PWR

G FR

LM R

Coolant is high pressure (15 MPa ), H 2 O liq uid; Lo w boi l i ng po in t, Good h eat trans f er

Coolant is high pressure (7-20 MPa), G as: He, CO 2 ; Medioc re heat trans f er

Coolant is low pressure (  0.5 MPa ) , L i qu id met a l: Na, P b ; Hi g h b o iling p o i n t, Excellent heat trans f er

Som e fract io n of coo l in g i nven to r y rem a ins afte r LO C A

>98% of coolant escapes in a com p le te depressurization

Use of guard vessel re tains adequate liquid inventory in reactor vessel

Large liquid reflood reservoir is ava i lable; Bo i lin g and heat of v a p ori zati o n facilit ate co o lin g

Lim i ted high pressure gas injec t ion available ; A t low pressure only a i r available in la rge am ount

Can design for pass ive decay heat rem oval by natura l convect ion to am bi ent a i r or water

P o st-L O C A g o al i s ti mely red u c ti on of conta i nm ent pressure to re duce leakage

Reducing p r e ssure af te r sh ut down does no t n e g a ti v e ly i m p act co ol ability

Maintainin g h i g h pressure facilitates lon g term cooling, es pecially via natura l convection

Lo w pressure is a l way s detrim enta l to heat tran sfer an d tran sp o r t; Co o l i ng cap ability is proportiona l to pressure; Adequate natura l convection only at several atm o sphe res

Sy s t em is low pressure

Refueling under water at 1 atm can re ly on an effective pass ive heat s i nk; Visual observa t i on p o ss ib le

Cool ing du r i n g depres sur i z e d refue l i ng i s challenging; Refueling a t high pressure creates m o re L O C A seque nces; Visua l observa t i on p o ss ib le

Refueling is at low press ure, but blind; H eat rem oval is assured

Benign reactor phys ics characteris tics ; Negative coolant void reactivity (in c o m m on with m o st the r m a l reactors)

Sm all de la y e d neu tr on fra ct ion and Dopp ler reactivity; Positive coolant void reactivity (in com m on with a l l fas t reactors ) ; Fue l co mp act io n an d recriti cality

Sm all de la y e d neu tr on fra ct ion ; Mo des t Doppler reactivity; Large loca l pos itive void reactivity; Fuel com p action and recriti cality