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

2. 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

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

4. 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

5. Example of calculation results

6. 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

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

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

9. Mitigation on KIc cont.

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

11. 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

12. 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 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

13. Shift of KI curve due to softening of transient

14. 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

15. Summary of mitigation in Loviisa 1 NPP

16. 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.

17. Comparative results of TKA calculation for WWER RPVs; influence of crack shapeТ 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