  We propose a two-dimensional (2d) quantum breakdown model of hardcore bosons
interacting with disordered spins, which resembles particles incident into
supersaturated vapor. The model exhibits a strong fragmentation into Krylov
subspaces in each symmetry sector. The Hamiltonian in each Krylov subspace maps
to a single-particle problem in a Cayley tree-like graph, which enters a 2d
many-body localization (MBL) phase beyond certain disorder strength $W_*$, as
indicated by Poisson level spacing statistics and entanglement entropy growing
as $\log t$ with time $t$. Our theoretical arguments suggest $W_*$ is finite or
zero for boson number $N_b\lesssim L^\gamma/\log L$ ($1/2\le \gamma \le 1$) as
system size $L\rightarrow\infty$. At zero disorder, the model also exhibits
fully or partially solvable features, such as degenerate quantum scar states.
