ML19221A241

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Updates R Balls 790417 Memo Detailing Rationale for High Counting Rate of B-F-3 Neutron Detecting Startup Channel
ML19221A241
Person / Time
Site: Crane 
Issue date: 04/20/1979
From: Stello V
Office of Nuclear Reactor Regulation
To: Richings H
Office of Nuclear Reactor Regulation
References
OLE-790420, NUDOCS 7905210055
Download: ML19221A241 (10)
v • d • e


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4/20/19 M. A. Schultz IAG Memo alA-23 ON THE HIGH COUNTING RATE OF ThE BF 3 NEUIRON DETECTING STARTUP CHANNEL

'((I:i_,This memo is an update of ihe memo of April 17, 1979 by R. Ball.

It addresses

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the problem of why the startup channels apparently read highcr than normal.

The two startup ch'nnels, as of 9:00 a.m. on April 19, 1979, were reading

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a 33 c/sec and 22 c/sec r[espictively.

The normal readings for these detectors was originally reported-to be about 1 c/sec af ter a long shutdown.

Later verbal information for a shutdown in January 1979 indicated stable counts of 6 and 5 cps.

for these channels at 5 days af ter shutdown.

All attempts to obtain the records of earlier shutdowns have failed.

. The reason for the pre'sent high channel readings has been under investigation

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~for several days and the following effects have been looked at:

i.

Buildup of Additional Neutron Sources a.

The growth of Cu-242 und Cu-244 was detennined from the ORIGEN code.

4 The code indicated a buildup of cnly 1.14 x 10 n/sec/MTU in th'e core which would appear to be too small to appreciably in #1uence the detectors.

b.

The creation of. a photo-neutron source from high energy garna rays v.

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.. interacting with the b 0 in the reactor was also considered.

La-140 2

was the principal isotope of concern as it emits a 2.5 Mev ganna ray.

Ball's original estimate of counting rate decay indicated a small isotope production having possibly a half-life of 20 days.

As more data was obtained with time, the.. curve of Figure 1 possibly indicates that a 12 day half-life emitter might indeed by present (maybe Ba-140 a precursor of La-140).

Figure 1 also suggests some sort of counting rate fl,cor -

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level might be reached at about 30 c/sec.

(Later data on 4/20 suggest that the counting rate might still be dropping off.)

This effect will -

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2.

ChangeofCounthatewithTemocrature The same data of Figure 1 was plotted against outlet coolant temperature, looking for 6ther correlations.

Figure 2 shows this plot, which is quite dif ficult to'explaFn.' The s' harp drop in counting rate at about 242 F as the core temperature was being reduced from 278 F to 231 F is puzzling and may simply be coincidental to the above time decay and have no meaning.

On the other hand, some interesting phenomenon involving changes in density in the ' reactor may be present.

The fact that both time and temperature were being simultan'eausly varied confuses the picture.

The count rate also dropped somewhat when the temperature was reduced from 231 F to 175 F.

3.

Movement of Original Sources Two fixed neutron sources of approximately 1.4 x 10 n/sec each were located roughly 90 away from each BF-3 cetector.

The hypothesis has been made that during the accident these sources moved closer to the detectors.

T It appears unlikely that both sources would rotate about the same amount y

a,.

to cause similar increases in both detector readings.

4.

Faulty Detectors One might well imagine the detectors being injured during the accident.

Unfortunately, there is no known mechanism to increase the sensitivity of these type tubes.

However, the possibility that the detectors read high because of'the huge gamma _ fields is real and was investigated.

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Faul ty_ Cetec tors (Cont. )

step was to run a pulse height selector curve en Channel 1.

This curve is shown on Figure 3 in comparison with the original pulse height selector [ ~

. _ = -,- curve taken early in the checkout of TMI-2.

The top curve which is the prfesent-day curve, indicates pile-up of gamma pulses at icw discriminator settings, r

and some drop-of f-at high settings.

This behavior is quite norma for BF-3 tubes when operating in a field of

-rir approximately 200-500 R/hr.

As each detector was shielded by a two-inch

=

thick annulus of lead, the non-shielded field at the detector location mighc well have been 20,000 R/hr, probably ccming mostly from the core.

The discriminator was set at 0.9v which is a reasonable setting and the tube

~

appeared to be operating quite well.

The change in counting rate caused by gamma rays would, therefore, be very small.

5.

Changes in Boron Concentration Changes in boron concentration have been going on in the primary loop for some time and measurement methods differ.

The concensus seems to be that the present concentration is somewhere between 3000 to 3500 ppm. And as of this date, Jeborating has been going on for the last 4 days at the rate of

y. -

m about FS ppm / day.

Two efvects might be postulatad.

The first of these is that the baron would absorb thermal nautrans and the counting rate would decrease.

This is not' the case as the detector arrangement is such that only fast neutrons moderated near the detector will be counted.

This rtatement has been confirmed by measurements at B&W, A more serious concern is that reduced boron concentration will increase the core multiplication.

The additional fast neutrons,would ~

then be counted.

The effects of 80 ppm in. reactivity are being investigated 14 201 l

t

~__

N A

_3_

Chances in Boron Concentration (Cont.)

5.

B&W rcports that the sensitivity to boron additicn of the at B&W and NRC.

Neither a gr ss' criginal core at 200 F and 3000 ppm is 12 pcm/ ppm of boron.

E(( m_.

_ decrease lir' ir[creash Tn^lccunting rate has been corrolated against the present boron manipulation in the core.

In effect, this sensitivity indicates that 4 days of deborating has theoreti-cally creat'ed a large change in reactivity of maybe 3.8% that went unnoticed

_ _ Hopefully, this means large sub-criticality.

by the detectors.

Increased Core Multiplication Because of Changed Geometry 6.

The original core was under moderated and any compaction of the core would be expected on first thoug'ht to reduce the core sub-critical multiplication.

However, the present gecmetry is clearly unknown and the fate of the burnable These rods containing boron carbide in an poison rods is also uncertain.

aluminum oxide matrix possibly have been exposed to the coolant and their solubility and dispersion characteristics are being checked at Oak Ridge.

In any evcnt, these rods contained approximately 3% in reactivity and their presence or absence'in the core is important, mr

~ l The overall contribution of core multiplication to the high counting rate

~

However, if tre entire increased counting rate is assumed is clearly unknown.

to be coming from increased multiplication, a back of the envelope calculation, dhful making the assumption that source and structure are unchanged (a bit of wi f the core to be 0.983.

This violates thinking), indicates the present keff the " Zinn" law that all reactors must have at least 2% in shutdown reactivity.,,,

l ~..-

f 0.983, then the deborating of

=. - ~

However, if the core were.indeed at a keff 3% in reactivity,wouid have_obviously been r.oticed.

w.

p.

~

141 202 t.

~~

N

,. CONCLUSIONS The instruments appear to be operating properly and give every indication of.

reading neutrons.

Hence, their rtadings cannot be ignored.

If the count rate

. = - -,

tontinues to decline' over-the next month, there is no problem and the core can be considered stable at a large subcritical reactivity.

If the counting rate does not continue to decline, then some ao'.iitional core multiplication must be assumed in order' ~to-be prudent.

Prudence also calls for inmediately suspending

-5

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