is the conservation of matter conserved in a radioactive decay reaction

no, energy is not conserved in a radioactive decay reaction,  mass is loss in the form of energy,

 in a radioactive decay which involve alpha beta or gamma , radiation

 masses an energy  is lost in the two processes of alpha beta  while only energy is lost in the gamma decay.

 When an  atom emits an alpha particle, a mass equivalent to the size of a helium nuclei is lost also additional energy. While in beta decay a mass equivalent to the size of an electron is lost in the process  whereas in a gamma emission only energy s lost

We can calculate this mass and loss  and energy lost by using the Einstein mass energy elation.

 Though a balanced nuclear equation may not show energy loss of mass , as the masses and charges on the product and reactant side are balanced both nonetheless mass and energy is loss.

 This mass lost can be accounted for if we we consider the actual atomic masses of these reacting atoms instead of the whole number atomic mass.

 For example n the equation below

²³⁴ ₉₀Th → ²³⁴ ₉₁Ac + ⁰ _-1e

 The mass number of thorium is 234 but the actual mass of the  thoium-234 is not a whole number but mixture of whole number and decimal places, the isotope thorium-234 has an actual mass of  234.0436. Helium actual mass is not 4 , a whole number but 4.002602 u ± 0.000002 u, the part not shown in the mass number is what is lost or converted to energy

 The decimal part in thorium, actinium above and helium is therefore lost as masses  and energy.

 Therefore in nuclear  reaction there are losses of mass and tremendous creation of energy, this is why the process produces very cheap power.