ML20037A925

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Exam of Vbwr Control Rods
ML20037A925
Person / Time
Site: Dresden Constellation icon.png
Issue date: 05/15/1958
From: Brandt F
GENERAL ELECTRIC CO.
To:
References
VAL-80, NUDOCS 8008130141
Download: ML20037A925 (14)
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EXAMINATION OF VBWR CONTROL RODS l

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VAL-80 Irradiations EXAMINATION OF VBWR CONTROL RODS By i

F. A. Brandt May 15, 1958 i

Approved By:

hN 4 Irradiation Projects Unit Irradiations Sub-section Address inquiries to Vallecitos Atomic Laboratory General Electric Company P. O. Box 846 Pleasanton, California

AUTHOR-COMPONENT StlBJECT CATEGORY REP. NO.

VAL-80 F. A. Brandt DATE

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Vallecitos Atalic Laboratory Reactor Components May, 19.i8 TITLE CL ASSIF.

EXAMINATION OF VBWR CONTROL RODS NO.PAGES 10 A8STRACT The control rods first used in the V3WR were composed of boron carbide blocks arranged in columns covered with a thin rectangular stainless steel sheath. After several months operation it was noted that boron carbide. par-ticles were collecting in the reactor water, and a change in control rod worth was noted.

Two of the rods were rcr.eved and brought to the Irradia-tions Sub-Section Hot Cells for examination to determine the condition of the boron carbide.

Several two inch discs were cut remotely from the stainless steel sheaths of the control rods numbers 5 and 7 to expose their interiors. Approxi-mately 507,of the B C plates were missing fron one column of rod number 7 4

(narrow), and there was evidence that some plates were missing from the other column. There seemed to be no missing plates in rod 5 (wide).

The remaining plates in the high flux zones were badly eroded, were easily scratched to a fine powder, and in rod 5 only were severely cracked.

It is concluded that in high flux areas the B C material was softened on the surface by the neutron 4

flux and was washed away as a fine powder by water streams circulating through the side seams of the rod sheaths, t

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l DISTRIBUTION M. Cain J. Violette L. Kornblith (3)

J. Wolcott E. Lee VAL Library S. Untermyer APED Library

3 EXAMINATION OF VBWR CONTROL ROUS F. A. Brandt PURPOSE The control rods first used in the VBWR were composed of boron carbide blocks arranged in columnt covered with a thin rectangular stainless steel-sheath. After deveral months operation it was noted that boron carbide par-ticles were collecting in the reactor water, and a change in control rod worth was noted.

Two of the rods were removed and brought to the Irradia-tions Sub-Section Hot Cells for examination to determine the condition of the boron carbide.

SUMMARY

-Several tvr. inch discs were cut remotely from the stainless steel sheaths of the control rods numbers 5 and 7 to expose their interiors. Approximately 50% of the B C plates were missing from one column of rod number 7 (narrow),

4 and there was evidence that some plates were missing from the'other column.

There semned to be no missing platea in rod 5 (wide). The remaining plates in the high flux zones were badly eroded, were easily scratched to a fine powder, and in rod 5 only were severely cracked.

It is concluded that in high flux areas the B C material was softened on the surface by the neutron 4

flux and was washed away as a fine powder by water. streams circulating through the side setns of the rod sheaths.

BACKGROUND The VBWR control rods are in the form of rectangular plates 76" long by

.4" thick and 6" or 12" wide (Refer to drawing 585D433). The active pertien of the plates extends 46 inches from the bottom and is composed of blocks 2 7/8" square by

.3" thick of high purity boron carbide of 2.2 g/cc minimum density purchased from the Norton Company. The boron carbide plates were placed in 2 or 4 columns along the length of the plates separated by stain-1ess steel dividing strips on their vertical edges only. The plate assembly was covered with a 50 mil stainless steel sheet on each side.

These had rolled edges along their length which were overlapped and spot welded at about 2 inch intervals. This joint was not watertight.

' SERVICE IN VBWR These rods were used to control the VBWR during the period July, 1957, to February, 1958. The number 7 rod (narrow) was located in the central por-tion of the core, was more or less stationary while the reactor was operating, and was fully inserted into the core. The number 5 rod was located in the east central portion of the core, was generally withdrawn approximately 50 percent of its active length, and was moved up and down in the core during operation.

The water in the VBWR core is at a temperature of 546 F and a pressure of 1000 psig during operation. Boron carbide particulate matter was discovered in the VBWR, and it.was noted that the control rod worth was i

chac,ing in February, 1958.

It was decided to shut down the reactor, replace the control rods, and to examine number 5 and 7 because they were in the high-est flux zone.

OPERATIONS AT IRRADIATIONS RML A special in-cell machine was constructed to hold the rods and cut two inch diameter holes through the cladding at selected locations.

Since the con-trol rods are long, an entire cell was used to carry out the operations. The holes were cut with a fly cutter mounted in a special milling head, and it proved very difficult to cut the holes especially at locations where the boron carbide was thin or absent, thus leaving no support for the metal.

It was necessary to use a new tool for each cut and several cuts were not successful.

OBSERVATIONS The material under the cut-outs was observed and photographed with a scanning periscope at 1/8X and 1.5X.

Figures 1 and 8 are sketches of the con-trol rods showing the location of the successful cuts. Figures 2 through 7 and 9 through 12 are photographs of the cuts at magnifications indicated under the pictures. A summary of the visual observations follows:

Control rod 7 (narrow) - Column 1 1.

Interior woe empty of baron carbide from the top to the middle of the rod (Figure 5).

2.

The boron carbide was continuous from the middle to the bottom.

3.

The boron carbide at the bottom appeared undamaged.

4.

The upper edge of the boron carbide at the middle appeared undam-aged (Figures 6, 7).

Control rod 7 (narrow) - Column 2 1.

The boron carbide at the top of the rod appeared undamaged (Figure 2).

2.

A boron carbide plate exposed at the middle was eroded to about hsif l

its previous thickness, and the outboard edge was badly eroded.

(Figures 3, 4, 7).

The plate could be moved about an inch along the length of the rod indicating that material was missing along the length of the column.

l Control rod 5 (wide) 1.

The boron carbide at the top of the rod appeared undamaged (Figure 9).

2.

The boron carbide at the middle appeared whole but was soft (Figure 10).

3.

The boron carbide at the bottom of the rod was badly cracked and eroded.

Erosion of the cracked edges showed that cracking occurred previous to removing the element (Figures 11, 12).

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. 2 The hardness of the material was checked with a slave-held-scratch' awl

- or tt slave finger.

All exposed B C surfaces which had been lbs high flux 4

areas were very soft and formed a *ine unifonn size powder where scratched.'

The empty area of control rod 5 was very cican with no debris of any sort visible. (Figure 5) Material under the weep holes at the top of the active area appeared unaffected (Figure 2).

The post-irradiation examination of control rods-5 and 7 from the VBWR indicates that boron carbide as fabricated for these rods will not withstand the combined effects of neutron irradiation and moving high temperature water.

KAPL-1403, "The Effect of Irradiation on Hot Pressed Boron Carbide" dated November 15,.1955, indicated tisat this material when irradiated :bs vacuo, in helium, and in sodium lost "hardntss and cracked at very low burnup". The loss of hardness was confirmed in this investigation and would indicate that the neutron irradiation was the basic cause of the damage to the VBWR rods and that the action of the water was subsequent.

It is believed that the water circulating freely through the many openings in the rods washed away 1

the surface material softened by neutron irradiation. Bearing out this con-jecture is the observation that the material under the weep holes at the top of the rod, where the neutron flux was_small and where the water could freely reach the boron carbide, was unaffected.

It is believed that the anomaly occurring in column 1 of rod 5 (Figures 6,17) where a hard, undam-aged, plate was found near the center was caused by this plate slipping down from the top of the rod. Cracking of the boron carbide was also observed 3

in this investigation but only in control rod 5 where conditions were more likely to produce thermal stresses. As stated previously, rod 5 was moving in the neutron flux field, whereas red 7 was stationary.

It is likely that rapid heat cycling and, therefore, thermal stresses are caused when the rod moves, and these would cause cracking in the brittle boron carbide.

It can be concluded that boron carbide of'this grade will soften on the surface in a neutron flux, may crack under thermal stresses, and when degraded in this fashion will erode rapidly in circulating water at ele-vated temperatures.

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