Pension wacc (again-:)

Hey guys, I know this has been visited many times but my question is a bitt different. WACC usually is We * cost e + Wd * costd (1-t) how come when we are using it to incorporate pensions wacc becomes the CAPM like formula of WACC = rf + bassets (MRP) (schweser book 2 pg 51) why is the risk free rate used (if anything it should be the after tax cost of debt) and it doesn’t incorporat the firm’s capital structure at all thx