Given enough radioactive materials you could get enough energy to sustain life. That's what happens in Earth's depths around thermal vents.
Also, the radioactive decay should last long enough to allow life to form. For instance, in our planet case the radioactive decay will keep the outer core liquid for another billion years give or take, after which we'll have no tectonic movements and no magnetic field anymore. The latter shouldn't be a problem in an universe without stars, but the former means that erosion would eventually smoothen out the whole planet (and have a smooth landmass completely underwater for Earth).
However, having enough radioactive materials that the surface remains hot enough to have liquid water is a problem on its own. You need too much radioactive material to achieve so, meaning that at the beginning (planet formation) you have 100% of the heat output, but after 24k year (assuming all the radioactive material is plutonium) you have 50% of it. So, if you assume that at some point you have enough heat to have liquid water one half-life later that shouldn't be enough. This doesn't take into account the internal heat that the planet contains after millions of planetoids impacts. However, it feels impossible anyway.
Better to go with tidal forces like other comments mentioned (like what happens to Io, one of Jupyter moons).
Also, the radioactive decay should last long enough to allow life to form. For instance, in our planet case the radioactive decay will keep the outer core liquid for another billion years give or take, after which we'll have no tectonic movements and no magnetic field anymore. The latter shouldn't be a problem in an universe without stars, but the former means that erosion would eventually smoothen out the whole planet (and have a smooth landmass completely underwater for Earth).
However, having enough radioactive materials that the surface remains hot enough to have liquid water is a problem on its own. You need too much radioactive material to achieve so, meaning that at the beginning (planet formation) you have 100% of the heat output, but after 24k year (assuming all the radioactive material is plutonium) you have 50% of it. So, if you assume that at some point you have enough heat to have liquid water one half-life later that shouldn't be enough. This doesn't take into account the internal heat that the planet contains after millions of planetoids impacts. However, it feels impossible anyway.
Better to go with tidal forces like other comments mentioned (like what happens to Io, one of Jupyter moons).