核弹头探测技术数值模拟研究

【作者】 伍钧；

【导师】 胡思得； 张本爱；

【作者基本信息】 中国工程物理研究院北京研究生部 ， 粒子物理与原子核物理， 2003， 博士

【摘要】 在核裁军过程中，核弹头探测是对核裁军条约执行情况进行核查的重要内容，它涉及到国家重大安全利益。当前核裁军虽然主要还是在美俄之间进行，但随着核裁军的进一步深入，中国参加未来深度核裁军也是可能的。由于核裁军的核查技术复杂，特别是核弹头的探测技术复杂，难度大，涉及国家的安全和国际核不扩散等敏感问题，因此在中国参加核裁军之前开展核查技术中的核弹头探测技术研究是一项非常重要的技术储备。 本论文从核弹头的中子探测和γ射线探测两个方面对几种典型结构的核弹头探测的方法进行了理论研究和数值模拟，论证其探测方法的可行性，讨论了其适用范围和限制条件。在被动中子探测数值模拟中，提出在确定了核弹头的泄漏中子计数后，采用迭代方法计算中子在核弹头模型中的增殖，由此在一定条件下可实施核弹头质量属性的反演。对几种主动探测核弹头方法进行的研究中，首先对不同能量各向同性外源中子照射核弹头的中子增殖行为进行了数值模拟，检验了用外源中子照射核弹头后，通过测量瞬发中子在核弹头中的增殖量，来证明核弹头中高浓铀存在的可行性；研究了用能量低于1MeV的外中子源照射核弹头，用测量核弹头中裂变产生的高于源中子能量中子数的方法探测核弹头，通过数值模拟分析了可行性。计算结果表明，核弹头裂变产生的较高能的中子数很少，需要较长的时间或较强的低能中子源才能探测到可靠的信号。 本文研究了用外中子源照射核弹头，诱发其裂变材料裂变，探测缓发中子的方法，以此判断高浓铀与低浓铀，从而为核查核弹头的真伪提供依据。在计算程序中实现了基于两步输运算法对缓发中子的数值模拟，物理图像清晰，计算简单。数值模拟表明，这种方法比计数瞬发中子总数有较大的优点，可用来探测铀为弹芯的核弹头。计算表明用天然铀替代高浓铀作的假弹头，它们的出壳缓发中子数相差约3倍，需在一定条件下，才可对真伪核弹头做出判断。 本文研究了用中子符合探测方式测量能量低于1MeV的外中子源在核弹头中诱发的裂变数，以判别模型中是否含有高浓铀，从而作为判断核弹头的真伪的可行性。推导并提出用中子输运计算与概率方法结合计算符合中子计数，对能量低于1MeV的外源中子照射核弹头诱发裂变的符合中子测量作了数值模拟。计算表明用探测符合中子的方法探测核弹头对区分真假核弹头有明显的优点，对于同一核弹头模型，裂变弹芯是高浓铀和用天然铀替代高浓铀时，符合中子计数相差1个量级，如通过其他途径了解核弹头的大致结构，符合测量的方法就可以有较高的置信度分辨真假核弹头。 我们研究了γ射线谱作弹头指纹以判别弹头类型的可行性、适用范围以及对探测时间、距离的要求。通过γ射线谱指纹测量，可以认定是否是同类弹头。中国工程物理研究院博士论文 对由特征Y射线谱反演核弹头信息的方法，分析了反演的可行性;提出为了保守核武器设计的秘密，对能谱测量应施加的限制。分析中考虑了由于核弹头结构未知对材料成分测量可能造成的误差。更多还原

【Abstract】 Nuclear warheads detection is an important verification technology for nuclear disarmament treaty in nuclear disarmament process, and this technology is also concerned to the national security interests. Although the current nuclear reduction process is proceeding mainly between the US and Russia, as the deep cut process goes further, it is possible that China would be involved in the nuclear disarmament process. Because the nuclear disarmament verification technologies are quite complex, particularly the technology for nuclear warhead detection is very difficult, and it concerns to the sensitive issues such as national security and nonproliferation of nuclear weapons, it would be benefitial for China to explore the technology of nuclear warhead detection before participating in the nuclear disarmament process. The study on this is the most important technological preparation for our participating in the further deep nuclear disarmament.In this thesis, the numeral simulation studies are performed on the measurement of neutrons and Gamma rays of some typical hypothetical nuclear warhead models. The feasibility of the measurements is studied, the situations that these measurements possibly can be applied are discussed, and the application conditions of these measurements are explored. In numeral simulation of passive neutron detection, it is proposed to use the iterative calculation in analyzing the neutron multiplication in nuclear warhead. On this basis, the mass of the plutonium pit could be obtained under some conditions. In analyzing the neutron active detection, the neutron multiplication in the nuclear warheads induced by outer neutron source with different neutron energys is studied with numeral simulation, and the feasibility of through counting neutrons to determine the presence of high enrichment uranium(HEU) in nuclear warhead has been proved. The method of detecting the warhead which contains HEU pit by counting the released neutrons whose energy is higher than that of source neutron is studied. The feasibility of this method has been studied. The results of numeral simulation show that the higher-energy neutrons counting rate is quite low, it requires more counting time or much stronger low energy neutron source to detect the reliable signals.Detecting nuclear warhead by counting delayed neutrons induced by neutron source is studied. The 2-step calculation method is proposed to resolve the delayed neutron transportation issue. This method can save computer’s calculation time, and is easy to operate. The results show that the method is better than the method of counting prompt neutrons indistinguishing real and fake nuclear warheads. If using the natural uranium to replace the HEU pit in warhead, the delayed neutron counting rate would be about 3 times lower.The coincidence neutron detection is proposed to be used in nuclear warhead detection. With the outer neutron source which can only induce 235U fission, it could possibly prove the presence of HEU in a warhead by counting the coincidence neutrons. This method possibly provides one of necessary attributions in identifying the real nuclear warheads. The method of numeral simulating the coincidence neutron detection is proposed with combining the neutron transportation and probabilistic calculation. The method is introduced and used to simulate the nuclear warhead induced fission by source neutrons with energy below 1 MeV. The results show that coincidence detection method has obvious benefit in detecting nuclear warheads. For the same type warhead, the coincidence counts rates of the model of warhead of HEU pit is 10 times as big as that of fake warhead of natural uranium pit. If we know the nuclear warheads configuration by other methods, the method of coincidence neutron detection could be used to distinguish the fake nuclear warheads from the reals.The feasibility of taking the Gamma ray spectrum as a fingerprint of nuclear warhead is studied. The range of the Gamma ray fingerprint could be applied to and the conditions under更多还原