Production of annular and compact type burnable absorber nuclear fuel pellets by powder metallurgy and sol gel route
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Date
2020
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Turkish Energy, Nuclear and Mineral Research Agency
Abstract
Günümüz nükleer reaktörleri farklı uygulamalar için farklı nötron soğurucularına sahiptirler. Nükleer endüstrinin en büyük zorluklarından biri, reaktörlerin performansını, güvenliğini ve ömrünü arttırmaktır. Bu alanda yakıt ömrünü uzatmak, yanmayı arttırmak ve güç yoğunluğu dağılımını optimize etmek için yeni malzemelerin araştırılmasına ve geliştirilmesine odaklanmış birçok çaba vardır. Bu amaçla bir nötron soğurucu malzeme genellikle UO2 nükleer yakıtına ilave edilir. Gadolinyum mükemmel bir yanabilir zehirdir çünkü nötron absorblanması için geniş bir kesit sunar ve ömrünün başlangıcında yakıtın aşırı reaktivitesinin dengelenmesini sağlar. Reaktör performansının daha uzun çevrim uzunlukları veya geliştirilmiş yakıt kullanımı yoluyla geliştirilmesi ihtiyacı, ticari nükleer enerji üretiminin başlangıcından beri belirgin taleplerden biri olmuştur. Bunun sonucunda ortaya çıkan çeşitli modifikasyonların başında, reaktör çekirdeğindeki ek parçalanabilir malzemenin (235U) arttırılması anlamına da gelen yakıtın ilk zenginlğinin arttırılması gelmektedir ve bu, ancak reaktör çekirdeğine ek nötron soğurucu malzeme eklenerek telafi edilebilmektedir. Bu dengeleme, başlangıçta sadece kontrol çubuklarına monte edilen nötron soğurucu malzemeler kullanılarak ve / veya reaktör soğutucusunda çözünebilir emici (borik asit) ilave edilerek sağlanabilmiştir. Kaynar Su Reaktörlerinde (BWR), soğutucu moderatörde çözünebilir soğurucunun kullanımı teknolojik nedenlerle yasaklanmıştır. Basınçlı Su Reaktörlerinde (PWR), soğutucu / moderatöre eklenen çözünebilir bir soğurucu olarak borik asidin kullanımı rutin olarak gerçekleştirilmektedir, fakat ilk yakıt zenginleştirilmesindeki artış borik asit konsantrasyonunu arttırılmasıyla süresiz olarak telafi edilememektedir. Belirli bir konsantrasyonun ötesinde, başlangıçta suyun termal olarak genleşmesi, çekirdek içindeki bor miktarını azaltır ve sonuç olarak, güvenli reaktör operasyonuna ilişkin kabul edilemez bir durum olan pozitif bir moderatör reaktivite katsayısı ile sonuçlanır. Yakıt çubukları içinde katı yanıcı soğurucuların (ya da yanabilir zehirin) kullanılmasının nedeni budur. Nükleer reaktörlerde yanabilir bir soğurucu zehir kullanılması, kor yaşamın başlangıcında gerekli negatif moderatör reaktivite katsayısını sağlar ve kor güç dağılımlarını şekillendirmeye yardımcı olur (TEC-DOC-844,1995; Yayli, 1995; Böhm et al., 1987; Zinkle & Was, 2013; Tobia et al., 2014). Yanabilir soğurucu zehir malzemesi yüksek bir nötron tesir kesitine sahip olmalı ve düşük soğurma tesir kesitli kız ürünlerini oluşturmalıdır. Ardından, ışınlama ilerledikçe, yanabilir zehir yakılır ve makroskobik absorpsiyon kesiti azalır. Nükleer bakış açısından, gadolinyum oksit, uygun bir şekilde tasarlandığında, yaklaşık olarak 235U tükenmesiyle eşleşebilen ve döngü sonunda reaktivite hatasını en aza indiren yanma oranına bağlı yüksek bir nötron emilim kesitine sahip mükemmel bir yanıcı zehirdir (Stogen, Nielsen & Grummer, 1982; Brandberg, 1973). Bu çalışmada UO2-Gd2O3 yanabilir soğuruculu peletleri elde etmek için iki yöntem kullanıldı. Birinci yöntem konvansiyonel toz metalurjisi ve ikincisi sol-jel yöntemidir. Bu çalışmada sinterlenmiş UO2-Gd2O3 peletlerinde mikroyapı ve U-Gd dağılımı incelemeleri SEM-EDAX yöntemleriyle yapıldı ve her iki yöntemle de üretilen ürünlerde homojen yapılar elde edildi. Sinterlenmiş UO2-Gd2O3 peletlerinde X- ışınları kırınımı yöntemiyle yapılan katı hal incelemelerinde, gadolinium UO2 matrisinde çözünmüştür. Sol-jel ve toz metalurjisi yöntemleri ile elde ettiğimiz yanabilir-soğuruculu kompakt ve delikli peletler, uluslararası standartlara uygun olarak elde edilmiştir.
The aim of this study is to produce UO2-Gd2O3 burnable absorber nuclear fuels of nuclear power reactors. Today's nuclear power reactors have different neutron absorbers for different applications. One of the major challenges of the nuclear industry is to improve the performance, safety and lifetime of reactors. In this area there are many efforts focused on the research and development of new materials in order to extend the fuel lifetime, increase the burn up and optimize the power density distribution. With this aim a neutron absorber material is usually incorporated into the UO2 nuclear fuel. Gadolinium is an excellent burnable poison because it presents a large cross section for neutron absorption and allows the compensation of the excess reactivity of the fuel in the beginning of its life. The need to improve reactor performance through longer cycle lengths or improved fuel utilization has been apparent since the beginning of commercial nuclear power generation. Among several modifications introduced as a consequence, the fuel initial enrichment has been increased, which means that the additional amount of fissile material (235U) in the reactor core has to be compensated by the introduction of additional neutron absorber material in the reactor core. This compensation was initially achieved only by using neutron absorber materials assembled in control rods or/and by addition of soluble absorber (boric acid) in the reactor coolant. In Boiling Water Reactors (BWR), the use of soluble absorber in the coolant/moderator was prohibited for technological reasons. In Pressurized Water Reactors (PWR), boric acid as a soluble absorber added to the coolant/moderator has been routinely used, but the increase in initial fuel enrichment cannot be indefinitely compensated by increasing the boric acid concentration. Beyond a certain concentration, thermal expansion of water at start-up reduces the quantity of boron in the core, resulting ultimately in a positive moderator reactivity coefficient, which is an unacceptable situation regarding to the safe reactor operation. This is the reason why the introduction of solid burnable absorbers (or burnable poison) within the fuel rods was considered. The use of a burnable poison in nuclear reactors provides the necessary negative moderator reactivity coefficient at the beginning of core life and help to shape the core power distributions (IAEA-TEC-DOC-844, 1995; Yayli, 1995; Böhm et al., 1987; Zinkle & Was, 2013; Tobia et al., 2014) The poison material should have a high neutron absorption cross section and form daughter products with low absorption cross sections. Then, as soon as the irradiation proceeds, the burnable poison burns up and the macroscopic absorption cross section decreases. From a nuclear viewpoint, gadolinia is an excellent burnable poison, having a high neutron absorption cross section coupled to a burn up rate that, if properly designed, can match approximately the 235U depletion, minimizing the reactivity penalty at the end-of-cycle (EOC) (Stogen, Nielsen & Grummer, 1982; Brandberg, 1973). In this study two methods were used to obtain UO2-Gd2O3 burnable-absorber fuels. The first method was conventional powder metallurgical route and the second method was sol-gel route. In this study, the investigation of the microstructure and U-Gd distribution in sintered bodies along with the achieved SEM-EDAX results showed that both methods provide homogeneous structures. The solid state investigations of sintered UO2-Gd2O3 pellets by x-ray diffraction showed that, gadolinium dissolved in UO2 matrix. Production of the burnable absorber compact and annular type nuclear fuel pellets by powder metallurgical and sol-gel routes showed that; we obtained the nuclear fuels in accordance with international standards.
The aim of this study is to produce UO2-Gd2O3 burnable absorber nuclear fuels of nuclear power reactors. Today's nuclear power reactors have different neutron absorbers for different applications. One of the major challenges of the nuclear industry is to improve the performance, safety and lifetime of reactors. In this area there are many efforts focused on the research and development of new materials in order to extend the fuel lifetime, increase the burn up and optimize the power density distribution. With this aim a neutron absorber material is usually incorporated into the UO2 nuclear fuel. Gadolinium is an excellent burnable poison because it presents a large cross section for neutron absorption and allows the compensation of the excess reactivity of the fuel in the beginning of its life. The need to improve reactor performance through longer cycle lengths or improved fuel utilization has been apparent since the beginning of commercial nuclear power generation. Among several modifications introduced as a consequence, the fuel initial enrichment has been increased, which means that the additional amount of fissile material (235U) in the reactor core has to be compensated by the introduction of additional neutron absorber material in the reactor core. This compensation was initially achieved only by using neutron absorber materials assembled in control rods or/and by addition of soluble absorber (boric acid) in the reactor coolant. In Boiling Water Reactors (BWR), the use of soluble absorber in the coolant/moderator was prohibited for technological reasons. In Pressurized Water Reactors (PWR), boric acid as a soluble absorber added to the coolant/moderator has been routinely used, but the increase in initial fuel enrichment cannot be indefinitely compensated by increasing the boric acid concentration. Beyond a certain concentration, thermal expansion of water at start-up reduces the quantity of boron in the core, resulting ultimately in a positive moderator reactivity coefficient, which is an unacceptable situation regarding to the safe reactor operation. This is the reason why the introduction of solid burnable absorbers (or burnable poison) within the fuel rods was considered. The use of a burnable poison in nuclear reactors provides the necessary negative moderator reactivity coefficient at the beginning of core life and help to shape the core power distributions (IAEA-TEC-DOC-844, 1995; Yayli, 1995; Böhm et al., 1987; Zinkle & Was, 2013; Tobia et al., 2014) The poison material should have a high neutron absorption cross section and form daughter products with low absorption cross sections. Then, as soon as the irradiation proceeds, the burnable poison burns up and the macroscopic absorption cross section decreases. From a nuclear viewpoint, gadolinia is an excellent burnable poison, having a high neutron absorption cross section coupled to a burn up rate that, if properly designed, can match approximately the 235U depletion, minimizing the reactivity penalty at the end-of-cycle (EOC) (Stogen, Nielsen & Grummer, 1982; Brandberg, 1973). In this study two methods were used to obtain UO2-Gd2O3 burnable-absorber fuels. The first method was conventional powder metallurgical route and the second method was sol-gel route. In this study, the investigation of the microstructure and U-Gd distribution in sintered bodies along with the achieved SEM-EDAX results showed that both methods provide homogeneous structures. The solid state investigations of sintered UO2-Gd2O3 pellets by x-ray diffraction showed that, gadolinium dissolved in UO2 matrix. Production of the burnable absorber compact and annular type nuclear fuel pellets by powder metallurgical and sol-gel routes showed that; we obtained the nuclear fuels in accordance with international standards.
Description
Keywords
Burnable absorber, Yanabilir-soğurucu, Nuclear fuel, Nükleer yakıt, Gadolinia, Gadolinyum oksit, Annular type pellets, Delikli tip yakıt, Sol gel, Sol jel
Citation
Yaylı, A. (2020). Production of annular and compact type burnable absorber nuclear fuel pellets by powder metallurgy and sol gel route. Turkish Journal of Nuclear Sciences, 32(1), 19-42.