İ.T.Ü. Triga Mark II reaktörünün reaktivite sıcaklık katsayısının deneysel olarak saptanması
Başlık çevirisi mevcut değil.
- Tez No: 55624
- Danışmanlar: DOÇ.DR. CİHAT BAYTAŞ
- Tez Türü: Yüksek Lisans
- Konular: Nükleer Mühendislik, Nuclear Engineering
- Anahtar Kelimeler: Reaktörler, Sıcaklık, Reactors, Temperature
- Yıl: 1996
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: Belirtilmemiş.
Özet
ÖZET Sıcaklığın malzemelerin bazı temel özelliklerini değiştirdiği bilinmektedir. Nükleer alanda da sıcaklık malzemelerde ve özelliklerinde bazı değişikliklere neden olur. Bunların en önemlisi reaktivite üzerindeki etkisidir. Bu özellik bazı reaktörlerde pozitif bazılarında ise negatiftir. Bu katsayının negatif olması reaktörün güvenli bir şekilde çalıştırılmasındaki temel etkenlerden bindir. Bu nedenle reaktör tasarımı sırasında bu katsayının negatif olması için gerekli düzenlemeler yapılmalıdır. Bu çalışmada bu katsayıya etki eden faktörler incelenmiş ve sonuçları yorumlanmıştır. TRIGA reaktörleri genellikle araştırma amacıyla inşa edildikleri ve çok değişik çalışma şartlarına maruz kalabileceği için güvenli çalışma özelliği çok önemlidir. Yakıtın özelliği nedeniyle bu reaktörlerde ani ve büyük pozitif reaktivite girişleri yapılabilir. Yüksek negatif reaktivite katsayısı nedeniyle reaktör bu çeşit reaktivite girişlerini frenler ve güvenli bir çalışma imkanı verir. Deneysel çalışma yapmadan önce benzer reaktörlerde yapılan çalışmalar incelenmiş ve bu bilgilerin ışığı altında İTÜ TRIGA reaktörünün sıcaklık katsayısı yapılan deneylerle saptanmıştır. Bu çalışmadan önce yapılan deney sonuçları kullanılarak ayar çubuğu kalibrasyon eğrisi elde edilmiş ve bu değerler yapılan çalışmalarda kullanılmıştır. vıı
Özet (Çeviri)
SUMMARY EXPERIMENTAL DETERMINATION OF THE TEMPERATURE COEFFICIENT OF THE REACTIVITY FOR İ.T.Ü. TRIGA MARK II REACTOR. Many of the parameters which determine the reactivity of a reactor are functions of the temperature of the fuel, moderator and coolant. Some of these parameters are thermal utilization, resonance escape probability and diffusion length. In other words, reactivity changes are due to changes in the effective cross sections, Doppler effect, a decrease in atomic density and overall changes in core dimensions. The temperature coefficient of reactivity is defined as dp aT = dT where p is the reactivity of the system and T is the temperature of a specific components. A reactor at operating conditions will not have a uniform temperature across its core. In a heterogeneous reactors, each component, like fuel, moderator, cladding, coolant reflector may be at different temperatures. Thus if T refers to the temperature of the fuel, aT is called the fuel temperature coefficient, if T is the moderator temperature aT is called the moderator temperature coefficient. Therefore, it is convenient to define a temperature coefficient associated with each of the separate components dp, 1 dT1' where a, is the feedback coefficient for the ith component. The total amount of reactivity feedback can be written as N p = 2>iATi where AT is the change in temperature of the ith component. p = at ATf + aT ATm + ac ATC + ar ATr + a.i AT where af, am, occ, a,, ai are the feedback coefficient of the fuel, moderator, coolant, reflector and the I remaining components.ocT = - and 1 dT P = 1-k“1, where k is the multiplication factor of the reactor, hence __^dk aT ”k2 dT k is usually close to unity and aT may be written as 1 dk a^kdT Since k is always positive, dk/dT has the same algebraic sign as aT is positive, dk/dT is also positive, and the multiplication factor of the reactor increases with increasing temperature. On the other hand, if ocT is negative, dk/dT is negative, and the multiplication factor decreases with increasing temperature. The behavior of a reactor following a change in temperature depends on the sign of <xT Reactors having positive temperature coefficients are unstable with respect to changes in temperature and reactors having negative temperature coefficients are stable with respect to the temperature changes. In solid-fueled reactors, the temperature coefficient of the fuel has a great importance. Because, the fuel temperature responds immediately to changes in power, whereas the temperature of the of heat from the fuel. For this reason, the fuel temperature coefficient is often called prompt temperature coefficient. The multiplication factor of a thermal reactor is given by k=k00 PT Pf where Kcprvrf p g and PT and PF are the slow and fast nonleakage probabilities. Pt=- ^~ T 1 + B2L2T and PF= e~BTT IXin k = In koo+ İn PT + In PF Differentiating this equation term by term then gives aT=aT(k<o)+aT(PT)+ aT(PF) Differentiating the logarithm of k*, gives aT(M=aT(TiT)+ aT(f)+ aT(p)+ aT(e) Its clear that in order to calculate the temperature coefficient of the reactor, it is necessary to determine the temperature coefficient of the parameters, tiT,f,p,?, L2T,xT and B2. If the fuel consist of a single fissile isotope where q and ex, are the average thermal fission and absorption cross sections. If, as in solid-fueled reactors, the fuel elements consist of a mixture of fissile and nonfissile isotopes, Zv“ğfn ^-fa”where vn and Zfn refer to the nth fissile isotope and Sa includes all includes in the fuel element. The thermal utilization of a homogeneous reactor is given by the formula SaF f = =- IaF + Iah If a reactor is quasihomogeneous and fuel is contained in thin fuel elements f is given by f = _ IaFVF EaFVF + SaMVM + IasVs and for a heterogeneous reactor the thermal utilization is given by f = _ Zap Vp IaFVF + ZaMVM^ where ^ is the disadvantage factor.By differentiating f and with calculations it could be shown that aT(f)=-(1-f)aT(U for solid-moderated reactors and aT(f)=-(1-f)[aT(0-pM] for liquid-moderated reactors. Where (3M is the coefficient of expansion of the moderator. The resonance escape probability in a homogeneous reactor depends on temperature by virtue of the Doppler effect. For a heterogeneous reactor,.it is given by p = exp NpVFI ^pFVp H*zMVv where I is the resonance integral Differentiating Inp gives aPromPt(p)=-aT(I)ln VpJ Thus the prompt temperature coefficient is primarily a function of the temperature coefficient of the resonance integral. Since a liquid moderator is ordinarily held in a metal tank of some sort, the ratio VFA/M remains constant during the change in temperature. The temperature coefficient of p can be written as l. aT(p) = -[-pF+pM+aT(l)]ln 9 J J3f«|3m and aT(P) = -[PM+aT(I)]l vPJ The fast fission factor is independent of temperature in a homogeneous reactor, with a heterogeneous reactor e may change slightly with temperature, but it is usually small compared with other reactor temperature coefficients. L2T is given by XILİ=Ş- and aT(L2T)=aT(D)-aT(Sa) For a heterogeneous reactor aT(L2T) = aT(l-f) + aT(L2T) where L-^, is the diffusion length of the moderator aT(L2T) is generally positive and aT(xT) is always positive The temperature coefficient of B2 is aT(B2)=-2ocT(x) where aT(x) is the ordinary coefficient linear expansion of the system. Thus ctT(B2) is always negative. the temperature coefficient of p is the most important factor contributing to the negative temperature coefficient of k». Also, the temperature coefficient of PT and PF are often small compared with the temperature coefficient of p. TRIGA reactor system has a big prompt negative temperature coefficient associated with the TRIGA fuel and core design. This temperature coefficient allows to operate safely with large step insertion of reactivity. The coefficient is prompt because the fuel is mixed with a large portion of the moderator and thus fuel and solid moderator temperatures rise simultaneously. The rise in temperature of the hydride found in fuel increases the probability that a thermal neutron in the fuel element will gain energy from atom. Aşjhe neutrons gain energy from the ZrH, their mean free path is increased. The probability of escape from the fuel element before capture is increased. In the water the neutrons are rapidly rethermalized so that the capture and escape probabilities are insensitive to the energy with which the neutron enters the water. For a standard TRIGA core, more then 50% of the temperature coefficient comes from the temperature dependent disadvantage factor and »20% each from Doppler broadening of the U238 resonances and temperature-dependent leakage from the core. To measure the temperature coefficient of the reactivity, it is necessary to know calibration diagrams of control rods. In our experiments, adjustment rod was used and made an experiment to draw calibration diagram of the adjustment rod. Firstly the position of all control rods and the fuel element and bulk temperatures are recorded. Then the reactor is xnoperated at allow power. While the reactor is operating in the steady state, adjustment rod is with drown up to a define height from the core. Now the reactor power and fuel temperature is increased.' A certain amount of reactivity (Ap) put into the reactor is compensated by the increased fuel element temperature (AT). The Ap can be found from the adjustment rod calibrations diagram and AT can be read from the instrumentation. The fuel element temperature is measured by thermocouples arranged in the fuel element. From Ap and AT, the - can be calculated which is essentially the temperature coefficient of reactivity. In this study, several different experiments were made and two important components of reactivity temperature coefficient, fuel and coolant, tried to be found. Then signs and amplitudes which were found were discussed.
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