Passive chemical injection system for controlling a nuclear reaction by delivering a neutron-absorbing chemical into a containment vessel

A method for controlling a nuclear fission reaction, comprising:

• operating a nuclear power system to generate a nuclear fission reaction, wherein the nuclear power system comprises:
• a reactor vessel defining a volume;
• a reactor core positioned within the volume and including one or more nuclear fuel assemblies configured to generate the nuclear fission reaction;
• a containment vessel having an inner surface sized to enclose the reactor vessel such that an open volume is defined between the containment vessel and the reactor vessel;
• and
• a passive chemical injection system positioned in the open volume and comprising a container sized to hold an amount of a neutron-absorbing chemical in solid form sufficient to at least maintain the nuclear fission reaction at a sub-critical state,
• wherein the container is configured to release the amount of the neutron-absorbing chemical in solid form directly into the open volume;
• releasing the amount of the neutron-absorbing chemical in solid form from the container directly into the open volume in response to a predetermined temperature and/or a predetermined pressure within the open volume such that the released amount of the neutron-absorbing chemical in solid form goes into solution upon mixing with coolant in the open volume and such that the solution fluidly communicates with the inner surface of the containment vessel;
• and
• circulating the solution from the open volume into the reactor vessel to maintain the fission reaction at least at the sub-critical state.