A nuclear reactor cooling circuit, comprising: a depressurisation valve (100,200), the depressurisation valve comprising: a main chamber (107,207) having a main valve (101,201), a pilot line (104,204) having a secondary valve (109,209) and a blowdown line (108,208); the main valve being located to seal a path of the cooling system, the main chamber (107,207) connected to the cooling circuit via the pilot line allowing coolant to enter the main chamber, and the blowdown line allowing coolant to escape from the main chamber, the pilot line having a lower fluid resistance than the blowdown line; and wherein, in normal operation, the main valve (101,201) is maintained in a closed position within the circuit by the fluid pressure from the coolant circuit passing through the pilot line (104,204) into the main chamber (107,207) at the back of a valve piston coupled to a valve stem (103,203) so as to apply a force on the valve piston and force a compression spring (102,202) down, and characterised in that the secondary valve (109,209) is thermally activated such that: if the fluid temperature rises above a certain limit the secondary valve closes and stops the flow of fluid into the main chamber; and the blowdown line allows fluid to escape from the main chamber reducing the pressure acting on the valve piston, such that the compression spring (102,202) can expand and the main valve (101,201) can open to allow the fluid to drain from the cooling circuit. a depressurisation valve (100,200), the depressurisation valve comprising: a main chamber (107,207) having a main valve (101,201), a pilot line (104,204) having a secondary valve (109,209) and a blowdown line (108,208); the main valve being located to seal a path of the cooling system, the main chamber (107,207) connected to the cooling circuit via the pilot line allowing coolant to enter the main chamber, and the blowdown line allowing coolant to escape from the main chamber, the pilot line having a lower fluid resistance than the blowdown line; and wherein, in normal operation, the main valve (101,201) is maintained in a closed position within the circuit by the fluid pressure from the coolant circuit passing through the pilot line (104,204) into the main chamber (107,207) at the back of a valve piston coupled to a valve stem (103,203) so as to apply a force on the valve piston and force a compression spring (102,202) down, and characterised in that the secondary valve (109,209) is thermally activated such that: if the fluid temperature rises above a certain limit the secondary valve closes and stops the flow of fluid into the main chamber; and the blowdown line allows fluid to escape from the main chamber reducing the pressure acting on the valve piston, such that the compression spring (102,202) can expand and the main valve (101,201) can open to allow the fluid to drain from the cooling circuit. a main chamber (107,207) having a main valve (101,201), a pilot line (104,204) having a secondary valve (109,209) and a blowdown line (108,208); the main valve being located to seal a path of the cooling system, the main chamber (107,207) connected to the cooling circuit via the pilot line allowing coolant to enter the main chamber, and the blowdown line allowing coolant to escape from the main chamber, the pilot line having a lower fluid resistance than the blowdown line; and wherein, in normal operation, the main valve (101,201) is maintained in a closed position within the circuit by the fluid pressure from the coolant circuit passing through the pilot line (104,204) into the main chamber (107,207) at the back of a valve piston coupled to a valve stem (103,203) so as to apply a force on the valve piston and force a compression spring (102,202) down, and characterised in that the secondary valve (109,209) is thermally activated such that: if the fluid temperature rises above a certain limit the secondary valve closes and stops the flow of fluid into the main chamber; and the blowdown line allows fluid to escape from the main chamber reducing the pressure acting on the valve piston, such that the compression spring (102,202) can expand and the main valve (101,201) can open to allow the fluid to drain from the cooling circuit. if the fluid temperature rises above a certain limit the secondary valve closes and stops the flow of fluid into the main chamber; and the blowdown line allows fluid to escape from the main chamber reducing the pressure acting on the valve piston, such that the compression spring (102,202) can expand and the main valve (101,201) can open to allow the fluid to drain from the cooling circuit.