Oscillations in Kinetics of the Reactor Pressure Vessel Steel Damage Disclosure as an Indication of Smart Nanostructure Presence
Evgeny
Krasikov*
*Corresponding author: Evgeny Krasikov, Research Assistant, National Research Centre, Kurchatov Institute, Kurchatov Sq 1, Moscow 123182, Russia. Tel: +74991969233; Email: ekrasikov@mail.ru
Received Date: 30 August, 2017; Accepted Date: 06 September, 2017; Published Date: 13 September, 2017
Citation: Krasikov E (2017) Nanostructure Evolution as Cause of the RPV Steel Non-Monotonic Radiation Embrittlement. J Nanomed Nanosci: JNAN-124. DOI: 10.29011/JNAN-124. 100024
1.
Abstract
2.
Keywords: Embrittlement Kinetics; Material Smart
Behavior; Nanostructure Self-Organization; Oscillation; Reactor Pressure Vessel, Steel
1.
Introduction
Modern nuclear power plants based on PWR type reactors. As the main safety barrier RPV have to be capable to working at any operating regimes including in an emergency. Radiation stability is a dominating factor of the RPV life cycle. In turn substantiation of the normative dependencies of the RPV materials radiation embrittlement was carried out in research reactors at neutron fluxes that were two and more orders higher than at RPV operation. Surveillance Specimens (SS) programmers are more adequate for RPV embrittlement control but at this case load factor may be also high (up to 20). So, scale of the flux effect influence touch upon hypersensitive questions of the RPV real radiation embrittlement. This context became the main motivation of the present work carrying-out and publication. Operating RPVs are subject to multi-factor influence. It is practically impossible to reproduce some of this factor (long-time bias, e.g.) in the framework of experimental investigations including RPV surveillance specimen’s tests. Detailed information that can be obtained by means of taking through RPV wall samples immediate from the decommissioned RPVs is more representative than received by any another way and therefore has a highest value.
2. Experimental Procedures and Test Results
Along with routine investigations in Russia systematic research on actual radiation embrittlement of the decommissioned pressure vessels via through samples (trepans) has been carried out. The earliest commercial PWR prototype unit Novovoronezh-1 (210 MWe) RPV after 20 years (1964-1984) of operation was trepanned in 1987. Then Novovoronezh-2 (365 MWe, 1969-1990), the oldest PWR type experimental prototype reactor EPR (1956-1986) and, finely, nuclear icebreaker Lenin” RPVs also were trepanned. Through-thickness RPV wall samples had been cut out using trepanning tools such as boring machine with annular drill, anodic-mechanical and electric resistance cut machines. Typical dimensions of trepans were 120-140 mm in diameter and 120-140 mm lengthwise. Test pieces and specimens were fabricated by means of electro-discharge wire cutting machine. The general dependence of the Transition Temperature Shift (TTS) on Radiation Embrittlement Coefficient (REC) AF is as follows:
TTS=AF×F (1)
where AF is the radiation embrittlement coefficient (REC - chemical factor) and F is the fast (E>0,5MeV) neutron fluence in units of 1018cm-2(fluence factor), n - coefficient (~1/3).
For Russian RPV Cr-Mo-V base steel
AF=A×(P+0,07Cu) (2)
where AF coefficient (800 at 270°C), P and Cu - are the mass concentrations of these elements.
One of the
Novovoronezh-1 RPV trepans for parallel study was delivered to IVO (Finland).
Team-work obtained serrated results (Figure 1)
force to lost in thoughts.
No less unexpected
became the results of a special research of the SS Kolskaya-3 NPP [1] that confirm the subsistence of the flux effect at
the initial section of the neutron exposition as well as data on post
irradiation tests of the trepans cut out of nuclear icebreaker “Lenin” RPV [2]. Particular curiosity to provoke paradox results
of the SS zero embrittlement obtained in Russia and abroad is shown in (Figure 2 and 3) [3-7].
Stress of incomprehension diminished when finding analogous data on non-monotonous SS degradation with commentary “Quite Atypical” [4]. In addition to this circumstances attention is drawn to the fact that the considerable scatter of the experimental data take place. (Figure 3) presents data scattering for SS of the Spanish nuclear power plants. One can note unexpected low embrittlement at the region of high fluence and zero embrittlement at the neutron fluence area of 2×1019сm-2.
3. Data Analysis and Discussion
Revealed non- monotony has to be inspired
by the peculiarity of the copper impurities containing metal degradation
process at low flux when after forced embrittlement at the initial stage
mitigation follows [8]. At the same time refinement of the steel up to
threshold level of 0,05%Cu that slows down flux effect changes the scatter
slightly; SS data scatter exceeds this parameter for data obtained in test
reactors (high flux) [9]. One can assume, that factor exists which generate
oscillating process in mechanism of embrittlement and become more apparent with
neutron flux lowering. Conversely, at high level of the flux it is hardly
noticeable because amplitude of а oscillation decrease while frequency
increase. As an analogue of temporal self-organization in solids under
irradiation one
can cite professor V.S. Chmelevskaya data on periodic change in length of the
plutonium specimens with different content of 238Pu
isotope (figure 4) [10].
Hypothetically we assume the presence of
the oscillation component in kinetics of the neutron embrittlement process that
arise from radiation defects periodic beginning and annealing, which in its
turn is a result of the metal structure temporal self-organization. Similar to
indicated in (Figure 2) effect we find out the
mention concerning structural and phase transitions in the amorphous and
nanocrystalline Ti50Ni25Cu25 alloy
upon high-pressure torsion [11]. (Figure 5) demonstrates volume fraction of crystalline
phase V as a function of the number of the mobile anvil rotations n for the
amorphous Ti50Ni25Cu25
alloy.
Assumed periodicity
of the embrittlement confirms the analysis of the dependence of the transition
temperature shift against neutron fluence from surveillance data for WWER-440/213
base metal and welds (figure 6).
Possibly linear
approximation is simplification, but one can notice the serrated character of
the dependence and high amplitude of the oscillations at the initial stage of
neutron exposition. In addition, an attempt was undertaking to consider the
dependence of the transition temperature shift against radiation embrittlement coefficient (chemical factor) AF from surveillance data for WWER-440/213
weld and base metal (figure 7).
In this case initial data were used for neutron fluences at the range of (40÷60) ×1018сm-2. Again, oscillating process become apparent. It is necessary note that data for different intensities and materials (weld and base metals) were used. It seems that oscillation factor may advance to the forefront.
4. Conclusion
Disclosure of the oscillations - sign of the self-organization - in kinetics of the Reactor Pressure Vessels (RPV) steel damage at fast neutron intensity decreasing testifies the periodic transformations of the metal structure and underlines the actuality of the thorough knowledge radiation damage mechanisms deepening. As a hypothesis we suppose that at some stages of irradiation damaged metal have to be partially restored by irradiation i.e. neutron bombardment. Nascent during irradiation structure undergo occurring once or periodically transformation in a direction both degradation and recovery of the initial properties. According to our hypothesis at some stages of metal structure degradation neutron bombardment became recovering factor. As a result, oscillation arise that in turn lead to enhanced data scatter.
So, one can assume, that factor exists which generate oscillating process in mechanism of embrittlement and become more apparent with neutron flux lowering. Conversely, at high level of the flux it is hardly noticeable because amplitude of а oscillation decrease while frequency increase. Hypothetically we assume the presence of the oscillation component in kinetics of the neutron embrittlement process that arise from radiation defects periodic beginning and annealing, which in its turn is a result of the metal structure temporal self-organization.
Revealing
of the wavelike process in the form of non-monotonous parts of the kinetics of
radiation embrittlement process - sign of self-organization, that testify to
periodic transformation of the structure - actualize the problem of more
precise definition of the RPV materials radiation embrittlement mechanisms and
give reasons for search of the ways to manage the radiation stability (Nano
structuring [12] and so on to stimulate the
radiation defects annihilation), development of the means for creating of more
stableness self-recovering smart materials.
Figure 1: Dependence of the
Novovoronezh-1 RPV Weld Metal TTS on the Fast Neutron Fluence (□ - NRC “Kurchatov
Institute”, ○- IVO Finland). Where ○ - flux~30×1011cm-2с-1,
● - flux ~4×1011cm-2с-1, □ - flux ~ 1,3×1011cm-2с-1.
Figure 2. Dependence of the Rovno-1 RPV Base Metal TTS on the Fast Neutron
Fluence. Neutron Flux~6×1010cm-2с-1 (Specimens from Upper Containers).
Figure 3. Dependence of the TTS on the fast neutron fluence from surveillance
data for Spanish nuclear power plants
base metal (○) and welds (●)
Figure
4.
Change in Length (Mm of the Plutonium Specimens with Different Content Of 238Pu Isotope Against Time (Hours): 1 - 7,3% 238Pu, 2 - 80% 238Pu.
Figure 5. Volume Fraction of Crystalline Phase V as a Function of the Number of the Mobile Anvil Rotations N for the Amorphous Ti50Ni25Cu25 Alloy.
Figure 6. Dependence of the Transition
Temperature Shift Against Neutron Fluence From Surveillance Data For WWER-440/213
Base Metal and Welds. Where ○ - flux~30×1011cm-2с-1, ● - flux
~4×1011cm-2с-1, □ - flux ~ 1,3×1011cm-2с-1.
Figure 7. Dependence of the Transition
Temperature Shift Against Radiation
Embrittlement Coefficient (Chemical Factor) AF
From Surveillance Data for WWER-440/213base Metal and Welds.
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2.
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9.
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10.
Chmelevskaya
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