Monday, May 3, 2010

Safety is designed into US Reactors--The Power of a Negative Temp. Coefficient of Reactivity

Let's step back and explore one of the fundamental concepts of reactor theory and the FACTS that make reactors in the US inherently safe. I am talking about the temperature coefficient of reactivity. Oh sure everyone knows about that. Well I think if they did know it would help to alleviate some of the concern with reactors supposedly being able to "blow" up or melt down in some China Syndrome event.

The sad news for the nay Sayers is that reactors are safer than ever and US reactors are designed such that they shutdown when something goes wrong. Current reactor technology uses less equipment and less automation, focusing on passive systems. When something goes wrong in a nuclear reactor temperature is likely to rise in the reactor core. A negative temperature coefficient of reactivity means that as temperature goes up...reactivity goes down. When reactivity goes down the reactor is essentially turning itself off like pulling your foot off the gas of your car.

Reactivity is the engine of fission in a reactor. Reactivity equals more neutrons per unit time (neutron density) and therefore more fission, therefore more energy released, therefore an increase in temperature. That increase in temperature is harnessed as steam to drive a turbine and create 20% of the power in the US.

A negative temperature coefficient of reactivity makes a reactor inherently stable. Example: As power demand increases on the turbine, more steam is used, the coolant circulating through the steam generator and the reactor is cooled slightly. As the temperature goes down the reactivity....goes up! So we push on the gas pedal and get more neutrons and energy as we increase fission and compensate for the temperature drop by increasing reactivity and reactor power to match steam demand.

As you can see this stability allows for a mitigated emergency response for a major casualty leading to an increase in temperature. If I lose reactor coolant and cannot cool the core as effectively the reactor will shutdown (to a point see emergency cooling below).

Contrast this with Chernobyl. Russian designed reactors had essentially a net overall positive temperature coefficient of reactivity (graphite moderator with water coolant thus positive steam void reactivity and positive reactivity of initial control rod motion [Ref1]). See where we are going here?!? Temperature goes up and reactivity goes up. Therefore power goes up and therefore temperature goes up.... leading to disaster. Chernobyl also did not have sealed containment. It also had an enormous reactor core which lead to fluctuating reactivity and flux..essentially three or four different reactors all within the same core behaving independently yet as a whole. All of this lead to a difficult to control reactor that was not inherently stable.

When the casualty hit, the reactor essentially was unable to be controlled (There are multiple factors) and fission products and gases were released to atmosphere (no containment) NRC analysis When disaster struck three mile island, containment was in place and there was very little release to the environment (maximum offsite radiation dose 0.1 rad and total population dose was approximately 10 person-rems [ref1]and NRC analysis plus an overall mitigated reactor response due to the negative temp. coefficient of reactivity.

US reactors have containment and inherently stable reactors. Other safety systems such as the emergency core cooling system (ECCS) ensure that the reactor is cooled even with a loss of coolant. Without emergency cooling the temp. coefficient of reactivity will not help as the uncovered fuel rods melt due to fission product heating leading to various exothermic chemical reactions between the molten material and the water steam mixture.

Test to follow next Tuesday........

For extra credit: I would be remiss in not clarifying that we are talking about the moderator (water coolant) temp. coefficient of reactivity above. The prompt temp. coefficient of reactivity describes the affect of the change of temperature of the fuel itself and determines the first response of a reactor to changes in either fuel temp or reactor power. The NRC requires all reactors to have a negative prompt temp. coefficient of reactivity.

Ref 1 Intro to Nuclear Engineering, John Lamarsh and Anthony Baratta

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