# First criticality and initial heat-up of the reactor core

#### Lucky mkhonza

1. how could one approach first criticality in practice?
2. what would happen if we start-up with full power conditions from first criticality?
3. what would the shutdown requirement be in comparison to an equilibrium core?

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#### Morbius

Dearly Missed
1. how could one approach first criticality in practice?
2. what would happen if we start-up with full power conditions from first criticality?
what would the shutdown requirement be in comparison to an equilibrium core?
Lucky,

Sounds like a homework problem.

First, when a reactor is shutdown; there is always a neutron source in the reactor. This
neutron source is a radioactive element that emits neutrons. If the reactor is shutdown,
or sub-critical; it won't support a self-sustaining chain reaction. So any chain reactions
that would start due to a stray neutron will die out. The instrumentation monitors neutrons,
and without neutrons how would the instrumentation work?

So a shutdown reactor always has a source in it. A sub-critical reactor will multiply
the source neutrons; called "sub-critical" multiplication; and will reach an equilibrium
level equal to the source rate divided by "k - 1"; where "k" is the criticality ratio.

If you know the neutron level, and the source level; you can compute "k".

You can then approach criticality which is the condition "k" = 1.

A fresh core has a higher reactivity than a shutdown core.

Dr. Gregory Greenman
Physicist

#### Astronuc

Staff Emeritus
1. how could one approach first criticality in practice?
2. what would happen if we start-up with full power conditions from first criticality?
3. what would the shutdown requirement be in comparison to an equilibrium core?
As Morbius mentioned, this does appear to be a homework problem in fuel cycle management and core operation.

Anyway - in short, the answer to question 1 is 'slowly'. How it is done depends on the type of reactor, but basically one withdraws the control elements (control rods in a PWR, CANDU or Fast Reactor, or control blades in BWR). The negative reactivity of the entire complement of control elements is much greater than the reactivity in the core.

In PWRs, boric acid is added as a chemical shim, which is not the case for BWRs. At startup of a PWR, most control elements are withdrawn and the boric acid is diluted. The core contains startup sources which emit neutrons and one monitors the core activity with neutron detectors both ex-core and in-core.

Heatup is accomplished by running the pumps which put about 10-15 MW of thermal energy into the cooling water. Auxiliary power sources are used until the turbine is brought online and the generator can provide electrical energy (here the assumption is that this a power reactor).

2. A reactor normally goes from cold zero power (CZP) to hot zero power (HZP) conditions prior to criticality (plant heat up) over a period of hours, then power ascension from HZP to hot full power (HFP) normally takes at least a couple of days.

3. what would the shutdown requirement be in comparison to an equilibrium core?
I'm not sure what is meant by this question. A certain minimum shutdown margin is always required. In the first core of a reactor, all the fuel is fresh, so there are no fission products to compete with the fissile material for neutrons. In the second core, there are at least two batches of fuel with some operation (low burnup, but some fission products) as well as the fresh fuel (reload), but the shutdown requirements are still the same nuclearwise. In reality, 'equilibrium core' is a myth - it is an ideal situation in fuel cycle/core management that is never realized, although some plants may have come close. Also, the enrichments of the first core are relatively low compared to subequent reload fuel, because all the fuel is fresh and there are no fission products to absorb neutrons. For subsequent cycle operation, part of the enrichment increase is due to consequence of offsetting the accumulation of fission products.

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