Tritium Management

Tritium (T or $^3$H) is a radioactive isotope used as fuel in fusion reactors. In DT fusion reactions, deuterium and tritium react to produce helium and high-energy neutrons. Tritium safety management is one of the most critical technical challenges for practical fusion reactors, requiring advanced technology and strict management systems.

This chapter provides a comprehensive overview of tritium, from fundamental physical and chemical properties to radiation protection, facility design, detection and measurement techniques, removal systems, accident response, and the international regulatory framework.

Physical and Chemical Properties

Atomic Structure and Mass

Tritium is an isotope of hydrogen with one proton and two neutrons in its nucleus. It has atomic number 1 and mass number 3. Comparing the hydrogen isotopes:

Isotope	Symbol	Protons	Neutrons	Atomic Mass (u)	Natural Abundance
Protium	H	1	0	1.007825	99.985%
Deuterium	D	1	1	2.014102	0.015%
Tritium	T	1	2	3.016049	Trace

Tritium’s atomic mass is approximately three times that of protium, but since the electron configuration is identical, its chemical properties are extremely similar to ordinary hydrogen. However, slight differences in reaction rates and equilibrium constants arise due to isotope effects from the mass difference.

Natural and Artificial Tritium Sources

Natural tritium is produced by nuclear reactions between cosmic rays and atmospheric nitrogen and oxygen:

$$^{14}\text{N} + n \rightarrow \, ^{12}\text{C} + T$$ $$^{16}\text{O} + n \rightarrow \, ^{14}\text{N} + T$$

The global natural production rate is approximately 70 PBq/year, with the steady-state inventory estimated at about 1.3 kg (1.3 EBq).

Artificial tritium sources include:

Source	Production Reaction	Annual Production (Estimated)
Nuclear reactors (heavy water)	$^2$H(n,γ)$^3$H	Several kg/reactor
Nuclear reactors (light water)	$^{10}$B(n,2α)T	Several hundred TBq/reactor
Nuclear weapons tests (past)	Various nuclear reactions	Peak of 200 kg
Lithium targets	$^6$Li(n,α)T	Depends on production facility
Accelerators	Spallation	Trace amounts

Commercial tritium production is primarily from Canadian CANDU reactors (heavy water reactors), supplying approximately 2.5 kg annually.

Thermodynamic Properties

The main thermodynamic properties of tritium molecules (T$_2$) and their mixed forms are shown below:

Property	T$_2$	HT	DT
Molecular weight (g/mol)	6.032	4.024	5.030
Boiling point (K)	25.04	22.92	23.67
Melting point (K)	20.62	17.63	19.71
Critical temperature (K)	40.44	35.91	37.84
Critical pressure (MPa)	1.85	1.57	1.70

At standard conditions (25°C, 1 atm), the density of tritium gas is:

$$\rho_{T_2} = \frac{P M_{T_2}}{RT} = \frac{101325 \times 6.032 \times 10^{-3}}{8.314 \times 298.15} \approx 0.247 \text{ kg/m}^3$$

This is about 1/5 of air density, so tritium tends to disperse upward when leaked.

The thermal conductivity of hydrogen isotope mixtures depends on molecular weight and is approximated by:

$$\lambda = \lambda_0 \sqrt{\frac{M_0}{M}}$$

Since tritium is heavier than hydrogen, it has lower thermal conductivity, which affects heat exchanger design.

Solubility and Hydrogen Isotope Exchange

Tritium dissolves in water and readily forms tritiated water through isotope exchange reactions with water. The solubility of tritium gas in water at 25°C is:

$$S_{T_2} = 0.0160 \text{ cm}^3(\text{STP})/\text{cm}^3\text{-H}_2\text{O}$$

The isotope exchange reaction with water is:

$$\text{T}_2 + \text{H}_2\text{O} \rightleftharpoons \text{HT} + \text{HTO}$$

The equilibrium constant for this reaction is temperature-dependent, at 25°C:

$$K_{eq} = \frac{[\text{HT}][\text{HTO}]}{[\text{T}_2][\text{H}_2\text{O}]} \approx 6.3$$

Therefore, molecular tritium readily converts to tritiated water in the presence of moisture, which is an important consideration in containment design.

The distribution coefficient for vapor-liquid equilibrium is:

$$\alpha_{l/g} = \frac{(T/H)_{liquid}}{(T/H)_{gas}}$$

At 20°C, α ≈ 0.9, meaning tritium is slightly concentrated in the gas phase.

Dissolution and Diffusion in Metals

Like hydrogen, tritium dissolves and diffuses in many metals. The solubility of hydrogen isotopes in metals follows Sieverts’ law:

$$c = K_s \sqrt{p}$$

Here, $c$ is the concentration in the metal, $K_s$ is Sieverts’ constant, and $p$ is the partial pressure in the gas phase.

Sieverts’ constant has temperature dependence expressed by an Arrhenius-type equation:

$$K_s = K_{s,0} \exp\left(-\frac{E_s}{RT}\right)$$

Tritium solubility parameters for major structural materials:

Material	$K_{s,0}$ (mol/m$^3$/Pa$^{0.5}$)	$E_s$ (kJ/mol)
Iron	$4.1 \times 10^{-3}$	28.9
Nickel	$4.5 \times 10^{-3}$	15.8
Copper	$3.4 \times 10^{-4}$	38.5
Tungsten	$9.0 \times 10^{-4}$	100.8
RAFM steel	$2.9 \times 10^{-3}$	26.1
Vanadium	$2.0 \times 10^{-2}$	-29.0
Titanium	$1.5 \times 10^{0}$	-53.0

The tritium diffusion coefficient in metals is also expressed by an Arrhenius-type equation:

$$D = D_0 \exp\left(-\frac{E_D}{RT}\right)$$

Representative diffusion parameters:

Material	$D_0$ (m$^2$/s)	$E_D$ (kJ/mol)
Iron (α phase)	$2.0 \times 10^{-7}$	10.5
Austenitic steel	$5.9 \times 10^{-7}$	53.9
Nickel	$4.8 \times 10^{-7}$	39.5
Copper	$1.1 \times 10^{-6}$	38.5
Tungsten	$4.1 \times 10^{-7}$	39.0

The permeation flux follows Fick’s law:

$$J = -D \frac{\partial c}{\partial x} = \frac{D K_s}{d} \left(\sqrt{p_1} - \sqrt{p_2}\right)$$

Here, $d$ is material thickness, and $p_1$ and $p_2$ are the partial pressures on the high and low pressure sides.

The permeation coefficient is defined as:

$$\Phi = D \cdot K_s = \Phi_0 \exp\left(-\frac{E_\Phi}{RT}\right)$$

This is an important parameter for material selection.

The time to reach steady-state permeation (lag time) is estimated as:

$$\tau_{lag} = \frac{d^2}{6D}$$

This is important for evaluating transient response.

Forms of Tritium Compounds

Main chemical forms of tritium encountered in fusion facilities:

Form	Chemical Formula	Characteristics	Biological Effect
Molecular tritium	T$_2$, HT, DT	Gas, permeable through metals	Low (poorly absorbed)
Tritiated water	HTO, DTO, T$_2$O	Liquid/vapor, high bioaffinity	High (easily absorbed)
Metal tritide	MT$_x$	Solid, used for storage	Medium (as dust)
Organic tritium compounds	C-T bond	Generated from lubricants, etc.	High (long retention)

Understanding the environmental behavior and conversion rates of each form is fundamental to safety assessment.

Radiation Characteristics

Beta Decay Mechanism

Tritium undergoes β$^-$ decay to $^3$He. During decay, a neutron converts to a proton, emitting an electron (beta particle) and an antineutrino:

$$^3\text{H} \rightarrow \, ^3\text{He} + e^- + \bar{\nu}_e + Q$$

The Q-value (decay energy) for this decay is:

$$Q = (m_{^3\text{H}} - m_{^3\text{He}})c^2 = 18.591 \text{ keV}$$

The emitted beta particles have a continuous energy spectrum, with the probability distribution at energy $E$:

$$N(E) \propto p E (Q-E)^2 F(Z,E)$$

Here, $p$ is the electron momentum, and $F(Z,E)$ is the Fermi function (Coulomb correction factor).

Key radiation parameters:

Parameter	Value
Maximum beta energy	18.591 keV
Average beta energy	5.69 keV
Most probable energy	3.0 keV
Half-life	12.312 years
Decay constant	$1.783 \times 10^{-9}$ s$^{-1}$

Tritium beta particles have very low energy, which gives them unique radiation protection characteristics.

Half-life and Decay Constant

The basic radioactive decay equation:

$$N(t) = N_0 \exp(-\lambda t)$$

From tritium’s half-life $t_{1/2} = 12.312$ years, the decay constant is:

$$\lambda = \frac{\ln 2}{t_{1/2}} = \frac{0.6931}{12.312 \times 365.25 \times 24 \times 3600} = 1.783 \times 10^{-9} \text{ s}^{-1}$$

The annual decay rate is:

$$\lambda_{yearly} = 1 - \exp(-\lambda \times 1 \text{ year}) \approx 0.0547 = 5.47\%$$

This means stored tritium decreases by approximately 5.5% annually, an important consideration for fuel management.

The time evolution of tritium inventory considering decay:

$$m(t) = m_0 \exp(-\lambda t)$$

The remaining fraction after 10 years:

$$\frac{m(10)}{m_0} = \exp(-1.783 \times 10^{-9} \times 10 \times 365.25 \times 24 \times 3600) = 0.569$$

In other words, approximately 43% decays in 10 years.

Specific Activity

Specific activity is defined as activity per unit mass:

$$A_{sp} = \frac{\lambda N_A}{M} = \frac{1.783 \times 10^{-9} \times 6.022 \times 10^{23}}{3.016}$$ $$A_{sp} = 3.56 \times 10^{14} \text{ Bq/g} = 9.62 \times 10^3 \text{ Ci/g}$$

Thus, one gram of tritium has approximately 9,620 curies (356 terabecquerels) of activity.

Conversely, 1 curie (37 GBq) of tritium is:

$$m = \frac{1 \text{ Ci}}{9620 \text{ Ci/g}} \approx 0.104 \text{ mg}$$

The relationship between concentration and activity (for gas at 25°C, 1 atm):

$$A = \frac{P \cdot V}{R \cdot T} \cdot \frac{2}{M_{DT}} \cdot A_{sp}$$

Tritium activity in 1 m$^3$ of DT gas at 1 Pa:

$$A = \frac{1 \times 1}{8.314 \times 298.15} \times \frac{2}{5.03} \times 3.56 \times 10^{14} \approx 5.7 \times 10^{10} \text{ Bq}$$

Beta Particle Range and Shielding

The range of low-energy beta particles in matter can be approximated by empirical formulas. The maximum range $R_{max}$ is:

$$R_{max} = 0.412 E^{1.265 - 0.0954 \ln E} \quad (\text{mg/cm}^2)$$

where $E$ is in MeV. For tritium beta particles (18.6 keV):

Medium	Density (g/cm$^3$)	Maximum Range
Air	0.00120	~6 mm
Water	1.00	~6 μm
Skin	1.10	~5 μm
Stainless steel	7.9	~0.8 μm
Plastic	1.2	~5 μm

The thickness of the skin’s stratum corneum (dead cell layer) is approximately 70 μm, which tritium beta particles cannot penetrate. Therefore, external exposure is practically negligible.

The average range under the continuous slowing down approximation (CSDA) is about 1/3 of the maximum range:

$$R_{avg} \approx \frac{R_{max}}{3}$$

Absence of Gamma Rays

Tritium beta decay does not emit gamma rays. This is a significant advantage for shielding design in fusion facilities. However, slight X-rays may be generated through bremsstrahlung:

$$P_{brem} = \frac{Z E_{max}}{3000} \approx 10^{-4}$$

Since bremsstrahlung efficiency is proportional to the absorber’s atomic number $Z$, using low atomic number materials (plastic, aluminum, etc.) minimizes this radiation.

Decay Products

The decay product of tritium, $^3$He, is a stable isotope with the following characteristics:

• Inert gas
• Accumulates as “ash” in fusion reactors
• Can be measured in trace amounts by mass spectrometry (isotope dilution method)

In sealed tritium-containing systems, $^3$He accumulation causes internal pressure rise:

$$P_{He}(t) = P_{T,0} \left(1 - \exp(-\lambda t)\right)$$

Periodic $^3$He removal is necessary for long-term storage.

Biological Effects and Exposure Assessment

Exposure Pathways

Human exposure to tritium occurs primarily through the following pathways:

Pathway	Main Chemical Form	Relative Importance
Inhalation	HTO, HT	High (HTO), Low (HT)
Skin absorption	HTO	Medium to High
Ingestion	HTO, OBT	High (via food/water)
Wound entry	All forms	Low (rare occurrence)

External exposure is negligible due to the short range of beta particles.

Biokinetic Models

The behavior of tritium once incorporated into the body varies greatly depending on its chemical form. ICRP (International Commission on Radiological Protection) provides several compartment models.

Biokinetics of Tritiated Water (HTO)

When ingested as tritiated water, it mixes completely with body water and is excreted exponentially. The two-component model:

$$R(t) = 0.97 \exp\left(-\frac{0.693 t}{10}\right) + 0.03 \exp\left(-\frac{0.693 t}{40}\right)$$

Here, $R(t)$ is the body retention fraction at time $t$ (days). The first component (97%) is excreted as body water with a biological half-life of 10 days, and the second component (3%) reflects incorporation into organic molecules with a 40-day half-life.

The effective biological half-life is:

$$T_{bio,eff} = \frac{0.97 \times 10 + 0.03 \times 40}{1} = 10.9 \text{ days}$$

Biokinetics of Organically Bound Tritium (OBT)

Organically bound tritium has longer biological half-lives:

$$R(t) = 0.5 \exp\left(-\frac{0.693 t}{40}\right) + 0.5 \exp\left(-\frac{0.693 t}{365}\right)$$

When incorporated into biomacromolecules such as DNA, local doses may be elevated.

The conversion rate from HTO to OBT in the body is estimated at approximately 2-3%/day.

Biokinetics of Molecular Tritium (HT)

Molecular tritium inhaled through the respiratory system is mostly exhaled without being absorbed:

• Lung retention time: Several minutes
• Blood absorption rate: ~0.005%
• In-body conversion rate to HTO: ~0.01%/hour (by intestinal bacteria, etc.)

Therefore, the dose coefficient for HT is approximately 1/10,000 that of HTO.

Dose Coefficients

Internal exposure dose is calculated as the product of intake and dose coefficient:

$$E = e(50) \times I$$

Here, $E$ is the committed effective dose (Sv), $e(50)$ is the 50-year committed dose coefficient (Sv/Bq), and $I$ is the intake (Bq).

Dose coefficients for adults based on ICRP Pub. 119:

Chemical Form	Intake Route	$e(50)$ (Sv/Bq)
HTO	Inhalation	$1.8 \times 10^{-11}$
HTO	Ingestion	$1.8 \times 10^{-11}$
HT	Inhalation	$1.8 \times 10^{-15}$
OBT	Ingestion	$4.2 \times 10^{-11}$
Skin absorption HTO	Dermal	$1.8 \times 10^{-11}$

Molecular tritium (HT) has a dose coefficient 4 orders of magnitude smaller because it is hardly incorporated into the body.

Dose coefficients by age:

Age	HTO Ingestion (Sv/Bq)	OBT Ingestion (Sv/Bq)
3 months	$6.4 \times 10^{-11}$	$1.2 \times 10^{-10}$
1 year	$4.8 \times 10^{-11}$	$1.2 \times 10^{-10}$
5 years	$3.1 \times 10^{-11}$	$7.3 \times 10^{-11}$
10 years	$2.3 \times 10^{-11}$	$5.7 \times 10^{-11}$
15 years	$1.8 \times 10^{-11}$	$4.2 \times 10^{-11}$
Adult	$1.8 \times 10^{-11}$	$4.2 \times 10^{-11}$

Infants have higher dose coefficients due to their higher water metabolism rate per body weight.

Exposure Dose Calculation Example

Dose when working for $t$ hours in an environment with air tritium concentration $C$ (Bq/m$^3$):

$$E = C \times B \times t \times e(50)$$

Here, $B$ is the breathing rate (approximately 1.2 m$^3$/h for adults).

For example, working 8 hours in an environment of 10$^5$ Bq/m$^3$:

$$E = 10^5 \times 1.2 \times 8 \times 1.8 \times 10^{-11} = 1.7 \times 10^{-5} \text{ Sv} = 17 \text{ μSv}$$

Total dose including skin absorption (skin absorption is approximately 50% of inhalation):

$$E_{total} = E_{inh} \times 1.5 = 25.5 \text{ μSv}$$

Dose Assessment from Urinary Tritium Concentration

Since body tritiated water equilibrates with body fluids, total body content can be estimated from urine concentration:

$$A_{body} = C_{urine} \times V_{body water}$$

Adult body water volume is approximately 42 liters:

$$A_{body} = C_{urine} \times 42 \text{ L}$$

The committed effective dose after a single intake is:

$$E = A_{body} \times e(50) = C_{urine} \times 42 \times 1.8 \times 10^{-11}$$

For continuous intake, the steady-state body activity is:

$$A_{ss} = \frac{\dot{I}}{\lambda_{eff}} = \frac{\dot{I} \times t_{1/2,bio}}{\ln 2}$$

Here, $\lambda_{eff}$ is the effective excretion constant.

Urine monitoring frequency is typically weekly to monthly. Response to abnormal values:

1. Confirmation by re-measurement
2. Investigation of exposure pathway
3. Dose assessment and recording
4. Work restrictions if necessary

Relative Biological Effectiveness (RBE)

The RBE of tritium beta particles is the ratio of biological effect to a reference radiation (usually gamma rays). Low-energy beta particles tend to have higher LET (linear energy transfer), and RBE may exceed 1:

$$\text{RBE} = \frac{D_{reference}}{D_{tritium}}$$

Studies indicate tritium RBE values:

• Chromosome aberration induction: 1.0-2.5
• Cell killing: 1.0-2.0
• Carcinogenesis: 1.0-3.0
• Genetic effects: 1.5-2.5

For regulatory purposes, ICRP adopts a radiation weighting factor $w_R = 1$ for tritium. However, some countries and experts advocate more conservative values (1.5-2).

The linear energy transfer (LET) of tritium beta particles is:

$$\text{LET} \approx 4.7 \text{ keV/μm}$$

This is higher than typical low-LET radiation (0.2 keV/μm) and provides the physical basis for RBE potentially exceeding 1.

Design of Tritium Handling Facilities

Basic Design Principles

Tritium facility design is based on the following principles:

1. ALARA principle: Keep exposure as low as reasonably achievable
2. Defense in depth: Prevent and mitigate accidents through multiple safety barriers
3. Passive safety: Safety functions that do not depend on active systems
4. Maintainability: Ease of remote operation and maintenance
5. Economics: Reasonable cost while ensuring safety

Defense in depth hierarchy:

Level	Objective	Means
1	Prevention of abnormalities	Conservative design, quality control
2	Prevention of escalation	Detection, alarms, safety systems
3	Accident response	Engineered safety features
4	Severe accident response	Accident management measures
5	Public protection	Offsite emergency response

Zoning Design

Facilities are zoned based on tritium handling quantities and contamination risk:

Zone	Tritium Concentration Indicator	Main Equipment/Areas	Access Restrictions
High-dose area	> 10$^7$ Bq/m$^3$	Main processing, storage systems	Special control
Medium-dose area	10$^4$-10$^7$ Bq/m$^3$	Analysis rooms, glovebox rooms	Controlled area
Low-dose area	< 10$^4$ Bq/m$^3$	Control rooms, corridors	Radiation control
Non-controlled area	< 10$^2$ Bq/m$^3$	Offices, general areas	No restrictions

Airlocks, changing rooms, and survey stations are installed for movement between zones.

Pressure differential management between zones:

$$\Delta P_{zone} = P_{higher} - P_{lower} < -25 \text{ Pa}$$

This prevents air outflow from high-contamination to low-contamination areas.

Ventilation and HVAC Systems

Ventilation design for tritium facilities is critical for preventing contamination spread:

Cascade ventilation: Progressively increasing negative pressure from non-controlled areas toward high-dose areas prevents backflow of contaminated air:

$$P_{low} < P_{medium} < P_{high} < P_{ambient}$$

Typical pressure differential settings:

Zone Boundary	Pressure Differential
Non-controlled/Low-dose area	-25 Pa
Low-dose/Medium-dose area	-50 Pa
Medium-dose/High-dose area	-100 Pa
Inside glovebox/Room	-200 to -500 Pa

Air change rates are typically 10-20 times/hour, with exhaust processed through tritium removal systems before release.

Ventilation system reliability:

• Fan redundancy (N+1 configuration)
• Continuous operation via emergency power
• Pressure monitoring and automatic control
• Multiple HEPA filters

Relationship between airflow and air change rate:

$$Q = n \times V$$

Here, $Q$ is flow rate (m$^3$/h), $n$ is air change rate (times/h), and $V$ is room volume (m$^3$).

Material Selection

The following material selection criteria are applied to minimize tritium permeation:

Low-permeability materials:

• Aluminum alloys (Al$_2$O$_3$ layer formation)
• Copper alloys
• Austenitic stainless steels
• Tungsten

Permeation barrier coatings:

• Alumina (Al$_2$O$_3$)
• Chromia (Cr$_2$O$_3$)
• Silica (SiO$_2$)
• TiN, TiC
• Er$_2$O$_3$

Permeation Reduction Factor (PRF) from coatings:

$$\text{PRF} = \frac{\Phi_{uncoated}}{\Phi_{coated}}$$

Good coatings achieve PRF = 100-10,000.

Relationship between coating thickness and PRF (ideal case):

$$\text{PRF} = 1 + \frac{\Phi_{metal} \cdot d_{coating}}{\Phi_{coating} \cdot d_{metal}}$$

In practice, coating defects (pinholes, cracks) result in PRF lower than theoretical values.

Containment Barriers and Gloveboxes

Multiple Containment Concept

Tritium containment in fusion reactors is based on a multiple barrier approach. Each barrier functions independently, ensuring that failure of a single barrier does not lead to direct environmental release.

Primary Barrier (Primary Containment)

Walls of equipment directly handling tritium:

• Vacuum vessel
• Piping, valves, pumps
• Heat exchangers
• Storage vessels

Design pressure is typically 1.5-3 times operating pressure, with stress analysis and fatigue evaluation. Welds undergo 100% non-destructive testing.

Design criteria:

$$P_{design} \geq 1.5 \times P_{max,operating}$$ $$t_{wall} \geq \frac{P \cdot D}{2 \sigma_{allow} \eta - P}$$

Here, $t_{wall}$ is required wall thickness, $D$ is inner diameter, $\sigma_{allow}$ is allowable stress, and $\eta$ is weld efficiency.

Secondary Barrier (Secondary Containment)

Contains leakage from the primary barrier:

• Gloveboxes
• Double-wall pipe structures
• Vacuum jackets
• Secondary containment vessels

Gloveboxes have airtight construction, with leak rates typically maintained at:

$$L < 0.1\% \text{ volume/hour}$$

Annulus pressure monitoring of double-wall pipes confirms primary barrier integrity:

$$\frac{dP_{annulus}}{dt} > threshold \Rightarrow \text{leak alarm}$$

Tertiary Barrier (Tertiary Containment)

Final containment by building structure:

• Tritium building
• Controlled area boundary
• Stack (exhaust chimney)

Building airtightness is designed at approximately:

$$L_{building} < 1\% \text{ volume/day}$$

Relationship between building concentration and environmental release rate:

$$\dot{Q}_{release} = C_{building} \times L_{building} \times V_{building}$$

Glovebox Design

Structural Design

Typical glovebox configuration:

• Enclosure material: Stainless steel (SUS304/316) 3-6 mm thick
• Window material: Tempered glass, polycarbonate 10-20 mm thick
• Gloves: Hypalon, butyl rubber, multi-layer construction
• Glove ports: Standard size 200-250 mm diameter

Design pressure differential:

$$\Delta P = P_{room} - P_{box} = 200 \sim 500 \text{ Pa}$$

This negative pressure ensures that even in case of glove rupture, contaminated air from inside the box does not leak into the room.

Inflow velocity during glove rupture (orifice flow):

$$v = C_d \sqrt{\frac{2 \Delta P}{\rho}}$$

With $\Delta P = 300$ Pa, air density 1.2 kg/m$^3$, and discharge coefficient 0.6:

$$v = 0.6 \times \sqrt{\frac{2 \times 300}{1.2}} = 13.4 \text{ m/s}$$

This inward flow prevents contamination spread.

Atmosphere Control

The atmosphere inside gloveboxes is controlled from the following perspectives:

Inert gas replacement:

• Replacement with nitrogen or argon
• Oxygen concentration < 10 ppm (prevents tritiated water formation)
• Moisture concentration < 10 ppm

Circulation purification system:

• Oxygen removal by catalyst
• Moisture removal by molecular sieves
• Organic removal by activated carbon

Typical circulation flow rate is:

$$Q = \frac{V_{box}}{\tau} \quad (\tau = 0.5 \sim 2 \text{ hours})$$

Time evolution of impurity concentration:

$$C(t) = C_0 \exp\left(-\frac{Q \eta}{V} t\right) + \frac{S}{Q \eta}\left(1 - \exp\left(-\frac{Q \eta}{V} t\right)\right)$$

Here, $\eta$ is purification efficiency and $S$ is generation rate.

Fire and Explosion Prevention

Hydrogen isotopes are flammable, with combustion range in air:

$$\text{LEL} = 4\% \quad \text{UEL} = 75\%$$

The minimum ignition energy is extremely small at approximately 0.02 mJ.

To prevent explosions:

• Maintain oxygen concentration below 4%
• Continuous oxygen concentration monitoring
• Inert gas purge system
• Pressure relief devices (rupture disks)

Explosion pressure estimate (worst case):

$$P_{max} = P_0 \times \frac{T_{combustion}}{T_0} \approx 8 \times P_0$$

Rupture disk design pressure accounts for this.

Tritium Storage Systems

Large quantities of tritium are stored as solid metal hydrides.

Metal hydride characteristics:

Material	Capacity (T/M ratio)	Dissociation Pressure (kPa at 300K)	Heat of Dissociation (kJ/mol)
ZrCo	3.0	10$^{-6}$	-82
LaNi$_5$	6.0	100	-31
TiMn$_{1.5}$	1.5	50	-28
Pd	0.6	3	-40
U	3.0	10$^{-3}$	-127

ZrCo (zirconium-cobalt alloy) has low dissociation pressure and moderate kinetics and is widely used for tritium storage.

Equilibrium pressure has temperature dependence described by the van’t Hoff equation:

$$\ln p = \frac{\Delta H}{RT} + \frac{\Delta S}{R}$$

Hydrogen is released upon heating and reabsorbed upon cooling.

Kinetics of absorption and release:

$$\frac{dx}{dt} = k \cdot f(x) \cdot g(p, p_{eq})$$

Avrami-Erofeev equation:

$$x = 1 - \exp(-(kt)^n)$$

Here, $n$ is a constant dependent on mechanism (2-3 for nucleation-growth models).

Tritium Accountancy and Inventory Management

Importance of Inventory Management

Tritium inventory management in fusion facilities is critically important for the following reasons:

1. Safety management: Basis for evaluating accident release quantities
2. Nuclear material control: Response to IAEA safeguards
3. Fuel economy: Efficient use of expensive tritium
4. Environmental management: Tracking and recording of releases

Measurement Systems and Methods

Gas System Inventory

Measurement of gaseous tritium:

$$m_T = \frac{P \cdot V}{R \cdot T} \cdot f_T \cdot M_T$$

Where:

• $P$: Pressure (measured)
• $V$: Volume (known)
• $T$: Temperature (measured)
• $f_T$: Tritium fraction (measured by mass spectrometry, etc.)
• $M_T$: Tritium atomic mass

Pressure measurement accuracy: ±0.5% (capacitance manometer) Temperature measurement accuracy: ±0.5 K Volume measurement accuracy: ±0.1% (calibrated vessels) Composition analysis accuracy: ±1-5%

Overall measurement accuracy:

$$\frac{\Delta m}{m} = \sqrt{\left(\frac{\Delta P}{P}\right)^2 + \left(\frac{\Delta V}{V}\right)^2 + \left(\frac{\Delta T}{T}\right)^2 + \left(\frac{\Delta f_T}{f_T}\right)^2}$$

Liquid System Inventory

Measurement of tritiated water:

$$A = C \times V$$

Activity concentration $C$ is measured by liquid scintillation counter.

$$C = \frac{(N - B) \times 60}{\epsilon \times V_s}$$

Here, $N$ is count rate, $B$ is background, $\epsilon$ is counting efficiency, and $V_s$ is sample volume.

Solid System Inventory

Tritium in metal hydrides:

$$m_T = m_{bed} \times x \times f_T \times \frac{M_T}{M_{metal}}$$

Here, $x$ is the hydrogen/metal atomic ratio.

Evaluation by calorimetry:

$$A = \frac{\dot{Q}}{E_{avg}} = \frac{\dot{Q}}{5.69 \times 10^{-3} \times 1.6 \times 10^{-16}}$$

Material Balance and Difference Evaluation

Periodic material balance evaluation:

$$\text{MUF} = (BI + X) - (EI + Y + D)$$

Where:

• MUF: Material Unaccounted For
• BI: Beginning Inventory
• X: Receipts
• EI: Ending Inventory
• Y: Shipments
• D: Decay loss

Acceptable MUF is statistically evaluated from measurement precision:

$$\sigma_{MUF} = \sqrt{\sum_i \sigma_i^2}$$

If MUF exceeds 3σ, investigation is required as an anomaly.

Holdup and Residual Inventory

Evaluation of tritium held up in piping and equipment:

$$m_{holdup} = \sum_i (c_i \times V_i)$$

Wall adsorption:

$$\Gamma = K_H \cdot p^n$$

Follows Henry-type (n=1) or Freundlich-type (n<1) isotherms.

Holdup accumulation after long-term operation:

$$m_{wall}(t) = m_{wall,\infty} \times (1 - \exp(-t/\tau))$$

For ITER, tritium adsorption on vacuum vessel inner walls is estimated at approximately 700 g.

Inventory Management System

Real-time inventory tracking system:

• Continuous measurement of pressure, temperature, and composition of each device
• Automatic recording to database
• Automatic material balance calculation
• Anomaly detection alarms

Update frequency:

• Process systems: 1 second to 1 minute
• Storage systems: 1 hour
• Comprehensive balance: 1 day to 1 week

Tritium Detection and Measurement Technology

Ionization Chambers

Ionization chambers are the standard method for tritium monitoring. Ion pairs generated by beta particles are collected between electrodes and measured as current.

Collection current is:

$$I = \frac{e \cdot A \cdot V \cdot E_{avg}}{W}$$

Where:

• $e$: Electron charge
• $A$: Activity concentration (Bq/m$^3$)
• $V$: Detector volume
• $E_{avg}$: Average beta energy (5.7 keV)
• $W$: Ion pair creation energy (~34 eV for air)

Numerical example ($A = 10^6$ Bq/m$^3$, $V = 1$ L):

$$I = \frac{1.6 \times 10^{-19} \times 10^6 \times 10^{-3} \times 5.7 \times 10^3}{34} = 2.7 \times 10^{-14} \text{ A}$$

Typical ionization chamber performance:

Parameter	Value
Detection volume	1-10 L
Measurement range	10$^2$-10$^{10}$ Bq/m$^3$
Response time	10-60 seconds
Accuracy	±10-20%
Minimum detectable concentration	~100 Bq/m$^3$

Proportional Counters

Proportional counters are used for higher sensitivity detection. Gas amplification allows detection of single beta particles as pulses.

Gas amplification depends on applied voltage:

$$M = \exp\left(\frac{\alpha (V - V_{th})}{p}\right)$$

Here, $\alpha$ is the Townsend coefficient, $V_{th}$ is threshold voltage, and $p$ is gas pressure.

Typical amplification factors are 10$^3$-10$^5$, enabling detection at background levels.

Liquid Scintillation Counting

Liquid scintillation counting (LSC) is used for quantifying tritium in environmental and biological samples.

Samples are mixed with organic scintillator (cocktail), and scintillation light from beta particles is measured by photomultiplier tubes.

Counting efficiency is:

$$\epsilon = \frac{N_{detected}}{A \cdot t}$$

For tritium’s low-energy beta particles, counting efficiencies of 25-50% are typically achieved.

Minimum Detectable Activity (MDA):

$$\text{MDA} = \frac{2.71 + 3.29\sqrt{B \cdot t}}{\epsilon \cdot t \cdot V}$$

Here, $B$ is background count rate, $t$ is counting time, and $V$ is sample volume.

Numerical example ($B = 20$ cpm, $t = 100$ min, $\epsilon = 0.3$, $V = 10$ mL):

$$\text{MDA} = \frac{2.71 + 3.29\sqrt{20 \times 100}}{0.3 \times 100 \times 10} = 0.56 \text{ Bq/mL}$$

Electrolytic enrichment can be combined for high-sensitivity measurements, improving detection limits by several hundred times.

Enrichment factor for electrolytic concentration:

$$F = \left(\frac{V_0}{V_f}\right)^{1-\beta}$$

Here, $\beta$ is the isotope separation factor (~0.05) and $V_0/V_f$ is the volume reduction ratio.

Helium-3 Ingrowth Method

For precise measurement of low-level tritium, the accumulation of the decay product $^3$He is measured.

After sealed storage for a period (weeks to months), $^3$He quantity is measured by mass spectrometer:

$$A_T = \frac{N_{He-3}}{\lambda T (1 - e^{-\lambda T})} \cdot \frac{1}{V}$$

Here, $T$ is the accumulation period.

Approximation for $\lambda T \ll 1$:

$$A_T \approx \frac{N_{He-3}}{T \cdot V}$$

This method is extremely precise (±1%) but requires weeks to months.

Laser Spectroscopy

New measurement technologies using laser spectroscopy are being developed:

Cavity Ring-Down Spectroscopy (CRDS):

• Precise measurement of absorption lines
• Real-time isotope ratio analysis
• Detection sensitivity: ppb level

Tunable Diode Laser Absorption Spectroscopy (TDLAS):

• Continuous monitoring capability
• Simultaneous multi-component measurement
• Response time: seconds

Real-time Monitoring Systems

Fusion facilities construct integrated monitoring systems combining multiple detectors:

$$A_{total} = \sum_i w_i \cdot A_i$$

Data from each detector is aggregated to a central control room, displaying:

• Real-time concentration distribution maps
• Trend graphs
• Alarm status
• Cumulative releases

Alarm levels are typically:

• Caution level: 50% of management target
• Alarm level: Management target
• Emergency level: 10× management target

Data archiving and analysis:

• Long-term trend analysis
• Abnormal pattern detection
• Preventive maintenance applications

Tritium Removal Systems

Catalytic Oxidation-Moisture Adsorption Method

This is the most widely adopted removal method, processing in a two-stage process.

Catalytic Oxidation

Platinum group catalysts (Pt, Pd, Rh) convert molecular tritium to water vapor:

$$\text{HT} + \frac{1}{2}\text{O}_2 \xrightarrow{\text{Pt/Al}_2\text{O}_3} \text{HTO}$$ $$\text{T}_2 + \frac{1}{2}\text{O}_2 \xrightarrow{\text{Pt/Al}_2\text{O}_3} \text{T}_2\text{O}$$

Reaction rate strongly depends on temperature, expressed by the Arrhenius equation:

$$k = A \exp\left(-\frac{E_a}{RT}\right)$$

Typical operating temperature is 150-400°C.

Catalyst performance parameters:

• Space velocity (SV): 1,000-10,000 h$^{-1}$
• Conversion efficiency: > 99.9%
• Catalyst lifetime: Several years to 10 years

Reactor design equation:

$$\frac{dC}{dz} = -\frac{k C}{u}$$

Required catalyst quantity:

$$W = \frac{Q}{SV}$$

Moisture Adsorption

Generated tritiated water is captured by molecular sieves (synthetic zeolites):

$$\text{MS} + n\text{HTO} \rightleftharpoons \text{MS} \cdot n\text{HTO}$$

Representative molecular sieves:

Type	Pore Size (Å)	Water Adsorption Capacity (g/g)	Recommended Use
3A	3	0.22	With organics present
4A	4	0.24	General purpose
5A	5	0.22	Large molecule handling
13X	10	0.28	High capacity

Adsorption equilibrium is approximated by the Langmuir isotherm:

$$q = q_m \frac{K p}{1 + K p}$$

Breakthrough time is:

$$t_b = \frac{W q_m}{Q C_0}$$

Here, $W$ is adsorbent mass, $Q$ is flow rate, and $C_0$ is inlet concentration.

Regeneration is performed by heating to 200-300°C with inert gas purge:

$$\text{MS} \cdot n\text{HTO} \xrightarrow{\text{heating}} \text{MS} + n\text{HTO}$$

Cryogenic Distillation

Cryogenic distillation is used for hydrogen isotope separation. Utilizing boiling point differences, H$_2$, HD, HT, D$_2$, DT, and T$_2$ are separated.

Boiling point data:

Molecule	Boiling Point (K)
H$_2$	20.4
HD	22.1
HT	22.9
D$_2$	23.7
DT	24.4
T$_2$	25.0

Separation factor α is:

$$\alpha = \frac{y_H / x_H}{y_T / x_T}$$

For the H$_2$/T$_2$ system, α ≈ 1.8.

Required theoretical stages are estimated by the Fenske equation:

$$N_{min} = \frac{\ln\left[(x_D / (1-x_D)) \cdot ((1-x_B) / x_B)\right]}{\ln \alpha}$$

Operating conditions:

• Temperature: 20-25 K
• Pressure: 100-200 kPa
• Reflux ratio: 5-20

ITER’s tritium plant plans approximately 180-stage distillation columns.

Palladium Membrane Permeation

Palladium selectively permeates hydrogen isotopes and is used for purification.

Permeation flux is:

$$J = \frac{\Phi}{d}\left(\sqrt{p_1} - \sqrt{p_2}\right)$$

Due to isotope effects, lighter isotopes permeate faster. Selectivity is:

$$S_{H/T} = \sqrt{\frac{M_T}{M_H}} \approx 1.73$$

Palladium-silver alloy (Pd-Ag 23%) has high hydrogen embrittlement resistance and is practically used.

Temperature dependence of permeation coefficient:

$$\Phi = 2.9 \times 10^{-8} \exp\left(-\frac{15.7}{RT}\right) \text{ mol/(m·s·Pa}^{0.5}\text{)}$$

Operating conditions:

• Temperature: 300-500°C
• Primary side pressure: 100-500 kPa
• Secondary side pressure: < 1 kPa

Water Detritiation System (WDS)

The following processes are used for decontaminating tritiated water:

Combined Electrolysis Catalytic Exchange (CECE)

$$2\text{HTO} \xrightarrow{\text{electrolysis}} 2\text{HT} + \text{O}_2$$

After electrolysis, tritium is concentrated in a catalytic exchange column:

$$\text{HTO(l)} + \text{H}_2\text{(g)} \rightleftharpoons \text{H}_2\text{O(l)} + \text{HT(g)}$$

Separation factor (25°C):

$$\alpha = \frac{(\text{T/H})_{liquid}}{(\text{T/H})_{gas}} \approx 6$$

Number of stages and separation performance:

$$\ln\left(\frac{C_{bottom}}{C_{top}}\right) = N \cdot \ln \alpha$$

Water Distillation

Atmospheric water distillation has a small separation factor (α ≈ 1.05), so vacuum distillation is being considered:

• Pressure: 10-50 kPa
• Separation factor: 1.1-1.2
• Multi-stage configuration required

Overall Performance of Removal Systems

Specifications of typical tritium removal systems:

Parameter	Value
Processing airflow	100-10,000 m$^3$/h
Inlet concentration	10$^4$-10$^{10}$ Bq/m$^3$
Removal efficiency	> 99.9%
Outlet concentration	< 10$^3$ Bq/m$^3$
Operating temperature	150-400°C (catalyst section)

Decontamination Factor (DF):

$$\text{DF} = \frac{C_{in}}{C_{out}} > 1000$$

Total decontamination factor for multi-stage systems:

$$\text{DF}_{total} = \prod_i \text{DF}_i$$

Tritium Behavior During Accidents

Classification of Leakage Events

Tritium leakage events anticipated in fusion facilities:

Category	Example Event	Release Order	Occurrence Frequency
Minor leak	Valve seal failure	< 1 GBq	10$^{-1}$/year
Medium leak	Pipe rupture	1-100 GBq	10$^{-2}$/year
Large leak	Vacuum vessel failure	100 GBq-1 TBq	10$^{-3}$/year
Design basis accident	Coolant leak	1-10 TBq	10$^{-4}$/year
Severe accident	Total loss of power + LOCA	> 10 TBq	10$^{-6}$/year

Safety system design requirements are specified for each event.

Indoor Dispersion Model

Tritium concentration changes in enclosed spaces are described by the following differential equation:

$$V \frac{dC}{dt} = S(t) - Q C - \lambda_r V C$$

Where:

• $V$: Room volume
• $C$: Concentration
• $S(t)$: Release rate
• $Q$: Ventilation flow rate
• $\lambda_r$: Deposition rate

Equilibrium concentration for steady release:

$$C_{ss} = \frac{S}{Q + \lambda_r V}$$

Concentration decay after instantaneous release:

$$C(t) = C_0 \exp\left(-\frac{Q + \lambda_r V}{V} t\right)$$

Half-time for ventilation:

$$t_{1/2} = \frac{\ln 2 \cdot V}{Q + \lambda_r V}$$

Atmospheric Dispersion Model

For environmental releases, atmospheric concentrations are estimated by the Gaussian plume model. For ground-level release:

$$C(x,y,0) = \frac{Q}{2\pi u \sigma_y \sigma_z} \exp\left(-\frac{y^2}{2\sigma_y^2}\right) \left[\exp\left(-\frac{H^2}{2\sigma_z^2}\right) + \exp\left(-\frac{H^2}{2\sigma_z^2}\right)\right]$$

Where:

• $Q$: Release rate (Bq/s)
• $u$: Wind speed
• $\sigma_y$, $\sigma_z$: Lateral and vertical dispersion parameters
• $H$: Effective release height

Dispersion parameters depend on atmospheric stability, following Pasquill-Gifford classification:

Stability	Conditions	$\sigma_y$ (100m)	$\sigma_z$ (100m)
A (Very unstable)	Strong insolation	22 m	20 m
D (Neutral)	Overcast/strong wind	8 m	4 m
F (Stable)	Nighttime/light wind	4 m	1.3 m

Dispersion parameters at distance $x$:

$$\sigma_y = a \cdot x^b, \quad \sigma_z = c \cdot x^d$$

Coefficients $a, b, c, d$ vary by stability class.

Conversion to HTO and Washout

Released tritium gas undergoes isotope exchange with atmospheric water vapor, gradually converting to tritiated water:

$$\frac{d[\text{HT}]}{dt} = -k_{ex} [\text{HT}] [\text{H}_2\text{O}]$$

Conversion rate depends on humidity and temperature; in summer, most converts to HTO within hours.

Estimated conversion half-time:

$$t_{1/2,conv} \approx \frac{10}{RH} \text{ hours}$$

Here, $RH$ is relative humidity (%).

Washout coefficient from precipitation:

$$\Lambda = \Lambda_0 I^{0.8}$$

Here, $I$ is precipitation intensity (mm/h), $\Lambda_0$ ≈ 10$^{-4}$ s$^{-1}$/(mm/h)$^{0.8}$.

Behavior in soil after surface deposition:

• Adsorption to surface soil
• Transfer to plants
• Percolation to groundwater

Dose Assessment

Public exposure doses during accidents are assessed through the following pathways:

1. Cloudshine (external exposure): Negligible for tritium
2. Inhalation intake
3. Skin absorption
4. Food ingestion (long-term assessment)

Inhalation dose:

$$E_{inh} = \int_0^T C(t) \cdot B \cdot e_{inh}(50) \, dt$$

Skin absorption is often evaluated as approximately 50% of inhalation:

$$E_{skin} \approx 0.5 \times E_{inh}$$

Total dose:

$$E_{total} = E_{inh} + E_{skin} + E_{ingestion}$$

Site boundary dose targets (example):

• Normal operation: < 10 μSv/year
• Design basis accident: < 1 mSv
• Severe accident: < 100 mSv (below evacuation criteria)

ITER Tritium Management System

ITER Overview

ITER (International Thermonuclear Experimental Reactor) is an international fusion experimental reactor under construction in Cadarache, France. As the world’s first device to achieve DT burning plasma, demonstrating tritium management technology is an important mission.

Key parameters:

Parameter	Value
Fusion power	500 MW
Energy gain Q	≥ 10
Pulse duration	400-3000 seconds
Tritium consumption rate	~0.5 g/pulse
In-plant tritium inventory	< 4 kg
Site boundary dose target	< 10 μSv/year

Tritium Plant

The ITER tritium plant consists of the following subsystems:

Storage and Delivery System (SDS)

• Storage in metal hydride beds
• Uses ZrCo alloy
• Maximum storage: ~700 g/bed
• Number of beds: ~20

Storage bed hydrogen absorption rate:

$$\frac{dm}{dt} = k_a \sqrt{p} (m_{max} - m)$$

Release rate upon heating:

$$\frac{dm}{dt} = k_d m \exp\left(-\frac{E_d}{RT}\right)$$

Absorption/release cycle time: ~2 hours

Tokamak Exhaust Processing (TEP)

Processes plasma exhaust gas and recovers tritium:

• Front End Permeator (FEP)
• Impurity removal columns
• Cryogenic adsorption pumps
• Catalytic reactors

Processing flow:

1. Hydrogen isotope separation from exhaust gas
2. Helium and impurity removal
3. Hydrogen isotope purification
4. Tritium concentration and storage

Processing capacity: Up to 200 Pa·m$^3$/s

Isotope Separation System (ISS)

Isotope separation by cryogenic distillation:

• Distillation column stages: ~180
• Operating temperature: 20-25 K
• D-T ratio adjustment

Separation performance:

• DT purity: > 98%
• Impurity concentration: < 100 ppm

Water Detritiation System (WDS)

Electrolyzes tritiated water and separates isotopes:

$$2\text{HTO} \xrightarrow{\text{electrolysis}} 2\text{HT} + \text{O}_2$$

Combined Electrolysis Catalytic Exchange (CECE) process:

• Electrolysis cells
• Catalytic exchange columns
• Processing capacity: ~15 kg/h

Separation performance: Decontamination factor > 10$^4$

Atmosphere Detritiation System (ADS)

Removes tritium from indoor air:

• Catalytic oxidizers (noble metal catalyst)
• Molecular sieve dryers
• Processing airflow: ~50,000 m$^3$/h

Normal operation: Circulation mode During accidents: Once-through mode

Analytical System (ANS)

Analytical equipment for process monitoring:

• Gas chromatographs
• Mass spectrometers
• Ionization chambers
• Laser Raman spectroscopy

Measurement accuracy: Composition ±1%, Pressure ±0.5%

Blanket and Tritium Breeding

ITER Test Blanket Modules (TBM) demonstrate tritium breeding:

Lithium breeding reactions:

$$^6\text{Li} + n \rightarrow \, ^4\text{He} + T + 4.78 \text{ MeV}$$ $$^7\text{Li} + n \rightarrow \, ^4\text{He} + T + n' - 2.47 \text{ MeV}$$

The $^6$Li reaction has a large cross-section for thermal neutrons, while the $^7$Li reaction occurs with fast neutrons.

Tritium Breeding Ratio (TBR) is necessary for reactor self-sufficiency:

$$\text{TBR} = \frac{\text{Tritium produced}}{\text{Tritium consumed}} > 1$$

ITER TBM target TBR: 1.05-1.15

ITER Safety Analysis

Release evaluation for design basis accidents:

Event Category	Assumed Release	Site Boundary Dose
Category I (Operational events)	< 0.3 g	< 1 μSv
Category II (Anticipated events)	< 1 g	< 10 μSv
Category III (Hypothetical accidents)	< 10 g	< 100 μSv
Category IV (Design basis accidents)	< 100 g	< 1 mSv

Designed to satisfy conditions requiring no evacuation.

Mitigation measures:

• Passive containment maintenance
• Automatic startup of tritium removal systems
• Isolation of ventilation systems

DEMO Tritium Management

DEMO Positioning

DEMO (Demonstration Power Plant) is the prototype reactor planned as the next step after ITER. As a pre-commercial stage, it will demonstrate:

• Power supply to the grid
• Tritium self-sufficiency
• Long-term continuous operation
• Economic viability prospects

DEMO Tritium Requirements

Comparison of ITER and DEMO:

Parameter	ITER	DEMO
Fusion power	500 MW	2000-3000 MW
Pulse duration	400 seconds	Continuous or several hours
Tritium consumption rate	0.5 g/pulse	~150 kg/year
Required TBR	N/A (external supply)	> 1.1
Initial tritium inventory	4 kg	5-10 kg

Tritium Self-Sufficiency

DEMO requires complete tritium self-sufficiency:

Tritium balance equation:

$$\frac{dm_T}{dt} = \dot{m}_{breed} - \dot{m}_{burn} - \lambda m_T - \dot{m}_{loss}$$

Self-sufficiency condition:

$$\text{TBR} > 1 + \frac{\lambda m_{inventory}}{\dot{m}_{burn}} + \frac{\dot{m}_{loss}}{\dot{m}_{burn}}$$

Typical values:

• Decay loss term: ~0.05
• System loss term: ~0.05
• Required TBR: > 1.1

Blanket Design

Blanket concepts being considered for DEMO:

Concept	Breeder Material	Coolant	Features
HCPB	Li$_4$SiO$_4$	He	Solid breeding, mature technology
WCLL	PbLi	Water	Liquid breeding, high TBR
DCLL	PbLi	He/PbLi	Self-cooling, high efficiency

Tritium extraction:

Tritium extraction from solid breeder materials:

• Purge gas (He + 0.1% H$_2$) circulation
• Extraction temperature: 400-600°C
• Extraction efficiency: > 95%

Tritium extraction from liquid breeder (PbLi):

• Vacuum degassing
• Permeators
• Bubble columns

DEMO Tritium Plant

DEMO requires a large-scale plant leveraging ITER experience:

Subsystem	ITER	DEMO (Estimated)
Fuel processing	1 kg/day	1 kg/hour
Water processing	15 kg/h	100 kg/h
Atmosphere processing	50,000 m$^3$/h	200,000 m$^3$/h

Technical challenges:

• Continuous operation reliability
• Extended maintenance intervals
• Remote maintenance technology
• Cost reduction

Commercial Reactor Outlook

For future commercial reactors:

• Tritium processing: kg/hour scale
• TBR margin: > 1.15 (accounting for uncertainties)
• Downtime: < 10%
• Tritium inventory minimization

Economic evaluation:

$$\text{COE} = \frac{(I + O\&M + F)}{E_{net}}$$

Tritium-related costs are estimated at 5-10% of total cost.

Regulations and Safety Standards

International Regulatory Framework

International regulations and guidance for tritium handling:

Organization	Document	Content
ICRP	Pub. 119	Dose coefficients
ICRP	Pub. 103	Basic recommendations on radiation protection
ICRP	Pub. 134	Tritium dose assessment
IAEA	GSR Part 3	Basic safety standards for radiation protection
IAEA	SSG-2	Safety of nuclear fuel cycle facilities
IAEA	SSR-4	Safety of nuclear facilities

Dose Limits

Dose limits based on ICRP recommendations:

Subject	Effective Dose Limit
Occupational exposure	50 mSv/year, 20 mSv/year averaged over 5 years
Public exposure	1 mSv/year
Pregnant women (occupational)	2 mSv to abdomen (after pregnancy declaration)

Management target values are typically set at 1/10 to 1/100 of limits.

Exposure reduction target for achieving ALARA:

$$\text{Collective Dose} \times \text{Monetary Value} < \text{Protection Cost}$$

ICRP recommended α value: ~$10,000-30,000/person·Sv

Concentration Limits

Examples of work environment and emission standards (Japanese regulations):

Item	Standard Value
Air concentration limit (inhalation)	8 × 10$^5$ Bq/m$^3$ (HTO)
Exhaust concentration limit	5 × 10$^1$ Bq/cm$^3$ (HTO)
Effluent concentration limit	6 × 10$^1$ Bq/cm$^3$ (HTO)
Drinking water standard	10 Bq/L (WHO recommendation)

Derived Air Concentration (DAC):

$$\text{DAC} = \frac{\text{ALI}}{2400 \text{ m}^3}$$

Here, ALI (Annual Limit on Intake) is calculated from occupational exposure limits.

HTO DAC calculation:

$$\text{ALI} = \frac{20 \times 10^{-3}}{1.8 \times 10^{-11}} = 1.1 \times 10^9 \text{ Bq}$$ $$\text{DAC} = \frac{1.1 \times 10^9}{2400} = 4.6 \times 10^5 \text{ Bq/m}^3$$

Fusion Facility-Specific Regulations

Fusion facilities have different characteristics from conventional reactors and nuclear fuel facilities, so dedicated regulatory frameworks are being discussed:

Distinctive points:

• No chain reaction (no criticality accident)
• Limited tritium inventory (kg order)
• High passive safety
• No high-level waste generated
• No runaway accident potential

In many countries, licensing is conducted under existing radiation protection laws or nuclear reactor regulations, but development of fusion-specific regulatory systems is progressing.

Situation in Japan:

• Act on the Regulation of Nuclear Source Material, Nuclear Fuel Material and Reactors
• Fusion reactors may not qualify as “nuclear reactors”
• Regulation under Radioisotope Regulation Act being considered

Quality Assurance and Inspection

Quality assurance program for tritium facilities:

Design phase:

• Safety analysis document review
• Design reviews (HAZOP, FMEA)
• Probabilistic Safety Assessment (PSA)
• Independent Verification and Validation (IV&V)

Fabrication and construction phase:

• Weld inspection (RT, UT, PT, MT)
• Pressure and leak testing
• Installation inspection
• Functional testing

Operation phase:

• Periodic inspections
• In-Service Inspection (ISI)
• Operating experience feedback
• Aging management

Example inspection frequency:

Item	Frequency
Leak inspection	Annual
Safety valve operation test	Annual
Alarm system test	Monthly
Monitoring calibration	Quarterly

Tritium Handling Supervisor

In Japan, use of radioactive isotopes including tritium requires appointment of a Radiation Protection Supervisor.

Supervisor responsibilities:

• Supervision of radiation hazard prevention
• Facility and equipment maintenance
• Worker education and training
• Exposure management
• Accident response
• Communication with regulatory authorities

Qualification requirements:

• Class 1 Radiation Protection Supervisor license
• Specialized knowledge in fusion field

Future Outlook

Tritium Self-Sufficiency Technology

Commercial fusion reactors require tritium breeding in blankets. Technical challenges to achieve target TBR:

• Li-6 enrichment technology (natural abundance 7.5% → 90%)
• High-performance breeding materials (Li$_2$TiO$_3$, Li$_4$SiO$_4$, LiPb, etc.)
• Tritium extraction technology
• Neutron multipliers (Be, Pb)
• Blanket structure optimization

Tritium balance:

$$\dot{m}_{bred} = \dot{m}_{burn} \times \text{TBR} - \lambda m_{inventory}$$

Self-sustainability condition:

$$\text{TBR} > 1 + \frac{\lambda m_{inventory}}{\dot{m}_{burn}} + \text{losses}$$

Numerical example (2 GW reactor):

• Burn rate: ~150 kg/year
• Inventory: 10 kg
• Decay loss: 0.55 kg/year
• System loss: 5 kg/year (assumed)
• Required TBR: > 1.04

Advanced Containment Technology

Advanced technologies for further reducing tritium permeation:

Multi-layer barrier coatings:

• Er$_2$O$_3$/W multilayer films
• PRF > 10,000
• High-temperature stability

Ceramic composites:

• SiC/SiC composites
• Low activation
• High-temperature strength

Liquid metal blankets:

• Self-cooling
• Immediate tritium extraction
• Neutron multiplication

Superconducting double-wall structure:

• Reduced tritium permeation at cryogenic operation
• Utilization of insulating vacuum layer
• Secondary containment function

Advanced Measurement Technology

Next-generation monitoring systems:

• Real-time isotope analysis by laser spectroscopy (ppt level)
• Micro ionization chamber arrays (improved spatial resolution)
• IoT/AI-optimized process monitoring
• Predictive maintenance via digital twin technology
• Machine learning-based anomaly detection

Remote sensing:

• Environmental monitoring by drones
• Integration with satellite data

Waste Management

Characteristics of tritium waste:

• Classified as low-level waste
• Relatively short half-life (12.3 years)
• Decay storage is effective

Decay storage period:

$$t = \frac{\ln(A_0/A_{limit})}{\lambda}$$

After 10 half-lives (~123 years), activity reduces to 1/1000.

Waste forms:

• Molecular sieves (impregnated with tritiated water)
• Metal hydrides
• Contaminated equipment
• Decontamination effluents

International Cooperation and Knowledge Sharing

Tritium technology is a common foundation for fusion development, and international cooperation is being promoted:

• Technology demonstration at ITER
• Broader Approach (BA) activities
• IEA Fusion Power Implementing Agreement
• Bilateral cooperation (Japan-EU, Japan-US, etc.)
• IAEA technical meetings

Technology transfer and human resource development:

• International training programs
• Database sharing
• Safety culture dissemination

Sharing tritium handling know-how and safety culture is essential for early realization of fusion energy.

Related Topics

• Confinement Methods: Overview - Fundamentals of plasma confinement
• Blanket - Tritium breeding materials
• ITER Project - World’s first burning plasma experiment
• Safety Overview - Basic concepts of fusion safety