ML20207L619
| ML20207L619 | |
| Person / Time | |
|---|---|
| Site: | Millstone  |
| Issue date: | 10/11/1988 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20207L616 | List: |
| References | |
| TAC-68212, NUDOCS 8810170367 | |
| Download: ML20207L619 (6) | |
Text
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,! N,Tp,0D_UCT DN By aoplicatien for license atendrent dated April 29, 1998, as supplemented June 24, 1988, Ncrtheast Nuclear Energy Company, et al. (the licensee),
requested a change to Technical Specifications 051 for Millstone Unit 2 that would allow a decrease in the concentration of boric acid in the Boric Acid Storage Tanks. Accordingly, the heat tracing for piping which contains a concentrated beric acid solutien would no longer be required to be raintained in an operable cerdition. The June.*4,1.988 sube.ittal provided additional inferration which reither alters the action nor the initipi detertninatten published in the Federal Fegister.
2.0 DISCUSSION billstone Unit 2 utilizes a concentrated beric acid system to control reactor core reactivity, independently of the centrol rods. A concentrated boric acid solution (approximately 6,25 weight percent boric acid) is stored in two 870 ft* boric acid storage tanks (BASTS).
Each EAST is equirred with a purrp, and an alternate gravity feed system, to supply concentrated beric acid to the charging purps and then into the reactor via the charging system.
Since the solubility'of boric acid is tenperature sensitise, all piping that contains concentrated boric acid rust be heated, via "beat tracing," to prevent the teftperature of the pipe from decreasing to a point where boric acid precipitation anei subsequent flew blockage would result.
The concentrated boric acid system is r,ot credited for reactivity control during any analyzed acci& rt.
Conuentrated boric acid is required, however, to traintain shutdown rargin.
Shutcown trargin is the degree to which the reactor is subtritical with all control rods inserted except the ecst reactive control rod which is assured to be fully withdrawn.
The beric acid heat tracirg at l'illstore Unit 2 has been found to be trouble-scrre and the need to keep it cerable has contributed to the radiation exposure to operating personnel. Accordingly, the licensee has proposed an operaticral system which wculd allcw a reduction in the boric acid concentra.
tion in the BASTS to the extent that precipitation would not occur under expected ambient cenditions (above 55'F). L'nder these conditions, the heat tracing en the concentrated boric acid piping would no longer be needed, i
8010170367 001011 i
LDR ADOCK 0%336 7
2 Borated water from the refueling water storage tank (RW37) would su,nolement the contents of the BASTS in order to maintain the required shutdown n rvr when cooling down the plant from post-power production to refueling cr.rc a.as (RCS temperature is less than or equal to 140'F).
During pitat cooldown, positive reactivity is introduced as a result of a negative isothermal temperature coefficient and the cecay of Xenon which was produced during power operation.
The only safety function credited to the boric acid in the BASTS is the above described raintenance of post-operation shutdown margin.
l 3.0 EVALUATION The licensee's approach to establishing the BAST and RWST requirements for post-power operation cooldewn was the following:
Required boron concentration was calculated for a cooldewn from 5'i7'F to 200'F while maintaining a shutdcwn margin of 2.9% delta K/K and a cooldown from 200'F to 130'F while maintaining a shutdown margin of 2.01 a
delta K/K.
For the cooldown from 557'F to 200'F, it was assured that BAST boric acid solution was added to corpensate for normal prinary coolant inventory shrinkage. No letdswn path was assun.ed.
Various BAST volumes, boric acid concentrations and RWST boron concentrations were assuned.
The resulting primary system boron concent' ration was shewn to be greater than
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required to raintain the 2.9% delta K/K shutdown margin for the assured
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range of BAST volumes, BAST boric acid concentrations, and RhST boron F
conuntrations.
l For the cooldown from 200'F to 130'F, two operating codes were considered, I
as follows:
(1) Prirary system makeup from the BAST, at 2.5 wtt boric j
acio, was undertaken.o cor.pensate for normal prinary coolant inventory j
shrinkage (assuring no letdenn path) and (2) Primary system "feed and j
blet.d" was undertaken using the RWST with a boron concentration of 1720 ppm.
In both cases, the resulting primary system boron concentration was 1
in excess of that calculated to be required to maintain a shutdown margin l
of 2.Ci delta K/r.
i Based upon the above, BAST volumes, BAST boric acid concentrations and RWST i
boron concentration: were established for the prirary system ccoldown fron I
557'F to 130'F.
j 3.1 QlculationofRequiredForonConcentration i
The cooldown of the primary cysten from 557'F to 200'F results in an increase in reactivity as a result of a negative isothernal teciperature coefficient and 1
the decay of xenon -135.
During the initial cooldown from 200'F to 150'F, it i
is assumed that xenon -135 decay is complete and thus, the positive reactivity i
i insertion is due solcly to the negative isothermal terperatiare coefficient.
In calculating the primary system boron conceritration necessary to maintain i
the specified shutdown margin, the licensee selected conservat.ive core physics L
]
and plant operating a sumptions so as to maximize the derand for boron.
These i
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. assumptions are described in Section 2 of the letter dated June 24, 1988. The physics and operacing assumptions were used as input to the Combustion Engineering code ROCS for the follcwing calculations:
Xenon worth Moderator deficit Inverse boron worth Pased upon the above, we conclude that the use of an approved computer code with appropriately conservative input provides a conservative estimate of the boren concentration required to maintain a shutdown margin of 2.9% delta K/K from 557'F to 200*F and 2.0% delta K/K from 200'F to 130'F.
3.2 Ca l c u l a t i,o n_ p,f, A c tua l fj n,i mum), Rp rp n,,Cp nc e n,t r,a t i p n The actual boron concentratfor 'or a primary system cooldown from 557*F to 200'F was determiried by calculating the reactor coolbnt system shrinkage with decreasing temperature and providing make-up free a boric acid source. The initial nake-up source is the BAST which is replaced by the RWST (switchover) when the initially assumed BAST inventory is depleted.
The assumption for calculation of actual boron concentrations were varied as follows:
BAST Concentration - Boric acid ccncentration was varied from 2.5 wt% to 3.5 wt%.
RWST Concentration - Boron concentration was varied from 1720 ppm to 2300 ppm.
BAST volume - The initial BAST volume was varied frcm 2155 gallons to 7899 gallons.
The above variations produced a total of 35 cooldown scenarios, each of which resulted in a temperature-dependent primary system boron concentration in excess of the calculated values (required to maintain a 2.9% delta K/K shutdown margin) as described in Section 3.1, herein.
The actual boron concentration for the primary system cooldown frcn 200"F to 130'F was also calculated by determining primary system coolant shrinkage.
Two cooldown scenarios were presented:
Boron addition from BAST only - With a BAST boric acid concentration of 2.5 wt%, a BAST volume of 3215 gallens was required for the cooldown.
Boron addition from RWST only - With the RWST at 1720 ppm boron, charging from the RWST with letdcwn flow (feed and bleed) required an initial 1600 gallons of RWST volume followed by an additional 3210 gallons of RWST volbre to account for primary system ccolant l
I l shrinkage from 200 F to 130*F. Thus, at an RWST boron concentration of 1720 ppm, a total of 4810 gallons of RWST would be required for the cooldown from 200*F to 130'F.
The required RWST volume is a small fraction of the RWST capacity which is approximately 475,000 gallons.
For each of the above cooldown scenarios, the resultant temperature-dependent boron concentration was greater than the calculated values (required to maintain a shutdown margin of 2.0% delta K/K) as described in Section 3.1, herein.
Based upon the above, we conclude the calculated actual (minimum) boron concentrations provide a basis for setting BAST and RWST requirements that will maintain the required shutdown margins when cooling down the primary t
system from 557'F to 130"F.
4.0 Te c h n i c a l,,Sp,e,c i,f,1,c,a,t,t o,n s NNECO has proposed changes to the TS that reflect the use of reduced concentra-tions of boric acid in the BAST.
4.1 TS 3/4.1,.2,1,,,",B,0,ra,t, ion,, Systems Flow Paths - Shutdown" The requirement for operability of heat tracing would be deleted from the Limiting Condition for Operation (LCO).
The seven-day surveillance to verify that heat traced portions of the boric acid system are at an accepteble temperature would be replaced with a 24-hour surveillance to ensure that each BAST and piping contents are above 55'F.
Section 2.1 of the licensee's June 24, 1988 submittal indicates that the solubility of boric acid in water varies from 2.52 wt% at 32*F to 3.49 wt% at 50'F.
Since the maximum BAST boric acid concentration assumed in the analysis (Section 3.2, herein) was 3.5 wt% boric acid, the assurance that ambient temper,eture is at least 55'F provides confidence that boric acid precipitation will not occur. Accordingly, the concentrated boric acid system heat tracing is to longer needed. The reference locations for determination of ambient temperature, elevations (-)5'-0" and (-)25'-6", represent locations within the l'illstone Unit 2 Auxiliary Building where concentrated boric acid system piping is located.
Based upon the above, the proposed change to TS 3/4.1.2.1 is acceptable.
4.2
,T S,,3/ a,,1,.,2,.,2,,,,",B o,r a,t,1,o,n,,5y,s,t,e,m,s, F,1 ow P a,t,h,s,,,,0pe ra t i n g "
a Existing Specification 3.1.2.2.1 (Modes 1 and 2) and 3.1.2.2.2 (Modes 3 and 4) and their associated surveillance requirements would be incorporated into one new TS. This change would also be reflected in TS 3.1.2.6, "Boric Acid Pumps-Operating" which references TS 3.1.2.2.a.
The LCO for this TS would require that sufficient boric acid flow paths, at a tenperature of at least 55'F, are available to perform boration of the reactor coolant systems (RCS) fromtheBAST(s)and he refueling water storage tank (RWST).
Redundant flow paths from the BAST to the charging pump suction would be assured by the requirement to have an operable boric acid purrp and gravity feed valve available for each tank which is used as a source.
The flow path from
d 1 the RWST to the charging pumps is required to be operable in Modes 1, 2, 3 and 4.
The surveillarce requirement to demonstrate operability of the heat tracing would be deleted. A surveillane.e requirement to verify at least once per 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> that the temperature of the BASTS and piping is above 55"F would be added. The remaining Surveillance Requirements, which add ess the exercising of valves and confinnation of correct valve position, would remain ur. changed.
The LCO for proposed TS 3/4.1.2.2 requires a combination of BAST and RWST availability to meet the cooldown requirements from 557 F to 200*F as described in Section 3.2, herein. Although the required BAST volume could be shared between the two BAST tanks, the availability of a single BAST tank is sufficient. The acceptability of the 55'F flow path temperature and the associated Surveillarce Requirement is addressed in Section 4.1, herein.
Based upon the above, proposed TS 3/4.1.2.2 and 3.1.2.6 are acceptable.
4.3 T S 3 / 4.,1,.,2,, 7,,
",B o ra,t,e d,,Wa,t e,r,,5 o u r,cp,s,,,,S,h,u t,d,o,w n,"
The proposed change to TS 3.1.2.7a would delete the reference to Figure 3.1-1, "Minimum Boric Acid Fakeup Tank Volume and Temperature as a Function of Stored Boric Acid Concentration." The proposed TS 3.1.2.7a would require the fcilowing if the BAST is to be used as a source of boron for cooldown from 200*F to 130*F:
A concentration between 2.5 and 3.5 weight percent boron A minimum volume of 3750 gallons, and A minimum boric acid storage tank temperature of 55'F.
These requirements are conservative with regard to the assumptions used for the cooljown scenario where the boron is supplied by the BAST (see Section 3.2, herein.)
In TS 3/4.1.2.7b, the volume of borated water in the RWST would be increased from 57,000 gallons to 47,300 gallons. As described in Section 2.3.3 of the licensee's June 24, 1988 submittal, 4810 gallons of borated water are required, if supplied only from the RWST, for a primary system cooldown from 200"F to 130'F.
The 4810 gallons value would be increased to 5350 gallons, for conservatism, and added to the RWST unusable volume of 51,950 gallons.
The resultant value of 53,700 gallons (of which only 5350 gallons is actually usable) is the basis for TS 3.1.P.7b.
In addition to the above, a propnsed change to TS 4.1.2.7 would require that, when the BAST (s) are the source of borated water, every 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> the BAST temperature is verified to be greater than 55'F.
The need to verify that the concentrated boric acid system is kept at a temperature greater than 55*F has been previously addressed.
Based upon the above, the proposed changes to TS 3/4.1.E.7 are acceptable.
4.4 iS 3 / 4.1,.,2,.,8,,,",B,0,ra,t,e d,,Wa,t,e,r,,5 o u r,c,e,s,,, Opp,r,a,t,1,ng,"
A proposed change to TS 3.1.?.8a would establish the minimum level of operability for the BASTS and associated gravity feed valves and pumps. The basis for the equipment operability is the initial phase of the prinary
. system cooldown from 557'F to 200"F when the BASTS are the source o" borated j
water (see Section 3.2, herein.) As with proposed TS 3/4.1.2.2, the required j
borated water, at a temperature greater than 55'F, can be shared between the BASTS. A single operable PAST, however, with appropriate volume and concentration of boric acid, is sufficient for the initial cooldown phase. An additional proposed surveillance TS 4.1.2.8d, would require verification that the BAST temperatures are greater than 55'F.
The need to keep the BASTS at a temperature greater than 55*F has been previcusly addressed.
Based upon the at'ove, the proposed changes to TS 3/4.1.2.8 are acceptable.
4.5 TS Figure _3.,1,-1,,"Pinimum BAST Volume vs. Stored BAST Concentration,(gy)"
Proposed TS Figure 3.1-1 shows minimum BAST volumes as a function of BAST boric acid concentraticn for various RWST boron concentrations. The data j
presented in TS 3.1-1 was generated for the primary system ccoldown from 557'F l'
to 200*F as described in Section 3.2, herein.
Based upon the above, the proposed TS Figure 3.1-1 is acceptable.
l 5.0 ENVIRONFENTAL CONSIDERATION This amendment changes a requirement with respect to the installation or use j
of a facility component located within the restricted area as defined in 10 CFR Parc 20 and chances surveillance requirements. The staff has determined that the aaendvent involves no significant increase in the arounts, and nc significant change in the types, of any effluents that may be released offsite.
l and that there is no significant increase in individual or cumulative occupational radiatico exposure.
The Ccmissicn has previously published a proposed finding that the amendment involves no significant hazards censideraticn and there has l
been no public ccnnent on such finding. Accordingly, the amendment meets the eligibility criteria for categorical exclusic.n set forth in 10 CFR 651.22(c)(9).
Pursuant to 10 CFR 551.22(b), no environmental impact statement or environnental i
assessment need be prepared in connection with the issuance of the amendment.
l
6.0 CONCLUSION
We have concluded, based on the considerations discussed above, that (1),there is reasonable assurance that the health and safety of the public will not be endangered by operation in the proposed manner, and (2) such activities will be ccnducted in compliance with the Conmission's regulations, and the issuance of the amendnent will oct be inimical to the comon defense ard security or to the health and safety of the public.
Dated:
October 11, 1988 Principal Contributor:
D. H. Jaffe
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