Chernobyl: reactor state and systems design

01 / Choose one

Why could the start of rod insertion increase reactivity in the lower core?

1. Graphite displaced neutron-absorbing water before sufficient absorber reached that region
2. All control rods always add reactivity when they move
3. Xenon instantly stopped absorbing neutrons

A. In the original geometry, graphite could replace water in the lower core during initial insertion. Absorption added higher in the core did not guarantee a negative net initial effect in the vulnerable state.

02 / Calculate · m²

One barn equals 10⁻²⁸ m². Write that area in m² using a decimal or scientific notation.

1 barn = 1 × 10⁻²⁸ m². This unit describes an effective interaction area, not a literal outline of a nucleus.

03 / Choose one

Two equal opposing neutron beams cross the same area. What happens to their scalar flux and net current?

1. Both are zero
2. The flux adds; the net current cancels
3. The current adds; the flux cancels

B. For this two-beam example, flux = rightward + leftward, and current = rightward − leftward. Equal positive beams give nonzero flux and zero net current.

04 / Choose one

The chain reaction has stopped. Which heat source can still remain?

1. Only fission that continues at the previous rate
2. Radioactive decay of products already present
3. None; zero chain-reaction power means zero heat

B. Fission products continue to decay, and hot materials retain thermal energy. Chemical reactions can contribute too. These are distinct from ongoing chain-reaction power.

05 / Choose one

What does an ORM value in equivalent rods describe?

1. A simple count of all rods physically touching the core
2. State-dependent reactivity worth expressed using a standard rod reference
3. A guarantee that any shutdown transition will be safe

B. ORM expresses reactivity worth in equivalent standard rods. It depends on the state and spatial neutron field, and it cannot by itself describe every shutdown trajectory.

06 / Reflection

A test begins with steady power and a trained crew. What else would you want to know before calling the test adequately controlled?

Look for a justified starting state, analysed transitions and failure cases, available independent protections, useful and timely indications, clear procedures and stop conditions, and authority to stop without production pressure. State what evidence is missing rather than assuming that steady power or training is sufficient.

• Separates a steady starting point from the safety of the whole transition
• Considers equipment, interfaces, procedures, and organizational decisions
• Asks whether protections share a common failure mechanism
• Names evidence or assumptions still needing verification