Abstract
Recent articles and news blogs have been written about nuclear waste management, specifically focusing on the alleged hazards of iodine-129. Here, we objectively review the specific details of iodine-129 as a radiation hazard. In contrast with the alarmist tone of these recent papers and news items, we conclude that iodine-129 is not nearly as concerning as has been projected. Rather, it appears to be a classic case of “much ado about nothing”.
We read with interest but disappointment a recent article in MIT News, which inappropriately rang alarm bells about the dangers of iodine-129 that were “identified” in a report in Nature Sustainability.1,2 The article begins by stating: “One of the highest-risk components of nuclear waste is iodine-129 (I-129), which stays radioactive for millions of years and accumulates in human thyroids when ingested.” 2 This reminds us of the old children’s folk tale of Chicken Little: “The sky is falling, we must tell the king.” The sky is not falling, and I-129 is a particularly innocuous radioisotope. Furthermore, I-129 is a low probability byproduct of the nuclear reaction – meaning that relatively little appears in nuclear waste. In contrast to the fission product yield of I-131 (2.83%), the cumulative fission product yield of I-129 from the thermal neutron fission of uranium-235 is approximately 0.706%, or about 1 in every 142 fissions.
Let us consider the characteristics of I-129:
Half-life: 1.57 × 107 years (4.95 × 1014 s).
Decay constant: 1.4 × 10-15 Bq (decays/s).
Specific radioactivity: 6.5 × 106 Bq/g.
Decay product: Xenon-129.
Decay mode: β-decay with a maximum beta energy of 0.189 MeV (189 keV) and an average energy of 40.9 keV.
Bodily accumulation: ∼92% thyroid, ∼8% elsewhere.
Biological half-life: 80 Days thyroid, 12 days elsewhere.
Regarding its long physical half-life (which indicates a very low radiation dose-rate at the concentrations we are considering here), I-129 is one of the seven long-lived fission products. But its very long half-life confers the lowest dose-rate of these seven. Additionally, at a yield of 0.706% it ranks number five of seven as far as fission byproduct yield. I-129 is the longest lived radioactive isotope of iodine.
The MIT article unfortunately focusses on the long half-life of I-129 – 15,700,000 years – and seems to overlook the radiobiology which essentially eliminates any risk. For example, 92% of ingested I-129 goes into the thyroid, but it does not remain there long: it is excreted with a biological half-life of about 80 days on average in healthy adults. 3 Moreover, the I-129 decay product is a β-particle (electron), averaging 40.9 keV with a maximum energy of 189 keV.
Biological damage is related to the energy deposited per unit length, and energy per unit length is quantified by
Furthermore, the range of these beta particles is quite limited. A 40.9 keV β- particle with an LET ∼0.82 keV/µm has a range of about 49.9 µm; it travels only 0.05 mm, then stops. It is not clear that the authors were aware of this very limited range. The beta particles from I-129 decay cannot even penetrate a layer of dead skin; the only risk is ingestion and deposition in the thyroid.
Importantly, I-129 does not emit high energy gamma rays like I-131 does. Iodine-131 decay produces several gamma ray energies, but the principal and highest intensity gamma ray has an energy of 364 keV. Additionally, some 637 keV gamma photons are emitted. Although I-129 decays entirely by beta emission, there are some gamma photons (with a modest energy of 39.58 keV) emitted when the daughter nucleus, Xe-129, relaxes from an excited intermediate state to its final stable ground state after the initial beta decay. Incidentally, the energies of the beta particles from I-131 (maximal energy of 606 keV and a mean energy of 190 keV) are far higher than those from I-129. These beta particles could have a maximum range reaching ∼2 mm in tissue, which is far deeper than the range of I-129 beta particles.
Consider the biological effects of the I-129 releases. The MIT article states that “France’s current practice of reprocessing released about 90 percent of the waste’s I-129 into the biosphere. . . [releasing] about 153 kg of I-129 each year:” This is about 7.14 × 1026 atoms/yr. Assuming continual release of I-129, this is ∼ 2.26 × 1019 atoms/s with ∼3.17 × 104 decays/s. Thus, every second, ∼31,700, 40.9 keV β-particles are released into the ocean. If we contrast this quantity with ∼8.4 × 1034 water molecules in an Olympic-sized swimming pool, it’s clear they are significantly diluted leaving the facility – then it effectively disappears in the English Channel.
If the US and other countries begin spent fuel reprocessing, there will be similar localized I-129 releases; but the data suggests future releases into the ocean, even thousands of times more than the French are doing, should not be a problem.
The remaining 10% of I-129 releases presumably comes from low level waste (LLW) facilities, where it is kept in near surface storage. I-129 is relatively mobile in wet settings, so it appears in the water outflow from these facilities. An NRC report published in August 2020, NUREG/CR-6567, 4 quantified the concentration of I-129 in LLW delivered from 45 nuclear power plants over 21 years (1988-2009). Based on the geometric mean of the NUREG data, we have calculated (in a report to be published) that I-129 activity in the LLW outflow water is not more than 6.7 × 10-3 Bq/ml – and probably substantially less – consistent with the NRC allowable activity levels 5 : 2 × 10-7 µCi/ml (7.4 × 10-3 Bq/ml).
This means that just one 40.9 keV β-particle appears in ∼150 mL of water (∼9.3 × 1024 water molecules). It is noteworthy that outflow occurs within a radiation controlled area and is substantially diluted by flowing bodies of water before leaving the facility; it then essentially disappears in rivers.
I-129 is continually added to LLW sites; but although each new delivery increases the total activity of I-129, it does not substantially increase the overall activity/ml.
A person or aquatic lifeform may still encounter a single I-129 atom from reprocessing and LLW outflow, despite such thorough dilution in the various bodies of water. However, a 40.9 keV β-particle will not penetrate the dead skin layer on a person; and if swallowed, it is 6 trillion times more likely to be naturally excreted than to decay in the thyroid. Nevertheless, as an unlikely worst case, imagine that an after hours intruder breaches the perimeter of a LLW storage facility and drinks one liter of outflow water directly; this is an estimated intake of 0.0669 Bq of I-129, of which 0.0618 Bq (9.4 ng) would go into the thyroid. Natural excretion would begin promptly; but consider the situation the instant the water is consumed: 0.0618 Bq represents a 40.9 keV β-particle released every 16 or so seconds within a sphere of radius ∼0.05 mm, ∼0.000004% of a 10 to 15 cc female thyroid, 7 and 9.4 ng of I-129 represents 550 parts per trillion in a 17 g female thyroid. It is impossible to imagine that such short range particles, so widely dispersed in the thyroid and released so infrequently might do significant DNA damage.
A final source of I-129 is from spent nuclear fuel rods, but these are stored in concrete canisters from which I-129 decay particles cannot escape. The MIT researchers worry what will happen if and when the canisters fail after ∼1000 years, but it seems a fool’s errand to imagine technology remaining the same for 1000 years: reprocessing before then may remove this as an issue – or if people are still around then, they can effectively address it.
The MIT researchers then go even further afield by fear-mongering and projecting 1,000,000 years hence – but the bottom line is that I-129 appears in small quantities, decays slowly, and has an extremely benign decay product.
The alarmism about I-129 in nuclear waste suggested by the MIT article is thus unfounded: it is “much ado about nothing.
