Plutonium (94) doesn't fuse out of water (8 + 1 + 1) very elegantly. You will have to use the control commands to make fine adjustments to the plutonium fusion. On the plus side, you are allowed to overproduce H2 and O2 as much as you like.
Initially you will need to split up the water in to a hydrogen atom pipeline and an oxygen atom pipeline. Use drive-by unbonding to strip the hydrogens as usual.
Two additional reactors are required. One to produce O2 and start the plutonium fusion, and one to produce H2 and finish the plutonium fusion.
The O2 reactor should take oxygen as an input and sit producing O2 unless a control command is activated. The control command should make it start fusing oxygen together. You can't directly produce plutonium from oxygen, but mercury (80) or radium (88) are very close. The H2 reactor is essentially the same but needs to take the mercury/radium and fuse hydrogen in to it, once again using a control command.
Edge triggered controls are useful here. See the structures overview for details.
Here is a reactor design for the final reactor.
It takes in hydrogen at the β input and outputs H2 in the ψ output. The red waldo gets the output of the prior reactor which I have marked as either mercury or radium depending on how precise I am with the control buttons. When control B is turned on, the red waldo passes the first trap and the blue waldo starts fusing hydrogen. When it is turned off, the red waldo passes the second trap and outputs the molecule, which should be plutonium if the timing is correct.
The prior reactor is almost identical to this apart from two minor differences. Firstly, it has to double bond the O2 using the additional bonding plates. Secondly, it does not take any starting input for the fusing process.