COMPARISON OF THE EMBRITTLEMENT METHODS FOR METAL OF REACTOR PRESSURE VESSELS

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1 ISSN ND days 17 / Дни на безразрушителния контрол 17 Year /Година XXV Nuber/ Брой 1 (16) June/Юни 17 COMPARISON O HE EMBRILEMEN MEHODS OR MEAL O REACOR PRESSURE VESSELS СРАВНЕНИЕ НА МЕТОДИТЕ ЗА ОКРЕХКОСТЯВАНЕ НА МЕТАЛА НА КОРПУСИТЕ НА РЕАКТОРИ Galia odorova Diova, е-ail: gdiova@npp.bg, NPP ozloduj Bulgaria Abstract: he nuclear aterials of Reactor Pressure Vessel (RPV) in Nuclear Power Plant (NPP) are subitted to nigh teperature and pressure. A neutron induced ebrittleent and theral ageing of RPV aterials are going. Scoping safety exploitation of NPP the ebrittleent process ust be study. he irradiation causes a shift of the initial critical brittleness teperature to the higher values ( ). here are two ways for receiving the critical teperature shift nuerical and experiental. he scope of current work is to be ade analysis of four nuerical ethods. he theoretical results are copared with data fro surveillance speciens. It has a good correlation between calculated data (by s and ) and experiental one. he study is intended to assist in the developent of the process of ebrittleent of the RPV aterials. eywords: Reactor Pressure Vessel (RPV), Nuclear Power Plant (NPP), ebrittleent process, irradiations, critical brittleness teperature 1. Introduction Reactor pressure vessel (RPV) beltline aterials are exposed to a high-energy neutron flux which results in ebrittleent and hardening. he change of properties is increase in yield strength, increase in brittle-ductile transition teperature deterined fro Charpy V-notch ipact or fracture toughness tests, decrease in upper shelf energy, tensile elongation and reduction of area. Scoping safety exploitation of NPP the ebrittleent process ust be study. he irradiation causes a shift of the initial critical brittleness teperature to the higher values ( ). he assessent of the critical teperature of RPV aterials and analysis of luences factors have iportant significance for safety work of NPP. here are two ways for receiving the critical teperature shift by nuerical and by experiental ethods. Nuerical ethods are developed based of the results and analysis fro RPV surveillance speciens [1,,, 4]. Neutron ebrittleent process of RPV aterials depends on the etal cheical coposition, the neutron dose and the operation teperature. he icrostructural testing proves a foration of Cu-Ni-Si-Mn-P clusters [5]. here can be seen Cu atos are at the center of clusters, Ni, Mn and Si atos are around the Cu core and P atos are at the periphery. RPV aterials which is used in reactors type VVER 1 are ferrit-perlit steels. Steel 15hHMAA and its welds are characterized by lower level of content of P and Cu and higher concentration of content of Ni in coparison to 15hMA steel and its welds. 55 he high-energy neutron flux causes a process of neutron and theral ebrittleent of RPV etal []. he theral ebrittleent depends on the coposition of aterials, operation tie (worked off tie) and teperature. he value of can be obtained by nuerical ethods using epirical forulae [1,,, 4], or experientally by ipact three point bending test [7, 8, 9] of unirradiated and irradiated surveillance Charpy speciens. he scope of this work is to ade a coparison between ebrettleent ethods. he results shows the ebrittleent process is intensive in case of calculations by 1, following fro these by s, 4 (in this succession) for base etal. or weld etal the ebrittleent process is ore intensive in case of calculation by 1, following fro the results according s, and 4 (in this succession). he ebrittleent rate is ore intensive for weld etal in coparison with base etal, but in the beginning of the unit s exploitation the speed of ageing (by ) for base etal is higher than weld etal. he study is intended to assist in the developent of the ebrittleent process of the RPV aterials.. ology our different coputing ethods are used for calculation [1,,, 4] and the results are copared with the data for which is received by experiental way.

2 Coputing ethods s 1,,, 4 are applied for calculation for base etal and weld etal for RPV type VVER 1 (RPVa and RPVb). hese ethods includes the luence of neutron dose and cheical eleent (Ni, Si, Mn, P, Cu), iportant for the ageing process. Only one ethod (ethod []) interprets difference between effects fro neutron and theral ageing. he following designation of paraeters are accepted in forulae used for calculation: - - is the initial critical teperature of etal, [ C ]; is critical teperature of etal after reactor operation [ C ]; - ( ) is the shift of the critical brittleness teperature due to theral ageing, [ C ]; - ( teperature due to theral ageing, [ C ] ; - t is work off tie [hours]; - (N) is the shift of the critical brittleness is the shift of the critical brittleness teperature due to cycle load, [ C ]; - N is nuber of cycles; - is the shift of the critical brittleness teperature due to neutron fluence, [ C ]; - is the neutron fluence with energy of neutrons n greater than,5mev at RPV, [ ] - 1 n / is a standardized coefficient; - A is the irradiation ebrittleent coefficient, [ C ]; - ω double standard deviation of ; - t - is the shift of the critical brittleness teperature when t ; - t O, t, b constants of aterials; - Ni, Mn, Cu и P is the concentration of cheical eleents in the coposition of aterial, [weighing units]; n D 7 is a standardized coefficient. -.1,[ ] [1] his coputing ethod is included in Russian docuent PNAEG [1] for calculation of of RPV base and weld etal during operation. It is derived upper 95 % boundary curve of experiental data, available at that tie. ollowing relations are applicable for reactors VVER 1: ( ) ( N) (1) It is accepted that and N and in this case 1 A. () A C for base etal; A C for weld etal he values of ( ) based on 1 includes a cuulative effect fro both neutron and theral ebrittleent of RPV etals..1. [] he is developed by CNII Proetey, RNC urchatovskij Institute, OB Gidropress and it is recoended by International Atoic Energy Agency []. his ethod is based on the investigation of the luence of high Ni content on RPV etal. It was established a process of theral ageing of RPV aterials is going during unit operation. he following ethod for calculation shift is proposed for reactors VVER 1 []. (, ( + ω A (4) for base etal,8; A 1,45 C for weld etal,8; A α.exp( α. C );[ ] () 1 eq C C eq Ni + Mn α. Si if Ni + Mn α. Si orc if Ni + Mn α. Si eq < α 1.7; α.88 ; α. 885 ( t + b able 1. he values of t t exp th to Material [ C] b [ ] C Base etal Weld etal, Ni>1,% Weld etal, Ni<1,%, b, t t t O (5) O of RPV etal suggests a separated assessent of the shift of critical teperature due to neutron fluence (a neutron par and due to theral ageing (a theral par. he values of fluence and the values of weight percents of the eleents nickel Ni, anganese Mn, silicon Si are iportant factors for neutron ebrittleent. he coposition of the aterials, teperature and worked off tie are iportant factors for the theral ebrittleent..1. [] t [ hours] O 18 6, ,1 18 6, 7 56

3 his ethod is developed trough any testing of surveillance speciens, providing by RNC urchatovskij Institute []. or first tie is taken into account the luence of the eleents Ni, Mn, Si on the ebrittleent rate. he ethod is published before establishent of the presence theral effect. suggests ( ) includes a total effect of neutron and theral ebrittleent. he ethod is not accepted as a standard; calculation of the shift of the critical teperature is referring for weld etal only. ( ) 1,5,7 ( ),5. Ni. Mn.(,64 Si). (7) [4] his ethod is developed by OA Shopen National Science Center harkov Institute of Physics and echnology, harkov, Ukraine (1997) for prediction of ebrittleen rate of RPV etals [4]. he is not standardized. here is taken into account the luence of the eleents nickel Ni, cupru Cu and phosphor P on the ebrittleent rate. 4 suggests includes both an neutron and a theral part of the ( ) ebrittleent. ( ),5 [. ( Ni). Cu + 4. P + 9. Ni]. D + 8 (9) 1+ D (Experiental ) he testing of surveillance speciens (SS) was copleted, in the frae of other project. here was applying the three point ipact bending testing of Charpy [7, 8, 9]. he obtained results for the shift of the critical teperature are used for the current investigation [1 1]. 1 (6) (8). Input paraeters he calculation of the neutron and theral ebrittleent rate was perfored for base etal BM (15hHMAA steel) and weld etal WM (SV 1ChNMA-A) of two RPV type VVER1 (a and b). he values of the neutron fluence used for calculations are in the range 18 n 18 n his values of the fluence and the operation tie t corresponds to the ean values for one fuel cycle of reactor type VVER 1, as follow: or base etal: 18 n,8.1 (for one fuel cycle); or weld etal: 18 n,79.1 ; ie t 7hours ; he neutron energy of the fluence is accepted E >, 5MeV.. Discussion 57.1 s 1, 5 he rate curves of the shift of the critical teperature ( ) by 1 for base etal (BM) and weld etal (WM) are shown on igure 1. he ean prognostic curve based on experiental data are presented on the sae figure. k, [ºC] k, [ºC] Neutron fluence. 1 n/ Neutron fluence. 1 n/ ig. 1. Dependency of ( ) on fluence for а) BM and b) WM, calculated by 1. he ean lines fro experiental data. he ebrittleent rate of BM is higher than WM rate due to the different cheical coefficients A. he trend curves based on the calculated data for base and weld etal are disposed vastly above the experiental curves. In the case 1 gives very conservative values of ( ) assessent of ebrittleent rate up to fluence n.1.. s, 5 he shifts of critical teperature due to the neutron fluence, by for both base and weld etal are presented on igure. he values of for both base and weld etal exponentially increases with the growth of the fluence. he trend curves ) for base etal (BMa and ) ( BMa,, SS_ SS_ coincides, because of A doesn t depend the etal cheical coposition for base etal. It can be seen the difference of the speed of neutron ebrittleent rate for weld etal ( and ). he ebrittleent rate is going faster for. or RPVb - the trend curves for base etal and weld etal coincides alost.

4 k, [ºC] ig.. Dependency of ( ) WM, calculated by, or a neutron inducted part of on fluence for BM and (, he shift of the critical teperature due to theral ebrittleent ( by for base etal and weld etal (BMa,,, ) are presented on igure. k(, [ºC] ig.. Dependency of ( part of on tie t (or theral inducted ) for BM and WM, calculated by. he theral inducted ( trend curves for base etal (BMa, ) prevails the trend curve for weld etal (, ). It can be seen a pick of values of the theral ebrittleent in a period years 5, after the theral ebrittleent rate is quickly reducing. After first years 1 11 the function ( has alost constant values during period of exploatation of NPP, for both base and weld etal. he su of the shift of critical teperature due to neutron and due to theral ebrittleent: ( Neutron fluence. 1 n/ for both base and weld etal are presented on igure 4. he theral inducted ebrittleent process prevails for a period of the first years 1 of NPP s work, according igure, after that prevails the neutron inducted ebrittleent process, igure 4. In the begging of NPP exploitation the ebrittleent rate of base etal BMa, prevails that of the weld etal,. After the values of for base etal are reducing slowly, the values of for weld etal growth. Тhe values of for quickly growths too. he speed of the neutron ebrittleent rate for is higher than BMa, and. BMa BMa,B Mb, ie t, [years] 58 k(,, [ºC] ig. 4 - Dependency of on worked off tie t for BM and WM, calculated by ethod. А coparison between the values of the shift of the critical teperature based on calculation by and experiental data fro surveillance speciens SS is ade. he results are shown on the igure 5a (BM) and igure 5b (WM). or base etal calculated results by are higher than the experiental one. he trend curves is prevailing the values fro experiental SS BMa data, in opposite to the case of base etal. or weld etal - the trend curve, based on the calculations by is high and quickly growths. for coincides (alos with he trend curve the ean lines fro experiental data SS_. k(,, [ºC] k(,, [ºC] ie t, [years] ie t, [years] ie t, [years] ig. 5 - Dependency of BMa BMa SS_ SS_ on worked tie t for a) BM and b) WM, calculated by Мethod. he ean lines fro experiental data fro surveillance speciens SS for a)bm and b)wm.

5 .. s, 5 A coparison between the calculated data by and the experiental data for the shift of critical teperature is ade, (igure 6). ( ) ig. 6. Dependency of ( ) on fluence for WM, calculated by. Experiental data fro surveillance speciens SS, ean curves. It can be seen on igure 6 the values of the shift of ( ) for is very high and prevails that of the surveillance speciens. he trend curve of the shift of ( ) for coincides (is very close) with ean curve fro the experiental data SS_. he results shows a good correlation of the calculated and the experiental data for. But for calculated values are higher than experiental one..4 s 4,5 he function ( ) on fluence, calculated by ethod 4 is presented on igure 7, as well experiental data fro surveillance speciens SS; for a) the base etal and for b) the weld etal. he results fro igure 7 shows the trends curve of the shift ( ), calculated by 4, coincides for BMa and ; as well for and. he assessent of the shift ( ) fro experiental data for base etal (, ) and weld etal (SS_, SS_) are higher than calculated data. k, [ºC] k, [ºC] Neutron fluence. 1 n/ SS_ Neutron fluence. 1 n/ BMa, k, [ºC] on fluence, calculated by ethod 4 for a)bm and b)wm. Experiental data fro surveillance speciens SS and ean curves. ig. 7. Dependency of ( ).5 s 1-5 he prognostic curve for the shift of critical teperature ( ) of the ebrittleent during unit operation, calculated according different ethods 1-4 are copared on igure 8 for a)base etal and for b) weld etal. he result fro the surveillance speciens SS, according 5 are shown on the sae figure. or base etal BM (igure 8a): Тhe values of the shift of the critical teperature ( ) are biggest by 1, followed fro these by ethods and 4 (in this order). or the first 1 years of NPP s work it can be seen a good correlation between 1) the data for the shift ( ) for BMa,, calculated according ethod 4 and ) the experiental data. It is observing the trend curve ( ) for BMa and, based on calculations by, is closely to the ean curve for. or weld etal (igure 8b): he ebrittleent rate of the curve ( ) is ost intensive for (by ). here is a very iportant exception ( ) calculated by for has very high values, prevailing even ( ), calculated by 1. he ebrittleent rate is intensive for, (calculated by 1), followed by the ebrittleent rate for, (by ), followed - for (by ) and finally the ebrittleent rate is less for, (by 4). ro igures 8a, 8b it can be seen the different speed of the ebrittleent process for base etal and weld etal. Calculated and experiental results defines the ebrittleent rate is ore intensive for weld etal in coparison with base etal. Coparatively the other ethods, 1 defines quickly growth of the rate of the ebrittleent process, (ig. 8a, 8b). It can be seen the values of the calculated data of the shift for (by both s and ), are placed ( ) Neutron fluence. 1 n/ closely to the experiental data., SS_ SS_ 59

6 k, [ºC] k, [ºC] Neutron fluence. 1 n/ Neutron fluence. 1 n/ ig.8. he prognostic curves for the shift ( ) according ethods 1-4 for a) BM and for b) WM. Experiental data fro surveillance speciens SS and ean curves., calculated 4. Conclusions here are presented 4 ethods for calculation of the shift. he trend curves of the critical brittleness teperature ( ) of the calculated ( ) 1,,,,,, 4, 4, 1, BMa,, 4, BMa,, BMa were copared with the experiental data fro surveillance speciens ( 5). 1 [1], according PNAEG 7--86, interprets iportant factors for the ebrittleent process the values of neutron fluence and type of etal (base or weld). he shift of the critical teperature ( ) calculated by 1 prevails results, calculated by the other ethods, as well as the experiental data. 1 is quite conservative for calculations of the ebrittleent rate due to irradiation of the etals. he reason is the approach for assessent of values of A doesn t depend fro cheical content of the etal. he factors, iportant for the ebrittleent, according [] are neutron fluence, weight percents of the eleents Ni, Mn, Si and worked off tie t of the nuclear aterials. [] defines a different parts of the neutron and theral ebrittleent. At the beginning of the exploitation of nuclear power plant the theral ebrittleent of BM and WM prevails, after that the theral ebrittleent decreases. After a period fro years 1 11 theral inducted part of ebrittleent process has alost constant values. he function - or neutron inducted part - growths exponential all the tie during the exploitation of the NPP s unit. In generally it can be say the 6 theral inducted ebrittleent process prevails for a period of the first 1 years of NPP work, after that prevails the neutron inducted ebrittleent. defines the luence of neutron fluence and weight percents of the eleents Ni, Mn, Si for the ebrittleent rate. gives coparatively low speed of the ebrittleent process. he assessent of the ebrittleent rate, calculated by 4, depends on the luence of neutron fluence and weight percents of the eleents Ni, Cu and P. 4 defines a lowest speed of the ebrittleent process, in coparison with other ethods. As copare the trend curves of based on all ethods for base etal, and the ean curves fro experiental data, it can be seen the function has biggest value and growth in case of calculation by 1, following fro the results according s, 4 (in this succession). or weld etal the function has biggest value and growth in case of calculation by 1, following fro the results according s, and 4 (in this succession). he ebrittleent rate is ore intensive for weld etal in coparison with base etal, but in the beginning of the unit s exploitation the speed of ageing (by ) for base etal is higher than of weld etal. 5. References [1] PNAEG Strain nors for equipent, type VVER, Moskow,1986; 86.pdf; Appendix [] Unified procedure for integrity and lifetie assessent of coponents and piping in WWER NPPs during operation Verlife, IEAE, 1; Nulife 14, Project RER 4; 4/7/47847.pdf#search%Verlife%procedures%; [] Radiation ebrittleent of VVER-1 RPV steels, Nikolaev Yuri A.,18 th International Conference on Structural Mechanics in Reactor echnology, China, 5. [4] Investigation of the kinetic of degradation of properties of welds and developent of odels for lifetie assessent of welds of RPV, type WWER in Ukraine, Mahnenko W.I., asatkin O.G., Welding on the hreshold of XXI century, Bulgaria, [5] Understanding on the Mechaniss Irradiation Ebrittleent of RPV steels and developeent of Ebrittleent Correlation, IAEA raining Workshop on Assessent of degradation Mechaniss of Priary Coponents in Water Cooled Nuclear Reactors, Akiyoshi Nooto, Madrid, Spain, 14. [6] Report for definition of echanical characteristic and structural condition of set with surveillance speciens, BAN, Bulgaria, 1995; [7] GOS : Welds. s for definition of echanical properties; ISO/C 164/SC 4/N 191; gostexpert.ru/gost/getdoc/98 [8] ISO/CD Steel Charpy V Pendulu ipact test Instruented test ethod; Standards Wordwide [9] ASM E -9 Standard est s for Notched Bar Ipact esting of Metallic Materials [1] ology for deterination of the RPV surfaces neutron flux (outside), type WWER 1; RB-18-1, GosAtoNadzor, Moscow, 1 [11] ology for calculations of the neutron flux, with energy over,5 MeV on the position both of the detectors and the surveillance speciens

7 [1] Calculations and Dosietry s for deterining Pressure Vessel Neutron luence, Draft Regulatory Guide DG- 15; U.S. Nuclear Regulatory Coission, 199 [1] Coputing Radiation Dose to Reactor Pressure Vessel and Internals, state-of-the-art Report, NEA/NSC/DOC (95)5, Nuclear Energy Agency,