NATIONAL CONTRIBUTIONS
S WATER CHEMISTRY IN SOVIET NUCLEAR POWER PLANTS o oo
Yu G. DRAGUNOV, Yu.V MARKOV, I.L. RYBALCHENKO, I.L. RYAZANTSEV, A.F. CHABAK
All-Union Scientific Research and Project Institute of Complex Power Technology, Leningrad, Union of Soviet Socialist Republics
Abstract
WWER reactors operate with water coolant which is alkahzed by a mixture of ammonia and pota-sium hydroxide Ammonia dissociates to give hydrogen which suppresses coolant radiolysis The narrow range of pH maintained ensures low material corrosion Corrosion product transport and deposition on fuel cladding surfaces are also suppressed The specified water quality is controlled by a system of ion exchange demmeralizers with mixed or separate beds of resins The dose rate from the primary circuit equipment is mainly due to inner surface deposits of activated corrosion products WWERs are equiped with horizontal steam generators with stainless steel OxISHIOT steam generator tubes With this design of steam generator and choice of SG tube material satis-factory operation of the equipment achieved with less stringent water chemistry specifications RBMK reactors are cooled with demmeralized water In order to suppress the corrosion of coolant circuit, oxygen is injected in the feed water circuit downstream of the condensate polishing plant and then removed from the water in the deaerator At present injection of hydrogen is proposed as the remedy for stainless steel piping IGSCC Current R&D work is directed towards further opti-mization of the chemical system and to improve the NPP safety and reliability
Introduction
Nuclear power program in the USSR is based on pressurized water-cooled reactors of vessel type (WWER) and boiling water-cooled graphite-moderated reactors of channel type (RBMK) Con-struction of heat generating nuclear power plants has also begun
Water chemistry regime for each reactor type depends on the reactor design
Experience has shown that the adopted water chemistries of WWER - and RBMK - reactors pro-vided high corrosion resistance of the structural materials, safe and reliable operation of the equipment, and low radiation doses
In view of the current trends towards higher fuel burnups, more flexible power regimes, and more stringent requirements for safety and reliability, water chemistry problems are considered to be very important, especially the influence of water chemistry on
The corrosion behaviour of the structural materials Corrosion product transport and deposition Radiation doses from the plant's equipment 1 Water Chemistry at NPPs with WWER-Reactors
WWER-reactors operate with water coolant alkalized by a mixture of ammonia and potassium hy-droxide Potassium hyhy-droxide provides the required pH value to decrease corrosion products
de-position on fuel and reduce the corrosion itself Under the core irradiation ammonia dissociates with formation of hydrogen and nitrogen Excess of hydrogen (30 - 60 ml/kg) provides for pro-found suppression of coolant radiolysis, as a result of which oxygen content is kept below 5 (img/kg The alkali concentration is coordinated with the boric acid content
This concept of water chemistry has the following features
High pH level is varied in a narrow range (Fig 1) Under these conditions the rates of material corrosion and corrosion product transport and deposition on fuel cladding are suppressed, and the risk of excess accumulation of strong alkali in the coolant is excluded
High hydrogen concentrations required for suppression of water radiolysis is reached with very simple means (no H2 gas)
Injection of hydrazine during refuelling periods suppresses the oxidative effect of air, when reactor vessel is open
Reducing conditions of the coolant prevent the radiation induced corrosion of zirconium al-loys
Water chemistry specifications for WWER-1000 are given Table 1
The specified coolant quality is controlled by a system of ion exchange demmeralizers with mixed or separate beds of resins
The filters are installed in the mam pump by-pass lines At some units high temperature mechani-cal filters are installed Capacity of each filter is up to 100 Vh Heating up and cooling down peri-ods of the clean-up system coincide with those of the reactor circuits Washing off,
decontamma-1 2 3 4
H3B03 (g/kg)
FIG 1 (K*) as function of boric acid concentration (pH calculations made on the basis of Meek data).
TABLE I SPLCIFICATIONS FOR REACTOR WATER CHLM1STRY (WWER 1000)
Parameter pH(25°C)
Halogens (CI' » f ) mg kg ' Dissolved oxygen, mg kg I Dissolved hydrogen (NTP), ml kg '
K'(tot) = { K « ) + (Na») + (Li*) as function of H3BÛ3 concentration mg eqv k9 ' NH3, mg kg '
H3B03 g kg l Coppper, mg kg '
Corrosion product (as Fe), mg kg '
Value 5 7 1 0 2
<0 1
<0005 30 60 0 0 5 0 4 5
>50 0 13 5
002
<02
tion or unloading of spent resins are performed during reactor shut down periods for planned maintenance and repair
Normally a cation exchanger in potassium and ammonia from the anion exchanger in BOj-form are operating continuously A cation exchanger in H-form is connected periodically, when potas-sium and ammonia concentrations in the coolant exceed the specified levels
Two groups of filters are provided one is in operation and the other is a standby The capacity of each line is 40 t/h
Long-term operation experience with fuel and post irradiation examinations m hot cells haven't revealed any case of fuel failure due to zirconium alloy corrosion Oxide layers on fuel cladding are usually less then 3 (im thick, and internal hydrogen content on irradiated fuel cladding is about 0 008 % Number of leaking fuel elements doesn't exceed 0 02 %
The dose rate from the primary circuit equipment is mainly due to inner surface deposits of acti-vated corrosion products (particularly, long-lived radionuclides), fission products are responsible only for a minor portion of the radiation dose Radiation levels from WWER steam generator headers reach 1 - 2 R h ' for reactor operation period > 1 year
The NPP's with WWERs are equipped with horizontal steam generators with vertical cylindrical collections headers and horizontal heat exchange tubing This design prevents accumulation of slurry in the region of the tube expanded joints, and hence, the danger of tube corrosion damage On these grounds austenitic stainless steel OX 18H10T has been used for SG tube fabrication (com-pared to high nickel alloys in the foreign designs) and satisfactory operation of the equipment achieved with less stringent water chemistry specifications The specifications of SG blowdown and feedwater are given in Table 2
The secondary water chemistry of WWERs is corrected with hydrazine hydrate, continuously in-jected into the feedwater circuit before the LP-heaters Injection is performed with a special fa-cility which maintains the required hydrazine levels in all periods of reactor operation
The NPPs with WWER reactors are provided with full flow condensate polishing plants, which con-sist of an (electromagnetic filter and a number of mixed bed demmeralizers) The combination of
TABLE 2 SPECIFICATIONS FOR WATER CHEMISTRY OF STEAM GENLRATORS (WWER)
Parameter
pH(25°C)
Conductivity, i»S cm ' Hardness, |ig eqv kg 1 Iron, ug kg '
Copper, wg kg ' Oxygen, ug kg t Sodium, ug kg ' N2H4.„g kg' Oils, ug kg ' Chlorides, ug kg '
Electrochemical potential of Pt, EPI mV
Value Feed water
7 5 8 5
<03
<02 15 5(3)") 10***) 5 40 70
100 NS 100 ±30***)
Slowdown water 78 8 8
<3 NS*)
NS NS 1000
NS NS 500 NS
*) Not specified
**) With LP heatm having stainless st«l piping
"*) Afterdeaerator
**") After the last LP heater
hydrazine injection with full flow condensate polishing ensures the specified quality of SG blow-down and feed-water
Long-term experience shows, that normally the number of plugged tube in SG's does not exceed 0 08 %
In case of considerable deviations of water chemistry specifications corrosion failures of heat transfer tube were observed
This depends not on duration of operation but on ingress of chlorides to the SG water 2 Water Chemistry at NPPs with RBMK-Reactors
RBMK-reactors are cooled with demmeralized water of high purity No additives or inhibitors are dosed into recirculation or secondary circuits to prevent their accidental break through into the reactor water
Operational experience has shown that the coolant quality m real systems corresponds to the specifications For these see Table 3
Considerable part of the NPP circuit (especially, that of the feedwater train) is made of pearlitic steels, which have higher corrosion rates compared to stainless steel OX 1 8H 1 0T Corrosion pro-ducts are released from these surfaces and can be carried out to the reactor core and activated there In order to suppress these processes oxygen (up to 200 ug kg ') is injected into the feed-water circuit downstream the condensate polishing plant, and then removed from the feed-water in the deaerator
TABLh 3 SPECIFICATIONS FOR REACTOR WATER CHEMISTRY (RBMK-1000)
Parameter pH (25 °C)
Conductivity, uS cm ' Chlorides, pg kg ' Iron, ng kg ' Copper, (jg kg ' Salts, |ig eqv kg '
Value 6 5 8 0
1 0
<100
<200
<50
<15
At present the injection of hydrogen into the system downstream the deaerator is proposed as the remedy for stainless steel piping IGSCC
With strict adherence of the adherence of the reactor and feedwater chemistries to the specifica-tions, fuel deposits 40 - 60 u,m thick are typical These mainly consist of iron oxides (up to 90 %) and are the heaviest in the region of initial boiling
The rate of (Zr + 1 % Nb) alloy corrosion in boiling water shows strong dependence on oxygen content and rather weak dependence on radiation Internal hydnding of fuel cladding doesn't ex-ceed 0 001 - 0 02 %, with hydrides oriented in a random or ring-like manner On the corroding areas the hydrides have shown radial arrangement
It is expected that radiolysis suppression may increase the operational reliability of both the fuel elements and (Zr + 2 5 % Nb) pressure tubes, the latter having much longer service life compared to that of the fuel elements
Current R&D work is directed towards further optimization of the chemical system and increase of the NPP safety and reliability Thus, much attention is paid to the physico-chemical processes in the NPP circuits and the development of new technologies for water chemistry control Complex studies are also carried out m the direction of increasing material corrosion resistance, prevention of corrosion product deposition on the heating surfaces; designing systems for high-temperature clean-up of the coolant, new equipment for water preparation, and automatic systems for chemi-cal and radiochemichemi-cal monitoring
HYDRAZINE REGIME FOR WWER-440 AND WWER-1000 PRIMARY CIRCUITS
V.I PASHEVICH, Yu.A. KHITROV, M.V. BELYAEV, N.V. NEMIROV, A.I. GRUSHANIN, N.D. KUKHAREV All-Union Scientific Research and Project
Institute of Complex Power Technology, Leningrad, Union of Soviet Socialist Republics
Abstract
Hydrazme regime in WWER type reactor primary circuit has been developed and proved in practice This new water chemistry regime has been tested at Kolskaya, Rovenskaya and Pakash NPPs Withthe use of hydrazme regime the corrosion product deposition constant is lowered in comparison to that of standard Ammonia-Boron-Potassium regime This resulted in lower level of contamination stability The use of Hydrazme regime immediately after standard regimes promo-tes peeling of corrosion products from fuel assemblies Also its use during shut down conditions prevents pitting of vessels without stainless steel fusion cladding Available data m the secondary circuit indicates that iron concentration exceeds the norm when hydrazme regime is used The main source of iron entering the feed water is through steam and water of highpressure prehea-ter and super heated steam separator Analysis of corrosion product deposits formed m the secon-dary circuit after the introduction of hydrazine-ammoma water chemistry has been carried and the results given The influence of hydrazme chemistry on the radiation chemical transformations of solution containing ^803 has been evaluted in this work
Observation and discussion
-Water chemistry in WWER type reactor primary circuit with continuous hydrazme hydrate dosing of the coolant is developed and proved in practice As a result of which the equipment surface contamination levels are reduced When hydrazme hydrate water chemistry is adopted, the corro-sion products from the primary circuit structural materials which are accumulated on fuel assem-blies (FA) gets removed and the deposition rate of newly formed corrosion products is reduced significantly Reduction in the amount of corrosion product in the intensive neutron zone reduces significantly the amount of radionuchdes formed from corrosion products Use of hydrazme wa-ter chemistry during shut down conditions (shut down, cooling, core refuelling and start up) pre-vents pitting of vessels without stailess steel fusion cladding
New water chemistry regime was proved to be good when tested at two units of Kolskaya, two units of Rovenskaya NPP and Paksh NPP (Hungary) Research works on hydrazme hydrate dosing into the coolant of the reactor under operation were started in 1975 at Kolskaya NPP When hy-drazme was introduced into the coolant, increase in the concentration of 60Co, 58Co, 59Fe, 5ICr,
s*Mn, 56Mn radionuchdes were observed, which indicated that these radionuclides were washed off the FA surface
Corrosion products formed under reactor operation conditions are found to deposit mainly on fuel assembly components (up to 85 - 90 %) Transport of corrosion products and radionuchdes from fuel assemblies occurs mainly during fast power unit shut down (e g upon emergency protection system functioning) During this period the total activity of the coolant rises by 100
-1000 times (Fig 1)
Ci/kg
2 / i 6 8 Time
1 At standard conditions for 2 months 2 At hydrazme conditions for 2 months 3 At hydrazme conditions for 4 months
Fig 1 Change in radioactive contamination of the coolant of 20 mm off load conditions
When standard boron ammonia potassium regime is used the constant for corrosion product de-position from the coolant was found to be (1 - 5) x 10 -* s ' The product withdrawal factor of the special water treatment system (SVO 1)is(1 2 ) x 1 05s ' Activated corrosion products are deposi ted mamyl on fuel assemblies With the use of hydrazme hydrate regime, the corrosion product deposition constant decreased to (1 5)x 1 0 5 s ' Comparable values of corrosion product deposi-tion rate on fuel assemblies and their withdrawal to SVG-1 system leads to contaminadeposi-tion stability at lower level, and in transient conditions the coolant activity increase doesn t takes place (Fig 2)
The units 3&4 of Kolskaya NPP are operated with hydrazme regime from their very start up Con lamination of their primary circuits is much lower than that of Novovoronezhskaya NPP and LOVII-sa NPP for the LOVII-same operation period (Fig 2)
The use of hydrazme regime at units 1 &2 of Rovenska NPP after their long term operation under boron ammonia water chemistry conditions decreased the gamma dose rate on the surfaces of steam generator header by 6 7 times (Fig 2) The use of hydrazme ammonia water chemistry im mediaately after standard regimes promote peeling of corrosion products from fuel assemblies and their carry over to SVO 1 system Also the use of hydrazme ammonia water chemistry in re actor shut down, off load periods and startup conditions prevented pitting of vessels without fusi on cladding The second generation WWER 440 reactor vessels and pressurize« are manufactured from 48 TC and 22 K type pearlitic steels without stainless steel fusion cladding Five such units we re built in USSR During reactor vessel inspection under operation conditions the cases of pitting were observed Corrosion damage depth was found to be dependent on the time of operation of the respective power unit
1 Units 1 &2 of Rovenskaya NPP 2 Unit 2 of Paksh NPP 3 Unit 1 of Lovnsa NPP 4 Unit 3 of Rovenskaya NPP
5 KOLA3 6 KOLA4
Years
Hydrogen fran the beginning
Fig 2 Steam generator header radio active contamination
The results of studies have shown that NO 2 and NOa ion concentration in reactor water increases NO 2- and NO 3 ion formation reaction rates depend on residual gamma radiation dose rate after reactor setup (i e from time of fuel assembly operation to start up moment) and oxygen entering the reactor water
Ionizing radiation and saturation with oxygen results m oxidation of ammonia dissolved m the coolant and the formation of nitrates and nitrites When hydrazme water chemistry is used the na-ture of radiation and chemical transformations m solution consisting of 12 g/l of H38O3, '40 mg"
of NHs and 0 3 g/l of NjH4 H2O changes under the influence of radiation and free access to air Residual radiation stimulates NÛ2 and NÛ3 ion reduction reactions in the presence of hydrazme and at temps 40 70 C Pitting also stops at 1 3 mole ratio of NO? and NOs ion concentrations With such concentration ratio maintained, reliable protection of reactor vessels manufactured from pearlite steel is provided Reactor vessel protection at Kolskaya NPP was achieved by this me-thod
The use of hydrazme water chemistry allowed to improve significantly the corrosion and radiati-on cradiati-onditiradiati-ons m WWER 400 SI 000 primary circuits for the whole operatiradiati-on period Resultsof longterm experience with hydrazme regime is available for WWER reactor secondary circuits
Water quality specifications for WWER reactor secondary circuits are given in Table 1
With the use of hydrazme the norm of iron could not be retained The main source of iron ente ring the feed water is through steam and water of high pressure preheater (HPP) and super hea ted steam separator (SHSS) That is why iron concentration in feed water greately exceeded the specified norm The iron concentration values are given m Table 2
Table 1
Water conditions of WWER 1 reactor secondary circuits
Specified
Iron concentration in feed water of WWER reactor secondary circuits at NPPs No NPP Iron Cone ug/kg
Khmelmtskaya 50 Novo-Voronezhskaya up to 40 Rovnenskaya up to 40 Zaporpzhskaya 50 Kalinmskaya 50
( (withoutcondensatepurification
An increase in hydrazmze concentration to 900 (ig/kg decreases iron concentration in feed water to 2 0 - 3 5 ug/kg within 5 - 1 0 days, while hydrazme dosage increase to 80-100 u.g/kgdid not lead to appreciably low iron concentration
The pH rise in secondary circuit should result in sharply decreasing pearlitic steel corrosion rate The mam difficulty with introduction of hydrazine ammonia water coolant chemistry is that copper containing alloys are present in secondary circuit and the NH^ form cationite is absesnt in home (industry H* cationite applied even at pH = 9 1 ± 0 1 results in very small filter cycles At Kalinmskaya NPP for a period of one month hydrazine at a concentration of 40 -SO |ig/kg and ammonium hydroxide at a cone of 300 - 400 ug/kg were dosed into feed water Iron concentrati-on in the steam generator feed water dropped to 10 (ig/kg level, copper cconcentrati-oncentraticoncentrati-on also did not exceed the norm Hence hydrazme-ammonium water chemistry was introduced at Kalinmska-ya NPP, the iron concentration exceeded the norm only after high pressure preheater was put into operation, but even in this case it drops to the norm within 3 days To examine the state of corrosi-on corrosi-on equipment surfaces in the seccorrosi-ondary circuit, the corrosicorrosi-on product deposits were sampled during repair periods, iron oxide chemical composition and phase content were determined Re-sults of the analysis are shown in Table 3 The deposits are mostly composesd of iron oxides, w ith all known phases present where as under hydrazine regime the set of iron oxides is limited to 3 phases viz magnetite, hematite and lepidocrocite Being predominantly made up of iron oxides, the deposits are common in their chemical composition, copper content is higher in steam genera-tor and on mean-pressure cylinder stagenera-tor blades Considerable amounts of nickel present in the deposits are difficult to explain In many cases nickel is found in larger amounts than copper To control the pearlitic steel and MHM-5-1 alloy corrosion rate, the samples were placed into the secondary circuit bypasses which are designed for taking representative samples Bypasses layout is shown in Fig 3 Studies showed that pearlitic steel is dangerous from the view point of corrosi-on during repairs, when equipment surfaces are affected by a damp atmosphere and are pe-riodically wetted which results in the development of pitting corrosion Average corrosion rates for representative samplse are given in Table 4 Maximum corrosion rate is observed when
The pH rise in secondary circuit should result in sharply decreasing pearlitic steel corrosion rate The mam difficulty with introduction of hydrazine ammonia water coolant chemistry is that copper containing alloys are present in secondary circuit and the NH^ form cationite is absesnt in home (industry H* cationite applied even at pH = 9 1 ± 0 1 results in very small filter cycles At Kalinmskaya NPP for a period of one month hydrazine at a concentration of 40 -SO |ig/kg and ammonium hydroxide at a cone of 300 - 400 ug/kg were dosed into feed water Iron concentrati-on in the steam generator feed water dropped to 10 (ig/kg level, copper cconcentrati-oncentraticoncentrati-on also did not exceed the norm Hence hydrazme-ammonium water chemistry was introduced at Kalinmska-ya NPP, the iron concentration exceeded the norm only after high pressure preheater was put into operation, but even in this case it drops to the norm within 3 days To examine the state of corrosi-on corrosi-on equipment surfaces in the seccorrosi-ondary circuit, the corrosicorrosi-on product deposits were sampled during repair periods, iron oxide chemical composition and phase content were determined Re-sults of the analysis are shown in Table 3 The deposits are mostly composesd of iron oxides, w ith all known phases present where as under hydrazine regime the set of iron oxides is limited to 3 phases viz magnetite, hematite and lepidocrocite Being predominantly made up of iron oxides, the deposits are common in their chemical composition, copper content is higher in steam genera-tor and on mean-pressure cylinder stagenera-tor blades Considerable amounts of nickel present in the deposits are difficult to explain In many cases nickel is found in larger amounts than copper To control the pearlitic steel and MHM-5-1 alloy corrosion rate, the samples were placed into the secondary circuit bypasses which are designed for taking representative samples Bypasses layout is shown in Fig 3 Studies showed that pearlitic steel is dangerous from the view point of corrosi-on during repairs, when equipment surfaces are affected by a damp atmosphere and are pe-riodically wetted which results in the development of pitting corrosion Average corrosion rates for representative samplse are given in Table 4 Maximum corrosion rate is observed when