What the public records show about Valar Atomics in Castle Country

What the document says

Valar's April 2025 post describes Ward One as a 100 kWt high-temperature gas reactor using TRISO fuel, with a planned operational lifetime of less than a month, and says: "Our analysis indicates that holding the spent fuel from this system for five minutes" gives the same radiation exposure as a CAT (CT) scan. The post does not say how much fuel is held, how long the reactor ran, how long the fuel cooled, how far it is from the body, or which dose is meant, and the analysis it cites has not been published. The text is the same in the live page (29 September 2026) and in Internet Archive captures of 23 March and 22 July 2026.

Given

• P₀ = 100 kW thermal (Valar post, Ward One paragraph)
• Fuel = TRISO fuel in a high-temperature gas reactor; fuel mass, enrichment and element type not stated (Valar post, Ward One paragraph)
• T = run of 1 to 30 full-power days (the post gives only a planned operating lifetime of less than a month); break-even run length also solved (Valar post; the range is an assumption)
• t_hold = 5 minutes = 300 s (Valar post)
• f = share of the core's fission products held: the whole core (the literal reading), 1/76 of the core (one of Ward 250's 76 fuel elements, as a stand-in), one 6 cm pebble holding 7 g of uranium (1/35,700 of a 250 kg core), and the break-even share (assumption; Ward 250 NSDA Table 6, p. 52; INL/EXT-20-60236 Table 2)
• t = cooling time after shutdown: 1 day to 1 year shown, break-even solved; less than 1 day not modelled (it would give more) (assumption (the post states none))
• M = 250 kg of uranium in the core (stand-in; a larger core spreads the fission products more thinly per gram) (Ward 250 NSDA Table 6, p. 52 (a different Valar reactor of the same 100 kW))
• E_f = 200 MeV per fission, so 3.12 × 10¹⁰ fissions per watt-second (Lamarsh & Baratta, Introduction to Nuclear Engineering, ch. 3)
• y, T½, gamma lines = U-235 thermal cumulative fission yields (e.g. Zr-95 6.50%, Ba-140 6.21%, Te-132 4.30%, I-133 6.70%); half-lives and principal gamma lines of 43 fission products (ENDF/B-VII.1 and JEFF-3.1 yields; ENSDF decay data via NuDat 3)
• μ/ρ, μ_en/ρ = graphite and uranium attenuation; air and soft-tissue energy absorption (tissue = 0.990 × water, ICRU-44) (NIST, Hubbell & Seltzer (NISTIR 5632))
• E/K_a = 1.00 to 1.43 Sv of effective dose per Gy of air kerma, front-on (AP) exposure (ICRP Publication 74, Table A.17)
• Pebble = 6 cm ball: 2.5 cm fuelled zone inside a 0.5 cm fuel-free shell; matrix graphite 1.70 g/cm³ (1.75 used); 7 g of uranium (the HTR-PM value; INL's benchmark pebble holds 9 g) (INL/EXT-20-60236 Rev. 1, Tables 2-3)
• d = body reference point 50 cm from the near surface of what is held (30 cm as a sensitivity) (assumption)
• CT = 1 to 10 mSv effective dose for typical diagnostic CT; FDA table values from 2 mSv (head) to 16 mSv (coronary CT angiogram) (FDA, What are the Radiation Risks from CT?, text and Table 1)

Working

1. F = 1.0 × 10⁵ W ÷ (200 MeV × 1.602 × 10⁻¹³ J/MeV) = 3.12 × 10¹⁵ fissions/s 30 full-power days: 0.1 MW × 30 d = 3.00 MWd = 8.09 × 10²¹ fissions; 1 full-power day: 0.10 MWd
   The radioactive fragments in spent fuel depend on how many uranium atoms were split, that is on power × running time, not on how much uranium was loaded. 200 MeV per fission is the textbook value; 193 MeV would raise every dose below by about 4%. The post's 'less than a month' is taken as a run of 1 to 30 full-power days.
2. A_i(T, t) = y_i × F × (1 − e^(−λ_i T)) × e^(−λ_i t) daughters (La-140 from Ba-140, Nb-95 from Zr-95, I-132 from Te-132 and others) by the two-member Bateman equations
   Activity of each gamma-emitting fission product after running for T and cooling for t, from U-235 fission yields and half-lives. Gases and iodine are taken to stay inside the intact fuel particles.
3. S_γ = Σ_i A_i × (gamma energy per decay)_i whole core, 30-day run: 164 W at 1 day of cooling, 25 W at 30 days, 0.49 W at 1 year
   Gamma-ray power given off by the fission products. Cross-check: half of the Way-Wigner decay-heat formula, 0.0622 × [t^−0.2 − (t + T)^−0.2] × 100 kW (t and T in seconds), gives 159 W at 1 day and 21 W at 30 days; gamma rays carry roughly half of fission-product decay heat. At 1 day lanthanum-140 (34%) and iodine-132 (21%) give most of it; at 30 days, lanthanum-140, zirconium-95 and niobium-95.
4. One CT in five minutes: (1 to 10 mSv) ÷ (5/60 h) = 12 to 120 mSv/h
   FDA gives typical effective doses of 1 to 10 mSv for diagnostic CT. The claim is counted as met at any five-minute dose up to 10 mSv, the top of that range.
5. Piece held = a solid ball of fuel: R = [3 f M ÷ (4π ρ_HM)]^(1/3), ρ_HM = 7 g ÷ 113.1 cm³ = 0.0619 g/cm³, M = 250 kg whole core R = 98.8 cm; 1/76 of the core R = 23.3 cm; one pebble: 2.5 cm fuelled zone inside a 0.5 cm graphite shell μ = 1.75 g/cm³ × (μ/ρ)_graphite + ρ_HM × (μ/ρ)_uranium = 0.139 per cm at 0.7 MeV
   f is the share of the core's fission products held. Ward One's fuel mass and form are not public, so Valar's Ward 250 figures (250 kg of uranium, 76 fuel elements; a different reactor) and a standard 6 cm pebble stand in. A ball of fuel with no gaps, at a graphite density above INL's 1.70 g/cm³, absorbs more of its own radiation than a real core would, which lowers every dose here.
6. φ(d) = (S_v ÷ 2μ) ∫₀^(π/2) sin α × (R/d) × (cos β ÷ cos α) × e^(−μs) × (1 − e^(−2μR cos β)) dβ, sin α = (R/d) sin β, s = path through the pebble's fuel-free shell (zero for a uniform ball)
   The exact rate at which gamma energy leaves the ball without scattering and reaches a point at distance d from its centre (S_v is the gamma power per cm³). Checks: it reproduces the closed-form contact result, the point-source result far away, and an independent shell integral to six digits.
7. Whole body: E = 300 s × Σ φ(R + 50 cm) × (μ_en/ρ)_air × (E/K_a)_AP Hand: H = 300 s × Σ φ(R) × (μ_en/ρ)_tissue, (μ_en/ρ)_tissue = 0.990 × (μ_en/ρ)_water
   Effective dose, the quantity FDA uses for CT, is estimated from the air dose at a point 50 cm from the near surface of the fuel, converted with ICRP Publication 74's front-on factors (1.0 to 1.4 Sv per Gy). The hand dose is the dose to the skin where it touches the fuel. It is not an effective dose and is not compared with the CT figure.
8. 30-day run, whole-body dose in five minutes: whole core: 334 mSv at 1 day of cooling, 54 mSv at 30 days, 0.99 mSv at 1 year 1/76 of the core: 64, 10.3 and 0.19 mSv one 7 g pebble: 0.77, 0.12 and 0.0024 mSv
   Against a 10 mSv CT, the whole core gives 33 times as much at 1 day and 5.4 times at 30 days; one pebble is below FDA's 1 to 10 mSv range. Hand on the fuel at 1 day: 1,379 mSv (core surface), 1,182 mSv (1/76) and 316 mSv (pebble). The whole core and the 1/76 piece (about 100 kg in this model) cannot be lifted; they are shown because the post says 'the spent fuel'.
9. 1-day run, whole-body dose in five minutes: whole core: 75 mSv at 1 day of cooling, 3.0 mSv at 30 days; 1/76 of the core: 14.6 and 0.56 mSv; one pebble: 0.18 and 0.006 mSv
   The low end of 'less than a month'. A shorter or lower-power run gives less (see the break-even run length below).
10. Solve E(f) = 10 mSv for the share f held: 30-day run, 1 day of cooling: f = 1/1,589, about 157 g of uranium or 22 pebbles' worth in a 250 kg core (a ball 17 cm across) 30-day run, 30 days: f = 1/81 (3.1 kg); 1-day run, 1 day: f = 1/156 (1.6 kg) for a 1 mSv CT: 8.7 g, 85 g and 50 g
    How small a piece makes five minutes equal one CT, as whole-body dose. Held 30 cm from the body instead of 50 cm, the 157 g falls to 48 g; against FDA's highest table value (16 mSv, coronary CT angiogram) it rises to 310 g.
11. One pebble-sized piece: E = 10 mSv when it holds 1/2,759 of the core's fission products (30-day run, 1 day of cooling)
    So the single-pebble result needs a core of at least about 2,760 pebbles' worth of fuel (about 19 kg of uranium at 7 g each). A 250 kg core holds about 35,700. Ward One's core size is not public.
12. Solve E(t) = 10 mSv for the cooling time t, and E(T) = 10 mSv for the run length T
    Whole core: 128 days of cooling after a 30-day run, 9.6 days after a 1-day run (365 and 62 days for a 1 mSv CT). 1/76 of the core: 31 days and 1.4 days. One day after shutdown, the whole core matches a 10 mSv CT only if the reactor ran about 2.1 full-power hours (8.7 kWd).
13. Hand dose, one 7 g pebble, 30-day run, 1 day of cooling: 316 mSv in five minutes; for the 157 g break-even piece: 833 mSv
    Skin dose at the point of contact from gamma rays alone; beta particles would add more if bare fuel were touched. It is a different quantity from a CT's whole-body effective dose. For scale, the NRC's annual limit for the skin of a worker's hands is 500 mSv (10 CFR 20.1201). The pebble's hand dose falls to 10 mSv after about 129 days of cooling (30-day run) or 9 days (1-day run).
14. Sensitivities, 30-day run, 1 day of cooling (factor on the whole-body dose): body 30 cm away instead of 50 cm: × 1.4 (core) to × 2.6 (pebble); scattered photons (buildup): × 1.35 to × 2.4 neptunium-239 from U-238 capture: × 1.04 to × 1.12; Ward 250's core volume instead of a solid ball: × 1.1 to × 1.2 9 g pebble (INL benchmark) instead of 7 g: × 1.27; only the 18 strongest emitters counted: × 0.89 to × 0.90
    Each choice the post leaves open was set to lower the dose; these factors show how far each could move the results. None of them moves a case across the 10 mSv line at 1 day of cooling.

Results: five minutes holding Ward One's spent fuel (mSv; a typical CT scan is 1 to 10 mSv whole-body)

Whole-body = effective dose, the quantity FDA gives for CT, at a point 50 cm from the near surface of the fuel. The last column is the skin dose where the hand touches the pebble: a different quantity, not comparable with a CT's whole-body figure. 'All the spent fuel' and 'one element' (about 100 kg in this model) could not be lifted by hand; they show what the post's words 'the spent fuel' mean taken literally. Scattered photons, beta particles and neptunium-239 are left out, so every figure is a lower estimate. Run length is in full-power days at 100 kW.

Result

With the post's 100 kWt and a run of 1 to 30 full-power days, five minutes with all of the reactor's spent fuel, 50 cm from the body, gives a whole-body effective dose of about 75 to 334 mSv one day after shutdown and 3 to 54 mSv after 30 days, against FDA's typical 1 to 10 mSv for a CT. It comes down to one 10 mSv CT only for a piece holding no more than about 1/1,600 of the core's fission products one day after a 30-day run (1/156 after a one-day run; 1/81 after 30 days of cooling), about 157 g of uranium or 22 pebbles' worth if the core held 250 kg; or, for the whole core, after about 10 to 128 days of cooling. One 7 g pebble gives about 0.8 mSv whole-body, below FDA's typical CT range, while the skin of the hand touching it receives about 316 mSv in the same five minutes, a different dose quantity. Scattered photons, beta particles and neptunium-239 are left out, so these are lower estimates.

Sources

• Valar Atomics is Suing the NRC (Valar Atomics, 2025-04-07) · 'Our Vision' section, Ward One paragraph
• Valar Atomics is Suing the NRC: Internet Archive capture of 23 March 2026 (Valar Atomics (via Internet Archive), 2025-04-07) · 'Our Vision' section, Ward One paragraph
• Valar Atomics is Suing the NRC: Internet Archive capture of 22 July 2026 (Valar Atomics (via Internet Archive), 2025-04-07) · 'Our Vision' section, Ward One paragraph
• What are the Radiation Risks from CT? (U.S. Food and Drug Administration, 2017-12-05) · Radiation Dose from CT Examinations; Table 1
• Radiation Dose from X-Ray and CT Exams (RadiologyInfo.org (Radiological Society of North America and American College of Radiology), 2025-04-15) · Effective radiation dose in adults (table, and its note citing ICRP Publication 103 on partial-body exposure)
• The nuclear fuel cycle (Nuclear explained) (U.S. Energy Information Administration, 2023-10-26) · The back end of the nuclear fuel cycle
• Ward250 Nuclear Safety Design Agreement, No. 100403, Rev 02 (status: Draft; issued for public release); used only for stand-in core figures (Valar Atomics, 2025-10) · p. 34 (3 MWd, 30 EFPD inventory basis); p. 52, Table 6 (100 kWth; core 1.5 m by 1.8 m; 76 fuel elements; 250 kg heavy metal)
• Benchmark for Fuel Shuffling and Depletion for Pebble-Bed Reactors, INL/EXT-20-60236 Rev. 1 (R. Sonat Sen) (Idaho National Laboratory (U.S. Department of Energy), 2020-09) · Tables 1-3 (6 cm pebble, 2.5 cm fuel zone, 0.5 cm fuel-free shell, matrix graphite 1.70 g/cm3, 9 g heavy metal per pebble)
• NuDat 3: half-lives, gamma-ray energies and intensities (ENSDF) (National Nuclear Data Center, Brookhaven National Laboratory) · ENSDF half-lives and gamma lines, entered from the evaluated data; NNDC's data pages answered our requests with 'too many requests', so they could not be archived for this check
• X-Ray Mass Attenuation Coefficients (Hubbell & Seltzer, NISTIR 5632): graphite, uranium, air, water (National Institute of Standards and Technology) · values from the published tables for graphite, uranium, air and water; NIST's server does not allow automated copies, so none is archived
• ICRP Publication 74: Conversion Coefficients for use in Radiological Protection against External Radiation (International Commission on Radiological Protection, 1996) · Table A.17 (effective dose per air kerma, front-on). The publication is not openly published, so no copy is archived; using a flat 1.0 Sv per Gy instead changes the results by about 2%
• 10 CFR 20.1201: occupational dose limits for adults (Legal Information Institute (Cornell)) · (a)(2)(ii): 50 rem (500 mSv) shallow-dose equivalent to the skin of any extremity per year
• A startup wants to build a massive nuclear-powered data center on public land in Utah (NPR, 2026-09-30) · read from a copy a resident saved; NPR does not allow automated copies, so none is archived here

Checked: Two independent calculations worked out the doses, each entering the nuclear data separately; a third check re-derived every case in one program and traced each difference between them to a stated modelling choice (distance measured from the fuel's centre or surface, density and size of the fuel ball, how many nuclides were counted, a point-source shortcut). With matching choices the third check reproduces both within 5%, and the two calculations' gamma-ray data agree within about 4%. The total gamma output agrees with the standard decay-heat formula within 3% one day after shutdown. Valar's post was compared across the live page (29 September 2026) and Internet Archive captures of 23 March and 22 July 2026: the text is identical. FDA's CT figures, EIA, Valar's Ward 250 safety paper, the INL pebble benchmark and the NRC's 10 CFR 20.1201 were read from archived copies. The fission-yield, decay and photon-attenuation tables come from the standard evaluated data (ENDF/ENSDF, NIST); the IAEA and NIST servers refuse automated copies and NNDC's answered 'too many requests', so those tables were not re-checked against an archived copy. An error of a few percent in them would move the results by about the same few percent; none of the sensitivity checks moves any case across the CT line. Valar has not published the analysis its post cites, so which reading it used cannot be checked.

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