Hand-considered hypotheses, 5901–6000
Nuclear plant systems and materials: fuel behaviour, cladding, coolant chemistry, heat exchangers, welds and inspection hardware, written one at a time. Each entry states a claim, names a physical or chemical mechanism, and gives a measurement that would separate it from its nearest rival; the logical form chosen for that entry is printed above the text, and no two entries share one, so that repetition of the argument rather than of the subject can be checked by eye. The claims are about mechanisms only: each is a statement about metal, water or instrument, and none turns on an argument from plant acceptance or from what an outside body would allow. None has been tested. No citation is invoked and no number is presented as measured: every magnitude offered is marked as the estimate it is. The instrument items belonging to the separate instrumentation list, and the sampling and estimator arguments of earlier sets, are deliberately absent here.
Fuel behaviour
Misattributed release mechanism
H5901. Fission-gas release measured during a low-temperature irradiation is normally assigned to volume diffusion with parameters taken from furnace anneals. At low temperature, though, the release is dominated by recoil and knockout from the outer few microns of the pellet, a process with almost no temperature dependence. The two are separated by irradiating a pellet carrying a thin deposited layer of known inventory on its surface, since the layer's release tracks the surface temperature weakly while diffusion-sourced release tracks the bulk temperature strongly.
Sensitivity ceiling set by a series resistance
H5902. The centreline temperature's sensitivity to fuel conductivity falls towards zero once the gap and the oxide together hold most of the temperature drop, so a better-conducting fuel and a poorer one converge as the heat flux rises. Two fuels of different conductivity in identical cladding, run to the same flux, ought to differ less as the flux rises, while a difference that stays constant means the gap is not the controlling resistance.
Non-monotonic response in burnup
H5903. Fractional gas release rises with burnup, then flattens, then rises again. The flattening is not a saturation of diffusion but the rim's fine-grained restructured zone storing gas at grain faces faster than it can release it. A burnup series at constant power, with the rim thickness measured separately, should place the plateau where the rim's grain faces reach their saturation coverage rather than where the pellet temperature falls.
Hidden term from reopened porosity
H5904. Swelling measured by immersion density treats all closed porosity as volume, but as burnup rises a fraction of the closed pores reconnects to the surface and the liquid enters them, so the measured swelling falls below the true solid swelling by exactly the reconnected volume. A gas-adsorption measurement of open porosity at each step supplies the correction, and without it the density-derived swelling curve bends the wrong way.
Artefact that survives a change of specimen
H5905. Laser-flash diffusivity of irradiated fuel is reported to degrade with burnup, yet part of the drop is the changing contact between specimen and graphite coating as the surface roughens. That contact is a property of the preparation, not of the ceramic. Roughening an unirradiated specimen to the same finish reproduces a similar drop, and measuring one specimen with two coatings and two pulse widths separates the contact term from the material's own.
Threshold fixed by oxygen potential
H5906. Caesium migration in a pellet is usually modelled as a volatility-driven process, but the migration stops once the local oxygen potential is high enough to bind caesium as a uranate, so the threshold is chemical and not thermal. Annealing pellets of identical temperature and different initial oxygen-to-metal ratio, then sectioning for the caesium profile, ought to put the mobility's turn-off at one oxygen potential rather than at one temperature.
Identity behind two activation energies
H5907. Fuel densification and the collapse of small pores under hot pressing are the same process observed at different pore sizes, and the two reported activation energies differ only because the driving curvature differs. A fuel with a deliberately bimodal pore distribution should densify in two stages whose rate ratio equals the ratio of the pore radii, which a genuinely distinct mechanism would not reproduce.
Parameter-count ceiling from a single test
H5908. A creep model of oxide fuel with grain size, porosity, pore shape and stoichiometry among its parameters cannot be identified from one constant-stress curve. Several combinations of those variables produce the same strain history, and only a stress drop or a temperature drop breaks the degeneracy. A fit that reports all four as independently determined has fitted the noise in the curve, which can be shown by fitting two halves of that curve separately.
Ratio of a range to a zone width
H5909. The restructured rim zone's thickness is set by the fission fragment's range, of the order of ten micrometres as an estimate, rather than by burnup, so the zone's width is nearly fixed while its porosity grows with burnup. A burnup series with width and porosity measured separately would leave the width constant while the porosity rises, which a burnup-driven growth model cannot produce.
Reversal of a trend where two carriers compete
H5910. Gadolinium substitution lowers the fuel's thermal conductivity at low temperature by scattering phonons. At high temperature the phonon mean free path is already limited by intrinsic scattering while the electronic contribution grows, so the dopant's relative penalty falls and may reverse. Two dopant levels measured from room temperature to the operating range should show the curves crossing if two carriers compete, and staying parallel if the dopant only scatters.
Quantity that belongs to its method
H5911. The melting temperature of irradiated fuel is inferred from a thermal arrest in a small specimen, and the arrest depends on the heating rate because the liquid's oxygen content equilibrates with the crucible during the ramp. Two heating rates in two crucible materials should give two arrests, so the reported melting point is a property of the schedule as much as of the fuel.
Correlation that is a shared cause
H5912. Grain size correlates with fission-gas release across a set of fuels, and the correlation is usually read as a mechanism. Both are set by the sintering schedule, which also fixes the closed porosity that actually retains the gas. Two fuels of the same grain size and different closed porosity should release differently, and two of different grain size and the same closed porosity should release alike, which reverses the causal reading.
Ceiling at a buffer's exhaustion
H5913. The fuel's oxygen potential stays low while the dioxide can absorb the excess oxygen that fission produces, and rises sharply once that buffer is exhausted, so the burnup at which volatile species begin to migrate is set by the buffer's capacity rather than by the inventory of fission products. A capsule with an oxygen getter and a solid-electrolyte probe would place the rise at the buffer's exhaustion, which scales with the initial stoichiometry deviation.
Path dependence that annealing erases
H5914. Two mixed-oxide pellets of identical plutonium fraction but different cooling through the cation-ordering range have different thermal conductivities, and the difference is not hysteresis because a re-anneal erases it. Conductivity is a function of the short-range order frozen in by the cooling path, so a specification written on composition alone does not determine the property.
Same signature from two kinetics
H5915. Gap closure appears as a change in the cladding's diameter rate, and it is produced either by fuel swelling, which is monotonic in burnup, or by cladding creep-down, which is stress-driven and transient, so the two give the same diametral trace. A power decrease after closure reverses the creep-down component and not the swelling one, so a single power cycle separates them.
Impossibility from a length that is fixed
H5916. Ridging of the cladding has been modelled as an instability with a preferred axial wavelength, but the ridges can only form where the pellet interfaces are. Their pitch therefore equals the pellet length and not a wavelength of the cladding. A test with two pellet lengths in identical cladding should move the ridge pitch with the pellet length, which an instability model would not allow.
Trade-off whose optimum moves with the duty
H5917. Chromia doping enlarges the grains and lowers gas release, but the same dopant lowers the conductivity and raises the centreline temperature, so no single grain size is best for both a base-load and a load-following history. The two criteria cross at a grain size that shifts with the power history, which makes a single recommended microstructure a compromise that is optimal for neither.
Hidden reservoir that discharges
H5918. Grain boundaries absorb fission gas early in life and release it later, so measured release is the difference between production and the boundaries' net filling, which makes early release lower than production and late release higher. A power ramp after a long low-power hold should produce a release transient above the instantaneous production rate, and the excess measures the reservoir that was filled.
Crossover between two swelling regimes
H5919. Fuel swelling passes from a regime governed by the closing of fabrication porosity to one governed by the generation of new porosity. The crossover is marked not by a change of slope but by a change in the swelling's temperature dependence, from negative to positive. A series run at two temperatures should cross at one burnup, and the crossing is the point at which the fabrication pores are gone.
Conservation bound on a getter's capacity
H5920. A second phase dispersed in the pellet is proposed to immobilise fission gas, but its capacity is bounded by the number of sites in its own lattice. That number is small compared with the gas produced at high burnup. An implantation experiment on the pure second phase should saturate, and the saturation inventory bounds what the additive can do regardless of how finely it is dispersed.
Cladding and structural materials
Threshold that is a ratio, not a constant
H5921. The threshold stress intensity for delayed hydride cracking is treated as a material constant, but it is the ratio of the hydride's fracture toughness to the stress concentration the hydride produces. It therefore moves with the hydrogen gradient ahead of the crack. Two specimens of one alloy with different hydrogen profiles should give two thresholds, and the difference should follow the gradient rather than the alloy.
Bound from solid solubility
H5922. The hydrogen available to build a hydride lens is bounded by the solid solubility at the local temperature, so a lens larger than that bound can only have grown by transporting hydrogen from elsewhere. Measuring the lens volume together with the surrounding concentration profile distinguishes growth by local precipitation from growth by diffusion, and the first cannot exceed the solubility bound.
Ranking inverted by the oxide's electronic character
H5923. Alloys ranked by autoclave corrosion resistance can invert under irradiation, because the ranking is set by the oxide's electronic conductivity rather than by its thickness. A thin oxide that conducts electrons passes oxidant to the interface, and a thick oxide that does not, blocks it. A photoelectrochemical measurement of each oxide's carrier type should predict the in-reactor ranking better than the autoclave thickness does.
Mass gain that is not all oxide
H5924. The weight gain of an oxidising zirconium coupon includes water taken up by the outer oxide's hydration, so the post-transition acceleration inferred from weight gain is partly a mass artefact. Comparing weight gain with a metallographic thickness and with the specimen's thermal desorption spectrum separates the hydrated fraction from the oxide actually formed.
Indistinguishable curves from spallation and breakaway
H5925. Breakaway oxidation and the spallation of an outer layer followed by regrowth produce the same sigmoidal weight-gain curve, so the curve's shape cannot identify the mechanism. Spallation releases the oxide's growth strain and breakaway does not. A length-change measurement on the same specimen therefore separates them where the mass measurement cannot.
Threshold in oxygen activity for nodule nucleation
H5926. Nodular corrosion requires a local oxygen activity above a threshold to nucleate, and below it the same alloy oxidises uniformly, so whether nodules appear is a statement about the local chemistry rather than about the alloy's average composition. A steam-hydrogen mixture at one temperature and a varied ratio should switch the nodule density on at one activity, and the switch should be sharp rather than graded.
Identity of two named galvanic effects
H5927. Accelerated corrosion beneath a deposit and the enhanced attack next to a dissimilar metal are the same galvanic mechanism with the current source named differently. Both need a local potential difference and a supply of oxidant. Reversing the couple's polarity in a controlled specimen should move the attack from one side to the other in both cases, which a deposit-specific mechanism would not do.
Parameter-count ceiling from texture symmetry
H5928. A hydride reorientation model with three orientation components cannot be identified from pole figures of the cladding's fibre texture, because at most two components are independent and the third absorbs the residual of the fit. Pole figures at two strains ought to show the third component changing while the others hold, which identifies it as a fitting freedom rather than a physical population.
Scaling with the square root of a rate
H5929. The depth of the hydride rim formed during cooldown is set by the ratio of the cooling time to the hydrogen diffusion time across the wall. That ratio is a single dimensionless group, so the depth should scale with the square root of the cooling rate and not with the final temperature. Three cooling rates ending at one temperature should give rim depths in that ratio, while a dependence on the final temperature would refute diffusion control.
Reversal of the sign of a length change
H5930. A tube oxidising at temperature changes length in two opposing ways, since the oxide's growth strain pushes the metal one way and the composite's thermal contraction pulls it the other, and the two cross at an oxide thickness of order a micrometre as an estimate. Continuous length measurement during isothermal oxidation should therefore show a turning point, and its position should move with temperature as the growth strain's relaxation does.
Quantity determined by the rig
H5931. The corrosion rate of a zirconium alloy in steam depends on the autoclave's refresh rate because the dissolved hydrogen's steady state is set by the balance between generation and purge, so the same coupon gives different rates in different rigs. Two refresh rates, with hydrogen measured at the outlet, should give two rates that collapse onto one curve when plotted against dissolved hydrogen.
Confounded by a shared heat treatment
H5932. The correlation between tin content and corrosion resistance in zirconium alloys is confounded by the second-phase particle size, which is set by the same heat treatment that fixes how much tin remains in solution. Two alloys of equal tin and different particle size should differ more than two of different tin and equal particle size, which inverts the usual attribution.
Non-additivity from a shared obstacle
H5933. Irradiation creep and thermal creep are added in correlations, but at low stress both are controlled by the same dislocation obstacles. The combined rate is therefore closer to the larger of the two than to their sum. A specimen crept under irradiation at a stress where the two rates are comparable ought to creep at a rate well below the sum, and the deficit should shrink as stress rises.
Anisotropy written by the fabrication route
H5934. The cladding's irradiation growth is set by the basal pole distribution, and that distribution can be reversed locally by a single roll pass, so the growth strain varies along the tube in a pattern that follows the fabrication route rather than the alloy. A texture map along the tube, compared with sectioned elongation, should show the strain following the texture, which an alloy-based model cannot explain.
Interface control with a fixed barrier
H5935. If oxide growth is limited by transport across a thin sub-stoichiometric layer at the metal interface rather than by diffusion through the whole oxide, the rate becomes independent of oxide thickness once that layer is established and thickening no longer slows it. Two specimens of very different oxide thickness, oxidised side by side, should then gain mass at the same rate, which bulk diffusion forbids.
Hidden state in the oxide's stress
H5936. Two coupons of the same oxide thickness can oxidise at different subsequent rates, and the difference is the oxide's residual stress, which relaxes by creep on a time scale comparable with the transition time. Curvature measurements at intervals during oxidation should show the stress falling before the rate changes, which puts the stress before the rate in causal order.
Load-history interaction that voids a sum
H5937. The fatigue life of cladding under strain-controlled cycling depends on the order of the blocks, because hydrides reorient during the tensile half-cycle and not the compressive one. A life-fraction sum over blocks of equal amplitude is therefore invalid. Two block orders with identical amplitudes should give two lives, and the difference should track the hydride reorientation measured between blocks.
Competition between a thermal and a stress drive
H5938. Hydrogen migrates down the temperature gradient and up the stress gradient, and the two oppose each other near a spacer contact, so the hydride lens forms where the sum vanishes rather than where either term is largest. A specimen with a controlled gradient and a known contact stress should place the lens at a computable offset from the cold point, and moving the stress should move the lens.
Trade-off whose optimum moves with cycling
H5939. A chromium coating resists oxidation in proportion to its thickness, but the same thickness stores more elastic energy in the coating's thermal expansion mismatch, so the coating that survives a steady exposure is not the one that survives cycling. Two thicknesses tested isothermally and cyclically should rank oppositely, which makes a single recommended thickness meaningless without the cycle shape.
Lower bound from a connected pore network
H5940. A silicon-carbide cladding cannot be made hermetic by perfecting the end plug alone, because the matrix retains a connected pore network whose permeability sets a floor on the leak rate. The floor falls steeply with density, so any hermeticity claim must name a density and a thickness at which the network closes, and a helium leak test on tubes of graded density would locate the closure.
Threshold at a percolation point
H5941. The first through-wall leak path in a ceramic cladding appears at a percolation threshold of its pore network rather than at a critical stress. The leak therefore precedes the crack, and the sequence is the reverse of the metallic case. Tubes sintered to different densities should begin to leak at one porosity and not at one pressure, which a fracture-controlled model would not predict.
Quantity fixed by the section plane
H5942. The thickness of the fuel-clad reaction layer is defined by a metallographic section, and the same bond measures thick or thin depending on the plane's obliquity and the etch, so comparisons between laboratories compare preparations. One bonded specimen sectioned at three angles should give three thicknesses that converge when corrected for obliquity, which tests the correction rather than the bond.
Coolant chemistry
Misattributed discrepancy in a calculated value
H5943. The difference between the calculated and the measured high-temperature pH of the primary coolant is usually charged to the measurement. It is the model's treatment of the borate's second dissociation that fails first, and the failure grows with temperature. Measuring pH at two boron concentrations at one temperature would place the discrepancy with the borate term rather than with the electrode's drift.
Ceiling from second-order recombination
H5944. The net oxidant inventory that can build up in the coolant is bounded by the bulk recombination of radicals, which is second order in their concentrations, so the steady state rises only as the square root of the energy deposition rate and no additive can lift the oxidant concentration above the ceiling that rate sets. Two deposition rates in one loop should give oxidant levels in the ratio of their square roots, and an additive should change the ceiling's coefficient rather than the exponent.
Ranking inverted by residence time
H5945. Additives are ranked by how well they suppress radiolytic oxidation in static capsules, and the ranking can invert in a flowing loop. A fast-reacting additive is consumed near the inlet and never reaches the hot leg. A loop with two sampling points should show the fast additive's protection decaying downstream while the slower one's persists, which reverses the capsule ranking.
Drift assigned to the wrong reservoir
H5946. The slow fall in lithium concentration that is attributed to burnup contains a term from lithium leached out of the pH sensor's reference compartment, so the depletion is partly an inventory transfer within the loop. A sensor with a solid-contact reference and a conventional one, run side by side, should give two depletion rates, and the difference is the leaching term.
Threshold at the deposit's solubility
H5947. Deposition of corrosion product on a fuel rod begins only when the local concentration exceeds the solubility of the phase that would form at the local subcooled temperature, so below that threshold no deposit forms however long the exposure. A loop with a controlled iron concentration and a fixed thermal-hydraulic profile should switch deposition on at one concentration, and the switch should move with the cold-leg temperature.
Artefact that follows a flow transient
H5948. The release of deposited corrosion product after a shutdown is read as chemical dissolution, but a large part is the resuspension of settled particles by the decaying flow, and the two are distinguished by the size distribution of what is released. Filters of graded pore size downstream of the core should retain a coarse mode during resuspension and none during dissolution.
Identity of two deposit effects
H5949. The axial offset anomaly and the power shift under a thick deposit are one mechanism at two length scales. The deposit's pores wick liquid to the hot surface and concentrate boron there, and the axial offset is that same concentration distributed over the core height. A loop with a heated section, a boron tracer and a deposit of controlled porosity should show the concentration factor tracking the pore size rather than the deposit's total mass.
Parameter-count ceiling from steady states only
H5950. A radiolysis model with several dozen reactions cannot be constrained by the steady-state concentrations of hydrogen, oxygen and peroxide, because many rate sets give the same steady state and only a transient can separate them. A step change in the energy deposition rate should produce a relaxation whose shape discriminates the sets, and a model fitted only to steady states will predict every set's transient equally well.
Sublayer against diffusion length
H5951. Whether an oxidant produced in the bulk reaches the wall is set by the ratio of the laminar sublayer's thickness to the species' reaction-diffusion length. When the sublayer is the thicker of the two, the wall's oxidation becomes independent of bulk chemistry. A channel at varied flow and fixed chemistry should show the wall rate changing only while that ratio is near one, and flattening on either side of it.
Reversal between a subcooled and a boiling surface
H5952. Zinc addition suppresses deposition on a subcooled surface by incorporating into the spinel and blocking further growth, while the same addition can accelerate deposition on a boiling surface, where the porous deposit's wick concentrates zinc and alters the surface charge. Two test sections, one subcooled and one boiling, in a single loop would show both signs, which a single-surface test cannot.
Mixed potential read as an equilibrium
H5953. The potential of a platinum electrode in the primary coolant is read as the redox potential of the water. It is a mixed potential set by several couples and by the electrode's own surface, so it responds to flow and to the electrode's history. Two electrode metals of different exchange current densities should give different readings if the potential is mixed, and one reading if it is an equilibrium.
Bound from an ion exchanger's capacity
H5954. The claim that a single pass through the purification system removes the activated corrosion product is bounded by the resin's capacity per unit volume and by the inventory's total mass, and the bound is of the order of tens of passes as an estimate. A bed of known capacity and a measured inlet concentration give that number without any assumption about the chemistry, and the number is large.
Acceleration that is a surface change
H5955. The accelerating recombination of hydrogen and oxygen on a metal surface is described as autocatalytic, but the rate rises because the surface's oxide is progressively reduced and the exchange current density rises with it. The product concentration is therefore not the cause. Two runs from the same initial chemistry, one with a pre-reduced surface, should give two induction periods that differ while the product's build-up is identical.
Lag from a buffered inventory
H5956. The boron concentration measured downstream lags a makeup because the ion-exchange resin buffers borate, and the lag time is set by the resin's distribution coefficient and the flow rather than by the loop's residence time. A step change in makeup across two bed volumes should give two lag times in proportion to the resin's mass, which a well-mixed loop model would not.
Two sources behind one measured species
H5957. The oxygen measured in a sodium system's cover gas is the sum of the dissolved oxygen and that released from the cold trap's deposits when the trap's temperature changes, so one reading does not identify one source. Stepping the cold-trap temperature at fixed sodium flow would give two plateaus whose difference measures the deposit's contribution.
Front position from a solubility product
H5958. Deposition on a heated surface occurs where the local solubility product crosses saturation, and that crossing moves axially when the dissolved hydrogen's partial pressure changes, without any change in the thermal hydraulics. A heated section with a hydrogen step, followed by a deposit profile, should show the front moving, which a flux-based deposition model would not predict.
Partition that is a flow property
H5959. The distribution of iodine between liquid and steam in a boiling channel is treated as an equilibrium partition coefficient. The volatile species forms at the interface at a rate competing with the liquid's mixing, so the measured carry-over is a property of the flow. Two channel lengths at one void fraction should give two carry-over fractions if interfacial formation is rate-limiting.
Circular definition in a calibration
H5960. The high-temperature pH reported by a glass electrode is defined by the buffers used to calibrate it, whose liquid junction potentials are assumed rather than measured, so an apparent pH excursion can be manufactured by changing the reference's concentration. Two references of different concentration, calibrated against the same buffers, would give two pH values differing by the junction potential alone.
Symmetry that forbids a redistribution
H5961. In an isothermal, well-mixed loop with no boiling, an additive that takes part in no surface reaction cannot change where deposition occurs, so any observed redistribution must come from the loop's own surfaces acting as a source. A loop with every surface passivated and a loop with one fresh iron source should differ only in the second, which localises the effect to the source rather than to the additive.
Time-scale separation that fails in a pore
H5962. The radiolysis steady state is reached far faster than a deposit grows, so the two are modelled separately, but the separation fails inside the deposit's pores, where the liquid's residence time exceeds the radical lifetime and the pore chemistry decouples from the bulk. A deposit of controlled thickness on a heated surface should show a wall rate that stops responding to bulk chemistry beyond a thickness of the order of the diffusion length, as an estimate.
Heat exchangers and steam generators
Floor rather than a continued decline
H5963. The heat-transfer coefficient of a fouled tube does not fall without limit. Once the deposit is thicker than its own capillary rise the pores fill, conduction through the liquid sets a floor, and boiling in those pores offsets part of the thickness. Run past the capillary limit, a fouling test ought to show the coefficient flattening, and the plateau should move with the deposit's pore size.
Asymptote that resumes
H5964. The asymptotic fouling resistance is a balance between deposition and removal, and the removal term weakens as the deposit ages and consolidates, so the asymptote is apparent and the resistance resumes rising after a delay. A test long enough to see the second rise, with the deposit's strength measured at intervals, would separate the balance from a true saturation.
Misattributed excitation frequency
H5965. U-bend tube vibration at low flow is attributed to vortex shedding, but the dominant excitation there is the pump's pressure pulsation transmitted through the inlet. The two separate because the vibration frequency follows the pump's speed and does not scale with cross-flow velocity. A test at two pump speeds and one flow should move the frequency with the pump and not with the flow.
Bound from the tube's own sag
H5966. The span between tube supports is bounded by the tube's sag under its own weight and the two-phase drag, and beyond that bound the tube contacts the support and the wear rate rises, so widening the span to reduce pressure drop trades directly against wear. A rig with three spans and a wear measurement at the support would show the wear rising sharply beyond one span, and the bound should scale with the tube's bending stiffness.
Ranking inverted between initiation and propagation
H5967. The ranking of alloys by resistance to stress-corrosion cracking in a slow-strain test can invert in service, because the test measures initiation in a smooth specimen while the tube fails by propagation through a cold-worked layer. A fracture-mechanics test on the same alloys in the same environment should give the opposite ranking, and the two together identify which stage the tubes have reached.
Hidden conductance in a crevice
H5968. The overall heat-transfer coefficient inferred for a steam generator tube contains a term from the tube-to-tubesheet crevice's steam gap, so part of the fouling deduced from a decline is a change in that gap's conductance as the crevice dries. Two tubes of different crevice geometry, in one bundle, should show different apparent fouling with the same deposit.
Threshold of flux for consolidation
H5969. A deposit on a boiling surface consolidates and loses porosity only above a local heat flux. Below it the deposit stays porous and can be removed by the flow, so one chemistry produces either a sludge or a scale depending on local flux. A test section with a flux gradient should show the deposit's porosity changing at one position, and the position should move with total power.
Change of flow distribution read as fouling
H5970. An apparent rise in a tube's thermal resistance can be produced by a baffle leak that reduces the shell-side velocity over part of the bundle, which is indistinguishable from fouling in the overall coefficient. Wall-temperature measurements at several axial positions on the same tube separate them, because a flow change alters the shell-side film while a deposit alters the wall's own resistance.
Identity of two named dissolution mechanisms
H5971. Flow-accelerated corrosion and erosion-corrosion in the condensate line are one dissolution mechanism with the mass-transfer term named twice. The test is the rate's dependence on velocity: dissolution gives a square-root dependence through the mass-transfer coefficient, while mechanical removal gives a much steeper one. A loop at four velocities, with specimen weight loss, should show the exponent, and the exponent names the mechanism.
Parameter-count ceiling from one curve
H5972. A fouling model with a deposition constant, a removal constant and a strength exponent cannot be identified from one fouling curve, because only a product and a ratio of the three appear in the fit. Two flow velocities, giving two curves, are the minimum for separating them, and a three-parameter fit to one curve is a re-parameterisation of a two-parameter fit.
Ratio of capillary to hydrostatic length
H5973. A tubesheet crevice dries only to a depth set by the ratio of the capillary pressure to the hydrostatic head. Beyond that depth the crevice stays wet and concentrates its solutes, and its chemistry is fixed by a length ratio rather than by the bulk. A crevice of variable depth in a boiling test should show concentration beginning at one depth, and the depth should move with pressure.
Crossover in a surface treatment's effect
H5974. A porous coating on a boiling surface raises the heat-transfer coefficient at low flux by supplying liquid to the nucleation sites and lowers it at high flux because the same pores cannot vent the vapour, so the coating's benefit crosses zero at one flux. A test at four fluxes should show the crossover, and the crossover should move with the coating's pore size.
Lag longer than the loop's residence time
H5975. The thermal response of a steam generator to a chemistry change is delayed by diffusion through the existing deposit, which takes longer than the loop's residence time, so a control action verified by the outlet temperature is verified too early. A step change with the outlet temperature logged for several deposit-diffusion times should show the response still evolving after the loop has turned over many times.
Baseline that is itself a calculation
H5976. The fouling factor is measured against a clean baseline computed from the design code, so a deposit present from the first day never appears. The factor measures departure from a model rather than from a clean tube. An as-built test with each tube's clean coefficient measured in situ, before any deposit, would set a real baseline, and the reported factors would shift.
Two populations read as one rate
H5977. Wall-thickness measurements on a steam generator tube are bimodal because fretting at the support and erosion by the two-phase flow remove metal at different rates, and a single wear rate fitted to the mixture is an average of the two. Separating the measurements by axial position and by the support's location should recover two distributions, and the mixture's mean should lie between them without belonging to either.
Bound from fin efficiency
H5978. An air-cooled condenser cannot be improved indefinitely by adding fin area, because fin efficiency falls as the fins grow longer and thinner, and the product of area and efficiency reaches a maximum at a spacing set by the air-side coefficient. A test at three fin spacings and one air velocity should show the product peaking, and the peak should move with the velocity.
Welds and joining
Cracking that follows the pass sequence
H5979. Reheat cracking in a thick ferritic weld is attributed to the global residual stress, but the cracks follow the coarse-grained heat-affected zone of the last passes. The driving stress is therefore the transformation-induced stress of those passes, and the pattern follows the pass sequence. Two welds of identical restraint and different pass order should crack in different places, which the global field cannot explain.
Bound set by dilution
H5980. The composition at a dissimilar metal weld's fusion boundary is bounded by the dilution ratio, so whether a hard martensitic layer can form is decided by the welding procedure rather than by a diffusion process. Three welds with dilution ratios spanning the bound should show the layer appearing at one ratio, and a heat treatment that does not change the dilution should not remove it.
Ranking inverted by heat input
H5981. Resistance to solidification cracking ranks alloys one way at low heat input and can rank them the other way at high heat input. The order is set by the volume fraction of eutectic and its ability to backfill a crack, and a high-heat-input weld produces a wider mushy zone needing more backfill. Two heats of one alloy welded at two heat inputs should rank oppositely, and the crack's surface composition should show whether eutectic or base metal supplied the liquid.
Hidden term in a magnetic phase measurement
H5982. The ferrite number of a duplex weld measured magnetically includes a contribution from deformation-induced martensite in the austenite, so the reading measures the specimen's strain history as well as its phase balance. A measurement before and after stress relief, on one specimen, should change if martensite is present and should not change if the reading is truly ferrite.
Threshold in cooling rate
H5983. Sensitisation of a weld's heat-affected zone requires the cooling rate through the carbide precipitation window to be below a threshold, above which the carbon stays in solution and the same composition resists. Three coupons cooled at three rates, then tested for intergranular attack, should show attack appearing below one rate, and the threshold should move with the alloy's carbon content.
Indication that is a boundary, not a crack
H5984. A delayed ultrasonic indication found after welding may be a crack or a specular reflection from a columnar grain boundary in the weld metal, and the two separate by the indication's response to a change in beam angle: a boundary reflects near one angle only, a crack over a range. A metallographic section at the indication should confirm which, and the angle dependence should have predicted the section's result.
Identity behind two liquid sources
H5985. Hot cracking and liquation cracking are one liquid-film rupture mechanism with the liquid's origin named differently, the eutectic during solidification and the constitutional liquation of second phases in the heat-affected zone. The film's composition names the source. A section through the crack with a composition profile should match one or the other, and one alloy can show both in a single weld.
Parameter-count ceiling from one measurement line
H5986. A weld residual-stress model with a dozen process and material parameters cannot be validated by a single line of diffraction measurements, because the measured components are fewer than the parameters and the model's output is only a projection onto them. A map on two planes is the minimum, and a model calibrated on one line will fit that line and predict the other badly.
Width that is a ratio of times
H5987. The unmixed zone at a dissimilar weld's fusion boundary is as wide as the ratio of the pool's stirring time to the diffusion time allows. That ratio is small, so the zone is a few grain diameters wide, as an estimate, and cannot be widened by preheating. A weld made with two stirring intensities should show the same width if diffusion sets it and different widths if stirring does.
Reversal of a surface stress
H5988. The surface residual stress of a weld can reverse sign after a stress-relief treatment, because the treatment relaxes the through-thickness gradient faster than it relaxes the surface layer's own stress, so a surface that was compressive becomes tensile. Diffraction measurements before and after the treatment, at two depths, should show the reversal and locate the depth at which it happens.
Lag from hydrogen redistribution
H5989. The toughness of a ferritic weld's heat-affected zone continues to fall for hours after welding while hydrogen redistributes from the weld metal into that zone, so a test made soon after welding samples a different distribution and cannot be extrapolated. Two specimens cut from one weld, one tested immediately and one after a hold, should differ, and the difference should vanish if the specimen is outgassed.
Symmetry that forbids a net bend
H5990. A circumferentially symmetric weld with symmetric heat input cannot produce a net axial bend, so any observed bow must come from an asymmetry in fit-up or restraint rather than from the weld. A weld made in a fixture with a known offset should bow in proportion to the offset, and a weld made in a symmetric fixture should not bow at all.
Inspection hardware
Signal loss split by its frequency dependence
H5991. The loss of ultrasonic signal through an austenitic weld is assigned to scattering, but a large part is the beam's redirection by the columnar grains' elastic anisotropy. The two separate because scattering grows with frequency while redirection does not. One probe at two frequencies, on the same path, gives two losses whose ratio names the mechanism.
Resolution bound set by the pulse, not the wavelength
H5992. The axial resolution of an ultrasonic inspection is bounded by the probe's pulse length in the material rather than by the wavelength, so raising frequency improves resolution only if bandwidth rises with it. Two probes of different centre frequency and equal bandwidth should resolve equally, and the one with the shorter pulse should resolve better regardless of frequency.
Hidden thermal term in a lift-off reading
H5993. An eddy-current probe's lift-off signal contains a term from the probe's own thermal expansion during a hot inspection. That term mimics a conductivity change because it moves the coil. A differential pair with common-mode rejection, or a measurement at two dwell times, separates the expansion from the material, since the expansion follows the probe's temperature and the material does not.
Threshold in film thickness against roughness
H5994. Below a couplant film thickness comparable with the surface roughness, ultrasonic transmission is set by asperity contacts rather than by the film's acoustic impedance, so the coupling loss becomes a function of surface finish and not of the couplant. Two surfaces of different roughness, with the same couplant and film thickness, should give two transmissions that converge as the film thickens.
Indication that does not move with the probe
H5995. A crack-like indication at a nozzle's inner corner can be a geometric reflection from the blend radius, and the two separate by moving the probe along the expected crack's length: a geometric reflector's response stays fixed in the probe's frame while a crack's response moves with it. A scan in two directions should show the difference, and a section at the corner confirms it.
Same amplitude loss, two causes
H5996. Probe wear and probe fouling both reduce the echo amplitude, but wear changes the probe's frequency spectrum while fouling changes only the amplitude, so the spectrum names the cause. A probe run until its amplitude falls, then cleaned and re-measured, should recover its amplitude if fouled and not if worn.
Parameter-count ceiling in flaw sizing
H5997. At most two independent flaw parameters can be recovered from a single pitch-catch scan, because the scan measures a projection of the flaw's geometry. A sizing claim with length, height, tilt and opening is therefore a fit to the noise in that projection. Two scans at different angles are the minimum for three parameters, and a four-parameter fit to one scan reproduces the scan and predicts nothing.
Visibility set by the ratio of opening to wavelength
H5998. A tight crack's visibility to an ultrasonic probe is set by the ratio of its opening to the wavelength and by its height, so a crack can be invisible at one frequency and obvious at another because the ratio crosses unity. A specimen with a known tight crack, scanned at three frequencies, would produce the indication at one of them, and a null result is then a statement about the probe rather than about the crack.
Beam angle that walks with temperature
H5999. The refracted angle of an ultrasonic beam in a wedge changes with temperature because the wedge's velocity rises while the component's falls. A probe that heats against a hot surface therefore steers its beam away from a small target during the scan. An echo from a side-drilled hole measured at two dwell times should fall as the probe warms, and cooling the wedge should restore it.
Bound from a product the material fixes
H6000. A probe cannot be specified to maximise penetration and resolution at once, because both are bounded by the product of bandwidth and attenuation that the material sets. A specification claiming both is claiming a product the metal forbids. Two probes of different bandwidth on one thick section should show the trade, and the better one is matched to the section's attenuation rather than to its resolution alone.
Status and plan
100 of 1,000. These were written one at a time and given forms chosen to differ in argument rather than only in subject: a mechanism credited to the wrong process, a ceiling set by a shared bottleneck, a ranking that inverts under a second criterion, an artefact that survives a change of specimen, a count of parameters that a single measurement cannot determine. The forms are labelled so the variety can be inspected instead of assumed.
None has been tested. The metal, the water and the instrument each present the same difficulty, that the measurement alters the thing measured at the length scale where the mechanism lives; the discriminating test is named in each entry so that a version too crude to separate the rivals can be recognised before it is run.
If one of these is worth an experiment, it is the entry on the hydride rim scaling with the square root of the cooling rate: three cooling rates, one final temperature, and a metallographic section. It needs a furnace with a controlled ramp and a saw, and it would settle whether the rim is a diffusion structure or a precipitation one.
Incoprea Research · hand-considered hypotheses 5901–6000 · 3 November 2026 · ~/Desktop/Nov3/