Sodium-cooled fast breeder reactors: why is a secondary (intermediate) sodium loop essential before the steam–water system?
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ATo achieve a very high degree of superheat in the steam
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BTo remove heat faster from the core by increasing flow area
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CTo reduce the primary-loop circulation pressure drastically
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DTo prevent water from contacting radioactive sodium (sodium–water reactions can be explosive)
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ETo allow operation without any steam generator altogether
Answer
Correct Answer: To prevent water from contacting radioactive sodium (sodium–water reactions can be explosive)
Explanation
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)