While journalists, bureaucrats, and anti-nuclear activists make a fuss about the radioactivity of tritium and create a holler when a few curies are released, tritium’s radioactivity is a trivial health hazard compared with the role it plays in nuclear weapons. As we shall relate, tritium is indispensable for “boosting” the yield of almost all modern fission (not fusion) explosives, a role that accounts for as much as 90% of the annual demand for tritium. But tritium’s radioactivity is an Achilles heel for its weapons application. Tritium’s short half-life (12.3 yr) lends itself to the gradual and effortless crippling of entire nuclear arsenals, simply by not replenishing decayed tritium.
The DT-boosted nuclear explosive
Nuclear weapons can be a single-stage fission device, or may also have a secondary thermonuclear stage. The discussion herein applies only to the fission-based stage, called the primary, that’s found in all nuclear weapons. The configuration in Fig. 1 is the basis of the primary, according to unclassified sources [1, 2, 3]. The fissile material can be plutonium isotopes, principally Pu-239, or U-235 or a mixture thereof.
The following is the sequence of events in a boosted fission primary, such as depicted in Fig. 1.
- Detonation of chemical high explosives (H.E.).
- A compression wave implodes the shell of fissile material in 10 μs and increases the density of the DT gas (typically 5 g) by a factor of 100, while neutrons are initiated.
- Fissions multiply neutrons some 70 times in 400 ns, producing nearly 1,000 GJ and heating most of the assembly, including the 5 g of DT, to 3 keV.
- At least 3/4 of the D and T nuclides fuse in 10 ns, producing 1,500 GJ and 5x1023 neutrons, about 30 times the number of existing fission neutrons. The 10 ns time for DT burnup is of the order of one fission doubling time, while the 14-MeV fusion neutrons have 3 times the speed of typical fission neutrons and the fission cross section is 50% larger. Hence essentially all subsequent fissions are generational descendants of the D-T neutrons, especially since each fusion neutron produces at least 4 fission neutrons in Pu or U-235 compared with just 3 for slower fission neutrons in Pu and 2.5 in U-235.
- D-T neutrons fission nearly 5x1023 nuclides, producing 2x1024 neutrons and 16 TJ of energy, or 4 ktons TNT-equiv.
- Before the primary totally disassembles, about half of the fission neutrons from event #5 cause further fission, producing another 8 ktons.
While only 5% of the total energy release is from fusion during event #4, essentially all of the fission energy release can be traced to fusion neutrons. With this extra neutron source, compression requirements can be relaxed, thus greatly reducing the mass of chemical explosives needed. Much higher explosive yields are possible, but the numbers herein reflect minimal fissile content and modest compression by H.E., and the yield is adequate to ignite the weapon’s secondary stage, if there is one.
The advantages of boosting are that 1) the Pu or U-235 charge can be smaller than with no boosting, 2) the mass of chemical H.E. can be greatly reduced, shrinking the warhead size so that it is small enough to be deliverable by missile, 3) reactor-grade Pu (high Pu-240 content) is perfectly usable, and 4) safety advantages against unwanted detonation are realized when the DT gas is stored separately from the weapon before use.
Some of the so-called “H-bomb” tests of India, Pakistan and No. Korea were probably tritium-boosted fission explosions, as distinct from the more difficult Teller-Ulam configuration requiring an elaborate secondary stage.
Proposals to ban tritium production
Tritium is radioactive with a half-life of 12.3 years. If the decayed tritium is not restored periodically, weapons’ primaries would provide only fizzle yields if detonated, a few hundred tons TNT-equivalent, and could not ignite the thermonuclear secondary.
Of course D is equally as important as T in the boosting process, but D does not decay and is easily extracted from ordinary water, being 1 part in 6,500 of natural hydrogen. In contrast there are no natural resources of tritium, which must be manufactured in fission reactors or particle accelerators. The tritium content of each weapon must be replenished after a few years, so that allowing it to decay indefinitely is a splendid target for arms control. Hence proposals have appeared for a freeze on tritium production.
The boosting technique was declassified in the 1970’s. The first published suggestion to implement a tritium production freeze that would effectively draw down weapons inventories was apparently made by Wilkie in 1984 [4]. Shortly after the last tritium-producing reactors in the US had been shuttered in the 1980’s, a worldwide ban on tritium production was proposed by Leventhal and Hoenig [5] and by J. Carson Mark, et al [6]. With a freeze on new tritium supply, the number of viable weapons would decrease monotonically as remnant tritium is transferred from retired weapons to replenish those in the active stockpile.
In the 1990’s Kalinowski and Colchen (K&C) made by far the most elaborate and complete study and recommendations for the cutoff of tritium supply and usage [7]. K&C went beyond reduction of arsenals by mere natural decay of tritium, and urged the active removal of tritium from weapons. K&C estimated that without tritium and no change to weapons composition or design the total yield of nuclear arsenals would decrease by two orders of magnitude, principally because the thermonuclear secondary could not be ignited by the greatly reduced yield of the primary. Rebuilding the primaries without tritium but with more Pu and vastly more chemical explosives for compression would make the weapons too large for delivery by missile, and both their fission and fusion yields might be drastically reduced.
Unplanned reduction of weapons tritium inventory
No international freeze agreement was ever negotiated. However, the end of tritium production in the US conveniently occurred a few years before the USSR unraveled and led to the START agreement between the US and USSR to reduce their nuclear stockpiles by a factor of five. For at least 15 years, the US simply transferred remnant tritium from retired weapons to replenish the stocks of those in active service, as the freeze proposals advocated. The US has lately resumed small-scale production using TVA reactors.
Nuclear stockpiles have always been dominated by the US and Soviet Union. Although the US stockpile peaked around 1965, the total size of the global arsenal peaked in 1986 at about 62,000 weapons, so that year is an appropriate starting point to monitor the decay of the original deployed tritium inventory.
Figure 2 shows the decay of the 1985 tritium inventory compared with the actual reduction in nuclear stockpiles [8] of the four nations that purposely reduced their inventories. The UK and France cut their peak strength nearly in half, a much smaller factor than effected by the US and Russia. The UK and French arsenals together amounted to only 1% of the total in 1985, but make up 4% of the total today.
The tritium scale in Figure 2 assumes that all the weapons in the 1985 stockpiles used tritium boosting in the first stage (the primary) and assumes a charge of 3 g per weapon, While that is probably incorrect, some weapons such as the “neutron bomb” reportedly each used tens of grams of tritium, which would compensate for those using none. Many two-stage weapons also use tritium boosting in the fission “spark plug” in the secondary. That use would increase the actual tritium inventory to as much as twice that indicated in the figure labeling, but does not affect the normalized decay curve. Actual tritium inventory has always been secret information in every country.
For any year there is some variability in the arsenal numbers given by different sources, because of different estimates about the actual number held by nations other than the US, and because of the uncertainty in distributing weapons among the deployed, stockpiled and to-be-dismantled categories.
Figure 2 shows that the global stockpile dropped by a factor of 5 from 62,000 to about 12,000, while the 1985 tritium inventory dropped by a factor of 9. The stockpile in 2005 was only 20% higher than would have been produced simply by tritium decay, but it was 65% higher in 2023. The US and Russia each have about 1,500 retired, non-deployed nuclear warheads included in this tally. If these weapons contain no tritium, the 2023 stockpile is only 20% higher than allowed by tritium decay.
If a tritium freeze had actually been implemented in 1988, then the number of tritium-boosted weapons would have been reduced, without international agreement, to about 60 to 80% of the number available for use today. Nevertheless, what the 1980’s proponents wanted to accomplish with a tritium freeze has essentially happened, but for other reasons.
Figure 2 omits the five nuclear powers that have never reduced their nuclear arsenals. Those arsenals were relatively insignificant in 1986, but expanded to a total of 900 weapons by 2023, and now add nearly 10% to the number shown in Fig. 2.
Prospects for a tritium freeze agreement
Nine-tenths of the annual global demand for tritium, both military and civilian, is used to maintain the effectiveness of nuclear arsenals by replenishing the boosting component.
In 2004 Kalinowski published an even more elaborate and detailed discussion of tritium inventories and proposed methods to reduce them, as well as methods to control the distribution and use of tritium [9]. Despite Kalinowski’s detailed and persuasive arguments, it seems that none of these recommendations has been adopted, as the continual production and extraction of tritium has apparently alarmed few. But with China and other Asian nations continuing to increase their stockpiles, there have been more recent calls for a tritium freeze [10].
Two dozen CANDU reactors around the world as well as many heavy water reactors in India unavoidably produce tritium in their D2O moderators by neutron absorption in deuterium, analogous to the inescapable production of plutonium in every uranium-fueled reactor by neutron absorption in U-238. Hence it’s not feasible to ban all production of tritium any more than to ban production of plutonium. It may be possible to ban the extraction of tritium from heavy water or lithium compounds, analogous to the de facto ban on reprocessing plutonium from spent fuel rods that’s tacitly observed in many countries including the US.
Meanwhile, the US has been producing tritium for weapons in one or more TVA reactors for nearly 20 years [11]. As for the other nuclear powers, Russia has two dedicated tritium production reactors named Lyudmila and Ruslan. France intends to produce weapons tritium from LWR’s at the Civaux nuclear power plant. India plans on building still more heavy-water moderated reactors, from which it extracts tritium for weapons. Israel, North Korea and Pakistan continue to produce tritium from small special-purpose fission reactors.
Summary and Conclusions
Tritium is an irreplaceable component of the primary of virtually all modern nuclear weapons, and thereby works hand-in-glove with plutonium to maintain the most dire threat to civilization. In principle this threat could be decisively curtailed by an international agreement simply to allow all tritium inventory to decay without replenishment. As it happened, a factor-of-5 reduction in deployed tritium occurred between 1988 and 2020 because of the negotiated reduction in nuclear arms that accompanied the demise of the Soviet Union. It is not realistic that a ban on tritium production itself can be negotiated, and a second inventory reduction may have to await another monumental geopolitical event that prunes nuclear arsenals.
Nevertheless, those who seek nuclear disarmament should look for opportunities to restrict tritium extraction and trafficking and not just maintain exclusive obsession with safeguarding plutonium and enriched uranium. After all, if a different practical method could be found to heat the DT package in the primary, the extremely challenging Pu or U-235 component could be replaced by widely available depleted or natural uranium, or even thorium, all of which can be fissioned by D-T neutrons.
References:
- Wikipedia entry for “Boosted fission weapon,” and references therein.
- “Boosted Weapons,” GlobalSecurity.org (online).
- André Gsponer and Jean-Pierre Hurni, “The physical principles of thermonuclear explosives,” Independent Scientific Research Institute, Geneva, Switzerland (1997) and 2009, online.
- Tom Wilkie, “ Old Age Can kill the Bomb,“ New Scientist, Feb. 6, 1984.
- P. Leventhal & M. Hoenig, “The Tritium Factor,” New York Times, Aug. 4, 1987.
- J. Carson Mark, T. D. Davies, M. M. Hoenig, and P. L. Leventhal. “The Tritium Factor as a Forcing Function in Nuclear Arms Reduction Talks.” Science 241, (September 2, 1988), 1166–68.
- Martin Kalinowski and Lars Colschen, “International Control of Tritium to Prevent Horizontal Proliferation and to Foster Nuclear Disarmament,” Science and Global Security, 1995, Vol. 5, p. 131 ff.
- Wikipedia entry for “Historical nuclear weapons stockpiles by country,” and references therein.
- Martin Kalinowski, “International Control of Tritium for Nuclear Nonproliferation and Disarmament,” Science and Global Security Monograph Series, v. 4. Boca Raton: CRC Press, 2004.
- Robert E. Kelley, “Starve nuclear weapons to death with a tritium freeze,” SIPRI report (August 2020), online.
- “Production of Tritium in Commercial Light-Water Reactors,” NNSA-USDOE report, June 2016, energy.gov (online).
Daniel L. Jassby
Retired from Princeton Plasma Physics Lab.
dljenterp@aol.com