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Use a common comparison for all of question2: retain the same number and height of fuel rods and approximately the same normalized rod-power shape; reduce fuel lattice pitch by making the active core cross-section smaller. Compare otherwise similar operating states of an initially under-moderated reactor, typical of a pressurized water reactor. Rod number, core height, reflector and control settings are not all fixed explicitly by the question, so these are stated assumptions. Keeping the external core envelope fixed while inserting more rods would be a different comparison.

Reactor reactivity normally decreases at unchanged controls and reference temperatures. Less water per fuel atom reduces neutron moderation and increases resonance absorption during slowing down. Reduced absorption in the water partly compensates, but on the under-moderated side the net lattice tendency is lower multiplication. This is the usual under-moderation argument, rather than a universal rule for arbitrary fuel lattices.^[ref]

Neutron leakage would usually tend to increase. The smaller transverse core extent increases the geometric tendency to leak, and reduced moderation generally increases the slowing-down distance. There are competing changes in fuel absorption, transport lengths and reflection. Thus a definite leakage fraction needs the actual core and reflector arrangement; the usual increase is a qualitative tendency, not a geometric theorem for every redesign.

The Doppler coefficient of reactivity remains negative under the ordinary uranium-fuel assumptions. Increasing fuel temperature broadens absorption resonances, increasing effective uranium-238 resonance capture and reducing reactor reactivity. This stabilizing fuel-temperature effect is distinct from moderator-density feedback.^[ref] Greater importance of resonance absorption can make this feedback more important in a tighter lattice, but its change in magnitude is not fixed by pitch alone: spectrum, resonance self-shielding, fuel composition, burnup and the reference fuel temperature all enter the derivative.

The fraction of fissions in uranium-238 tends to increase. The harder neutron spectrum gives more opportunity for fast fission before neutrons slow below the uranium-238 fission threshold. This concerns the fraction of the total nuclear fission rate, not an assertion that the total thermal power increases; that power is held fixed.

A tighter core is likely to have a more demanding cooling response, but the specification does not determine a unique peak cladding temperature or reflood time in a loss-of-coolant accident. Several effects compete.

At the same pre-accident power and comparable irradiation history, the total decay heat source remains of broadly comparable scale; increasing core power density does not multiply the total decay power by the inverse change in core volume. The core nevertheless has less water within its tighter rod lattice. Its local thermal buffer is smaller, and the same heat must be removed through narrower passages. If the normal-operation fuel temperature is higher, more stored fuel thermal energy must also be removed after shutdown.

Reduced passage size and increased hydraulic resistance can impede emergency-coolant penetration and steam discharge during reflood. Local two-phase flow, entrainment and boiling heat transfer can change, so single-phase pump-curve reasoning is insufficient during blowdown and reflood. Less local water can favor earlier uncovering, but blowdown flow and the much larger inventory elsewhere in the unchanged primary circuit also matter. It would be unjustified to infer a proportionate reduction in the entire primary-circuit inventory.

Loss of water also removes neutron moderation. In the assumed under-moderated reactor, this normally contributes negative reactor reactivity, helping suppress fission power. It does not eliminate decay heat after shutdown. The thermal challenge must therefore be assessed separately from the sign of this feedback.

A quantitative comparison needs break characteristics, initial states, emergency injection performance, bypass paths and a transient two-phase calculation. The defensible conclusion is potentially reduced thermal margin and more difficult reflood, requiring reassessment of the emergency core cooling system, rather than an unconditional prediction that every large-break transient becomes worse in every metric.