# Decay power and spent fuel pond

Hello to all, i need more details about followings problems:
Why nuclear spent fuel ponds must be cooling down and which is maximum allowable temperature in cooling agent?
How we can evaluate decay power using theoretical formula?

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Astronuc
Staff Emeritus
Hello to all, i need more details about followings problems:
Why nuclear spent fuel ponds must be cooling down and which is maximum allowable temperature in cooling agent?
How we can evaluate decay power using theoretical formula?
The spent fuel pool provides shield of the spent fuel. The water is cooled such that it does not boil away since the water provides shielding.

Knowing the discharge burnup and date of discharge, one can determine the decay heat of a given assembly. Batches of assemblies are discharged on an annual, 18 month or 24 month basis. Assemblies in a given batch will have a range of burnups.

With reactors using low leakage core designs, usually three or four cycle assemblies are operaing on the core periphery at less than 40 or 50% core average power, so the decay from short-lived isotopes would be less than higher power assemblies near in the core interior.

Decay heat power can be estimated using any number of formulations from simple to complex. Simplest of all might be the Way-Wigner formula. For licensing calculations, typically the ANS standard 5.1, “Decay Heat Power for Light Water Reactors" is used. This has been issued and revised several times. The older version (1971) is the simplest (it is really just a table) while the newer versions require more knowledge of the core and operating history. Best estimate analyses may use the ORIGEN computer code.

Keep in mind that for spent fuel pool analysis you are interested in the decay heat generated months and years after removal from the core, while for reactor accident analysis you really care about the decay heat at the time of reactor shutdown and for the first minutes thereafter. The decay heat model you select should consider this, so you can make some informed judgement of whether your analysis is conservative, and by how much.

QuantumPion
Gold Member
Spent fuel pools are at atmospheric pressure, so the maximum temperature they can reach is the boiling point of water.

The decay heat load for a fuel pool will be on the order of 1-10 MBTU/hr under normal conditions and anywhere from 20 to 60 MBTU/hr during an outage, when a full core of fuel is temporarily unloaded into the pool. Exact figures depend on the size of the fuel pool and core loading.

jim hardy
Gold Member
2019 Award
Dearly Missed
Spent fuel pools are at atmospheric pressure, so the maximum temperature they can reach is the boiling point of water.
that's the physical limit.

The pool is large and normally it's accessible to workers so there's common sense limits imposed administratively.
You don't want steam condensing all over the room and dripping all over the machinery.
Should somebody fall in you don't want him scalded to death.
So pools are generally kept at or below the temperature of warm bathwater.

Regulatory limits (See NUREG-0800 Standard review plan section 9.1.3) generally limit you to something like 140F or 150F for your maximum heat load and maximum abnormal heat load respectively.

So, for keep water temperature below limit established, are used special circuit for cooling(i.e pumps, heat changers)?

So, for keep water temperature below limit established, are used special circuit for cooling(i.e pumps, heat changers)?
In a BWR, the spent fuel pool cooling system is a safety related system and is powered by the emergency power system, meaning the emergency generators will run the system.

The fuel pool has fuel pool cooling pumps. Heat is exchanged from the fuel pool to the reactor closed cooling water system, or directly to the lake/river/ocean. The reactor closed cooling water system transfers its heat directly to a lake/river/ocean. There are typically 2 pumps of each system per reactor, and the entire system is single failure proof, and designed to function post accident with a loss of all offsite power and a single failure.