F Bardet

J-L Bretelle
email: jean-luc.bretelle@edf.fr

N Perdrigé

F Dacquait

Chemical Degassing on EDF units -Feed Back Experience and Method

The chemical degassing feasibility of all French units RCS during outages is usually performed on the four 1500 MWe French units since July 2004 and is planned for 2007 on the 34 units of 900 MWe and the 20 units of 1300 MWe units. It helps to an optimized shutdown planning management, especially in case of unplanned shutdowns, and helps to limit the thermal stress corrosion of RHR at temperature higher than 120°C by avoiding temperature step extension after bubble collapsing to remove hydrogen increase. Chemical degassing is performed at temperature equal or lower than 80°C, by hydrogen peroxide injection if 3 < H 2dissolved < 35 cm 3 /kg. H 2 O 2 calculation can be stoechiometric or increased by 20 % depending on the SG tubes material and the full power operation duration before the shutdown. Purification flow is adapted to each design type units characteristics to easily manage a hypothetical early oxygenation. Early oxygenation management consists in forecasting the dilution of the VCT (Volume Control Tank) gaseous phase by nitrogen to avoid dangerous H 2 /O 2 gaseous mixture and to obtain oxygenation criteria on VCT. Three kinds of dilution can be used: nitrogen burping with high levels amplitude, continuous nitrogen flushing or discontinuous flushing with a maximal constant VCT level but pressure variation.

I. INTRODUCTION

The chemical degassing feasibility of all design French units RCS during outages has been proved by tests performed from 2002 to 2004 on EDF units. It has been usually performed on the four 1500 MWe French units since July 2004 and is planned for 2007 on the 34 units of 900 MWe and the 20 units of 1300 MWe units.

The use of chemical degassing improves the management of refueling outages and of unplanned shutdowns on operation conditions and radioprotection point of view. It is also used:

• to limit the thermal stress corrosion of RHR by shorter operation time of RHR at temperatures higher than 120°C, • to better manage H 2 increases after bubble collapsing, • to perform a new way of bubble collapsing.

This paper describes the main characteristics of the French chemical degassing method. The method considers:

• the main risks associated with the chemical degassing and the usual outages like material, surface contamination, H 2 /O 2 gaseous mixture …, • in-core corrosion products quantity with which the hydrogen peroxide may react, • the SG tubes material.

It is adapted to the main characteristics of each type of French units. To improve the chemical degassing duration and to limit the contamination risk, H 2 O 2 volume calculation takes into account the reaction between H 2 O 2 and corrosion products and the purification flow rate is adapted. New criteria have been established for the gaseous phases management.

II. CONTEXT

During cold shutdown for refuelling outages, hydrogen is removed from RCS and biphasic capacities before oxygenation to avoid any dangerous gaseous H 2 /O 2 mixture. RCS dissolved hydrogen is usually degassed by physical means: nitrogen burpings on VCT (Volume Control Tank), degassing of pressurizer gaseous phase by the sampling line between pressurizer gaseous phase and BRS head tank, BRS degasser. In gaseous phases, gaseous hydrogen must be less than 2% and in RCS, oxygenation criteria for dissolved hydrogen is H 2dissolved < 3 cm 3 /kg at T≤ 80°C if oxygenation is performed by oxygen peroxide injection and achieved by VCT venting. On French units, hydrogenated gas (H 2gaseous > 2%) are set to hydrogenated gaseous wastes tanks and aerated gas are evacuated by the chimney.

In some operation cases, high hydrogen concentrations can be observed during shutdowns.

• Bubble collapsing at 130°C needs to maintain high hydrogen concentrations (H 2 > 10 cm 3 /kg) in order to limit the RHR operating duration at temperatures higher than 120°C without increasing RCS contamination risk • For some kinds of unplanned shutdowns, without RCS opening, hydrogen is maintained in RCS at high levels (> 10 cm 3 /kg). If RCS opening is finally needed, large quantities of dissolved hydrogen must be degassed. Slow hydrogen degassing could be detrimental to the contamination risk management and to the work planning. • Some hydrogen increases can occur during refuelling shutdown :

-At RHR connexion, if gaseous hydrogen has come from pressurizer gaseous phase by common sampling lines because of valves which are no more hydrogen tight, -after the bubble collapsing, if the pressurizer has not been completely degassed. In those cases, physical degassing of high hydrogen concentrations is performed at temperatures higher than 120°C. It takes a lot of time, increases the thermal stress corrosion of RHR and delays the operation planning.

After keeping high hydrogen concentrations as long as possible, chemical degassing by hydrogen peroxide injection at temperature equal or lower than 80°C is performed in some foreign countries [1]. The chemical degassing feasibility on all design French units RCS has been proved by four tests (one per each design).

Considering the positive feed back experience of the four tests and the solutions accorded by the chemical degassing in the fields of operation management improvement and of RHR thermal stress limitation, it was decided in 2003 to propose the chemical degassing to all design French units, with a priority to the 1500 MWe units because of their pressurizer degassing problems. The industrial chemical degassing method takes into account the feed back experience of the four tests and the results of the risks analysis about staff security, materials, fission products behaviour, corrosion products management, contamination and gaseous H 2 /O 2 mixtures.

III. RISKS ANALYSIS

III.A. Materials

Hydrogen at concentrations in a range from 3 to 35 or 50 cm 3 /kg at temperature equal or lower than 120°C and for short steps (few hours) has no influence on SCC of stainless steel materials or Alloy 600.

Chemical degassing increases injected H 2 O 2 volumes during shutdowns but the injection temperature is lower than 120°C, which limits the impact of higher H 2 O 2 volume on corrosion.

III.B. Fission products behaviour

Gaseous fission products like xenon are removed by physical degassing (hydrogen or nitrogen burpings on VCT, BRS degasser and use of the degassing line between pressurizer gaseous phase and the surge head tank). Iodine mainly reacts as xenon does but can be removed by lithiated mixed bed resins in reducing conditions [2]. Fission products can increase into RCS depending on RCS pressure decreases. It means that fission gas physical degassing must be performed during the first shutdown steps. The gaseous wastes are set to the hydrogenated gaseous wastes tanks.

Because of environment, planning and security points of view, fission products must be mainly removed before oxygenation and H 2 O 2 injection.

Chemical degassing uses the same reactant, H 2 O 2 , as the oxygenation. The temperature of both operations are closed and there is no pressure variation between those operations. Chemical degassing can not remove fission products and because of the gas treatment, fission gas must be removed before chemical degassing. Fission products activities must respect oxygenation criteria ( 133 Xe< 4 GBq/t and 131 I < 2 GBq/t) before chemical degassing. The figure 1 presents the behaviour of iodine and xenon during the chemical degassing and the oxygenation performed on Civaux 2 NPP in august 2004.

III.C. Corrosion products

The four tests and the first chemical degassings on 1500 MWe-units have shown that more than 70% of the injected H 2 O 2 for chemical degassing react with dissolved hydrogen. The other part reacts with corrosion products, illustrated by an increase of volumic activities, as shown by the figure 2.

Volumic activities increase represents a small part of the corrosion products available to be dissolved. The chemical degassing has a low influence on the activity peak during oxygenation : Saint-Laurent B 2 NPP RF18 oxygenation peak after chemical degassing : 140 GBq/t, to be compared to a ca1culated 160 GBq/t without chemical degassing peak (calculation made by the CEA RELTRANS code). On units whose shutdowns have no several hours step at temperatures between 170°C and 80°C, the part of corrosion products reacting with H 2 O 2 during chemical degassing is about 20 to 30 %, whatever the injections number. For units with 690 SG tubes and whose 58 Co oxygenation peaks are lower than 50 GBq/t after a full cycle, actual feed back experience indicates that this part is about 10-15%. For units where shutdowns present very long steps at temperature lower than 170°C and with hydrogen concentration higher than 10 cm 3 /kg, like Bugey 5 NPP in 2003, the corrosion part can increase to 30-40%. This increase could be explained by structure modifications of some corrosion products kinds, usually insoluble during normal shutdowns, to some soluble species in presence of oxidizing reactants. [1] Chemical degassing must take into account the corrosion products behaviour which depends on SG tubes material and power duration.

The forms and the quantities of corrosion products produced during a cycle and having an influence on shutdown management, mainly depend on SG tubes material and on full power duration. Thus, fore one kind of material with a given history, the longer is the full power duration, the greater is the activated corrosion products quantity. Activated corrosion products are usually considered as a simple part of RCS total corrosion products.

When an oxygenation is performed during unplanned shutdown after more than six full power months, 58 Co oxygenation peak value mainly represents more than 50% of the peak expected after a full production cycle. Corrosion products quantities which can react with dissolved oxygen are very important. For oxygenation performed during the first six months of a cycle, 58 Co oxygenation peak values have a very large range and the feed back experience is not enough to can determine such a rule. [3] For SG tubes made of 600 nickel based alloy, 58 Co oxygenation peaks stay in the same range, as the figure 3 shows it for some 1300 MWe-units.

Smaller values can be explained by unplanned oxygenation performed during the first cycle months and which has reduced the RF oxygenation peak value. Fig. 3

III.D. Contamination risk

Saint-Laurent B2 NPP test feed back experience has shown that a RCS contamination had occurred during the five hours period from H 2dissolved < 3 cm 3 /kg criterion reaching to the oxygenation. This time period has been used to obtain H 2gaseous < 2% on VCT gaseous phase and to prepare air venting on VCT. The three other tests and the 1500 MWe-units feed back experience show that one to three hours after the chemical degassing can be needed to perform oxygenation (H 2 O 2 injection). This period can be very critical for the contamination risk management. Thus, performing an nitrogen burping on VCT before reaching 80°C and having well trained operators can help to quickly obtain H 2gaseous < 2% on VCT after chemical degassing.

To manage contamination risks during chemical degassing, it is important to keep in mind that, the more important are the out of flux zones contamination risks during temperature steps, the longer is the step, the lower is the temperature, the higher is the volumic activity" [3]. This behaviour is particularly important when the primary coolant contains very low level of dissolved hydrogen (less than 5 cm 3 /kg) and is not oxygenated. Contamination can occur during sudden decreases of purification flow rate and during sudden temperature decreases. It means that, according to the general shutdowns procedures,

• Temperature step at temperature lower than 180-170°C without lithium and with small amount of hydrogen, are reduced or avoided, to limit corrosion products solubilization, • Purification flow rate is as high as possible but takes into account the management of the other risks (activity peak at the chimney, H 2 /O 2 gaseous mixture …) which can ask to reduce the flow rate for a short while. Chemical degassing is performed at temperature equal or lower than 80°C, which are a good compromise between planning respect and contamination risk. The temperature is low and this helps to the corrosion products dissolution. The higher is the number of H 2 O 2 injections, the longer is the temperature step. It is important to limit the chemical degassing duration by limiting the H 2 O 2 injection number. As a consequence, to limit the injections number, the H 2 O 2 volume calculation must take into account the reaction between H 2 O 2 and the corrosion products.

• For units with more than six months of full power if they have 600 SG tubes or 690 SG tubes with 58 Co oxygenation peak higher than 50 GBq/t, an excess of 20 % of H 2 O 2 compared to the stoechiometric volume is injected. • For units with less than six months of full power, the injected H 2 O 2 volume is stoechiometric. • For units with 690 SG tubes and with 58 Co peak lower than 50 GBq/t, the injected H 2 O 2 volume is stoechiometric In a second step, H 2 O 2 volume calculation can be reviewed, after feed back experience (four shutdowns with full power duration between some hours to four months).

Purification flow rate must be as high as possible and must take into account the management of others risks.

III.E. H 2 /O 2 gaseous mixture

III.E.1 Normal situation

On biphasic capacities, a dangerous H 2 /O 2 gaseous mixture is a risk usually managed during shutdowns on French units because of air venting on main capacities. This risk is associated too to the chemical degassing. This risk is managed for the chemical degassing in the same way as for oxygenation : biphasic capacities, VCT excepted, must respect the usual oxygenation criteria (commonly H 2gaseous < 2%) before chemical degassing at temperature equal or lower than 80°C. VCT gaseous phase has its own criterion : H 2gaseous < 15 %, which is a compromise between continuous hydrogen introduction in VCT due to the sprinkling flow rate, the later oxygenation criterion reaching (H 2gaseous < 2%), the gaseous wastes volume and the VCT gaseous phase nitrogen dilution duration. For oxygenation, criterion on VCT remains H 2gaseous < 2%. Nitrogen burpings with a great levels range help to quickly obtain this criterion. Nitrogen burping can be performed before reaching 80°C. The 20 % H 2 O 2 excess is injected on units with very large amount of corrosion products and compensates the H 2 O 2 /corrosion products reaction. There is no risk of oxygen increase.

Hydrogen and oxygen evolution -Civaux 2 RF05

Nevertheless, on line oxygen measurement is performed and recorded. The devices (hydrogen-meter and oxygen-meter) allow on-line measurements and help to an early detection of an "early oxygenation".

III.E.2 Early oxygenation

If H 2 O 2 injected volume is over evaluated, hydrogen disappears and dissolved oxygen could appear in RCS if no reaction between H 2 O 2 and corrosion products took place, which is a very hypothetic case. This dissolved oxygen would degas into the VCT which contains more than 2% of gaseous hydrogen. A H 2 /O 2 gaseous mixture would form in VCT gaseous phase. Such a situation is called "early oxygenation".

To prevent an "early oxygenation" : • H 2gaseous < 15 % on VCT • H 2gaseous < oxygenation criteria on other biphasic capacities (mainly, H 2gaseous < 2%) • Availability of the oxygen monitor on the inlet to gaseous wastes treatment system, • Availability of redundancy mean for on-line monitors. Early oxygenation is declared when hydrogen has disappeared and oxygen appears and is stable, although there is more than 2 % of gaseous hydrogen in the VCT. The management of an "early oxygenation" consists to forecast to dilute the VCT gaseous phase by nitrogen to avoid dangerous gaseous mixture and to obtain as quickly as possible H 2gaseousVCT < 2 %. VCT gaseous wastes are set to hydrogenated gaseous wastes tanks. To manage an early oxygenation in good conditions (gas volumes, duration …), gaseous oxygen concentration must remain smaller than 3.0 %. Three methods can be used to dilute the VCT gaseous phase :

• Nitrogen burping (levels variation)

• Continuous nitrogen flushing • Discontinuous nitrogen flushing (constant level but pressure variation by nitrogen inlet and gas discharge to gaseous wastes treatment system). One of the three solutions is operated as soon as the early oxygenation is detected and confirmed by the chemist. The first dilution must have been performed before gaseous oxygen concentration becomes higher than 3.0%. It means that after a burping, the maximal VCT level, just before decreasing the level to introduce nitrogen, must be reached before O 2gaseous = 3.0 %. The minimal value of the time period called ∆t1 in the figure 6 must be higher than the monitor information added to the chemistry time period (detection and confirmation) and to the level increase. The design case is taken for a maximal volume equal to the reactant injection tank volume which integers an error of five litres, an excess of 20% and an error on the hydrogen measurements. The table 1 gives the main characteristics of an early oxygenation management.

Table 1. Main characteristics of an early oxygenation management according to the design-type units. To avoid oxygen concentration higher than 3.0 % in a time period shorter than the minimal time period, purification flow rate must be reduced to 14 t/h on 3 loops reactors. On four loops reactors, VCT volumes are greater than those of the three loops reactors and the reaction is more rapidly detected by monitors. The calculation have used conservative hypothesis by not decreasing at first the VCT level, to introduce nitrogen and to dilute the VCT gaseous phase. But, for an easier early oxygenation management, it is recommended to decrease first the VCT level as low as possible and then to perform maximal nitrogen burping levels variations (for an 1500 MWe unit : levels variation from 20 to 80%).

If continuous or discontinuous nitrogen flushing are the management way, the VCT level must be increased to 80% to have a maximal efficiency (small gaseous space to dilute). The normal nitrogen flow rate is higher than the needed dilution flow rate. The time period to perform the level increase is the same for the three methods. The main difference between the three methods is the gaseous wastes volume.

III.F. Environment

During the four feasibility tests, hydrogen was remained in RCS. Hydrogenated gaseous wastes volumes were smaller than the usual volumes after a physical degassing (reduction from 25 to 40%). This conclusions still remains for units which must perform unplanned oxygenation and have no time to physically degas the RCS. In case of hydrogen increase after bubble collapsing, chemical degassing avoids greater gaseous wastes volume production.

Chemical degassing uses hydrogen peroxide which is the reactant used for the oxygenation. There is no new waste type. The injected H 2 O 2 volume is too small to have an influence on the liquid wastes volume. By reducing burpings during the first times of the shutdown, it can reduce the liquid wastes to be treated in the BRS head tank.

As a conclusion, chemical degassing does not increase the volumes of gaseous and liquid wastes and could limit them when hydrogen is remained in RCS.

IV. METHOD

Because of the specific French gaseous wastes management, chemical degassing is strictly separated from oxygenation. It is performed at temperatures equal or less than 80°C by hydrogen peroxide injection. It can be used on units with fission products if their volumic activities respect the oxygenation criteria before the chemical degassing.

Hydrogen management till the end of the bubble collapsing depends on the nature of the shutdown (planned or unplanned) and on the available volume in gaseous wastes tanks.

After bubble collapsing, RCS dissolved hydrogen is measured and according to the concentration value, a check up is done before temperature decreasing to 80°C.

• 3 < H 2dissolved < 35 cm 3 /kg in RCS,

• H 2gaseous < 15 % on VCT,

• H 2gaseous < oxygenation criteria on other biphasic capacities (mainly, H 2gaseous < 2%)

• Availability of the oxygen monitor on the inlet to gaseous wastes treatment system and of RCS oxygen and hydrogen monitors If dissolved hydrogen remains higher than 3 cm 3 /kg, a new injection is performed with similar calculation rules. If the hydrogen peroxide volume introduced into the reactants injection tank is wrong, the tank is drained, rinsed and filled with the right H 2 O 2 volume.

V H2O2 stoechiom. = 794 , 9 V C n V M C (L/mol) REN (cm3/kg) H O H REN t H REN 2 2 2 REN 2 × × ∆ = × × ∆ θ θ θ θ t M (L) If Θ REN = 25°C, (L/
V H2O2 majoré = 794 , 9 V C 2 , 1 n V M C 2 , 1 (L/mol) REN (cm3/kg) H O H REN t H REN 2 2 2 REN 2 × × ∆ = × × ∆ θ θ θ θ t M x x (L) If Θ REN = 25°C,
When 0 < H 2dissolved < 3 cm 3 /kg, purification flow rate is increased to 27 t/h on three loops reactors and nitrogen burping or flushing is performed on VCT. For a maximal efficiency, burpings present a maximal levels variation (20 to 80%).

Figure 7. H 2 O 2 volume to remove dissolved hydrogen on a 1500 MWe French unit V. CONCLUSIONS AND FURTHER USES Chemical degassing has been successfully performed on the French 1500 MWe units since July 2004 and the feasibility has been proved on the three other design type units. It helps to an optimized shutdown planning management, especially in case of unplanned shutdowns, when physical degassing cannot be easily and efficiently performed. Chemical degassing helps to limit the thermal stress corrosion of RHR at temperature higher than 120°C by avoiding temperature step extension after bubble collapsing to mitigate hydrogen concentration increase.

Chemical degassing and oxygenation still remain separated operations because of the gaseous wastes.

Chemical degassing is simply performed at temperature equal or lower than 80°C, by hydrogen peroxide injection if dissolved hydrogen is in the range 3 -35 cm 3 /kg. H 2 O 2 calculation can be:

• Stoechiometric if the power duration is less than six months, • Stoechiometric if SG tubes are made in 690 nickel based Alloy and the unit has 58 Co oxygenation peak smaller than 50 GBq/t after a full cycle, • Increased by 20 % if the power duration is higher than six months and if the SG tubes are made of 600 nickel based Alloy or of 690 nickel based Alloy with 58 Co oxygenation peak greater than 50 GBq/t after a full cycle Purification flow rate which is the VCT sprinkling flow rate is adapted to each design type units characteristics to easily manage a hypothetic early oxygenation. Three kinds of dilution can be used: nitrogen burping with high levels amplitude, continuous nitrogen flushing with an maximal VCT level and a flow rate monitors on gaseous wastes or discontinuous flushing with a maximal constant VCT level but pressure variation.

Chemical degassing, by its very rapid hydrogen removing, gives more choices for chemical conditioning for unplanned shutdowns. To limit the thermal stress corrosion of RHR, bubble collapsing is performed again at 130°C but with 10 to 35 cm 3 /kg of hydrogen to limit contamination risk (expected value: 15 cm 3 /kg).

VI. REFERENCES

[1] Pressurized Water Reactor Primary -Water Chemistry Guidelines -Volume 2 Revision 5 -EPRI, Palo Alto, CA. 12884.

[2] J-L. BRETELLE and alii, Study of various Chemical Species Behaviour for Contamination Risk -Water Chemistry in nuclear Reactor Systems -SFEN Avignon, France, April 2002.

[3] F. DACQUAIT and alii, Corrosion Products transfer in French PWRs Shutdown Systems -SFEN Avignon, France, April 2002.

[4] F. CARRETTE and alii, Impact of the Surface State of Steam Generator Tubes on the Release of Corrosion Products in Pressurized Water Reactors, Water Chemistry in nuclear Reactor Systems -Jeju Island, Korea, October 2006.

Fig. 1 .

 1 Fig. 1. Behaviour of fission products during chemical degassing and oxygenation on Civaux 2 NPP RF04 -August 2004

Fig. 6 .

 6 Fig.6. Evolution of VCT (Volume Control Tank) levels during early oxygenation management by N 2 burping

(

  Proceedings of the International Conference on Water Chemistry of Nuclear Reactors Systems, 23-26/10/06, Jeju Island (Korea), Session 2 Paper 9, 2006

F. Bardet, J.L. Bretelle, N. Perdrige, F. Dacquait, "Chemical degassing on EDF units -Feed back experience and method",

Fission gas behaviour during chemical degassing and oxygenation on Civaux 2 RF 04

	Fig. 2. Evolution of volumic activities during chemical
	degassing and oxygenation on Bugey 5 NPP
	900														Xe133
															(MBq/t)
	800														Xe135
	600 700	Chemical degassing at 17h40											I133 (MBq/t) I131 (MBq/t)
															(MBq/t)
	500
	400						Oxygenation on
							15/08/04 at 19h50
	300
	200
	100
	0
	15/08/04	15/08/04	15/08/04	15/08/04	15/08/04	15/08/04	15/08/04	16/08/04	16/08/04	16/08/04	16/08/04	16/08/04	16/08/04	16/08/04	16/08/04	16/08/04
	12:00	13:55	15:50	17:45	19:40	21:36	23:31	01:26	03:21	05:16	07:12	09:07	11:02	12:57	14:52	16:48

58Co oxygenation peak on units with SG tubes in 690 TT alloy

  . Evolution of58 Co refuelling oxygenation peaks on some 1300 MWe-NPPs with Alloy 600 SG tubes On units whose SG tubes are made of 690 nickel based alloy (SGR or original SGs),58 Co oxygenation peaks are mainly about 200 to 300 GBq/t during the first two or three outages, then they decrease and reach very low values like less than 5 GBq/t, as it is described by the figure 4. Smaller oxygenation peak means lower release of 690 SG tubes. Corrosion products quantities reacting during oxygenations are low on 690 SG tubes units whose58 Co oxygenation peak are smaller than 50 GBq/t after a full power cycle (cycle length equal or higher than nine months). [4] Fig.4. Evolution of 58 Co refuelling oxygenation peaks on some NPPs with Alloy 690 SG tubes Even if58 Co oxygenation peak can be very low, corrosion products available for reaction with oxygen or H 2 O 2 are still present and have influence on the oxygen value. As an example, 30 L of H 2 O 2 at 30% injected in a 1500 MWe-unit RCS at 80°C give about 10 mg/kg of dissolved oxygen. On those units, where58 Co oxygenation peak are very low (3 GBq/t at Civaux 2 NPP RF 05), the maximal oxygen value after H 2 O 2 injection is about 7 mg/kg (figure5), proving that corrosion products have reacted with H 2 O 2 . The oxygen saturation is reached by VCT venting.

	Volumic activities evolution during chemical degassing and oxygenation
			at Bugey 5
	1000000
			7h30 : oxygenation
		Chemical degassing
		2 injections of H2O2
	100000
	10000
			Gamma total (MBq/t)
			58Co (MBq/t)
	1000
	37725,5 37725,667 37725,833	37726	37726,167 37726,333 37726,5 37726,667 37726,833	37727
					300000
					250000											900 MWe-unit G
																900 MWe unit H
				Activité Co58 en MBq/t	100000 150000 200000											1500 MWe -unit I 1500 MWe-unit L
					50000
					0
					0	1	2	3	4	5	6	7	8	9	1 0	1 1	1 2	1 3	1 4
											cycle w ith 690TT SG tubes

58 Co oxygenation peak examples-1300 MWe-units 600 TTSG tubes

														A
														B
		350000												C
														D
		300000												E
	Volumic activity (MBq/t)	100000 150000 200000 250000												F
		50000
		0
	Fig. 5. Evolution of dissolved oxygen during oxygenation on Civaux 2 NPP RF 05	4	5	6	7	8	9	10 cycle	11	12	13	14	15	16
	As a conclusion, the quantities of solubilized species
	during chemical degassing are : • Smaller than those of the oxygenation, because
	available oxygen quantity for corrosion products is
	lower during chemical degassing, • A small part of the global species quantity, which
	can react with oxidizing agents. The main part is
	solubilized during oxygenation.
	Chemical degassing has a low influence on
	oxygenation peak values.


lmpact of Steam Generator primary circuit contamination products This paper analyses the impact of Steam Generator Replacement (SGR) on PWR primary circuit contamination by corrosion products. It presents a comparison of deposited and released activities between three new SG tube materials : alloys 600TT, 690TT and 800. This comparison allows to describe the typical evolutions of 58Co and óoCo contamination due to SGR.

The discussion highlights the importance of conosion product deposits on primary surfaces before SGR and the manufacturing process of SG tubes.

Introduction

During reactor maintenance, for most PWRs, the collectivi dose is mainly due to 58Co and 60Co. As shown in Figure l,58Co is predominant in dose rate for the f,rrst reactor cycles. Progressively, its contribution decreases giving way to óoco, which accumulates because of its radioactive half-life. Nevertheless for some reactors (circles in Figure l), after several cycles, the contribution of 58Co may reach the level of the first cycles.

Because of Inconel 600 Mill-Annealed tube corrosion problems (intergranular attack, stress corrosion cracking, ...), PWR steam generators have to be replaced. The new tube material can be Thermally-Treated Inconel 600 or 690, and Incoloy 800.

Regarding contamination, Inconel heat treatment should reduce the release [], alloy 690 high Chromium content (about 29Vo Cr, 59% Ni, l0% Fe) is meant to reduce generalized corrosion and alloy 800 low Nickel content (about 33% Ni, 2lo/o Cr, 44o/o Fe) should reduce the 58Co source.

The impact of these three tube materials on contamination is compared and typical evolutions of5tco and 60Co contamination due to SGR are described and analysed.

Deposited activities

To analyse SGR impact on contamination, deposited activities on out-of-flux primary surfaces have been measured, before and after SGR, for 13 PWRs in France and abroad : o l0 SGRs with alloy 690TT tubes which have undergone different manufacturing processes, r I SGR with alloy 600TT tubes, ¡2 SGRs with alloy 800 tubes. These PWRs have operated with coolant pH3s6.s of 7'0 or 7.2 and at base load or load follow.

The deposited activities have been measured on the hot legs, the crossover legs and the SG tube bundles by gamma spectrometry with the EMECC system [START_REF] Eimecke | Ensemble de Mesure et d'Etude[END_REF].

Because of their important contribution to dose rate in the PWRs concerned, only the two main cobalt isotopes, 58Co and óoco, are taken into account. Let us recall that ssCo (radioactive half-life: 7l days) comes from 58Ni (68% of natural Nickel) and óoco (radioactive half-life : 5.3 years) comes from t'Co 1100% of naturalCobalt).

58Co contamination Figure 2 and Figure 3 show the variation of the 58Co deposited activities on the primary coolant pipes (hot legs and crossover legs) for the three types of SGR. Whatever the new tube material is (alloys 600TT, 690TT or 800), the 58Co deposited activity on the primary piping increases for one or two cycles after SGR, and then decreases. Figure 4 shows the different hot leg contamination rises in PWRs that had a SGR with alloy 690TT. At the first cycle, after SGR with Inconel, the 58Co deposited activity on the SG tubes is generally lower than before SGR (* Figure 5). Then, it increases for one or two cycles and decreases. For the reactor < C > (SGR with alloy 800), after a rise at the first cycle after SGR, the 58Co deposited activity on the SG tubes slightly decreases on the cold side but highly drops on the hot side. Thus, at the sixth cycle after SGR, the cold side is six times more contaminated than the hot side. After a SGR with alloy 800, the oxide structure must be such as its solubility is very sensitive to temperature. It should be observed that there is practically no contamination difference between the two sides of lnconel SG As seen in Figure 6 (reactor ( G >l), after a SGR with alloy 690TT, the 58Co deposited activity on the tube bundles can reach a low level comparable to the one measured on the hot side of alloy 800 SG tubes. Thus, for a same SG tube material (alloy 690TT), the tube bundle contamination can be really different. The manufacturing process of the SG tubes could explain these differences. lndeed, e.g. for the reactor < A > before SGn 1e Figure 5), the alloy 600M4 SG tubes manufactured by < Y > are about four times more contaminated than those manufactured by < X ). It is important to recall that these SG tubes have undergone the same operating conditions (fluid chemistry, thermohydraulic, ...).

The impact of SGR on 5tco contamination distribution in the primary circuit is illustrated in Figure 7. This figure

shows that, because of SGR, there is a higher 58Co deposited activity increase on the primary coolant pipes than on the tube bundles. The 60Co deposited activities on the primary system are represented from Figure 8 to Figure l0 for the three types ofSGR.

Whatever the SG tube material is, since SGR the @Co contamination has decreased on the primary coolant piping (c' Figure I and Figure 9). For the reactor < A >, the @Co deposited activity tends to stabilize after 4 cycles of reduction. For the crossover legs of the reactor < C >, the decrease practically follows the radioactive decay. The 60Co contamination rises on the new tube bundles for the three types of tube material (* Figure l0) ; except on the SG hot side of the reactor ( C ) (SGR with alloy 800), where the 60Co deposited activity decreases after an increase. As well as for the 58Co contamination, the SG hot side is less contaminated by óoco than the cold side (three times less). It should be noted that the reactor < H > (alloy 690TT SG tubes) presents very low peaks and total released activities. It is not the result of the shutdown procedure but it is due to low nickel deposits on the fuel rods very likely thanks to lower temperatures of the fuel rod claddings. Figure 13 shows the dose rate around the primary coolant pipes for PWRs having undergone a SGR with Inconel. The dose rate increases generally for one or two cycles and then decreases because of the 58Co deposited activity variation.

The dose rate at the primary coolant pipes reflecting the combined effect of the radionuclides and especially the ambient dose rate [4], it can be stable after a SGR even if 58Co deposited activity on the primary coolant pipes rises (e.g.reactor(FD). Figure 12 shows the SGR impact on the 58Co concentration peak during refuelling shutdown. Let us recall that the 58Co peak comes from the dissolution of deposited Nickel (Ni0 or NiO) on the fuel rods because of primary system oxygenation [START_REF] Anthoni | Behaviour of the corrosion products during PWR shutdowns[END_REF].

The 58Co peak variation due to a SGR is similar to the 58Co deposited activity on the primary coolant piping : the 58Co peak rises generally for one or two cycles after SGR, and then decreases.

The lower peak of the reactor < C > is not due to the SGR with alloy 800 tubes (the peak was already low before the SGR even when the SG tube material was alloy 600M4) but because of the shutdown procedure. Indeed, the total Thanks to the low 60Co contamination on the SG tubes, the dose rate around the SG tube bundles is low for the first cycle after SGR (ø Figure l4). Because of SGR, Nickel release increases in the primary system up to three cycles. This rise is shown in an increase of the 58Co deposited activity on the primary coolant loops and an increase of 58Co activity released from fuel rods during shutdown with oxygenation. Then, the ssCo contamination decreases for several cycles and generally reaches lower levels than before SGR. These levels do not seem to mainly depend on alloy composition : oa SGR with alloy 600TT can lead to a ssCo contamination similar to a SGR with alloy 690TT'

.the 58Co deposited activity on the alloy 690TT SG tubes can be comparable to the one on the alloy 800 SG tubes, rthe decrease of ssco contamination, after a SGR with alloy 800, is less than expected; except on the hot side of primary coolant loóps. This relative high58Co contamination shows that there is still a large source of Nickel after a SGR with alloY 800. After a SGR, the ssCo variations could be due to : oa release and a trapping of SG tubes depending on the manufacturing process ofnew SG tubes Il] [5]' ¡a higher release of SG tubes during formation of the inner protective oxide layer, aa ( memory effect > due to Nickel deposits in the primary system before SGR, this effect decreasing with this Nickel reserve. In any case, the contamination levels reached after SGR are not due to reactor coolant chemistry as long as pH3es's lies between 7.0 and 7.4. As there is no correlation with reactor operation (load follow or base load).

As indicated in Figure 17, it should be noted the similitude of the 58Co total deposited activity evolutions before and after SGR : only the levels are different. Figure 15 and Figure 16 show the 58Co contribution to the dose rate in the vicinity of the crossover legs and SGs, respectively.

Just before SGR, óoCo contributes to 70o/o'80o/o of the dose rate at primary system. Because of SGR, the ssCo contribution increases and generally becomes the major one. The 58Co contribution reaches the levels preceding SGR with alloy 800 much faster than SGR with lnconel.

.+A / 690TT -ae B / 600TT -*-C / 800 I Þt -2sGR246

CYclesfrom SGR Co deposited activity on the new SG tubes increases for several cycles but by a lower rate than before SGR, as indicated in Figure 18. This is due to a lower cobalt content of the new SG tubes and to the improvement of their manufacturing process. By the way, it means that SG tubes are the main source of cobalt as long as there is no excessive wear of stellite hard-facing material.

Thanks to this lower cobalt release, 60 Co deposited activity on the primary coolant pipes decreases. But because of the long half-life of 60 Co, the contamination levels after SGR mainly result from the state before SGR SGR leads to an activity distribution, on the one hand, from the SG tubes to the primary piping, and on the other hand, from 60 Co to 58 Co. Thus generally, the 58 Co contribution becomes preponderant for some cycles, giving way to 60 Co, which accumulates because of its halflife.

Conclusion

Steam Generator Replacement influences level, distribution and contribution to the dose rate of 58 Co and 60 Co in PWR primary circuit. These impacts vary for the first cycles after SGR. They are dependent on contamination levels before SGR, material and manufacturing process of new SG tubes, primary system parts and radionuclides taken into account, if the other parameters are constant.

This study can be summarized as follows : corrosion product deposits in primary system before SGR and manufacturing process of new SG tubes seem to be the two main key factors for distribution and levels of contamination after SGR. Although tube material (alloys 600, 690, 800) is important, it seems to be of second order, as reactor coolant chemistry as long as pH 300°C lies between 7.0 and 7. [START_REF] Beslu | Trend of plant radiation fields of French reactors : analysis and perspectives[END_REF] The first key factor applies especially to 60 Co because of its radioactive half-life. SGR with alloy 600TT can lead to a contamination comparable to the one due to SGR with alloy 690TT. And contamination after a SGR with alloy 690TT can be similar to the one after a SGR with alloy 800, alloy 800 bringing not all the expected profit.

Generally after several cycles, a SGR enables to reach contamination levels lower than before SGR.

Figure 2 Figure 4 :

 24 Figure 2 : 58Co deposited activities on the hot legs

Figure 5 :Figure 6 :

 56 Figure 5 : 58Co deposited activities on the SG tubes

Figure 7 :

 7 Figure 7 : Ratio primary pipes/SGs for 58Co

  Figure I : 60Co deposited activities on the hot legs I

  Figure l0 :60Co deposited activity on the SG tubes

Figure I 1

 1 Figure I 1 gives the variation of the 60Co contamination distribution in the primary circuit because of SGR. The increase of the ratio < primary coolant pipes/SG tubes > is due to the very low level of the 60Co deposited activity on the new SG tubes.

  Figure I I : Ratio primary pipes/Scs for 60Co

  Figure l3 : Dose rate at the reactor coolant pipes

Figure 16 :

 16 Figure 16 : 58Co contribution to the dose rate at the SG

Figure l4 :

 l4 Figure l4 : Dose rate at the SG tube bundles 5u Co and o" C o C ontr ibut ion

Figure l5 :Figure 17 :

 l517 Figure l5 : 58Co contribution to the dose rate at the crossover legs

Figure 18 :

 18 Figure 18 : 60 Co total deposited activity on the out-of-flux surfaces of primary circuit

  The reactor

	Replacement	ON PWR
	by radioactive	corros¡on
	F. Dacquait and S.

  Anthoni, CEA, Centre d'Etudes de Cadarache, France P. Ridoux, EDF, SEPTEN, France C. Brun and N. Engler, Framatome, France

	< A > is representative of PWRs that have undergone a
	SGR with alloy 690TT.
	50
		+
	3¿o o o CI o tt	+	Boundaries 50%-50% for dose rates
	È30 o o Ero o ü10		+	+	8 3,gE; t I I	o o
	0	rl
	0		246810'12		14 16
						2000	53

Gycle

Figure I : Ratio ttcofoco for deposits This paper will show that this increase is the result of Steam Generator Replacement (SGR).

Water Chemistry of Nuclear Reactor Sysfems 8. BNES,

  tubes. If there is any difference, it is lower and the hot side is generally more

	contaminated,
	+A	/ 690TT
	-o-B / 600TT l+-Cl 800
	Cold side
	side
	-2SGR2468
	Cycles from SGR