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Starve nuclear weapons to death with a tritium freeze

Wikimedia/US National Nuclear Security Administration

Proponents of nuclear non-proliferation will be pleased in late 2020 as the 2017 Treaty on the Prohibition of Nuclear Weapons (TPNW) crosses the threshold of 50 ratifications and enters into force. Under this treaty, it will be illegal to hold nuclear weapons, and the parties will have to dismantle any in their possession. The problem with the TPNW approach is that no state that actually possesses nuclear weapons has signed it or is likely to. An alternative to this ‘all-or-nothing’ approach is needed—a freeze on the production of tritium is a different way to manage nuclear disarmament that will gradually bring an end to nuclear weapons.

Tritium and its critical role in nuclear weapons 

Tritium is a radioactive isotope of hydrogen (hydrogen-3). It is used in the nuclear weapon systems of most of the five nuclear weapon states (NWS)—China, France, the United Kingdom, the United States and Russia—today to ‘boost’ the yield of a fission weapon or fission primary. It is important to note that elemental tritium is not used in hydrogen bombs. 

While tritium is necessary for boosted nuclear weapons to function, it is not a nuclear material as defined by international statute. It is a radioactive gas and decays with a half-life of 12.3 years. That means that half of this material disappears every 12.3 years.

This relatively short half-life distinguishes tritium from the fissile materials used in nuclear bombs, such as plutonium and highly enriched uranium (HEU). For many years diplomats have tried to negotiate a fissile material cut-off treaty (FMCT) to prohibit the further production of fissile materials. But an FMCT would leave stockpiles frozen at high levels without increase or decrease. Unlike the FMCT, a proposed tritium cut-off treaty (TCOT) would begin to reduce weapons stocks immediately because of tritium’s natural decay.

Neutron bombs, believed to be in the stockpiles of China and Israel, would be among the first casualties of a TCOT. The neutron bomb uses large quantities of tritium to produce a huge flood of neutrons designed to kill living organisms but produce a drastically reduced blast. The USA abandoned this type of weapon because of its high tritium consumption, many times that of a boosted weapon.

Why a tritium cut-off treaty could work

A TCOT would be more likely to be accepted politically as it does not demand immediate and total disarmament. Under such a treaty, the reduction in nuclear stockpiles would take place gradually, inexorably and without human intervention. Radioactive decay does the job—the essential material simply disappears. Unlike uranium and plutonium—which have half-lives of thousands of years—tritium decays quickly enough to force a natural arms reduction. The process is fast enough to be seen as a genuine step towards nuclear disarmament. It is slow enough for states to monitor its effectiveness and to withdraw from a TCOT if they feel it is not working.

Note that even under a nuclear weapon ban treaty such as the TPNW, disarmament would be a slow process. The actual dismantlement of nuclear weapons takes decades. For example, in 2013, thousands of Russian weapons were eliminated permanently by burning their HEU components in power reactors to produce electricity (ironically in the USA). It took about 20 years to dismantle the weapons, verify their dismantlement and convert the uranium to reactor fuel. No matter what treaty path is chosen this is an illustrative timeline.

Furthermore another agreement for the conversion of plutonium to fuel for electricity, the Plutonium Management and Disposition Agreement between the USA and Russia, eventually broke down because the USA was unable to build the factory to do the job. Much of the Russian factory was completed, ironically again, with US financial aid. This agreement was signed in 2000 and abrogated in 2016. Good intentions were not enough. 

Politicians are more likely to embrace a treaty that begins slowly but visibly. The NWS regard nuclear weapons as vital to their security so total disarmament without a verification experience is seen as precipitous. Freezing tritium production gives each state time to observe the working of a TCOT, to implement and monitor any safeguards from the International Atomic Energy Agency (IAEA) and the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO), and to convince their public and their military that the move is in its national interest. 

This is similar to the voluntary suspension of nuclear explosive testing. The rate of testing slowed drastically for a number of years. When the major states simply stopped testing, the positive value of this voluntary move was apparent to policymakers and was easy to convey to the public and the military establishment (even without the entry into force of the 1996 Comprehensive Nuclear-Test-Ban Treaty (CTBT)). No state gained an advantage over any other during the period when testing slowed and ceased. It was seen as no longer necessary. This was a cooperative effort on the part of the NWS without acceding to the CTBT.

There are also some major implications for cost savings, such as in the above-mentioned case of repurposing Russian uranium weapons for generating electricity in the USA. The cost of dismantlement was hugely offset by the enormous value of the uranium fuel produced. There are also major implications for changes in military doctrine. In the first year or so of a tritium freeze, tritium reserves can be used to refresh most weapons in the stockpile. Within a few years the dwindling supply will need to be distributed to fewer and fewer weapons and many will need to be removed from active service. This will have major impacts on doctrine. 

Take, for example, the nuclear triad that the USA touts as necessary: ballistic missile submarines (SSBNs), intercontinental ballistic missiles (ICBMs) in silos and bombers. Justifying a triad would become increasingly difficult under a TCOT as warheads cease to function. This is the essence of irreversible denuclearization: constant pressure to redistribute dwindling tritium stocks until it is no longer possible to have an effective strategic stockpile.

Countries considering building new nuclear delivery systems would need to take pause if there will be no warheads for them to deliver. The greatest resistance to a TCOT will come from the military-industrial complex that builds enormously expensive weapon systems, such as a new fleet of SSBNs or a new generation of ICBMs with the same vulnerabilities as the existing arsenal. Cost savings of deferring the planned US nuclear weapons modernization can run into trillions of dollars.

The effectiveness of a tritium freeze

What is the effect of removing tritium from a nuclear warhead? A good example is revealed in unclassified information about the UK's Trident warhead. The full yield of the warhead is about 100 kilotons as designed and deployed. There is a version that only has the fission-stage boosted primary for relatively small engagements. But if the tritium is removed from the fission stage, then the yield drops to only 0.3 kt. Although 0.3 kt is not total disarmament, it is small enough to be militarily insignificant, especially when launched from a multibillion-dollar platform: a Trident submarine. Simply put, modern nuclear weapons without tritium are not military weapons.

Monitoring and verifying a tritium cut-off treaty

Verifying a tritium freeze could be accomplished with arrangements similar to IAEA safeguards on plutonium and HEU. Tritium would need to be defined to be a weapon-usable material of the same class as nuclear materials. Verification would be similar to the IAEA’s existing mission of verifying legal stocks of existing nuclear materials. It would require new technologies and training for inspectors, but this is not an insurmountable problem.

Tritium has legitimate civilian uses, largely exploiting its beta particle emission for such uses as self-illuminating lights requiring no electric power source, medical procedures and as a radioactive tracer. Tritium is considered to be a future source of electricity in fusion reactors, although this application has been a goal for at least 60 years and is unlikely to be realized for many more decades. Legitimate uses of tritium can easily be accommodated under an IAEA-type monitoring programme just as large quantities of uranium and plutonium are monitored for the civilian power cycle. The civilian market for tritium is tiny compared to the fissile material nuclear fuel cycle. It would not be a huge burden to monitor.

Tritium is a radioactive gas similar to normal hydrogen. Its radioactive properties mean that there is no point in clandestinely stockpiling it because it is constantly disappearing through decay. Tritium that escapes to the environment can be monitored using gas-sampling systems similar to the ones fielded today by the CTBTO. The CTBTO’s worldwide network of samplers is designed to detect other radioactive substances. Adding tritium detectors to a functioning and reliable existing network would be feasible and not costly. 

Tritium has historically been produced in dedicated military production reactors at temperatures and pressures much lower than commercial electric power-production reactors. The USA was forced to develop new technology to produce tritium in some US commercial power reactors. The technological hurdles were difficult. Furthermore, such an activity in a nuclear power reactor subject to IAEA safeguards would be notable, distinctive and subject to investigation and verification.

Challenges bringing states on board

Bringing in other states that possess nuclear weapons, particularly India and Pakistan, will be the most difficult task. For example, the five NWS have essentially voluntarily implemented an FMCT-like posture. Since China is not transparent it may be an exception. Significantly, India and Pakistan are actively producing HEU and plutonium, both for nuclear bombs and for the propulsion of future SSBNs. Pakistan, in particular, has been a major impediment to achieving an FMCT. These two states are actively increasing their nuclear weapon stockpiles through fissile material production and it will be difficult to persuade them to end tritium production. They may also have weapons in their stockpiles that do not use tritium at all, reducing the value of a TCOT.

Israel, on the other hand, does not even admit to having nuclear weapons, a poorly kept political secret. SIPRI estimates that Israel has 90 nuclear weapons, largely based on plutonium. Israel’s tritium production, if any, comes from its 56-year-old Dimona reactor, which is approaching the end of its lifetime. Israel could easily agree to a tritium freeze because its production is going to end in the near future. Israel will already be busily searching for ways to keep its stockpile effective in the absence of tritium or it may come to depend on its ample conventional strength. 

The case of Israel is unusual but illustrative. The Israeli stockpile is presumed to have many thermonuclear weapons. Yet Israel has not used nuclear weapons in its disagreements with its neighbours. Instead it uses high-precision conventional explosives to target individual buildings and sites from which it believes threats emanate, for example in Gaza. Its powerful nuclear weapons have little application beyond deterrence in its current military posture.

Denuclearization begins on day one 

Freezing tritium supplies is an attractive disarmament step because it begins inexorably on day one of the treaty going into force. It does not make any state suddenly vulnerable to attack from another. All TCOT states parties will begin to see natural radioactive decay reduce their stockpiles on an identical basis. After 12.3 years each state will have exactly half of its tritium left and will have had to make some hard choices about dismantling its nuclear weapon stockpile. A great deal can change in 12 years: entire weapons systems will become obsolete and remaining military objectives will be constantly reassessed. The USA will hold three presidential elections. Under these circumstances, politicians and the public will see measurable progress and reduced threats. 

A negotiated TCOT is an attractive adjunct to other approaches to disarmament. It begins the process of making weapons obsolete on the first day of its implementation. Every day that a TCOT exists, more and more weapons will become disabled. At first there will be an effort to redistribute dwindling tritium stocks to the weapons considered most essential. This effort, in itself will illustrate why legacy cold war weapons systems no longer matter. It will become increasingly apparent that the huge stockpiles targeted on adversary missile systems are obsolete in an age of precise conventional targeting against military objectives and asymmetric warfare against small, often urban targets. Implementation of a TCOT could mean that, after 25 years, 75 per cent of nuclear weapons that use tritium will have disappeared.

Implementation of a TCOT would require a new definition of tritium as a material similar to existing nuclear materials. Verification and inspections are missions familiar to the CTBTO International Monitoring System and the IAEA. A cooperative working agreement between these two international organizations would be necessary.

A complete nuclear weapon ban treaty, such as the TPNW, is a lofty and worthwhile goal. But its total immediate implementation is a difficult option for the NWS, with their shared monopoly on powerful weapons. The TCOT offers a simpler option based on radioactive decay that will immediately, slowly but inexorably eliminate large nuclear weapon stockpiles and highlight the need for rethinking nuclear deterrence.


Robert E. Kelley is a Distinguished Associate Fellow at SIPRI.