimated by interpolating between the properties of the pure components.
However, even the mixtures of compatible greases may exhibit slight departure from the interpolated values. It is quite common, for example, for the viscosity of the mixture to be somewhat lower than the interpolated value.
This softening of the grease will not cause lubrication problems in the Limitorque valve operator because the grease is enclosed in a gear case and will be retained even if its consistency changes. However, in the interim to further assure that adequate lubrication is being maintained in the mixed condition, we concur with Duke Power Company's consultant that the gear cases should be periodically inspected.
8609110344 8609 PDR 34 8090269 PDR
-2 Texaco Marfak 0 grease is not as resistant to radiation as Nebula EP grease and will degrade faster. In general, when a soap-gelled grease is exposed to a radiation dose which exceeds its stability limit, gelling compounds start to decompose first. This produces softening of the grease. Further irradiation may produce cross-link polymerization of oil which results in an increase of viscosity and eventual hardening of the grease. Both these effects are harmful to lubricating characteristics. In the case of Marfak 0 greasg, its gelling compound (sodium soap) has a stability limit of about 5.5 x 10 rags which means that at higher radiation doses it starts to soften. At 1 x 10 rads, its mineral oil componept starts to polymerize producing the hardening effect. The dose of 5.5 x 10 rads is, therefore, a lower radiation exposure limit at which this grease starts to degrade. Duke Power Company has determined that during thirty minutes of exposure to the post-accident radiation environment, 5the Limitorque valve operators will receive a radiation dose of about 9.2 x 10 rads. Assuming a conservative maximum radiation dose in the containment during normal plant operation of about 60 rads per hour, the valve could be exposed for about eight and one-half years before the limiting dose for Marfak 0 grease is exceeded when the post-accident dose is added to the accumulated dose incurred from normal operation.
(We understand that this is a relatively recent practice at Oconee; no more than four years).
Therefore, the Nebula/Marfak grease mixture in the Limitorque valve operators could be exposed for prolonged-time to a radiation environment before any significant degradation of its lubricating characteristics could occur under accident conditions.
Although the above evaluation indicates that no immediate concern exists for the grease mixture to lose its lubricating properties and impede valve operator performance, it should be recognized that mixing different types of lubricating greases is not good engineering practice as uncertainty in lubricating properties is introduced, and thus the affected Limitorque valve operators should have their grease changed to a qualified brand recommended by the manufacturer. However, we concur that this change can be performed during scheduled plant outages without unnecessarily compromising plant safety for the reasons indicated above.
Dated:
SP Principal Contributor: K. Parczewski