ML14247A202
| ML14247A202 | |
| Person / Time | |
|---|---|
| Site: | Byron  |
| Issue date: | 06/13/2014 |
| From: | Walton R NRC/RGN-III/DRS/OLB |
| To: | |
| References | |
| Download: ML14247A202 (31) | |
Text
Question 91 When moving irradiated fuel assemblies in the Refueling Cavity, the minimum water level above the vessel flange can be monitored __(1)__.
The Tech Spec basis for the minimum water level is to __(2)__.
________(1)________ _______(2)________
A. locally and in the MCR limit iodine fission product release B. locally and in the MCR provide longer time to core boil C. ONLY in the MCR limit iodine fission product release D. ONLY in the MCR provide longer time to core boil Answer: A Answer Explanation:
A is CORRECT: TS 3.9.7 requires at least 23 feet above the vessel flange when moving irradiated fuel. The top of the cavity is 26 feet above the vessel, and the TS basis is to lower iodine activity.
The local and MCR indicators are both individually used in some surveillances and procedures, making it plausible that only one exists. Time to core boil is calculated and monitored, but is not a basis for TS required minimum level.
B is incorrect: Time to core boil is calculated and monitored, but is not a basis for TS required minimum level.
C is incorrect: The local and MCR indicators are both individually used in some surveillances and procedures, making it plausible that only one exists.
D is incorrect: The local and MCR indicators are both individually used in some surveillances and procedures, making it plausible that only one exists. Time to core boil is calculated and monitored, but is not a basis for TS required minimum level.
Answer reference (question was not open reference): TS 3.9.7, Refueling Cavity Level Applicant: Ryan Frank, Docket Number 55-33761 Applicants comment:
I selected Answer (B) due to the fact that I had the basis of LCO 3.9.5 - RH & Coolant Circulation High Water Level in mind. Specifically, Tech Spec Basis (B 3.9.5-3) includes:
With no forced circulation cooling, decay heat removal from the core occurs by natural convection to the heat sink provided by the water above the core. A minimum refueling water level of 23 ft above the reactor vessel flange provides an adequate available heat sink.
Suspending any operation that would increase decay heat load, such as loading a fuel assembly, is a prudent action under this condition. Therefore, actions shall be taken immediately to suspend loading of irradiated fuel assemblies in the core.
It continues to say on page (B 3.9.5-4):
With the RHR loop requirements not met, the potential exists for the coolant to boil and release radioactive gas to the containment atmosphere.
==
Conclusion:==
I believe without asking for the basis of the specific technical specification 3.9.7, the question left an opportunity to consider a different LCO. LCO 3.9.5 includes language to capture both iodine fission product release as well as time to core boil, while LCO 3.9.7 (that the answer key relied on) contains only language specific to iodine fission product release. Consequently, I believe there are two correct answers, both (A) and (B).
Facility recommendation:
The question was written and validated with the basis of TS 3.9.7, Refueling Cavity Water Level in mind. However, that technical specification was not specified in the question, leaving the question open to the interpretation of the examinee. TS 3.9.5 does apply with > 23 feet in the refueling cavity, and the basis for it does discuss adequate level to maintain time to core boil and decay heat removal.
The facility agrees with the applicants comment. The question should be changed to allow both choices A and B as correct answers.
Both TS 3.9.5 and the basis document are attached.
QUESTION 97 Unit 1 is at 100% power.
Which valve failure(s) will result in the Emergency Core Cooling System UNABLE to meet its safety function?
A. 1CV182, RCP Seal flow backpressure control valve, failed OPEN.
B. 1SI8812A, RWST to RH pump suction valve, and 1SI8811A, RH pump CNMT sump suction valve, BOTH failed OPEN.
C. 1SI8807A and 1SI8807B, CV and SI pump crosstie valves, failed CLOSED.
D. 1CV8804A and 1SI8804B, RH to CV and SI pump suction valves, failed CLOSED.
Answer: D Answer Explanation:
A is incorrect: Plausible, in that if the seal injection throttle valves are too far open, injection flow could be lower than design. There is a surveillance to verify throttle valve position.
B is incorrect: Plausible in that a failure to close this valve would result in RWST draining to the CNMT sump on recirc actuation. The water is fully available to the RH pumps when in the sump.
C is incorrect: These are parallel suction header crosstie valves to allow all CV and SI pumps to be supplied from 1 RH pump in the event of a pump trip. However, other valve alignments will supply water to the pumps even if they failed. This is evidenced by the existence of 1SI8924, a single valve in series with these two paralleled valves.
D is CORRECT: The listed valves must be able to be opened for the RH pumps to supply the CV and SI pumps for cold leg recirculation.
Answer reference (question was not open reference): TS Basis 3.5.2, Emergency Core Cooling System Applicant: Ryan Frank, Docket Number 55-33761 Applicants comment:
I selected Answer (B) due to the fact that I envisioned the RWST draining/drained to the containment sump based on the valve lineup in this answer choice and believed this challenged/extinguished the safety function of ECCS.
According to Byron Operating Department Policy Statement (No. 600-04, Rev 25):
The RWST will gravity drain to the containment sump [via an open 1SI8811A/B] at a rate of 7000 gpm/train in 30 minutesto achieve 40% RWST level. [as time progresses, the RWST will eventually empty at the same rate.]
According to the Byron UFSAR (B/B 6.3-31, 32):
Following a small break LOCA, the RWST will deplete much slower than a large break LOCA since high RCS pressure prevents RHR pump injection and containment spray pump actuation on HI-3 containment pressure is not expected. Under these conditions, switchover to recirculation is not time critical since a relatively large amount of time is available. However, if the containment spray pumps actuate during the injection mode, the RWST level LO-2 alarm could be reached in a relatively short period of time, necessitating switchover to the recirculation mode. If RCS pressure remains above RHR pump shutoff head pressure under these conditions, only the SI and CV pumps are capable of providing core cooling flow. [but the RWST volume is inadequate or non-existent]. Therefore, completing the manual actions to align RHR pump discharge to the suction of the SI and CV pumps becomes time critical since the SI and CV pumps will lose their suction source unless manual switchover actions are completed prior to reaching the RWST empty level. [the suction source is already lost from the RWST.]
According to Tech Spec Basis (B 3.5.4-2), with respect to the notion that the 1SI8812A is failed open in the answer choice I selected, it says:
When the suction for the ECCS and Containment Spray System pumps is transferred to the containment sump, the RWST flow paths must be isolated [i.e. 1SI8812A closed] to prevent a release of the containment sump contents to the RWST, which could result in a release of contaminants to the atmosphere and the eventual loss of suction head for the ECCS pumps.
It continues to say on page (B 3.5.4-7):
With the RWST inoperable for reasons other than Condition A (e.g., water volume), it must be restored to OPERABLE status within 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br />. In this Condition, neither the ECCS nor the Containment Spray System can perform its design function. Therefore, prompt action must be taken to restore the tank to OPERABLE status or to place the unit in a MODE in which the RWST is not required.
Lastly, the available NPSH for the CV and SI pumps is also challenged. According to the UFSAR (B/B 6.3-8):
The net positive suction head for the safety injection pumps and the centrifugal charging pumps is evaluated for both the injection and recirculation modes of operation for the design-basis accident. The end of the injection mode of operation gives the limiting net positive suction head available (minimum static head). The net positive suction head available is determined from the elevation head and vapor pressure of the water in the refueling water storage tank, the tank air space pressure, and the pressure drop in the suction piping from the tank to the pumps.
First
Conclusion:
The stem of the question only said that Unit 1 is at 100% power. It did not say that an SI had just been actuated or was in progress. I had assumed that the RWST drained itself at least adequately enough to the containment sump such that the available NPSH for the CV and SI pumps would be challenged and potentially not have an adequate suction source to deliver flow for the injection phase of a LOCA. While all the RWST water may be in the containment sump, if RCS pressure stays high enough, the RH pumps will never deliver water to the RCS until cold leg recirc is manually aligned, and thus the safety function of ECCS to deliver core cooling at this time does not exist. The time necessary to manually align for cold leg recirc before the RWST is empty is further challenged due to the low/lowering/empty volume of the RWST. Furthermore, shutdown margin and the ability to maintain the reactor subcritical during a secondary LOCA
would be challenged due to the lack of borated water getting to the reactor during the injection phase.
To the extent that there may be doubt regarding to the validity of answer (B) despite the referenced material above, the UFSAR (B/B 6.3-31) offers this information to challenge answer (D) and suggests that 1CV8804A and 1SI8804B failing closed is of no consequence with respect to ECCS meeting its ability to deliver core cooling during the cold leg recirculation phase, upon which the answer key relies:
following a large break LOCA. In the event the manual actions to align RHR pump discharge to the suction of the SI and CV pumps are not completed prior to reaching the RWST empty level, required core cooling capability will be maintained by the RHR pumps.
Second
Conclusion:
While the failed closed positions of 1CV8804A and 1SI8804B will prevent cold recirculation via the CV and SI pumps, the UFSAR says that ECCS will still be able to maintain core cooling capability via the RH pumps with the water in the containment sump alone. In which case, I would argue that the safety function to maintain core cooling by ECCS is not totally extinguished with respect to answer (D).
Third
Conclusion:
Lastly, there is one more argument to support the notion that a loss of safety function for ECCS exists. According to TRM, Appendix O, 3.5.4 RWST is a support system for 3.5.2 ECCS (Operating). The continued draining of the RWST lower than 89% T.S. impacts not just one train of ECCS, but both. All ECCS pumps take suction off a common discharge header from the RWST. If the available volume isnt adequate (or in existence) for one pump, then its not available for any of the other ECCS pumps either. It appears this would be outside the design basis for Byron, which is probably also why 10CFR50.54X is referenced in the Byron Operating Department Policy Statement (No. 600-04) for gravity draining the RWST during a security threat.
Facility recommendation:
The distractor was written with the idea that the RWST would completely drain to the containment sump, as the applicant correctly surmised. While the RWST level would be less than TS 3.5.4 required level, the inventory would be available to the ECCS system via the Containment Recirculation Sumps.
An automatic signal caused by a Safety Injection signal in conjunction with Low-2 RWST level (below 47%) will automatically align both RH pumps suctions to the containment sump. Further manual actions are necessary to close the RWST to RH pump suction valves, and to align the RH pump discharge to the high head injection pumps suction.
With RWST level below 9%, the operators would stop the high head injection pumps that are aligned to the RWST based on the Operator Action Summary page of 1BEP ES-1.3, Transfer To Cold Leg Recirculation procedure. While this would render the high head injection pumps incapable of immediately providing flow on a Safety Injection actuation, the Byron emergency procedure network would direct operators to use 1BEP ES-1.3 to align the RH pump discharge to the high head injection pumps suction, restoring high head injection flow.
The facility recommends making no change to the grading of the question.
The relevant pages of the following procedures referenced in the applicants and facilitys comment are attached:
Operations Policy 600-04 UFSAR 6.3 Emergency Core Cooling System Basis for TS 3.5.4 RWST TRM Appendix O, Safety Function Determination Program 1BEP ES-1.3 Transfer To Cold Leg Recirculation