[Grade 12/1st-year University Physics: Nuclear physics] How much energy can be utilized from the deuterium in water?

[u/CroCop27]

Hi! So for the last month of my first term in uni, our physics professor handed out 31 questions to help study for next exam until the end of next month and I'm hard-stuck on the last one and would love some help (I emailed him but since its Christmas break he hasn't answered and both of my friends from uni gave up on the last question after many failed attempts), the question is originally in Swedish but ill do my best to translate it as accurately as possible. The question is in the field of nuclear physics but as of now we have not gone into molar masses and avogadros constant so this should be doable without it.

Question:

In a body of water there is 6500 the amount of hydrogen-atoms as deuterium-atoms. Assume you can utilize all the deuterium atoms in the 1m3 body of water to produce energy according to this reaction equation:

How much energy can you extract/obtain?

Now in my book I have the masses for neutrons, deuterium and helium nucleus, the water I need to calculate by myself and I get it to 18,01u per molecule but not sure how correct it is or if its even needed but I did it anyways. After that I went into getting the mass defect and I got it to 0,00349u, using the massdefect I could calculate how much energy I get per reaction (I got a hint from a friend to do so but I still don't really understand it?) but after getting the energy for the mass defect I'm kind of lost, I tried using the mass of one water molecule to get the number of molecules in water and the total number of particles of deterium knowing it has a ratio of 1/6500 but I still go wildly wrong answers form 0,02 GJ to 5 TJ to 20TJ

I checked what the answer is in a answer sheet the professor posted but it did not include a method only that the final answer is 3 TJ.

So yeah I would love to get some help here :)

TYVM

Edit: Forgot picture

Edit: Ok nvm I finally solved it, I think.. Got it to 2,6 TJ which I assume rounded up to 3 TJ from the significant figure of "1" in "1 cubic meter of water"

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