ML19221A641

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Task 23:Examine High Counting Rate of BF3 Neutron Detector
ML19221A641
Person / Time
Site: Crane Constellation icon.png
Issue date: 04/20/1979
From: Shultz M
INDUSTRY ADVISORY GROUP
To:
References
OSC-790420, TASK 23, TASK-23, NUDOCS 7905230498
Download: ML19221A641 (9)
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IAG Memo #IA-23 ON THE HIGH COUNTING RATE OF THE BF3 C

NEUTRON DETECTING STARTUP. CHANNEL This memo is an update of the memo o_f April 17, 1979 by R. Ball.

It addresses the problem of why the startup channels apparently read higher than nonnal.

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

,33 c/sec and 22 c/sec respectively. The normal readings for these detectors was originally reported to be about 1 c/sec after a long shutdown. Later verbal information for a shutdown in Janua'ry 1979 indicated stable counts of 6 and 5 cps for these channels at 5 days after shutdown. All attempts to obtain the records of earlier shutdowns have failed.

The reas.on for the present high channel readings has been under investigation for several days and the following effeci.s have bee, looked at:

b 1.

Buil<iuo of Additional Neutron Sources a.

The growth of Cu-242 and Cu-244 was determined from the ORIGEN code.

4 The code indicated a buildup of only 1.14 x 10 n/sec/MTU in the core which would appear to be too small to appreciably influence the detectors.

b.

The creation of a photo-neutron source from high energy gama rays interacting with the D 0 in the reactor was also considered. La-140 2

was the principal isotope of concern as it emits a 2.5 Mev gama 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 166 O U.

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$g g 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

.g have to be watched with time and the data continue to be collected.

2.

Charae of Count-Rate with Temperature The same data of Figure 1 was plotted against outlet coolant temperature,

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looking for other correlations. Figure 2 shows this plot, which is quite difficult to explain. The sharp drop in counting rate at about 242 F as 0

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 simultaneously varied confuses the picture. The count rate also dropned somewhat when the temperature was reduced from 231 F to 175 F.

'1 3.

Movement of Original Sources 9

s Two fixed neutron sources of approximately 1.4 x 10 n/sec each were

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located roughly 90 away from each BF-3 detector. The hypothesis has been made that during the accident these sources moved closer to the detectors.

It appears unlikely that both sources would rotate atmut the same amount 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 gama fields is real and was investigated. The first i

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This curve is

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shown on Figure 3 in comparison with the original pulse height selector curve taken early in the checkout of THI-2..The top curve which is the present-day curve, indicates pile-up of gamma pulses at low discriminator settings, and some drop-off at high settings.

This behavior is quite normal for BF-3 tubes when operating in a field of 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 might well have been 20,000 R/hr, probably coming 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 ccacensus seems to be that the present concentration is somewhere between 3000 to 3500 ppm. And as of this date, deborating has been going on for the last 4 days at the rate of

.. about 80 ppm / day.

Two effects might be postulated.

The first of these is that the boron would absorb thermal neutrons and the counting rate would decrease. This is ont the case as the detector arrangement is such that only fast neutrons mc.-rated rear the detector will be counted.

This statement has been confirmed by measurements at B&W. A more serious concern is that reduced bcron concentration will increase the core multiplication. Ths additional fast neutrons wo0ld

~

The effects of 80 ppm in reactivity are being investigated then be counted.

166 128

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'M W M.'..~ ^.

~ a g.3 a y m :a

5. ~ Changes in Boron Concenter. tion (Cont.)

/

@h at B&W and NnC., B&W reports lthat the sensitivity to boron addition of the original, core at 200 F and 3000 ppm is 12 pcm/ ppm of boron.

Neither a gross decreas,e or increara in counting' 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 created a large change in reactivity of maybe 3.8% that went unnoticed by the detectors. Hopefully, this n.eans large sub-criticality.

6.

Increased Core Multiplication Because of Changed Geometry The.criginal core was under moderated and any compaction of the core would be expected on first thought to reduce the core sub-critical multiplication.

However, the present geometry is clearly unknown and the fate of the burnable poison rods is also uncertain. These rods containing boron carbide in an aluminum oxide matrix possibly have been exposed to the c7olant and their solubility and dispersiori characteristics are being checked at Oak Ridge.

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

The overall contribution of core multiplication to the high cuunting rate is clearly unknown.

However, if the entire increased counting rate is assumed to be coming from increased multiplication, a back of the envelope calculation, making the assumption that source and structure are unchanged (a bit of wishful thinking), indicates the present k of the core to be 0.983. This violates eff the " Zinn" law that all_ reactors must have at least 2% in shutdown reactivity.

However, if the core were indred at a k of 0.983, then the deborating of gff 3% in reactivity would have obviously been noticed.

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

/

reading neutrons. Hence, their readings cannot be ignored.

If the count rate continues 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, tnen some additional core multiplication must be

, assumed in order to be. prudent. Prudence also calls for imediately suspending deboration.

m e

9 166 130

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