Nuclear materials continued
Nuclear Materials continued
2 Moderators慢化剂 Requirements - Low mass number低质量数 - Very low neutron capture cross- section极低吸收 截面 - High scattering cross- section高散射截面 · Candidate materials候选材料 Hydrogen and its isotopes deuterium Water, heavy water, paraffin wax Carbon graphite Beryllium Being toxic, presently not suitable for application in any form
2 Moderators 慢化剂 • Requirements – Low mass number 低质量数 – Very low neutron capture cross-section 极低吸收 截面 – High scattering cross-section 高散射截面 • Candidate materials 候选材料 – Hydrogen and its isotopes deuterium • Water, heavy water, paraffin wax – Carbon • graphite – Beryllium • Being toxic, presently not suitable for application in any form
The requirements of the moderator for a thermal reactor, namely(即,也就 tE)low mass number, very low neutron capture cross-section and high scattering cross-section, limit the choice to only a few materials Hydrogen and its isotope deuterium, carbon and beryllium are the only elements that are suitable Hydrogen and deuterium, being gases, are not sufficiently dense and must be used in the form of compounds, water and heavy water being the obvious choices(表示结果,氢和氘是气体,密度不高,必须以化合物的形式使 用,所以水和重水是明确的选择。 The use of hydrocarbon(烃,碳氢化合物) has been tried, but has not been successful and such materials are not used as moderators It is interesting to recall, however, that Fermi used paraffin wax in his early experiments in the 1930s to slow down neutrons and study their interactions with the elements , so he was one of the first scientists to be aware of the effects of neutron moderation.但是,回顾历史,十九世纪三十 年代费米在早期的实验中曾经利用固体石蜡来慢化中子,从而研究中子与元素的 交互作用,所以,他是最早意识到中子慢化效应的科学家之
The requirements of the moderator for a thermal reactor, namely (即，也就 是) low mass number, very low neutron capture cross-section and high scattering cross-section, limit the choice to only a few materials. Hydrogen and its isotope deuterium, carbon and beryllium are the only elements that are suitable. Hydrogen and deuterium, being gases, are not sufficiently dense and must be used in the form of compounds, water and heavy water being the obvious choices (表示结果). 氢和氘是气体，密度不高，必须以化合物的形式使 用，所以水和重水是明确的选择。 The use of hydrocarbon (烃，碳氢化合物) has been tried, but has not been successful and such materials are not used as moderators. It is interesting to recall, however, that Fermi used paraffin wax in his early experiments in the 1930s to slow down neutrons and study their interactions with the elements, so he was one of the first scientists to be aware of the effects of neutron moderation. 但是，回顾历史，十九世纪三十 年代费米在早期的实验中曾经利用固体石蜡来慢化中子，从而研究中子与元素的 交互作用，所以，他是最早意识到中子慢化效应的科学家之一
Beryllium has a very low neutron capture cross-section(0.009barns), high melting point (about 1300oC)and good strength, and at one time it seemed possible that it would find an application either as the moderator or the fuel cladding in thermal reactors.铍有极低的中子俘获截面、高熔点和良好的 强度,曾一度好像可能成为热堆慢化剂或者燃料包壳。 However, it and its compounds are toxic, and beryllium itself has low ductility(延展性) and poor corrosion resistance(抗腐蚀性).但是,其化合物 具有毒性,并且铍本身的延展性比较低,抗腐蚀性也比较差。 Beryllium oxide Beo also has undesirable properties As a result of this neither bery ilium nor its oxide have found any use in power reactor, and it is unlikely that they will be used in the future. tAAR 镀及其氧化物都无法在动力堆中使用,而且将来也不会派上用场。 The choice of moderators for thermal reactors is thus limited to three materials-water, heavy water and carbon in the form of graphite.热堆的慢 化剂的选择范围被局限在三种材料一水、重水和石墨状态的碳
Beryllium has a very low neutron capture cross-section (0.009barns), high melting point (about 1300oC) and good strength, and at one time it seemed possible that it would find an application either as the moderator or the fuel cladding in thermal reactors. 铍有极低的中子俘获截面、高熔点和良好的 强度，曾一度好像可能成为热堆慢化剂或者燃料包壳。 However, it and its compounds are toxic, and beryllium itself has low ductility (延展性) and poor corrosion resistance (抗腐蚀性). 但是，其化合物 具有毒性，并且铍本身的延展性比较低，抗腐蚀性也比较差。 Beryllium oxide BeO also has undesirable properties. As a result of this neither beryllium nor its oxide have found any use in power reactor, and it is unlikely that they will be used in the future. 结果， 铍及其氧化物都无法在动力堆中使用，而且将来也不会派上用场。 The choice of moderators for thermal reactors is thus limited to three materials-water, heavy water and carbon in the form of graphite. 热堆的慢 化剂的选择范围被局限在三种材料－水、重水和石墨状态的碳
21 Water Advantages The right choice for the moderator of thermal reactor, and it can also serve as the coolant Excellent neutron slowing down properties Abundant, cheap and easily obtainable with high purity disadvantages Capture cross-section is rather high 0.66 barns, reactor requires enriched uranium for criticality. Unfavorable thermodynamic characteristics.不理想的热力学特性 High pressure is needed to prevent boiling at high temperature Pressure of 150 to 160 bar are typical of PWRs Caution t is important to maintain water purity保持水的纯度很重要 · To minimize corrosion减轻腐蚀 To prevent radioactive transformation防止放射性转变
2.1 Water • Advantages – The right choice for the moderator of thermal reactor, and it can also serve as the coolant. – Excellent neutron slowing down properties. – Abundant, cheap and easily obtainable with high purity. • disadvantages – Capture cross-section is rather high, 0.66 barns, reactor requires enriched uranium for criticality. – Unfavorable thermodynamic characteristics. 不理想的热力学特性 • High pressure is needed to prevent boiling at high temperature. • Pressure of 150 to 160 bar are typical of PWRs. • Caution – It is important to maintain water purity 保持水的纯度很重要 • To minimize corrosion 减轻腐蚀 • To prevent radioactive transformation 防止放射性转变