ML20082B855

RO 50-20/1995-2:on 950320,operation of Reactor in natural- Circulation Mode W/O Required Two Operable Nuclear Safety Level Channels Set to Scram at or Less than 100 Kw,Mitr TS 3.7

ML20082B855
Person / Time
Site:	MIT Nuclear Research Reactor
Issue date:	03/30/1995
From:	Bernard J, Lau E, Newton T NUCLEAR REACTOR LABORATORY
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
5-20-1995-2, NUDOCS 9504060073
Download: ML20082B855 (5)

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NUCLEAR REACTOR LABORATORY

. AN INTERDEPARTMENTAL CENTER OF MASSACHUSETTS INSTITUTE OF TECHNOLOGY

' O. K. HARUNG 138 Abany Street, Cambridge. Mass. 02139 4 296 J. A. BERNARD. JR.

Director ' Telefax No. (617) 253 7300 cirector of Reactor Operatens Telex No. 921473-MIT-CAM Tel. No. (617) 253 4211/4202 March 30,1995 U.S. Nuclear Regulatory Commission Washington, D.C. 20555 Attn: Document Control Desk

Subject:

Reportable Occurrence 50 20/1995-2, Operation with Fewer than the Required Number of <100 kW Nuclear Safety System Level Channels Gentlemen:

The Massachusett: Institute of Technology hereby submits this ten-day report of an occurrence at the MIT Research Reacter (MITR) in accordance with paragraph 7.13.2(d) of the Technical Specifications. An initial report was made by telephone to NRC

~ Headquarters, Mr. Alexander Adams, Jr. at about 1600 on March 21,1995, and to NRC Region I, Mr. Thomas F. Dragoun, at about 1640 the same day. This was followed at 1715 by a more detailed report that was teleconferenced to include both the NRC Headq uarters and NRC Region I (Dr. Seymour H. Weiss, Mr. Alexander ' Adams, Jr., and Mr. Thomas F. Dragoun). Additional information was provided on both 03/23/95 and 03/24/95. It was postulated at that time that the calibration of the low-range amplifiers was incorrect. This has subsequently been verified. ,

1 The format and content of this report are based on Regelatory Guide 1.16, l Revision 1.

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1. Report No.: 50-20/1995 2 2a. Reoort Date: 30 March 1995 2b. Date of Occurrence: 20 March 1995

3. Facility: MIT Nuclear Reactor Laboratory 138 Albany Street Cambridge,MA 02139 7

1. 1 9504060073 PDR 950330 9

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ADOCK 05000020 I PDR

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Reportable Occurrence 50-20/1995-2 ,

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4. Identification of Occurrence: l l

Operation of the reactor in natural-circulation mode without the required two operable nuclear safety level channels set to scram at or less than 100 kW, MITR Technical Specification No. 3.7.

5. Conditions Prior to Occurrence:

'Ihe reactor had been prepared for operation in the natural-circulation mode in order to perform an approved experiment. This mode of operation is pennitted under MITR Technical Specification No. 2.2. The maximum allowed power level is 100 kW and there is no forced flow of coolant through the core. MITR Technical Specification No. 3.7 also xrtains to operation in the natural-circulation mode and one ofits requirements is t 1at there be two neutron flux level channels set to scram at or below 100 kW. This is normally accomplished by replacing two of the three high-range safety channel amplitkrs with low range amplifiers that are capable of scramming the reactor at 100 kW or less. This had been done and the reactor instrumentation had been tested in accordance with the "Startup for Less than 100 kW Operation" checklist, PM 3.1.3.

6. Description of Occurrence:

Reactor startup began at 1222. The low-range amplifiers (Channels #5 and

1. 6) were adjusted in accordance with PM 3.1.3 to cause a scram at 75 kW. The high range amplifier (Channel #4) was set at 5.5 MW. The procedure for conduct of the experiment required a series of flux measurements at various low power levels. The major portion of the experiment was conducted at low power levels

(<50 kW) and, for these power levels, the response of the low-range amplifiers was expected to be quite low. Accordingly, there was no reason to believe that the amplifiers were not fully operable. The maximum power attained during the experiment was 70 kW as indicated by Channel #7, which is a compensated ion chamber that is used to provide indication to the console operator. Two other Channels (#8 and #9) confirmed this. The response of the low-range amplifiers at power levels of 50 70 kW appeared low and subsequent analysis confirmed this.

As a result, the actual scram serpoints on the Channel #5 and #6 low-range amplifiers were 420 kW and 230 kW respectively.

7. Description of Apoarent Cause of Occurrence:

The MIT Research Reactor's nuclear safety system consists of six independent channels that are designated as Channels #1 through #6. Channels

1. 1-#3 provide period scrams. These were fully functional during the occurrence reported here. Channels #4-#6 provide power level scrams. The minimum required complement of nuclear instrumentation is two period and two level l channels. Monitoring of the reactor power by the console operator is performed using three other channels that are not part of the safety system.

Each level channel consists of a neutron-sensitive ion chamber, a voltage supply, and a scram amplifier together with its power supply. Two types of scram amplifiers are used. For forced-circulation cooling of the core, 'high-range' amplifiers are installed in all three level Channels (#4-#6). These are set to scram at 5.5 MW For natural-circulation cooling, two of the high-range amplifiers are

p' s 3A Reportable' Occurrence 50-20/1995-2 Page 3 of 5 s- ...

i gysically removed ese amplifiers and, in their are connected to the place, same ion two ' low-range' chambers as their amplifiers corresponding are inserted.

high-range am )lifiers. 'Ihe result is that, when operating in the natural-circulation mode, Channe . #4 is a high-range amplifier set to scram at 5.5 MW and Channels

1. 5 and #6 are low range amplifiers set to scram at or below 100 kW.

In order to determine the scram setpoints of the low-range amplifiers, the most recent equilibrium indication of the corresponding high-range channel is used -

along with an " amplification factor" to determine a value at which to set the low-range scrams. The equation below describes this process:

low range scuing = .+ offset equilibrium power The above formula is specified in PM 3.1.3 together with an amplification factor of 80. The calculation of the settings for the low-range amplifiers was donc properly in accordance with that procedure.

On 03/28/95 and 03/29/05, the low range amplifiers were placed in parallel with the high-range amplifiers and the reactor was operated in the forced-circulation mode. . Comparisons were made of the readings on each low-range amplifier with Channel #7's output and it was found that the amplification factors for low-range Channels #5 and #6 were actually 10.8 and 7.7 respectively.

8. . Analysis of Occurrence:

This occurrence involved a failure to maintain the minimum complement of low-range safety channels during operation in the natural-circulation mode. No challenge of the safety system took place during this occurrence. Accurate indications of the reactor power were available to the reactor operator at all times via Channels #7, #8, and #9 which were fully functional, and power never exceeded .

70 kW.

All of the other components of the reactor's safety system that were required for operation in the natural circulation mode were fully operable. This included the ~

three period scram Channels (#1 - #3) as well as two primary coolant outlet ,

temperature scrams. It is known that these would have caused a scram had the  !

reactor power approached 100 kW. In addition, the level scrams on Channels #5 l and #6 were fully functional and would have scrammed the reactor at approximately 400 kW and 230 kW, respectively.

It should be noted that operation of the MIT Research Reactor in the natural circulation mode is rare. It is used primarily for refuelings, and the installation / removal of in core experiments. For these operations the reactor is never critical and the reactor's reactivity state is the maximum possible negative value (i.e., all blades fully inserted and reflector dumped). The low-range amplifiers are a technical specification requirement but, under this reactivity condition, they serve no purpose and have no safety significance. Operation of this type usually occurs for a few hours per month.

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Reportable Occurrence 50-20/1995-2 Page 4 of 5 Critical operation in the natural circulation mode is even rarer. Four types of activities are possible:

(a) Intermediate Criticals Daring Refuelings: If a refueling is complex and/or if it involves significant reactivity, an intermediate critical is performed to confirm the accuracy of the reactivity calculations. These occur every few refuelings, perhaps once or twice a year. The reactor power is limited to I kW by Technical Specification No. 3.1 during such criticals and is kept at a few hundred Watts for only a few minutes.

(b) Blade Calibrations: By procedure these can be donc under either natural-circulation or full-flow conditions. No use of the natural-circulation mode for this purpose has occurred in the last ten years.

(c) Reactor Physics Exoeriments: These include xenon transients and subcritical multiplication for MIT courses. These can be ; ne under natural-circulation or full flow conditions. No use of the natural-circt > tion mode for this purpose has occurred in the last ten years.

(d) Flux Synthesis Exoeriments: This experiment, which is now complete, was run in the natural-circulation mode at power levels above ikW in December 1993, A pril 1994, and March 1995. These are believed to be the only instances wherein the low-range amplifiers have been used for critical operation at power levels in excess of 1 kW during the last ten years.

9. Corrective Action:

(a) The low-range amplifiers were bench-tested for functionality and found operable. In addition, the as-built amplifiers were compared to the design drawings. No discrepancies were found.

(b) The high-range am 31ifiers and their associated circuitries were verified to be fully functional anc in proper calibration.

(c) The amplification factors of the low range amplifiers was determined both by electronic testing and by reactor operation in parallel with the high-range amplifiers. The amplification factors were determined to be 10.8 and 7.7 for Channels #5 and #6 respectively.

(d) A requirement to verify the am on a periodic basis (annual) is to beplification instituted.factor of be This will thedone low byrange amplifiers operating these amplifiers in parallel with the high-range amplifiers in the reactor under full flow conditions. (Initial tests done on 03/28/95-03/29S5.

Permanent procedure to be in effect by 04/30/95.) This item of corrective action resolves the root cause of the occurrence.

(e) PM 3.1.3, "Startup for Less than 100 kW Operation" is to be modified to reflect the correct amplification factor. (Temporary change made; permanent change to be made by 04/07/95).

In addition to the above, all of the safety amplifiers, including the low-range amplifiers, are scheduled for replacement later this year. The new equipment will L i

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, Reportable Occurrence 50-20/1995-2 L

• - Page 5 of 5 be capable of indicating the detector response over the full range of reactor operation, from startup to full power.

10. Failure Data:

ROR #50-20/1993-1 involved the failure of a relay in one of the low-level  ;

scram amplifiers, resulting in the loss of detection signal to the amplifier. 1 Sine ly,

, ffy Qt f Q Edward S. La , NE Thomas H. ewton, Jr., PE Asst. Superintendent for Engineering Asst. Superintendent for Opera: ions MIT Research Reactor MIT Research Reactor

/Q O J hn A. Bernard, Ph.D. j '

Director of Reactor Operations MIT Research Reactor j JAB /gw  ;

1 cc: MITRSC (with enclosures) l USNRC - Project Manager, NRR/ONDD USNRC - Region I- Project Scientist, Efiluents Radiation Protection Section (ERPS)

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