Nuclear engineering — If a nuclear reactor produces more fissile nuclear fuel than it consumes, what is this type of reactor called?

Introduction:
A key concept in nuclear engineering is whether a reactor merely sustains a chain reaction or also creates more fissile material than it burns. Reactors designed to generate excess fissile fuel are essential for closing the fuel cycle and improving resource utilization of fertile isotopes such as U-238 or Th-232.

Given Data / Assumptions:

• The reactor converts fertile material into fissile material via neutron capture and subsequent decay (e.g., U-238 → Pu-239).
• We distinguish among criticality state (subcritical/critical/supercritical) and breeding behavior.
• Terminology: fissile vs. fertile; breeding ratio > 1 implies net production of fissile material.

Concept / Approach:

A breeder reactor achieves a breeding ratio greater than 1, meaning the rate of creation of new fissile atoms exceeds the rate at which fissile atoms are fissioned. This is typically accomplished by surrounding the core with a blanket of fertile material that absorbs excess neutrons and transmutes into fissile fuel. The reactor may be fast (no moderator) or thermal (with a moderator), though fast breeders are common due to higher conversion probabilities for U-238.

Step-by-Step Solution:

Verification / Alternative check:

Reactor physics texts define the breeding ratio and classify breeder designs (e.g., fast breeder reactors with liquid-metal coolants) specifically to denote net fissile growth.

Why Other Options Are Wrong:

A: 'Critical' refers to neutron population steady state (k-effective = 1), not fuel breeding. C: 'Fertile' describes isotopes like U-238/Th-232, not the reactor type. D: 'Heterogeneous' refers to core geometry/material arrangement (fuel and moderator separated), not breeding behavior. E: 'Subcritical converter' would not breed more fissile material than consumed.

Common Pitfalls:

Confusing criticality (a power/kinetics concept) with breeding (a fuel cycle concept). A reactor can be critical without being a breeder and vice versa regarding design intent.

Final Answer:

breeder