def heat_equation(Cu, Cth, Ck, density=2700, c1=9.52, c2=2.56, c3=3.48):
    """
    Heat production equation from Beamish and Busby (2016)
    density is the density of the 
    
    density: rock density in kg/m3
    Cu: weight of uranium in ppm
    Cth: weight of thorium in ppm
    Ck: weight of potassium in %
    
    Returns: Radioactive heat production in W/m3
    """
    return (10e-5)*density*(c1 * Cu + c2 * Cth + c3 * Ck)