Monitoring Degradation of Phenolic Resin-Based 

Neutron Absorbers in Spent Nuclear Fuel Pools 

Technical Letter Report 

Matthew A. Hiser, USNRC 

April L. Pulvirenti, USNRC 

Mohamad Al-Sheikhly, University of Maryland College Park 

U.S. Nuclear Regulatory Commission 

Office of Nuclear Regulatory Research 

June 2013
 

i 

Executive Summary 

This report compiles and analyzes the available information on Carborundum and Tetrabor to 

assess the adequacy of surveillance methods for monitoring degradation of phenolic resinbased absorbers. Carborundum and Tetrabor are very similar neutron absorber materials that 

incorporate boron carbide powder in a binder of phenolic resin. Carborundum neutron absorber 

material was manufactured in two forms: thick rigid plate and thin flexible fiberglass-reinforced 

sheet. Tetrabor was produced by a different manufacturer, but is described as being similar to 

Carborundum plate. Qualification testing was performed on both Carborundum and Tetrabor 

prior to installation to assess their ability to perform in the spent fuel pool environment. Limited 

information is available regarding the Tetrabor qualification testing, while much more information 

is available regarding the Carborundum qualification testing. Operating experience of phenolic 

resin-based neutron absorbers includes wrapper plate bulging, loss of B-10 areal density, and 

results from coupon and in situ neutron attenuation testing. There have been operating 

experience events of rack wrapper plate bulging due to offgassing from phenolic resin-based 

neutron absorbers in sealed panel cavities. Coupon testing, including visual examinations, 

dimensional and weight loss measurements, and neutron attenuation testing, has found steady 

weight loss over time, but no degradation outside the acceptance criteria has been identified by 

visual examination or neutron attenuation testing of coupons. However, the in situ neutron 

attenuation testing campaign performed at Palisades found significant reductions in B-10 areal 

density below the minimum credited amount. The B-10 areal density plotted as a function of 

gamma dose for Carborundum plate at Palisades does not appear to show a strong correlation 

between loss of B-10 and absorbed gamma dose, potentially indicating other factors may 

influence boron carbide loss. 

In order to understand the qualification testing results and operating experience, this report 

describes the mechanisms of radiolysis of phenolic resins in the spent fuel pool environment. In 

brief, alpha, beta, and gamma radiation breaks bonds and creates free radicals in the phenolic 

resin, leading to scission of the polymer backbone and degradation of the polymer structure. In 

the presence of water and oxygen, degradation is accelerated by the introduction of more 

aggressive chemical species, including hydroxyl radicals and hydrogen peroxide. A key step in 

the progression of degradation is the loss of polymer material from the matrix through erosion 

and dissolution, leading to release of boron carbide from the panel. Erosion and dissolution of 

the polymer material may be influenced by the rate of flow and water exchange that occurs 

between the panel cavity and pool. Greater flow and pool water exchange would increase the 

degradation and dissolution of the phenolic resin by providing fresh water to the panel cavity. 

The flow rate is highly variable and dependent on the original rack design or modifications made 

to the racks during service. 

Boraflex, a silicone-based polymer neutron absorber, shows a strong correlation between loss 

of B-10 areal density and absorbed gamma dose. While the radiation chemistry of phenolic 

resin-based materials would lead one to expect a similar correlation for Carborundum and 

Tetrabor, the in situ neutron attenuation data collected for Carborundum at Palisades does not 

exhibit this trend. In the Palisades in situ neutron attenuation data, panels with apparently 
 

ii 

similar absorbed gamma doses had no indicated degradation while others had indications of 

significant degradation. 

It is expected that flow and pool water exchange in phenolic resin-based absorber panel cavities 

could be a dominant factor in the boron carbide release process. While many spent fuel racks 

containing Boraflex are open to constant exchange with bulk pool water, phenolic resin-based 

neutron absorber rack panel cavities were usually designed to be sealed from the pool. Panel 

cavities in the Palisades spent fuel racks were only vented to the pool through small holes 

drilled in the wrapper plate to relieve pressure from offgassing after installation. These holes 

were typically drilled at the top of the wrapper plate and would provide little or no flow or pool 

water exchange with the cavities surrounding the phenolic resin-based panel. Potentially, the 

lack of correlation with absorbed gamma dose may be explained by the effect of flow and pool 

water exchange factors on the process of boron carbide release. However, because the flow 

and pool water exchange factors at Palisades are unknown, the amount and rate of B-10 

release may not be predicted by gamma dose alone. 

For several forms of qualification testing, discrepancies between the qualification test data and 

operating experience encountered during service were identified. These discrepancies are 

attributed to the accelerated nature of qualification testing, which was non-conservative in two 

ways: a much higher dose rate and a much shorter exposure time in water. 

The report analyzes several forms of surveillance testing for Carborundum. For in-pool coupon 

testing, given uncertainties in the exact nature of the panel cavity environment and the boron 

carbide release process, it is difficult to assess whether the design of in-service coupons is 

representative of the service environment. For in situ direct measurement neutron attenuation 

methods such as BADGER, there are numerous uncertainties (Chapman and Scaglione, 2012). 

With adequate consideration for these uncertainties, in situ data could be valuable in assessing 

the B-10 loss of individual panels of phenolic resin-based absorbers. However, given the 

inconsistencies in rack design and flow rates for Carborundum pools, there is additional 

uncertainty associated with extrapolating the B-10 loss of a limited number of phenolic resin 

panels to the degradation and B-10 losses of all the panels in a spent fuel pool. 

Therefore, the report concludes that even though both Boraflex and Carborundum are polymerbased neutron absorbing materials, for Carborundum the loss of B4C from the degraded 

polymer matrix does not show a strong correlation to absorbed gamma dose. The lack of 

correlation between gamma dose and B4C loss has two significant implications. First, it 

precludes the development of a predictive model to inform which panels to interrogate with an 

in-situ B-10 areal density measurement method. Second, one cannot predict the degradation of 

Carborundum panels throughout the spent fuel pool from a few discrete B-10 areal density 

panel measurements. 

 
 

iii 

Table of Contents 

Executive Summary ....................................................................................................................... i

Table of Contents ......................................................................................................................... iii

List of Figures ............................................................................................................................... v

List of Tables ................................................................................................................................ vi

1. Introduction ............................................................................................................................... 1

2. Background ............................................................................................................................... 2

2.1 Spent Fuel Pool Storage ..................................................................................................... 2

2.1.1 Neutron Absorber Materials .......................................................................................... 2

2.1.2 Management of Boraflex Degradation .......................................................................... 3

2.2 Carborundum and Tetrabor ................................................................................................. 5

2.2.1 Carborundum ................................................................................................................ 5

2.2.2 Tetrabor ........................................................................................................................ 6

3. Qualification Testing and Operating Experience ....................................................................... 7

3.1 Qualification Testing ............................................................................................................ 7

3.1.1 Carborundum ................................................................................................................ 7

3.1.2 Tetrabor ...................................................................................................................... 10

3.2 Operating Experience ........................................................................................................ 10

3.2.1 Bulging of Rack Wrapper Plate ................................................................................... 10

3.2.2 Coupon Testing Results ............................................................................................. 12

3.2.3 In Situ Testing Results ................................................................................................ 18

4. Radiation Chemistry of Phenol Formaldehyde Resins ........................................................... 21

4.1 Degradation of Polymers in the Spent Fuel Pool Environment ......................................... 21

4.1.1 Direct Radiation Effects: Scission and Cross-linking .................................................. 21

4.1.2 Radiolysis of Water ..................................................................................................... 23

4.1.3 Effect of Oxygen Presence ......................................................................................... 24

4.2 Radiation Chemistry of Phenol Formaldehyde Resins ...................................................... 25

4.2.1 Degradation Process in the Absence of Water or Oxygen ......................................... 25

4.2.2 Degradation Process in the Presence of Water .......................................................... 26

4.2.3 Degradation Process In the Presence of Oxygen ....................................................... 28

4.2.4 Degradation Products of the Phenol Formaldehyde Resin ......................................... 30

4.2.5 Radiation Dose Tolerance of Phenol Formaldehyde Resins ...................................... 30
 

iv 

4.3 Effect of Variables in the Surrounding Environment .......................................................... 32

4.3.1 Water Temperature ..................................................................................................... 32

4.3.2 Filler Materials: Fiberglass, Graphite, and Boron Carbide Particles ........................... 33

4.3.3 The Presence of Oxygen ............................................................................................ 33

4.3.4 Pool chemistry: pH, Soluble Boron, and Metal Ions ................................................... 34

4.3.5 Water Exchange between Panel Cavity and Pool ...................................................... 34

4.3.6 Flow Rate within the Panel Cavity .............................................................................. 37

4.4 Summary ........................................................................................................................... 37

5. Discussion ............................................................................................................................... 39

5.1 Radiation-Assisted Degradation of Phenolic Resins and Empirical Data .......................... 39

5.2 Rack Conditions and Boron Carbide Release ................................................................... 39

5.3 Adequacy of Qualification Testing and Surveillance Methods .......................................... 42

5.3.1 Qualification Testing ................................................................................................... 42

5.3.2 Total Organic Carbon Monitoring ................................................................................ 43

5.3.3 Coupon Testing .......................................................................................................... 44

5.3.4 In Situ Testing ............................................................................................................. 45

6. Conclusion .............................................................................................................................. 47

References .................................................................................................................................. 49

 
 

v 

List of Figures 

Figure 2-1 Examples of absorber panel wrapper plates.......................................................... 2 

Figure 2-2 PDMS chemical structure...................................................................................... 3 

Figure 2-3 Chemical structure of phenol formaldehyde resin.................................................. 5 

Figure 3-1 Hydrogen gas evolution as a function of gamma dose........................................ 10 

Figure 3-2 Weight loss data and projection over time from ASA Carborundum 

coupons at Calvert Cliffs...................................................................................... 14 

Figure 3-3 Nominal B-10 areal density in Carborundum plate panels as a function 

of gamma dose.................................................................................................... 19 

Figure 3-4 Minimum B-10 areal density in Carborundum plate panels as a function 

of gamma dose.................................................................................................... 19 

Figure 4-1 Scission of ether bond in phenol formaldehyde resin by ionizing radiation......... 26 

Figure 4-2 Hydrogen abstraction in phenol formaldehyde resin by ionizing radiation........... 26 

Figure 4-3 Reactions of hydroxyl radicals with phenol formaldehyde resins......................... 28 

Figure 4-4 Reactions of phenol formaldehyde resin free radicals producing peroxyl 

radicals followed by superoxide elimination......................................................... 29 

Figure 4-5 Reactions of phenol formaldehyde resin free radicals producing peroxyl 

radicals followed by the tetraoxide intermediate formation and 

decomposition...................................................................................................... 29 

Figure 4-6 Methylene hydrogen abstraction by peroxyl radical............................................. 30 

Figure 4-7 Effects of radiation on polymers as a function of absorbed dose........................ 31 

Figure 4-8 Degradation of mechanical properties of polymers as a function of 

absorbed dose..................................................................................................... 32 

Figure 4-9 Reaction of hydroxyl radical and hydrogen peroxide with phenyl........................ 35 

 
 

vi 

List of Tables 

Table 2-1 Effects of gamma irradiation on PDMS polymer in Boraflex.................................. 4 

Table 2-2 Composition of Carborundum neutron absorber materials.................................... 5 

Table 2-3 Summary of Carborundum and Tetrabor neutron absorber material 

characteristics and installations............................................................................. 6 

Table 3-1 Composition of Gas Evolved from Carborundum Plate Material........................... 8 

Table 3-2 Change in Weight of Carborundum Plate and Sheet Material as a 

 Function of Dose.................................................................................................... 8 

Table 3-3 Change in Dimensions and Volume of Carborundum Plate Material as a 

 Function of Dose.................................................................................................... 9 

Table 3-4 Change in Dimensions and Volume of Carborundum Sheet Material as a 

 Function of Dose.................................................................................................... 9 

Table 3-5 Gas Composition from Test Specimen in Haddam Neck..................................... 11 

Table 3-6 Summary of Wrapper Plate Bulging Events......................................................... 12 

Table 3-7 Areal density results from blackness testing of Tetrabor coupons at 

Kewaunee............................................................................................................ 16 

Table 3-8 Summary of Coupon Testing Programs and Results........................................... 17 

Table 4-1 Radiation Induced Changes to Common Polymers............................................. 22 

Table 4-2 Summary of degradation process in possible environments............................... 38 
 

1 

1. Introduction 

NRC regulations, particularly 10 Code of Federal Regulations 50.68 and General Design 

Criterion 62, require that the subcriticality of spent fuel pools be maintained during both normal 

operation and under design basis accident scenarios. Over time, many licensees have installed 

high density spent fuel storage racks that incorporate neutron absorber panels containing boron 

carbide to ensure subcriticality. These neutron absorbers have come in various forms, including 

borated stainless steel, aluminum-based composite materials such as Boral and Metamic, and 

polymer-based materials such as Boraflex, Carborundum, and Tetrabor. In general, polymerbased absorbers have experienced the most significant degradation in service resulting in 

reduced neutron absorption. The degradation mechanisms and rates for silicone-based 

Boraflex, which was installed in dozens of spent fuel pools, have been studied extensively. 

However, the degradations mechanisms and rates for phenolic resin-based absorbers, 

Carborundum and Tetrabor, have not been studied as comprehensively. 

The objective of this report is to analyze the available information on Carborundum and Tetrabor 

to assess the adequacy of surveillance methods for monitoring degradation of phenolic resinbased absorbers. This analysis considers material composition/structure, the radiation and 

aqueous spent fuel pool environment, testing results, operating experience, and irradiated 

polymer degradation theory. 

 
 

2 

2. Background 

This section briefly summarizes the background of spent fuel pool storage, the need for neutron 

absorbers, and the experience of Boraflex degradation. It also provides a detailed description of 

the Carborundum and Tetrabor absorber materials and the spent fuel pool environment to which 

they are exposed. 

2.1 Spent Fuel Pool Storage 

When most of the current fleet of nuclear power plants were designed and built, spent fuel pools 

were designed to provide short-term storage for spent fuel before shipping to a reprocessing 

facility. The pools were designed to use geometrical spacing to ensure subcriticality of the 

limited number of assemblies anticipated to be stored on-site. However, as the reprocessing 

and a long-term waste repository options did not materialize, spent fuel pools began to fill up. In 

order to increase storage capacity to accommodate the accumulating spent fuel assemblies, 

many licensees chose to replace the original storage racks in the spent fuel pool with high 

density storage racks that incorporated neutron absorber panels between the spent fuel 

assemblies to ensure subcriticality per NRC regulations. 

2.1.1 Neutron Absorber Materials 

Solid neutron absorber materials integrated into high density storage racks have been 

developed in a variety of forms, but all fundamentally utilize Boron-10 (B-10) atoms to absorb 

neutrons and prevent criticality. Therefore, the neutron absorption capability of these materials 

is generally characterized by their B-10 areal density, which is measured in grams of B-10 per 

square centimeter (g/cm2

). B-10 is generally present in the chemical form of boron carbide (B4C) 

within an aluminum matrix or a polymer matrix. 

Borated stainless steel incorporates the B-10 atoms 

into the alloy composition. 

Neutron absorber panels are generally enclosed within 

a panel cavity by a wrapper plate that may or may not 

allow interaction with the water in the spent fuel pool 

as illustrated in Figure 2-1 (Haley, 2012). Therefore, 

the absorber materials may be exposed to borated or 

unborated water or kept dry. These materials may also 

be exposed to alpha, beta, gamma, and neutron 

radiation from the spent fuel. The solid neutron 

absorber materials can degrade or deform over time in 

the spent fuel pool environment, reducing their B-10 

areal density and capability to absorb neutrons. 

Among the neutron absorber materials credited by licensees, polymer-based materials such as 

Boraflex, Carborundum, and Tetrabor have experienced the most significant degradation. The 

degradation mechanisms and rates of silicone-based Boraflex have been studied extensively 

Figure 2-1 Examples of absorber panel 

wrapper plates (Haley, 2012) 
 

3 

and are briefly described in the following section, while the degradation of phenolic resin-based 

Carborundum and Tetrabor are the primary subject of this report. 

2.1.2 Management of Boraflex Degradation 

While the polymer in Boraflex is very different from the phenolic resin used in Carborundum and 

Tetrabor, it is useful to understand the performance of Boraflex when considering the 

degradation of Carborundum and Tetrabor in the same spent fuel pool environment. A more 

detailed description of Boraflex and its degradation in the spent fuel pool environment can be 

found in “Boraflex, RACKLIFE, and BADGER: Description and Uncertainties” (Agencywide 

Documents Access and Management System (ADAMS) accession number ML12216A307) 

(Haley, 2012). Boraflex is composed of ~50% boron carbide (B4C), ~25% elastomeric polymer 

polydimethyl siloxane (PDMS), and ~25% crystalline silica (SiO2) filler. Boraflex panels are 

generally 0.025 – 0.125 inches thick depending on the desired B-10 areal density and cover the 

entire active fuel length with one panel. 

The flexible PDMS polymer serves as the binder for the boron carbide and silica and is 

therefore the essential component for the integrity of the composite material. The chemical 

structure of a PDMS polymer 

chain can be seen in Figure 2-2. 

In the presence of radiation, 

PDMS undergoes three chemical 

processes: abstraction (bondbreaking) within or of a methyl 

group, cross-linking between 

polymer chains, and scissioning 

of the polymer backbone. Gamma radiation causes the abstraction of H or CH3 radicals, which 

provide sites for cross-linking and lead to offgassing and bubbles in the material. Cross-linking 

induces shrinkage, gap formation, increases density and hardness, and reduces ductility. 

Simultaneously, polymer chain scissioning takes place, which breaks the backbone of the 

polymer chain and eventually leads to dissolution of the PDMS binder (Haley, 2012). At low to 

moderate gamma radiation doses, cross-linking is the dominant effect over scissioning and 

causes Boraflex to shrink and densify (Haley, 2012). Cross-linking also leads to the formation of 

gaps due to shrinkage around locations where the panel is constrained or pinned by the 

wrapper plate. Once gamma dose reaches 1 x 1010 rad, cross-linking effects saturate and 

scissioning effects, which do not saturate, begin to dominate. After 1 x 1010 rad gamma 

exposure, PDMS has converted to a matrix of amorphous silica and has thus lost its ductility 

and become hard, brittle, and very susceptible to cracking. Table 2-1 below summarizes the 

effects of gamma irradiation on PDMS in Boraflex (Haley, 2012). 

Figure 2-2 PDMS chemical structure (Haley, 2012) 
 

4 

Dose [Gy] Dose [rads] Cross-linking Effects on material properties 

<1⋅104 <1⋅106 elastomeric 

levels negligible effects 

1⋅104 – 5⋅106 1⋅106 – 5⋅108 increasing increasing Shore A hardness 

5⋅106 – 1⋅108 5⋅108 – 1⋅1010 increasing increasing Shore D hardness, density, 

and shrinkage 

9⋅107 – 1.5⋅108 9⋅109 – 1.5⋅1010 becomes 

saturated 

fully hard and brittle, ceramic-like; 

no further densification or shrinkage 

>1⋅108 >1⋅1010 saturated continued scissioning degradation, 

including increased porosity 

The scissioning process is affected by the aqueous environment. When a polymer chain 

scissions, the fragments frequently recombine and therefore no permanent damage occurs. 

However, in the aqueous spent fuel pool environment, dissolved oxygen reacts with the radical 

sites created by scissioning. This prevents recombination at the site of the scission and causes 

permanent damage to the polymer by shortening the polymer chains. In this manner, the water 

and radiation environments work synergistically to cause greater degradation than would be 

possible by each separately (Haley, 2012). Boraflex degradation is largely dependent on 

gamma radiation exposure and water flow. As gamma exposure increases polymer chain 

scissioning, water flow through openings around the wrapper plate increases dissolution of the 

degraded polymer matrix leading to the proportional loss of B-10 from the panel and thus 

reduced neutron absorption capability. As Boraflex degradation progresses, the breakdown of 

PDMS causes the slightly soluble silica to be released to the spent fuel pool, which can be 

measured as silica concentration in the pool at large. 

With this understanding of a series of directly proportional correlations among gamma dose, 

polymer matrix degradation, dissolution of slightly soluble silica, and loss of boron carbide, a 

computer model known as RACKLIFE was developed. Based on absorbed gamma dose and 

spent fuel pool silica concentration, RACKLIFE can predict B-10 areal density for each panel in 

the pool (Haley, 2012). Absorbed gamma dose for each panel is estimated based on tracking 

the activity levels of the spent fuel assemblies surrounding that panel. RACKLIFE can then 

estimate the gamma exposure, and thus the degree of degradation and boron carbide loss, of 

each specific panel relative to the rest of the panels in the pool. Observed silica concentration is 

used to calibrate the model to account for total degradation in the entire Boraflex panel 

population of the pool. This model has many uncertainties, particularly the estimate of the 

exchange of water between panel cavities and the surrounding pool volume, which limit its 

ability to predict degradation in absolute terms. However, given the high correlation of absorbed 

gamma dose to degradation, the relative estimate of individual panel degradation to other 

panels in the pool may be useful for targeting the most degraded panels in the population. In 

practice, RACKLIFE is used in conjunction with Boron-10 Areal Density Gauge for Evaluating 

Table 2-1 Effects of gamma irradiation on PDMS polymer in Boraflex (Haley, 2012) 
 

5 

Figure 2-3 Chemical structure of phenol formaldehyde 

resin 

Racks (BADGER) in situ neutron attenuation testing to directly assess absorber efficacy and 

further calibrate RACKLIFE (Haley, 2012). 

2.2 Carborundum and Tetrabor 

Carborundum and Tetrabor are very similar neutron absorber materials that incorporate boron 

carbide powder in a binder of phenolic resin. The chemical structure of phenol formaldehyde, 

comprises a polymer of phenol molecules linked by either methylene or ether bridges (Painter 

and Coleman, 2009). A representative structure of the resin can be seen in Figure 2-3 below. 

Carborundum and Tetrabor panels were manufactured and installed in high density spent fuel 

storage racks in U.S. spent fuel pools in the late 1970s and early 1980s (EPRI, 2009). 

2.2.1 Carborundum 

Carborundum neutron absorber material 

was manufactured by the Carborundum 

Company in two forms: plate and sheet. 

Carborundum plate material is “a thick 

rigid plate” consisting of “granular boron 

carbide bonded in a phenol formaldehyde 

resin” (The Carborundum Company, 

1980). The plate material is composed of 

73% natural boron carbide and 27% 

phenolic resin by weight as shown in 

Table 2-2 (EPRI, 2009). For mechanical 

strength reasons, the Carborundum plate 

material is relatively thick (up to 0.25 

inches) and short (30 inches in length). 

Carborundum plate therefore requires several pieces stacked on top of one another to 

cover the active length of a fuel assembly (EPRI, 2009). The relatively thick plates result 

in high B-10 areal densities of about 0.095 g/cm2

 (The Carborundum Company, 1980). 

Table 2-2 Composition of Carborundum neutron absorber 

materials (EPRI, 2009)
 

6 

Carborundum sheet material is “a thin flexible glass fabric reinforced composite” containing 

natural boron carbide powder in a matrix of phenol formaldehyde resin (The Carborundum 

Company, 1980). The sheet material is composed of 64% boron carbide, 18% phenolic resin, 

and 18% fiberglass reinforcement by weight (EPRI, 2009). Carborundum sheet material 

incorporates fiberglass reinforcement to reduce the thickness to less than 0.1 inches with a B-10 

areal density of 0.015 – 0.025 g/cm2 (NRC, 1980; NRC, 2000). Carborundum sheet material, 

unlike Carborundum plate, covers the entire active fuel length in one sheet (EPRI, 2009). 

Carborundum plate material was installed in spent fuel pools at Haddam Neck (Connecticut 

Yankee), Millstone 1, and Palisades. Carborundum sheet material was installed in spent fuel 

pools at Calvert Cliffs 1, Crystal River 3, and LaCrosse. 

2.2.2 Tetrabor 

Tetrabor neutron absorber material was manufactured by Electroschmelzwerk Kemptem (ESK), 

a major boron carbide supplier in Germany, which is now owned by Ceradyne (EPRI, 2009). 

Tetrabor neutron absorber plates are composed of [natural] B4C granule (powder) material 

mixed with a phenolic resin and catalyst (Energy Northwest, 2010). The 2009 EPRI report on 

neutron absorber materials, however, states Tetrabor “is manufactured with a phenolic binder 

with fillers of boron carbide and graphite” (EPRI, 2009). Based on these descriptions, some 

batches of Tetrabor neutron absorber material may contain graphite. Plates are manufactured 

by pressing into the approximate shape, followed by heating to 1050°C for 12 hours ”to pyrolyze 

the resin material” (Energy Northwest, 2010). 

Tetrabor absorber plates used domestically are 0.21-0.24 inches thick and 19 inches in length 

with a B-10 areal density of 0.0959 g/cm2

 (Energy Northwest, 2010; Dominion Energy 

Kewaunee, Inc., 2009). Similar to the Carborundum plate material, Tetrabor plates are relatively 

short sections which are stacked on top of one another to cover the entire active fuel length. 

Two U.S. plants have installed Tetrabor for use in their spent fuel pools: Columbia Generating 

Station and Kewaunee. In addition to Carborundum plate, Haddam Neck also used Tetrabor, 

but is now decommissioned with its spent fuel pool dismantled. A summary of the key 

characteristics of Carborundum and Tetrabor neutron absorbers and the plants that installed 

these materials can be seen in Table 2-3 below. 

Material Components 

Typical 

Thickness 

(inches) 

Typical Length 

(inches) 

Typical B-10 

Areal Density 

(g/cm2

) 

Carborundum 

plate 

B4C powder 

Phenolic resin binder 0.25” Up to 30” 0.095 

Carborundum 

sheet 

B4C powder 

Phenolic resin binder 

Fiberglass reinforcement 

<0.1” Full length of 

active fuel 0.015 – 0.025 

Tetrabor 

B4C powder 

Phenolic resin binder 

Graphite (possibly) 

0.21 – 0.24” Up to 19” 0.0959 

Table 2-3 Summary of Carborundum and Tetrabor neutron absorber material characteristics and installations 
 

7 

3. Qualification Testing and Operating Experience 

Qualification testing results and operating experience reports serve as two key sources of 

information regarding the performance of phenolic resin-based neutron absorbers in the spent 

fuel pool environment. Qualification testing was performed on both Carborundum and Tetrabor 

prior to installation to assess their ability to perform in the spent fuel pool environment. Samples 

were exposed to water and radiation in order to simulate the spent fuel pool environment. 

Operating experience is accumulated after installation based on reports of the performance of 

Carborundum and Tetrabor in service. Operating experience may result from surveillance 

testing performed to ensure neutron absorber performance or may simply be discoveries made 

during the course of normal plant operations, such as movement of spent fuel assemblies. 

3.1 Qualification Testing 

3.1.1 Carborundum 

The Carborundum Company tested Carborundum boron carbide sheet and plate material at the 

High Flux Isotope Reactor (HFIR) Spent Fuel Storage Site (SFSS) (The Carborundum 

Company, 1980). The HFIR SFSS was chosen for this testing due to the high gamma radiation 

fluxes of 106

-108 rad per hour, which allowed a total exposure of 1011 rad in about 100 days 

(The Carborundum Company, 1980). The test arrangement was designed to provide a large 

volume of uniform flux of full spectrum gamma radiation with minimum neutron flux. Strength 

(flexural for the brittle plate material and ultimate tensile for the ductile sheet material), elastic 

modulus, dimensions, and weight data were collected for four exposure conditions: plate and 

sheet material in deionized and 2500 ppm borated water environments. A range of gamma 

fluxes from 109

 – 1011 rad were used (The Carborundum Company, 1980). A total of 450 plate 

samples and 480 sheet samples were involved in the testing to allow at least 12 specimens to 

be measured per data point for measurements of strength, elastic modulus, dimensions, and 

weight (The Carborundum Company, 1980). Gas evolution and boron leachability data were 

also collected over more limited exposure setups with fewer samples. 

Test results found that strength and elastic modulus decreased with increasing gamma 

exposure for both materials in both environments. At exposures up to 1010 rad in deionized and 

borated water environments, plate material strength reduced by around 30%, compared to only 

about a 10% reduction for the sheet material (The Carborundum Company, 1980). At higher 

exposures up to 1011 rad in deionized water, both sheet and plate materials experienced greater 

strength losses of about 60 and 65%, respectively (The Carborundum Company, 1980). In 

separate experiments, soak testing of both plate and sheet material with no exposure to 

radiation showed that a significant portion of the observed strength losses were due solely to 

exposure to water, which is typical for fiberglass-reinforced composites (The Carborundum 

Company, 1980). 

In addition to mechanical testing, gas evolution and boron leachability testing were also 

performed to establish the chemical stability of the Carborundum boron carbide materials in the 

spent fuel pool environment. Gas evolution testing showed a decreasing amount of gas 
 

8 

evolution with increasing dose. It also showed oxygen (O2) being consumed, while hydrogen 

(H2) and carbon dioxide (CO2) are produced as gamma dose increases as seen in Table 3-1 

(The Carborundum Company, 1980). The majority of the gas evolution data was collected on 

Carborundum plate material with little data on the sheet material. Boron leachability data 

showed only a slight increase in the amount of soluble boron during irradiation compared to the 

unirradiated state: 0.047% in 30 days and 0.025% in 25 days, respectively (The Carborundum 

Company, 1980). 

 Exposure Level 

(107

 gray γ) 

(109

 rad γ) 

 1.44 2.95 3.62 5.54 

Component LDC* PSU** LDC* LDC* PSU** PSU** LDC* LDC* 

H2 13 37 50 69 59 77 75 71 

O2 16 5 3 1 2 1 1 2 

N2 68 51 38 18 27 12 10 11 

CO2 2 6 7 11 11 9 12 14 

CO N.D. Tr Tr Tr Tr Tr Tr Tr

CH4 Tr N.D. Tr Tr Tr Tr N.D. Tr

Total 100 100 99 100 100 100 100 100 

*Analyzed at Lexington Development Center Tr = Trace 

**Analyzed at Pennsylvania State University N.D. = Not Detected 

Weight results found a 5.5% loss for the Carborundum plate material and a 20.2% loss for the 

sheet material after exposure to 1 x 1011 rad in deionized water as seen in Table 3-2 (The 

Carborundum Company, 1980). Exposures in borated water were suspended at 1 x 1010 rad 

after showing less change in weight and dimensions than those exposed to deionized water. 

Volume changes based on dimensional measurements increased with dose as seen in Tables 

3-3 and 3-4. Carborundum plate’s volume increased by around 3-5% at dose levels of 5 x 1010

to 1 x 1011 rad. Carborundum sheet showed volume increases indicating swelling at lower doses 

(1 x 1010 rad) before strongly decreasing in volume by 10 to 15% at the highest gamma 

exposure doses (The Carborundum Company, 1980). 

Gamma 

Dose, rad 

Sheet Plate 

Initial*, g Final*, g Weight 

change, % Initial*, g Final*, g Weight 

change, % 

1 x 109 1.761 1.757 -0.2% 9.584 9.596 0.1% 

1 x 1010 1.787 1.767 -1.1% 9.566 9.527 -0.4% 

1 x 1010 1.687 1.660 -1.6% 9.281 9.232 -0.5% 

3 x 1010 1.718 1.653 -3.8% 9.71 9.552 -1.6% 

5 x 1010 1.735 1.511 -12.9% 9.456 9.239 -2.3% 

8 x 1010 1.704 1.399 -17.9% 9.562 9.097 -4.9% 

1 x 1011 1.675 1.337 -20.2% 9.482 8.961 -5.5% 

*Based on measurements at each exposure on a minimum of 12 samples 

Table 3-1 Composition of Gas Evolved from Carborundum Plate Material (The Carborundum 

Company, 1980)

Table 3-2 Change in Weight of Carborundum Plate and Sheet Material as a Function of Dose 

(The Carborundum Company, 1980) 
 

9 

Gamma 

Dose, rad 

Initial 

Length*, 

cm 

Final 

Length*, 

cm 

Initial 

Width*, 

cm 

Final 

Width*, 

cm 

Initial 

Thick.*, 

cm 

Final 

Thick.*, 

cm 

Initial 

Volume, 

cm3

Final 

Volume, 

cm3

Volume 

change, 

% 

1 x 109 7.615 7.620 1.280 1.280 0.546 0.548 5.322 5.345 0.43% 

1 x 1010 7.612 7.645 1.280 1.285 0.541 0.544 5.271 5.344 1.38% 

1 x 1010 7.615 7.666 1.285 1.298 0.536 0.538 5.245 5.353 2.07% 

3 x 1010 7.633 7.666 1.278 1.283 0.549 0.544 5.355 5.351 -0.09% 

5 x 1010 7.617 7.747 1.273 1.293 0.538 0.549 5.217 5.499 5.42% 

8 x 1010 7.635 7.732 1.267 1.278 0.541 0.549 5.233 5.425 3.66% 

1 x 1011 7.633 7.757 1.270 1.285 0.544 0.549 5.273 5.472 3.77% 

*Based on measurements at each exposure on a minimum of 12 samples

Gamma 

Dose, rad 

Initial 

Length*, 

cm 

Final 

Length*, 

cm 

Initial 

Width*, 

cm 

Final 

Width*, 

cm 

Initial 

Thick.*, 

cm 

Final 

Thick.*, 

cm 

Initial 

Volume, 

cm3

Final 

Volume, 

cm3

Volume 

change, 

% 

1 x 109 7.668 7.630 1.354 1.367 0.119 0.122 1.236 1.272 2.99% 

1 x 1010 7.617 7.617 1.354 1.354 0.119 0.124 1.227 1.279 4.20% 

1 x 1010 7.559 7.577 1.308 1.308 0.119 0.122 1.177 1.209 2.77% 

3 x 1010 7.557 7.564 1.298 1.293 0.124 0.122 1.216 1.193 -1.90% 

5 x 1010 7.557 7.574 1.313 1.306 0.122 0.114 1.211 1.128 -6.85% 

8 x 1010 7.579 7.577 1.303 1.285 0.122 0.109 1.205 1.061 -11.91%

1 x 1011 7.549 7.539 1.298 1.265 0.122 0.107 1.195 1.020 -14.64%

*Based on measurements at each exposure on a minimum of 12 samples 

The report observes that at higher levels of radiation exposure the polymeric bonding phase 

becomes embrittled and weakened, reducing “the tenacity with which the boron carbide 

particles are held” and leading to a tendency for material to rub off during handling for both the 

sheet and plate materials (The Carborundum Company, 1980). The report calls this phenomena 

“attrition” and attributes the majority of the weight losses observed to attrition during packaging, 

testing, or other handling, with only minor roles played by gas evolution, boron solubility, and 

transmutation. The report also cites another study reporting that the exposure of phenolic 

polymers to radiation leads to swelling, hardening, embrittlement, and “a water soluble 

degradation product, ‘which causes the material to disintegrate in water’” (Sisman, 1955). 

Despite the observed attrition and report from Sisman, the Carborundum report provides no 

further discussion or explanation of why Carborundum is not expected to “disintegrate” over 

time exposed to water and radiation in the spent fuel pool. 

One of the key observations from the Carborundum Company testing program is as follows: 

“Analysis of the sheet material after exposure to 109

 gray (1011 rad) showed a reduction in boron 

content of 15%, compared to the noted 20% weight loss of the material.” (The Carborundum 

Company, 1980). It is not clear whether “boron content” refers to B-10 areal density or a mass 

balance of boron, nor does the report describe which method was used to collect this data. It is 

also not clear how or whether this one observation might support the use of weight loss as an 

indicator of “reduction in boron content.” 

Table 3-3 Change in Dimensions and Volume of Carborundum Plate Material as a Function of Dose 

(The Carborundum Company, 1980)

Table 3-4 Change in Dimensions and Volume of Carborundum Sheet Material as a 

Function of Dose (The Carborundum Company, 1980)
 

10 

Figure 3-1 Hydrogen gas evolution as a function of 

gamma dose (EPRI, 2009) 

3.1.2 Tetrabor 

Less information is available regarding qualification testing for Tetrabor neutron absorber 

material than the previously described testing of Carborundum. Tetrabor was gamma irradiated 

from a Co-60 source (1.25 MeV average 

energy) to a maximum dose of 2.66 x 1010 

rad at 1.6 x 106 rad per hour at a 

temperature of 50°C (EPRI, 2009). The 

results of hydrogen evolution during 

irradiation are plotted in Figure 3-1 showing 

hydrogen evolution saturating at about 

0.2% after a gamma dose of 6 x 109

 rad 

(EPRI, 2009). Thermal cycling of 16 cycles 

from room temperature to 350°F at 

50°F/hour showed no significant change in 

the modulus of rupture (EPRI, 2009). Water 

immersion testing without radiation 

exposure for 16 days at elevated 

temperature (exact temperature not 

reported) showed a 65% reduction in the 

modulus of rupture and a 30% reduction in 

the modulus of elasticity (EPRI, 2009). 

3.2 Operating Experience 

Operating experience regarding Carborundum and Tetrabor neutron absorber materials may be 

a result of complications of normal spent fuel handling activities as well as specific surveillance 

programs consisting of coupons, in situ neutron attenuation testing, and total organic carbon 

spent fuel pool water sampling. Operating experience includes wrapper plate bulging, loss of B10 areal density, and results from coupon and in situ neutron attenuation testing. 

3.2.1 Bulging of Rack Wrapper Plate 

There have been numerous reported instances of bulging of the rack wrapper plate due to gas 

generation from phenolic resin-based neutron absorbers in sealed neutron absorber cavities. 

This plate bulging has typically been identified during normal spent fuel movement, when it is 

observed that a fuel assembly has become stuck inside a storage rack cell and cannot be 

removed, or when an assembly cannot be easily inserted into an empty rack cell. In most cases, 

wrapper plate bulging has been managed by drilling vent holes in the top of the neutron 

absorber panel cavity. Venting prevents bulging by allowing gases to escape, but also exposes 

absorber panels to spent fuel pool water, which may have other effects on the absorber 

material. 
 

11 

Table 3-5 Gas Composition from Test Specimen in Haddam Neck 

(NRC, 1978)

Carborundum Plate at Haddam Neck (Connecticut Yankee) 

Haddam Neck Nuclear Power Plant (since dismantled) installed Carborundum plate neutron 

absorber materials in 1977. Excessive drag when moving spent fuel assemblies led to the 

discovery of wrapper plate bulging as described in Haddam Neck Licensee Event Report (LER) 

78-004 on April 28, 1978 (Connecticut Yankee, 1978). The bulging was attributed to gas 

generation from the Carborundum plate absorber material causing a pressure buildup within the 

neutron absorber cavity and subsequent wrapper plate deformation (Connecticut Yankee, 

1978). The LER also states: “Connecticut Yankee Atomic Power Company (CYAPCO) believes 

that offgassing could be the result of radiolytic decomposition of water and/or organic resin used 

to bind the B4C powder in plate form.” Short-term corrective actions included drilling vent holes 

“2 1⁄2 inches from the bottom of the storage cell” to relieve the gas pressure buildup. 

The LER also includes discussion of test specimens that were inserted in the spent fuel pool in 

fall 1977 and experienced offgassing (Connecticut Yankee, 1978). Gas chromatography results 

of the composition of gas present in the test specimen holder after offgassing can be seen in 

Table 3-5. It is not clear from the LER, but may be reasonably inferred that the starting gas was 

air. Therefore, the results appear to show that hydrogen, carbon dioxide, and methane are 

produced, while oxygen is consumed and nitrogen is unaffected. The reduction in nitrogen 

volume percent may be understood by the increase in the total amount of gas in the specimen 

holder due to the production of the other gases, rather than any reduction in the amount of 

nitrogen present. 

Gas Volume Percent 

 Starting gas (air) Offgas sample 

Hydrogen (H2) < 1 18 

Oxygen (O2) 21 3 

Carbon Dioxide (CO2) < 1 8 

Nitrogen (N2) 78 69 

Methane (CH4) < 1 1 

 

Tetrabor at Kewaunee 

Kewaunee Power Station installed Tetrabor neutron absorber material in 1979 (Dominion 

Energy Kewaunee, Inc., 2009). On December 19, 1980, Kewaunee observed swelling and 

bulging of the wrapper plate encasing two coupon specimens and installed fuel storage racks 

due to offgassing from the Tetrabor material into the sealed panel cavities (Wisconsin Public 

Service, 1980). The licensee chose to drill vent holes in the top of the racks to relieve the 

pressure and prevent further bulging (Wisconsin Public Service, 1981). The licensee also 

reported “the results of a laboratory analysis of the gas generated” showing by volume about 

65% hydrogen and 30% nitrogen to be present (Wisconsin Public Service, 1981). 

Carborundum Plate at Palisades 

Palisades Nuclear Plant installed Carborundum plate neutron absorber material between 1979 

and 1981 (Entergy, 2008). Between 1988 and 2007, the plant experienced fourteen instances of 
 

12 

wrapper plate bulging impacting the movement of spent fuel assemblies (Entergy, 2008). 

According to the licensee: “Originally, the annulus [panel cavity] was designed to be water tight, 

but cell wall swelling due to internal gas production from gamma exposure necessitated the 

drilling of 3/16 inch vent holes in the upper region of each cell” (Entergy, 2008). There is little 

further information regarding the wrapper bulging available. In situ testing performed in 2009 

revealed a significant reduction in neutron absorption of the Carborundum plates, which is 

discussed in greater detail in Section 3.2.3 (Northeast Technology Corp, 2009). 

Summary 

There have been three well-documented operating experience events of rack wrapper plate 

bulging due to offgassing from phenolic resin-based neutron absorbers at Haddam Neck, 

Kewaunee, and Palisades, which are summarized in Table 3-6. It should be noted that there are 

several other plants with these materials that have not documented any wrapper plate bulging 

events based on the literature review done for this report. However, among the plants without 

documented wrapper plate bulging events, Crystal River 3, Calvert Cliffs 1, and Columbia are 

known to have initially installed vented panel cavities as a preventative measure. 

Plant Material Year Corrective 

Action(s) Comments 

Haddam 

Neck 

Carborundum 

plate 1978 Drill vent 

holes 

Gas chromatography of gases from test 

specimen holder showed by volume 18% H2, 

3% O2, 8% CO2, 69% N2, and 1% CH4

Kewaunee Tetrabor 1980 Drill vent 

holes 

Generated gases found to be about 65% 

hydrogen and 30% nitrogen 

Palisades Carborundum 

plate 

1988 - 

2007 

Drill vent 

holes 

Continued bulging possibly due to 

incomplete venting 

3.2.2 Coupon Testing Results 

Crystal River 3, Calvert Cliffs 1, Kewaunee, Columbia, and LaCrosse have Carborundum and 

Tetrabor installed as neutron absorbers and use coupon testing surveillance programs to 

monitor the degradation of the in-service absorber materials. These coupons are placed in the 

spent fuel pool in order to simulate the environmental exposure and configuration of the inservice panels. To achieve this, they are exposed to pool water and spent fuel radiation typically 

within sheathing that may be vented similar to the in-service panels. Coupon testing is generally 

performed every few years and may include visual examination, dimensional measurements, 

weight/density measurement, and neutron attenuation testing. 

Carborundum Sheet at Crystal River 

Crystal River 3 Nuclear Generating Plant installed Carborundum sheet coupons along with 

Carborundum sheet neutron absorber material in their spent fuel racks in 1981 (Progress 

Energy, 2009). The coupons are all sheathed with a vent hole similar to the actual rack 

configuration (Progress Energy, 2009). There are two sizes of coupons, which are otherwise 

identical: small (1” x 2”) Type A coupons and larger (2” x 10”) Type B coupons (Progress 

Table 3-6 Summary of Documented Wrapper Plate Bulging Events 
 

13 

Energy, 2009). Crystal River’s coupon testing program includes a visual examination and weight 

loss testing, but no dimensional measurements or neutron attenuation testing. Crystal River 

committed in 2007 to perform coupon testing and in situ testing alternately every ten years, so 

that one surveillance test, either in situ or coupons, is performed every five years (Progress 

Energy, 2010).The most recent surveillance performed in 2004 showed an average weight loss 

of less than 7% (Progress Energy, 2010). Weight measurements showed a decrease of less 

than 2% in average weight from 1993 to 2004 (Progress Energy, 2010). 

However, Crystal River also reported a sample in 2004 that failed the acceptance criteria of less 

than 20% weight loss with a weight loss of 21%. The licensee explained this result as “an 

anomaly” from being “damaged (eroded) by water rinsing/lancing for decontamination during 

removal of the holder from the spent fuel pool” (Progress Energy, 2010). This coupon was 

located “directly opposite the vent/inspection hole in the packet,” and the only part of the coupon 

damaged “was the area opposite the vent/inspection hole in the packet” (Progress Energy, 

2010). The licensee believes that this type of localized degradation is not expected to occur for 

the in-service material because the vent/inspection holes in the installed racks are located 

above the active fuel length (Progress Energy, 2010). 

Carborundum Sheet at Calvert Cliffs Unit 1 

Calvert Cliffs Nuclear Power Plant Unit 1 installed storage racks with Carborundum sheet 

material and coupons in or shortly after 1980 (NRC, 1980). No documentation was found 

discussing the sheathing of the coupons. Coupons are tested by visual examination and length, 

width, and weight measurements every four years, and benchtop neutron attenuation testing 

every eight years (NRC, 2008). Calvert Cliffs’ coupon program comprises an Accelerated 

Surveillance Assembly (ASA) and a Long-Term Surveillance Assembly (LTSA) (Constellation 

Energy, 2007). The ASA conservatively accelerates exposure by moving coupons every outage 

to a new location with freshly discharged fuel, thereby maximizing radiation exposure. The 

LTSA, on the other hand, is initially surrounded by freshly discharged fuel, but is simply left in 

the same location surrounded by the same radioactively decaying fuel to more realistically 

simulate the exposure of in-service panels (Constellation Energy, 2007). The Calvert Cliffs 

surveillance program does not utilize in situ neutron attenuation testing (NRC, 2008). 

A total of six coupons from the ASA have been tested and the weight loss results can be seen in 

Figure 3-2 (Constellation Energy, 2008). The most recent ASA coupon tested in 2005 showed 

9.5% weight loss, which is consistent with a linear projection for weight loss over time 

(Constellation Energy, 2008). Limited understanding of the mechanism causing the weight loss 

introduces uncertainty into the continued linearity of the trend. 
 

14 

Carborundum Sheet at La Crosse 

La Crosse installed spent fuel storage racks containing Carborundum sheet material and 

coupons in 1980 (Dairyland Power Cooperative, 2010). The in-service Carborundum sheet 

material is “essentially sealed” by stainless steel wrapper plates that are tack welded along the 

edges, while the coupons are exposed to flowing water on two edges and are moved during 

each outage next to the fresh, newly discharged fuel assemblies (Dairyland Power Cooperative, 

2010). The coupon testing program has primarily consisted of visual examination and weight 

loss measurements at least every six years. Weight loss results have generally shown less than 

6% weight loss, except for four samples in 2005 that were measured with weight loss of 10 – 12 

%. The licensee noted that coupons tend to show erosion from exposure to water along the 

exposed edges of the coupons, with up to 1/16 inch of fiberglass exposed along the edges due 

to erosion of the polymer matrix. They also observed that exposed coupons were “a little more 

friable” than archival material (Dairyland Power Cooperative, 2010). 

In 1997, La Crosse sent two coupons for more extensive testing by NETCO after 16.3 years of 

exposure to near-maximum gamma fluxes (Dairyland Power Cooperative, 2010). NETCO 

performed a visual examination, dimensional, weight, and density measurements, a radioassay, 

and neutron attenuation testing. Neutron attenuation testing showed a B-10 areal density of 

Figure 3-2 Weight loss data and projection over time from ASA Carborundum 

coupons at Calvert Cliffs (Constellation Energy, 2008) 
 

15 

0.0211 and 0.0193 g/cm2 for exposed samples compared 0.0207 g/cm2 for the archived sample, 

indicating a 2% increase and 7% decrease in B-10 areal density for the two exposed samples 

compared to the archive (Dairyland Power Cooperative, 2010). 

Tetrabor at Kewaunee 

A coupon testing program has been in place for the Tetrabor neutron absorber material at 

Kewaunee since 1982 with testing at least every four years since 1982 (Dominion Energy 

Kewaunee, Inc., 2009). Coupons are 1 inch x 4 inch x 0.24 inches, vented in stainless steel 

sheathing, and placed in locations near freshly discharged fuel (Dominion Energy Kewaunee, 

Inc., 2009). Coupon testing consists of visual examination, dimensional and thickness 

measurements, and quantitative benchtop blackness testing (Dominion Energy Kewaunee, Inc., 

2009). “The boron-10 (B-10) areal density is calculated based on blackness testing results” by 

comparing “the test rig control plate data of different thicknesses to test coupons from the spent 

fuel pool” (Dominion Energy Kewaunee, Inc., 2009). 

Areal density results can be seen in Table 3-7 below (Dominion Energy Kewaunee, Inc., 2009). 

There is a fairly large amount of variability from one testing date to the next, particularly looking 

at the data between 1990 and 1995. In this date range, areal density results for all seven 

coupons appear to increase and then decrease together. This may indicate issues with the 

calibration associated with the testing procedure and may reduce confidence in the absolute 

values for B-10 areal density generated. Overall, the trend in areal density appears to be 

decreasing over time with data shown in Table 3-7,; however, the coupons continued to meet 

the acceptance criteria of 0.086 g/cm2

 B-10 areal density 
 

16 

Tetrabor at Columbia 

Columbia Generating Station installed Tetrabor neutron absorber material between 1979 and 

1983 (Energy Northwest, 2010). Columbia’s installation of Tetrabor is very unique, because “the 

Spent Fuel Rack enclosures are vented above storage pool water surface through an 

arrangement of tubing and sampling valves at the pool curb” (Energy Northwest, 2010). This 

system prevents pressure buildup in the panel cavities, but also allows the Tetrabor to be 

sealed from the spent fuel pool water. Columbia reported finding “less than 1⁄4 cup of water” 

when venting the line connected to the panel cavities, which may indicate leakage from the pool 

into the panel cavities (Energy Northwest, 2010). 

Columbia has performed visual examination of coupons every four years since 1999 and has 

observed “no signs of degradation” (Energy Northwest, 2011). Columbia’s program includes 

coupons both sealed from the spent fuel pool water and vented to the pool water configurations 

to simulate a leak into the panel cavities (Energy Northwest, 2010). Isotopic abundance 

analyses performed in 1995 showed at least 20 atomic percent of isotope B-10 out of all boron 

atoms in all samples (Energy Northwest, 2011). Columbia committed in 2011 to perform coupon 

Table 3-7 Areal density results from blackness testing of Tetrabor coupons 

at Kewaunee (Dominion Energy Kewaunee, Inc., 2009) 
 

17 

visual examinations every four years, coupon neutron attenuation testing “at an interval not to 

exceed ten years,” and in situ testing at least every ten years (Energy Northwest, 2011). 

Summary 

Coupon testing has been performed at several plants that credit phenolic resin-based 

absorbers, including visual examinations, dimensional and weight loss measurements, and 

neutron attenuation testing as shown in Table 3-8. Coupon testing results have shown steady 

weight loss over time in Carborundum sheet coupons at Crystal River and Calvert Cliffs, 

including one sample with over 20% weight loss. Visual examinations at some plants have not 

shown visual degradation. Finally, neutron attenuation testing of Tetrabor coupons at Kewaunee 

have shown acceptable neutron attenuation over 25 years of testing, but also a high degree of 

variability. 

Plant Material 

Environment Coupon Testing 

Methods Results In-Service 

Material Coupons

Crystal 

River 3 

Carborundum 

sheet 

Vented to 

pool 

Vented to 

pool 

Visual exam and 

weight loss testing 

Average coupon weight 

loss of 7% in 2004; one 

failed coupon showed 

21% weight loss 

Calvert 

Cliffs 1 

Carborundum 

sheet 

Vented to 

pool Unknown

Visual exam; length, 

width, and weight 

measurements; and 

neutron attenuation 

testing 

Accelerated coupon in 

2005 showed 9.5% 

weight loss after about 25 

years 

La Crosse Carborundum 

sheet 

Vented to 

pool 

Vented to 

pool 

Visual exam and 

weight loss testing 

Neutron attenuation 

testing in 1997 showed 

7% reduction in B-10 

areal density for one 

sample; Coupons in 2005 

showed 12% weight loss 

Kewaunee Tetrabor Vented to 

pool 

Vented to 

pool 

Visual exam, 

dimensional and 

thickness 

measurements, and 

blackness testing 

Blackness testing areal 

density results show 

variability over time, and 

overall downward trend 

Columbia Tetrabor 

Vented to 

air above 

pool 

Both 

sealed 

and 

vented to 

pool 

Visual exam and 

neutron attenuation 

testing 

No visual signs of 

degradation; isotopic 

abundance analysis in 

1995 normal 

Table 3-8 Summary of Coupon Testing Programs and Results 
 

18 

3.2.3 In Situ Testing Results 

In situ neutron attenuation testing has been used to directly monitor B-10 areal density in pools 

both with and without coupons. The most common in situ testing method is the Boron-10 Areal 

Density Gauge for Evaluating Racks (BADGER) system, which was originally developed by 

NETCO for use in conjunction with the RACKLIFE computer code to monitor Boraflex 

degradation. However, BADGER operates by measuring neutron absorption and is therefore 

potentially applicable to any absorber material integral to spent fuel pool racks. BADGER places 

a Californium-252 source on one side of the panel and four BF3 neutron detectors on the 

opposite side of the panel to measure neutron count rate and estimate B-10 areal density 

(Northeast Technology Corp., 2009). A technical letter report describing the uncertainties 

associated with the BADGER system, “Initial Assessment of Uncertainties Associated with the 

BADGER Methodology,” can be found in ADAMS under accession number ML12254A064. 

Carborundum Plate at Palisades in 2008 

As discussed in Section 3.2.1, Palisades experienced fourteen instances of wrapper plate 

bulging from 1988 to 2007. During the license renewal process, Palisades presented several 

justifications to demonstrate adequate management of aging effects for Carborundum. 

Palisades stated that it had been monitoring total organic carbon (TOC) in the spent fuel pool 

since 2000 and found “very low, stable TOC values (typically less than 0.2 ppm)” (Nuclear 

Management Company, 2005). The licensee concluded from this monitoring “that the B4C 

absorber [Carborundum plate] is not degrading” and that “periodic confirmatory sampling” would 

provide assurance that no degradation is occurring (Nuclear Management Company, 2005). In 

addition, the licensee stated that it is and would continue monitoring a coupon testing program 

of the same absorber material at another spent fuel pool to provide information about the 

performance of the material at their plant (Nuclear Management Company, 2005). 

To confirm that the bulging was not due to degradation of the Carborundum plates and to gain 

NRC’s approval for license renewal, the licensee committed to perform BADGER testing on 

selected cells in the Palisades spent fuel pool before the beginning of the period of extended of 

operation. The first BADGER campaign, conducted in July 2008, indicated that B-10 areal 

densities of several panels in the pool were below the minimum areal density assumed in the 

criticality analysis (Northeast Technology Corp., 2009). A follow-on BADGER campaign was 

conducted in March 2009 to more rigorously measure the panel areal densities (Northeast 

Technology Corp., 2009). The nominal and minimum B-10 areal densities of the sixty panels 

tested can be seen plotted as a function of gamma radiation dose in Figures 3-3 and 3-4. 

Minimum B-10 areal density values are calculated conservatively with respect to count rate 

uncertainty by assuming the minimum count rate from the calibration panel and the maximum 

count rate from the observed panel. For this BADGER campaign, test panels were compared to 

a 0.0705 g/cm2

 maximum calibration standard panel. Therefore, any data points with a count 

rate less than that for the highest calibration standard (indicating a higher B-10 areal density) 

were simply recorded as 0.0705 g/cm2

 to avoid extrapolation. 
 

19 

This testing data was calibrated using a Boraflex panel standard, which may affect the accuracy 

of the absolute B-10 areal density values (Chapman and Scaglione, 2012), but should not 

preclude comparative analysis of the data points. The scatter of the data in Figure 3-3 does not 

appear to show a strong correlation between gamma dose and nominal B-10 areal density. As 

discussed in Chapter 2, BADGER measurements of Boraflex shows that gamma dose leads to 

proportional Boraflex polymer matrix degradation, which, through a time and flow dependent 

silica transport model, leads in turn to proportional loss of B-10. Therefore, B-10 areal density 

Figure 3-3 Nominal B-10 areal density in Carborundum plate panels as a 

function of gamma dose (Northeast Technology Corp., 2009) 

Figure 3-4 Minimum B-10 areal density in Carborundum plate panels as a 

function of gamma dose (Northeast Technology Corp., 2009) 
 

20 

can therefore be proportionally correlated to gamma dose. However, the same trend is not 

observed for Carborundum plate at Palisades. While only one data set, the Palisades results 

suggest that gamma dose alone is not a clear indicator of reduction in B-10 areal density. This 

lack of correlation is further discussed in Chapter 5. 

As described previously, Palisades used TOC sampling and monitoring of a coupon testing 

program at another plant to manage this degradation. However, significant degradation of the 

Carborundum plate was detected by in situ neutron attenuation testing without any indication 

from TOC or the coupons at another plant, indicating the failure of these techniques in this 

instance. 

Carborundum Plate at Millstone 1 in 2007 and 2011 

Millstone 1 is known to have performed two BADGER campaigns, one in 2007 and another in 

2011 (NRC, 2010; NRC, 2011). Unlike the 2008 Palisades BADGER campaign, detailed reports 

from these campaigns at Millstone 1 were not available for this report. An NRC inspection report 

mentions that the 2007 BADGER campaign at Millstone 1 found a reduction in B-10 areal 

density (NRC, 2010). A second BADGER testing campaign is mentioned in an NRC inspection 

report as having taken place in June 2011 at Millstone 1, but no results are available (NRC, 

2011). 

Summary 

BADGER testing campaigns have only been performed at two plants with phenolic resin-based 

absorbers, both of which were Carborundum plate material. The BADGER testing campaign of 

Carborundum plate at Palisades found significant reductions in B-10 areal density below the 

minimum credited amount. Based on the information available, it is not clear whether this 

degradation is unique to Carborundum plate material or more generally applicable to 

Carborundum sheet and Tetrabor materials, which also use phenolic resin binder material. It is 

important to note that the significant degradation of the Carborundum plate at Palisades 

occurred without any indication from TOC or the coupons at another plant, indicating the failure 

of these techniques in this instance. 

 
 

21 

4. Radiation Chemistry of Phenol Formaldehyde Resins 

In order to better explain the qualification testing data and operating experience, it is important 

to understand the degradation mechanisms of phenol formaldehyde resin in the spent fuel pool 

environment. This section will describe the degradation process of phenolic resin-based neutron 

absorbers employed in spent fuel pools using well-established radiation chemistry theory. This 

discussion focuses on the effects of radiation, water, and oxygen on phenolic resin polymers, 

and also considers other variables such as temperature, pool chemistry, and local flow. 

4.1 Degradation of Polymers in the Spent Fuel Pool Environment 

There are several important processes that affect the degradation of polymers in the spent fuel 

pool environment. First, ionizing radiation has a direct effect on the polymeric material by 

breaking bonds and inducing scission and cross-linking. Second, radiolysis of water produces 

hydroxyl radicals and hydrogen peroxide, which can attack polymer bonds. Finally, the presence 

of dissolved oxygen allows for oxidation of free radicals that can enhance degradation (Wilson, 

1974). 

4.1.1 Direct Radiation Effects: Scission and Cross-linking 

Ionizing irradiation induces the formation of anions, cations, and free radicals in polymers on a 

time scale of about 10-12 to 10-6 s. The early events in the interaction of ionizing radiation with 

polymers are the production of ionized and excited polymer molecules. These ions and excited 

polymers undergo various reactions to produce free radicals, additional ions, and hydrogen 

atoms. The most probable mechanisms of radical and cation formation in the radiolysis of 

polymers, for which there are varying degrees of evidence, include the following in Equations 1-

10: 

 (1) Excitation כܪܴ ื ܪܴ

ܴܪ ื ܴܪା ൅ ݁ି Ionization / cation formation (2) 

 (3) ܪ ൅ •ܴ ื כܪܴ

 (4) ܪ ൅ •ܴ ื כܪܴ ื ݁ି ൅ ାܪܴ

•ܴܪ ื ݁ି ൅ ାܪܴ

 (5) 

ଶܪܴ ื ݁ି ൅ ାܪܴ

ା ൅ ܴ• (6) 

ଶܪܴ

ା ൅ ݁ି ื ܴ• ൅ ܪଶ (7) 

ଶܪܴ

 (8) ܪ ൅ ܪܴ ื ݁ି ൅ ା

 (9) ଶܪ ൅ •ܴ ื ܪ ൅ ܪܴ

ܴ• ൅ ܴ• ื ܴ െ ܴ Cross-linking (10) 

Where: 

ܴܪ = polymeric material ܴכܪ = excited molecule 
 

22 

ܴ• = free radical ܴܪା, ܴܪଶ

ା = cations 

In general, upon the irradiation of polymers, the following processes will take place: 

1. Cross-linking: the formation of covalent bonds between the polymer chains 

2. Scission: bond-breaking in the backbone of the polymer molecule 

3. Changes in the concentration and the nature of double bonds 

4. Oxidative and degradation reactions (in the presence of oxygen) 

5. Evolution of gases such as hydrogen (H2) and methane (CH4)

The predominance of these processes depends on the chemical structure of the polymers, the 

irradiation dose, dose rate, the linear energy transfer (LET) of the irradiation, the presence of 

oxygen, and the presence of antioxidants. 

Whether a particular polymer undergoes scission or cross-linking upon irradiation depends 

entirely on its backbone’s chemical structure. The presence of quaternary carbon atoms 

(bonded to four other carbon atoms) in the backbone of the polymer enhances radiation-induced 

scission, while the presence of secondary and tertiary carbon atoms (bonded to two or three 

other carbon atoms) in the backbone enhances cross-linking reactions as seen in Table 4-1 

below (Woods and Pikaev, 1993). Bonds broken by radiation-induced scission can be repaired 

by recombination, and the backbone’s chemical structure affects the recombination process to 

repair scission damage. 

Polymer R1 R2

Radiation-Induced 

Change 

Polyethylene H H Cross-linking 

Polypropylene CH3 H Cross-linking 

Poly(vinyl chloride) Cl H Cross-linking 

Polyacrylonitrile CN H Cross-linking 

Polyacrylates CO2R H Cross-linking 

Polystyrene C6H5 H Cross-linking 

Poly(vinylidene chloride) Cl Cl Scission/Degradation 

Polyisobutylene CH3 CH3 Scission/Degradation 

Polymethylstyrene C6H5 CH3 Scission/Degradation 

Poly(methyl methacrylate) CO2CH3 CH3 Scission/Degradation 

Polytetrafluoroethylene Varies Varies Scission/Degradation 

Natural rubber Varies Varies Cross-linking 

Cellulose and derivatives Varies Varies Scission/Degradation 

Table 4-1 Radiation Induced Changes to Common Polymers (Woods and Pikaev, 1993) 
 

23 

4.1.2 Radiolysis of Water 

Radiolysis is the disassociation of water molecules in the spent fuel pool water by interaction 

with beta and gamma radiation from the spent fuel to break chemical bonds. Radiolysis 

produces radicals, aqueous electrons, and stable compounds that can be seen in Equation 11 

below: 

 ܪଶܱ ఊ,ఉ

௤௔݁ ,•ܪ , • ܪܱ ሱሮ

ି , ܪଷܱା, ܪଶܱଶ, ܪଶ (11) 

Radiolysis of water by gamma radiation has the following radiation-chemical yields in micro 

moles produced per joule of deposited energy (Spinks and Woods, 1990): 

ܩሺ ܱܪ • ሻ ൌ ܩ൫݁௔௤

ି ൯ൌܩሺܪଷܱାሻ ൌ 0.28

ܩሺܪ•ሻ ൌ 0.062

ܩሺܪଶሻ ൌ 0.042

ܩሺܪଶܱଶሻ ൌ 0.082

Water radiolysis changes the chemical environment in which the neutron absorbers are located. 

Hydroxyl radicals ( ܱܪ ( • are very powerful oxidizers that react with aromatic compounds via 

addition to the phenyl rings to produce various isomeric adduct radicals (•

Ar(OH)), which begins 

the degradation of the polymer structure. Meanwhile, aqueous electron (݁௔௤

ି ), are very strong 

reducing agents, which react with the phenyl structure as well as ether, ketone, and carboxylic 

groups. However, in the presence of oxygen, aqueous electrons may be scavenged very 

rapidly by the soluble oxygen in water, producing superoxide radicals with reaction rate constant 

of 2 x 1010 M-1s-1 as seen in Equation 12: 

 ݁௔௤

ି ൅ ܱଶ ՜ ܱଶ

•ି (12) 

Therefore, effectively all aqueous electrons will be scavenged by the dissolved molecular 

oxygen. 

Hydrogen atoms are also scavenged by the dissolved oxygen, producing perhydroxyl radicals 

ଶܱܪ)

•) at a diffusion-controlled reaction rate of 1-2 x 1010 M-1s-1 in Equation 13: 

 ܪ൅ܱଶ ՜ ܪܱଶ

• (13) 

ଶܱܪ

• has a pKa value of 4.7 that governs the following reaction seen in Equation 14: 

ଶܱܪ 

• ൅ ܪଶܱ՞ܱଶ•ି ൅ ܪଷܱା (14) 

Therefore, at pHs less than 4.7, there will be more ܪܱଶ

• than ܱଶ•ି present. ܪܱଶ• is more reactive 

than ܱଶ

ଶܱܪ . ି•

• will abstract hydrogen from the polymer structure to form corrosive hydrogen 

peroxide (ܪଶܱଶ) as follows in Equation 15: 
 

24 

ଶܱܪ 

• ൅ܴܪ ՜ ܪଶܱଶ ൅ ܴ• (15) 

At pHs greater than 4.7, most of the ܪܱଶ

• will be converted to ܱଶ•ି, which is less reactive than 

ଶܱܪ

• and reacts bimolecularly to also produce the corrosive ܪଶܱଶ in Equation 16: 

 2ܱଶ

•ି ଶுమை

ሱۛۛሮ ܪଶܱଶ ൅ ܱଶ (16) 

ܪଶܱଶ is one of the main corrosive radiolytic products of nuclear reactor operation as well as 

spent fuel aqueous storage. Both ܪܱଶ

• and ܱଶ•ି will produce hydrogen peroxide, but at pHs less 

than 4.7, degradation will be increased due to ܪܱଶ

• abstracting hydrogen from the polymer 

structure. 

In the presence of impurities such as Fe2+, H2O2 will decompose to produce more hydroxyl 

radicals through Fenton’s reaction seen in Equation 17, which will serve to further enhance the 

polymer degradation: 

(17) ିܪܱ ൅ ଷା݁ܨ ൅ • ܪܱ ՜ ଶା݁ܨ ൅ ଶܱଶܪ 

4.1.3 Effect of Oxygen Presence 

The presence of oxygen plays a major role in the degradation of polymers through peroxidation 

seen in Equation 18. The vast majority of the free radicals created by radiation, particularly the 

carbon-centered free radicals, react with oxygen to produce the corresponding peroxyl radicals 

ଶܴܱܪ)

•). Oxygen greatly impedes cross-linking reactions seen in Equation 19 by reacting very 

quickly with the carbon-centered radicals before they can cross-link with one another. 

ଶܱ • ൅ ܪܴ

௞ଵ

ሱሮ ܴܱଶ (18) Peroxidation ܪ ൅ •

ଶ • ௞ܪܴ 2

ሱሮ ܴ െ ܴ ൅ ܪଶ Crosslinking (19) 

As oxygen concentration increases, the peroxidation reaction dominates over cross-linking 

reactions. Peroxidation of free radical leads over time to degradation of the polymer molecules. 

HRO2

●

 can also initiate a chain reaction seen in Equation 20 by abstracting a hydrogen atom 

from the neighboring molecules producing another free radical and organic peroxide: 

ଶܴܱܪ 

 (20) •ܴ ൅ܪଶܴܱܪ ՜ ܪܴ ൅ •

Another important reaction of the peroxyl radicals seen in Equation 21 is the formation of an 

intermediate molecule called tetraoxide: 

ଶܴܱܪ ଶܴܱܪ ൅ •

 (21) ܪܴܱܱܱܱܴܪ ՜• 

The tetraoxide will then decompose to aldehyde and alcohol as seen in Equation 22: 

 ܴܱܱܱܱܴ ՜ ܴᇱ

 (22) ଶܱ ൅ ܪܱ"ܴ ൅ ܪܱ
 

25 

These products will undergo further decomposition and degradation reactions. 

In the open spent fuel pool, the oxygen concentration is expected to remain relatively constant 

with a level of ~ 2.2 x 10-3 mol L-1 due to the atmosphere above serving as an oxygen source. 

However, the panel cavity may not be as oxygen-rich as the pool itself depending on the 

amount of exchange between the panel cavity and pool water. 

4.2 Radiation Chemistry of Phenol Formaldehyde Resins 

As described previously in Chapter 2, a detailed description of the phenolic polymer materials 

used in Carborundum and Tetrabor is not available, so a generic phenol formaldehyde resin is 

assumed for this discussion. The chemical structure of phenol formaldehyde resin is shown in 

Section 2.2.1 and includes primarily phenol, ether, and methyl groups. 

4.2.1 Degradation Process in the Absence of Water or Oxygen 

This section discusses the degradation process in the absence of water or oxygen. This 

environment is expected to be present in the bulk interior of the neutron absorber material 

whether in a vented or sealed configuration as well as at the surface of the neutron absorber 

material if sealed from the spent fuel pool water. The direct effects of the ionizing radiation of 

the spent fuel pool on phenol formaldehyde resins are a result of beta particles, gamma rays, 

and alpha particles. 

Radiation-Induced Degradation 

Given the light elements present in the phenol formaldehyde resin, beta particles (fast electrons) 

and gamma rays (high-energy photons) from the spent fuel are not expected to interact 

significantly with atomic nuclei. However, gamma and beta will interact with the electrons of the 

elements in phenol formaldehyde and impact the chemical bonds that are present. It is well 

known that the presence of the aromatic group (in this case, phenol) in the polymer structure 

greatly enhances its resistance to ionizing radiation. The pi (π) structure in the aromatic group is 

very effective at dissipating the energy of the ionizing radiation. It is also known that the OH 

groups of the phenol structures act as excellent hydrogen donors (antioxidants) to the 

radiolytically-produced free radicals, thereby converting them to stable molecules by the 

hydrogen transfer mechanism. However, despite containing the resilient pi structure and 

antioxidant OH groups, phenol formaldehyde resins also contain ether bonds, which are very 

susceptible to ionizing radiation, in their backbone. As a result, it is expected that the ether 

bonds connecting phenyl groups (phenyl-C-O-C-phenyl) undergo a scission reaction in the 

presence of ionizing radiation such as beta particles and gamma rays from spent fuel. 

In addition to beta particles and gamma rays from the spent fuel, alpha particles produced from 

neutron absorption by B-10 may also contribute to degradation of phenol formaldehyde resins. 

Alpha particles from the spent fuel are absorbed before reaching the neutron absorber material, 

and neutrons from the spent fuel have insignificant direct effects on the phenolic resin due to the 

low molecular weight and small neutron cross section of the atoms composing the phenolic 

resin. However, when neutrons from the spent fuel are absorbed by B-10 within the absorber 

material, alpha particles are produced by the following nuclear reaction seen in Equation 23: 
 

26 

1

n + 10B → 4α (1.47 MeV) + 7Li (0.84 MeV) (23) 

Alpha particles lose their energy rapidly when passing through liquid or solid materials and have 

a very limited penetration (or range) in such condensed systems. However, in this case, the 

alpha particle is produced within a boron carbide particle embedded within the neutron absorber 

material. Similar to beta particles and gamma rays, the interactions of alpha particles with 

matter primarily cause ionization. Alpha particle irradiation is considered high linear energy 

transfer (LET), since alpha particles lose their energy very rapidly when passing through a 

material. 

Beta particles, gamma rays, and alpha particles are all expected to have similar radiation 

chemistry effects on the phenol formaldehyde resin. Scission through the ether bonds is most 

likely and can be seen in Figure 4-1 below. Scission through the ether bonds breaks the 

polymer backbone, shortens, and leads to the formation of phenoxyl and carbon-centered free 

radicals. Breaking the polymer backbone shortens the polymer chain length and degrades the 

polymer as chain length decreases with successive breaks in the backbone. 

Another reaction that may occur is hydrogen abstraction as seen in Figure 4-2 below. In this 

reaction, the radiation excites the polymer structure, leading to abstraction of the hydrogen atom 

from the methyl group. However, due to the chemical structure of the phenol formaldehyde 

resin, the probability of hydrogen abstraction is much lower than scission the ether bond. 

This section focused on the direct effects of radiation of phenol formaldehyde resin, but the 

degradation process may be affected by the presence of water or oxygen. It is known that 

several plants have vented their neutron absorbers to the pool, which would place them in 

contact with water and likely oxygen. The role of water and oxygen in the degradation process 

of phenol formaldehyde resin will be discussed in the following sections. 

4.2.2 Degradation Process in the Presence of Water 

Although phenol formaldehyde resins are insoluble in water, they are in contact with water at 

their surfaces if submerged. Therefore, it is expected that the radiolytically-produced species 

from water discussed in Section 4.1 would interact with the phenol formaldehyde resins at these 

interfaces. The most important radiolytically-produced species is the strongly oxidizing hydroxyl 

radical (●

OH), which reacts rapidly with phenol formaldehyde resins through addition to the 

phenol group and abstraction of hydrogen (Neta and Steenkin, 2003). 

Figure 4-1 Scission of ether bond in phenol formaldehyde resin by ionizing radiation 

Figure 4-2 Hydrogen abstraction in phenol formaldehyde resin by ionizing radiation 
 

27 

Addition to the Phenol Group 

As mentioned earlier, hydroxyl radicals react with the phenyl group at a very high reaction rate 

constant with a value of 1x 1010 M-1s-1. The hydroxyl radical reacts by addition to yield 

dihydroxycyclohexadienyl radicals as shown in Figure 4-3 below, which then may undergo a 

spontaneous, acid-catalyzed, or base-catalyzed dehydration to yield phenoxyl radicals. 

Phenoxyl radicals do not react with oxygen since they are oxidizing radicals and are primarily 

short-lived intermediates. Phenoxyl radicals undergo a dimerization reaction with a secondorder decay of the order of 1 X 109

 M-1s-1 producing various dimers. Since the electron 

(unpaired spin) is delocalized on the oxygen and on the ortho-carbon and para-carbon, various 

dimers can be produced through the following cross-linking reactions seen in Equations 24 and 

25: 

െܥ଺ܪହܱ• െ ൅ െ ܥ଺ܪହܱ•െ ՜ െܥ଺ܪହܱܥ଺ܪସܱܪ െ Oxygen with Carbon (24) 

െܥ଺ܪହܱ• െ ൅ െ ܥ଺ܪହܱ•െ ՜ െܪܱܥ଺ܪସܥ଺ܪସܱܪ െ Carbon with Carbon (25) 

The carbon with carbon cross-linking reaction will be predominant because of the absence of a 

delocalized electron. 

Hydrogen Abstraction 

In addition to reacting with the phenol group, hydroxyl radicals will abstract hydrogen to produce 

carbon-centered radicals as shown in Figure 4-3 below. The abstraction of hydrogen takes 

place from two sites: methylene and ether. 

1. At the methylene site, a hydrogen atom reacts with the hydroxyl radical to form water 

and a carbon-centered radical as shown below in Equation 26 and seen in Figure 4-3: 

—Phenyl—CH2—Phenyl— 

ைு

ሱሮ —Phenyl—●CH—Phenyl— + H2O (26) 

In the absence or low concentration of oxygen, the carbon-centered free radical will undergo a 

cross-linking reaction. However, in the presence of oxygen, it is not expected that the crosslinking reaction would take place. 

2. At the ether site, a hydrogen atom reacts with the hydroxyl radical to form water and a 

carbon-centered radical as shown below in Equation 27 and seen in Figure 4-3: 

—Phenyl—CH2—O—CH2— Phenyl

ைு

ሱሮ Phenyl—●CH—O—CH2— Phenyl— + H2O (27) 

Because of the delocalization of the unpaired spin on the carbon, oxygen, and phenyl group, it 

is expected that the (—Phenyl—●

CH—O—CH2— Phenyl—) free radical is relatively stable and 

its cross-linking reaction rate is slow. However, it is also expected to undergo scission as seen 

in Equation 28, because this bond is weak and susceptible to breaking. 

 —Phenyl—●

CH—O—CH2— Phenyl— → —Phenyl—●CH2 + O=CH— Phenyl— (28) 

In the absence or low concentration of oxygen, —Phenyl—●

CH2 is expected to undergo crosslinking reactions. However, the presence of the neighboring phenyl will enhance the 

delocalization of the unpaired spin and make this free radical less reactive with a slow cross-
 

28 

linking reaction rate. The presence of water alone will enhance degradation on the surface of 

the polymer as compared to a dry state. 

4.2.3 Degradation Process In the Presence of Oxygen 

In general, phenol formaldehyde resins produce carbon-centered free radicals by reaction with 

hydroxyl radicals as discussed in the previous section. In the presence of oxygen, these free 

radicals react with molecular oxygen to produce the corresponding peroxyl radicals. However, 

the reactivities of the free radicals toward oxygen depend strongly on their chemical structure 

and the presence of the delocalization of the unpaired spin. The delocalization of the unpaired 

spin decreases the reductive tendency of the carbon-centered radical and hence makes it less 

reactive with oxygen. As Figure 4-3 shows, the interactions of phenol formaldehyde with 

hydroxyl radicals produce primarily free radicals with delocalization of the unpaired spin. 

The subsequent reactions that the phenol formaldehyde peroxyl radicals can undergo are 

superoxide elimination, bi-molecular reaction, and hydrogen abstraction. These reactions with 

oxygen increase the degradation rate by providing more opportunities for degrading reactions to 

occur. After an addition to the phenol group reaction with a hydroxyl radical, molecular oxygen 

can react with the phenol group followed by a superoxide (O2

●―)- elimination reaction leading to 

the formation of the cyclohexene structure as shown in Figure 4-4 below. After a methylene 

hydrogen abstraction reaction with a hydroxyl radical, a bi-molecular reaction with the 

production of a tetraoxide intermediate can occur as shown in Figure 4-5. The tetraoxide 

intermediate undergoes a decomposition reaction with the formation of alcohol and ketone 

structures, based on Russell’s and Bennett’s type mechanisms as seen in Figure 4-5. Oxygen 

can also lead to a chain reaction of methylene hydrogen abstractions as shown in Figure 4-6. 

After a methylene hydrogen abstraction reaction with a hydroxyl radical, oxygen reacts at the 

methylene to form a peroxyl radical. This leads to a hydrogen abstraction reaction from the 

neighboring phenol formaldehyde molecules by the phenol formaldehyde peroxyl, which 

produces carbon-centered free radicals as seen in Figure 4-6. 

Figure 4-3 Reactions of hydroxyl radicals with phenol 

formaldehyde resins
 

29 

Figure 4-4 Reactions of phenol formaldehyde resin free radicals 

producing peroxyl radicals followed by superoxide elimination 

Figure 4-5 Reactions of phenol formaldehyde resin free radicals producing peroxyl 

radicals followed by the tetraoxide intermediate formation and decomposition 
 

30 

It is important to note the very rapid rate of reactions involving oxygen. The reaction rate 

constants of carbon-centered free radicals with the dissolved oxygen are estimated to be 107

-

108

 M-1s-1. These reactions are very fast, but still slower than the reaction of alkyl carboncentered free radicals with oxygen, which have reaction rate constants around 109

 M-1s-1. 

Meanwhile, oxygen reacts most quickly with aqueous electrons at a rate constant of 2 x 1010 M1

s-1. This indicates that most of the oxygen will be consumed by scavenging electrons, but the 

other degradation reactions involving oxygen are also very rapid. 

4.2.4 Degradation Products of the Phenol Formaldehyde Resin 

The degradation products described in Sections 4.2.1, 4.2.2, and 4.2.3 are further broken down 

by radiation as well as chemical processes. Various other intermediate products and free 

radicals are produced at intermediate steps, but are not soluble or expected to be removed from 

the material prior to further decomposition. Ultimate degradation products from this process will 

include water-soluble low molecular weight compounds such as phenols, aldehydes, and 

muconic acids and partially water-soluble gases such as hydrogen (H2), carbon dioxide (CO2), 

methane (CH4), and carbon monoxide (CO). Depending on the type of ionizing radiation and 

polymer additives, the total radiation chemical yields (G-values) of the gases may range from 

0.1 to 1.0 molecules/100eV (Milinchuk, Tupikov, and Kemp, 1989). 

4.2.5 Radiation Dose Tolerance of Phenol Formaldehyde Resins 

While radiation chemistry theory can describe conceptually and qualitatively the degradation of 

phenol formaldehyde resins by radiation, water, and oxygen, experimental data is needed to 

quantitatively measure degradation as a function of dose. Figure 4-7 shows that phenol 

formaldehyde resin loses 50% of its ductility after irradiation in air to 106

 – 5 x 107 Gy (108 – 5 x 

109

 rad) depending on dose rate. Likewise, Figure 4-8 shows that irradiation in air of phenol 

formaldehyde resins to a dose level of 3 x 107

 Gy (3 x 109 rad) leads to 50% loss of flexural 

Figure 4-6 Methylene hydrogen abstraction by peroxyl radical 
 

31 

strength. This indicates a severe degradation of mechanical properties and consequently of the 

chemical structure of the polymer. 

Figure 4-7 Effects of radiation on polymers as a function of 

absorbed dose (Clegg and Collyer, 1991, reprinted with 

permission from Springer Science and Business Media, B.V.) 
 

32 

Based on the Palisades BADGER testing report, licensees have exposed phenolic resin-based 

neutron absorbers to dose levels of higher than 108

 Gy (1010 rad), which is significantly higher 

than the doses shown in Figures 4-7 and 4-8. It should also be noted that the results shown in 

Figures 4-7 and 4-8 were performed in air and therefore reflect only the effects of radiation and 

oxygen. It is expected that degradation will be enhanced with the additional stressor of water. 

4.3 Effect of Variables in the Surrounding Environment 

It is important to understand the effects of different environmental variables on the degradation 

process of phenol formaldehyde resins in the spent fuel pool environment. This may allow for 

better understanding of what conditions are bounding or conservative relative to others, which 

may be essential when assessing the adequacy of surveillance methodologies. Several key 

variables considered in the sections below include water temperature, filler material, the 

presence of oxygen, pool chemistry, the exchange rate between the panel cavity and the pool, 

and flow rate within panel cavity. 

4.3.1 Water Temperature 

Pool water temperature is expected to affect the oxidation rate, the decomposition rate of 

hydrogen peroxide, and the oxygen content of the water; all of which affect the degradation 

process of the phenolic resin. 

In general, higher temperatures enhance and increase the reaction rate of oxidation-induced 

degradation. At high temperatures, peroxidation of the carbon-centered free radicals will be 

enhanced leading to the formation of more peroxyl radicals. Furthermore, the increased 

decomposition of the phenolic aldehyde tetraoxide (ROOOOR) leads to more degraded 

products. The abstraction of hydrogen atoms from the phenol formaldehyde resin by the peroxyl 

radicals will also be enhanced, which will lead to the formation of more carbon-centered radicals 

and hydroperoxide as seen in Figure 4-6 and the following reaction: RO2

●

 + RH → R● + RO2H. 

Figure 4-8 Degradation of mechanical properties of polymers as a function of 

absorbed dose (Tabata, Ito, and Tagawa, 1991, reprinted by permission of CRC 

Press © 2013 
 

33 

This reaction initiates a chain reaction that creates more free radicals and hydroperoxide 

(RO2H) and faster degradation. 

As described previously, gamma radiolysis of water produces hydrogen peroxide (H2O2) with a 

radiation chemical yield (G-value) of 0.75-0.8 molecules per 100 eV (0.8 μmole per joule). At 

high temperatures, hydrogen peroxide decomposes to form more hydroxyl radicals (●

OH). An 

increased hydroxyl radical concentration enhances the degradation of the phenol formaldehyde 

resin because hydroxyl radicals are a very oxidizing species. The decomposition of one 

molecule of H2O2 leads to the formation of 2 ●

OH radicals: H2O2 → 2 ●OH. 

The solubility of oxygen in water at room temperature and at atmospheric pressure is about 0.23 

x 10-3 M. This solubility of oxygen decreases dramatically as the temperature of the system 

increases. For example, at atmospheric pressure, the concentration of oxygen in water 

decreases from 8.8 mg per liter at room temperature to 5.6 mg per liter at 50o

C. The soluble 

oxygen molecule in aqueous solutions is consumed with about 0.75 kGy (75 kRad) of absorbed 

dose radiation dose. The consumption of soluble oxygen in water is a result of the reaction of 

the aqueous electron (eaq-

) with molecular dissolved oxygen in water: eaq- + O2  O2

●―. Since 

the spent fuel pools are open to air, it is expected that the system reaches equilibrium with the 

atmosphere. Therefore, increasing temperature will decrease the amount of dissolved oxygen in 

the water, but there should still be sufficient oxygen in the pool water to initiate the oxidative 

degradation based on reaching equilibrium with the air above the pool. 

4.3.2 Filler Materials: Fiberglass, Graphite, and Boron Carbide Particles

As described in Chapter 2, Carborundum sheet material is described as containing fiberglass 

reinforcement, while Tetrabor is described as possibly containing graphite. In addition, all of the 

phenolic resin-based neutron absorbers contain boron carbide particles. Fiberglass and graphite 

are not expected to affect the radiation chemistry of phenol formaldehyde resin due to their 

nuclear and chemical stability in the presence of ionizing radiation. As discussed earlier, the 

presence of boron carbide produces alpha particles which degrade the phenol formaldehyde 

resin, but other components of the neutron absorber panel are generally stable. 

Another key distinction between Carborundum sheet compared to Carborundum plate and 

Tetrabor is that Carborundum sheet is much thinner. This difference is not expected to alter the 

fundamentals of the degradation process beyond the effect on the surface area to volume ratio. 

Dissolution occurs at the surface of the material where it interfaces with water, so the thinner 

Carborundum sheet would be expected to be more susceptible due to its greater surface area to 

volume ratio. 

4.3.3 The Presence of Oxygen 

As discussed earlier in the report, oxygen has a crucial and effective role in the degradation of 

phenol formaldehyde resin. The amount of oxygen present in the panel cavity may be 

dependent on the amount of exchange between the panel cavity and pool, because fresh 

oxygenated water from the pool is expected to be the source of oxygen as it is consumed in the 

degradation process. Given sufficient exchange between the panel cavity and pool, molecular 
 

34 

oxygen should be present in both the panel cavity water and in the bulk phenol formaldehyde 

resin. Therefore, the peroxidation of the phenol formaldehyde carbon-centered radicals should 

occur in both the bulk and the surface of the resin, leading to continuous degradation of the 

resin in the bulk and at the surface. However, more degradation is expected to occur on the 

surface due to greater availability of hydroxyl radicals and oxygen on the surface than in the 

bulk. 

4.3.4 Pool chemistry: pH, Soluble Boron, and Metal Ions 

Panel cavity chemistry around the phenolic resin-based neutron absorber materials can have a 

significant impact on the degradation process. However, the exact chemical conditions in the 

panel cavity are not well-known. The pH of panel cavity water may be influenced by radiolysis of 

water from spent fuel radiation as well as exchange between the panel cavity and pool and the 

presence of soluble boron. The pH decreases during radiolysis of water by producing hydronium 

(H3O+

) with a radiation chemical yield of G(H3O+) = 0.29 μmol J-1. Without significant pool water 

exchange to reduce hydronium concentrations, they cause pH to decrease. In addition, the 

presence of boric acid (soluble boron) in PWR pools also reduces pH. In general, more acidic 

conditions would be expected to enhance the degradation, because of the role of perhydroxyl 

radicals as discussed in Section 4.1.2. 

Radiolysis also produces hydrogen peroxide (H2O2), which is a very powerful oxidizer that can 

degrade the phenol formaldehyde resins. While hydrogen peroxide is produced through the 

radiolysis of water, it also decomposes to hydroxyl radicals and reacts with metals and organic 

materials in the pool such as phenol formaldehyde resin. Additionally, dissolved metal ions such 

as ferrous ions (Fe2+) may react with hydrogen peroxide to form more hydroxyl radicals by 

Fenton’s reaction. This reaction enhances the hydroxyl radical concentration, which leads to 

greater degradation of the phenol formaldehyde resin. 

4.3.5 Water Exchange between Panel Cavity and Pool 

It is expected that the amount of water exchange between the panel and pool affects the 

degradation rate of the polymer matrix. However, the actual amount of water exchange between 

the panel cavity and the pool is not well-known and will vary from pool to pool depending on 

rack design. Therefore, the following subsections describe the potential scenarios of significant 

water exchange and no or insignificant water exchange. 

Water-Filled Panel Cavity with Pool Water Exchange 

In the case of a panel cavity with significant pool water exchange, fresh water from the bulk pool 

entering the panel cavity would have high oxygen concentrations. This replenishment of oxygen 

in the panel cavity can enhance the peroxidation of the radiolytically-produced carbon-centered 

free radicals on the surface of the phenol formaldehyde resin and therefore increase the 

degradation rate of the phenol formaldehyde resin. Conversely, a continuous exchange of water 

may also reduce the buildup of hydrogen peroxide (H2O2) concentrations in the panel cavities 

since they can be regarded as open systems. This decrease in the concentration of the 

corrosive radiolytically-produced hydrogen peroxide would consequently decrease the 

degradation rate of the phenol formaldehyde resin. 
 

35 

Water-Filled Panel Cavity without Pool Water Exchange 

In the case of no or insignificant pool water exchange, the panel cavity may be treated as an 

essentially closed system. This situation may exist in the case of very small or clogged vent 

holes that allowed water to enter the cavity, but allow little or no pool water exchange. Assuming 

an essentially closed system filled with stagnant water in a radiation field, three key effects are 

expected: hydrogen peroxide will build up, pH will decrease, and oxygen will be consumed. 

As described previously, hydrogen peroxide (H2O2) is a very corrosive and oxidative radiolytic 

product of the radiolysis of water, which is produced inside the panels by gamma irradiation. 

Due to the continued production of hydrogen peroxide and lack of exchange with the pool water, 

hydrogen peroxide concentration will increase as a function of dose. Hydrogen peroxide 

concentration can be estimated based on the radiation chemical yield (G = 0.75 mole per 100 

eV) and the absorbed dose in Gy as seen in Equation 29: 

 GሺHଶOଶሻ ൌ 9.648 ݔ 10 ଺ቀ

C୦ୣ୫୧ୡୟ୪ ୷୧ୣ୪ୢ ሺ୫୭୪.୪୧୲ୣ୰షభሻ

Aୠୱ୭୰ୠୣୢ D୭ୱୣ ሺG୷ሻ ୶ ஡ ቁ (29) 

Assuming the water pool receives a gamma dose of 108

 Gy and the density of the H2O2 is 1.45 

g/cm3, the concentration of the produced H2O2 is 113 moles per liter, which is extremely high. 

Usually, while hydrogen peroxide is being produced through the radiolysis of water, it also 

decomposes to hydroxyl radicals and reacts with metals and organic materials in the pool such 

as phenol formaldehyde resin. Its corrosive and oxidative nature will effectively degrade the 

phenol formaldehyde resins by the following mechanism involving hydroxyl radicals and 

hydrogen peroxide as illustrated in Figure 4-9.7 

This mechanism involves the addition of a hydroxyl radical to the pi (π) structure of the phenyl 

ring followed by opening of the ring. The phenols are further oxidized by hydroxyl radicals and 

hydrogen peroxide, leading to decarboxylation and the formation of muconic acids. These 

muconic acids are prone to further oxidation by hydroxyl radicals and hydrogen peroxide. The 

cumulative effect of these types of oxidation/degradation reactions inside the panels is 

dissolution of the resin. 

Figure 4-9 Reaction of hydroxyl radical and hydrogen peroxide with phenyl (Patai, 1983) 
 

36 

Continuous irradiation without pool water exchange also increases the hydronium (H3O+) 

concentration according to: H2O ఊ

՜ •OH, H•, eaq-, H3O+, H2O2, H2. The continuous increase in 

hydronium concentration leads to a continuous decrease in the pH. It is known that phenol 

formaldehyde resin has a reasonable resistance to acidity (Schnabel, 1992). However; it is 

expected that the combination of radiation and low acidity can cause increased degradation. 

Continuous irradiation without pool water exchange leads to dissolved oxygen consumption due 

to reaction with aqueous electrons and hydrogen atoms produced by radiolysis of water. 

Assuming an initially air-saturated solution, complete scavenging of the aqueous electrons and 

hydrogen atoms by oxygen (O2 = 0.24 millimol L-1) occurs. For a closed system, the O2

consumption yield G(-O2) is 0.32 μmol J-1. The dose required to consume all dissolved oxygen 

can be estimated by Equation 30: 

௃ ቀ ݁ݏ݋ܦ 

௞௚ ൌ ݕܩቁ ൌ 

଴.ଶସ ௫ ଵ଴షయ ௠௢௟

௅ൗ

ቀଵ ௞௚

௅ൗ ቁ ௫ ሺଷ.ଶ ௫ ଵ଴షళ ௠௢௟

௃ൗ ሻ

 (30) ݕܩ ଶ10 ݔ 7.5 ൌ

Therefore, all oxygen will be consumed in the submerged panels at a very low dose of only 0.75 

kGy (0/75 kRad). Without oxygen present, the degradation of the phenol formaldehyde resin 

from irradiation of water will still occur from both the direct interactions of the hydroxyl radicals 

and the oxidation by hydrogen peroxide as described above. 

Water Exchange Effect on Dissolution in Water 

As degradation progresses, the degraded phenol formaldehyde resin may be removed from the 

panel surface by dissolving in the bulk water of the panel cavity. Degraded radiolytic resin 

products, such as phenols and muconic acids, dissolve very easily in water. This dissolving 

process enhances further degradation by exposing more resin from the bulk polymer directly to 

the water. In panel cavities with significant pool water exchange, the concentration of the 

degraded resin dissolved in the water of should remain below saturation due to dilution with 

fresh pool water, promoting further dissolution Conversely, in panel cavities without significant 

pool water exchange, the stagnant water may eventually be saturated with degraded resin 

products due to the lack of diluting pool water exchange. Therefore, dissolution of degraded 

resin will occur in stagnant water, but the dissolution process is slowed when the concentration 

of degraded resin products approaches solubility limits. Dissolution is expected to occur more 

rapidly with significant pool water exchange by maintaining the concentration of degraded resin 

below the solubility limit.. Therefore, it is expected that the degraded phenol formaldehyde resin 

in panels with significant pool water exchange would dissolve faster than resin in panels without 

significant pool water exchange. 

The gases produced from the irradiation of water and phenol formaldehyde resin (such as 

hydrogen gas (H2), carbon dioxide (CO2), methane (CH4), and carbon monoxide (CO)) dissolve 

to a certain extent in water. However; solubility in water varies from one gas to another, and gas 

produced above the solubility limit will come out of solution and form gas bubbles. It is therefore 

expected that their release to the environment does not depend only on their G-values, but also 

on their solubility in water. 
 

37 

4.3.6 Flow Rate within the Panel Cavity 

Another key factor that may affect the erosion and dissolution process is the flow rate within the 

panel cavity. The nature of water flow rates within the panel cavity of plants with phenolic resinbased absorbers is not well known. However, it is important to consider how flow within the 

panel cavity may affect the degradation rate and the physical erosion of degraded resin. 

Increasing flow rate within the panel cavity is expected to enhance the kinetics of the erosion 

and dissolution process by chemically increasing the interaction between the degraded polymer 

and flowing water. In the case of Boraflex, flow is known to be a key factor to increasing the 

severity of degradation. Therefore, flow rate should also be considered as a potentially 

important factor in the erosion and dissolution process. 

4.4 Summary 

Radiation chemistry theory shows that phenol formaldehyde resins are expected to degrade in 

the spent fuel pool environment. Radiation directly interacts with the polymer itself to break 

bonds and cause scissioning of polymer chains. Radiation also affects the water environment 

through radiolysis by creating aggressive hydroxyl radicals and corrosive hydrogen peroxide. 

The presence of oxygen increases the opportunities for chemical reactions that degrade the 

polymer structure. Other important variables that may significantly impact the degradation 

process, but are not well-known include water exchange between the panel cavity and pool and 

flow rate within the panel cavity. 

Table 4-2 below summarizes the expected degradation process in four potential panel cavity 

environments: dry without oxygen, dry with oxygen, wet without oxygen, and wet with oxygen. 

With the information available, it is not possible to quantify the degradation rate. However, Table 

4-2 shows the relative degradation rate in each of the four environments. It is expected that the 

presence of water and oxygen accelerate degradation, while a dry environment without oxygen 

is expected to show the least degradation. However, it is important to note that these rates are 

only relative. A dry environment without oxygen may still cause significant degradation. 
 

38 

Environment Primary Causes of Degradation Relative 

Degradation Rate 

Wet with oxygen 

• Beta and gamma radiation from spent fuel 

• Alpha particles from B-10 neutron reaction 

• Hydroxyl radicals and hydrogen peroxide 

• Oxygen 

Highest 

Wet without oxygen 

• Beta and gamma radiation from spent fuel 

• Alpha particles from B-10 neutron reaction 

• Hydroxyl radicals and hydrogen peroxide 

High 

Dry with oxygen 

• Beta and gamma radiation from spent fuel 

• Alpha particles from B-10 neutron reaction 

• Oxygen 

Lower 

Dry without oxygen • Beta and gamma radiation from spent fuel 

• Alpha particles from B-10 neutron reaction Lowest 

Table 4-2 Summary of degradation process in possible environments 
 

39 

5. Discussion

Chapter 3 surveyed the current empirical knowledge of the behavior of boron containing 

phenolic resin composite neutron absorber materials under spent fuel pool conditions: initial 

qualification studies, testing of in-pool coupons, and in-pool operational experience of racks. 

Chapter 4 provided a mechanistic background of the radiation-assisted degradation of phenol 

formaldehyde resin polymers. Chapter 5 will compare the empirical knowledge with mechanistic 

understanding to assess whether initial qualification testing, in-situ testing, or coupon programs 

can adequately capture and predict the degradation of phenolic-resin based neutron absorbers. 

5.1 Radiation-Assisted Degradation of Phenolic Resins and Empirical Data 

The degradation product and mechanical property observations from qualification testing and 

operating experience in Chapter 3 are consistent with the degradation mechanisms from 

radiation chemistry theory in Chapter 4. Radiation chemistry theory predicts the production of 

hydrogen (H2) and carbon dioxide (CO2) as the ultimate degradation products due to bondbreaking in the phenol formaldehyde resin by gamma radiation. This is in good agreement with 

the empirical data described in Section 3.1. Qualification testing and operating experience have 

found offgassing from Carborundum and Tetrabor to be primarily hydrogen and carbon dioxide. 

For example, Palisades observed continued swelling problems from 1988 to 2007 even after 

drilling vent holes indicating continued offgassing for an extended period of time. 

Radiation chemistry theory shows that gamma radiation causes degradation of phenol 

formaldehyde polymers by scissioning polymer chain backbones. Degradation is expected to 

occur in the presence of spent fuel radiation (particularly gamma) even in a dry sealed 

environment. However, the presence of water and oxygen should influence the kinetics of the 

polymer degradation process. Degradation is expected to occur more rapidly in wet 

environments with oxygen present than in dry environments without oxygen. Degradation 

physically weakens the phenolic resin by shortening polymer chains and is consistent with 

observations of “friability” of these polymers after irradiation described in the qualification testing 

of Carborundum in Chapter 3 and in the coupons from La Crosse. 

5.2 Rack Conditions and Boron Carbide Release 

The mechanism of releasing boron carbide from radiation degraded polymer is postulated to be 

influenced by two effects. First, bulk dissolution of water-soluble phenolic resin degradation 

products such as phenol, as described in Section 4.3.5, may remove enough resin to cause 

particles of insoluble boron carbide to be released from the panel by gravity. If stagnant water 

is present in the panel cavity, dissolution and simple gravity release can potentially occur even if 

there is little or no water exchange with the bulk pool. Second, boron carbide release may be 

effected by water flow within the panel cavity and exchange between the panel cavity and the 

surrounding pool water. Water flow, in turn, is highly dependent on rack configuration. Racks 

containing Boraflex are open to constant flow and exchange with bulk pool water and show a 

strong correlation between B-10 areal density and absorbed gamma dose. In contrast, 

phenolic resin-based neutron absorber rack panel cavities are generally less exposed to flow 
 

40 

and pool water exchange and do not show a strong correlation between B-10 areal density and 

absorbed gamma dose. Therefore, gamma dose alone may not be a good predictor of B-10 

areal density for phenolic resin panels depending on water flow and exchange conditions. The 

postulated influence of flow and pool water exchange in the panel cavity on the release of boron 

carbide is discussed in greater detail below. 

Section 2.1.2 reviewed the mechanism for the degradation of Boraflex polymer-B4C neutron 

absorber panels. In general, the degradation is a two-step process. First, gamma radiation 

degrades the backbone of the PDMS matrix from a flexible polymer to brittle amorphous silica 

which is only slightly soluble in water. Second, water flow in the pool erodes the panel, 

releasing both silica and boron carbide from the panel at known rates proportional to each other. 

Through a series of directly proportional processes based on a silica transport model, the B-10 

areal density loss from a panel as measured by in situ testing can be correlated from boron 

carbide release proportionally to polymer degradation and then to the panel’s gamma exposure 

history. This proportionality of the boron carbide release rate to silica release has not been 

observed directly, but is inferred based on the correlation between B-10 areal density and 

gamma dose found for Boraflex, and is integral to the model for RACKLIFE (Haley, 2012). 

If the rate of boron carbide loss in phenolic resins is proportional to polymer matrix degradation, 

similar to what is inferred for Boraflex, then it would be expected that the B-10 areal density of a 

phenolic resin panel would follow that of Boraflex; i.e., B-10 areal density should decrease with 

increasing gamma dose. However, the BADGER B-10 areal density data plotted as a function 

of gamma dose for Carborundum plate at Palisades, shown in Figure 3-3, does not appear to 

indicate a strong correlation between loss of B-10 and dose. At Palisades, panels with low 

gamma dose may show large losses of B-10, while panels with high gamma dose still retain B10 above the maximum calibration areal density of the 0.0705 g/cm2

 standard. While this is only 

one data set, the apparent lack of a correlation suggests that the boron release process for 

Carborundum plate, and by extension for Carborundum sheet and Tetrabor, may not occur 

proportionally with radiation-induced polymer degradation as it does for Boraflex. Two 

postulated reasons for this behavior, variations between panel cavities with respect to flow and 

pool water exchange and solubility of the degradation products are discussed below. 

As described in Chapters 3 and 4, significant degradation of the mechanical properties of 

phenolic polymers has been observed at dose levels in the range of 109

 – 1010 rad as well as 

the production during irradiation of “a water soluble degradation product, which causes the 

material to disintegrate in water’” (Sisman, 1955). Although there is clear evidence that the 

polymer structure is degraded at these high doses, degradation of the phenolic resin matrix 

does not necessarily lead to a proportional amount of B-10 loss from the panel, unlike Boraflex, 

as evidenced by the absence of a correlation between B-10 areal density and gamma dose 

observed at Palisades. Operating experience with Boraflex shows that both flow and exchange 

with the pool have significant effects on the rate of dissolution of polymer materials. Flow in the 

panel cavity accelerates the dissolution process by mixing otherwise stagnant water. Greater 

water exchange with the pool can increase the driving force for dissolution by removing 

dissolved degradation products from the panel cavity and introducing fresh water which can 

dissolve the panel further, as discussed in the technical letter report on RACKLIFE (Haley 
 

41 

2012). Although the assertion that boron carbide release from the panel is dominated by flow 

and pool water exchange rather than by polymer degradation from gamma dose alone cannot 

be confirmed directly, it can be inferred from the consistent correlation of loss of B-10, as 

observed by in-situ methods such as BADGER, with accumulated gamma dose. 

It is expected that flow within the cavity and water exchange with the bulk pool will have similar 

dissolution effects on phenolic resins as they do on Boraflex, as described in Chapter 4. Similar 

to Boraflex, the boron carbide release process from degraded phenolic resins likely involves an 

erosion or dissolution interaction with the water that removes the fragments of degraded 

polymer binder, exposing and releasing the boron carbide. However, there are two key factors 

which affect the predictability of B-10 release in Boraflex racks and phenolic resin-based 

absorber racks 

The first key factor is the degree of interaction between the panel cavity and the bulk pool. 

Boraflex rack panel cavities are generally open to the pool with only skip or spot welds attaching 

the wrapper plate to the rack. This allows for significant water exchange and even areas of 

strong flow into the panel cavity. Meanwhile, phenolic resin-based neutron absorber rack panel 

cavities, as described in Chapter 2, were usually designed to be sealed from the pool, and were 

only vented to the pool through small holes drilled in the wrapper plate to relieve pressure from 

offgassing after installation. 

A single small hole will only allow a small amount of water exchange with the bulk pool. Without 

flow and water exchange to spur the erosion and dissolution process, boron carbide may be 

retained in a severely degraded polymer matrix due to factors that are difficult to observe, 

model, or predict. An additional confounding factor in the boron release process is that the 

degradation products of polymer resins such as phenols are very soluble in water compared to 

amorphous silica, the degradation product of the silicone rubber matrix in Boraflex. Bulk 

dissolution of degraded phenolic resin matrix in water may lead to release of boron carbide 

particles by gravity even in a no-flow environment, although that effect may be slowed or limited 

by saturation of the stagnant water with degradation products. Therefore, while high flow may 

accelerate loss of B4C, low or no flow does not necessarily prevent the loss of B4C.. Potential 

loss of B4C in the absence of flow contributes additional uncertainty to the effect of flow and rack 

environment on extent of boron carbide release. 

The series of correlations from gamma dose to measured B-10 loss depends both on 

predictable polymer degradation and on proportional release of B-10 from the degraded 

polymer. Therefore, if the lack of flow and pool water exchange leads to a lack of proportional 

B-10 loss, then gamma dose will no longer be a reliable predictor of B-10 loss. This may explain 

why, at Palisades, an in situ test such as BADGER detected an acceptable level of B-10 present 

in severely degraded phenolic resins that are exposed to high gamma doses above 1010 rads. 

The second key factor in predicting B-10 release from Boraflex racks or phenolic resin racks is 

their consistency of design. Although there were several variations in design, all Boraflex racks 

allowed for the significant exchange between the panel cavity and the bulk pool. In contrast, 

phenolic resin rack designs can vary greatly from pool to pool. For example, a rack that leaked 

or was drilled with large holes would be expected to experience less water exchange with the 
 

42 

bulk pool than an open Boraflex rack. A panel in a rack that had very small or potentially 

clogged vent holes drilled may experience little to no exchange with the bulk pool, leading to 

partial or full submersion in fairly stagnant pool water. A panel in a rack that remained airtight 

during venting, as at Columbia, may be exposed to a dry but oxygenated environment. The 

effect of each of these postulated environments on degradation of the polymer matrix and on 

the release of B-10 is discussed in greater detail in Section 4.3. 

The inconsistent flow in panel cavities and inconsistency in rack designs leads to uncertainties 

in the amount and rate of B-10 release as a function of dose, which will affect the adequacy of 

the surveillance methods. 

5.3 Adequacy of Qualification Testing and Surveillance Methods 

The lack of clear understanding of the boron carbide release process and inconsistency of 

environments based on varying rack designs makes speculation or predictive modeling about 

the state of phenolic resin-based neutron absorber panels difficult. Therefore, the adequacy of 

qualification testing and surveillance methods becomes paramount to assessing the neutron 

absorption capabilities of these panels. 

Qualification testing of Carborundum and Tetrabor materials was performed at the time they 

were installed to establish their performance over the range of radiation doses anticipated for 

service. Initial qualification testing programs are intended to provide the technical basis to 

show that the material will perform throughout its design life for the specified environment. 

Surveillance methods are employed to ensure neutron absorber performance during service 

and should be designed to detect degradation in a manner that captures the degradation 

mechanism or is at least reliably representative of the condition of the in-service material, if not 

conservatively bounding. The three surveillance methods described in Chapter 3 that have been 

employed to monitor the performance of phenolic resin-based neutron absorbers are monitoring 

of total organic carbon (TOC) in the spent fuel pool, coupon testing, and in situ testing. 

5.3.1 Qualification Testing 

Qualification testing was performed on Carborundum and Tetrabor neutron absorber materials 

as described in Chapter 3. Limited information is available regarding the Tetrabor qualification 

testing, although it is known that hydrogen gas was evolved during gamma irradiation as 

described in greater detail in Section 3.1.2. Much more information is available regarding the 

Carborundum qualification testing, which was carried out at gamma fluxes of 106

 – 108 rad per 

hour up to a total exposure of 1011 rad in about 100 days (The Carborundum Company, 1980). 

The Carborundum qualification testing program concluded that Carborundum would maintain its 

performance up to 1011 rad with only 20% weight loss and 15% boron loss. However, BADGER 

testing at Palisades showed greater than 50% boron loss at about 1010 rad, representing much 

greater loss of neutron absorption at ten times lower dose. 

The BADGER results from Palisades and Millstone 1 may be explained by shortcomings in the 

applicability of the Carborundum qualification testing to in-service materials. Most importantly, 

the qualification testing was greatly accelerated compared to the amount of time in service. 
 

43 

These materials have been used in service for over 30 years, while qualification testing was 

condensed to 100 days. This accelerated testing is non-conservative in two ways. First, in order 

to reach a 1011 rad dose in 100 days, very high dose rates approaching 108

 rad per hour were 

used. However, it is well-established that for a given accumulated dose, damage is less severe 

at higher dose rates due to greater likelihood of recombination as described in Section 2.4.2, 

Section 4.1., and Woods and Pikaev, (1993). At low dose rates, the radical-oxygen reaction that 

causes polymer degradation is predominant, but at higher dose rates, the radical-radical 

reaction that causes cross-linking is predominant. Therefore, qualification testing with high dose 

rates over a short time may result in phenolic resin binder that is less degraded than it would be 

in-service with lower dose rates for a longer time. 

Second, as discussed in Section 5.2, one BADGER data set suggests that the loss of boron 

carbide from phenolic resin-based absorbers may not be dependent only on degradation from 

radiation exposure, but also strongly affected by dissolution and erosion in water. Therefore, the 

exposure time in water of only 100 days would not be conservative or representative of 

degraded phenolic resin binder exposed to pool water and flow for over 30 years. These two 

non-conservative aspects of the qualification testing may help explain why the Carborundum 

qualification testing results are not consistent with operating experience. 

5.3.2 Total Organic Carbon Monitoring 

There is one occurrence available in the literature of using total organic carbon (TOC) 

monitoring as a surveillance method for phenolic resin-based neutron absorbers. As discussed 

in Chapter 3, during the license renewal process in 2005 Palisades described a TOC monitoring 

program in their spent fuel pool as evidence of the lack of degradation of their Carborundum 

plate neutron absorbers. Palisades informed the NRC of periodic sampling since 2000 of spent 

fuel pool water showing low, stable levels of TOC less than 0.2 ppm. Subsequent BADGER 

testing of the absorber materials in 2008 found significant degradation of the Carborundum plate 

material at Palisades, including greater 50% loss of B-10 areal density in some panels. The 

finding of significantly degraded Carborundum plate material indicates that TOC monitoring at 

Palisades did not provide effective indication of degradation of the Carborundum plate 

materials. 

This TOC monitoring may be considered an analogue to silica concentration measurements 

used to monitor Boraflex degradation. Through the RACKLIFE program, Boraflex degradation 

and subsequent loss of B-10 can be calculated from measuring the concentration of the 

Boraflex degradation product, reactive silica, through a well-established model which accounts 

for all forms of silica released from a degraded panel of Boraflex (Haley, 2012). In addition, the 

RACKLIFE model assumes that release of boron carbide from the panel is proportional to the 

release of silica from the panel. As discussed in Chapter 4, the degradation of phenolic resins 

produces a variety of degradation polymer fragments along with hydrogen and carbon dioxide 

gases. These degradation products may bubble from the pool water or remain in sealed racks 

where it is not available to TOC analysis. The discrepancy between the TOC monitoring as 

described and the release of boron carbide measured by BADGER can be explained because, 

unlike silica with Boraflex, it appears that TOC monitoring for phenolic resins does not 
 

44 

necessarily detect all the products of radiation-induced degradation of phenolic resins. Current 

understanding of the events at Palisades shows that TOC monitoring is not an effective 

surveillance method for the degradation of phenolic resin-based neutron absorbers. 

5.3.3 Coupon Testing 

Crystal River 3, Calvert Cliffs 1, La Crosse, Kewaunee, and Columbia credit phenolic resinbased neutron absorbers and utilize coupon testing programs as a surveillance method to 

monitor degradation of the absorber materials. These coupons are placed in the spent fuel pool 

in order to simulate the environmental exposure and configuration of the in-service panels. To 

achieve this, they are exposed to pool water and spent fuel radiation within sheathing that may 

be vented similar to the in-service panels. Coupon testing may include visual examination, 

weight and dimensional measurements, and neutron attenuation testing. 

The primary considerations of the effectiveness of coupon surveillance programs are how 

representative the coupons are of the population of in-service panels and whether the coupon 

test procedures can adequately detect degradation. In order to capture the degradation of the 

service material, the coupon surveillance programs must be designed to expose the coupons to 

the same stressors that degrade the service material. In order to determine those stressors, the 

primary degradation mechanism must be known. For example, if the degradation mechanism is 

known to be dependent on radiation dose, then coupons that are exposed to the same or 

greater radiation dose as the service material will be representative of the in-service panels. 

Coupon testing methods must also be able to detect expected degradation. If a coupon 

degrades by weight loss without losing volume, then measuring dimensions of the coupon may 

not capture the degradation. Further, uncertainties in coupon testing must be considered. For 

example, if a coupon is sensitive to damage by abrasion, there is uncertainty whether a 

negative test result such as weight loss is reflective of degradation in service, or by damage 

from handling of the coupon during the test procedure, as reported by Crystal River as seen 

Section 3.2.2. Also, measuring the B-10 areal density of coupons by laboratory neutron 

attenuation testing must consider the uncertainties of the attenuation test. An example of this is 

the inconsistencies in the neutron attenuation results of the Kewaunee coupons in Section 

3.2.2. 

For phenolic resins, there is good understanding that the degradation of the overall panel begins 

with degradation of the polymer binder due to polymer chain scission from gamma radiation, 

which is exacerbated in the presence or absence of oxygen and water. However, as described 

in Section 5.2, the factors which control the subsequent release of boron carbide particles and 

loss of neutron absorption are not well understood, making it difficult to assess whether the 

performance of coupons are representative or conservative regarding the boron carbide release 

process of in-service panels. For example, if the coupon assembly does not represent pool 

water exchange as in the in-service panels, then coupon performance may not be a reliable 

indicator of panel performance. 

Since the mechanism of boron carbide release is not well-established, operating experience 

was used to assess the effectiveness of coupon surveillance in this report. However, only 
 

45 

limited direct information is available on the condition of current in-service phenolic resin-based 

absorber materials to provide a reliable comparison. One key operating experience event 

involving coupon testing programs occurred at Palisades in 2008. During the license renewal 

process in 2005, Palisades informed the NRC that it was monitoring the coupon testing program 

at another plant with the same absorber material. The coupon testing from the other plant gave 

no indication of significant degradation, but as described previously, Palisades found 

significantly degraded absorber material by in situ testing in 2008. This limited experience 

demonstrates the limitations of relying on the comparison of coupons from another pool to 

predict the release of boron carbide and loss of neutron absorber performance, particularly 

considering the inconsistencies in rack design as discussed in Section 5.2. 

5.3.4 In Situ Testing 

In situ methods such as BADGER testing has been used primarily to measure B-10 areal 

density in neutron absorber materials at plants that credit Boraflex; but it is also known to have 

been conducted at two plants with phenolic resin-based absorbers (Palisades and Millstone 1). 

BADGER uses a neutron source and detectors on opposite sides of a panel in adjacent storage 

cells to measure neutron transmission count rates which are then correlated to B-10 areal 

density, albeit with significant uncertainty (Chapman and Scaglione, 2012). Given adequate 

considerations for these uncertainties, an in situ method such as BADGER can provide a 

comparative assessment of the B-10 areal density of individual panels of phenolic resin-based 

absorbers. 

In situ methods are valuable in that they directly measure the ability of an in-service neutron 

absorber panel to perform its function of absorbing neutrons without introducing uncertainties 

from pool chemistry models or potentially non-representative coupons. However, in situ neutron 

attenuation methods cannot observe degradation of the phenolic polymer binder unless that 

degradation has led to boron carbide loss, nor can these methods provide predictive 

information. Therefore, while an in situ method may assess the neutron absorbing capabilities of 

a single panel, without a consistent method to relate individual panel results to environmental 

conditions such as gamma dose, there is uncertainty in its ability to extrapolate the B-10 areal 

density results of a limited number of panels to the degradation and B-10 losses of all the 

panels in a spent fuel pool. 

In the case of Palisades, BADGER results showed that B-10 content cannot be correlated to 

one specific property, gamma dose. However, this lack of correlation between B-10 content and 

gamma dose may not necessarily extend to other spent fuel pools containing phenolic resinbased neutron absorbers. As described in Section 5.2, the absence of a correlation at 

Palisades may be explained by a lack of predictable release of B-10 from the degraded polymer 

due to a low or inconsistent flow between the rack cavity and the pool. It is possible that 

another pool may have a different rack design where flow is sufficiently strong and consistent 

that the release of B-10 is proportional to polymer degradation and dose. In that case, the 

series of proportional correlations which relate gamma dose to B-10 loss may exist, similar to 

the series of correlations observed for Boraflex. Each spent fuel pool should examine its rack 
 

46 

design in this light to determine whether the B-10 areal density results of an in situ method can 

be extrapolated from a few panels to the entire spent fuel pool. 

BADGER results from the two spent fuel pools tested with phenolic resin-based absorbers 

represent the only information available that directly assesses the performance of phenolic 

resin-based neutron absorbers in-service. The only two BADGER campaigns known to have 

been conducted have shown significant panel degradation at both pools. Although in-situ 

methods are known to have uncertainty in results for individual panels and for the extrapolation 

of results to the pool, it can still provide at least a qualitative measure of B-10 areal density. 

Therefore, these results of significant phenolic resin absorber degradation at both plants tested 

by BADGER, which represent the only information available to directly assess absorber 

performance, are noteworthy and concerning. 

 
 

47 

6. Conclusion 

This report assesses the adequacy of current surveillance methods to monitor the degradation 

of phenolic resin-based neutron absorbers in the spent fuel pool environment. Background 

information describing the absorber materials, qualification testing results, and operating 

experience is compiled in Chapters 2 and 3. Qualification testing showed some degradation in 

mechanical properties and weight loss at doses of 1010 to 1011 rad, but not excessive 

degradation. Meanwhile, operating experience includes two instances of BADGER testing 

showing greater than 50% loss of boron carbide. Interestingly, BADGER testing data from 

Palisades does not appear to show a strong correlation between B-10 areal density and 

absorbed gamma dose, potentially indicating other factors may influence boron carbide loss in 

these materials. 

Radiation chemistry theory provides a mechanistic theory of the radiolysis of phenolic resins in 

the spent fuel pool environment in order to understand the qualification testing results and 

operating experience. Radiation breaks bonds and creates free radicals in the phenolic resin, 

leading to scission of the polymer backbone and degradation of the polymer structure. In the 

presence of water and oxygen, degradation is accelerated by the introduction of more 

aggressive chemical species, including hydroxyl radicals and hydrogen peroxide. A key factor in 

the progression of degradation may be the amount of flow and water exchange that occurs 

between the panel cavity and pool. Greater flow and pool water exchange may increase the 

degradation and dissolution of the phenolic resin by providing fresh water to the panel cavity. 

The Palisades BADGER data that does not appear to show a strong correlation between B-10 

areal density and absorbed gamma dose may be explained by the predominance of flow and 

pool water exchange factors on the process of boron carbide release. Phenolic resin-based 

neutron absorber rack panel cavities were usually designed to be sealed from the pool and were 

only vented to the pool through small holes drilled in the wrapper plate to relieve pressure from 

offgassing after installation. This effect of flow and pool water exchange in phenolic resin-based 

absorber panel cavities, in addition to gamma dose, may be a dominant factor in the boron 

carbide release process. Some boron carbide may be released by dissolution and gravity, but 

without flow and water exchange to spur the erosion and dissolution process, even severely 

degraded phenolic resins that are exposed to high gamma doses above 1010 rads and 

susceptible to erosion and dissolution may not release boron carbide proportionally to gamma 

dose. 

Discrepancies between qualification testing results and operating experience are believed to be 

due to the accelerated nature of qualification testing, which was non-conservative in two ways: a 

much shorter exposure time in water and a much higher dose rate. Operating experience at 

Palisades and understanding of the expected degradation products shows that TOC monitoring 

is ineffective. It is difficult to assess the adequacy of coupons given uncertainties in the exact 

nature of the panel cavity environment and the boron carbide release process. Given adequate 

considerations for uncertainties (Chapman and Scaglione, 2012), BADGER may be similarly 

effective on phenolic resin-based absorbers as on Boraflex. However, uncertainties associated 

with extrapolating boron carbide loss results from measurement of a limited number of panels to 
 

48 

all the panels in a spent fuel pool are significant when the degradation mechanism is not well 

understood. 
 

49 

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50 

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