Power Plant Engineering

340 POWER PLANT ENGINEERING

10.26.1 Reactor Design: RBMK-1000

Boiling Water Reactor

Electric generating capacity 1000 Megawatt.

Thermal output of core about 2000 Megawatt.

1661 zirconium fuel rods, holding mix of U-238 and U-235; Plutonium-239 is a byproduct,

which can be extracted by reprocessing the fuel rod material. Each fuel rod is enclosed in a heat transfer

water channel.

211 Boron control rods with 8 fuel rods/control rod

Graphite core 1700 tons, made up of graphite bricks

10.26.2 Control of the Reactor

1. Graphite Core, moderates neutron flux from fuel rods


2. Boron control rods to reduce neutron flux for shutdown


3. Thermal transfer control — closed circuit water/steam loop, multiple water pumps Nitrogen/
   Helium gas within containment — low thermal conductivity and oxygen exclusion — pressure and gas
   mixture are controlled. emergency core cooling water system (ECCS)

10.26.3 Chernobyl Reactor Operations

Computer for fine control, operator controls set points of feedback controllers Central power

authority dictated operating levels in managing power grid Unnecessary shutdown meant 600,000 ruble

revenue loss, firing of person responsible.

Plant engineers found the plant unstable at low power levels,

Local practice was to manually pull control rods if downward fluctuation threatened spontane-

ous shutdown. Response time to scram: 18 seconds (theoretically it was claimed to be 3 seconds).

Regulations against manual control routinely excepted.

10.26.4 ACCIDENT\SAFETY PLANS

Published odds million to 1 against an accident. Authoritarian control staff & engineers do not

question safety. Accident planning was around a scenario of 1 or 2 fuel rod/water channels bursting. No

plan included a graphite fire. Administration building had emergency bunker under it. Reactor building

was a water tight containment building.

10.26.5 Evacuation

Plant director had authority in principle to order evacuation of Pripyat. However a standing order

made any nuclear accident a state secret.

10.27 Safety Problems in Chernobyl Reactor Design

10.27.1 System Dynamics

A problem with RMBK-1000 reactor design is that the time constants for changes in thermal
output are short. Control depends on computer regulated feedback control systems. The human operator
could not react fast enough to manually control it without the automatic controls.