A second Cs barrier in the fuel matrix captures Cs as pollucite and immobilizes it within the fuel form — without redesigning the qualified TRISO particle.
The getter is added to the graphite matrix; the qualified particle is unchanged.
Including 1 h in air, as well as 1 h and 3 h in Ar.
Electron diffraction and EDS identify pollucite in the reacted specimen.
Why
Cs that leaves the kernel deposits in the primary circuit. About 80 % of the plate-out dose rate there comes from caesium, which drives occupational dose during maintenance (JAERI-M 91-198).
Thicker coatings would help. But TRISO requalification would be required.
HTTR shielding evaluations attribute the major part of the deposited radioactivity in the primary circuit to caesium. The same evaluation base gives two different assumed through-coating failure fractions — 1×10−2 for shielding design and 4×10−4 for maintenance dose — both carrying margin over the reported as-manufactured value.
Release from failed particles adds to the diffusive term. The getter addresses both, since it acts on Cs that has entered the matrix regardless of how it got there.
Strengthening physical confinement by thickening the coating layers is feasible, but it reduces fuel uranium density and limits design flexibility — and any change to the particle triggers a fresh qualification campaign, from irradiation testing through post-irradiation examination.
Graphite adsorbs Cs. We want it chemically bound.
What
Aluminosilicate is added to the graphite matrix so Cs can be chemically captured as pollucite before it leaves the fuel element. Cs is immobilized within the fuel form.
Potential benefit of confining Cs within the fuel form
Unlocking new value for HTGRs
Safety benefits
Less Cs in the primary circuit during normal operation and accidents.
Economic benefits
Lower maintenance doses, shielding requirements, and decommissioning waste.
Potential licensing benefits
Coating the particle surface would need its own process development and durability data. Coating primary-circuit components only redistributes the contamination — it does not capture Cs at the source. The matrix is already in the release path and already goes through a powder-blending step.
Placement: uniform dispersion, local enrichment near particles, a getter-rich zone, or a graded distribution. Fuel form: pebble, cylindrical or annular compact, block-type compact, FHR pebble. Physical form: fine or granulated powder, agglomerates, coated getter particles, pre-dispersed composite powder.
And you get there without touching the qualified particle.
Evidence
Step 01
Graphite + 1 mol% of aluminosilicate (Al2O3·3SiO2) + metallic Cs. Heating at 670 °C × 1 h under Ar (just below the boiling point of Cs) to make the Cs-captured specimen.
Step 02
1600 °C under Ar (simplified AOO condition) × 1 h, 3 h; 1600 °C under air (simplified air-ingress accident condition) × 1 h. Then: is Cs retained? → checked by SEM-EDS and TEM.
Cs peaks remain in all heated specimens — including 1 h in air. No Cs was detected in the getter-free reference.
With the getter, cesium stays. No getter, no Cs retained.
No Cs detected in the getter-free reference.
Cs was retained detectable after 1600 °C exposure: 1 h and 3 h in Ar, and 1 h in air.
FIB-TEM electron diffraction and EDS identify Al2O3 and CsAlSi2O6 (pollucite) in the reacted specimen.
Under Ar, SiO2 tends to react with carbon to form SiC, while Al-bearing species can be lost through reduction and volatilisation. Under air, local low-pO2 conditions can promote SiO(g) formation. The observed Si/Al shift is therefore consistent with matrix-element redistribution.
Cost and benefit
Even under conservative assumptions, the performance penalty remains small.
Reactivity
|Δk∞| ≤ 0.12 %
zero within 3σ — all three cases.
Thermal conductivity
−3.12 % / −0.46 %
Even if every particle failed: compact / pebble. Maxwell–Eucken estimate; 0.03 % or less at the 1 % failed-particle case.
Maintenance dose
η = 0.63
Capture efficiency that would halve the primary-circuit dose rate in the preliminary assessment.
Getter mass per fuel element, the estimated effect on thermal conductivity, and the effect on k∞, by assumed failure fraction
The table is the same basis used in the TopFuel 2026 presentation. [1] Demkowicz et al., HTR-2016, INL/CON-16-38197 (2016), Table 2. [2] IAEA-TECDOC-1674 (2012), §5.1.4.2 — scram level 7×10⁴ MBq/m³ ≡ 1 % failed particles.
Failure-fraction basis: 1.1×10−5 — AGR-1 PIE: zero TRISO failures in about 2.98×105 particles, 95 % upper bound [1]. 1×10−2 — HTTR scram level, 1 % failed particles [2].
Bounding Cs inventory: HTTR-type particle; UO2 kernel 600 µm diameter; 20 % enrichment; all U-235 fissioned; Cs fission yield 20 %.
Getter mass: stoichiometric requirement of 2 mol Al2O3·3SiO2 per 3 mol Cs forming CsAlSi2O6.
Reactivity: one calculation covers both fuel forms because the coated particle — and therefore the getter-to-fuel ratio — is identical; MVP-3 (continuous-energy Monte Carlo) / JENDL-4.0, HTR-PROTEUS; spectral differences are second-order — negligible at this magnitude.
Thermal conductivity: Maxwell–Eucken estimate for non-conducting spherical inclusions, assuming λ0 = 60 W·m−1·K−1 for A3-3 matrix graphite and no heat-conduction contribution from Al2O3·3SiO2. Treating the getter as non-conducting bounds the penalty from above, and the relative penalty does not depend on λ0. In a pebble the getter sits in the overcoat — 12 % of the matrix; in a compact the overcoat is most of the matrix, which is why the two columns differ.
Caesium generated is the cumulative fission count times the Cs yield; the pollucite stoichiometry fixes two moles of aluminosilicate per three moles of Cs. Because the getter demand is set by the particle, the getter-to-fuel ratio is the same in a pebble and in a compact.
MVP-3 with JENDL-4.0 on an HTR-PROTEUS pebble cell, getter dispersed in a 200 µm overcoat. Three loadings were run; all three returned zero within a 3σ statistical uncertainty of about ±0.12 %.
The remaining dose rate is y = 1 − η fCs, where η is the fraction of Cs captured by the getter (not yet measured) and fCs is the Cs share of the plate-out dose rate, 0.5 y after shutdown. fCs = 0.902 at the primary water cooler, 0.799 across the primary circuit, 0.551 at the IHX, and 0.002 at the double pipe / RPV inner surface. The dose falls with capture efficiency — up to 80 % across the primary circuit. We plan to estimate η experimentally.
Estimated from the HTTR maintenance-evaluation source (JAERI-M 91-198), with effective dose-rate constants from the Isotope Handbook, 12th ed.
Confining Cs within the fuel form could reduce the Cs source term, add margin in accident consequence evaluations, and provide an additional chemical barrier supporting defense-in-depth. A lower primary-circuit source term has been discussed in connection with shielding wall thickness, emergency planning distance, stack height, and waste classification at decommissioning. These depend on the licensing position taken for a given plant and are offered here as directions for assessment, not as established credits.
Where we need you
◆ = fuel-form specific. Cannot be closed without the fuel designer's input. Underlined: tasks currently in progress.
01
A second Cs barrier can be added without redesigning TRISO. Aluminosilicate captures Cs as pollucite, confirmed after 1600 °C exposure in Ar and air.
02
Confining Cs within the fuel form unlocks new value for HTGRs. Less Cs in the primary circuit; lower maintenance dose, shielding and decommissioning waste; potential licensing benefits.
03
The estimated performance penalty is small. No measurable reactivity penalty; and a thermal-conductivity penalty that stays small even under conservative assumptions.
04
The remaining step is fuel-form integration. Getter loading, matrix properties and processing window must be optimized with the fuel designer.
The chemistry is demonstrated. The next step is integration into a real fuel design.
We can start from a single specimen made to your spec.
Answer as many of the three design questions as you can.
Anything left blank can be discussed directly.
Fuel matrix fabrication → Cs retention testing → irradiation stability → fuel-form validation.
Frequently Asked Questions
No. The concept places the aluminosilicate getter in the graphite fuel matrix. The qualified TRISO particle itself is not changed.
Cs was retained detectable after 1600 °C exposure in Ar and air. FIB-TEM identified CsAlSi2O6 (pollucite) in the reacted specimen. No Cs was detected in the getter-free reference used in the comparison.
Less Cs in the primary circuit during normal operation and accidents; lower maintenance doses, shielding requirements and decommissioning waste; and, potentially, licensing benefits — a reduced Cs source term, additional margin in accident consequence evaluations, and an additional chemical barrier supporting defense-in-depth. These are directions for assessment, not established credits.
Not yet. Quantitative retention, mass balance and control testing are the next validation needs. The maintenance-dose calculation therefore treats capture efficiency η as a parameter rather than a measured value.
The required mass scales with the assumed Cs inventory. In the current bounding calculation, the 1 % failed-particle case corresponds to 4.5 mg per compact and 3.3 mg per pebble; the all-particles-failed case corresponds to 0.45 g and 0.33 g, respectively.
The reactivity calculation returned zero within 3σ for all three loading cases (|Δk∞| ≤ 0.12 %). The Maxwell–Eucken thermal-conductivity estimate gives −0.03 % / −0.005 % at the 1 % failed-particle case and −3.12 % / −0.46 % at the all-particles-failed case for compact / pebble.
Fuel-form integration: optimize getter loading, matrix density/strength/conductivity/porosity, the fabrication-temperature window, in-pile stability and reactor-specific fuel-performance/source-term assessment with the fuel designer.
