ML19275A851
| ML19275A851 | |
| Person / Time | |
|---|---|
| Site: | Crane  |
| Issue date: | 10/16/1979 |
| From: | Herbein J METROPOLITAN EDISON CO. |
| To: | Vollmer R NRC - TMI-2 OPERATIONS/SUPPORT TASK FORCE |
| References | |
| GQL-1234, NUDOCS 7910190298 | |
| Download: ML19275A851 (14) | |
Text
.
Metropolitan Edison Company Q
[.
ff {
Post ^ffice Box 542 w..
Reading Pennsylvania 19640 215 929-3601 Writer's Direct Dial Number October 16, 1979 GQL 1234 D!I-2 Support Attn:
R. Voll.ner, Direc kr U. S. Nuclear Regulatory Commission Washington, D.C.
20555
Dear Sir:
Three Mile Island Nuclear Station, Unit 2 (TMI-2)
License No. DPR-73 Docket No. 50-320 RCS Corrosim/ Chemistry Program In response to your letter of September 17, 1979 concerning primary system chemistry and corrosion program the following information is provided:
"he primary item of concern related tt, Reactor Coolant System (RCS) Chemistry /
Corrosion control is chloride stress corrosion of austenitic stainless steel.
This concern is centered around the potential for rapid corrosion cracking if the variables for chloride stress corrosion are allowed to reach applicable values. The variables of concern are: time, temperature, stress, chlorides, or/ gen and pH.
The temperature at which chloride stress corrosion occurs is not well defined.
The plant is presently at an average core temperature of about 17COF.
At this temperature, the possibility of chloride stress corrosica may still be of concern. When the Mini-Decay Heat Removal system is placed into operation primary temperature vill be reduced to less than 1h00F to further reduce the possibility of chloride stress corrosion.
Chlor _ des in the RCS are higher than desirable (3-5 ppm recently). They have not been controlled because of the lack of purification capability and the undesirability of feed and bleeding due to the large quantities of radioactive water which would result.
Chlorides are controlled to the extent possible by limiting the cencentration in the RCS makeup water to 1.0 ppm.
RCS dissolved orygen is controlled by maintaining a hydrogen inventory of 5-15 cc/kg the limit for oxygen in the RCS is.1 ppm.
Oxygen is maintained at less than this value to minimi::e the possibility of chloride stress corrosion and general corrosion. Difficulties have recently been experienced in maintaining continuous operation of the makeup water vacuum degassifier, due to leakage of solenoid valves in this system. As a result of interruption of makeup deaeration, approximately 2600 gallon of h.5 ppm oxygen bearing makeup have been added to the RCS over the last three weeks.
Corrections of these difficulties are being g
pursued presently with high priority.
7910100298 1L81 2c,3 Metrccobian Ed son Company is a Member of the General P chc Ut at.es System u
R. 7ollmer, Director
-2_
RCS pH is maintained > T.5 which provides for a margin of.5 pH units above the pH of 7.0 referred to in Section 6.1.1 of the NRC's standard review plen concerning pH for post LOCA core cooling vater.
Further information on plant conditions, chemistry and corrosion control is contained in the attached BW TMI-2 vater chemistry recommendations and requirements.
Sincerely, h
J. G. Herbein Vice President-Nuclear Operations JGH:LWH:tas Enclosures 1185 264
TMl-2 !!AiLR CHU'ISTRY l.
GEf1E RAL This document proviocc information on BMl's recernendations *and requirements
~
for the water chemistry at TMI-2, with the following conditions:
a) t iition A -
Reactor coolant system in natural circulation at pressures in the range of 100-300 psig and temperatures of approximately 100-180 F with one steam generator operating under a vacuum or in a solid condition to remove decay heat and letdown and makeup via the makeup and purification system.
b) Condition 8 -
Reactor coolant system at pressures of approximately 70-100 psig and temperatures of approximately 100-130 F with the "ltini"-decay heat cooling system operating to remove decay heat, the steam generators isolated and the standby pressure control system operating for RCS pressure control and fresh RC makeup addit:ons.
2.
REACTOR C00LAMT SYSTEM (RCS) 2.1 General The RCS water chemistry specifications for both Conditiors A and B are as follows:
pH at 77F 27.5 Boron 300;-4203 ppm liydrogen 5-15 std cc/kg Chloride See Section 2.4 Fluoride See Se-tion 2.5 0xygen See Sec tion 2.6 Sodium See Section 2.2 2.2 pH and Sodium or Sodiu.a Hydroxice The pH must be maintained at 27.5 in order to minimize the possibility of chloride stress corrosion of austenitic s' iless steel.
1-l nile a specific upper limit is not listed, an effort should be made to keep the i185 265
p!! balca 9.5.
It is also recor.nended that the pH should actually be maintained at >8.0 in order to assure that the pH is >7.5 at all times.
The pH is adjusted by the use of sodium hydroxide additions to the RCS.
The relationship bet' cen pH and caustic concentrations for various boric acid concentrations are shown in Figure 1, while the graph is for solu-tions.lso containing I w/o sodium thiosulfate, the presence of the sod-ium thiosulfate does not have any significant effect on the pH.
The pH limit of >7.5 at 77 F is also a technical specification limit.
2.3 Boron and Boric Acid The boron range of 3000-4000 ppm is equivalent to a range of 17,000-22,800 ppm as boric acid and this range is specified to maintain a sub-critical condition for the postulated fuel configurations in the vessel region.
Actually, however, B&W recommends that the actual boron or boric acid concentration be maintained on the high side of the specification range to provide a further assurance of a subcritical condition at all times with a slow mixing rate in the RCS.
The technical specification for boron is 3000-4500 ppm with the upper limit based on the solubility limit of boric acid at 32 F.
2.4 liydrogen, Oxvcen and Hvdrazine A dissolved hydrogan range of 5-15 std cc H /kg w ter is specified 2
to help control oxygen that may result from the radiolysis of the coolant or that may come in with the fresh rakeup.
A hydrogen con-centraction semawhat in excess of 15 std cc/kg water does not create any problems.
Up to now, the hdyrogen has been "self-sustaini,,g" since the March 28 incident, i.e., it has not been necessary to make any adjust-ments to maintain a dissolved hydrogen residual of 5 std cc/kg.
In the event that hydrogen co'ntrol becomes a problem, hydrazine additions should be made to indirectly increase the hydrogen o ncentration for the tenper-tures in the RCS at Condition A and Condition B, the reaction rate between hydrazine and oxygen is slow, but the reaction rate is more rapid ~in 1185 266
.radia6 ion field which makes the hydrazine an effective RCS cxygen scavenger ?ven though the radiation field causes the hydrazine to decompose.
Hydrogen is a hydrazine decomposition product in a radiation field with the amount generated being dependent on t.hether the solution is aerated or deaerated as shown below:
(deaeratedsolution) 2ft H
)2NH 2+H2 24 3
fl H
)II+2H
( earated solution) 24 2
2 For every pound of pure hydrazize ad'ded' to the RCS - the first reaction
~
foms about 0.6 cc H /kg water in the RCS whereas the second reaction 2
forms about 2.3 std cc H /kg water in the RCS.
2 Oxygen is undesirable in the RCS because it prcmotes the general corrosion of RCS materials, but the real concern is that oxygen is a factor in the chloride stress. corrosion of austenitic stainless steels.
It's desirable to limit the oxygen to 0.1 ppm max for this reason.
2.5 Chlorides As stated in Section 1.2.2, chlorides can cause stress corrosion cracking of austenitic stainless steel and is dependent on temperature, stresses, time, chlorides, oxygen, and to some extent, pH.
The temperature ranges for Condition A and Condition B are in a borderline area where the possibility of chlo -ide stress corrosion is still of concern and cannot be cc:.pletely ignored.
Therefore, the chlorides should be controlled to the extent possible.
The chlorides are controlled by limiting the chlorides in the makeup water to the RCS ts 1.0 ppm cax.
2.6 Fluorides As with chlorides, fluorides can cause stress corrosion of austenitic stainless steels and should be contro11e. by limiting the fluorides in the makeup water to the RCS to 1.0 ppm max.
3.
RCS SUPPORTING SYSTEMS - CC"DITIO 1A 1l hen the RCS is operating at Condition A as defined in Section 1, the supporting systems can vary particularly from the standpoint of the source of the fresh nakeup water (i.e. RC bleed holdup tanks, BiM, etc.).
Hence, ra ther than specify the chemistry for all of these various sources, only the general 1185 267
requirements for the fresh RCS makeup are covered here, and these requirec.ents are identical to those listed in Section 4.3.2 for the charging water storage tank.
4.
RCS SUPPORTII;G SYSTEMS - CO::DITIO:1 B 4.1 Ceneral This section covers the water chemistry for Condition B when the Mini-Decay Heat Removal System and the Standby RCS Pressure Control System are operating.
4.2 Mini Decay Heat Removal System When operating, the mini decay heat removal system circulates water directly through the reactor core and thus the system has the same water chemistry as that for the RC System as discussed in Section 2.
4.3 Standby RCS Pressure _ Control System The standby RCS pressure control system provides a backup for maintaining the RCS pressure when the need arises.
It consists of three surge tanks, a charging water storage tank, a borated water batching tank and associated pumps, valves, piping, and instrumentation.
The three surge tanks are connected in series with two tanks completely filled with water and the third tank is naintained under a nitrogen pressure.
The charging water storage tank is in effect a makeup tank for the RCS and is the point where chemicals are added to the RCS.
4.3.1 Borated Mater Batching Tank The borated water batching tank is used to makeup the boric acid -
sodium hydroxide solution to maintain the specified boron and pH levels in the RCS via the standby RCS pressure control system.
When a batch of solution is made up in the tank, the resultant water naistcy is:
Boric Acid 4 wt% nominal Sodium Hydroxide 0.5 wt ; nominal Demineralized water shall be used in caking up the solution. 1185 268
4.3.1.1 Boric Acid The 4 wt% boric acid is approximately equivalent to 7000 ppm boron which when diluted in the charging tank by a factor of two will result in a boroa conccistration of about 3500 ppm which is midway between the 3000-4000 ppm specification for the charging tank and the RCS.
The boric acid used to makeup the batch tank shall meet the follc aing specifications *:
Maximum Constituent Minimum "
ppm
' Boric Acid (H 80
Fluoride (F) 0.0004 4
Sulfate (S0 )
0.003 30 4
Phosphate (PO )
0.003 30 4
Iron (Fe) 0.0008 8
Heavy Metals (as Pb) 0.0002 Uater Insoluble 0.005 50
- United States Borax & Chemical Corporation - Special-Quality Grade Coric Acid and Stauffer Chemical Ccmpany -
fluclear Grade Boric Acid meet these specifications.
4.3.1.2 Sodium Hydr _ oxide As shown in Figure 1
, a solution containing 4 wt", :a0H when diluted by a factor of two (i.e. to 20,000 ppm boric acid and 2500 ppm (0.0625 colar) sodium hydroxide results in a pH of slightly greater than 8.0 at 77F which follows the goal stated in Section 2.2 of maintaining a nominal RCS pH of <8.0 at 77F.
The 2500 ppm l:a0H concentration is equivalent to about 1400 ppm sodiuq.
1185 269
-s-
The sodium hydroxide is expected to react with the boric acid and form tetraborate by the reaction:
277+7HO 2Na0li + 4l10 U8 B0 2
3 3 Thus, the 4 wt/: boric acid and 0.5 wt/5 NaCH mixed in a solution, about 12,500 ppm sodium tetraborate are formed with an exress of about 24,500 ppm boric acid.
Sodium hydroxide added to the borated water batch tanks should be equivalent to rayon grade or equivalent if it is purchased as solution or should be regent grade if it is purchased as pellet or flake.
4.3.2 Charging Mater Storage Tank The charging water storage tanks is the primary sour:e of fresh makeup for the RC system and thus is one of the main control points for controlling the RCS water chemistry.
The specifications for this tank are as follows:
Boron (See Section 4.3.2.1) pit at 77F (See Section 4.3.2.2) 0xygen 0.1 ppm Max.
Chlorides 1.0 ppm Max.
Fluorides 1.0 ppd 1 Max.
itydrazine (See Section 4.3.2.3)
Any water added directly to this tank, shall be supplied from a demineralized water source.
4.3.2.1 Scron The boron concentration shall be that required to maintain the RCS boron concentration in the specified range of 3000-4000 pga.
Therefore, it is prudent to maintain the boron concentration in the charging water storage tank at 3000-4000 ppm.
Of course, it may be necessary to exceed this range at times to make proper adjustments to the RCS baron levels.
. 1185 270
4.3.2.2 pH As with the bcron, the pH shall be that rcquired to maintain the RCS pH at E7.5 and preferably at 1.8.0.
The pH is controlled with sodium hydroxide.
0.ygen and Pydrazine 4. 3. 3 7
Control of oxygen in the RCS makeup water source is of primary concern because it nakes it easier to control the oxygen in the RCS.
The recorrended oxygen specifi-cation for the makeup and consequently the charging tank water is 0.1 ppm max.
A hydrazine residual of 3005 of the stoichiometric requira. ants to react with the oxygen should be maintained in the charging water, i.e., if the oxygen level is 0.1 ppm, thus the hydrazine level should be maintained at 0.3 ppm or higher.
As statec: in Section 2.4, hydrazine is added when necessary to maintain the hydrogen levels in the RCS.
In this case, it will probably be necessary to increase the hydrazine levels in the pressure control system.
The oxygen in the charging tank water is reduced by main-taining water in a heated condition at 180F and spraying the water into the tank.
4.3.3 Surge Tanks TF 1 water chemistry for the surge tanks is the same as that for the charging water storage tanks.
The only other consideration is that the nitrogen used to pressurize these tanks should be low in oxygen content in order to limit the absorpticn of oxygen in to the water.
The recommended specifications for oxygen in the nitrogen supplied to these tanks is 0.05 vol. 5 "ax.
5.
STEAl1 GE.';ERATOR FEED'..%TER 5.1 Gene ra_1_
The steam generator #cedwater chemistry depends on whether (1) the steam generator is operating or stea iing under a vacuum maintained with a condenser.,2) the steam generator is operating in full solid condition with the feedwater system also operating in solid closed loop condition, i~ ~
l185 271
% '* Hem
,e.e mm m e m m.
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- 3) feed. tater system is in isolated layup condition '..-hen the liini-Decay Heat P.emoval System is operating to remove decay heat from the RCS.
5.2 Feedwater With Steam Generator Steamina Under Vacoun or with Steam
[e}isto[05ritlMn311 S[oll]d ConditEn((CondTtion A)
~-
The specifications for these two operating modes are as follows:
pH at 77F 9.3 - 10.5 Dissolved Oxygen 100 ppb fax.
Cation Conductivity lumho/cm !!ax.
Hydra zi.ie 0.1 ppm tiin.
In the case of operating a OTSG in a solid condition, the quality of the feedwater will be low for a period of time after this mode of operation is initiated.
The reason is that flooding of the steam generator will result in the dissolution of any tube deposits into the water and subsequent carryover into the externel closed cooling loop.
With a demineralizer operating to cleanup the external loop and feedwater, the feedwater and s' team generator water should reach a condition where the feedwater and steam generator water have the same quality and consequently the same specifications.
5.2.1 pH_
The pH nust be maintained above 9.3 (9.3-10.5) to prevent low temperature corrosion of the steam generator materials should sulfur compounds and/or oxygen be present.
Sulfur compounds in an acid environment could cause tube damage.
If oxygen is present, a similar result is possible in addition to the general corrosion of the carbon steel surfaces.
The pH is adjusted by the use of aronia.
A pH of 9.3 at 77F is equivalent to an acronia concentration of 1.5 ppm.
5.2.2 OxySen_
Oxygen is generally an accelerator for corrosion reactions involvirc carbon steel and inconel surfaces.
Since it may be difficult to determine the true extent of chemical contanination of the OTSG's, oxygen control is necessary because the potential for causing local detrinental enviroments by contamination - oxygen interaction. is high. 1185 272
5.2.3 Hydra zine Hydrazine should be added to prcvide a reducing environment and retard of the oxidation of carbon steel.
At the tenperatures of the feedeater and the water in the OTSGs, little if any oxygen reacticn should occur.
However, hydrazine is beneficial in maintaining iron oxide films in a protective reduced form.
5.2.4 Cation Conductiviiv, Cation conductivity is a general measure of the water purity when cationic control additives are present (e.g., ammonia and hydrazine) and thus responds to the presence of anionic contaminants These contaminants could produce a corrosive er Jronment in the OTSG r.nd should be controlled.
5.3 Feedwater With System Isolated in Wet Layup Condition (Condition B)
With the Mini QH system operation, the feedwater system should be layed up (under a nitrogen blanket, if not full) and maintained with water treated with amr.onia and hydrazine as follows:
pH at 77F 9.5-10.5 Ammonia 10 ppa initially 2-20 ppm range ~
Hydrazine 100 ppm initially 50-200 ppm range Chloride 1 ppm Max.
This chiaride specification applies if a stainless steel feedlater heater is ic3 up.
a 6.
STEAM GENERATOR WATER 6.1 General As described in Section 5.1. the steam generators can operate in a steaming condition with a vacuum being maintained by a condenser (e.g., main condenser) or in a solid condition with a closed external loop.
At other times (e.g., with the nini DH system operating), the units and maintained in wet layup condition.
~'~
1185 273 em.
e 6. 2.
Stean_ Generator S_teg ing Unde n1 Vacuug for this condition the OTSG water Specifications are:
pH at 77 F 9.5-10.5 Cation conductivity 10paho/cm Max.
,1.0 ppm Max.
Sodium 2.0 ppm Max.
6.2.1 EH As stated in Section 5.2.1, the OTSG pH must be maintained on the basic side to prevent low temperature corrosion of the steam generator caterials should sulfur compounds and/or oxygen be present.
This is especially true when steaming in the OTSG results in the concentration of contaminants in the feed.;ater.
6.2.2 Cation Conductivity With the presence of ammor.ia and hydrazine, cation conductivity is the most practical measure of the water quality and the anionic contaminants in the water.
6.2.3 Sodium and Chlorides Sodium and chlorides are undesirable because they can lead to damage to the steam generators.
Also, these two items can be a general indication of the contanination levels in the steam generator water.
6.2.4 Chenistry Control Uhen out-of-specifications occur in the steam generator, the conditions should be corrected by bicudown or feed and bleed operations. 1185 274
6.3 Steam Generator Operating Uith A Solid Conditi_on The specifications for the steam generator water in this condition is the same as those for the feedwater listed in Section 5.2.
6.4 Steam Generator In f.n Isolated Uetlayup Condition Uher a steam generator is isolated, it is layed up by filling it with water conditioned with ammonia and hydrazine under a nitrogen blanket (if possible) or as follows:
pH at 77 F 9.5 - 10.5 Ammonia 10 ppm initially 2-20 ppm range Hydrazine 100 ppm initially 50-200 ppm range Cation c.onductivity 10 pmho/cm max Sodium 2 ppm max Chlorides 1 ppm max This is basically the same layup chemistry listed in Section 5.3 for the feedwater and the comments listed for the feedwater in that section also apply here for the layup of a steam generator.
The additional re-quirements for limits on sodium and cholorides Secause of the concern of the effects of these chemicals on steam generator materials.
7.
liAKEUP AND FILL WATER FOR STEAM GENERATORS AND FEEOUATER SYSTEMS Uater added to the steam generators or to the feedwater shall meet the folleuing requirements:
Dissolved Oxygen 0.1 ppm max.
Cation Conductivity 1 paho/cm Water with low dissolved oxygen levels is desired to limit the contri-butions frca this source to the oxygen in the feedwater and steam gen-erator water, oxygen promotes the general corrosion of steam generator and feedwater system materials:
cation conductivity is the measure of the quality of the water.
The 1.0 paho/cm max corresponds to the feedwater specifications when a steam generator is operating. 11Bli 275
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