The reactor as a radiation source Reactor pressure vessel (RPV) surveillance

Transcription

1 The reactor as a radiation source Reactor pressure vessel (RPV) surveillance Course on Operation of Nuclear Reactors 3 rd lecture Dr. Szabolcs Czifrus associate professor Budapest University of Technology and Economics Institute of Nuclear Techniques (BME NTI)

2 The reactor as a radiation source

3 Sources of radiation in the case of an operating reactor Neutron sources Fission neutrons from induced fission Neutrons from (,n) and (,n) reactions Gamma photon sources Prompt fission photons Photons from decay of fission products (delayed) Prompt capture gamma photons Photons from decay of activated materials (delayed)

4 Sources of radiation in the case of a shut down reactor Neutron sources Fission neutrons from spontaneous fission Neutrons from (,n) and (,n) reactions Gamma photon sources Photons from decay of fission products Photons from decay of activated materials Remnant heat is created mainly by decay of fission products (beta and gamma heating) It is of crucial importance that remnant heat be removed from the core

5 Energy spectrum of prompt fission neutrons Watt spectrum: ( E) a e E / b sinh c E

6 Energy spectrum of neutrons born in the induced fission of 244 Cm, 252 Cf and 235 U

7 Data of isotopes capable of spontaneous fission Isotope Izotóp Felezési Half life idő Felezési Half life idő for a spontaneous spontán hasadásra fission T sf Decay const. for spont. sf fission s -1 (1/s) 235 U 7, a a 2, , Fajlagos Specific neutronhozam, yield neutron (n/gram s) neutron sg Amount Mennyisége, * (g/tonns of g U) tonna U 236 U 2, a 2, a 8, , U 4, a 8, a 2, , Np 2, a > a <2, <5, Pu 87,74a 4, a 4, ,30 2, Pu 2, a a 2, , Pu 6570 a 1, a 1, ,07 9, Pu 14,4 a <6, a >3, >9, Pu 3, a 6, a 3, ,15 1, Am 432,1 a 1, a 2, , Am 7380 a 2, a 1, , Cm 162,8 d 7, a 3, ,61 2, ,6 244 Cm 18,11 a 1, a 1, ,80 1, Cf 2,64 a 85 a 2, ,77 2, Megjegyzés: * - PWR reaktorra MWnap/tonna kiégési szintnél A kivastagított számok egysége: hasadás/sg

8 The most important photoneutron (,n) reactions Energy dependence of the cross section of the 2 H(,n) 1 H reaction Energy dependence of the cross section of the 9 Be(,n) 8 Be reaction

9 Energy distribution of prompt gamma photons from fission E, MeV N(E), /hasadás /fission 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0 4,5 5,0 5,5 6,0 6,5 3,1 1,9 0,84 0,55 0,29 0,15 0,062 0,065 0,024 0,019 0,017 0,007 0,004 M(E), MeV/hasadás MeV/fission 1,55 1,90 1,26 1,10 0,725 0,45 0,217 0,260 0,108 0,095 0,094 0,042 0,026 Összesen Total 7,028 7,827

10 Energy distribution of gamma photons emitted by fission products Energia- Energia-intervallum, Effektív energia, M(E) Energy group csoport MeV MeV Energy interval (MeV) 0,1-0,4 0,4-0,9 0,9-1,35 1,35-1,8 1,8-2,2 2,2-2,6 2,6 Average energy (MeV) MeV/fission MeV*/Ws 0,4 0,8 1,3 1,7 2,18 2,5 2,8 MeV hasadás 0,645 3,87 0,645 1,06 0,677 0,290 0,032 MeV* Ws 2, , , , , , , Összesen: Total 7,219 2, */ A * gamma-fotonok Energy of the gamma energiája photons a reaktor for által 1 Ws termelt energy 1 produced Ws energiára in the vonatkoztatva reactor

11 Specific neutron source strength of the spent fuel of a VVER-1000 reactor as a function of burnup (n/gs) Isotope Burnup, MWday/kg Total

12 Spatial distribution of neutron flux and gamma dose rate around a reactor core Neutron flux (n/m 2 s) Gamma dose rate (mr/h) Primary Secondary Water Steel Air Boundary of reactor core Pressure vessel Steel structures Distance from centre of core, cm

13 The total radioactive inventory of a reactor of 1000 MWe power Isotope type Fission products Operating NPP % Actinides Activation products 23% % Inventory of spent fuel (peta Bq) Shutdown % % % 150 days after % 165 3% 36 1% 10 years after % 95 20% 4 1%

14 Specific neutron source stregth of PWR spent fuel due to spontaneuos fission as a function of burnup At shutdown 2 years cooling Burnup, MWday/tonns of U

15 Specific neutron source stregth of PWR spent fuel due to spontaneuos fission as a function of cooling time Total Cooling time, years

16 Remnant heat power compared to the heat power at shutdown 1 Cooling time, s Cooling time, s

17 Reactor pressure vessel (RPV) surveillance

18 Why is the RPV important? The RPV determines the lifetime of the whole power plant (it is practically impossible to replace) The most important engineered barrier If there is a rupture (break), it is very likely that a core melt damage occurs

19 How to design? Ultra conservative? Very safe Oversized Impossible to transport Impossible to handle Not the proper solution Be conservative, but not ultra conservative! Normal dimensions The vessel can be trasported and handled However, it must be checked and tested regularly Furthermore, a complex program called RPV surveillance is needed

20 What happens to the RPV during its lifetime? Thermal stress Heating up, cooling down Mechanical stress High pressure Vibrations, oscillations However, the most important is the effect of fast neutrons

21 The effect of fast neutrons Solid materials can break in two different ways: - Brittle - Ductile Above a certain temperature (transition temperature), normally they break in a ductile manner Due to irradiation by fast neutrons, the transition temperature increases The effect depends on: neutron fluence of high energy neutrons integrated over time neutron flux temperature material composition of steel Ductile fracture Brittle fracture

22 Ductile and brittle fractures Ductile fracture: Ductile fracture is characterised by plastic deformation that precedes failure of the part. Ductility is usually understood to mean the ability of a material to accept large amounts of deformation (mainly tensile) without fracture. It is the antithesis of brittleness Brittle fracture: Brittle fracture occurs with little or no gross plastic deformation occurring in the component. It is characterised by the very small amount of energy, which is absorbed, and by the crystalline appearance of the surfaces of the fracture (break). Brittle fracture mostly results in catastrophic failure. Brittle fracture is influenced by defects, fatigue, stresscorrosion, and embrittlement.

23 Embrittlement by neutrons: shift of the ductile-brittle transition temperature, T t = A(chemical composition, T irradiation, ) ( ) n n ~

24 What happens to the crystal material (metal of the RPV)? The approximately E>0.5 MeV neutrons knock out atoms from their place in the crystal lattice If the energy transferred by the neutrons >40eV E min, then a Frankel-pair is created The magnitude of this phenomenon is characterized by the displacement per atom The probability of reordering is a function of the temperature of the metal In this respect, it is advantageous that the pressure vessel is at a high temperature

25 Embrittlement The sequence of basic embrittlement processes: (a) creation of primary radiation damage defects: (b) formation of nanoscale solute and defect clusters (iron atoms not shown); (c) pinning of dislocations and hardening by nanofeatures; (d) hardening enhanced cleavage fracture; at a (e) stress concentration. Solute clusters due to irradiation

26 How to monitor the influence of fast neutrons? The energy needed to break a piece of metal decreases at a given temperature Charpy impact test:

27 Surveillance specimens to monitor the radiation damage So called surveillance speciments are manufactured from exactly the same material as that of the reactor pressure vessel These specimens are put into containers along with monitor foils to determine the neutron fluence during the irradiation So called chains are formed The chains of speciments are located into the RPV The location is closer to the reactor core tha the wall of the vessel 1 year for the specimen ~ 4 to 11 years to the vessel This can predict the transition temperature shift for the future of the RPV f, specimen f, vessel 4 11

28 Location of the surveillance specimens Specimens are located here

29 Encapsulation of the surveillance specimens Specimens próbatestek, 15H2MFA 4 10 B C titán 55 6 C A A B C-C metszet (felülnézet) aluminum alumínium saválló steel acél A-A metszet (oldalnézet) B-B metszet (oldalnézet)

30 Weld material samples Not only the base material (the actual steel of the RPV) should be monitored The weld material differs from the base material There must be welds at certain locations around the RPVs Welds must not be at the most irradiated areas

31 Requirements concerning the material of the pressure vessel Microruptures are always present It is very important that these microruptures should be stable if a shock occurs The ruptures must not spread further than ¾ of the thickness of the vessel (RPV) wall The vessel can be renewed : this process is called annealing

32 What can lead to a rupture? Transients Operation/startup of the Emergency Core Cooling System The LOCA is less dangerous However, SB LOCA may cause a Pressurized Thermal Shock (PTS) due to the fact that the pressure inside the RPV is high and the temperature of the vessel is also high

33 Micro fractures Initiating event High pressure Low temperature Mechanical stress Thermal stress High stress intensity Fracture instability Fast neutron flux Cold RPV Structural change due to irradiation Higher probability of brittle fracture RPV rupture Impurities

34 Inspection of the pressure vessel During the scheduled maintenance periods at re-fueling time Periodic in-service inspections provide most relevant criteria of the integrity Reliably detect defects and realistically measure and characterize them Non-destructive testing (NDT) must be applied Remote underwater contact ultrasonic inspection equipment Eddy current method: applied for clad surface examinations Visual inspection is used for examination of the vessel inner surface Advanced phased array ultrasonic technique applies for base material examination Advanced nondestructive techniques applied for the examination of reactor welds