ML19350C200

Responds to Questions Re Power Fluctuations at Triga Mark Ii.Fluctuations Are Generic to Triga Reactors Cooled by Natural Convection

ML19350C200
Person / Time
Site:	Kansas State University
Issue date:	03/26/1981
From:	Faw R KANSAS STATE UNIV., MANHATTAN, KS
To:	John Miller Office of Nuclear Reactor Regulation
References
NUDOCS 8103310562
Download: ML19350C200 (2)

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Standardization and Special Projects Branch ,- ' ' , 1. i .,

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ATTN: James R. Miller , f, Division of Licensing

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U.S. Nuclear-Regulater Commission Washington, D. C. 2A.a5

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RE: Docket 50-188,Lic)'9l~s.

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Gentlemen:

This is in response to questions raised about power fluctuations at the KSU TRIGA MkII Nuclear Reactor Facility.

When operating in.the power range, typically 200 kWt, we experience fluctuations in power of random amplitude ranging to a maximum of about 20 kWt. The : fluctuations are not periodic, but are. slow and typically 30 seconds peak-to-peak. The KSU reactor core is cooled by natural convection. .The primary water is cooled by an external heat exchanger.

Return water is-discharged above the core,-transverse to the natural convection plume. This is done deliberately to break up the plume and delay transfer df 16N to the pool surface. If the e'xternal forced flow of coolant'is interrupted, reactor power drops and fluctuations cease. If operation is at 200 kW when external cooling is interrupted, the power drops.quickly'(few-seconds) to about 150 kW. If operation is at 10 kW'when external cooling is interrupted, the power drops slowly (about three minutes) to about 7 kW.

We.have discussed these power fluctuations'with General Atomics (G.A.) personnel:and with operators of other TRIGA reactors. -We are advised by G.A. that these fluctuations are generic to TRIGA reactors cooled by natural' convection. Similar fluctuations-have been reported by Reed College [ Proceedings, TRIGA User's Conference, San Diego :1980,-

p.2-57].

The: concern over; indications.of power-fluctuations seems not to be-whether' cooling flow causes fluctuations,-but.whether-cooling. flow affects the, ability'to monitoripower fluctuations. The main question ~ [OhD is whether coolant ' flow might cause motion of detectors (compensated IJ ion' chambers) leading to erroneous' indications. We are convinced that.

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S this_is no.t the case. During apparent power fluctuations, all ion chambers. respond coherently,~.and these' chambers are located in separate /g quadrantsLaround the core.

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Standardization and Special Projects Branch March 26, 1981 Page.2 A second concern over coolant-flow-induced power fluctuations is whether they might be caused by flow-induced displacement of

^ control rods. Since fluctuations take place only in the high-power range, this would seem not to be the case. In this regard, we measure control rod insertion times with coolant flow in operatioa. . and for training purposes at frequencies much greater than -called for- by Technical Specifications. Never have we ob-served effects of coolant flow on control insertion times.

We submit that the power. fluctuations are not unique to the KSU TRIGA Reactor,,that we are.able to monitor the fluctuations, that we.are operating within Technical Specifications, and that the. fluctuations do not risk.the integrity of the reactor nor the safety of personnel.

We~are presently-of the opinion that the fluctuations are caused by. changes in natural convection patterns within the

. coolant-channels,;and consequent fuel-temperature changes, pro-moted by turbulent coolanc flow across the top of the core trans-

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verse to the-. convection plume. As our operation schedule permits,

.we' plan'to investigate these fluctuations more thoroughly in order to: characterize better their nature:and cause.

-Sincerely,

/ :b Richard'E..Faw,nDirector-KSU Nuclear Reactor Facility

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- cc: United States JNuclear -Regulatory _ Commission JRegion'IV

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