Question 1 of 7: Tritium Production Mechanisms and Radiation-Protection Tenets

Question 1: Tritium Production Mechanisms and Radiation-Protection Tenets (12 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Given. Six neutron-induced/fission production pathways for tritium (³H) in a light-water PWR such as Turkey Point; the ICRP three-tenet radiation protection framework (Justification, Optimization, Dose Limitation); the reported Biscayne Bay tritium leak (up to 215× background).

Find. (a) the balanced nuclear reaction equation for each of the six production paths; (b) the physical basis of tritium's radiological hazard, tied to its decay process and half-life; (c) how the three protection tenets apply to reducing the leak.

Approach. For each path, identify the target nuclide and neutron energy regime, then enforce conservation of atomic number $Z$ and mass number $A$ term-by-term to fix the reaction products (light nuclides for (i)–(v); the ternary-fission channel off ${}^{235}$U for (vi)). For (b), trace tritium's $\beta^-$ decay spectrum and range in tissue to locate where the hazard actually resides. For (c), map each tenet onto a concrete mitigation action at the plant.

1. Part (a)(i)–(vi) — tritium-production reaction equations. Balancing $Z$ and $A$ on each side gives: $$\text{i.}\quad n + {}^{10}_{\ 5}\text{B} \longrightarrow {}^{3}_{1}\text{H} + 2\,{}^{4}_{2}\text{He}$$ a fast-neutron $(n,2\alpha)$ reaction on the boron used in control rods and soluble boric acid (reactivity control); it is the classic route by which boron chemistry itself manufactures tritium. $$\text{ii.}\quad n + {}^{14}_{\ 7}\text{N} \longrightarrow {}^{3}_{1}\text{H} + {}^{12}_{\ 6}\text{C}$$ a fast-neutron $(n,t)$ reaction (threshold near 4 MeV) on nitrogen dissolved in the coolant (from entrained air or hydrazine water-chemistry additives). $$\text{iii. and v.}\quad n + {}^{6}_{3}\text{Li} \longrightarrow {}^{3}_{1}\text{H} + {}^{4}_{2}\text{He}$$ the same $(n,\alpha)$ equation serves both listed pathways: at thermal energy the cross-section is huge ($\sigma\approx940$ b) so even a trace of ${}^{6}$Li (present as a boric-acid impurity, or deliberately as LiOH for pH control) contributes measurably despite the low thermal flux fraction; at fast energy the cross-section is far smaller, but the fast-neutron flux is much larger, so the fast channel is not negligible either — both are logged as distinct "mechanisms" precisely because neither dominates outright. $$\text{iv.}\quad n + {}^{2}_{1}\text{H} \longrightarrow {}^{3}_{1}\text{H} + \gamma$$ thermal radiative capture $(n,\gamma)$ on the small natural-abundance deuterium already present in ordinary ("light") water. $$\text{vi.}\quad n + {}^{235}_{\ 92}\text{U} \longrightarrow {}^{236}_{\ 92}\text{U}^{*} \longrightarrow X + {}^{39}_{\ 18}\text{Ar} + {}^{3}_{1}\text{H} + 2n$$ a rare ternary-fission channel in which the compound nucleus splits into three fragments (here ${}^{39}$Ar and ${}^{3}$H, as instructed, plus a third, heavier fragment $X$) instead of the usual two. Conservation fixes $X$'s charge and mass: $Z_X = 92-18-1=73$, $A_X = 236-39-3-2=192$ (taking two prompt neutrons as representative), i.e. a fragment near ${}^{192}$Ta — the exact partner nuclide and neutron multiplicity vary channel to channel, but $Z$ and $A$ must always balance this way.
2. Part (b) — why tritium is a hazard. ${}^{3}$H decays by $\beta^-$ emission (${}^{3}\text{H}\rightarrow{}^{3}\text{He}+\beta^-+\bar\nu$) with a long physical half-life ($T_{1/2}=12.32$ years) but a very low beta endpoint energy (18.6 keV maximum, ∼5.7 keV average). Such a low-energy beta has a range in tissue of only a few tens of micrometres — less than the thickness of the skin's outer dead layer — so tritium is essentially harmless as an external source; it cannot even reach living tissue from outside the body. The hazard is entirely internal: tritium behaves chemically exactly like ordinary hydrogen, so it is readily incorporated as tritiated water (HTO) via ingestion, inhalation, or skin absorption, and once inside the body it distributes uniformly through total body water (∼60% of body mass) and exchanges into organic molecules, irradiating tissue from within at short range for as long as it remains resident. Its biological half-life as HTO (∼10 days) is far shorter than its 12.32-year physical half-life, so any single intake clears relatively quickly — but a continuing leak, as reported at Turkey Point, means chronic, repeated low-level intake rather than a single dose.
3. Part (c) — applying the three tenets to the leak. Justification asks whether continuing the practice that produces the leak (operating the unlined cooling-canal system as-is) still yields a net benefit once the newly-measured leakage is accounted for — if not, the practice (or at least its current unmitigated form) is no longer justified and must be modified, independent of cost. Optimization (ALARA — As Low As Reasonably Achievable) applies once continued operation is justified: reduce the leak to the lowest level reasonably achievable given cost and benefit, e.g. installing canal liners, groundwater recovery/interceptor wells, or enhanced monitoring and treatment, weighing the incremental dose reduction against the cost of each measure. Dose Limitation is the independent backstop: regardless of how "optimized" or economically justified the canal system is, no member of the public may receive more than the regulatory dose limit (e.g. 1 mSv/y) from the leak — this is a hard compliance ceiling, not subject to cost–benefit trade-off, and monitoring data (like the reported 215× background readings) must be checked against it directly.