高放废物地质处置库中膨润土的侵蚀机理和模型研究综述

doi:10.11867/j.issn.1001-8166.2017.10.1050

在核废料处置库安全使用的设计年限(数万年至十数万年)内,受地下水动力和化学作用,膨润土被侵蚀,导致缓冲/回填层致密性降低、渗透性增加,危及核废料处置库安全,引起核元素扩散和对流迁移,甚至造成核泄漏灾害。从膨润土的侵蚀机理、侵蚀模型、侵蚀试验和侵蚀控制措施等方面,系统地综述了膨润土侵蚀的研究成果,指出当前针对膨润土侵蚀研究的不足,提出新的研究方法和手段,为更好地开展膨润土侵蚀研究奠定基础。

关键词: 核废料, 地质处置库, 缓冲/回填层, 膨润土, 侵蚀

The performance of the bentonite buffer in nuclear waste repository concept relies to a great extent on the buffer surrounding the canister having sufficient dry density. Loss of buffer material caused by erosion remains as the most significant process reducing the density of the buffer. In the worst case, the process is assumed to last as long as the free volume between the pellets in the pellets filled regions is filled with groundwater. Erosion rate and mass erosion are calculated based on the erosion model, and the measures are presented to prevent the geological disaster due to bentonite erosion. The groundwaters may solubilise the smectite particles in the bentonite and carry them away as colloidal particles. A dynamic model is developed for sodium gel expansion in fractures where the gel soaks up groundwater as it expands. The model is based on a force balance between and on smectite particles, which move in the water. Attractive van der Waals forces, repulsive electric diffuse layer (DDL) forces, gravity and buoyancy forces and forces caused by the gradient of chemical potential of the particles act to move the particle in the water. The effect of the fracture width and the frictions between particles and water and surrouding rock is analysed based on erosion model. The DDL forces strongly depend on the type of clay minerals and the type of ion and concentration in the water surrounding the particles. In the designed safe use of nuclear waste disposal (tens of thousands of years to hundreds of thousands of years), the safety of nuclear waste disposal is affected by the hydrodynamic and chemical effects, and bentonite erosion. Due to the bentonite erosion, the buffer/backfill layers become loose, and their permeability increases, which causes the nuclear element diffusion and convection, and even the nuclear disaster. In this paper, the mechanisms, models, experiments and control measures of bentonite erosion were systematically summarized. The current deficiencies of bentonite erosion were pointed out, and new methods were put forward to carry out the research for bentonite erosion. The measures were presented to prevent the geological disaster due to bentonite erosion through changes. The project is not only academic innovation, but also has a large practical significance. The research results of this project can be widely applied to the design, construction and maintenance of the bentonite buffer in nuclear waste repository.

Key words: Nuclear waste, Geological disposal, Buffer/backfill layer, Bentonite, Erosion.

中图分类号:

P642.1

图1 黏土侵蚀模型 ^{[

12
]}

Fig.1 Erosion model of cohesive soils ^{[

12
]}

表1 膨润土的临界凝结离子浓度(pH=6.5)

Table 1 Critical Coagulation Concentration of bentonites (pH=6.5)

试验方法	黏土土 /水质量比	CCC/(mmol/L)		参考文献
试验方法	黏土土 /水质量比	[Na⁺]		[Ca²⁺]
TOM	0.015	18	0.45	[17]
TTS	0.025	10~15		[18]
AM	0.1	10	0.355	[19]
RM	0.025	5	0.4	[20]
	0.5	15	2
	1	20	3
	2	30
		100	1	[21]
VI	0.2	250	2	[22]
CKM	4	52		[23]

表1 膨润土的临界凝结离子浓度(pH=6.5)

Table 1 Critical Coagulation Concentration of bentonites (pH=6.5)

试验方法	黏土土 /水质量比	CCC/(mmol/L)		参考文献
试验方法	黏土土 /水质量比	[Na⁺]		[Ca²⁺]
TOM	0.015	18	0.45	[17]
TTS	0.025	10~15		[18]
AM	0.1	10	0.355	[19]
RM	0.025	5	0.4	[20]
	0.5	15	2
	1	20	3
	2	30
		100	1	[21]
VI	0.2	250	2	[22]
CKM	4	52		[23]

图2 膨润土凝胶的应力—应变关系

Fig.2 Stress-strain curves of bentonite colloids

图3 膨润土凝胶的剪切强度与含水量的关系 ^{[

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]}

Fig.3 Relation of shear strength to water content of bentonite colloids ^{[

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]}

图4 m和 n与含水量的关系 ^{[

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]}

Fig.4 Relation of parameters of m and n to water content ^{[

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]}

图5 Kunigel V1膨润土的自由膨胀特性 ^{[

46
]}

Fig.5 Free swell of Kunigel V1 bentonite in different concentrations ^{[

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]}

图6 不同类型膨润土的自由膨胀特性 ^{[

47
]}

Fig.6 Free swell of different bentonite ^{[

47
]}

图7 膨润土侵蚀质量与流水质量的关系 ^{[

50
]}

Fig.7 Relation of erosion mass to water flow mass of bentinites ^{[

50
]}

图8 横穿裂隙渗流侵蚀试验装置图 ^{[

52
]}

Fig.8 Apparatus of bentonite erosion ^{[

52
]}

图9 膨润土侵蚀迁移距离与时间的关系 ^{[

52
]}

Fig.9 Radial extrusion distance vs. time ^{[

52
]}

图10 膨润土侵蚀过程中的膨胀力和浊度变化规律 ^{[

24
]}

Fig.10 Swelling pressure and turbidity in the process of bentonite erosion ^{[

24
]}

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Mechanisms and Models for Bentonite Erosion Used for Geologic Disposal of High Level Radioactive Waste: A Review