Schramke J A, Santillan E F U, Peake R T
SC&A Inc., 2200 Wilson Blvd, Suite 300, Arlington, Virginia 22201.
U.S. Environmental Protection Agency, Office of Air and Radiation.
Appl Geochem. 2020 May;116. doi: 10.1016/j.apgeochem.2020.104561.
The Waste Isolation Pilot Plant (WIPP), a deep geologic repository located 660 meters underground in bedded salt, is designed to isolate U.S. defense-related transuranic waste from the accessible environment. Plutonium isotopes are the most important radionuclides in WIPP waste. Plutonium solubility in WIPP brines (ionic strengths from 5.3 to 7.4) is strongly dependent on its oxidation state, with much lower solubilities associated with Pu(III) and Pu(IV) than with the higher Pu(V) and Pu(VI) oxidation states. The large quantity of metallic iron in WIPP waste and waste containers is expected to undergo anoxic corrosion, producing strongly reducing conditions and high hydrogen gas pressures after repository closure and brine intrusion. Because reducing conditions will prevail in the WIPP repository, the most important long-term oxidation states will be Pu(III) and Pu(IV). We performed a literature review to evaluate the effects of WIPP chemical and physical processes (not colloidal) on plutonium oxidation states that included reactions with reducing agents such as iron solids and aqueous species and radiolysis of solids and aqueous species. The results of this review indicate that equilibrium between Pu(III) solids and aqueous species will control dissolved plutonium concentrations in WIPP brines. We also performed geochemical modeling calculations using the ThermoChimie database to support this assessment of plutonium oxidation states in the long-term WIPP repository. Control of plutonium solubilities by Pu(III) solid instead of Pu(IV) solid may lead to higher predicted plutonium concentrations in brines potentially released to the ground surface by an inadvertent drilling intrusion into the long-term WIPP repository. The results of this study demonstrate that Pu(III) solid solubilities provide a reasonable upper bound for dissolved plutonium concentrations in WIPP brines.
废物隔离试验工厂（WIPP）是一个位于地下660米的层状盐岩中的深部地质处置库，旨在将美国与国防相关的超铀废物与可进入的环境隔离开来。钚同位素是WIPP废物中最重要的放射性核素。钚在WIPP盐水（离子强度为5.3至7.4）中的溶解度强烈依赖于其氧化态，与Pu(III)和Pu(IV)相比，Pu(V)和Pu(VI)等较高氧化态的溶解度要低得多。WIPP废物和废物容器中的大量金属铁预计会发生缺氧腐蚀，在处置库封闭和盐水侵入后产生强还原条件和高氢气压力。由于WIPP处置库中将普遍存在还原条件，最重要的长期氧化态将是Pu(III)和Pu(IV)。我们进行了文献综述，以评估WIPP化学和物理过程（非胶体）对钚氧化态的影响，这些过程包括与铁固体和水性物质等还原剂的反应以及固体和水性物质的辐射分解。该综述结果表明，Pu(III)固体与水性物质之间的平衡将控制WIPP盐水中溶解钚的浓度。我们还使用ThermoChimie数据库进行了地球化学建模计算，以支持对WIPP长期处置库中钚氧化态的这一评估。由Pu(III)固体而非Pu(IV)固体控制钚的溶解度，可能会导致通过意外钻探侵入WIPP长期处置库而潜在释放到地面的盐水中预测的钚浓度更高。这项研究的结果表明，Pu(III)固体溶解度为WIPP盐水中溶解钚的浓度提供了一个合理的上限。
