A new concept for compact radiation shielding: Reactive sintered tungsten borocarbides

迄今为止，二氧化碳排放造成的人为气候变化对人类的生存威胁最大。气候变化的影响已经很明显，比如极端天气的增多、极地冰层的减少和海平面的上升。由于全球对电力的需求持续超过供应，发电造成了化石燃料燃烧产生的大部分二氧化碳排放。可再生能源（风能、太阳能、水力）受到天气依赖性的限制，以及能源储存和土地使用等相关问题。核聚变在全球发电脱碳方面发挥着重要作用，但辐射屏蔽是一个限制因素。创造微型太阳是一部分，但必须存在能够承受实际聚变反应堆聚变条件的材料。W基合金和其他难熔金属是聚变反应堆的当前解决方案，但发电聚变反应堆的工程要求超过了当前材料的要求。本研究的目标是证明基于烧结碳化钨和反应烧结硼化物（cWC-RSB）概念的紧凑型球形托卡马克（cST）辐射屏蔽的可行性。cWC-RSB可以弥合当前材料与发电cST的工程要求之间的差距。cWC-RSB通过将重元素（W）和轻元素（C、B）组合而具有优异的辐射吸收性能，并且通过将WC与延展性金属粘合剂组合而具有强度和韧性。然而，迄今为止，cWC从未在核反应堆中使用，因为使用Co（和Ni）金属作为粘合剂合金阻止了cWC作为辐射的使用，因为Co和Ni是活化危害。在2014年，我发现非活化FeCr合金适合作为cWC粘结剂合金，RSB的开发是在研究cWC中的硼添加剂之后进行的。与单独的cWC相比，组合的cWC-RSB屏蔽罩总体上具有更大的辐射衰减。第一个目标是评估热机械性能和屏蔽候选物的安全情况，包括高温氧化和热冲击，以及最坏情况下的情况，如热屏蔽暴露于空气中。在900 ℃ ~ 1200 ℃温度范围内，硅涂层的氧化速率比钨涂层的氧化速率慢4个数量级。我将评估cWC-RSBs的性能超过低温失效（> 1200 ℃）的温度预测发电聚变反应堆。虽然自20世纪30年代以来存在关于cWC的热机械性能的大量数据，但考虑到其新奇，关于RSBs的数据很少，并且RSBs的热机械性能在工业化之前是众所周知的是至关重要的。RSBs的新奇意味着对它们的化学和制造路线知之甚少。目前的处理方法并没有完全优化致密，无裂纹的RSBs。第二个目标的目的是填补这些空白，使用计算的相图方法（CALPHAD）预测最合适的组合物和实验设计（DoE）的方法，最有效的加工试验。这项研究表明，当当前解决方案中存在关键差距时，如何从现有材料和技术中获得新的解决方案。最近的中子和伽马衰减的WC和RSB为基础的屏蔽概念的模拟显示出相当大的希望。然而，迄今为止，关于cWC的辐射响应的数据很少，并且没有关于RSBs的数据。对于第三个目标，我打算建立在目前的研究使用模拟cST通知辐射实验和实验工作模拟cST内的条件范围，包括离子轰击，带电粒子和二次辐射。来自模拟聚变反应堆中cWC-RSB屏蔽件的数据以及证明的抗氧化性表明，cWC-RSB材料在辐射衰减和安全性方面超过了当前的辐射屏蔽候选者。

There has been no greater existential threat to humanity to date from anthropogenic climate change as a result of CO2 emissions. The effects are already apparent in terms of more extreme weather, loss of polar ice and rising sea levels. Power generation contributes to much of the CO2 emissions from fossil fuel burning as the worldwide demand for power continues to outstrip supply. Renewable energy (wind, solar, hydro) is limited by weather dependency, with associated issues such as energy storage and land use. Nuclear fusion has a significant role in decarbonizing global power generation but radiation shielding is a limiting factor. Creating miniature Suns is one part, but materials must exist that can withstand fusion conditions for practical fusion reactors. W-based alloys and other refractory metals are current solutions in fusion reactors, but the engineering requirements for power-generating fusion reactors exceed those in current materials. The goal of this fellowship will demonstrate the feasibility of radiation shielding based on the Cemented Tungsten Carbides and Reactive Sintered Borides (cWC-RSB) concept in Compact Spherical Tokamaks (cSTs). cWC-RSBs can bridge the gap between current materials and the engineering requirements for a power-generating cST. cWCs-RSBs have excellent radiation absorption properties by combining heavy (W) and light elements (C, B) with the strength and toughness by combining WC with a ductile metal binder. However, cWCs have never been used in nuclear reactors to date since the use of Co (and Ni) metal as a binder alloy prevented the use of cWCs as radiation due to Co and Ni being activation hazards. In 2014, I discovered that non-activating FeCr alloys are suitable as cWC binder alloys, with RSB development following on investigating boron additions in cWCs. Combined cWC-RSB shields have greater radiation attenuation overall, compared to cWCs alone. The first objective evaluates the thermo-mechanical properties and the safety case for shielding candidates, including high-temperature oxidization and thermal shock to in terms of worst-case scenarios, such as exposure of hot shielding to air. Experimental data on Si-coated cWCs showed that Si-coating retarded oxidization rate by 4 orders of magnitude relative to tungsten in the temperature range 900C-1200C. I will evaluate the properties of cWC-RSBs over cryogenic to failure (> 1200C) temperatures predicted for power-generating fusion reactors. While considerable data on the thermo-mechanical properties exist for cWCs since the 1930s, there is little on RSBs, given their novelty and it is crucial that thermo-mechanical properties of RSBs are well-known prior to industrialization.The novelty of RSBs means that very little is known about their chemistry and routes to fabrication. Current processing methods are not fully optimized for dense, crack-free RSBs. The second objective aims to fill these gaps using the calculation of phase diagram method (CALPHAD) for predicting the most suitable compositions and design of experiment (DoE) methodology for the most efficient processing trials. This research demonstrates how new solutions can be derived from existing materials and techniques when a critical gap in current solutions is apparent. Recent simulations of the neutron and gamma attenuation of WC- and RSB-based shielding concepts show considerable promise. However, there is little data on the radiation response of cWCs and none on RSBs to date. For this third objective, I intend to build on current research using simulated cSTs to inform radiation experiments and experimental work simulating the range of conditions inside a cST, including ion bombardment, charged particles, and secondary radiation. Data from cWC-RSB shields in a simulated fusion reactor alongside demonstrated oxidization resistance indicates that cWC-RSB materials exceed current radiation shielding candidates in terms of radiation attenuation and safety.