Question 1 of 7: Hanford Tunnel Collapse — Historic Radiation Level, Decay, and Inspection Planning

Question 1: Hanford Tunnel Collapse — Historic Radiation Level, Decay, and Inspection Planning (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. 1997 DOE report: radiation level "in excess of 5 roentgens per hour" at the tunnel (treated as $X=5$ R/hr for the estimate); tunnel collapses May 2017, 20 years after the report; assumed effective half-life of the contaminant $T_{\text{eff}}=90$ years; inspection task requires 2 hours of hands-on work near the tunnel.

Find. (a) exposure rate in Gy/hr; (b) dose-equivalent rate in Sv/hr; (c) the likely radionuclide content of the waste; (d) the dose rate at the time of collapse (2017); (e) a worker-rotation inspection plan; (f) an assessment of the airborne-release risk and its effect on the plan; (g) suitable field instruments.

Approach. Convert the historic exposure-rate reading to absorbed dose using the standard air-kerma (W/e) factor, treat the radiation as photon-dominated so the dose equivalent is numerically equal to the absorbed dose, decay that rate forward 20 years with the given effective half-life, then size a crew rotation against the Canadian annual occupational dose limit for nuclear energy workers.

1. Part (a) — exposure converted to absorbed dose in air. The roentgen is a unit of exposure (ionization produced in air), not absorbed dose; it converts to air-kerma via the mean energy per ion pair in air, $W/e=33.97$ J/C, giving the standard factor $f=2.58\times10^{-4}\ \text{C/kg per R}\times 33.97\ \text{J/C}=0.00876$ Gy per R: $$D_{\text{air}} = X\cdot f = 5\ \tfrac{\text{R}}{\text{hr}}\times 0.00876\ \tfrac{\text{Gy}}{\text{R}}$$ $$\boxed{D_{\text{air}} \approx 0.0438\ \text{Gy/hr}}$$
2. Part (b) — dose equivalent. The 5 R/hr reading is a photon (gamma) field — the roentgen is defined specifically for x/gamma radiation — and the ICRP radiation weighting factor for photons of any energy is $w_R=1$, so the dose equivalent is numerically identical to the absorbed dose: $$\boxed{H = D_{\text{air}}\cdot w_R \approx 0.0438\ \text{Sv/hr}}$$
3. Part (c) — likely radioactive content. The tunnel served a facility that extracted plutonium from spent fuel (a PUREX-type reprocessing line, per the article), so the waste is not simply "leftover uranium" — the extraction removes most of the uranium and plutonium, leaving behind the raffinate: the fission-product inventory (notably the long-lived, high-activity pair ¹³⁷Cs and ⁹⁰Sr, plus ⁹⁹Tc, ¹²⁹I and shorter-lived species), the minor actinides that were not chemically separated (neptunium, americium, curium), and activation products from structural/process materials exposed to the neutron flux. This is exactly the class of "high-level waste" residue expected from historic plutonium-extraction reprocessing.
4. Part (d) — dose rate at collapse (May 2017). The report is dated 1997; the collapse is 20 years later. Applying exponential decay with the given effective half-life: $$D(2017) = D_{\text{air}}\cdot e^{-\ln2\cdot t/T_{\text{eff}}} = 0.0438\ \text{Gy/hr}\times e^{-\ln2\cdot 20/90}$$ $$\boxed{D(2017) \approx 0.0375\ \text{Gy/hr} \approx 37.5\ \text{mSv/hr}}$$ (numerically the same in Sv/hr, by the same $w_R=1$ argument as part (b)).
5. Part (e) — inspection crew plan. Use the Canadian nuclear-energy-worker (NEW) annual effective-dose limit of 50 mSv (CNSC Radiation Protection Regulations) as the per-worker budget for this single task. At the current rate the maximum time any one worker can spend near the tunnel before using up that whole year's allowance is $$t_{\max} = \frac{50\ \text{mSv}}{37.5\ \text{mSv/hr}} \approx 1.33\ \text{hr} \approx 80\ \text{min}$$ so a single worker cannot complete the 2-hour job alone. Splitting the task between $$\boxed{n = 2\ \text{workers, each working 1.0 hr}}$$ keeps each worker's task dose at $1.0\ \text{hr}\times 37.5\ \text{mSv/hr}\approx 37.5$ mSv, comfortably under the 50 mSv annual limit and leaving headroom for any other occupational exposure that worker receives later in the year — the ALARA-preferred choice over the bare minimum of two workers running right up to the limit.
6. Part (f) — Lyman's release comment and its impact on the plan. The assessment is reasonable: the earth cover was not just aesthetic, it also acted as shielding and as a barrier that kept any loose surface contamination from becoming airborne. A collapse that exposes the tunnel interior to open air breaks both roles — it can raise the local external dose rate (less shielding overburden) and, more importantly, creates a pathway for resuspension of loose contamination as airborne particulate, which is an internal (inhalation) hazard that a simple external dose-rate reading does not capture at all. This changes the inspection plan in part (e): before committing the crew to the time-budgeted external-dose plan above, the plan must add continuous air sampling / real-time particulate monitoring at the opening, respiratory protection (at minimum a powered air-purifying respirator) for every entrant, and a contamination-control step-off pad, since the workers' true dose could otherwise be under-estimated by external gamma-rate alone.
7. Part (g) — monitoring instruments. A portable pressurized ion-chamber survey meter (e.g. a "cutie-pie" type) is well suited to mapping the external gamma exposure rate at the tunnel opening, since ion chambers read accurately over the wide, high dose-rate range expected here; each entrant should also carry a personal electronic (direct-reading) dosimeter with an audible dose-rate alarm so an unexpected hot spot is caught immediately. Given part (f)'s airborne-release concern, a portable continuous air monitor (CAM) with particulate filter and alpha/beta counting should also be deployed at the opening to catch any resuspended contamination before it reaches the crew.