ML20012D965

RO 50-20/1990-1:on 900312,incorrect Calculation of Estimated Critical Position (ECP) Attained During Reactor Startup. Caused by Failure to Check Second Portion of ECP Calculation Performed by Individual in Training

ML20012D965
Person / Time
Site:	MIT Nuclear Research Reactor
Issue date:	03/22/1990
From:	Bernard J, Kwok K MASSACHUSETTS INSTITUTE OF TECHNOLOGY, CAMBRIDGE
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
50-20-1990-1, NUDOCS 9003290166
Download: ML20012D965 (4)

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! NUCLEAR REACTOR LABORATORY

, , - AN INTERDEPARTMENTAL CENTER OF

. MASSACHUSETTS INSTITUTE OF TECHNOLOGY

' o.K, HARLING 138 Albany Street, Cambridge, Mass. 02139 ~ J.A. BERNARD, JR.

Diroc'or Telefax No. (617)253-7300 Director of Reactor Operations '

Telex No. 92-1473-MIT CAM Tel. No. (617) 253-421 t/4202 -

March 22,1990 s

U. S. Nuclear Regulatory Commission Washington, D.C. 20555 Attn: Document ControlDesk -

Subject:

Reportable Occurrence 50-20/1990-1, Incorrect Calculation of Estimated Critical

..v, Position '

Gentlemen:

Massachusetts Institute of Technology hereby submits this ten-day report of an occurrence'at the MIT Research Reactor in accordance with paragraph 7.13.2(d) of the ~

Technical Specifications. An initial report was made by telephone to Region I (Dr. P. K.

' Eapen) on 13 March 1990.

The format and content of this report are based on Regulatory Guide 1.16, .

Revision 1.

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1. ' Report No.: 50-20/1990-l' 2a.- Reoort Date: 22 March 1990

< 2b. Date of Occurrence: 12 March 1990

?. " Facility: MIT Nuclear Reactor Laboratory 138 Albany Street Cambridge, MA' 02139

4. Identification of Occurrence:

During a reactor startup on 12 March 1990, the MIT Research Reactor was briefly operated on an unusually short reactor period. - As a result, an automatic safety system shutdown occurred.' No technical specifications were violated and all safety system equipment functioned properly. The o;3eration in this period range was not in accordance 1with internal operating procedures in t 1at a dynamic period of 10.4 seconds was attained -

while that allowed by normal operating procedure is 30.0 seconds with allowance for l

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shorter periods during special tests. The technical specification for the limiting safety ,

system setting is 3 seconds.

5.  : Conditions Prior to Occurrenc_e:

The reactor had been operated normally the previous week and, following a

- weekend shutdown,'was being started up in accordance with the standard operating procedures, f

af 6.' Description of Occurrence:

, As part of the routine reactor startup procedure, an estimated critical position (ECP) ,

is calculated. This is done by first determmmg the net reactivity change that has occurred since the last startup. For example, allowance is made for such factors 'as refuelings, sample changes, primary temperature, and xenon. Once this figure is obtained, it is added (or subtracted as the case may be) to the critical data recorded from the most recent, previous, xenon-free startup to obtain the anticipated critical position for the present startup. For the startup on 03/12/90 a trainee, the console operator, and the reactor supervisor each independently calculated the net reactivity change, which is the first part of the pmcess of determining the ECP. All three did the~ calculation correctly. However, for the second part of that process, both the operator and the supervisor relied on the traince's s , calculation. Unfortunately, the trainee used critical data from a xenon-equilibrium.

l' condition when that for a xenon free (or near xenon-free) condition should have been used.

At 1100, the console operator and the supervisor commenced a reactor startup on

. the assumption that criticality would be attained at about 11.04 inches on the reactor's shim bank. In reality, the correct ECP was 8.60 inches. As the startup progressed, the operator g observed an increasingly short period but attributed it to noise on the instrument channels.

The operator and supervisor also failed to recognize the significance of several other 4

- indicators of approaching criticality including'the need to upscale both the startup channels i and an audible count rate meter. Also, they might have predicted the approach to criticality if they had taken heed of the changes in count rate level on the startup channels.

At 1120, the reactor was automatically shut down by the safety system. The reactor l- supervisor reported the cause of the shutdown as instmment noise and obtained approval for a restart from the Reactor Superintendent. At 1135, the restart began. At 1150 the operator and supervisor again observed an increasingly short period. This time they g recognized it as a true signal and immediately made the reactor suberitical in accordance with existing written procedures that cover an error in the ECP calculation. The

Superin endent was agam notified, the reactor was shut down, and the Director of Reactor Operations was notified.

L 7.- Dgscriotion of Apparent Cause of Occurrence:

This occurrence was the result of several factors. These were as follows:

.y a) The console operator and reactor supervisor did not check the second

[ portion of the ECP calculation which had been performed by an individual L; m traimng.

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b) Excessive reliance was placed on the ECP as the means of identifying -

criticality, c) The console operator and supervisor did not take notice of the indications that the reactor was approachmg cri;icality. These included the earlier-than-usual upscaling of the mstrumentation and the response of the reactor period -

to contml blade motion.

~ d). The existence f L actual short period was' attributed to instrument noise.

Each of the above involves a human error. It should be noted that all equiament functioned properly. However, in the startup range, some _' noise' on some of the c iannels has becc observed in the past. -

8. Analysis of Occurrence:

This occurrence resulted solely in a violation ofinternal operating procedures. No technical specification violations occurred. Following the occurrence, the trip points of the -

three period channels in the safety system were verified. They were found to trip at 10.5, 10.3, and 10.4 seconds. The specification is 10-11 seconds. The limiting safety system

, setting for period is 3 seconds. The excess tractivity present at the time of the automa'.ic shutdown was estimated to be 325 mbeta which corresponds to a steady period of 16 seconds. (Holn: The dynamic effect of blade withdrawal is sufficient to create an.

Instantaneous 10.4 second period given a steady one of 16 seconds.) The reactor neutronic power at the time of the scram was 340 watts. The reactor's heat removal system had been configured for 5 MW, the licensed operating level. Hence, at all times the reactor _was safely within the limiting conditions for operation.

9. Conective Action:

The following corrective actions have been taken:

a) Reponed the occurrence to the Chairman and to several members o'f the MIT.

, Reactor Safeguards Committee (MITRSC). ,

b) Held a meeting with alllicensed personnel on 03/19/90 to discuss both the occurrence and , roper operating practice. Both the MITRSC Chairman and the Director of t le NRL attended and spoke at the meeting stressing the need to give attention to the details of safe operation.

n c) Modified the reactor startup procedure to require two independent calculations of the ECP by licensed operators. (Previously, it was required ,

that the calculation only be reviewed by the supervisor. It is now required that the console operator and supervisor each do the calculation separately.

. Action' implemented via temporary directive 03/19/90. Action to be L complete 05/31/90).

d) Stressed to all operators the importance of sound operating practices including not to rely on trainees and to thoroughly investigate all abnormal instrument readings.

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kl e) ~ Directed electronics personnel to investigate noise effects on the reactor  !

startup instrumentation. ,

L p 10.- Failure Data:

here have been no related occurrences.

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. Sincerely, o Kwan S. Kwok .

.s Superintendent .

MIT Research Reactor

.k o ' d<  !

ohn 'A. Bernard, Ph.D.

. Director of Reactor. Operations

' MIT Research Reactor r.

1-JAB /CRH 1

cc: MITRSC -

USNRC -- Region I- Chief, . ,

_ Reactor Projects Section 3B P- -

USNRC - Region I Project Engineer,-' -;

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Reactor Projects Section 3B -

L USNRC - Senior Resident Inspector, . -

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- Pilgrim Nuclear Station :

USNRC - Project Manager, .s B - Standardization and Non-Power Project Directorate -

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