EnBG . Energiemanagement für KMU Risk of Uncontrollable Re-Criticality due to a Steam Generator Tube Rupture NURIS 2015, 1st International Conference On Nuclear Risk, Viena 17. April 2015 Dipl.-Ing. Helmut J. L. Mayer, EnBG Energiebüro Gorxheimertal Photo: Südhessen Morgen 17. April 2015, NURIS 1 Content EnBG . Energiemanagement für KMU Purpose: Publish the risk of non-borated water inside the reactor core 1. Steam generator tube rupture (SGTR) according to the emergency operation procedures probability of occurrence procedure 2. SGTR and shutdown of main cooling pumps (MCP) probability of occurrence procedure 3. Scientific investigations and main results (BMUB, GRS, RSK, AST, ISR, ATHLET-analyses, ROCOM-tests) 4. Summary and conclusions 17. April 2015, NURIS 2 Probability of SGTR EnBG . Energiemanagement für KMU Probability of SGTR 3,000 to 16,000 tubes in one steam generator (SG), 2 to 4 SGs in one pressurized water reactor (PWR) End of 2012: worldwide 366 PWRs in operation (Reference: ATW Kernenergie-Weltreport-2012) Very high stresses and strains, high pressure, temperature, transients, bending load and so on … Significant tube thinning (Reference: RSK) Stress corrosion cracking Wikipedia (Reference: RSK) Multiple ruptures have to be assumed (Reference: RSK) Highest probability of all loss of coolant accidents 17. April 2015, NURIS 3 SGTR according to emergency operating procedures (example) EnBG . Energiemanagement für KMU Damaged SG Intact SGs Quelle: GRS, Internet Procedure 1. 2. 3. 4. Emergency plant shut down Secondary pressure rises to 72 bar Primary pressure is sprayed down to 80 bar The damaged SG will be locked by the valves at the secondary side 7. 8. 9. With the intact SGs the plant is cooled down by the steam bypass station Long-time heat discharge by the heat discharge system The damaged tube may be repaired in the depressurized primary system 17. April 2015, NURIS 4 SGTR according to emergency procedures (example) EnBG . Energiemanagement für KMU Damaged SG Intact SGs Quelle: GRS, Internet Thermodynamic conditions after spray process (example) Secondary side of all SG: 72 bar, correlating temperature of saturated steam at 288 ° C Primary side: 80 bar (sat. steam-temp. 295 ° C), average coolant temperature at 290 ° C Supercooling of the MCP in normal operation: 99 bar (155 bar – 56 bar correlating to 271°C) Supercooling of the MCP during SGTR: merely 7 bar (80 bar – 73 bar correlating to 289°C) 17. April 2015, NURIS 5 Probability of shutdown of main coolant pumps EnBG . Energiemanagement für KMU Loss of external power sources: Regular emergency case, specified in emergency procedures Possible causes in the primary side: Undershoot of the spray process Different possible measuring tolerances (e. g. foaming of the SG-water secondary side) Delayed closing or stuck opening of spray valve(s) (Three Miles Island Accident) Leakage of spray valve (corrosion products, dissolving during the very rare spray process) Stiffness of seldom used spray valves Possible causes in the secondary side: Overshoot of the non-safety-designed steam bypass station Inaccurate measurements, possibly in opposite directions between the primary and secondary side Possible causes due to human errors: Several imprecise manual operations, e. g. when tracking the primary pressure on the secondary pressure There are a lot of reasons in this extraordinary situation (conclusion from many years of operating experience) Failure of only one device, no multiple failures and no unusual human errors assumed 17. April 2015, NURIS 6 SGTR followed by a shutdown of main coolant pumps EnBG . Energiemanagement für KMU Damaged SG Intact SGs Non-borated Water Quelle: GRS, Internet Non-borated relatively cold water, may flow through the defective tube and may enter the primary loop as well as the reactor core PWR: For sufficient subcriticality it is absolutely essential to have borated coolant in the core! What happens to the criticality of the reactor core? 17. April 2015, NURIS 7 History and scientific investigations and tests until today EnBG . Energiemanagement für KMU Nov. 2012 Recognition of the seriousness by my novel “Der Störfall” Feb. 2013 Information provided to the „Reaktor-Sicherheitskommission“(RSK) May 2013 Forwarded to the „Bundesministerium für Umwelt, Naturschutz, Bau und Reaktorsicherheit“(BMUB) Apr. 2013 BMUB commissioned the Gesellschaft für Reaktorsicherheit (GRS) Apr. 2013 GRS is investigating the incident including ATHLET-Analysis and ROCOM-Tests des Helmholtz-Zentrum Dresden-Rossendorf (HZDR); ( support by the author/initiator was refused by BMU and GRS!) Jan. 2014 Supported by the “Büro für Atomsicherheit” (Prof. Renneberg) and the “Institute for Safety and Risk Research” (ISR, Report of 23. Jan. 2014) May 2014 Presentation and discussion with the „RSK-Ausschuss Anlagen- und Systemtechnik“ (RSK-AST, minutes of the 97. and the 103. meeting) May 2014 RSK-Discussion (minutes of the 446. and the 471. meeting) Feb. 2015 The author/initiator was informed by BMUB with a letter. Main results: There are still different views. RSK-AST advises a comprehensive clarification. BMUB issued an order to the RSK. 17. April 2015, NURIS 8 Main contradictions EnBG . Energiemanagement für KMU Number of damaged tubes RSK assumes that ruptures of more than one tube will occur (RSK-Report 447. Meeting) only the case of one rupture has been investigated Critical boron concentration (Control rods inserted! At hot conditions!) RSK assumes that a “reactor core will go critical again at 600 ppm” (RSK-report 446. meeting, during hot conditions!) 850 ppm are required by BMU (RSK report 446. meeting) GRS calculated a plug of 8.000 kg with less than 100 ppm (GRS-report of 16. Sept. 2013) What happens to the criticality of the reactor core at 100 ppm? 17. April 2015, NURIS 9 Individual assessment EnBG . Energiemanagement für KMU My personal assessment: Contradictions between RSK requirements and the GRS analysis Predominant theoretical and formalistic approach A lot of different sequences are possible, but only 1 case considered, only 1 nuclear power plant, only 1 plant technology, only 1 emergency operation procedure, only 1 course of actions ……… There is too much inconsistency to my practical experience. There is no comprehensive solution. 17. April 2015, NURIS 10 Summary and conclusions EnBG . Energiemanagement für KMU I´m afraid, but it´s not just fantasy, it could come to reality High probability x severe impact = very high risk More than two years extensive research and discussions by GRS, RSK, AST, ISR, BMUB, ….. Identified and initiated by the author but cooperation with the author/initiator was not accepted, he did not get a chance to be involved There is an urgent need for external specialists to work in the field of nuclear safety! And it is extremely important that independent experts get involved and organize events such as NURIS! Thanks a lot for NURIS - and thanks for your attention! 17. April 2015, NURIS 11
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