The Past, The Present, The Future

November 1986 Shelter Object completed; Units 1,2 back online June 1982 Unit 3 goes online April 1972 officially named March 1984 November 1996 January 1967 Unit 4 goes Unit 1 shut down September 2010 Gosplan online permanently Construction begins recommends a new on New Safe May 1978 November Confinement to power plant at Unit 1 goes 1987 Unit 3 Kopachi, replace Shelter online back online Object

1967 1972 1977 1982 1987 1992 1997 2002 2007 2012 2017

February 1970 May 1979 Construction of Unit 2 goes Pripyat begins online; Pripyat becomes a city April 1986 Unit 4 destroyed, August 1972 Pripyat is December 2000 Construction begins evacuated Unit 3 shut down on Unit 1 permanently; end of power-producing September 1982 October 1991 era at ChNPP Partial core melt, Fire destroys Unit 1 Unit 2

4 3 2 1

Fuel Fuel

Chain-reacting systems are familiar (e.g., fire). Feedback, stability, and control concepts in nuclear reactors may be compared to the behaviors of fires. CONTROLLING FIRES Fuelis combustible Fuel is combustible Reactivity depends on 1. Increase or reduce the material that reacts with heat transfer efficiency efficiency of heat transfer material that reacts with between fuel air when exposed to heat, air when exposed to heat, (separation of fuel pieces), 2. Decrease efficiency by putting producing more heat, producing more heat, heat leakage (shape and water, or damp fuel, in the fire which can cause which can cause size of the system), and (water evaporation removes heat). combustion in additional combustion in additional availability of fuel and air. 3. Increase efficiency by moving fuel. fuel. fuel pieces closer together, or by burning dryer fuel.

Fuel is atomic nuclei that Fuel is atomic nuclei that Reactivity depends on CONTROLLING REACTORS explode when they absorb explode when they absorb neutron transport, slow-moving neutrons, 1. Increase or reduce the slow-moving neutrons, including moderation efficiency of neutron transfer producing more neutrons producing more neutrons (separation of fuel between fuel nuclei (generally fast-moving (generally fast-moving pieces), neutron leakage 2. Decrease efficiency by putting ones). The moderator slows neutron-absorbing material ones). The moderator (shape and size of the (typically called control rods) down fast neutrons so they slows down fast neutrons system), and availability between fuel elements. will cause fission in so they will cause fission in of fuel. 3. Increase efficiency by additional fuel. additional fuel. withdrawing the control rods. FIRES FEEDBACK: POSITIVE FEEDBACK: NEGATIVE FEEDBACK: POISONING: Feedback refers to a system A perturbation to a parameter of A perturbation causes a Fires and nuclear reactors AND influencing its own the system causes a response response that moves the both generate byproducts behavior. that moves that parameter in the parameter in the opposite that interfere with—or NUCLEAR same direction as the stimulus. direction of the stimulus. “poison”—the chain reaction. REACTORS Can promote instability. Favors stability.

Time Event / Action Result ~12:00 AM Failure to maintain computerized Reactor almost shuts down. Xe-135 April 26 automatic control of reactor poison builds up, suppressing ability to 1986 power at intended power level. resume intended power.

~1:00 AM Operator attempts to maintain Only neutron absorber left in core is power. He must remove almost boiling water. Positive power reactivity all the control rods. feedback scenario. Power begins rising.

1:23 AM Test concludes, operator presses Control rod tips push water out of the button to insert all control rods. core, adding reactivity because the tips (Unknown whether he does this are made of a poor absorber. Reactor because of power surge, or power rises uncontrollably; core damage because he was attempting a impedes further control rod entry. normal shutdown.) ***EXPLOSION*** The operator withdrew most control rods to counteract poisoning, leading to positive power reactivity feedback as water boiled in the core. When the operator attempted to replace the control rods, their tips accelerated the power surge and control was lost. OPERATOR DESIGN SAFETY RISK

Operator Fault: Flawed Design: Deficient “Safety Culture”: Residual Risk: ChNPP staff performed RBMK had dangerous Phrase coined by INSAG The constellation of contrary to regulation characteristics, did not encompasses faults technical / human and technical meet official safety specific to Chernobyl and circumstances leading instruction from the mandates; some views the wider system of to accident was designer, contributing suggest catastrophe nuclear power regulation preemptively unknown to the accident and its “built in”. Upgrades and management in and practically consequences. Six addressed reactor USSR; the Soviet industry, unknowable; nuclear ChNPP staff convicted issues most significant by its priorities, ideology, power is “inherently of misconduct in the to accident. and organization, was dangerous” even when Chernobyl Trial (1987), accident-prone. handled competently; but the trend since has from an alternative been exculpatory. viewpoint, “Science requires victims.”

CHERNOBYL 18

CHERNOBYL 21

Isotope Activity Am-241 3,782 pCi/g Ba-133 42 pCi/g Cs-137 78,274 pCi/g Eu-154 48 pCi/g Gd-153 21 pCi/g Ra-226 384 pCi/g