Tutorial Irradiation Embrittlement and Life Management of RPVs RPV Life Assessment and Mitigation R. Ahlstrand

The RPV is very difficult (but possible) to replace; economic issue The RPV is the most important component when considering life time of a NPP The RPV is very difficult (but possible) to replace; economic issue Life time assessment of a RPV is based on integrity analyses which includes the following elements: Process knowledge Thermo hydraulics Stress analyses Fatigue and corrosion Fracture mechanics Material characteristics and degradation Reactor physics Neutron fluence evaluation ISI; In Service Inspection Operating procedures Operator education

The basis for RPV life time assessment is shown in this figure

t = (ΔTk/Af)3 * 1/F The fracture mechanics assessment is based on: Determination of the fracture toughness curve, KIc, of RPV material Evaluation of the embrittlement of the material due to neutron irradiation Thermo hydraulics of selected incident e.g. LBLOCA Evaluation of cooling of “down comer” of the RPV due to cooling (ECC) Calculation of temperature distribution in the RPV wall Calculation of stress distribution at a “postulated” crack location Calculation of SIF (stress intensity factor) of the crack front The safe life time of the reactor can be determined from the maximum acceptable shift (ΔTk) t = (ΔTk/Af)3 * 1/F

Example of calculation results

Mitigation possibilities: Mitigation on KIc The material toughness curve KIc and the calculated KI curve must not coincide. Mitigation possibilities: Mitigation on KIc Neutron fluence rate reduction by using low leakage core or replacement of peripheral fuel elements by dummies

Influence of core management on neutron fluence and shift in transition temperature

Mitigation on KIc cont. Annealing of the RPV for recovering of material toughness

Mitigation on KIc cont.

Re-embrittlement of weld 502 (IAEA Round Robin program) Mitigation on KIc cont. Re-embrittlement of weld 502 (IAEA Round Robin program)

Mitigation on KIc cont. The core region of all 1st generation VVER 440 RPVs have been annealed (NV 3 RPV was annealed 2 times) Two reactors of 2nd generation of VVER 440 have been annealed (Loviisa 1 and Rivne 1) Research reactor BR3 in MOL has been annealed by “wet annealing” An old Army reactor in the USA was annealed a long time ago but not used after that The VVER 440 RPV can be annealed without deformation of RPV nozzles since they are far from the core region In more compact RPVs annealing can be a problem due to the risk of deformation of nozzles and primary piping

2. Mitigation on KI The calculated SIF (stress intensity factor) can be reduced by softening thermal transients and reducing pressure loading in a possible incident Increasing of temperature of the ECCS (emergency core cooling system) water. This will decrease thermal stresses and accordingly calculated KI loading The water of the hydro accumulator can be increased to 100 degrees and ECCS water tank to 50-60 degrees in a VVER 440 In VVER 1000 the same measures can be used The flow and pressure of the ECCS pumps can be adjusted in order to slow down cooling rate of RPV Adjusting PORV (pressurizer relief valve) parameters Mixing of core cooling water Change ECCS injection location in order to reduce thermal loading in RPV down comer Decrease the risk of pressure increase by proper operational procedures (e.g. avoid cold over pressurization or closing of pressurizer safety valve) Proper operator education

Shift of KI curve due to softening of transient

Other mitigation means Utilizing “warm pre-stress” at the crack tip during thermal transient Loading of crack tip in higher temperature causes compressive stresses at crack tip in further cool down Crack initiation is avoided unless re-pressurization takes place. Utilised to some degree in a few countries Pre-stressing of the RPV locally in the core region Pre-stressing of the brittle region e.g. core weld will reduce stresses at the inner surface of the RPV. Not utilized yet. Fine tuning of KI calculations and models Crack shape Utilization of cladding toughness properties Optimising of FEM mesh at crack tip Optimising mixing of ECC water Optimising heat transfer assumptions

Summary of mitigation in Loviisa 1 NPP

In USA the RPV life assessment is based on a Probabilistic approach; the PTS rule 10CFR50.61 “Fracture Toughness Requirements for Protection Against Pressurized Thermal Shock Events”. This rule is based on a low enough TWCF (Through wall crack frequency) in the RPV. For a longitudinal crack the RTPTS is 250 degrees as shown in the figure. For this limit the TWCF is 5*10 -6 per reactor year. In the main nuclear operating countries in Europe the deterministic approach as described previously is applied. In some countries new standards on RPV life time management are under development. In Russia a new standard proposal MKP-CXP-2000 has recently been launched officially. In VERLIFE which is a standard proposal for VVER owner countries RPV life assessment is an important issue.

Comparative results of TKA calculation for WWER RPVs; influence of crack shape Т ка , о С ВВЭР-1000 ( Течь 1 контура) ВВЭР-440 с наплавкой Течь 2 контура) ВВЭР-440 без наплавки Разрыв ПП ПГ) Code used Постулиру- емые дефекты Расчет по условиям (8) и (9) Уточненный расчет условиям (8) Постулируемые (8) и (9) Уточненный расчет 48 52 151 60 84 172 190 70 85 200 216 1 88 94 80 168 MKP-CXP-2000 PNAE G-7-002-86