1
Q
What are the two main natural sources of background radiation?
A
Terrestrial radiation (U, Th, K in rocks/soils) and cosmic radiation (muons/neutrons from space).
2
Q
Typical global natural background dose?
A
~2.4 mSv/yr (UNSCEAR).
3
Q
Typical terrestrial dose?
A
1–2 mSv/yr, up to 5+ mSv/yr in granites.
4
Q
Why do terrestrial radiation levels vary?
A
Rock type, soil gas permeability, radon levels, building materials.
5
Q
What radionuclides dominate terrestrial background?
A
U-238, Th-232, K-40.
6
Q
What is radon and why is it important?
A
Radioactive gas from U-decay; main source of natural dose; varies strongly by geology.
7
Q
Typical radon contribution to dose?
A
1 mSv/yr average; >10 mSv/yr in hotspots.
8
Q
Why does cosmic radiation vary with altitude?
A
Thinner atmosphere → less shielding → higher particle flux.
9
Q
Cosmic radiation at sea level?
A
0.3 mSv/yr.
10
Q
Cosmic radiation during a long-haul flight?
A
2–5 µSv/hr.
11
Q
Why is cosmic dose higher at the poles?
A
Weaker geomagnetic shielding → more cosmic rays reach atmosphere.
12
Q
Legal public annual dose limit from nuclear activities?
A
1 mSv/yr (ICRP).
13
Q
Typical design target for repository during operation?
A
<0.3 mSv/yr.
14
Q
Long-term post-closure dose target?
A
0.01–0.1 mSv/yr.
15
Q
Expected real repository flux?
A
Often <0.001 mSv/yr.
16
Q
How does natural dose compare to regulated nuclear releases?
A
Natural variability is 100–1000× higher than regulated doses.
17
Q
Chernobyl total release?
A
~5.2 EBq.
18
Q
Chernobyl radiation in 1986 hotspots?
A
>1000 µSv/hr.
19
Q
Chernobyl radiation today?
A
0.2–5 µSv/hr.
20
Q
Fukushima total release?
A
~940 PBq.
21
Q
Fukushima 2011 off-site dose rate?
A
20–300 µSv/hr.
22
Q
Fukushima decontaminated area dose today?
A
0.2–1 µSv/hr.
23
Q
Global weapons testing fallout dose in 1960s?
A
+0.1–0.2 mSv/yr.
24
Q
Global fallout dose today?
A
<0.01 mSv/yr.
25
What are the three artificial contributors to present-day environmental radiation?
Nuclear medicine, nuclear power operations, past accidents/fallout.
26
Nuclear medicine average dose contribution?
0.5–1 mSv/yr per capita.
27
Environmental impact of nuclear medicine?
Very small: <0.01 mSv/yr (short-lived isotopes decay quickly).
28
Nuclear electricity generation public dose?
0.0001–0.02 mSv/yr.
29
Past accident contribution today? Chernobyl
~0.01 mSv/yr from Cs-137 & Sr-90 residues.
30
Which radionuclide dominates long-term accident contamination?
Cesium-137 (t½ = 30 years).
31
What caused the WIPP 2014 radiological release?
Drum packed with organic kitty litter + nitrate salts** → exothermic reaction → burst.
32
What radioisotopes were released at WIPP?
**Am-241** and **Pu-239/240**.
33
Underground contamination level at WIPP?
**>1000 Bq/m³** in panel air.
34
Maximum worker dose at WIPP?
**~5 mSv**.
35
Public dose at WIPP?
<0.01 mSv.
36
Depth of WIPP repository?
~650 m in bedded salt.
37
Key lesson from WIPP?
Human error in packaging can defeat engineered barriers; strict QC essential.
38
Why is salt (e.g., WIPP) chosen for disposal?
Self-sealing, low permeability, stable, no flowing groundwater.
39
Why is clay (e.g., Opalinus Marl) ideal?
Low permeability, diffusion-dominated transport, self-sealing fractures.
40
What is a typical natural dose at high altitude cities (Denver)?
**~0.7 mSv/yr** (cosmic).
41
Why do building materials affect radiation?
Some contain U/Th/K; recycled materials can concentrate radionuclides.
42
What is the main source of man-made radiation dose to the public?
**Nuclear medicine**, not nuclear power.
43
Which natural factor contributes the most to background dose?
Radon gas (typically ~50% of total).
44
What is the half-life of Cs-137?
30 years.
45
What percentage of Cs-137 remains after 60 years?
**~25%** (two half-lives).
46
Why does radiation drop quickly after accidents?
Many radionuclides (e.g. I-131, Xe-133) have short half-lives.
47
What does UNSCEAR stand for?
United Nations Scientific Committee on the Effects of Atomic Radiation.
48
What does ICRP stand for?
International Commission on Radiological Protection.
49
Typical global radon dose indoors?
1–2 mSv/yr.
50
What is the global average total dose from all artificial sources today?
**~0.6 mSv/yr**.
51
Dose from a chest CT scan?
**~5–10 mSv** (contrast with everyday doses).
52
Dose from eating a banana?
**0.1 µSv** (K-40).
53
What is background radiation mainly measured in?
Dose rate: **µSv/hr** or **mSv/yr**.
54
Primary mechanism for radionuclide migration in clay repositories?
**Diffusion**, not flowing water.
55
Why is radon often higher indoors than outdoors?
Accumulates in confined spaces with low ventilation.
56
Radon mitigation method?
Sub-floor ventilation / positive pressure / sealing cracks.
57
Why does cosmic dose vary with solar cycle?
Solar activity affects cosmic ray shielding — more solar wind = lower cosmic rays.
58
59
What is the BUSC concept and why is it favoured internationally?
BUSC = Basement Under Sedimentary Cover. Waste is placed in crystalline basement overlain by thick, low-permeability sediments. The very low slope (~0.6°) of the basement causes near-stagnant groundwater, giving long residence times essential for containment. Used as conceptual model in Nirex studies.
60
Why is the Maryland (USA) section considered an ideal BUSC analogue?
It shows a deep, crystalline basement with minimal hydraulic gradient, thick sediments, and extremely slow groundwater flow. Vertical exaggeration demonstrates how flat and unreactive the hydrogeology is—ideal for reducing leakage risk.
61
How is the West Cumbria cross-section distorted compared to true BUSC?
Compared to Maryland: horizontal scale compressed 20×, terrain height exaggerated 20×, sedimentary dips steeper by 40×. Overall geometry distorted by factor 400, producing vigorous, complex flow not typical of BUSC.
62
Why is modelling groundwater flow in West Cumbria inherently uncertain?
Requires integrating borehole heads, fracture networks, porosity, and conductivity. BVG fractures cause results to vary by orders of magnitude. Small changes in conductivity produce huge changes in predicted travel time (3 m/yr vs 300 m/yr).
63
What does the modelling workflow (Model → Calibrate → Validate → Predict) show about reliability?
Calibration to borehole data was difficult; many models required unrealistic parameters to fit observations. Validation often failed. Predictive simulations therefore have low confidence, leading to large uncertainty in risk.
64
What does the final flow risk curve reveal?
The probability distribution extends far beyond the regulatory target of 10⁻⁶/yr. There is a wide range of possible outcomes, some strongly unsafe, due to fracture flow uncertainty and upward gradients.
65
Why must a GDF be placed in reducing, alkaline conditions?
Many radionuclides (U, Pu, Tc) form insoluble species under reducing conditions (e.g., U(IV)). In oxidising conditions, they convert to mobile, soluble forms (e.g., U(VI) complexes), increasing migration risk.
66
What evidence shows West Cumbria is oxidising?
Fracture surfaces coated with haematite (iron oxide) representing long-term oxidising flow; borehole waters also oxidising; sulphide presence is limited. Ancient geology, fractures, and current conditions all show lack of reducing environments.
67
Why is predicting -250 mV redox conditions unrealistic?
No natural reducing processes were observed. All measured Eh values reflect oxidation, so assuming persistent reducing conditions is unsupported. Models assuming -250 mV are not evidence-based.
68
Why did Nirex initially choose West Cumbria, and why was it rejected?
Sellafield already holds major waste inventory; thus disposal nearby was logistically appealing. However: poor hydrogeology, oxidising conditions, fractured rock, and modelling uncertainty led to scientific rejection and political rejection in 1997 and again in 2013.
69
What was the purpose of the 2010–2014 national restart?
To search for a site with community consent plus geological suitability. West Cumbria re-entered discussions, but public resistance and scientific concerns persisted.
70
What areas are now being considered (2021–2023)?
Three areas in Copeland & Allerdale, excluding the Lake District National Park; no use of old coal mines. Search continues under new “Working Group” approach.
71
What is the core reason West Cumbria is unsuitable for radioactive waste disposal?
The site relies entirely on engineered barriers; natural hydrogeology and geochemistry actively oppose containment (upward flow, oxidising fractures, high conductivity). A safe GDF requires geology that supports containment—not fights it.
72
What is recommended instead?
Identify geological settings with: slow, downward/stagnant flow; reducing chemistry; low fracture conductivity—so natural conditions help retain radionuclides for >100,000 years.
73
What is the primary aim of geological disposal?
To isolate ILW/HLW from the biosphere for up to 1 million years, meeting regulatory risk limits (10⁻⁶/yr). Not to find the “best” site, but a site that meets safety criteria.
74
Why may the UK need multiple disposal sites?
Waste types differ (LLW, ILW, HLW), geological conditions vary regionally, and community consent may only occur locally.
75
Why is surface storage not a final solution?
Vulnerable to climate change, terrorism, corrosion, and societal instability; requires perpetual maintenance and funding.
76
Why is deep geological disposal preferred over alternatives?
It uses natural geological stability to isolate waste without requiring perpetual human intervention. Other ideas (subduction, ocean dumping, space launch) have unacceptable environmental or safety risks.
77
Why is waste disposal politically difficult?
Transboundary concerns, local community resistance, “not in my backyard” attitudes, and uncertainty over geological predictions.
78
Why have Sweden and Finland succeeded?
Transparent process, local veto power, staged investigation, strong trust in institutions, and local financial benefits.
79
What is the UK’s required risk limit for radioactive waste disposal?
Annual risk of death must be ≤ 10⁻⁶ per year for 1 million years into the future.
80
Who regulates waste disposal in the UK?
NDA/RWM develop projects; Environment Agency regulates; Local Authorities manage planning; DESNZ/DEFRA approve final decisions.
81
What controversial change occurred in 2010?
Planning law removed the ability to debate the “national need” for a repository at inquiries, reducing democratic scrutiny.
82
What is the near field?
The engineered zone (100–600 m deep) where waste containers, backfill, and tunnel linings create multiple engineered barriers.
83
Why use bentonite clay?
Swells with water to seal fractures, absorbs radionuclides, provides mechanical support, and limits flow.
84
Why use copper canisters (KBS-3)?
Copper corrodes extremely slowly (<1 mm per 10,000 years), ensuring long containment of spent fuel.
85
What is the far field?
The surrounding natural rock mass that provides independent long-term containment via low permeability, stable chemistry, and slow groundwater.
86
What is the geologist’s role in GDF design?
Ensure the natural geological system supports containment — predictable flow, reducing conditions, few fractures.
87
Why must the near field be assumed to fail?
Over geological timescales, engineered barriers will degrade; far field must be the ultimate safety mechanism.
88
Advantages of salt deposits for disposal?
No groundwater, self-healing creep closes voids, long-term stability.
89
Advantages of claystone (e.g., Switzerland)?
Extremely low permeability, strong sorption capacity, predictable long-term evolution.
90
Advantages & disadvantages of crystalline rock?
Strong and stable but heavily fractured → requires robust engineered barriers.
91
Why is borehole disposal being considered?
Uses natural reducing conditions at depth (shales), cheaper (~50% less than tunneling), isolates waste vertically.
92
What are concerns with borehole disposal?
Limited waste volume, challenging retrieval, untested for large national inventories.
93
What determines whether a GDF “succeeds”?
A balance between: (1) good geology, (2) robust engineering, (3) public consent, (4) cost feasibility, and (5) regulatory compliance.
94
Why is leakage inevitable?
No barrier lasts forever; performance must be predicted and managed, not prevented absolutely.
95
What key questions must risk assessment answer?
What leaks? How much? How fast? When? Where does it go? How can it be monitored?
96
Where and when did the Chernobyl disaster occur?
Reactor 4, Chernobyl Nuclear Power Plant, Pripyat, Ukraine (USSR), 26 April 1986.
97
What type of reactor was involved- Chernobyl?
RBMK-1000 graphite-moderated, water-cooled reactor.
98
What inherent reactor design flaw contributed to the accident- Chernobyl?
Positive void coefficient → reactivity increases when water turns to steam, causing runaway power.
99
Why were graphite-tipped control rods dangerous- Chernobyl?
When inserted, they initially increased reactivity, creating a power spike.
100
What operational error triggered instability before the accident- Chernobyl?
Operators lowered power far below safe limits → xenon poisoning → reactor unstable.
101
Which safety systems were disabled during the test- Chernobyl?
Automatic shutdown systems, emergency cooling, and protection interlocks.
102
What caused the explosion- Chernobyl?
A rapid reactivity surge, sudden steam generation → steam explosion + reactor vessel rupture.
103
What caused the long-lasting fire- Chernobyl?
Exposure and ignition of the graphite moderator.
104
What operational error triggered instability before the accident- Chernobyl?
Operators lowered power far below safe limits → xenon poisoning → reactor unstable.
105
Which safety systems were disabled during the test- Chernobyl?
Automatic shutdown systems, emergency cooling, and protection interlocks.
106
What caused the explosion- Chernobyl?
A rapid reactivity surge, sudden steam generation → steam explosion + reactor vessel rupture.
107
What caused the long-lasting fire- Chernobyl?
Exposure and ignition of the graphite moderator.
108
How many people died from Acute Radiation Syndrome- Chernobyl?
28 workers and firefighters within weeks.
109
How many cases of ARS were confirmed- Chernobyl?
134
110
What long-term cancer is most strongly linked to Chernobyl?
Thyroid cancer, especially in children ingesting I-131 via milk.
111
Approximate number of excess thyroid cancer cases- Chernobyl?
~6,000.
112
Why were children particularly vulnerable- Chernobyl?
Higher milk consumption + smaller thyroid gland + active cell growth.
113
What was the “Red Forest”- Chernobyl?
Pine forest near the plant killed by intense radiation, turning reddish-brown.
114
What happened to wildlife over time- Chernobyl?
Rebounded strongly due to human evacuation, despite contamination.
115
Why does Cs-137 continue to cause problems?
Long half-life + mobility in soils + uptake into plants/mushrooms → enters food chain (e.g., wild boar).
116
What is the Exclusion Zone- Chernobyl?
30 km radius around the plant, still largely uninhabited.
117
What was the main exposure pathway for children- Chernobyl?
Ingestion of I-131 in contaminated milk.
118
Why was inhalation less significant long-term- Chernobyl?
I-131 decays quickly (8 days), and particulate fallout settled rapidly.
119
How did radionuclides spread across Europe- Chernobyl?
Atmospheric transport → plumes moved via changing wind patterns.
120
Why does Cs-137 persist in forests longer than in urban areas- Chernobyl?
Organic soils bind Cs strongly; slow turnover keeps it bioavailable for decades.
121
What was the 1986 “sarcophagus”- Chernobyl?
Emergency concrete enclosure built over Reactor 4 to reduce airborne contaminants.
122
Why was a new structure required- sarcophagus chernobyl?
The original sarcophagus was unstable, cracking, and releasing dust.
123
What is the New Safe Confinement (NSC)- Chernobyl?
A 108 m high steel arch built in 2016 to enclose and dismantle the old sarcophagus safely.
124
What radioactive waste problem still exists inside Chernobyl?
Fuel-containing materials (FCMs) like corium “lava”, including the “Elephant’s Foot”.
125
What are FCMs?
Fuel-Containing Materials: mixtures of molten core, concrete, steel (highly radioactive “lava”).
126
Why are FCMs dangerous?
Contain long-lived isotopes; brittle; release dust; difficult to remove.
127
What long-term challenge does Chernobyl highlight for global nuclear policy?
Need for geological disposal, not long-term surface containment.
128
Why will parts of the Exclusion Zone remain radioactive for centuries- Chernobyl?
Persistence of Cs-137, Sr-90, and plutonium isotopes.
129
What safety culture problems contributed to the accident- Chernobyl?
Poor training, rule violations, inadequate design reviews, hierarchical pressure to complete tests.
130
Why is transparency essential in nuclear oversight- Chernobyl?
Chernobyl showed secrecy delays response, increases exposure, and undermines public trust.
131
What does Chernobyl teach about “unknown unknowns”?
Real-world accidents can reveal failure modes not predicted in models (fires, FCM formation, long-term plume behaviour).
132
How did Chernobyl influence global nuclear safety?
Led to IAEA reform, improved emergency procedures, reactor design changes, and stress tests.
133
What is the multi-barrier concept in radioactive waste disposal?
A system where engineered near-field barriers and the natural far-field geological environment work together to prevent radionuclide migration.
134
Name the two major components of the multi-barrier system.
Near-field (engineered barriers) and far-field (geological environment).
135
Sketch the general structure of a repository.
Surface → shafts → host rock → bentonite → canister → wasteform.
136
What does “predictability” mean in nuclear waste geoscience?
Long-term stability and predictability of geological and hydrogeological conditions over 10⁴–10⁶ years.
137
What tectonic conditions increase predictability?
Low seismicity, no volcanism, minimal uplift, stable crustal block.
138
Why is hydrogeological predictability important?
Future changes in groundwater flow (e.g., glaciation) can accelerate radionuclide transport.
139
Which repositories were chosen for exceptional predictability?
Onkalo (Finland) and Forsmark (Sweden).
140
What does the near-field include?
Wasteform, canister, bentonite buffer, engineered materials, immediate host rock.
141
Why is HLW vitrified?
Immobilises radionuclides in a stable borosilicate glass matrix.
142
What is ILW typically encapsulated in?
Cementitious materials.
143
Describe the KBS-3 canister design.
Cast-iron inner insert + copper outer shell.
144
Key functions of bentonite in near-field?
Swelling seal, low permeability, sorption, reducing conditions, physical isolation.
145
Why is thermal modelling important?
HLW heat affects bentonite hydration and local geochemistry.
146
Which geochemical condition immobilises radionuclides?
Reducing conditions.
147
How does cement affect near-field chemistry?
Produces hyperalkaline (pH > 12) porewaters altering mineralogy and radionuclide mobility.
148
Why is canister corrosion modelling important?
Oxidising water accelerates corrosion, increasing release risk.
149
Favourable far-field hydrogeological properties?
Low permeability, long flow paths, stagnant groundwater, limited fracture connectivity.
150
Why are fractures problematic?
They act as preferential flow paths for radionuclides.
151
Why was Sellafield considered unsuitable?
Complex fractured geology & unpredictable hydrogeology.
152
What is sorption? Why important?
Radionuclides bind to minerals, slowing migration.
153
Which host rock has the highest sorption capacity?
Claystone (e.g., Cigéo, France).
154
What is geochemical buffering?
Host minerals maintain stable pH and redox, limiting radionuclide solubility.
155
Why is deep groundwater favourable?
It is saline, reducing, and chemically stable over long timescales.
156
Name three common host rocks.
Claystone, crystalline granite, rock salt.
157
Why is claystone used at Cigéo?
Ultra-low permeability & high sorption.
158
Why is rock salt favourable (e.g., WIPP)?
Self-sealing creep & dryness → very low permeability.
159
What is KBS-3 and where used?
Copper/iron canisters in bentonite within granite (Finland, Sweden).
160
What caused the 2014 WIPP accident?
Chemical reaction with organic absorbent leading to drum breach.
161
What does the WIPP incident teach?
Near-field chemistry must be strictly controlled.
162
Why is Cigéo notable?
Exploits claystone’s exceptional hydrogeochemical properties.
163
What are the three pillars of repository geoscience?
Predictability, groundwater behaviour, geochemistry.
164
Which conditions minimise radionuclide mobility?
Reducing chemistry, low permeability, high sorption, stable near-field.
165
What is the ultimate goal of repository design?
Keep radionuclides isolated until decay renders them harmless.
Environmental Problems and Issues
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