TR-2 reaktöründe kontrol çubuğu kalibrasyonunun karşılaştırılmalı incelenmesi

Abstract

Reaktivite ve reaktivite değişimi, reaktör dizaynında, kontrolünde ve güvenliğinde kullanılan önemli bir kavramdır. Bir reaktörün işletilmesi ve kontrolü genellikle kontrol çubukları vasıtasıyla reaktöre pozitif ve negatif reaktivite verilmek suretiyle sağlanır. Kontrol çubuklarının reaktör kalbinde bulundukları pozisyonlara göre reaktivite değerlerinin bulunması, kontrol çubuk kalibrasyonu olarak adlandırılır. Bu amaçla kullanılan reaktivite ölçme metodlarından başlıcaları pozitif peryod, çubuk düşürme, kaynak çekme, çubuk salınma, rossi alpha ve ortalamaya göre değişim, nötron darbesi, yakıt yerine zehir koyma metodlarıdır. Bu tekniklerden en yaygın olarak kullanılan pozitif peryod ve çubuk düşürme metodu, TR-2 Reaktörünün kontrol çubuklarına uygulanmıştır. Pozitif peryod metodu kullanılarak yapılan deneysel çalışmada reaktöre verilen pozitif adım reaktivite; 1- Gücün iki kat olma zamanından hesaplanan kararlı peryodun inhour eşitliğinde yerine konulmasıyla, 2- Deney sırasında bilgisayara kaydedilen nötron yoğunluğu-zaman verilerine göre çizilen eğri yardımıyla bulunan inhour denkleminin kökünün, ilgili bilgisayar programına veri olarak girilmesiyle bulunmuştur. Pozitif peryod metodunda kontrol çubuğunun her yukarı çekilişinde reaktöre verilen adım şeklindeki reaktivite değeri 30-300 pcm değerleri arasında sınırlı olup, çubuğun tümünün yukarı çekilmesine kadar bu işlem devam ettiğinden, bu metodun uygulanması uzun zaman gerektirmektedir. Bu metot alçak bir güç seviyesinde uygulanır. Çünki yüksek güç sebebiyle artan reaktör sıcaklığı, reaktivite hesabında hataya sebep olur. Çubukların gölgeleme etkilerinin azaltmak için kalibrasyonu yapılan çubuğun dışındakiler, reaktörün her kritik olması sırasında aynı seviyelere çekilmelidir. Kararlı peryodun hesabı, geçici rejim hareketinin sönümlenmesinden sonra, yapılmalıdır. Ancak reaktör güç artışından dolayı sıcaklık artışının reaktiviteyi değiştirebileceği göz önüne alınarak, çok uzun bekleme samanı sakıncalıdır. Çubuk düşürme deneyinde bir kontrol çubuğunun en üst konumundan reaktör kalbine düşürülmesiyle ortama verilen negatif reaktivite, bilgisayara kaydedilen nötron yoğunluğunun bozunum eğrisinden hesaplanmıştır. Çubuk düşürme metodunun uygulanması basit olmakla birlikte analizi karışıktır. Çubuk düştükten ve ani geçici rejim hareketinden sonraki nötron akı seviyesinin doğru tesbiti zordur. Bu da reaktivite hesabında hata kaynağıdır. Bu metotla bir kaç bin pcm mertebesinde reaktivite ölçmek mümkündür. TR-2 Reaktörünün kontrol çubuklarına uygulanan pozitif peryod ve çubuk düşürme deneyleri sonucunda bulunan reaktivite değerleri birbirine yakın olup, her iki metodun birbiriyle uyumlu olduğu söylenebilir.

Reactivity and reactivity change is an important concept in the design, control and safe operation of the reactor. Operating and controlling of a reactor is usually provided by giving positive or negative reactivity to the reactor with control rods. If we consider the reactivity as a measure of the deviation of a reactor from the critical state, it can be shown that the decay of neutron density will be a function of reactivity. After additive positive or negative reactivity to the critical reactor, the reactivity worth can be found by observing the time behaviour of neutron density. The knowledge of the reactivity of control rods is important for safety and practical purpose. Control rods are used for start up, steady-state operation and shut down of the reactor. Therefore, their reactivity worths must be known accurately before the reactor start-up. It is necessary to know the total worth of a rod. Also, it is desirable to know the reactivity worth of a control rod at any of its position in the reactor. The practical purpose of rod calibration is to detect reactivity changes that occur in the reactor. Normal reactivity changes can occur from effects such as temperature, fuel burnup, fission product poisoning and poison burnup. The magnitude of the change can be determined from the calibration of the rods and the critical rod setting. The rod calibration curves are determined by measuring the value of reactivity corresponding to a control rod withdrawal distance. They give important data for safe operation and management of the reactor. VII Before the initial reactor 6tart up, it is required that the reactor be loaded to critical. Then, a desired reactivity amount is inserted into the reactor core by adding further fuel assemblies to supply the reactivity necessary for the routine operation of the reactor. The excess reactivity should always be considerably less than that which can be suppressed by the control rods. Obtaining reactivity values of the control rods in different position of the reactor core is called control rod calibration. Applied the reactivity measuring methods in use for this purpose are mainly positive period, rod drop, source jerk, rod oscillator, rossi alpha and variance to mean, pulsed neutron, fuel poison substitution methods. The most common methods, positive period and rod drop are carried out for TR-2 Reactor in this thesis study. In the positive period method, reactivity changes are measured by observing positive asymptotic periods of the reactor. These asymptotic periods are related to reactivity by the Reactor Kinetic Equations and the Inhour Equation. When a control rod is withdrawn a certain distance from the critical position, the positive reactivity corresponding to the withdrawal length is added to the reactor approximately stepwise. After a transient excursion, the reactor- power increases exponentially with the stable period corresponding to the added positive reactivity. In the experiment with positive period method, the value of the positive step reactivity given to the reactor is found by; 1- Measuring the doubling time of the reactor power to find stable period and using this in the Inhour Equation. 2- Recording and plotting change of neutron concentration in time by a data acquisition system and finding the root of Inhour Equation. And then using this root in a computer code. VIII If there is a neutron source in the reactor, the k in the critical state is slightly smaller than unity. The effect of the neutron source on the measurement must be negligible. Therefore, the power level in the critical state should be set. For the positive period method, the reactivity given with every withdrawal of control rod is limited between 30-300 pcm. Since all the length must scanned, the method requires a lot of steps and time. This method must be applied at low power levels. Because the temperature of the reactor increases with the increasing power, this causes error in the reactivity measurements. For the value of the stable period, one has to wait until transients become negligible. But also long waiting times must be avoided for the temperature effects on reactivity arising with the power increase. If the reactivity given to the reactor at one time in the calibration is too large, the reactor power increase too fast. A large error in the measurement of the doubling time arises and operation of reactor may not be controlled. On the other hand if too small reactivity is added to the reactor, the difference of k from unity at the initial state causes error in the measurement and also a long time is needed to measure the doubling time. Therefore the amount of reactivity to be added at one time for the measurement must be determined so that the reactor period becomes in the range of 20 s. and 30 s. The effectiveness of a control rod depends upon the value of the neutron flux at the point of insertion of the rod. A single control rod is most effective at the center of a reactor and less effective at the outer edge or in the reflector. When the control rod pattern is changed, the neutron flux distribution in the reactor changes. Then the reactivity worth of a material located in the reactor also changes even if its position does not change. To avoid shadowing effects of other control rods, these must be held on the same level with respect to each other. IX Control rods must be placed symmetrically in a reactor core in order to prevent excessive distortion of the thermal neutron flux and to minimize thermal stresses. The neutron flux is low at the outer part of a reactor core, so the absorption of thermal neutrons are small. As the beginning part of a control rod is inserted into the reactor core, the negative reactivity per unit length is small. After a certain length of control rod is inserted, the change in reactivity is about linear. When the end part of control rod is almost inserted, the negative reactivity per unit length again becomes small. For safe operation of a reactor, the rod calibration curves must be prepared ready. This curves display the reactivity worth of the rods as a function of position. If the critical rod positions are known, the calibration curves can be used to determine the reactivity in the reactor, also they are required some experimental work. In same cases it is desirable to know the amount of rod motion which will provide a certain period. The rod drop method is based upon subcritical measurements. When the rod which is to be calibrated is dropped into the core, the reactor is shut down by the insertion of step negative reactivity. The resulting decay of the neutron flux is observed and related to the reactivity. For the measurements, a dedector which has a fast response is necessary. The neutron detector must be located where its response will be proportional to total neutron flux in the assembly both before and after the drop. The equations used in this method are derived for the condition of no external neutron source. Therefore, the source must be withdrawn before the rod is dropped. This is more important in this method as compared to the period method, since one requires to avoid the subcritical multiplication of the source neutrons. X In the rod drop experiment, the negative reactivity given to the system is calculated from recording and plotting decay of neutron concentration data. Although it is easy to make the rod drop experiment, its analysis is complex. Determination of the neutron flux just after the transient time is difficult and this causes error in the reactivity. With this method it is possible to measure reactivities such as a few thousand pcm. Results of rod drop and positive period experiments on TR-2 Reactor are in good agreement with each other, in terms of reactivities obtained.