Flaw indications in the reactor pressure vessels of Doel 3 and Tihange 2

Transcription

1 Flaw indicatins in the reactr pressure vessels f Del 3 and Tihange 2 1. Cntext Technical infrmatin nte Del 3 and Tihange 2 are tw f the seven Belgian nuclear reactrs perated by Electrabel, a GDF-SUEZ Grup cmpany. In June 2012, during a new type f in-service inspectin cnducted fr the first time in Belgium, several thusands f flaw indicatins were detected in the base metal f the Del 3 reactr pressure vessel, lcated mainly in the upper and lwer cre shells. As a precautin, similar inspectins were cnducted in September 2012 n the Tihange 2 unit, whse reactr pressure vessel is f identical design and cnstructin. Flaw indicatins were detected as well, but t a lesser extent. The pressure vessel is a key-cmpnent in a reactr unit, and its failure is nt cvered by safety studies. As a result, the licensee decided t keep bth units in cld shutdwn state, cre unladed, at least until in-depth analyses have been achieved and submitted t the Federal Agency fr Nuclear Cntrl (FANC) in view f a pssible restart f the peratin. With the supprt f internal and external experts, the licensee started an investigatin f the precise nature and rigin f these indicatins in the summer f 2012, and built its wn analysis t determine whether r nt the reactr units in questin culd safely resume peratin in spite f the detected flaws. The demnstratin f the licensee was recrded in tw safety case reprts and backed by a number f technical dcuments, leading the licensee t cnclude that bth Del 3 and Tihange 2 reactr units were eligible fr immediate restart. In parallel, the licensee als prpsed several additinal measures designed t further increase the safe peratin f the units, t mnitr the pressure vessels state alng time r t extend its initial material testing prgram. Meanwhile, the FANC built up a dedicated rganisatin and cmmissined several natinal and internatinal expert grups t seek scientific and technical advice in rder t elabrate an independent, funded and balanced judgement abut the issue. Alng the assessment prcess, the expert grups raised a number f questins that were discussed with the licensee and its technical supprts. Frm thse discussins, a number f pen issues were raised abut the manufacturing f the reactr pressure vessels, the suitability f the in-service inspectin technique, the pssible evlutin f the flaws during future peratin, the characterizatin f the material prperties, and the structural integrity f the reactr pressure vessels under penalizing ladings. 2. FANC Findings Based n the data prvided by the licensee and the cnclusins released by Bel V, AIB- Vinçtte and the natinal and internatinal experts grups abut the flaws f the Del 3 and Tihange 2 reactr pressure vessels, the Federal Agency fr Nuclear Cntrl draws the fllwing glbal cnclusins fr each tpic: 1

2 Regarding the manufacturing f the reactr pressure vessels: Based n the sle manufacturing files, the presence f flaw indicatins since the manufacturing stage cannt be cnfirmed as the indicatins which were detectable at this stage were nt reprted in the final inspectin reprts f the manufacturing f the reactr pressure vessels. Regarding the in-service inspectins: Sme uncertainty still exists regarding the capability f the in-service inspectin techniques t prperly detect and characterize all present flaws in the reactr pressure vessels. Regarding the metallurgical rigin and evlutin f the indicatins: The mst likely rigin f the indicatins identified in the Del 3 and Tihange 2 reactr pressure vessels is hydrgen flaking due t the manufacturing prcess. Significant evlutin ver time f hydrgen flakes due t the peratin f the reactr units is unlikely. Regarding the material prperties: Mre experimental data n tensile and tughness prperties f the materials are needed t validate the apprach fllwed in the structural integrity assessment. Regarding the structural integrity f the reactr pressure vessels: The apprach adpted by the licensee t justify the structural integrity f the reactr pressure vessels still needs t be cmpleted r validated fr sme tpics. The prbabilistic assessment apprach prvided by the licensee is used nly fr infrmatin. Regarding the additinal measures and actins prpsed by the licensee: The additinal peratinal measures prpsed by the licensee are relevant. The in-service inspectin prgram prpsed by the licensee shuld fcus particular attentin n the mst adverse flaws. Sme uncertainties still remain in the structural integrity assessment and call fr additinal experimental verificatin. In the current state f knwledge and given the available data, the pen issues identified alng the assessment prcess d nt represent cnditins that require a definitive shutdwn f the Del 3 and Tihange 2 reactr units. Hwever, these pen issues lead t sme uncertainties that might reduce the cnservatism f the licensee s safety demnstratin and hence impair the level f cnfidence in the safe perability f the reactr units in questin. As a cnsequence, the Federal Agency fr Nuclear Cntrl cnsiders that, in the current state, the Del 3 and Tihange 2 reactr units may nly restart after the requirements listed in the next paragraph 3 hereafter are fulfilled by the licensee. The licensee shall elabrate an actin plan t meet thse requirements, including a methdlgy and assciated acceptance criteria where applicable. This actin plan shall be apprved by the Belgian nuclear safety authrity. Once the licensee has implemented its actin plan, the FANC, tgether with Bel V and AIB-Vinçtte, will evaluate whether all the safety cncerns at the rigin f the requirements are slved and whether the related reservatins can be lifted. On this basis, the FANC will mtivate its decisin abut the pssible restart f the Del 3 and Tihange 2 reactr units in a final evaluatin reprt. 2

3 This psitin applies nly t the Del 3 and Tihange 2 reactr units and des nt extend t ther nuclear reactrs ptentially cncerned elsewhere in the wrld. The evaluatin f their safety remains within the jurisdictin f the cmpetent natinal authrities. 3. FANC requirements The suggestins, bservatins and cnclusins f these different rganisatins and wrking grups were evaluated by the Federal Agency fr Nuclear Cntrl. Wherever apprpriate and relevant, the FANC decided t use this input in the frmulatin f the specific requirements fr the licensee. The FANC issues the fllwing requirements fr each tpic. Regarding the manufacturing f the reactr pressure vessels: Given that the whle dcumentatin currently available was explited and n additinal finding can be derived frm that material, the FANC issues n further requirement n this tpic. Regarding the in-service inspectins 1 : As a prerequisite t the restart f bth reactr units, the shrt-term requirements n inspectins mentined in the AIB-Vinçtte assessment shall be fulfilled by the licensee: The licensee shall re-analyze the EAR acquisitin data fr Tihange 2 in the depth range frm 0 t 15 mm in the znes with hydrgen flakes t cnfirm whether r nt sme f these technlgical cladding defects have t be cnsidered as hydrgen flakes. The licensee shall demnstrate that n critical hydrgen flake type defects are expected in the nn-inspectable areas. The licensee shall demnstrate that the applied ultrasnic testing prcedure allws the detectin f the higher tilt defects in the Del 3/Tihange 2 data (2012 inspectins) with a high level f cnfidence. The licensee shall present the detailed reprt f all macrgraphical examinatins including the sample with the 45 T reflectins and shall als analyze and reprt additinal samples with 45 T reflectivity. The licensee shall include a set f defects partially hidden by ther defects fr macrgraphic examinatin, t cnfirm whether the sizing methd cntinues t functin well. The licensee shall re-analyze the tilts f the defects in the VB-395/1 blck with the same methd as applied n-site. As sn as pssible after the restart f bth reactr units: The licensee shall achieve a full qualificatin prgram t demnstrate the suitability f the in-service inspectin technique fr the present case. The qualificatin shall give sufficient cnfidence in the accuracy f the results with respect t the number and features (lcatin, size, rientatin ) f the flaw indicatins. Where apprpriate, the prcess shall be substantiated by apprpriate 1 The FANC wuld like t stress that the requirements related t the verificatin f the nn-destructive examinatin prcedure and the review and fllw-up f their qualificatin prgram is the respnsibility f AIB-Vinçtte, which is the authrized inspectin agency in Belgium.. 3

4 experimental data using representative specimens. The full qualificatin prgram shall be achieved befre the next planned utage fr refuelling. Regarding the metallurgical rigin and evlutin f the indicatins: After the restart f bth reactr units: The licensee shall perfrm fllw-up in-service inspectins during the next planned utage fr refuelling t ensure that n evlutin f the flaw indicatins has ccurred during peratin. Regarding the material prperties: The licensee shall cmplete the material testing prgram using samples with macr-segregatins cntaining hydrgen flakes. This experimental prgram shall include: small-scale specimen tests: lcal tughness tests at hydrgen flake crack tip, lcal tensile tests n ligament material near the flakes; large-scale (tensile) specimen tests. The licensee shall perfrm additinal measurements f the current residual hydrgen cntent in specimens with hydrgen flakes, in rder t cnfirm the results f the limited number f tests achieved s far. Fr example, the licensee has estimated an upper bund n the amunt f residual hydrgen that might still be present in the flaws. The licensee shall demnstrate that the chsen material prperties are still valid, even if the upper bund quantity f hydrgen wuld still be present in critical flaws. As sn as pssible after the restart f bth reactr units: A further experimental prgram t study the material prperties f irradiated specimens cntaining hydrgen flakes shall be elabrated by the licensee. The licensee shall further investigate experimentally the lcal (micr-scale) material prperties f specimens with macr-segregatins, ghst lines and hydrgen flakes (fr example lcal chemical cmpsitin). Depending n these results, the effect f the cmpsitin n the lcal mechanical prperties (i.e. fracture tughness) shall be quantified. The licensee shall further evaluate the effect f thermal ageing f the zne with macr-segregatin. Regarding the structural integrity f the reactr pressure vessels: Taking int accunt the results f the actins related t the previus requirement n the detectin f the higher tilt defects during in-serviceinspectins, the licensee shall evaluate the impact f the pssible nn-reprting f flaws with higher tilts n the results f the structural integrity assessment. The licensee shall cmplete the n-ging material testing prgram by testing larger specimens cntaining hydrgen flakes, with the fllwing 2 bjectives: Objective 1 : Tensile tests n samples with (inclined) multiple hydrgen flake defects, which shall in particular demnstrate that the material has sufficient ductility and lad bearing capacity, and that there is n premature brittle fracture. 4

5 Objective 2 : An experimental cnfirmatin f the suitability and cnservatism f the 3D finite elements analysis. Regarding the actin plan prpsed by the licensee: In additin t the actins prpsed by the licensee and the additinal requirements specified by the FANC in the previus sectins, the licensee shall perfrm a lad test f bth reactr pressure vessels. The bjective f the test is nt t validate the analytical demnstratin n the reactr pressure vessel itself but t demnstrate that n unexpected cnditin is present in the reactr pressure vessels. The methdlgy and assciated tests (acustic emissin and ultrasnic testing...) will be defined by the licensee and submitted t the nuclear safety authrity fr apprval. The acceptance criterin will be that n crack initiatin and n crack prpagatin are recrded under the pressure lading. 5