The content of nuclear uranium fuel contains approximately 99.3% U- 238 and 0.7% U-235 by mass. The uranium reactors used today contain 5% U-235 by mass. Therefore, by enriching uranium, the mass of 0.7% is increased to 5%. If we did not want to enrich uranium and the amount of U-235 by mass was 5%, how many years ago would we have to go before today? (U-235 half-life: 7.04*10^8 years, U-238 half-life: 4.5*10^9 years)