ML20154B091

Topical Rept Evaluation of TR-UR-85-225, ASEA-ATOM Control Rod Blades for Us Bwrs. Rept Acceptable for Ref in License Applications

ML20154B091
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Issue date:	02/20/1986
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ML20154B074	List: ML20154B070 ML20154B091
References
TAC-60093, NUDOCS 8603040224
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ENCLOSURE EVALUATION OF ASEA-ATOM TOPICAL REPORT ON CONTROL ROD BLADES (TACS 60093)

By letter dated September 24, 1985, ASEA-ATOM (A-A) submitted for staff review Licensing Topical Report (LTR) TR-UR-85-225 entitled " ASEA-ATOM Control Blades for U.S. BWRs". This report was written in support of A-A's desire to obtain generic approval for the installation of the ASEA-ATOM blades in U.S. reactors.

We have reviewed the LTR and prepared the following evaluation.

1. Background By letter dated July 18, 1983, Comonwealth Edison made application to install eight ASEA-ATOM control rods in the Dresden 3 reactor. In support of.that application they submitted ASEA-ATOM Report TR-BR-82-98, Rev. 1. That report described two different types of ASEA-ATOM control blades both of which were to be inserted into the Dresden 3 reactor. The staff prepared an evaluation (Reference 1) of these blades which concluded that they were acceptable for use in Dresden 3. The evaluation addressed materials compatibility, nuclear design, rod maneuvering, and blade surveillance. A copy of the relevant portion of that evaluation is attached as Appendix A to the present evaluation.

2. Description of Report Topical Report TR-UR-85-225 is an update of the report submitted with the Dresden application. Two additional blade designs are described. Certain additional analyses which have been performed are also discussed. The four blade types which are described in the report are:

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a. A blade which contains only B4 C as the absorber material. (Type 1)

b. A blade in which a short section at the top has hafnium as the absorber rather than B4C. (Type 2) l 8603040224 860220 PDR TOPRPEMVAy i C l 1

c. A blade like Type 2 except that the amount of B4C is reduced. (Type 3)

d. A blade with the B4C content further reduced. (Type 4)

Blade Types 1 and 2 were described in the Dresden submittal. Blade Types 3 and 4 are provided to permit the user to specify blades which have nearly the same worth (to within a few percent) as the currently used B C blades. Blade 4

Type 1 is no longer offered in the U.S.

The mechanical design of the blades is described in Set.cion 2 of the report along with analyses of blade stresses, operating temperature and stiffness.

Design limits are specified and expected values are calculated. Section 4 (there is no Section 3) of the report discusses the nuclear characteristics of the blade designs. The comparisons of the reactivity characteristics between the standard blade and the various A-A blades are presented for cold, xenon-free and hot, voided and unvoided conditions. The methods used are described. A difference from the analysis of the Dresden 3 rods is the use of updated hafnium cross-sections which result in a reduction in the reactivity

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worth of the hafnium portion of the blades.' Dependence of the comparisons on burnup and xenon concentration was analyzed and is presented. The effect of the blades on the detailed assembly power distribution is presented along with the discussion of absorber depletion rates in the blades.

Section 5 summarizes the experience with A-A blades in European reactors, Section 6 discusses blade life limitations including reduction in reactivity worth, buildup of gas pressure, boron carbide swelling and radiation embrittlement. Section 7 discusses operation aspects and Section 8 presents the safety evaluation.

3. Summary of Evaluation The emphasis in our evaluati'on is on those aspects of the report which are i different from the Dresden 3 submittal. These include the additional blade l types (3 and 4), the changed hafnium cross sections and the potential use of a full complement of blades in the core.

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3.1 Mechanical Design The mechanical design of the blades is essentially the same as those inserted

into Dresden 3 and appetved in Appendix A to this evaluation. The design-variations required for blade Types 3 and 4 do not alter that approval since the changes in the design are to the depth; of the absorber containing holes in the blades. This does not affect blade mechanical properties.

The design analyses show that the specified limits on stress and absorber I

temperatures are met and that the increased stiffness of the A-A blades over the standard ones is not significant with respect to blade behavior. We find

'he mechanical design of the A-A blades to be acceptable.

3.2 Nu: ear Characteristics Compared to the standard BWR control blade the A-A blade has the following

, differences which affect neutronic behavior.

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1. The A-A blades are lighter in weight which tends to increase scram speed. This is a positive contribution to safety. The increase in scram speed is somewhat mitigated by the presence of friction pads rather than rollers on the blades but on balance the scram speed is increased.

2. The reactivity worth of the A-A blades is greater than that of the standard blade by an amount which is dependent on the blade type.

Blade Type 1 is worth approximately 7 to 9 percent more than the standard blade. The worth of the Type 2 blade is about the same amount greater than the standard blade (the hafnium portion contributes little to the total blade worth). The Type 3 blade is worth only about 3-4 percent more than the stand.srd blade while the Type 4 blade is worth only 1-3 percent more. These differences ,

result in shutdown margin increases of about 1 percent for blade Types 1 and 2, 0.4 percent for Type 3 and 0.2 percent for Type 4.

At full power conditions the absolute reactivity worth increases are about 0.4% for Types 1 and 2 and less than 0.1% for Types 3 and 4.

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3. The presence of the stronger absorber in the A-A blade results in a steeper flux gradient in assemblies surrounding an inserted control blade. This effect is slightly less for Type 3 and 4 blades than for Types I and 2. The conclusions of Appendix A are still valid.

The effect of these differences is either benign (increased scram speed, j increased shutdown margin) or may be easily accounted for in core analyses (increased at-power rod worth and increased flux peaking). We conclude that these changes in nuclear characteristics are acceptable.

Calculations have been performed to show that the differences in nuclear characteristics between the standard and A-A blades are only slightly dependent on core burnup. In addition calculations which used various fuel types (average enrichments) show that the conclusions are independent of fuel type.

3.3 Blade Life Limitations Several potential limits on blade life are discussed. The most obvious is reduction in blad. worth as a result of absorber burnout. The presence of

. higher initial boron loading results in an increase in b' lade life of a factor of 1.6 for the A-A blade when compared to the standard blade if the same

, criterion of a 10 percent reduction of reactivity worth in any 3-foot section of the blade is used. Analysis of other potential limiting effects has accounted for this increased life.

A potential limit on blade life is the buildup of gas pressure in the blade from the release of helium in the (n, ) reaction in boron. Conservative calculations show that the design pressure limit is not reached for the expected control blade lifetime.

l Another mechanism which may be life-limiting is that of boron carbide swelling.

J Observations of A-A blades in operating reactors have shown that the nuclear i life-time limit is reached before any cracking of the blade due to this phenomenon is reached. We conclude that the A-A blades may be safely operated for their effective life-time. )

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4. Regulatory Position

We conclude that ASEA-ATOM control blade designs described in TR-UR-85-225 are acceptable for use in U.S. Boiling Water Reactors when properly accounted for in plant specific safety analyses. This conclusion is based on the following:

1. The A-A blades are mechanically compatible with the existing drives and handling equipment.

2. A small number of blades are currently being operated in Dresden 3 (see Appendix A). These blades are subject to an extensive

surveillance program to assess their performance. Visual examination and dimensionc1 measurements were made after one cycle ( 18 months) exposure and no deterioration was found in the blades. The results of the program will be available in time to detect any serious problems before they develop in other reactors.

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3. The differences in neutronic behavior between the A-A blades are either benign or can be accounted for in safety analyses and operational planning.

4. Analyses have shown that the blades may be safely operated during their reactivity based life-time.

We will, however, require on a case-by-case basis from at least the first few plants that install the A-A blades, that the licensee connit to a confirmatory surveillance program involving visual inspecti,on of control blades during refueling outages for the life of the blades included in the inspection program. Results of the surveillance programs will be applicable to the q continued acceptability of the ASEA-ATOM control blades as a function of exposure life.

This report may be referenced in licensing actions involving the installation and operation of ASEA-ATOM A-A control blades in U.S. Boiling Water Reactors subject to the above conditions.

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REFERENCE

1. Letter D. Crutchfield, NRC, to Dennis L. Farrar. Commonwealth Edison, dated March 9,1984, (Amendment 47 to the Dresden Unit 3 license).

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J APPENDIX A EVALUATION OF USE OF ASE.a-ATOM BLADES 3

IN ORESDEN 3 Y

2.0 EVALUATION OF THE USE OF THE ASEA-ATOM CONTROL BLADES 2.1 GENERAL DESCRIPTION OF BLADE DESIGN AND PRESENT OPERATING SE00ENCE -

' The ASEA-AT0ft (A-A) blades to be installed in Dresden Unit 3-have been .

designed to be mechanically compatible with existing blades. The blade profile is quite close to the standard blade and the velocity limiter and drive coupling portions are identical. The blades may be manipu-f lated with the same handling tools as used on the standard blade. The blade weight is slightly less than the standard blade. The absorber material is vibratory-compacted B C but the blade design pemits sig-nificantly more boron to be place 8 in the blade.

Dresden Unit 3 is currently operating with the Exxon single sequence centrol strategy. This means that the seme control rods remain in the core throuchaut the Cycle (as opposed to periodic. secuence changes in previcus cycles). The A-A rods will be among those renaining fa the core in order to raximize thei. exposure.

2.2 MTERIALS COMPATIBILITY There will be two types of A-A blades used in D3b9. Four of the eig'ht

. blades will have only B C as an absorber material, and four will have f ~

bothBCandhafniummekalasabscrbermaterials.

3 The hafnium will comprile only the top six inches of the absorber sectic.) of these four

' blades. This design provision has been made to allow additional blade lifetime and reduce internal pressure in the blades. The use of hafnium in control blades has previously been approved for GE test blades in Peach Bottom, and is an alternative for the silver-indium-cadmiun (Ag-In-Cd) used in Hestinghouse reactors. The staff is unaware of any ma-terials problems associated with the use of hafnium, and finds this aspect

, of the. design acceptable.

The absorber in the A-A blade design is cuntained in horizontally drilled absorber holes in low-carbon stainless steel sheets. The staff's review of the mechanical design of the blades included a request for additional -

infomation (Ref. 3) related to the potential for blocking of the individual i

slits which interconnect these holes to equalize internal gas pressure in .

each blade wing. The applicant's response (Ref. 4) provides adequate '

assurance that there is no potential mechanism for blocking gas comuni- -

cation between the B4C holes.

In addition, the staff' evaluated additional information furnished by Comnn- .

wealth Edison (Ref. 4) on the conservatism of a 10 percent helium release rate

' (from B C) on blade temperature calculations, maxinum internal gas pressure, mechani, cal strength and strain design reouirements, use of gridpads, and the '

seismic design. Comonwealth Edison (Ref. 4) provided justificatien that

! each of these concerns has been addressed satisfactorily.in the design of the j control blades.

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2.3 N_UCLEAR DESIGN CHARACTERISTICS The nuclear design characterist.ics of the improved A-A control blades ,

has been performed by A-A with the PHOENIX lattice and depletion

• code. While this code has not been reviewed by the staff, a-sufficient .

description of it has been included (Ref 2) to pennit the conclusion

! that it is acceptable for use in performing the comparsions between the neutronic characteristics of the standard and A-A blades that are presented. - -

The code has be'en used to compare reactivity worths at cold xenon-

-- free conditions and hot voided and unvoided conditions as a function of fuel burnup. In addition power distribution effects and absorber depletion effects have been studied. The conclusions of the analyses 2 are discussed below.

The presence of a larger boron inventory in the rods implies a greater i

reactivity worth. The calculations by A-A have shown that the worth of the all B 4C rods is 6 to 9 percent greater than that of the standard i

rods. A control blade containing all hafnium would have about the same

. worth as the standard blade.

An important effect of the increased rod worth is to increase the shutdown maroin. However, the increase in shutdown margin will be small for Dresden

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3 since there are only eight of the stronger control rods and they will be placed in low worth regions of the core. Another effect of the increased -

baron content is a steeper flux gradient in assemblies surrounding an inserted control blade. The maximum difference is in the wide-wide corner

' and is about 5 percent. The difference at the LPRM location is only about O.5 percent. These differences are accounted for in the reload analyses.

The increased blade worth may cause the consequences of a rod withdrawal or rod drop event to be more severe. The effect of the presence of the A-A rods' in Dresden 3 will be addressed for 'each reload containing them. -

The increased baron loadin'g of the blades also provides a ' longer exposure lifetime. A-A calculations show that the improved blade will have a 60 percent greater life if end-of-life is defined as a 10 percent reduction in blade worth. If the lifetime is determined on the basis of equal end-of-life worths, the improved rod would have more than twice the lifetite of the standard rod.

2.4 CONTROL ROD MANEUVERING The A-A control rods are essentially identical in exterior envelope to the standard rods. The all B C g rods are about 12 pounds lighter than current rods and the rods with hafnium tips are about 7 pounds lighter. -

Thus, the insertion speed should be greater for these rods. However, the presence of friction pads rather than rollers and an open central structure (increasing flow resistence).tends to offset the smaller weight.

It is concluded that the insertion (scram) spe'ed will not be significantly '

affected by the inproved rods. The scram speed will be measured as part of the startup testing program.

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- 2.5 PLADE SURVEILLANCE PROGRAM l' By letter dated November 10, 1983 (Ref. 5) the licensee informed ,

the staff that evidence of cracking with some loss of B C 4 had occurred in -

, ' similar rods being used in a Swedish reactor. Based on the proposed .

i positioning of the eight lead A-A rods in the Dresden 3 core, the burn-up of the rods in the Swedish reactor, at the time the cracking was discovered, was greater than that which will occur during two 18 month

. cycles in Dresden 3. However, the lead rod burnup will be greater after i three 18 month cycles than the burnup of the rods in the Swedish reactor.

l " Despite this, the staff has concluded that, because there are differences  ;

between the two sets of rods, concerns relating to their use are alleviated.

In eddition, the licensee has proposed an extensive monitoring program 4 while they are being used at Dresden 3 so that indications of inferior i . performar.ce will be detected promptly. These. factors are.significant '

enough for the staff to conclude that-the Swedish problems would not be expected at Dresden 3. First, the stainless steel in the rods to be -

i used in Dresden 3 has been fabricated with tighter chemistry control than 1 that used in the blades used in the Swedish reactor. Second, nondestructive

• i examinatten of the Dresden 3 A-A rods will be coeducted following each usage cycle. Tests will be performed to check dimensional stability.

. corrosion effects and the integrity cf the B,C containment. A high re-i solution TV camera will be used for visual inspection, a guaging fixture for dimensional stability and a neutron transmission measurement for

!- demonstrating B 4 C presence. After the third 18 months cycle, an extensive P examinatien of one or more rods will be made after their removal from the 4

core.

Based on the above and upon the fact that four of the rods use hafnium I instead of B C in the top six inches making them less susceptible to ,

4 IGSCC from B C swelling, the staff has concluded that there is not a j cracking-rel ted safety concern from use of A-A rods in Dresden 3.

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2.6

SUMMARY

1 On the basis of its review the staff has concluded that the use of the A-A

. improved control blades in Dresden 3 is acceptable. This conclusion

is based on the following considerations

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. 1. The improved blades are mechanically and hydraulically'conpatible i j .

with the present control blades. l

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2. Only eight of the rods will he installed in the reactor.

! 3. The nuclear characteristics of the blades have been detennined by

acceptable methods.

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4.. The presence of the blades will be taken into account in the design l end analysis of core reloads.

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5. Sufficient experience has been had with the red design in other i

(Swedish and Finnish) BWRs to pemit the conclusion that they will operate without significant-deterioration. '

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6. A satisfactdry surveillance program has been established-to monitor .

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