Sodium-cooled fast breeder reactors: why is a secondary (intermediate) sodium loop essential before the steam–water system?

Introduction / Context:
Fast breeder reactors (FBRs) often use liquid sodium as primary coolant to preserve a fast neutron spectrum and enable high heat transfer at low pressure. Integrating this with a water/steam power cycle requires careful interface design to manage chemical reactivity and radiological safety.

Given Data / Assumptions:

• Primary coolant is radioactive sodium after core contact.
• Water/steam is used to drive turbines on the secondary/tertiary side.
• Sodium and water react vigorously, generating heat and hydrogen, leading to potential explosions.

Concept / Approach:
A secondary (intermediate) sodium loop sits between the primary sodium and the steam–water system. This loop transfers heat through a sodium–sodium heat exchanger, so any sodium–water leak in the steam generator does not involve radioactive primary sodium, reducing radiological consequences and protecting the core from water ingress.

Step-by-Step Solution:
Recognise the chemical hazard: sodium + water → violent reaction + hydrogen.Note the radiological hazard of primary sodium.Insert an intermediate loop to isolate primary sodium from water.Hence, the main reason is to prevent water–primary sodium contact and explosions.

Verification / Alternative check:
SFR design literature consistently emphasises the intermediate loop as a key safety and availability feature, localising any steam generator leaks to non-radioactive sodium.

Why Other Options Are Wrong:

• Superheat degree depends on steam-side design; not the primary reason.
• Heat removal rate depends on overall design; the intermediate loop is for isolation, not simply higher flow.
• Circulation pressure is already low with sodium; isolation is the driver.
• No steam generator: incorrect—steam generators are required for Rankine cycles.

Common Pitfalls:
Underestimating sodium–water reactivity; assuming the intermediate loop is optional in power SFRs.

Final Answer:
To prevent water from contacting radioactive sodium (sodium–water reactions can be explosive)