From Wikipedia, the free encyclopedia
Tritium (
or
; symbol
T
or
3H
, also known as
hydrogen-3) is a
radioactive isotope of hydrogen. The
nucleus of tritium (sometimes called a
triton) contains one
proton and two
neutrons, whereas the nucleus of
protium
(by far the most abundant hydrogen isotope) contains one proton and no
neutrons. Naturally occurring tritium is extremely rare on Earth, where
trace amounts are formed by the interaction of the atmosphere with
cosmic rays. It can be produced by irradiating
lithium metal or lithium-bearing ceramic pebbles in a
nuclear reactor. Tritium is used as a
radioactive tracer, in
radioluminescent light sources for watches and instruments, and, along with
deuterium,
as a fuel for nuclear fusion reactions with applications in energy
generation and weapons. The name of this isotope is derived from
Greek, Modern τρίτος
(trítos), meaning 'third'.
Decay
-
and it releases 18.6
keV of energy in the process. The
electron's kinetic energy varies, with an average of 5.7 keV, while the remaining energy is carried off by the nearly undetectable
electron antineutrino.
Beta particles
from tritium can penetrate only about 6.0 mm of air, and they are
incapable of passing through the dead outermost layer of human skin. The unusually low energy released in the tritium beta decay makes the decay (along with that of
rhenium-187) appropriate for absolute neutrino mass measurements in the laboratory (the most recent experiment being
KATRIN).
Production
Lithium
Tritium is produced in
nuclear reactors by
neutron activation of
lithium-6. This is possible with neutrons of any energy, and is an
exothermic reaction yielding 4.8 MeV. In comparison, the
fusion of deuterium with tritium releases about 17.6 MeV of energy. For applications in proposed fusion energy reactors, such as
ITER, pebbles consisting of lithium bearing ceramics including Li
2TiO
3 and Li
4SiO
4, are being developed for tritium breeding within a helium cooled pebble bed (HCPB), also known as a breeder blanket.
-
High-energy neutrons can also produce tritium from
lithium-7 in an
endothermic (net heat consuming) reaction, consuming 2.466 MeV. This was discovered when the 1954
Castle Bravo nuclear test produced an unexpectedly high yield.
-
Boron
High-energy neutrons irradiating
boron-10 will also occasionally produce tritium:
-
A more common result of boron-10 neutron capture is
7Li
and a single
alpha particle.
Deuterium
Tritium is also produced in
heavy water-moderated reactors whenever a
deuterium nucleus captures a neutron. This reaction has a quite small absorption
cross section, making
heavy water a good
neutron moderator,
and relatively little tritium is produced. Even so, cleaning tritium
from the moderator may be desirable after several years to reduce the
risk of its escaping to the environment.
Ontario Power Generation's
"Tritium Removal Facility" processes up to 2,500 tonnes (2,500 long
tons; 2,800 short tons) of heavy water a year, and it separates out
about 2.5 kg (5.5 lb) of tritium, making it available for other uses.
Fission
Fukushima Daiichi
In June 2016 the Tritiated Water Task Force released a report on the status of tritium in tritiated water at
Fukushima Daiichi nuclear plant,
as part of considering options for final disposal of this water. This
identified that the March 2016 holding of tritium on-site was
760
TBq (equivalent to 2.1 g of tritium or 14 mL of tritiated water) in a total of 860000 m
3
of stored water. This report also identified the reducing concentration
of tritium in the water extracted from the buildings etc. for storage,
seeing a factor of ten decrease over the five years considered
(2011-2016), 3.3 MBq/L to 0.3 MBq/L (after correction for the 5% annual
decay of tritium).
According to a report by an expert panel considering the best approach to dealing with this issue,
Tritium
could be separated theoretically, but there is no practical separation
technology on an industrial scale. Accordingly, a controlled
environmental release is said to be the best way to treat
low-tritium-concentration water.
Helium-3
-
Cosmic rays
Tritium occurs naturally due to
cosmic rays interacting with atmospheric gases. In the most important reaction for natural production, a
fast neutron (which must have energy greater than 4.0
MeV) interacts with atmospheric
nitrogen:
-
Worldwide, the production of tritium from natural sources is 148
petabecquerels
per year. The global equilibrium inventory of tritium created by
natural sources remains approximately constant at 2,590 petabecquerels.
This is due to a fixed production rate and losses proportional to the
inventory.
Production history
The production of tritium was resumed with
irradiation of rods containing
lithium (replacing the usual
control rods containing
boron,
cadmium, or
hafnium), at the reactors of the commercial
Watts Bar Nuclear Generating Station
from 2003–2005 followed by extraction of tritium from the rods at the
new Tritium Extraction Facility at the Savannah River Site beginning in
November 2006.
Tritium leakage from the rods during reactor operations limits the
number that can be used in any reactor without exceeding the maximum
allowed tritium levels in the coolant.
Properties
Tritium figures prominently in studies of
nuclear fusion because of its favorable reaction
cross section and the large amount of energy (17.6 MeV) produced through its reaction with deuterium:
-
All atomic nuclei contain protons as their only electrically charged
particles. They therefore repel one another because like charges repel.
However, if the atoms have a high enough temperature and pressure (for
example, in the core of the Sun), then their random motions can overcome
such electrical repulsion (called the
Coulomb force), and they can come close enough for the
strong nuclear force to take effect, fusing them into heavier atoms.
The tritium nucleus, containing one proton and two neutrons,
has the same charge as the nucleus of ordinary hydrogen, and it
experiences the same electrostatic repulsive force when brought close to
another atomic nucleus. However, the neutrons in the tritium nucleus
increase the attractive strong nuclear force when brought close enough
to another atomic nucleus. As a result, tritium can more easily fuse
with other light atoms, compared with the ability of ordinary hydrogen
to do so.
The same is true, albeit to a lesser extent, of deuterium. This is why
brown dwarfs (so-called failed
stars) cannot utilize ordinary hydrogen, but they do fuse the small minority of deuterium nuclei.
Radioluminescent 1.8 curies (67 GBq)
6 by 0.2 inches (152.4 mm × 5.1 mm) tritium vials are thin,
tritium-gas-filled glass vials whose inner surfaces are coated with a phosphor. The vial shown here is brand-new.
Like the other isotopes of
hydrogen,
tritium is difficult to confine. Rubber, plastic, and some kinds of
steel are all somewhat permeable. This has raised concerns that if
tritium were used in large quantities, in particular for
fusion reactors, it may contribute to
radioactive contamination, although its short half-life should prevent significant long-term accumulation in the atmosphere.
The high levels of atmospheric
nuclear weapons testing that took place prior to the enactment of the
Partial Test Ban Treaty
proved to be unexpectedly useful to oceanographers. The high levels of
tritium oxide introduced into upper layers of the oceans have been used
in the years since then to measure the rate of mixing of the upper
layers of the oceans with their lower levels.
Health risks
Tritium is an isotope of hydrogen, which allows it to readily bind to
hydroxyl radicals, forming
tritiated water (
HT
O), and to carbon atoms. Since tritium is a low energy
beta emitter, it is not dangerous externally (its beta particles are unable to penetrate the skin), but it can be a radiation hazard when inhaled, ingested via food or water, or absorbed through the skin. HTO has a short
biological half-life in the human body of 7 to 14 days, which both reduces the total effects of single-incident ingestion and precludes long-term
bioaccumulation of HTO from the environment.
The biological half life of tritiated water in the human body, which is
a measure of body water turn over, varies with the season. Studies on
the biological half life of occupational radiation workers for free
water tritium in the coastal region of Karnataka, India, show that the
biological half life in the winter season is twice that of the summer
season.
Environmental contamination
Tritium
has leaked from 48 of 65 nuclear sites in the US. In one case, leaking
water contained 7.5 microcuries (280 kBq) of tritium per litre, which is
375 times the EPA limit for drinking water.
According to the U.S.
Environmental Protection Agency, self-illuminating exit signs improperly disposed in municipal landfills have been recently found out to contaminate waterways.
Regulatory limits
The legal limits for tritium in
drinking water vary from country to country. Some figures are given below:
Tritium drinking water limits by country
| Country
|
Tritium limit (Bq/l)
|
| Australia
|
76103
|
| Japan
|
60000
|
| Finland
|
30000
|
| WHO
|
10000
|
| Switzerland
|
10000
|
| Russia
|
7700
|
| Ontario (Canada)
|
7000
|
| United States
|
740
|
The American limit is calculated to yield a dose of 4.0
millirems (or 40
microsieverts in
SI units) per year. This is about 1.3% of the natural background radiation (roughly 3,000 μSv).
Use
Self-powered lighting
The beta particles emitted by the radioactive decay of small amounts of tritium cause chemicals called
phosphors to glow. This
radioluminescence is used in
self-powered lighting devices called
betalights, which are used for night illumination of firearm sights,
watches,
exit signs, map lights, knives and a variety of other devices. Tritium has replaced
radioluminescent paint containing
radium in this application, which can cause
bone cancer and has been banned in most countries for decades. As of 2000, commercial demand for tritium is 400 grams per year and the cost is approximately US$30,000 per gram.
Nuclear weapons
Neutron initiator
These are devices incorporated in
nuclear weapons which produce a pulse of neutrons when the bomb is detonated to initiate the
fission reaction in the fissionable core (pit) of the bomb, after it is compressed to a
critical mass by explosives. Actuated by an ultrafast switch like a
krytron, a small
particle accelerator drives
ions of tritium and deuterium to energies above the 15
keV or so needed for deuterium-tritium fusion and directs them into a metal target where the tritium and deuterium are
adsorbed as
hydrides. High-energy
fusion neutrons
from the resulting fusion radiate in all directions. Some of these
strike plutonium or uranium nuclei in the primary's pit, initiating
nuclear chain reaction.
The quantity of neutrons produced is large in absolute numbers,
allowing the pit to quickly achieve neutron levels that would otherwise
need many more generations of chain reaction, though still small
compared to the total number of nuclei in the pit.
Boosting
Before detonation, a few grams of tritium-deuterium gas are injected into the hollow "
pit"
of fissile plutonium or uranium. The early stages of the fission chain
reaction supply enough heat and compression to start deuterium-tritium
fusion, then both fission and fusion proceed in parallel, the fission
assisting the fusion by continuing heating and compression, and the
fusion assisting the fission with highly energetic (14.1
MeV)
neutrons. As the fission fuel depletes and also explodes outward, it
falls below the density needed to stay critical by itself, but the
fusion neutrons make the fission process progress faster and continue
longer than it would without boosting. Increased yield comes
overwhelmingly from the increase in fission. The energy released by the
fusion itself is much smaller because the amount of fusion fuel is so
much smaller. The effects of boosting include:
- increased yield (for the same amount of fission fuel, compared to detonation without boosting)
- the possibility of variable yield by varying the amount of fusion fuel
- allowing the bomb to require a smaller amount of the very expensive
fissile material – and also eliminating the risk of predetonation by
nearby nuclear explosions
- not so stringent requirements on the implosion setup, allowing for a smaller and lighter amount of high-explosives to be used
The tritium in a
warhead is continually undergoing radioactive decay, hence becoming unavailable for fusion. Furthermore its
decay product,
helium-3, absorbs neutrons if exposed to the ones emitted by nuclear
fission. This potentially offsets or reverses the intended effect of the
tritium, which was to generate many free neutrons, if too much helium-3
has accumulated from the decay of tritium. Therefore, it is necessary
to replenish tritium in boosted bombs periodically. The estimated
quantity needed is 4 grams per warhead. To maintain constant levels of tritium, about 0.20 grams per warhead per year must be supplied to the bomb.
One
mole
of deuterium-tritium gas would contain about 3.0 grams of tritium and
2.0 grams of deuterium. In comparison, the 20 moles of plutonium in a
nuclear bomb consists of about 4.5 kilograms of
plutonium-239.
Tritium in hydrogen bomb secondaries
Since tritium undergoes radioactive decay, and is also difficult to
confine physically, the much larger secondary charge of heavy hydrogen
isotopes needed in a true
hydrogen bomb uses solid
lithium deuteride as its source of deuterium and tritium, producing the tritium
in situ during secondary ignition.
During the detonation of the primary
fission bomb stage in a thermonuclear weapon (
Teller-Ullam staging), the
sparkplug, a cylinder of
U-235/
Pu-239
at the center of the fusion stage(s), begins to fission in a chain
reaction, from excess neutrons channeled from the primary. The neutrons
released from the fission of the sparkplug split
lithium-6
into tritium and helium-4, while lithium-7 is split into helium-4,
tritium, and one neutron. As these reactions occur, the fusion stage is
compressed by photons from the primary and fission of the U-238 or
U-238/U-235 jacket surrounding the fusion stage. Therefore, the fusion
stage breeds its own tritium as the device detonates. In the extreme
heat and pressure of the explosion, some of the tritium is then forced
into fusion with deuterium, and that reaction releases even more
neutrons.
Since this fusion process requires an extremely high temperature
for ignition, and it produces fewer and less energetic neutrons (only
fission, deuterium-tritium fusion, and
7
3Li
splitting are net neutron producers),
lithium deuteride is not used in boosted bombs, but rather for multi-stage hydrogen bombs.
Controlled nuclear fusion
Analytical chemistry
Tritium is sometimes used as a
radiolabel.
It has the advantage that almost all organic chemicals contain
hydrogen, making it easy to find a place to put tritium on the molecule
under investigation. It has the disadvantage of producing a
comparatively weak signal.
Electrical power source
Use as an oceanic transient tracer
Aside from
chlorofluorocarbons,
tritium can act as a transient tracer and has the ability to "outline"
the biological, chemical, and physical paths throughout the world oceans
because of its evolving distribution.
Tritium has thus been used as a tool to examine ocean circulation and
ventilation and, for such purposes, is usually measured in Tritium Units
where 1 TU is defined as the ratio of 1 tritium atom to 10
18 hydrogen atoms, approximately equal to 0.118 Bq/liter.
As noted earlier, nuclear weapons testing, primarily in the
high-latitude regions of the Northern Hemisphere, throughout the late
1950s and early 1960s introduced large amounts of tritium into the
atmosphere, especially the
stratosphere.
Before these nuclear tests, there were only about 3 to 4 kilograms of
tritium on the Earth's surface; but these amounts rose by 2 or 3 orders
of magnitude during the post-test period.
Some sources reported natural background levels were exceeded by
approximately 1,000 TU in 1963 and 1964 and the isotope is used in the
northern hemisphere to estimate the age of groundwater and construct
hydrogeologic simulation models.
Recent scientific sources have estimated atmospheric levels at the
height of weapons testing to approach 1,000 TU and pre-fallout levels of
rainwater to be between 5 and 10 TU. In 1963
Valentia Island Ireland recorded 2,000 TU in precipitation.
North Atlantic Ocean
While
in the stratosphere (post-test period), the tritium interacted with and
oxidized to water molecules and was present in much of the rapidly
produced rainfall, making tritium a prognostic tool for studying the
evolution and structure of the
hydrologic cycle as well as the ventilation and formation of water masses in the North Atlantic Ocean.
Bomb-tritium data were used from the Transient Tracers in the Ocean
(TTO) program in order to quantify the replenishment and overturning
rates for deep water located in the North Atlantic. Bomb-tritium also enters the deep ocean around the Antarctic.
Most of the bomb tritiated water (HTO) throughout the atmosphere can
enter the ocean through the following processes: a) precipitation, b)
vapor exchange, and c) river runoff – these processes make HTO a great
tracer for time-scales up to a few decades. Using the data from these processes for 1981, the 1 TU isosurface lies between 500 and 1,000 meters deep in the
subtropical regions and then extends to 1,500–2,000 meters south of the
Gulf Stream due to recirculation and ventilation in the upper portion of the Atlantic Ocean. To the north, the isosurface deepens and reaches the floor of the
abyssal plain which is directly related to the ventilation of the ocean floor over 10 to 20 year time-scales.
Also evident in the Atlantic Ocean is the tritium profile near
Bermuda
between the late 1960s and late 1980s. There is a downward propagation
of the tritium maximum from the surface (1960s) to 400 meters (1980s),
which corresponds to a deepening rate of approximately 18 meters per
year.
There are also tritium increases at 1,500 meters depth in the late
1970s and 2,500 meters in the middle of the 1980s, both of which
correspond to cooling events in the deep water and associated deep water
ventilation.
From a study in 1991, the tritium profile was used as a tool for studying the mixing and spreading of newly formed
North Atlantic Deep Water (NADW), corresponding to tritium increases to 4 TU. This NADW tends to spill over sills that divide the
Norwegian Sea
from the North Atlantic Ocean and then flows to the west and
equatorward in deep boundary currents. This process was explained via
the large-scale tritium distribution in the deep North Atlantic between
1981 and 1983.
The sub-polar gyre tends to be freshened (ventilated) by the NADW and
is directly related to the high tritium values (> 1.5 TU). Also
evident was the decrease in tritium in the deep western boundary current
by a factor of 10 from the
Labrador Sea to the
Tropics, which is indicative of loss to ocean interior due to turbulent mixing and recirculation.
Pacific and Indian Oceans
In
a 1998 study, tritium concentrations in surface seawater and
atmospheric water vapor (10 meters above the surface) were sampled at
the following locations: the
Sulu Sea, the
Fremantle Bay, the
Bay of Bengal, the
Penang Bay, and the
Strait of Malacca.
Results indicated that the tritium concentration in surface seawater
was highest at the Fremantle Bay (approximately 0.40 Bq/liter), which
could be accredited to the mixing of runoff of freshwater from nearby
lands due to large amounts found in coastal waters. Typically, lower concentrations were found between
35 and
45 degrees south latitude and near the
equator.
Results also indicated that (in general) tritium has decreased over the
years (up to 1997) due to the physical decay of bomb tritium in the
Indian Ocean.
As for water vapor, the tritium concentration was approximately one
order of magnitude greater than surface seawater concentrations (ranging
from 0.46 to 1.15 Bq/liter).
Therefore, the water vapor tritium is not affected by the surface
seawater concentration; thus, the high tritium concentrations in the
vapor were concluded to be a direct consequence of the downward movement
of natural tritium from the stratosphere to the troposphere (therefore,
the ocean air showed a dependence on latitudinal change).
In the
North Pacific Ocean,
the tritium (introduced as bomb tritium in the Northern Hemisphere)
spread in three dimensions. There were subsurface maxima in the middle
and low latitude regions, which is indicative of lateral mixing
(advection) and
diffusion processes along lines of constant
potential density (
isopycnals) in the upper ocean. Some of these maxima even correlate well with
salinity extrema.
In order to obtain the structure for ocean circulation, the tritium
concentrations were mapped on 3 surfaces of constant potential density
(23.90, 26.02, and 26.81).
Results indicated that the tritium was well-mixed (at 6 to 7 TU) on the
26.81 isopycnal in the subarctic cyclonic gyre and there appeared to be
a slow exchange of tritium (relative to shallower isopycnals) between
this gyre and the anticyclonic gyre to the south; also, the tritium on
the 23.90 and 26.02 surfaces appeared to be exchanged at a slower rate
between the central gyre of the North Pacific and the equatorial
regions.
The depth penetration of bomb tritium can be separated into 3
distinct layers. Layer 1 is the shallowest layer and includes the
deepest, ventilated layer in winter; it has received tritium via
radioactive fallout and lost some due to advection and/or vertical
diffusion and contains approximately 28% of the total amount of tritium.
Layer 2 is below the first layer but above the 26.81 isopycnal and is
no longer part of the mixed layer. Its 2 sources are diffusion downward
from the mixed layer and lateral expansions outcropping strata
(poleward); it contains about 58% of the total tritium.
Layer 3 is representative of waters that are deeper than the outcrop
isopycnal and can only receive tritium via vertical diffusion; it
contains the remaining 14% of the total tritium.
Mississippi River System
The impacts of the nuclear fallout were felt in the United States throughout the
Mississippi River System. Tritium concentrations can be used to understand the
residence times
of continental hydrologic systems (as opposed to the usual oceanic
hydrologic systems) which include surface waters such as lakes, streams,
and rivers.
Studying these systems can also provide societies and municipals with
information for agricultural purposes and overall river water quality.
In a 2004 study, several rivers were taken into account during
the examination of tritium concentrations (starting in the 1960s)
throughout the Mississippi River Basin:
Ohio River (largest input to the Mississippi River flow),
Missouri River, and
Arkansas River.
The largest tritium concentrations were found in 1963 at all the
sampled locations throughout these rivers and correlate well with the
peak concentrations in precipitation due to the nuclear bomb tests in
1962. The overall highest concentrations occurred in the Missouri River
(1963) and were greater than 1,200 TU while the lowest concentrations
were found in the Arkansas River (never greater than 850 TU and less
than 10 TU in the mid-1980s).
Several processes can be identified using the tritium data from
the rivers: direct runoff and outflow of water from groundwater
reservoirs.
Using these processes, it becomes possible to model the response of the
river basins to the transient tritium tracer. Two of the most common
models are the following:
- Piston-flow approach – tritium signal appears immediately; and
- Well-mixed reservoir approach – outflow concentration depends upon the residence time of the basin water
Unfortunately, both models fail to reproduce the tritium in river
waters; thus, a two-member mixing model was developed that consists of 2
components: a prompt-flow component (recent precipitation – "piston")
and a component where waters reside in the basin for longer than 1 year
("well-mixed reservoir").
Therefore, the basin tritium concentration becomes a function of the
residence times within the basin, sinks (radioactive decay) or sources
of tritium, and the input function.
For the Ohio River, the tritium data indicated that about 40% of
the flow was composed of precipitation with residence times of less than
1 year (in the Ohio basin) and older waters consisted of residence
times of about 10 years.
Thus, the short residence times (less than 1 year) corresponded to the
"prompt-flow" component of the two-member mixing model. As for the
Missouri River, results indicated that residence times were
approximately 4 years with the prompt-flow component being around 10%
(these results are due to the series of dams in the area of the Missouri
River).
As for the mass flux of tritium through the main stem of the Mississippi River into the
Gulf of Mexico, data indicated that approximately 780 grams of tritium has flowed out of the River and into the Gulf between 1961 and 1997,
an average of 7.7 PBq/yr. And current fluxes through the Mississippi
River are about 1 to 2 grams per year as opposed to the pre-bomb period
fluxes of roughly 0.4 grams per year.
History
