Advantages of Primary System Flow Control During Reactor Scram in an LMFBR

Following a reactor scram in an LMFBR, the sodium flow rate through the core should ideally be reduced at such a rate as to hold the core ΔT essentially constant if thermal stresses are to be minimized. Three different methods of varying primary coolant flow following scram were investigated.

1. normal pump coastdown
2. braking force applied to the primary pump
3. fast-closing throttle valve in the primary coolant circuit.

It was determined that proper action of a primary system throttle valve can eliminate severe thermal shocks to the core following scram and pump trip out. A pump brake will reduce, but not eliminate, the excessive stresses generated in the upper core region following reactor scram and free rotating pump coast-down.