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Folklore Debunking

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Folklore Debunking A Critical Assessment of Radiation Folklore Bruce Heinmiller Dosimetry Services Chalk River Laboratories UNRESTRICTED / ILLIMITÉ It ain’t so much the things we don’t know that get us into trouble. It’s the things we know that just ain’t so. Artemus Ward (according to Thomas Gilovich in How We Know What Isn’t So) Variations in quote and attribution exist. There’s a take-home message here. What Folklore? • K-40 dose mis-information widely propagated by nuclear energy proponents; putative doses compared to public doses from nuclear energy production • Mis-information widely propagated about source of, and shielding of, cosmic rays • And a few others Folklore-contaminated Items: • Nuclear association “information” materials • University “information” websites • News articles/magazines; BBC, CBC, other TV • Journals; conference reports • Testimony at public hearings • Presentations/speeches • Reports from utilities and other corporations • List servers/blogs; posters; books … Why Give “Anti-nukes” a Pass? • Too easy • Not very interesting • No accountability • Quantitative claims are typically versions of the “Texas Sharpshooter” technique. A Few Quantities and Units Absorbed Dose, D = dε / dm J/kg (Gy) Effective Dose = W T W R D T , R J/kg (Sv) T R For this talk, Gy = Sv is good enough (except where I think it needs to be noted otherwise) Activity = The expectation value of the number of nuclear transformations in a given quantity of material per unit time s-1 (Bq) (N λ in the case of N atoms of a single radionuclide) NCRP Report No. 160: “Ionizing Radiation Exposure of the Population of the United States” (data 2006) Total = 6.2 mSv/y per capita (average) Adult K-40 Activity BNL study: K mass (g) = = (5.52 – 0.014 A) A = age (y); W = mass (kg); H = height (m) ICRP-89: Men: 1.76 m; 73 kg Women: 1.63 m; 60 kg For A = 40: (and using 31.7 Bq K-40 / g K) Men: 4160 Bq Women: 2730 Bq Average: 3445 Bq Folklore from Distortion of Fact -- Fact Becomes Factoid Factoid: “Average adult *or even “person”+ contains 140 g *4440 Bq+ of potassium.” Fact: Quoted datum is for Reference Man who is: young and male -- both attributes that are predictive of higher potassium content. Result: Factoid is in error by 29% (1000 Bq). Comparative Veracity of Two Popular Statements: Correct Not Correct The claim that an average adult [or person] contains 140 g of potassium Widely Propagated Folklore • Sleeping with someone results in a dose of 20 µSv/y [2 mrem/y] (from gamma rays emitted by the potassium in their body). • Stated explicitly or implicitly by comparison • Variations: 10, 30 and 40 µSv/y • Nuc. Reg. Com. : 1 µSv/y Spousal Irradiation – Debunking the Folklore Source/Target Geometry for Monte Carlo Modelling Reference Adult Whole-body Nominal Radionuclide Inventories (Bq) Nuclide Male Female Average K-40 4160 2730 3445 C-14 a 3400 3320 3360 Rb-87 b 550 360 455 Others c 200 200 200 Total 8310 6610 7460 a Based on cosmogenic 200 Bq/kg C. Equilibrium largely restored from atmospheric weapons testing. Fossil fuel contribution to decreased C-14 isotopic abundance. b Discrepancy in references. Probably correct. c H-3, Be-7, U-series, Th-series, and others. Activities highly variable. Simplified K-40 Decay Scheme 89.3% β- to Ca-40 (stable) 10.7% e.c. to Ar-40 (stable) 585 keV average β energy 1.461 MeV γ ray Decay is to ground state. Few c.e.; γ yield 10.7% per n.t. Bremsstrahlung Annihillation photons from β+ << 1% of β energy <0.2%. Folded into the 10.7%. No other significant radiations No other significant radiations Reference Man K-40 Self Dose Rate # n.t./y = (4440 /s) (3.156E7 s/y) = 1.4E11 /y β self dose /y assuming uniform activity: (1.4E11 /y)(0.893)(0.585 MeV)(1.6E-13 J/MeV) / 70 kg = 0.17 mGy/y (NCRP 94 gives 0.16 mGy/y) γ self dose /y: (1.4E11 /y)(0.107)(1.461 MeV)(A.F.)(1.6E-13 J/MeV) / 70 kg = (0.05)(A.F.) mGy/y (A.F. ≈ 0.5 using MIRD) = 0.025 mGy/y (NCRP 94 gives 0.02 mGy/y) = 20 µGy/y = 20 µSv/y Max. Dose Rate to Spouse from Ref. Man: Rate of Photon Energy Escaping into 4π sr: (0.05 mGy/y)(1 – A.F.=0.5)(70 kg) = 1.75 mJ/y Quick Estimate of Maximum Dose Rate to Spouse: [Closest-packing co-ordination number of a circle is 6.] (1/6)(1.75 mJ/y)(A.F.)(8 h / 24 h) / 60 kg = (1.6)(A.F.) µGy/y = 1 µGy/y for A.F. = 0.6 [?] Overestimate: contact; 4π/6 sr too large; M irr. F (Most widely propagated folklore: 20 µGy/y) Monte Carlo Results (all results in µGy/y) 24 h/d Dose 8 h/d Dose Male (M) Photon Self Dose 16 Female (F) Photon Self Dose 12 M F Touching Geometry 3 1 F M Touching Geometry 2 0.7 M F 10-cm Separation 2 0.7 F M 10-cm Separation 1 0.3 M F 30-cm Separation 1 0.3 F M 30-cm Separation 0.6 0.2 20 µSv/y debunked … • … and easily, too! • So how does it survive/propagate? examine at end of talk • Defensible dose rate estimate appears to be 0.2 or 0.3 µSv/y. But wait! The body is a source, but it’s also a shield (and a scatter medium). Terrestrial Photon Dose to Cylinder With vs. Without Another Present – MCNP Results • Touching Geometry: -9% (lower dose with two present) • 25-cm Separation: -4% (lower dose with two present) • UNSCEAR Indoor Terrestrial Doses (but assuming 8 h / d) World average: 170 µSv/y U.S.A. (very low): 80 µSv/y Let’s use 100 µSv/y as a reference. Touching Geometry: -9 µSv/y vs. nominally +0.9 µSv/y from K-40 25-cm Separation: -4 µSv/y vs. nominally +0.3 µSv/y from K-40 So, you’re 10 times a greater shield than a source! (assuming no recent nuclear medicine administration) Radiation Dose From Ingestion of Potassium-rich Foods Radiation Folklore Goes Bananas Putative doses calculated for ingestion of potassium-rich foods, using ICRP adult ingestion DCF for K-40. New unit conceived: “Banana Equivalent Dose” Sample cal’n: (6.2 nSv/Bq) (31.7 Bq/g) (0.422 g) = 83 nSv Rounded to 0.1 µSv Propagated Folklore: Eating a banana = 0.1 µSv ICRP DCF = 6.2E-9 Sv/Bq (K-40 adult ing.) Let’s figure out what this assumes. Crude calculation (uniform activity) using ICRP data: 1 / λbio ≈ 43 d # n.t./Bq ≈ (43 d) (8.64E4 s/d) (1 /s) = 3.7E6 3.7E6 [(0.893)(0.585 MeV) + (0.107)(1.461 MeV)(0.5)] x (1.6E-13 J/MeV) / 70 kg = 5.1E-9 Gy ( /Bq ) Evidently, ICRP DCF assumes a long retention time for K in the body. [for the atoms ingested] Proposed Simplified(!) Potassium Metabolic Model • Mean intake is 3.3 g/day in Reference Man • 85% excreted in urine at, say, 4 hours • 11% excreted in feces at, say, 24 hours • 4% excreted in sweat et al. at, say, 24 hours [?] • Equivalent single retention time (=# n.t./Bq): 0.85(4 h) + 0.15(24 h) = 7 hours • (Unusual heavy sweating before ingestion may result in K deficit and subsequent longer retention) [?] Here’s the thing … • If action A is claimed to deliver dose D, then one should feel justified in assuming that dose D is that dose that is above the baseline dose that would have been delivered had action A not occurred. • K is under homeostatic control via kidney function, implying that ingestion of X Bq of K results in excretion of 0.85 X Bq of K a few hours later, and 0.15 X Bq (generously) a day later if, in both cases, the K has the natural isotopic composition (0.0117% K-40). Consider Three Scenarios: Case 1: Ingestion of carrier-free (isotopically pure) K-40. ICRP DCF applies. Case 2: Ingestion of natural K before or in the absence of Case 1. Derive dose from, say … ICRP DCF x 7 h / [(43 d)(24 h/d)] = 0.7% DCF. Case 3: Ingestion of natural K after Case 1. Net dose will be negative if previous Case-1 ingestion was significant and recent enough that the body is still isotopically rich in K-40. Banana Summary: The K atoms from the banana when ingested have a mean retention time of 43 days. But, ingesting them triggers the excretion of K atoms (most from the inventory present in the body before the banana was eaten). We assumed mean excretion occurs 7 hours after ingestion. If so, the dose would be 0.7% of the widely propagated folklore dose. Widely Propagated Folklore about Cosmic Radiation • Cosmic ray dose being attributed to the sun • Earth’s magnetic field being critical in protection of life on earth from cosmic rays Let’s see. (But with a few distractions) Prelude to Discovery of Cosmic Radiation • 1895 discovery of x-rays • 1896 discovery of radioactivity (in U) • 1897 discovery of the electron • 1898 discovery of Po, Ra • the Earth is radioactive • c. 1900 discovery of Rn (“emanation”) • the atmosphere is radioactive Curie Museum Early 20th Century Radiation Folklore Did these (and other) great scientists’ work delay the discovery of cosmic rays? (an aside: Radiological Protection Practices c.
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