Nuclear Propulsion Engines such Plasma fusion whatever contaminate the environment the fly (terrestrially atmosphere at-least). Nuclear reaction emit too many neutrons that would damage the elements the the engine is constructed with making the engine radioactive.

Lithium is used in Nuclear reactors to absorb neutrons that then causes heat that can be converted into electricity.

However how many neutrons can Lithium absorb?

Elements that are bigger become radioactive when absorbing too many neutrons.

However speculatively specking it may be possible to build up stable non-radioactve elements into larger elements (bigger protons volumes) within particle accelerators

within the Protons Electron Neutrons model of atomic physics

The transmutation equation could prhap be written

+++> P++++++> {(x)P + (x)E + (e?)N} => [(x+1)P + (x)E + (e?)N]

These synthetic elements contain fewer neutrons than naturally accuring elements. They could possible be used as neutron bomb defense shield's that don't weigh as much as lead'.

They would also prevent your nuclear propulsion engine from turning into radioactive active junk that splits like uranium in-two corrupt radioactive elements that ruin the tensile strength of metals/materials within the spacecraft.