RadCluster v2.1.2: Frenkel-pair conservation correction

RadCluster v2.1.2: Frenkel-pair conservation correction

The problem

The regression case im5vm2 had a Frenkel-pair residual δ_FP = 1.24×10⁻² at t = 10⁻² s. That is above the 10⁻³ level that signals a coding error.

Cause

In all three C++ right-hand sides (rhs_case2, rhs_case1, rhs_bin_moment in radcluster_code/cpp_utils/materials/eurofer97/rate_kernels.cpp), SIA thermal emission I₍ₙ₊₁₎ → Iₙ + I₁ moved the emitting cluster down one size. It never added the emitted monomer to I₁; vacancy emission does this, through emit_mono. One SIA was lost per emission event, and this is the whole residual:

• the vacancy arm of δ_FP closed to 10⁻¹²;
• the time integral of the emitted-monomer flux equals the SIA arm's deficit at every output time.

Fix

The monomer is now returned to I₁. On im5vm2 at 10⁻² s:

	before (v2.1.1)	v2.1.2
δ_FP	1.24×10⁻²	2.9×10⁻⁷
δ_He	1.0×10⁻¹⁵	1.9×10⁻¹⁴
C_SIA_tot (m⁻³)	2.0648×10¹⁸	2.0754×10¹⁸ (+0.5 %)
C_VAC_tot, C_He_tot, mean sizes, swelling		unchanged to 2×10⁻⁶

Reproducing earlier results. Set sim.input_data.reactions["sia_emission_monomer"] = "dropped" (the default is "returned"). This restores the arithmetic before v2.1.2 bit for bit: im5vm2 then gives δ_FP = 0.012442841061051264 exactly. In the solver parameter file the key is sia_emission_monomer; it is 0 when absent.

A second residual remains in long loop-coarsening runs. There, the loop distribution can outgrow the size axis, and content leaves through the upper boundary. Check δ_FP against the axis length.

The correction was found in the migration of RadCluster into GSD (v2.0.0, Stage 5). GSD's declared equations cannot lose content, and they match this C++ kernel at round-off. See README §2.9.