Random thermal loads in the thermal fatigue assessment of nuclear piping

doctoral dissertation

Oriol Costa Garrido (Author), Leon Cizelj (Mentor), Iztok Tiselj (Co-mentor)

Abstract

Thermal fatigue is a structural damage mechanism which has been among the causes of reactor coolant leakages in nuclear power plants. A well-recognized source of thermal fatigue in the safety related nuclear piping is the turbulent mixing of fluids at different temperatures. The thermal fatigue assessment of components requires an interdisciplinary approach. This involves the evaluation of the fatigue driving loads from the fluid temperatures, heat transfer to the pipe and stress amplitudes within the pipe wall. The fatigue analyses are then needed to predict the fatigue life using the estimated driving loads and experimental fatigue resistance data. It is clear that the accuracy of fatigue life estimates strongly depends on the accuracy of the estimated fatigue driving loads, especially fluid temperature histories. These can be obtained from complex and extremely time-consuming experiments or computational fluid dynamic (CFD) simulations of turbulently mixing fluids. These could only offer spatially incomplete temperature histories, which are typically limited to minutes or hours at the best, and reported in compact terms of the first two statistical moments combined with power spectral densities. Extrapolation of compact and comparatively short fluid temperature histories to the expected fatigue life time of months or years is inherently uncertain and lacks deeper understanding and proper treatment in the open literature. The major goal of the dissertation is therefore to contribute to the prediction and understanding of the uncertainties involved in the assessment of the safe life of pipes exposed to thermal fatigue due to the turbulently mixing fluids. To this end, the dissertation explores a range of simplified and complex state-of-the-art numerical methods, including one-dimensional (1D) approximations with one- and multi-frequency (spectral) approaches and turbulent three-dimensional (3D) CFD models with and without conjugate heat transfer. An improved spectral loading approach for the generation of synthetic fluid temperatures at affordable computational costs is proposed. Synthetic fields of temperatures acting near the surface of pipes during turbulent fluid mixing are employed in 3D structural analyses of pipes to verify the persistence of nearly equi-biaxial stress states in the pipe surface away from geometrical, structural and material discontinuities. Then, the improved spectral approach is employed to generate a wide variety of fluid temperature histories resulting in a set of estimated fatigue lives. These are further analyzed to quantify the magnitudes of inherent uncertainties. Linear 1D heat diffusion and thermal stress estimates are used together with fatigue assessment codified rules in the uncertainty analysis.

Keywords

material science;thermal fatigue;structural damage;nuclear safety;spectral methods;

Data

Language:	English
Year of publishing:	2016
Typology:	2.08 - Doctoral Dissertation
Organization:	UL FMF - Faculty of Mathematics and Physics
Publisher:	[O. C. Garrido]
UDC:	620.1:621.039(043.3)
COBISS:	2969700
Views:	595
Downloads:	243
Average score:	0 (0 votes)
Metadata:

Other data

Secondary language:	Slovenian
Secondary title:	Naključne toplotne obremenitve pri oceni toplotne utrujenosti cevi v jedrskih elektrarnah
Secondary abstract:	Toplotno utrujanje je pojav, ki povzroča strukturne poškodbe materialov in je eden od vzrokov za nastanek puščanja reaktorskega hladila v jedrskih elektrarnah. Dobro poznan vir toplotnega utrujanja v ceveh, ki so pomembne za jedrsko varnost, je turbulentno mešanje tekočin različnih temperatur. Ocenjevanje toplotnega utrujanja komponent zahteva interdisciplinarno obravnavo. Ta vključuje oceno obremenitev - temperaturnih polj tekočine, ki povzročajo utrujanje, oceno prenosa toplote med tekočino in cevjo ter oceno mehanskih napetosti v stenah cevi. Na podlagi ocenjenih obremenitev in izmerjenih krivulj odpornosti materiala na utrujanje nato z analizami utrujanja napovemo življenjsko dobo komponent. Jasno je, da je natančnost ocene življenjske dobe utrujanja močno odvisna od natančnosti ocene obremenitev, predvsem časovnega poteka temperaturnih polj. Le-ta lahko pridobimo s pomočjo kompleksnih in časovno zelo zahtevnih eksperimentov ali simulacij turbulentnega mešanja tekočin s pomočjo računske dinamike tekočin (CFD). Na ta način dobljena temperaturna polja so ali prostorsko nezvezna ali pa so časovno omejena tipično na nekaj minut oz. kvečjemu ur. Poleg tega so dostopni podatki običajno predstavljeni le v kompaktni obliki, npr. s prvima dvema statističnima momentoma in gostoto spektralne moči. Ekstrapolacija kompaktnih in relativno kratkih časovnih temperaturnih polj glede na pričakovano življenjsko dobo komponent, ki je reda velikosti nekaj mesecev oz. let, je inherentno obremenjena z negotovostjo, njena poglobljena obravnava v dostopni literaturi pa je pomanjkljiva. Zato je glavni cilj disertacije prispevati k napovedim in razumevanju negotovosti pri ocenjevanju varne življenjske dobe cevi, ki so izpostavljene toplotnemu utrujanju zaradi turbulentnega mešanja tekočin. V disertaciji v ta namen raziščemo nabor najsodobnejših poenostavljenih in kompleksnih numeričnih metod, med katere sodijo enodimenzionalni (1D) približni modeli, ki jih rešujemo z eno- ali več-frekvenčnim (spektralnim) pristopom, ter turbulentni tridimenzionalni (3D) modeli CFD, ki upoštevajo vezan oz. nevezan prenos toplote. Predlagamo izboljšan, računsko nezahteven spektralni pristop za generacijo sintetičnih temperaturnih polj tekočine. Sintetična temperaturna polja, ki delujejo tik ob površini cevi, uporabimo v 3D termo-mehanskih analizah, s katerimi preverjamo obstojnost približno izo-dvoosnega napetostnega stanja na površini cevi daleč stran od geometrijskih, strukturnih in materialnih nezveznosti. V nadaljevanju z izboljšanim spektralnim pristopom izračunamo širok nabor temperaturnih signalov tekočine, s katerimi ocenimo pripadajoče življenjske dobe utrujanja. Z analizo teh rezultatov nato kvantificiramo inherentne negotovosti ocen. Pri analizi negotovosti poleg linearnega 1D modela difuzije toplote in termičnih napetosti uporabimo standardizirana pravila za ocenjevanje utrujanja.
Secondary keywords:	materiali;toplotno utrujanje;toplotno razslojevanje;napoved življenjske dobe utrujanja;spremenljiva amplituda utrujanja;večosno utrujanje;negotovosti;strukturne poškodbe;jedrska varnost;spektralne metode;Jedrske elektrarne;Disertacije;Cevi;Toplotna utrujenost;
Type (COBISS):	Doctoral dissertation
Thesis comment:	Univ. v Ljubljani, Fak. za matematiko in fiziko, Oddelek za fiziko
Pages:	144 str.
ID:	11334331