# Presentation: 330441

## Radiation Effects on Component Materials in Nuclear Thermal Rocket EnginesNASA Academy Final Presentation

**Katherine A. Yakubisin **
**Research Associate**
**PI: Karl Nelson**
**ER11 / Nuclear and Advanced Propulsion Branch**
**8/12/2005**

## Outline

**Background**
**Objectives**
**Radiation effects on metals **
**Radiation effects on polymers**
**Radiation in NTP**
**Use of metals in NTP**
**Use of copper in NTP**
**Use of polymers in NTP**
**Conclusions**

- PRE-DECISIONAL - For planning and discussion purposes only

- 8/12/2005

## Background

**Nuclear Thermal Propulsion**
**High specific impulse yields less mass required for launch to Mars**
**Uranium-235 fission reaction to heat liquid hydrogen propellant ROVER/NERVA program developed NTP engines in 1960s**
**Point of departure for MSFC**
-  LANL small engine
**Component Materials**
**Despite shield, still receive fluence**
**Nozzle, turbopump, lines, seals, valves, tie tubes**

- Regen Nozzle

- Radiative Nozzle

- Tie Tubes

- Slats

- Reflector

- Core

- Shield

- Chamber

- PRE-DECISIONAL - For planning and discussion purposes only

- 8/12/2005

## Objectives

**Main Objectives**
**Understand radiation effects on materials**
**Make judgments on materials projected for use outside the core based on radiation properties **
**Process**
**Literature study on radiation hardening of materials**
**Obtain list of materials used on chemical engine components**
**Literature study of data from ROVER/NERVA program and other programs testing radiation effects on materials**
**Compiling questions and suggestions**
**Final report**
- Thorough explanation of radiation effects
- Analysis of each individual material
**	**

- PRE-DECISIONAL - For planning and discussion purposes only

- 8/12/2005

## Radiation Effects on Metals

**Gamma Rays and Ionizing Radiation**
**Electron cloud structure in metals**
**Uniform heating**
**Neutron Radiation**
**Atomic collisions with fast neutrons yield lattice vacancies and interstitials**
**Molten region resulting from high energy density**
**Formation of displacement spike**
**Effects of Vacancies and Interstitials**
**Decreased density**
**Reduced plasticity and ductility**
**Increased hardness (yield strength)**
**Changes in ductile to brittle transition temperature**

- PRE-DECISIONAL - For planning and discussion purposes only

- 8/12/2005

## Radiation Effects on Polymers

**Gamma Rays and Ionizing Radiation**
**Primary source of damage**
**Degradation depends on bond structure**
**Covalent vs. ionic bonds**
**Structural Effects **
**Usually irreversible**
**Radiation-induced excitation localized to specific bond**
**Cross linking**
- Increases molecular weight
**Chain scission**
- Decreases molecular weight
- Decrease in Young’s modulus and yield stress
**Vacuum Effects **
**Sublimation/evaporation**
**Removal of absorbed gas layer on surface**

- cross linked polymer

- PRE-DECISIONAL - For planning and discussion purposes only

- 8/12/2005

## Neutron Absorption Cross Sections

| Element | σa thermal (barns) | σs thermal (barns) |
| --- | --- | --- |
| Be | .0095 | 7 |
| Mg | .063 | 4 |
| Zr | .18 | 8 |
| Al | .235 | 1.4 |
| Nb | 1.1 | 5 |
| Mo | 2.6 | 7 |
| Fe | 2.53 | 11 |
| Cr | 3.1 | 3 |
| Cu | 3.8 | 7.2 |
| Ni | 4.6 | 17.5 |
| V | 4.9 | 5 |
| Ti | 6.1 | 4 |
| Mn | 13.3 | 2.3 |
| W | 19 | 5 |
| Ta | 21 | 5 |

- Measure of the probability that a nuclear reaction will occur

- PRE-DECISIONAL - For planning and discussion purposes only

- 8/12/2005

## Radiation in NTP

- Neutron flux around turbopump area:
- 1011-1012 n/cm2s
- Fluence:
- 3.6x1015-3.6x1016 n/cm2

- Neutron flux around nozzle area: 3x1012 n/cm2s
- Fluence:
- 1.08x1017 n/cm2

- Gamma flux around turbopump area:
- 103 rad/s
- Fluence: 3.6x107 rad

- Gamma flux around nozzle area:
- 3x104 rad/s
- Fluence: 1.08x109 rad

- PRE-DECISIONAL - For planning and discussion purposes only

- 8/12/2005

## Use of Metals in NTP

**Low neutron-absorption cross sections**
**More deleterious effects in cryogenic temperatures**

- PRE-DECISIONAL - For planning and discussion purposes only

- 8/12/2005

## Use of Copper in NTP

**Coppers Qualities**
**Low strength, especially compared with Inconel**
**Low CTE and high thermal conductivity**
- Lower melting point
**12.8 hour half life**
**3.8 barns cross section**
- Nickel: 4.6 barns
- Titanium: 5.8 barns
**Copper vs. Nickel**
**Similar structure**

- PRE-DECISIONAL - For planning and discussion purposes only

- 8/12/2005

## Use of Polymers in NTP

***Use of Polymers in NTP***

**Effects vary by polymer classification**
**Epoxy, phenolic, and polyester glass fabric laminates acceptable in radiation (at 10****8**** rad)**
**Elastomers, Teflon, Kel-F, and many others experience total degradation at low doses of radiation (around 10****7**** and 10****8**** rad)**
**Cryogenic temperatures **
**     ****slow polymer degradation**
**Most polymers cannot be **
**     ****used in components**

- PRE-DECISIONAL - For planning and discussion purposes only

- 8/12/2005

## Conclusions

**Analysis of metals**
**Most appear adequate for use in NTP components.**
- Cobalt should not be used.
**Copper**
**Appears reasonable to proceed with nozzle design using copper**
- Similar to nickel
- Good thermal properties
- Must be cooled and supported in non-nickel structural jacket
**Analysis of polymers**
**Do not appear satisfactory for use in NTP components**
- Vespel suitable
- Glass epoxies acceptable

- PRE-DECISIONAL - For planning and discussion purposes only

- 8/12/2005

## Questions?

- PRE-DECISIONAL - For planning and discussion purposes only

- 8/12/2005